DEMOGRAPHY IN THE CACTACEAE
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The Botanical Review 69(2): 173–203
Demographic Trends in the Cactaceae
HÉCTOR GODÍNEZ–ÁLVAREZ
Unidad de Biología, Tecnología y Prototipos
Facultad de Estudios Superiores Iztacala
Universidad Nacional Autónoma de México
Avenida de los Barrios 1, Los Reyes Iztacala
Tlalnepantla 54090, Ap. Postal 314, Edo. de México, Mexico
TERESA VALVERDE
Departamento de Ecología y Recursos Naturales
Facultad de Ciencias
Universidad Nacional Autónoma de México
Ciudad Universitaria
Mexico City, D.F. 04510, Mexico
AND
PABLO ORTEGA–BAES
Laboratorio de Investigaciones Botánicas
Facultad de Ciencias Naturales
Universidad Nacional de Salta
Buenos Aires 177, Salta 4400, Argentina
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
X.
XI.
Abstract/Resumen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Distribution and Abundance Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seed Germination and Seedling Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Growth and Reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Size Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Population Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Elasticity Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Matrix Models and Matrix Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix 1: Demographic Information, Endangerment, and Geographical
Distribution of Cactus Species with Different Life-Forms . . . . . . . . . . . . . . . .
Copies of this issue [69(2)] may be purchased from the NYBG Press,
The New York Botanical Garden, Bronx, NY 10458–5128, U.S.A.;
nybgpress@nybg.org. Please inquire as to prices.
Issued 16 December 2003
© 2003 The New York Botanical Garden
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I. Abstract
Although our biological knowledge regarding cactus species is thorough in many areas,
only in recent years have ecologists addressed their demographic behavior. Here we attempt a
first review of the present knowledge on cactus demography, including an analysis of the published information on species with different growth forms and life-history traits. Our review
shows that cactus distribution ranges are determined by environmental heterogeneity and by
species-specific physiological requirements. Temperature extremes may pose latitudinal and
altitudinal distribution limits. At a more local scale, soil properties dramatically affect cactus
distribution. Most cacti show a clumped spatial distribution pattern, which may be the reflection of a patchy resource distribution within their heterogeneous environments. The association of cacti with nurse plants is another factor that may account for this aggregated distribution.
Many cacti grow in association with these perennial nurse plants, particularly during early lifecycle phases. The shade provided by nurse plants results in reduced evapotranspiration and
buffered temperatures, which enhance cactus germination and establishment. In some cases a
certain degree of specificity has been detected between particular cactus species and certain
nurse plants. Yet some globose cacti may establish in the absence of nurse plants. In these
cases, rocks and other soil irregularities may facilitate germination and establishment.
Cacti are slow-growing species. Several abiotic factors, such as water and nutrient availability, may affect their growth rate. Competition and positive associations (i.e., mycorrhizae
and nurse–cacti association) may also affect growth rate. Age at first reproduction varies greatly
in relation to plant longevity. In general, cactus reproductive capacity increases with plant size.
Populations are often composed of an uneven number of individuals distributed in the different
size categories. This type of population structure reflects massive but infrequent recruitment
events, apparently associated with benign periods of abundant rainfall.
A few cactus species have been analyzed through the use of population-projection matrices.
A total of 17 matrices were compiled and compared. Most of them reflect populations that are
close to the numerical equilibrium ( = close to unity). Elasticity analyses revealed that the
persistence of individuals in their current size category (“stasis”) is the demographic process
that contributes the most to population growth rate. Also, adult categories (rather than juveniles
or seedlings) show the largest contributions to . No differences were apparent regarding this
matter between cacti with different life-forms. This review shows that our knowledge of cactus
population ecology is still incipient and rather unevenly distributed: some topics are well developed; for others the available information is still very limited. Our ability to preserve the
great number of cactus species that are now endangered depends on our capacity to deepen our
ecological understanding of their population processes.
Resumen
A pesar de que nuestro conocimiento biológico sobre las cactáceas es basto en muchas
áreas, ha sido sólo hasta fechas recientes que los ecólogos han abordado el estudio de su
comportamiento demográfico. En este artículo presentamos una primera revisión del conocimiento actual sobre demografía de cactáceas, incluyendo un análisis de la información publicada
sobre especies con diferentes formas de crecimiento y características de historia de vida. Nuestra
revisión muestra que el área de distribución de las cactáceas se ve afectada por la heterogeneidad
del ambiente y por los requerimientos fisiológicos de cada especie. Los valores extremos de
temperatura fijan límites latitudinales y altitudinales de distribución a muchas cactáceas. A una
escala más local, las propiedades del suelo juegan un papel fundamental. La mayoría de los
cactus muestran una distribución espacial agregada, lo cual es un reflejo de la distribución
DEMOGRAPHY IN THE CACTACEAE
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aparchonada de los recursos en los ambientes altamente heterogéneos que habitan. La asociación
de los cactus con plantas nodriza es otro de los factores que explica su distribución agregada.
Muchos cactus crecen en asociación con estas plantas nodriza, particularmente durante los
primeros estadios. La sombra de estas plantas perennes reduce la evapotranspiración y amortigua
la temperatura, lo cual incrementa la germinación y el establecimiento de los cactus. En algunos
casos se ha detectado especificidad entre especies particulares de cactus y ciertas plantas nodriza.
Aun así, algunos cactos globosos pueden establecerse en ausencia de plantas nodrizas. En
estos casos, las rocas y otras irregularidades del terreno podrían facilitar la germinación y el
establecimiento.
Los cactus son especies de lento crecimiento. Varios factores abióticos, como el agua y la
disponibilidad de nutrientes, pueden afectar su tasa de crecimiento. La competencia y las
asociaciones positivas (i.e., formación de micorrizas y asociación con plantas nodriza) también
pueden afectar su tasa de crecimiento. La edad a la primera reproducción varía mucho con
relación a la longevidad de las plantas. En general, la capacidad reproductiva de los cactus
aumenta conforme aumenta su tamaño. Las poblaciones frecuentemente están compuestas de
individuos distribuidos de manera irregular entre las diferentes clases de tamaño. Este tipo de
estructura poblacional refleja eventos masivos pero poco frecuentes de reclutamiento,
aparentemente asociados a períodos benignos de abundantes lluvias.
Sólo unas cuantas especies de cactus se han estudiado a través de matrices de proyección
poblacional. En este estudio se compilaron y se compararon 17 matrices. La mayoría de ellas
reflejan poblaciones que se encuentran cerca del equilibrio numérico ( = cerca de la unidad).
Los análisis de elasticidad revelaron que la persistencia de los individuos en su misma categoría
de tamaño (“estasis”) es el proceso demográfico que mayormente contribuye a la tasa de
crecimiento poblacional. También se vio que las categorías de adultos (y no las de juveniles o
plántulas) fueron las que mostraron un mayor aporte a . No se encontraron diferencias a este
respecto entre especies de cactus con diferente forma de crecimiento. Esta revisión muestra que
nuestro conocimiento sobre la ecología poblacional de los cactus es aún incipiente y está
distribuido de manera dispareja: algunos temas están bien desarrollados mientras que para
otros la información disponible es aún muy limitada. La posibilidad de conservar el gran número
de especies de cactáceas que se encuentran amenazadas depende de nuestra capacidad para
profundizar en el entendimiento ecológico de sus procesos poblacionales.
