1. Although the biomass of freshwater macrophytes consumed by invertebrate herbivores (excluding ... more 1. Although the biomass of freshwater macrophytes consumed by invertebrate herbivores (excluding crayfish) is usually low, there is growing evidence that invertebrates do exert a structuring effect on macrophyte communities. To explain this, we postulated that the effect of invertebrates may be concentrated on macrophytes during their regenerative phase.2. We tested this hypothesis by means of a mesocosm-based experiment, in which we investigated the effects of different densities of pond snails [Lymnaea stagnalis (L.)] on macrophytes regenerating from the natural sediment propagule bank.3. After 2 months, a diverse macrophyte community had established in the absence of snails, mainly from sexual propagules. Under moderate snail grazing (4 individuals m−2), the ultimate biomass of macrophytes was similar, but its species composition differed dramatically. Only a few unpalatable taxa, such as Ceratophyllum demersum and Nymphaeaceae, persisted. Moreover, the relative success of macrophytes regenerating from vegetative rather than sexual propagules improved. Under higher snail grazing (20 m−2), all macrophytes disappeared before the end of the experiment.4. These results confirm that snails at natural densities can have a strong effect on the ultimate structure of macrophyte communities by selectively consuming some species at a juvenile stage. Therefore, the regenerative phase can be seen as a window of opportunity for invertebrate grazers, which can have a qualitative effect on communities that is disproportionate to the biomass consumed.
The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation ... more The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation in palatability of three freshwater macrophytes: Potamogeton lucens, Elodea canadensis and E. nuttallii. For each species, 2–5 populations were investigated in spring and in summer. Preliminary results showed that the feeding rate of similarly-aged snails bred under standard conditions was stable over time. In contrast, snail feeding rate on the three macrophyte species decreased from spring to summer, which was therefore interpreted as a decrease in plant palatability. This decrease was probably due to tissue maturation, as suggested by the concomitant increase in the dry matter content of leaves of the three species. The high palatability of the species studied during the spring may prove detrimental in cases of strong herbivore pressure, and could have consequences for macrophyte distribution among aquatic habitats.
Although patterns of seedling selection by herbivores are strongly influenced by plant age and th... more Although patterns of seedling selection by herbivores are strongly influenced by plant age and the expression of anti-herbivore defence, it is unclear how these characteristics interact to influence seedling susceptibility to herbivory. We tracked ontogenetic changes in a range of secondary metabolites (total phenolics, alkaloids and cyanogenic glycosides) commonly associated with seedling defence for nine sympatric British grassland species. Although there was marked variation in concentrations of secondary metabolites between different species, we found a consistent increase in the deployment of phenolics, alkaloids and cyanogenics with seedling age for six of the seven dicotyledonous species examined. The two grass species by contrast exhibited low levels of secondary metabolites across all developmental stages, possibly due to an investment in structural (silica phytoliths) defence. Our results corroborate species-specific patterns of seedling herbivory observed in field studies, and offer some explanation for the relatively high sensitivity to herbivore attack frequently observed for relatively young seedlings compared with their older conspecifics. Our results also support predictions made by the growth–differentiation balance hypothesis regarding ontogenetic changes in resource allocation to anti-herbivore defence for a range of potential chemical defences and across a range of sympatric plant species presumably subject to broadly similar selective pressures at the regeneration stage.
Abstract. The success of invasive species has been attributed to the ability to displace other sp... more Abstract. The success of invasive species has been attributed to the ability to displace other species by direct competition. We studied growth and possible competition between the two macrophyte species Elodea nuttallii and E. canadensis, because the former has been observed to replace the latter in the field. Additional experiments were conducted in aquaria with mixed plantings of Elodea species. Species growth was measured and competitive abilities of each species determined by applying the reciprocal yield model to mean plant weight and length. In monocultures the growth rates of the two species were similar, while in mixtures the growth rate of E. canadensis was significantly lower than that of E. nuttallii. E. canadensis was more sensitive to intraspecific than to interspecific neighbours, whereas E. nuttallii was indifferent to the presence of neighbours. Differential growth characteristics of Elodea species can explain the displacement of E. canadensis by E. nuttallii under eutrophic field conditions.
1. The palatability of aquatic macrophytes to the snail Lymnaea stagnalis was investigated in the... more 1. The palatability of aquatic macrophytes to the snail Lymnaea stagnalis was investigated in the laboratory. Eight species of macrophyte were selected from habitats that differed in either flood disturbance regime or nutrient status.2. In a non-choice test, single macrophyte species were offered to individual snails. The average amount of plant dry mass consumed per Lymnaea dry mass ranged from 3.6 ± 1.4 (±SE) to 63.6 ± 13.9 mg g–1 day–1 across plant species. In a choice test, all eight plant species were presented simultaneously to sets of five snails. The average total consumption was 66.1 ± 3.8 mg g–1 day–1 and the maximum average consumption for a single plant was 26.2 ± 3.6 mg g–1 day–1.3. In both tests, the amount consumed by snails differed significantly between the plant species. The species growing in undisturbed habitats were the least consumed. Habitat nutrient status was unrelated to plant palatability.4. These results suggest that macrophyte species growing in habitats that are rarely disturbed by floods allocate a greater proportion of their resources to resisting herbivory.
