Springtails

Background

The California Coastal Prairie has the highest biodiversity of North America's grasslands, but also has the highest percentage of urbanization.  The most urbanized part of the California Coastal Prairie is its southernmost area, in Los Angeles County.  This southernmost region, known as the Los Angeles Coastal Prairie, was historically dotted with vernal pools, and has a unique biodiverse composition.  More than 99.5% of its estimated original 95 km (23,475 acres), as well as almost all its vernal pool complexes, have been lost to urbanization.

The Madrona Marsh Preserve, in Torrance, California, safeguards approximately 18 hectares (44 acres) of Los Angeles Coastal Prairie and includes a complex of vernal pools.  Its aquatic biodiversity had been studied, predominantly to genus level, but its terrestrial macro-invertebrates were virtually unknown, aside from butterfly, dragonfly, and damselfly observations.

Results

In order to better understand the biodiversity at the Madrona Marsh Preserve, a minimally-invasive macro-invertebrate inventory was conducted. The results of this inventory, with 689 invertebrate organisms recorded, covering eight phyla, 13 classes, 39 orders, and 222 families, are presented in this document.

Springtail communities have been shown to be good indicators of environmental health, although it can occur that the influence of site is higher than that of vegetational types, pointing to the need for a comprehensive exploration of the regional pool before reaching conclusions based on species composition.

Relationships between anecic earthworms (Lumbricus terrestris and Aporrectodea giardi) and the collembolan species Heteromurus nitidus (Templeton, 1835), which is known to be attracted to earthworms, were investigated in an 8-week laboratory experiment. Our aims were (1) to assess whether earthworms influence the population dynamics of H. nitidus, and (2) to study pathways of influence and how earthworm effects are modified by humus forms and predators. Using microcosms with three defaunated humus forms, then provided with earthworms and predators, we intended to demonstrate that, amongst possible favourable effects of earthworms on springtail populations, earthworm activity may provide greater access and more pathways for springtails to explore soil and avoid predation. We expected that the effects of predators (centipedes) on the abundance of H. nitidus would increase from less (calcic mull) to more (moder) compact soil, and we hypothesized that earthworms would reduce predation pressure on H. nitidus by providing escape routes through increased macroporosity. Humus forms and earthworms only affected the population size of H. nitidus under high predation pressure, when collembolan numbers were higher in calcic mull than in moder, and were increased by the presence of earthworms. These results corroborate the hypothesis that earthworms, by increasing soil macroporosity, improve the escape routes for Collembola and thus evade predation. In moder humus earthworms increased the density of H. nitidus whether predators were present or not, so we cannot exclude that earthworms were also directly beneficial to H. nitidus. However, the hypothesis of a functional relationship mediated by soil macroporosity seems relevant since it was supported by differences observed when considering body size. When two size classes were distinguished within populations of H. nitidus (1) the positive effect of earthworms in moder was observed only on larger Collembola (O1 mm), (2) the density of the larger Collembola was decreased by predation only in moder and not in mull, (3) the effects of predators on the smaller individuals were not influenced by the presence of earthworms whatever the humus form, and was not decreased by the presence of earthworms. Nevertheless, factors other than macroporosity may operate as the presence of earthworms in acidic mull led to an unexplained decrease in the abundance of small-sized H. nitidus.

Five bioassays (inhibition of lettuce germination and growth, earthworm mortality, inhibition of springtail population growth, avoidance by springtails) were compared, using four coke factory soils contaminated by PAHs and trace elements, before and after biotreatment. For each bioassay, several endpoints were combined in an ‘ecoscore’, a measure of test sensitivity. Ecoscores pooled over all tested bioassays revealed that most organisms were highly sensitive to the concentration of 3-ring PAHs. When four soils were combined, behavioural tests using the springtail Folsomia candida showed higher ecoscores, i.e. they were most sensitive to soil contamination. However, despite overall higher sensitivity of behavioural tests, which could be used for cheap and rapid assessment of soil toxicity, especially at low levels of contamination, some test endpoints were more sensitive than others, and this may differ from a soil to another, pointing to the need for a battery of bioassays when more itemized results are expected.

We sampled collembolan communities in 11 coal mine spoil tips (locally called 'terrils') from the North of France, and their surroundings, in various stages of vegetation development (bare soil, herb, shrub and tree vegetation). We wondered whether species, functional trait (Community Weighted Mean trait or CWM) composition and trait diversity (Rao's quadratic entropy) responded to the particular environment of coal mine spoil tips (here called 'terril' effect) and to successional effects and which of these three sets was the best indicator of soil and vegetation effects, taking into account that spoil tips were spatially distant and idiosyncratic (e.g. varying in height, area, past history and surrounding environment). We showed that species, but not traits, were strongly influenced by site effects, while traits were influenced by both 'terril' and successional effects to a significant extent, making traits more robust than species when trying to discern patterns of community response to soil and vegetation changes. Functional diversity did not show any significant multi-trait response to either 'terril' or successional effects, but several functional traits showed an increase in diversity with succession. The overall response of trait diversity was intermediate between species and trait responses. The 'terril' effect was mainly mediated by a coarser soil texture (51% coarse sand compared to 14% in the surrounding environment) which, combined with harsher microclimate conditions, stems in a better representation of functional traits linked to subterranean life, such as shorter body length and increased representation of parthenogenesis. Successional effects (in particular the increase in diversity shown by several functional traits varying with vertical distribution) were mediated by the increasing development of organic horizons but other factors like microclimate were seemingly involved, too. More generally this study showed that shift to subterranean life was an adapted response of collembolan communities to harshness of the environment and that vertically stratified ecosystems allow a more complete expression of their functional diversity.