II. Introduction
Cacti are a very diverse and complex plant family comprising more than 2000 species, most
of which are native to arid and semi-arid environments in the Americas (Bravo-Hollis & SánchezMejorada, 1978, 1991). The extraordinary adaptations of these plants to low water availability
and other stressful conditions have made them the subject of a number of morphological and
physiological studies, including aspects related to their photosynthetic pathways, their waterstorage ability, and the various other ways in which they optimize their water-use efficiency
(among others, Bravo-Hollis & Sánchez-Mejorada, 1978, 1991; Gibson & Nobel, 1986; Nobel,
1988; Altesor et al., 1992). This information, along with a host of botanical records and taxonomic analyses, has been tremendously valuable in aiding our understanding of the biology
and evolution of this fascinating plant family. Yet the lack of ecological information on cacti is
still notorious in the biological literature. In recent years a number of authors have published
ecological studies on cacti, covering aspect such as reproductive biology (Tinoco-Ojanguren &
Molina-Freaner, 2000; Fleming et al., 2001), plant–pollinator and plant–disperser interactions
(Fleming & Valiente-Banuet, 2002; Godínez-Álvarez et al., 2002), the ecophysiology of ger-
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mination and early growth (Rojas-Aréchiga & Vázquez-Yanes, 2000; Ruedas et al., 2000; Flores
& Briones, 2001; Rojas-Aréchiga et al., 2001), and cactus–nurse plant interactions (McAuliffe,
1984a, 1984b; Valiente-Banuet & Ezcurra, 1991; Valiente-Banuet et al., 1991a, 1991b; FloresMartínez et al., 1994, 1998). Additionally, recent articles have addressed the demography of
cactus populations, including species with contrasting distribution ranges, growth forms, and
life-history traits. It is this kind of ecological information that we are concerned with in the
present article. We believe that our present knowledge of the demography of cactus species
deserves a first attempt to search for patterns and trends that may influence the future development of this particular area of plant ecology. A detailed evaluation of the present status of our
knowledge will allow us to establish priorities and encourage researchers to develop ideas
about particular issues that may eventually fill the gaps on this subject.
Part of the need to study the demographic trends emerging in the Cactaceae is related to the
fact that there is a high number of threatened species in this plant family (Anderson & Taylor,
1994; Hunt, 1999). In fact, the whole family is included in appendix 2 of the CITES book of
endangered species (Hunt, 1999). Some of the reasons that may account for this are: 1) Many
cacti are highly restricted in their distribution and occupy very specific habitats, which makes
them prone to extinction by habitat destruction and land-use change (Esparza-Olguín et al.,
2002); 2) They seem to be particularly sensitive to disturbances due to their low individual
growth rates and highly vulnerable early stages of development (Hernández & Godínez-Álvarez,
1994); 3) Illegal collection and trade are an increasing pressure on many cactus populations,
especially since they have become more fashionable in recent years; and 4) Their habitats are
frequently associated with poor areas in developing countries, where the pressure of increasing
human populations on land-use change toward farming and cattle ranching are very strong.
Thus the increasing demographic information on cactus species will certainly aid in the design
of conservation and management plans and will offer a tool with which to evaluate the conservation status of poorly known species in this plant family.
In addition to conservation and management interests, demographic information is the basis on which our understanding of population dynamics and life-history traits is supported
(Silvertown et al., 1993). Recent demographic information on cactus species is adding significantly to our understanding of plant population dynamics in nature, since they include longlived species that inhabit dry tropical areas, two particularities that are poorly represented in
the demographic literature. In this article we attempt a first review of the demographic features
of cactus species, according to the literature published to date, which is summarized in Appendix 1. A decade ago Silvertown et al. (1993) published their first literature review on plant
demographic trends. In that review, in which interesting trends between demography and lifeform emerged, only one cactus, Carnegiea gigantea, was included. In this article we use the
approach developed by Silvertown et al. (1993) to analyze the information on cactus demography published mainly in the last decade. We use a comparative approach that focuses on cactus
species with different life-forms, since a correlation between life-form and some demographic
properties appears to be emerging among desert plants (Flores & Briones, 2001). We consider
the following life-forms, according to the classification used by other authors (Gibson & Nobel,
1986): columnar cacti (species with long, column-like, ribbed stems measuring >2.0 m in height);
barrel cacti (plants with ribbed stems, with a maximum height of 0.5–2 m); globose cacti (species with single or multiple hemispherical stems); cylindropuntias (opuntias with cylindrical
stems); and platyopuntias (opuntias with flattened stems). In addition to data on population
dynamics, we include valuable information on other aspects of cactus population ecology, such
as distribution and abundance patterns, data on growth, reproduction, and population structure, and survival and establishment during early life-cycle phases. Our aim is to search for
DEMOGRAPHY IN THE CACTACEAE
177
trends and patterns that may contribute to our understanding of the factors that determine longterm population dynamics in nature, which still remain a central issue in ecology (Horvitz &
Schemske, 1995).
III. Distribution and Abundance Patterns
Studies on the distribution and abundance patterns of cacti have shown that these ecological
attributes may vary both temporally and spatially (Bowers et al., 1995; de Viana, 1996–1997;
Bowers, 1997b; Mourelle & Ezcurra, 1997; Parker, 1988a, 1993). With regard to the distribution of cacti at a regional scale, environmental heterogeneity as well as the particular physiological requirements of each species appear to determine their distribution range. For instance,
the columnar cacti Carnegiea gigantea, Lophocereus schottii, and Stenocereus thurberi occupy different habitats along the topographic and edaphic gradients of the Sonoran Desert
(Parker, 1988a). In addition, the northern distribution limits of these species appear to be determined by the occurrence of low temperatures, which severely damage their stem apices (Nobel,
1980). Mourelle and Ezcurra (1997) also mention that variation in environmental factors such
as temperature, topography, and rainfall may be associated with the occurrence of different
Argentine cacti.
At a local scale, the three spatial distribution patterns described in the ecological literature
(random, uniform, and clumped) have been found in cactus populations (Gulmon et al., 1979;
de Viana et al., 1990; Martínez et al., 1993; Huerta & Escobar, 1998; Valverde et al., 1999;
Cody, 2000). Yet clumped distributions appear to be far more common than random or uniform
distributions. It has been suggested that the aggregation of individuals that has been found in
cacti with different life-forms (i.e., Capiapoa cinerea, Ferocactus hystrix, Ariocarpus trigonus,
Mammillaria gaumeri, M. magnimamma, M. oteroi, Turbinicarpus pseudopectinatus, Trichocereus pasacana, and Neobuxbaumia macrocephala) may be related to the patchy distribution
of conditions that enhance seedling establishment and plant growth (de Viana et al., 1990;
Valverde et al., 1999; Martínez et al., 2001; Esparza-Olguín et al., 2002), which is bound to
exert a particularly strong effect on those species that require the shade of a nurse plant. Other
potential mechanisms that may also account for a clumped distribution pattern are restricted
seed dispersal (Martínez et al., 1993, 1994) and patchy distribution of seeds in the soil (Hutto
et al., 1986; de Viana, 1996–1997; de Viana et al., 2001). Additionally, some studies have
shown that young and adult plants within a population may have different distribution patterns.
Intraspecific interactions, such as competition leading to self-thinning processes, may be responsible for this variation (de Viana et al., 1990).
In regard to abundance patterns, it is difficult to find a general trend among cactus species
because population densities may vary greatly, from very few individuals per hectare to thousands per hectare (Valiente-Banuet & Ezcurra, 1991; Martínez et al., 1993, 1994; Schmalzel
et al., 1995; de Viana, 1996–1997; Huerta & Escobar, 1998; Mandujano et al., 1998; Valverde et
al., 1999; Esparza-Olguín et al., 2002). The density of a population is the result of its birth and
death rates, which may be affected by a host of biotic and biotic factors at regional and local
scales. Apparently, massive but infrequent recruitment events are associated with favorable
rainfall and temperature periods. In particular, infrequent events such as “El Niño” may increase the number of new recruits in some populations. This has been suggested for the columnar cacti Carnegiea gigantea and Stenocereus thurberi (Parker, 1993; Pierson & Turner, 1998),
the barrel cacti Echinocactus polycephalus, Ferocactus acanthodes, and F. cylindraceus (Jordan & Nobel, 1981; Goldberg & Turner, 1986; Bowers et al., 1995; Bowers, 1997b), and the
platyopuntias Opuntia acanthocarpa, O. basilaris, and O. erinacea (Bowers et al., 1995).
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Contrastingly, Haman (2001) reports that El Niño events have an adverse effect on two species
of Opuntia (O. echios and O. galapageia) in the Galapagos Islands, by increasing seedling,
juvenile, and adult mortality due to water logging following heavy rainfall. In addition to rainfall patterns, other factors such as the length of the dry period, the availability of safe sites for
establishment and growth, the presence of trees and shrubs in the habitat, soil erosion, and the
effect of livestock grazing are important in determining cactus density at a particular site (Turner
et al., 1966; Jordan & Nobel, 1981, 1982; Nobel, 1989; Valiente-Banuet & Ezcurra, 1991;
Parker, 1993; Bowers et al., 1995; Bowers, 1997b; Pierson & Turner, 1998; Esparza-Olguín et
al., 2002).