1We examined the possibility of using the dry matter content (DMC) of macrophytes (the ratio of d... more 1We examined the possibility of using the dry matter content (DMC) of macrophytes (the ratio of dry mass to wet mass) as an integrative variable to predict their palatability to generalist invertebrate grazers.2We assessed the palatability of 20 macrophyte species, using the snail Lymnaea stagnalis (L.) in non-choice feeding experiments. Three of the species were studied at two different dates in the year, at two or four sites.3The average dry mass consumed by L. stagnalis ranged widely between species, and was negatively correlated to plant DMC. At the intraspecific level, the dry mass consumed varied over time but was not related to site location. Again, the dry mass consumed was negatively correlated to plant DMC.4The DMC of the macrophytes studied explained about 30% of interspecific variability, and >80% of seasonal variability, in snail consumption rate. Therefore this trait could be used as a shortcut to predict variations in macrophyte palatability, especially at the intraspecific level. At the interspecific level, the relationship between DMC and palatability might be weakened by the presence in some plants of low molecular weight chemical deterrents.We examined the possibility of using the dry matter content (DMC) of macrophytes (the ratio of dry mass to wet mass) as an integrative variable to predict their palatability to generalist invertebrate grazers.We assessed the palatability of 20 macrophyte species, using the snail Lymnaea stagnalis (L.) in non-choice feeding experiments. Three of the species were studied at two different dates in the year, at two or four sites.The average dry mass consumed by L. stagnalis ranged widely between species, and was negatively correlated to plant DMC. At the intraspecific level, the dry mass consumed varied over time but was not related to site location. Again, the dry mass consumed was negatively correlated to plant DMC.The DMC of the macrophytes studied explained about 30% of interspecific variability, and >80% of seasonal variability, in snail consumption rate. Therefore this trait could be used as a shortcut to predict variations in macrophyte palatability, especially at the intraspecific level. At the interspecific level, the relationship between DMC and palatability might be weakened by the presence in some plants of low molecular weight chemical deterrents.
1. Although the biomass of freshwater macrophytes consumed by invertebrate herbivores (excluding ... more 1. Although the biomass of freshwater macrophytes consumed by invertebrate herbivores (excluding crayfish) is usually low, there is growing evidence that invertebrates do exert a structuring effect on macrophyte communities. To explain this, we postulated that the effect of invertebrates may be concentrated on macrophytes during their regenerative phase.2. We tested this hypothesis by means of a mesocosm-based experiment, in which we investigated the effects of different densities of pond snails [Lymnaea stagnalis (L.)] on macrophytes regenerating from the natural sediment propagule bank.3. After 2 months, a diverse macrophyte community had established in the absence of snails, mainly from sexual propagules. Under moderate snail grazing (4 individuals m−2), the ultimate biomass of macrophytes was similar, but its species composition differed dramatically. Only a few unpalatable taxa, such as Ceratophyllum demersum and Nymphaeaceae, persisted. Moreover, the relative success of macrophytes regenerating from vegetative rather than sexual propagules improved. Under higher snail grazing (20 m−2), all macrophytes disappeared before the end of the experiment.4. These results confirm that snails at natural densities can have a strong effect on the ultimate structure of macrophyte communities by selectively consuming some species at a juvenile stage. Therefore, the regenerative phase can be seen as a window of opportunity for invertebrate grazers, which can have a qualitative effect on communities that is disproportionate to the biomass consumed.
The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation ... more The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation in palatability of three freshwater macrophytes: Potamogeton lucens, Elodea canadensis and E. nuttallii. For each species, 2–5 populations were investigated in spring and in summer. Preliminary results showed that the feeding rate of similarly-aged snails bred under standard conditions was stable over time. In contrast, snail feeding rate on the three macrophyte species decreased from spring to summer, which was therefore interpreted as a decrease in plant palatability. This decrease was probably due to tissue maturation, as suggested by the concomitant increase in the dry matter content of leaves of the three species. The high palatability of the species studied during the spring may prove detrimental in cases of strong herbivore pressure, and could have consequences for macrophyte distribution among aquatic habitats.