Ecological processes are the result of both local dynamics and processes acting at the broader scale of the surrounding landscape. The potential influence of the surrounding landscape includes effects on community composition as number of species, habitat generalist species, habitat specialist species, fast dispersal species, slow dispersal species. The study took place in the Morvan Regional Park within the Bioassess European project. Strict-herbaceous and generalist species have faster dispersal ability while forest-preferring, strict-forest and herbaceous-preferring species have slower dispersal ability. When the surrounding landscape matrix was associated with habitat preference it exerted an effects on alpha diversity of species strict or preferential of forest or herbaceous habitat. Gamma diversity increases but alpha diversity decreases with landscape heterogeneity. The negative impact of surrounding landscape heterogeneity at 1 km² scale on alpha diversity may be due to response time-lag from slow dispersal species and the inability of specialist species to cross a hostile habitat.

Although considerable work has been done investigating the factors that underlie species trait assemblages in emblematic groups such as plants and vertebrates, more remains to be done in belowground invertebrate species. For this group, in particular, relationships between species traits and habitat preferences are still a matter of debate. To eludicate upon the relationships, springtails were sampled in a heterogeneous landscape centered on the Sénart forest, near Paris (northern France), which represented the largest possible array of five environmental gradients (humus forms, vegetation, moisture, vertical strata, and seasons) over which Collembola are known to be distributed. The distance between samples varied from a few cm to several km. Canonical correspondence analysis using species (128) as observations and species trait attributes (30) and habitat indicators (82) as dependent and independent variables, respectively, allowed us to discern whether species habitats and species trait assemblages were related and which trends could be found in trait/environment relationships. It was concluded that, within the studied area, species habitats were significantly associated with species trait assemblages. The main gradient explaining the distribution of species traits combined the vertical distribution of habitats (from the mineral soil to plant aerial parts), and the openness of the environment, suggesting that a complex of many ecological factors is at play. For the ecological traits of Collembola, this gradient corresponded to an increasing contribution of sensory and locomotory organs, bright color patterns, size and sexual reproduction, all attributes associated with aboveground life under herbaceous cover. Another important , although secondary contrast concerned traits associated with habitats far from soil but that were concealed (corticolous vs. all other habitats). Soil acidity and water did not contribute significantly to trait distribution, at least within the limits of our database.

Springtails (Hexapoda: Collembola) are known to live in a wide range of soil conditions, but changes occurring in their communities under the influence of heavy metal pollution are still poorly documented. The present study was undertaken in order to discern main coenological trends along a gradient of metal pollution, downwind of a zinc smelter located in the North of France. Three sites were compared within an abandoned field planted with poplar twenty years ago. The total zinc content of the topsoil varied from 4000 to 35 000 mg/kg according to distance to the smelter outlet. Changes in humus forms (shift from mull to mor) and in the degree of opening of the poplar stand (mainly caused by death or stunting of planted trees in the vicinity of the smelter) explained most of the observed variation in abundance and species composition. Sun species living at the ground surface appeared at the most polluted sites where poplar declined and was replaced by a sward of metal-tolerant herb species. The increase in abundance of the whole collembolan community was mainly due to Mesaphorura macrochaeta, the gut contents of which indicated that this species avoided the root mat and fed mainly in the mineral soil underneath. Shifts in food habits and/or in habitat were observed in several other common species, pointing to the importance of avoidance behaviour as a mechanism by which collembolan species may endure heavy metal pollution.

Metal immobilization may contribute to the environmental management strategy of dredged sediment landfill sites contaminated by metals. In a field experiment, amendment effects and efficiency were investigated, focusing on plants, springtails and bacteria colonisation, metal extractability and sediment ecotoxicity. Conversely to hydroxylapatite (HA, 3% DW), the addition of Thomas Basic Slag (TBS, 5% DW) to a 5-yr deposited sediment contaminated with Zn, Cd, Cu, Pb and As resulted in a decrease in the 0.01 M Ca(NO 3) 2-extractable concentrations of Cd and Zn. Shoot Cd and Zn concentration in Calamagrostis epigejos, the dominant plant species, also decreased in the presence of TBS. The addition of TBS and HA reduced sediment ecotoxicity and improved the growth of the total bacterial population. Hydroxylapatite improved plant species richness and diversity and decreased antioxidant enzymes in C. Epigejos and Urtica dïoica. Collembolan communities did not differ in abundance and diversity between the different treatments.