Soil characteristics appear to play an important role in the distribution and abundance of
cactus species in arid and semi-arid landscapes (Parker, 1991). The edaphic conditions in which
some species live may be highly specialized. It has been observed that columnar cacti such as
Cephalocereus columna-trajani, Escontria chiotilla, Neobuxbaumia tetetzo, Pachycereus fulviceps, P. pringlei, Stenocereus gummosus, and S. thurberi, as well as globose cacti such as
Mammillaria crucigera, are associated with particular soil types (Meyrán, 1980; Valiente-Banuet
et al., 1995; Ortega-Baes, 2001; Contreras & Valverde, 2002; Valiente-Banuet & GodínezÁlvarez, 2002). Certain edaphic properties, such as soil texture, may dramatically affect water
availability, thus preventing the establishment of certain species while favoring others, depending on their particular water requirements and tolerances for germination and early growth.
IV. Seed Germination and Seedling Establishment
A great number of studies have addressed the germination behavior of cactus species under
both experimental and natural conditions. Since Rojas-Aréchiga and Vázquez-Yanes (2000)
recently published a thorough review of the germination ecology of cacti, in this section only
some general aspects will be discussed.
The main factors that affect seed germination are water, temperature, and light. However,
embryo immaturity, salinity, seed age, plant hormones, inhibitory compounds in the testa, plant
domestication, and other factors may also affect this process (Rojas-Aréchiga & VázquezYanes, 2000; Rojas-Aréchiga et al., 2001). Of all these factors, water availability appears to be
the most important for germination of cactus seeds, since it is the most limiting factor under the
conditions that prevail in deserts. Dubrovsky (1996, 1998) found that seeds of the columnar
cacti Carnegiea gigantea, Pachycereus pecten-aboriginum, Stenocereus gummosus, and S. thurberi, as well as seeds of the barrel cactus Ferocactus peninsulae, germinated more rapidly and
accumulated higher biomass when they were subjected to hydration–dehydration cycles. This
phenomenon is known as “seed hydration memory” and may facilitate the germination process,
leading to an increase in the probability that seedlings will survive (Dubrovsky, 1996).
Most studies of seed germination of cactus species have been conducted under controlled
conditions, and only a few have considered their germination behavior in the field. Further
field research is necessary in this area, since there are important aspects of seed ecology of
which the demographic implications are ignored. One of these aspects is the potential of cactus
seeds to remain viable in the soil for long periods of time, forming long-term buried seed banks
(Rojas-Aréchiga & Vázquez-Yanes, 2000; Rojas-Aréchiga & Batis, 2001). The only available
information on this subject suggests that seeds of the barrel cactus Ferocactus wislizeni may
remain viable in the soil for at least 18 months; postdispersal seed predation in this species is
high, although seeds may escape from predators when hidden among rocks (Bowers, 2000).
Also, Ortega-Baes (2001) found viable seeds of Escontria chiotilla of unknown age (at least 12
months old) buried in the soil, and field experiments with Mammillaria magnimamma showed
DEMOGRAPHY IN THE CACTACEAE
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that its seeds may remain viable in the soil for more than a year (Valverde et al., in press).
Additionally, several Mammillaria species have been reported to retain fruits in their stems,
which may play a similar ecological role as a buried seed bank (Rodríguez-Ortega & Franco,
2001).
Studies of seed germination and seedling survival of cactus species under natural conditions have shown that the presence of perennial plants is necessary for the successful recruitment of new individuals (Niering et al., 1963; Turner et al., 1966; Steenbergh & Lowe, 1969,
1977; Jordan & Nobel, 1981, 1982; McAuliffe, 1984a; Valiente-Banuet & Ezcurra, 1991;
Mandujano et al., 1998). Under the harsh environmental conditions that prevail in deserts,
these perennial nurse plants function as islands within which physical and/or biotic conditions
are beneficial for the establishment and growth of young cacti (Callaway, 1995). The nurse
plants decrease the contrast between maximum and minimum soil temperatures (Shreve, 1931;
Jordan & Nobel, 1981; Hutto et al., 1986; Valiente-Banuet et al., 1991a; Arriaga et al., 1993;
Suzán et al., 1996), provide protection against predators (Steenbergh & Lowe, 1977; Hutto et
al., 1986; Valiente-Banuet & Ezcurra, 1991), increase soil-nutrient availability under their canopies (García-Moya & McKell, 1970; Nobel, 1989), and provide a shady environment in which
evapotranspiration is substantially reduced (Turner et al., 1966; Steenbergh & Lowe, 1977;
Nolasco et al., 1997; Godínez-Álvarez & Valiente-Banuet, 1998). Additionally, they constitute
roosting sites for dispersers, thereby attracting important amounts of seed dispersal (Steenbergh
& Lowe, 1977; Yeaton, 1978; Hutto et al., 1986; Godínez-Álvarez et al., 2002).
The literature on the nurse-plant phenomenon in cacti is summarized in Table I. The first
point we may note from this table is that the association with perennial plants occurs in all of
the life-forms that are represented in the cactus family. For instance, Suzán et al. (1996) reported that 17 cactus species from the Sonoran Desert with different life-forms grow in association with the long-lived tree Olneya tesota. Also, Arriaga et al. (1993) reported that the
number of individuals of Stenocereus thurberi and Ferocactus peninsulae was significantly
higher under the canopy of perennial plants than in open spaces in the tropical dry forests of
northwestern Mexico. In the Tehuacán Valley of central Mexico, five cactus species with different life-forms were found to be associated with different perennial plants (Valiente-Banuet
et al., 1991a). In South America, the columnar cactus Trichocereus atacamensis shows a positive association with trees and shrubs (de Viana, 1996–1997; de Viana et al., 2001). In most of
these cases it has been suggested that the shade provided by nurse plant is the main factor that
enhances seedling establishment by buffering extreme temperatures and reducing evapotranspiration.
The identity of nurse plants so far reported is varied (Table I). Different plant species may
play this role, including shrubs, trees, grasses, agaves, and even cacti (McAuliffe, 1984b; Hutto
et al., 1986; Nobel, 1989; Valiente-Banuet et al., 1991a; Suzán et al., 1996; Mandujano et al.,
1998; de Viana et al., 2001; Ortega-Baes, 2001). The number of cactus seedlings and juveniles
established beneath the canopies of some nurse-plant species is higher than expected by chance,
according to their relative abundance in their plant communities (Hutto et al., 1986; ValienteBanuet et al., 1991a, 1991b; Mandujano et al., 1998; de Viana et al., 2001; Ortega-Baes, 2001).
This pattern suggests that, in some cases, the nurse–cactus association may have reached a
certain degree of specificity, which may be related to particular traits (i.e., morphology, plant
architecture, phenology) that determine differences in the quality of each perennial plant as
nurse for different cactus species (Callaway, 1998). Certain traits, such as the presence of
spines, deep roots, long-lived leaves, secondary metabolites, and nitrogen-fixation ability, among
others, may favor the establishment of new individuals under the canopy of specific perennial
plants (McAuliffe, 1984a; Suzán et al., 1996; Callaway, 1998).
Species
Columnar cacti
Carnegiea gigantea*
Lophocereus schottii*
Neobuxbaumia macrocephala
N. tetetzo*
Pachycereus pringlei
Stenocereus thurberi
Ambrosia deltoidea
Cercidium microphylum
Encelia farinosa
Larrea tridentata
Olneya tesota
Prosopis juliflora
Caesalpinia melanadenia
Acacia cochliacantha
Fouquieria formosa
Mimosa luisana
M. polyantha
Zizyphus pedunculata
Celtis pallida
Lycium andersonii
L. berlandieri
Olneya tesota
Zizyphus obtusifolia
Lippia graveolens?
Caesalpinia melanadenia
Castela tortuosa
Eupatorium odoratum
Mimosa luisana
Olneya tesota
Prosopis articulata
Haematoxylon brasiletto
Jatropha vernicosa
Tecoma stans
Life-cycle stage Mechanism
Source
S–J–A
Shade
Frost protection
Predation avoidance
Turner et al., 1966
Steenbergh & Lowe, 1969; Hutto et al., 1986
Franco & Nobel, 1989
S–J
S–J–A
Shade
Shade
Valiente-Banuet et al., 1991a
Ortega-Baes, 2001
S–J
Shade
Frost protection
Suzán et al., 1996
Parker, 1989
S–J?
S–J
Shade?