Although patterns of seedling selection by herbivores are strongly influenced by plant age and th... more Although patterns of seedling selection by herbivores are strongly influenced by plant age and the expression of anti-herbivore defence, it is unclear how these characteristics interact to influence seedling susceptibility to herbivory. We tracked ontogenetic changes in a range of secondary metabolites (total phenolics, alkaloids and cyanogenic glycosides) commonly associated with seedling defence for nine sympatric British grassland species. Although there was marked variation in concentrations of secondary metabolites between different species, we found a consistent increase in the deployment of phenolics, alkaloids and cyanogenics with seedling age for six of the seven dicotyledonous species examined. The two grass species by contrast exhibited low levels of secondary metabolites across all developmental stages, possibly due to an investment in structural (silica phytoliths) defence. Our results corroborate species-specific patterns of seedling herbivory observed in field studies, and offer some explanation for the relatively high sensitivity to herbivore attack frequently observed for relatively young seedlings compared with their older conspecifics. Our results also support predictions made by the growth–differentiation balance hypothesis regarding ontogenetic changes in resource allocation to anti-herbivore defence for a range of potential chemical defences and across a range of sympatric plant species presumably subject to broadly similar selective pressures at the regeneration stage.
Abstract. The success of invasive species has been attributed to the ability to displace other sp... more Abstract. The success of invasive species has been attributed to the ability to displace other species by direct competition. We studied growth and possible competition between the two macrophyte species Elodea nuttallii and E. canadensis, because the former has been observed to replace the latter in the field. Additional experiments were conducted in aquaria with mixed plantings of Elodea species. Species growth was measured and competitive abilities of each species determined by applying the reciprocal yield model to mean plant weight and length. In monocultures the growth rates of the two species were similar, while in mixtures the growth rate of E. canadensis was significantly lower than that of E. nuttallii. E. canadensis was more sensitive to intraspecific than to interspecific neighbours, whereas E. nuttallii was indifferent to the presence of neighbours. Differential growth characteristics of Elodea species can explain the displacement of E. canadensis by E. nuttallii under eutrophic field conditions.
1. The palatability of aquatic macrophytes to the snail Lymnaea stagnalis was investigated in the... more 1. The palatability of aquatic macrophytes to the snail Lymnaea stagnalis was investigated in the laboratory. Eight species of macrophyte were selected from habitats that differed in either flood disturbance regime or nutrient status.2. In a non-choice test, single macrophyte species were offered to individual snails. The average amount of plant dry mass consumed per Lymnaea dry mass ranged from 3.6 ± 1.4 (±SE) to 63.6 ± 13.9 mg g–1 day–1 across plant species. In a choice test, all eight plant species were presented simultaneously to sets of five snails. The average total consumption was 66.1 ± 3.8 mg g–1 day–1 and the maximum average consumption for a single plant was 26.2 ± 3.6 mg g–1 day–1.3. In both tests, the amount consumed by snails differed significantly between the plant species. The species growing in undisturbed habitats were the least consumed. Habitat nutrient status was unrelated to plant palatability.4. These results suggest that macrophyte species growing in habitats that are rarely disturbed by floods allocate a greater proportion of their resources to resisting herbivory.
1We examined the possibility of using the dry matter content (DMC) of macrophytes (the ratio of d... more 1We examined the possibility of using the dry matter content (DMC) of macrophytes (the ratio of dry mass to wet mass) as an integrative variable to predict their palatability to generalist invertebrate grazers.2We assessed the palatability of 20 macrophyte species, using the snail Lymnaea stagnalis (L.) in non-choice feeding experiments. Three of the species were studied at two different dates in the year, at two or four sites.3The average dry mass consumed by L. stagnalis ranged widely between species, and was negatively correlated to plant DMC. At the intraspecific level, the dry mass consumed varied over time but was not related to site location. Again, the dry mass consumed was negatively correlated to plant DMC.4The DMC of the macrophytes studied explained about 30% of interspecific variability, and >80% of seasonal variability, in snail consumption rate. Therefore this trait could be used as a shortcut to predict variations in macrophyte palatability, especially at the intraspecific level. At the interspecific level, the relationship between DMC and palatability might be weakened by the presence in some plants of low molecular weight chemical deterrents.We examined the possibility of using the dry matter content (DMC) of macrophytes (the ratio of dry mass to wet mass) as an integrative variable to predict their palatability to generalist invertebrate grazers.We assessed the palatability of 20 macrophyte species, using the snail Lymnaea stagnalis (L.) in non-choice feeding experiments. Three of the species were studied at two different dates in the year, at two or four sites.The average dry mass consumed by L. stagnalis ranged widely between species, and was negatively correlated to plant DMC. At the intraspecific level, the dry mass consumed varied over time but was not related to site location. Again, the dry mass consumed was negatively correlated to plant DMC.The DMC of the macrophytes studied explained about 30% of interspecific variability, and >80% of seasonal variability, in snail consumption rate. Therefore this trait could be used as a shortcut to predict variations in macrophyte palatability, especially at the intraspecific level. At the interspecific level, the relationship between DMC and palatability might be weakened by the presence in some plants of low molecular weight chemical deterrents.
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