Shade
Esparza-Olguín et al., 2002
Valiente-Banuet et al., 1991a
Valiente-Banuet & Ezcurra, 1991
Flores-Martínez et al., 1994
S–J
Shade
Increased soil nutrients
Shade
Valiente-Banuet et al., 1995
Carrillo-García et al., 2000
Parker, 1987
Arriaga et al., 1993
S–J–A
THE BOTANICAL REVIEW
Cephalocereus hoppenstedtii*
Escontria chiotilla*
Nurse plant
180
Table I
Association between nurse plants and different cacti species
(* = species with a higher number of recruits growing in association with nurse plants than expected by chance; S = seedlings; J = juveniles;
A = adults; Mechanism = advantages offered by the nurse plant to the growing cacti; ? = uncertainty in the proposed aspect)
Trichocereus atacamensis*
Barrel cacti
Echinocereus conglomeratus
E. engelmannii*
Echinomastus erectrocentrus
F. cylindraceus
F. peninsulae
Hamatocactus hamatacanthus
Peniocereus greggii
P. striatus
Globose cacti
Ariocarpus trigonus
Coryphantha pallida*
–
Castela tortuosa
Eupatorium odoratum
Caesalpinia melanadenia
Castela tortuosa
Castela tortuosa
Eupatorium odoratum
Shade
De Viana, 1996–1997
De Viana et al., 2001
–
Silvertown & Wilson, 1994
Predation avoidance
Shade
McAuliffe, 1984b
Suzán et al., 1996
S
Shade
Increased soil nutrients
Franco & Nobel, 1989
S–J–A
S–J–A
–
Increased soil nutrients
Shade
–
Nobel, 1989
Arriaga et al., 1993
Silvertown & Wilson, 1994
S–J–A
S–J–A
Shade
Shade
Suzán et al., 1996
Suzán et al., 1996
S–J–A
S–J–A
Shade?
Shade
Martínez et al., 1993
Valiente-Banuet et al., 1991a
S–J–A
–
S–J–A
Shade
–
Shade
Valiente-Banuet et al., 1991a
Rodríguez-Ortega & Ezcurra, 2001
Valiente-Banuet et al., 1991a
–
S–J–A
S–J–A
181
Mammillaria casoi*
M. carnea
M. colina*
Euphorbia antisyphilitica
Hamatocactus hamatacanthus
Opuntia leptocaulis
Opuntia fulgida
Celtis pallida
Lycium andersonii
L. berlandieri
Olneya tesota
Zizyphus obtusifolia
Ambrosia dumosa
Ephedra aspera
Hilaria rigida
Hilaria rigida
Haematoxylon brasiletto
Euphorbia antisyphilitica
Opuntia leptocaulis
Celtis pallida
Lycium andersonii
L. berlandieri
Olneya tesota
Zizyphus obtusifolia
S–J–A
DEMOGRAPHY IN THE CACTACEAE
Ferocactus acanthodes
Aphyllocladus spartoides
Larrea divaricata
Prosopis ferox
182
Table I, continued
Species
Globose cacti, continued
M. gaumeri
M. microcarpa
M. oteroi
M. thornberi
O. echinocarpa
O. leptocaulis
O. ramosissima
Platyopuntias
O. rastrera*
O. streptacantha
–
Opuntia fulgida
Quercus castanea
Celtis pallida
Lycium andersonii
L. berlandieri
Olneya tesota
Zizyphus obtusifolia
Hilaria rigida
Krameria sp.
Hilaria rigida
Krameria sp.
Larrea tridentata
Hilaria rigida
Krameria sp.
Cordia parvifolia
Hilaria mutica
Jatrpoha dioica
Prosopis glandulosa
Acacia schaffneri
Life-cycle stage Mechanism
Source
S–J
Shade?
–
S–J–A
–
Shade
Leirana-Alcocer & Parra-Tabla, 1999
McAuliffe, 1984b
Martínez et al., 2001
Suzán et al., 1996
–
–
Cody, 1993
–
–
Cody, 1993
S–J
–
Seed dispersal site?
–
Yeaton, 1978
Cody, 1993
S–J
Shade
Predation avoidance
Mandujano et al., 1998
Shade
Flores & Yeaton, 2000
S–J–A
THE BOTANICAL REVIEW
Cylindropuntias
Opuntia acanthocarpa
Nurse plant
DEMOGRAPHY IN THE CACTACEAE
183
Despite the fact that nurse plants may offer cacti some protection during the initial life-cycle
phases, mortality is tremendously high at this stage. Survivorship curves of cactus seedlings
growing under the canopies of different trees and shrubs are mainly type III (sensu Pearl, 1928;
see Appendix 1; Turner et al., 1966; Steenbergh & Lowe, 1969; Valiente-Banuet & Ezcurra,
1991; Mandujano et al., 1998; Esparza-Olguín et al., 2002; Valverde et al., in press). Yet the
intensity of seedling mortality is lower beneath the canopies of nurse plants than in open spaces.
For instance, seedlings of Mammillaria gaumeri associated with nurse plants showed a constant mortality rate through time (i.e., type II), whereas those growing in bare soil showed a
clear type III curve (Leirana-Alcocer & Parra-Tabla, 1999).
Although the evidence discussed so far may suggest that the nurse plant–cactus association
is prevalent throughout the Cactaceae family (Table I), we must bear in mind that there are a
number of examples in which new cactus seedlings have been reported to establish in the
absence of nurse plants. Curiously, these species are mainly globose cacti, such as Ariocarpus
fissuratus, Epithelantha bokei, Mammillaria magnimamma, M. lasiacantha, M. pectinifera, and
Turbinicarpus pseudopectinatus, which may grow in open spaces (Nobel et al., 1986; Arriaga et
al., 1993; Martínez et al., 1994; Valverde et al., 1999; Rodríguez-Ortega & Ezcurra, 2001). For
other desert plants the presence of rocks on the ground, as well as other irregularities, appear to
be of vital importance for seed germination and/or seedling survival. Rocks may reduce solar
radiation and thermal loadings, as well as prolong the presence of soil moisture in their immediate vicinity (Larmuth & Harvey, 1978). Thus the importance of rocks and other surface irregularities (cavities, holes, etc.) as potential facilitators of germination and seedling survival
in different cactus species is one of the areas to which further research should be directed.
V. Growth and Reproduction
After seed germination and seedling establishment, growth and reproduction of cacti may
be affected by extrinsic factors such as biological interactions and environmental conditions, as
well as by intrinsic factors such as genetic variation, allometry, and metabolic rates. Biological
interactions with other plants, animals, or microorganisms may have a positive or negative
impact on cacti. During early growth, positive interactions such as the association with vesicular-arbuscular mycorrhizal fungi can enhance biomass production by increasing nutrient
absorption efficiency. This appears to be the case of the columnar cactus Pachycereus pectenaboriginum in a tropical dry forest in Mexico (Rincón et al., 1993). The presence of mycorrhizae has also been reported in Mammillaria magnimamma (Ruedas et al., 2000), although
their effect on the growth of this species has not been analyzed. Other biotic interactions may
negatively affect the growth of cacti; such is the case of intra- and interspecific competition.
Paradoxically, these interactions are enhanced under the canopies of trees and shrubs where
seedling establishment is most successful. McAuliffe and Janzen (1986) documented the effect
of intraspecific competition for limited water in dense aggregations of young Carnegiea gigantea
plants associated with the canopy of perennial shrubs; in this case, the effects of competition
were decreases in water uptake, water storage, apical growth, and reproductive potential
(McAuliffe & Janzen, 1986). It has also been reported that some columnar cacti (C. gigantea
and Neobuxbaumia tetetzo) may establish intense interspecific competition for water with their
nurse plants, producing a decline in leaf, flower, and fruit production in the nurse trees and
shrubs (McAuliffe, 1984a; Flores-Martínez et al., 1994, 1998). This type of interaction has
also been documented between the barrel cactus Ferocactus acanthodes and the perennial
bunchgrass Hilaria rigida (Franco & Nobel, 1989), between Opuntia leptocaulis and the shrub
Larrea tridentata (Yeaton, 1978), and between O. rastrera, the perennial tussock grass Hilaria
mutica, and the shrub L. tridentata (Briones et al., 1998). Further research is needed to deepen
184
THE BOTANICAL REVIEW
our understanding of the way in which both positive and negative interactions affect cactus
growth.
In addition to biotic interactions, cactus growth may be affected by environmental conditions. The literature on this subject indicates that winter precipitation and frost decrease the
growth rate of Lophocereus schottii and Stenocereus thurberi (Parker, 1988b). In contrast, high
summer and low winter temperatures, soil-moisture availability, and pre- and postsummer dry
periods are among the main environmental factors that affect the growth rate of Carnegiea
gigantea (Steenbergh & Lowe, 1977). Experimental analyses of the early growth of cacti also
show that it is affected by environmental conditions. For instance, a high concentration of
nutrients significantly increased seedling growth in Mammillaria magnimamma, Pachycereus
hollianus, and P. pringlei (Godínez-Álvarez & Valiente-Banuet, 1998; Carrillo-García et al.,
2000; Ruedas et al., 2000). Effects of direct solar radiation were contradictory, for it increased
seedling growth in M. magnimamma (Ruedas et al., 2000) but decreased the growth of
Neobuxbaumia tetetzo, Opuntia rastrera, P. hollianus, P. pringlei, and S. thurberi seedlings
(Nolasco et al., 1997; Godínez-Álvarez & Valiente-Banuet, 1998; Mandujano et al., 1998;
Carrillo-García et al., 2000).
Genetic variation among individuals, as well as allometry, are also among the factors that
affect growth rates. With regard to allometry, there is some variation in the annual growth rate
of Carnegiea gigantea related to changes in plant form associated with ontogeny (Hastings &
Alcorn, 1961; Steenbergh & Lowe, 1977). Thus, after seedling establishment, individuals grow
rapidly until they reach a height of about 2 m. This fast growth is the result of an increase in the
photosynthetic surface area and water-storage capacity of individuals. However, growth rate
decreases when plants are between 2 and 6 m tall, since they start using some of the products of
photosynthesis to generate reproductive structures. In larger plants growth occurs mainly through
the proliferation of branches, while apparently the main stem reaches a maximum height (Shreve,
1910; Hastings & Alcorn, 1961; Steenbergh & Lowe, 1977).
The growth pattern is different for other species of columnar cacti, such as Lophocereus
schottii and Stenocereus thurberi. In these species the annual growth rate increases with increasing plant size (Parker, 1988b). Differences in growth rate among species of columnar
cacti may be related to the particular branching pattern of each species. For instance, L. schottii
and S. thurberi grow through the production of new stems, whereas Carnegiea gigantea produces a single stem with several branches.
Since all cactus species are slow-growing, long-lived plants, there is little information about
the relationship between size and age (Table II), and therefore age-related life-history traits
have been difficult to investigate. The available information shows that age at first reproduction is variable, depending on the species, life-form, and plant longevity. In the long-lived
columnar cacti, plants reproduce for the first time when individuals are 33 years old in Carnegiea
gigantea (Steenbergh & Lowe, 1977), 70 years old in Cephalocereus columna-trajani (ZavalaHurtado & Díaz-Solís, 1995), and more than 90 years old in Neobuxbaumia macrocephala
(Esparza-Olguín et al., 2002). For a shorter-lived globose cactus (Mammillaria magnimamma)
and a platyopuntia (Opuntia engelmannii), first reproduction occurs when individuals are 4
and 9–11 years old, respectively (Bowers, 1996a; Valverde et al., in press).
Once plants have reached reproductive size, fecundity appears to increase with increasing
size (Fig. 1; Appendix 1). This pattern was observed in species with different life-forms, such
as the globose cacti Coryphantha robbinsorum, Mammillaria crucigera, M. magnimamma
(Schmalzel et al., 1995; Contreras & Valverde, 2002; Valverde et al., in press), the platyopuntia
O. rastrera (Mandujano et al., 2001), the barrel cactus Echinomastus erectrocentrus (Johnson,
1992; Johnson et al., 1992), and the columnar cacti Carnegiea gigantea, Escontria chiotilla,
DEMOGRAPHY IN THE CACTACEAE
185
Table II
Age or size at which first reproduction occurs in cacti with different life-forms
Age
(years)
Size
(meters)
Columnar cacti
Carnegiea gigantea
Cephalocereus columna-trajani
Lophocereus schottii
Stenocereus thurberi
33
70
–
–
2.2
3.3
>2.0
2.0–2.5
4–10a
Barrel cacti
Echinomastus erectrocentrus
–
0.024
Johnson et al., 1992
Globose cacti
Coryphantha robbinsorum
Mammillaria magnimamma
–
4
0.013
–
Schmalzel et al., 1995
Valverde et al., in press
9–11
6–13b
Bowers, 1996a
Species
Opuntioid cacti
Opuntia englemannii
Source
Steenbergh & Lowe, 1977
Zavala-Hurtado & Díaz-Solís, 1995
Parker, 1989
Parker, 1987
a
Plant size measured as the number of branches.
b
Plant size measured as the number of cladodes.
Lophocereus schottii, Neobuxbaumia macrocephala, N. tetetzo, and Stenocereus thurberi
(Steenbergh & Lowe, 1977; Parker, 1987, 1989; Godínez-Álvarez et al., 1999; Ortega-Baes,
2001; Esparza-Olguín et al., 2002). It has been suggested that this pattern is related to an
increase in the number of branches as plants age, at least in columnar cacti (Steenbergh &
Lowe, 1977).
Reproductive phenology varies between species with different life-forms (see Appendix 1).
Species in the different genera of globose cacti, such as Mammillaria, Sclerocactus, and
Turbinicarpus (Martínez et al., 1994; Lockwood, 1995; Contreras & Valverde, 2002; Valverde
et al., in press), begin to reproduce in early winter, and their reproductive periods last for more
than 5 months. In contrast, species in the different genera of columnar cacti may reproduce in
autumn/winter (Cereus, Myrtillocactus, Pachycereus, Pilosocereus, Selenicereus, Stenocereus) or
spring/summer (Carnegiea, Cephalocereus, Escontria, Hylocereus, Lophocereus, Neobuxbaumia,
Pachycereus, Pilosocereus, Polaskia, Stenocereus, Subpilocereus), and their reproductive periods
last for 2 to 4 months (Steenbergh & Lowe, 1977; Weiss et al., 1994; Valiente-Banuet et al., 1996,
1997a, 1997b; Nassar et al., 1997; Casas et al., 1999; Rojas-Martínez et al., 1999; Ruíz et al., 2000;
Fleming et al., 2001; Esparza-Olguín et al., 2002). Other columnar cacti, however, may produce
flowers and fruits throughout the year (Praecereus, Stenocereus, Weberbaurocereus, Weberocereus;
Sahley, 1996; Nassar et al., 1997; Tschapka et al., 1999; Ruíz et al., 2000). Species in the genus
Opuntia produce flowers and fruits in spring/summer (O. brunneogemmia, O. compressa, O. discata, O. imbricata, O. lindheimeri, O. phaecantha, O. polyacantha, O. rastrera, O. stricta, O. viridirubra; Grant et al., 1979; Spears, 1987; Osborn et al., 1988; McFarland et al., 1989;
Mandujano et al., 1996; Schlindwein & Wittmann, 1997) or in winter (O. spinosissima; NegrónOrtíz, 1998). Finally, barrel cacti in the genera Echinocereus, Echinomastus, and Ferocactus
reproduce mainly during spring/summer (Breckenridge & Miller, 1982; Johnson, 1992; McIntosh, 2002).
Field observations regarding flower and fruit production in some cactus species (Carnegiea
gigantea, Echinomastus erectrocentrus, Lophocereus schottii, Mammillaria crucigera, M. mag-
186
THE BOTANICAL REVIEW
Fig. 1. Examples of the relationship between fecundity and size for cacti with different life-forms.
a. The globose cactus Mammillaria crucigera. b. The columnar cactus Neobuxbaumia tetetzo. c. The
platyopuntia Opuntia rastrera.
DEMOGRAPHY IN THE CACTACEAE
187
nimamma, Neobuxbaumia macrocephala, N. tetetzo, Pachycereus pringlei, Stenocereus queretaroensis, S. thurberi; Steenbergh & Lowe, 1977; Johnson, 1992; Johnson et al., 1992; Pimienta
et al., 1998; Godínez-Álvarez et al., 1999; Fleming et al., 2001; Contreras & Valverde, 2002;
Esparza-Olguín et al., 2002; Valverde et al., in press) have shown that there is interannual
variation in the timing and intensity of reproductive events. These variations may be related to
particular environmental factors, such as the presence of winter rainfall, the early start of the
rainy period, and/or a high total annual precipitation (Bowers, 1996b; Valverde et al., in press).
The number of fruits produced per plant and the number of seeds per fruit vary according
to species and life-form (Table III). For instance, fruits of columnar cacti may produce more
than 1000 seeds, whereas fruits of globose cacti generally produce fewer than 100 seeds. The
number of seeds per fruit in barrel cacti, cylindropuntias, and platyopuntias varies between
10 and 200.
In addition to sexual reproduction, many cacti are capable of spreading vegetatively (Parker
& Hamrick, 1992; Mandujano et al., 1998). This ability is common in the genus Opuntia,
although plants in other genera, such as Lophocereus, Myrtillocactus, Pachycereus, and
Stenocereus, may also spread through ramet production (pers. obs.). The relative importance of
vegetative spread compared with sexual reproduction is related to different factors such as soil
type, the presence of nurse plants, herbivory, and flooding (Mandujano et al., 1998). Opuntia
engelmanni plants apparently adjust the relative proportion of new cladodes and flowers in
response to the previous year’s resource expenditure (Bowers, 1996c).
VI. Size Structure
Population structure is the result of size- (or age-) specific birth and mortality rates
(Silvertown, 1987). The particular size- (or age-) structure found in a population may be determined by several extrinsic and intrinsic factors. Among these, seed production, germination
rates, soil seed banks, vegetative spread, herbivory, and weather patterns may play an important role (Silvertown, 1987). Histograms representing population structure for different cactus
species often show an uneven distribution of individuals among size classes (Fig. 2; Appendix
1), which suggests that recruitment occurs in pulses, apparently associated with infrequent
favorable conditions for germination and establishment (Mandujano et al., 2001; EsparzaOlguín et al., 2002). These favorable conditions appear to be related to particular precipitation
and temperature combinations for species such as Carnegiea gigantea, Echinocactus horizonthalonius, Ferocactus cylindraceus, Neobuxbaumia macrocephala, and Opuntia echios (Reid et
al., 1983; Bowers, 1997b; Pierson & Turner, 1998; Hicks & Mauchamp, 2000; Esparza-Olguín
et al., 2002). As noted above, recruitment pulses of some species may occur in rainy years
associated with particular climatic events such as El Niño (Hicks & Mauchamp, 2000). In the
case of Opuntia rastrera, germination and establishment are enhanced when nurse plants are
readily available and herbivory is low (Mandujano et al., 1998).
Other cactus species show a type of population structure in which the number of individuals
decreases monotonically with size or age (i.e., Ferocactus acanthodes, Neobuxbaumia tetetzo,
Opuntia echios; Jordan & Nobel, 1981; Godínez-Álvarez et al., 1999; Hicks & Mauchamp,
2000; Fig. 2; Appendix 1). This type of population structure suggests growing populations that
are regenerating constantly, in which mortality rate decreases with size or age (Martínez-Ramos
& Álvarez-Buylla, 1995).
Populations that spread throughout a variety of environmental conditions, such as those
occupying altitudinal gradients, may show distinct population structures in different habitat
types. This is the case in Opuntia echios, in which populations in a low arid habitat show a
188
THE BOTANICAL REVIEW
Table III
Mean number of seeds per fruit and standard errors for species of cacti with different life-forms
Species
Columnar cacti
Carnegiea gigantea
Cereus peruvianus
Lophocereus schottii
Neobuxbaumia macrocephala
N. mezcalaensis
N. tetetzo
Pachycereus pringlei
Pilosocereus moritzianus
Stenocereus griseus
S. gummosus
S. stellatus
S. thurberi
Subpilocereus horrispinus
S. repandus
Weberbaurocereus weberbaueri
Barrel cacti
Echinomastus erectrocentrus
Melocactus violaceus
Globose cacti
Mammillaria crucigera
M. magnimamma
Cylindropuntias
Opuntia imbricata
Platyopuntias
Opuntia brunneogemmia
O. engelmannii
Opuntia phaeacantha
O. polyacantha
O. rastrera
O. stricta
O. viridirubra
Standard
error
Number
of fruits
sampled
2263
1358
1241.8
1349.8
1946.7
1952
126
465
552
496
933
1288.1
1330.4
1329.1
2496
1121
1581
674
750
934
1969
536.9
351.3
1123
1483
1067
108
140
112.7
99.3
131.8
–
6.3
209
30
32.9
51.1
91.7
162.2
141.1
165.3
91
62.2
99.8
15
35
161.3
101.5
77.3
51.6
93
–
18
15
25
51
44
10
168
25
10
10
35
23
15
18
15–27
27
15–27
25
20
20
19
28
8
15–27
15–27
14
86.6
21.8
4.4
1.6
–
16
Johnson, 1992
Figueira et al., 1994
20
93.48
93.5
12.6
36.8
42
112
69
69
Contreras & Valverde, 2002
Valverde et al., in press
62
8.4
36
McFarland et al., 1989
55.2
143.9
171.6
5.8
21.5
16.8
20
20
20
Schlindwein & Wittmann, 1997
Bowers, 1997a
30
30
31
30
36
–
–
–
20
Osborn et al., 1988
Seeds
per fruit
50.2
72.7
25.1
9.0
208
60.6
65.3
67.5
63.7
6.9
5.7
3.6
2.5
0.13
3.7
4.8
4.5
3.7
Source
Steenbergh & Lowe, 1977
Fleming et al., 2001
Silva, 1988
Parker, 1989
Esparza-Olguín et al., 2002
Valiente-Banuet et al., 1997a
Valiente-Banuet et al., 1997a
Godínez-Alvarez et al., 2002
Fleming et al., 2001
Nassar et al., 1997
Silvius, 1995
Nassar et al., 1997
León de la Luz & Cadena, 1991
Casas et al., 1999
Parker, 1987
Fleming et al., 2001
Nassar et al., 1997
Nassar et al., 1997
Sahley, 1996
Osborn et al., 1988
Mandujano et al., 1996
Spears, 1987
Schlindwein & Wittmann, 1997
DEMOGRAPHY IN THE CACTACEAE
189
Fig. 2. Population size structures. a. The barrel cactus Echinocactus horizonthalonius. b. The globose
cactus Mammillaria crucigera. c. The columnar cactus Neobuxbaumia macrocephala. d. The columnar
cactus Neobuxbaumia tetetzo.
smooth decline in the number of individuals from small to large size classes, whereas populations at higher altitudes show a bimodal structure with few plants of intermediate sizes (Hicks
& Mauchamp, 2000).
VII. Population Dynamics
With the increasing popularity of projection matrix models, plant population ecology has
grown significantly in recent decades. However, most of the populations for which projection
matrices have been constructed correspond to herbaceous species from temperate habitats (e.g.,
Bierzychudek, 1982; Valverde & Silvertown, 1998). There are only a few references for cactus
populations that include the use of matrix models. Yet the information available is quite valuable, for it includes cactus species with different growth forms for which demographic information has been gathered during several years and/or at various locations (Table IV). For each
species, we consider the matrices of different years and/or sites separately, instead of constructing one average matrix per species. We believe that this approach allows us to analyze the
variation in demographic behavior both within species and between species. If average matrices were built, the within-species variation would be totally obscured, and flawed conclusions
could arise regarding the differences between species.
From the information presented in Table IV, it may be noted that population growth rate ()
is close to unity for most records; only in two populations is it significantly lower than unity
(Carnegiea gigantea and Mammillaria crucigera 2), and only in one population is it significantly larger than unity (Mammillaria magnimamma 2). This suggest that most cactus populations studied are close to the numerical equilibrium. Yet annual projection matrices may be a
limited tool for detecting long-term increases or declines in population numbers, especially in
190
Table IV
Summary of population dynamics data for 17 cactus populations of eight species.
Population growth rates () were calculated from projection matrices from n individuals each. Elasticity values e1 to e6 correspond to matrix
entries representing the seed bank (e1), sexual reproduction via seed and/or seedling production (e2), clonal spread (e3), retrogression (e4),
stasis (e6), and growth (e6). Elasticity values per demographic process are reproduction (R), survival (S) and growth (G);
and per life-cycle stage are seedlings (S), juveniles (J), and adults (A).
Elasticity values
Per demographic process
Species
n
e1
e2
e3
e4
0.540
15
1.000 1900
0
0.009
0.979
0.994
206
206
0.006
0.006
1.011
1.04
400
400
0.020
0.025
0.956
1.333
0.967
0.945
203
203
206
206
0.003
0.207
2E–05
0.006
0.015
0.019
7E–06
0.125
0.977
0.896
230
230
0.001
0.002
0.002
0.006
1.093
1.006
1.042
63
39
63
0.059
0.018
0.058
1.04 3000
0.997 250
0.001
1E–04
0.014
0.001
0.052
0.021
0
0.05
0.100
0.050
e5
e6
R
S
G
S
J
A
Source
0.99
0.914
0
0.078
0
0.000
0.99
0.914
0
0.078
0.13
0.080
0
0.387
0.865
0.534
1
2
0.837
0.885
0.105
0.088
0.006
0.006
0.889
0.906
0.105
0.088
0.033
0.093
0.182
0.163
0.785
0.744
3
0.791
0.763
0.189
0.212
0.020
0.025
0.791
0.763
0.189
0.212
0.040
0.045
0.760
0.755
0.2
0.2
4
0.934 0.044
0.334 0.436
0.999 7E–04
0.713 0.156
0.003
0.207
2E–05
0.006
0.948
0.353
0.999
0.838
0.044
0.436
0.001
0.156
0.003 0.010
0.208 0.323
1E–04 2E–04
0.006 0.047
0.987
0.469
0.999
0.947
5
0.984
0.971
0.013
0.021
0.001
0.002
0.986
0.977
0.013
0.021
0.002
1E–04
0.004
0.003
0.994
0.997
6
0.762
0.927
0.769
0.179
0.054
0.173
0.059
0.018
0.058
0.762
0.927
0.769
0.178
0.054
0.173
0.215
0.053
0.157
0.265
0.067
0.140
0.521
0.879
0.703
7
0.570
0.780
0.280
0.150
0.001
0.020
0.670
0.830
0.330
0.150
0.015
0.010
0.200
0.045
0.785
0.94
8
Sources: 1 = Steenbergh & Lowe, 1977, in Silvertown et al., 1993; 2 = Godínez-Álvarez et al., 1999; 3 = Esparza-Olguín et al., 2002; 4 = Ortega-Baes, 2001;
5 = Valverde et al., in press; 6 = Contreras & Valverde, 2002; 7 = Schmalzel et al., 1995; 8 = Mandujano et al., 2001.
a
One population in each case, sampled in different years. b Two different sites sampled within the same time period. c Two sites sampled in two different years.
d
Each record correspond to a different site, and in each case the average results of several years is given.
THE BOTANICAL REVIEW
Carnegia gigantea
Neobuxbaumia tetetzo
Neobuxbaumia macrocephala a
1
2
Escontria chiotilla b
1
2
Mammillaria magnimamma c
1
2
3
4
Mammillaria crucigera a
1
2
Coryphanta robbinsorum d
1
2
3
Opuntia rastrera d
1
2
Per life-cycle stage
DEMOGRAPHY IN THE CACTACEAE
191
long-lived species in which relevant population processes may take place at the scale of decades (Pierson & Turner, 1998; de Kroon et al., 2000).
As noted in previous sections, the analysis of transition matrices revealed that mortality
decreases and reproduction increases with increasing plant size in all species. Additionally,
plant growth rates are slow, which may be noted by consistently low transition and high “stasis” probabilities in most population matrices. Only the matrices for Mammillaria species show
rapid growth transitions, in which plants may grow more than one size category from one year
to the next. In some species “regressions” to smaller size categories are possible due to branch
death, particularly in globose species. Among the species analyzed, vegetative spread occurs
only in Opuntia rastrera, although, as noted above, other species may also show this ability.
Another aspect that may be noticed from the analyzed projection matrices is the almost total
absence of a seed bank in the models, with the exception of O. rastrera. Although it appears
that the seeds of various cactus species have the ability to remain viable in the soil for more
than a year (see section IV), it is believed that most seeds either germinate (given their high
germinability) or die shortly after they are shed.
A. ELASTICITY ANALYSES
The relative contribution of different transitions to the value of has traditionally been
evaluated through elasticity analyses (de Kroon et al., 1986). In addition to evaluating the
elasticity of individual matrix entries, it is possible to add up elasticity values of several matrix
entries corresponding to the same demographic process in order to analyze the relative contribution of each process to population growth rate (Silvertown et al., 1993). Here we subdivided
the life cycle of cactus species into six different types of transitions, following Silvertown et al.
(1993): persistence of seeds in the seed bank (e1), sexual reproduction (e2), clonal propagation
(e3), retrogression to smaller size classes (e4), persistence or “stasis” in the current size class
(e5), and growth to larger size classes (e6). Table IV shows the values of e1–e6 for the 17 matrices (corresponding to eight species) available in the literature for cactus species. Several trends
are apparent from these data: as noted above, seed banks and vegetative propagation are absent
in most cases (with the exception of Opuntia rastrera 2); sexual reproduction contributes very
little to population growth rate; and the most important contributions are in e5, which correspond to the persistence of individuals in their current size class. These trends are also apparent
when the different elasticity values were added up to account for the three main demographic
processes considered by Silvertown et al. (1993): fecundity (e1 + e2), survival (e4 + e5), and
growth (e3 + e6) (Table IV). Thus, plotting the different populations in the demographic triangle, we found that most of them fell toward the survival end (Fig. 3, top), which coincides
with the pattern observed for long-lived perennials (Silvertown et al., 1993). Only one population, Mammillaria magnimamma 2, fell closer to the center of the triangle, which implies a
balance between the contribution of growth and survival and a relatively high contribution of
fecundity to population growth rate. This population is the one that exhibits the largest . Thus,
as observed by other authors (Valverde & Silvertown, 1998; de Kroon et al., 2000), those
populations with larger s tend to be located closer to the center of the triangle, whereas lower
s generally occupy the far-right corner of the triangle (Fig. 4, top).
The position of the different populations in the demographic triangle does not seem to be
related to either life-form or longevity, as was suggested by Rosas-Barrera and Mandujano
(2002). For instance, the Carnegiea gigantea and the Mammillaria magnimamma 3 populations (a columnar long-lived species and a globose shorter-lived species, respectively) are located at the far-right corner of the triangle, almost in the vortex, while those populations located
closer to the center of the triangle include Escontria chiotilla 2, Coryphantha robbinsorum 1,
Opuntia rastrera 1, and Mammillaria magnimamma 2, representing three different life-forms.
192
THE BOTANICAL REVIEW
Fig. 3 (above). Triangular ordination of elasticity values for the 17 matrices reported in Table IV. The
three axes correspond to fecundity, growth, and survival (top) and seedlings, juveniles, and adults (bottom). See the text for further explanation.
Fig. 4 (facing page). Elasticity values for the 17 matrices reported in Table IV ordained on a triangular plot representing different demographic processes (top) and different life-cycle stages (bottom), along
with the value for each matrix represented in the y axis.
194
THE BOTANICAL REVIEW
In addition to evaluating the contribution of particular demographic processes, as proposed
by Silvertown et al. (1993), it is possible to use elasticity values to analyze the contribution of
different life-cycle phases to population growth rate. Here we have considered three life-cycle
phases: seedlings (newly established individuals, generally corresponding to the first and, occasionally, also the second and the third size classes); juveniles (categories of well-established
individuals, with mortality probabilities lower than those of seedlings but no sexual reproduction); and adults (reproductive individuals belonging to larger size classes) (Table IV). In most
of the cactus populations we detected a relatively larger contribution of adults to population
growth rate, compared with those of seedlings and juveniles (Fig. 3, bottom). Only the two
Escontria chiotilla populations (located toward the top of the triangle in Fig. 3, bottom) show
a high contribution of juveniles to , whereas Mammillaria magnimamma 2 and Coryphantha
robbinsorum 1 show also a substantial contribution from seedlings to . The latter high elasticity values for the seedling phase coincide with the largest s (Fig. 4, bottom). This pattern is not
particularly clear, however, since some populations with very low values have relatively high
seedling-elasticity values (i.e., Carnegiea gigantea) and some populations with a relatively
high show very low elasticity values for the seedling phase (Table IV; Fig. 4, bottom).
B. MATRIX MODELS AND MATRIX SIMULATIONS
In some of the demographic studies compiled in Table IV, authors also report the results of
matrix simulation exercises in which particular matrix entries were modified to explore the
potential effect of these changes on population growth rate. A common result of these exercises
is that a substantial increase in fecundity does result in an increase in population growth rate,
despite the fact that fecundity entries generally show low elasticities, whereas changes in other
matrix entries generally have a low impact on (Schmalzel et al., 1995; Ortega-Baes, 2001;
Contreras & Valverde, 2002; Esparza-Olguín et al., 2002).
Mandujano et al. (2001) combined the demographic information for Opuntia rastrera for
eight years and at two sites in order to generate different projection models; that is, the time
invariant (most generally used) yearly matrices, average matrices for several years, periodic
matrices, and stochastic projections. They conclude that the values obtained using these
different methods are quite similar. However, they assert that average matrices are not an adequate way of exploring environmental heterogeneity, because they do not take into account
variability in demographic behavior across years. In this case the authors recommend the use of
periodic matrices because they integrate interannual variation and give more consistent elasticity values (Mandujano et al., 2001). Stochastic simulations have also been used in Mammillaria magnimamma (Valverde et al., in press). The authors suggest that this type of approach
may be useful in projecting long-term population dynamics of cactus species, especially as an
aid to understanding not only long-term population growth rates but also transient dynamics.
This approach may be especially fruitful when the probability of occurrence of environmental
phenomena that most dramatically affect mortality and recruitment are known and can be incorporated in the stochastic projection (Valverde et al., in press).
VIII. Conclusions and Perspectives
The review presented here leaves us with the certainty that cactus population ecology is
definitely a consolidating subject within plant ecology. The number of publications and the
amount of ecological information on the subject is already substantial and is growing day by
day. Yet certain topics are clearly more developed than others. The ecology of seed germination
is perhaps one of the subjects to which greater attention has been directed, although we still
DEMOGRAPHY IN THE CACTACEAE
195
lack information about the potential role of seed banks. In recent years the nurse–cactus association has also been the subject of a substantial amount of work; however, we need further
research in order to evaluate the importance of this phenomenon for cacti with different lifeforms. On the other hand, analysis of cactus population dynamics through matrix models is
rather incipient. On this same topic, the relative importance of sexual reproduction versus
vegetative propagation in the maintenance of those populations that may show clonal growth is
still unknown.
In regard to the different life-forms represented in the Cactaceae, columnar cacti have received the greatest attention; ecological research on the globose and the barrel cacti, as well as
the cylindropuntias and platyopuntias, has been less intense. In addition, most of the literature
refers to North American and Central American cacti, and little is known about South American
species.
In exploring the literature on cactus population ecology, it has been interesting to discover
that biotic interactions such as nurse–cactus associations, pollination and seed dispersal by
animals, seed predation, herbivory, and competition, appear to be important organizing forces
in desert and semi-desert communities (Valiente-Banuet & Godínez-Álvarez, 2002; ValienteBanuet et al., 2002). Previous work on desert ecology had failed to recognize this aspect, since
it was believed that the main organizing forces of communities in stressful habitats were limiting abiotic factors (Noy-Meir, 1973, 1974, 1979/1980; Grime, 1979). However, the evidence
compiled in this review suggests that biotic forces may exert dramatic effects on desert populations, therefore functioning as key factors determining community composition and structure.
We conclude with a reflection on cactus conservation. As we mentioned before, it is well
known that many cactus species are vulnerable or in danger of extinction. Yet, of all the cactus
species for which any kind of population ecology information is available, only a few have
been classified as rare, vulnerable, or endangered (see Appendix 1). Only through a better
understanding of population processes will we be able to account for ecological phenomena
such as rarity and population decline, which are widespread among endangered plants. Thus
we must intensify our efforts to deepen our understanding of cactus population dynamics if we
are to offer any tools with which to preserve the biodiversity of this fascinating plant family.
IX. Acknowledgments
We are grateful to M. C. Mandujano for supplying us with a copy of Rosas-Barrera &
Mandujano (2002) even before it was published. We thank M. Franco and J. Silvertown for
allowing us to use the matrix they compiled for Carnegiea gigantea (used in Silvertown et al.,
1993) from previously published work. We are also grateful to A. Valiente-Banuet, who supervised the work of P. Ortega-Baes on Escontria chiotilla.
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Globose cacti
Ancistrocactus tobuschii
Ariocarpus trigonus
Coryphantha robbinsorum
Mammillaria crucigera
M. gaumeri
1
3
1
2/3
M. magnimamma
3
1
3
M. microcarpa
Neolloydia pseudopectinata
3
3
1
2
Barrel cacti
Copiapoa cinerea
Echinocactus horizonthalonius
Echinocereus engelmanni
E. triglochidiatus
1/3
3
CITES IUCN
I
I
3
1
Endangered
species
1
v
e
r
1
3
1
1
3
1
Geographical
distribution Source
N*
N*
N
Lockwood, 1995
Martínez et al., 1993
Schmalzel et al., 1995
N*
N*
N*
N*
Contreras & Valverde, 2002
Leirana-Alcocer & Parra-Tabla,
1999
Valverde et al., 1999; Ruedas et
al., 2000; Valverde et al., in
press
McAuliffe, 1984b
Martínez et al., 1994
N*
I
1
3
v
v
S
N
Gulmon et al., 1979
Reid et al., 1983
3
e
i
N
N
McAuliffe, 1984b
Reid et al., 1983
1
THE BOTANICAL REVIEW
Species
Pattern of Recruit- SurvivorSize
Population
distribution ment
ship
Fecundity structure dynamics
202
XI. Appendix 1: Demographic information, endangerment, and geographical distribution of cactus species with different life-forms.
Pattern of distribution: 1= random, 2 = regular, 3 = clumped; Recruitment: 1 = under the canopy of trees and shrubs, 2 = in spaces deprived of
vegetation; Survivorship: 1 = mortality is higher in the last size/age categories than in the first ones, 2 = mortality remains constant with age/size,
3 = mortality is higher in the first age/size categories than in later ones; Fecundity: 1 = fecundity increases with size/age,
2 = fecundity increases until a certain size/age and then remains constant, 3 = fecundity increases with size/age, reaching a maximum,
then decreases; Size structure: 1 = number of individuals decreases with size/age, 2 = number of individuals increases with size/age,
3 = number of individuals varies with size/age; Population dynamics: 1 = species analyzed with matrix models; CITES: I = species listed
in appendix I; IUCN: e = endangered, v = vulnerable, r = rare, i = indeterminate;Geographical distribution: N = North America,
S = South America, * = endemic to one country of the geographical region.
1
Echinomastus erectrocentrus
Ferocactus acanthodes
1
F. cylindraceus
F. histrix
F. wislizeni
Columnar catci
Carnegiea gigantea
Pachycereus pringlei
Stenocereus thurberi
Trichocereus pasacana
1
1
3
1
3
1
1
1
1
3
1
N
3
1
3
N
N*
N
3
N
1
N*
3
3
3
1
1
1
1
i
3
e
N*
N
S
1/3
i
S*
1
1
3
1
3
1
N*
N
N*
N*
1
3
N
1
3
Opuntioid cacti
Opuntia echios
O. rastrera
3
e
3
1
N*
Johnson, 1992; Johnson et
al., 1992
Jordan & Nobel, 1981;
Franco & Nobel, 1989
Bowers, 1997b
Huerta & Escobar, 1998
Reid et al., 1983
Niering et al., 1963; Turner
et al., 1966; Steenbergh
& Lowe, 1969; Hutto et
al., 1986; Franco & Nobel, 1989; Parker, 1993;
Silvertown et al., 1993;
Pierson & Turner, 1998
Zavala-Hurtado & DíazSolís, 1995
Ortega-Baes, 2001
Parker, 1989
Esparza-Olguín et al., 2002
Valiente-Banuet & Ezcurra,
1991; Valiente-Banuet et
al., 1991a, 1991b;
Godínez-Álvarez e al.,
1999
Carrillo-García et al., 2000
Parker, 1987, 1993
De Viana, 1996–1997
DEMOGRAPHY IN THE CACTACEAE
Cephalocereus columnatrajani
Escontria chiotilla
Lophocereus schottii
Neobuxbaumia macrocephala
N. tetetzo
3
I
Hicks & Mauchamp, 2000;
Hamann, 2001
Mandujano et al., 1998,
2001
203