Publications

Trophic cascades – indirect effects of predators that propagate down through food webs – have been extensively documented in many ecosystem types. It has also been shown that predator diversity can mediate these trophic cascades, and separately, that herbivore biomass can influence the stability of primary producers. However, whether predator diversity can cause cascading effects on the stability of lower trophic levels has not yet been studied. We conducted a laboratory microcosm experiment and a field mesocosm experiment manipulating the presence and coexistence of two heteropteran predators and measuring their effects on zooplankton herbivores and phytoplankton basal resources. We predicted that if the predators partitioned their zooplankton prey, for example by size, then co-presence of the predators would reduce zooplankton prey mass and lead to 1) increased biomass of, and 2) decreased temporal variability of phytoplankton basal resources. We present evidence that the predators partitioned their zooplankton prey, leading to a synergistic suppression of zooplankton. In turn, this enhanced zooplankton suppression led to only a weak, non-significant increase in the central tendency of phytoplankton biomass, but significantly reduced its variability. Our results demonstrate that predator diversity may indirectly stabilize basal resource biomass via a “diversity-stability trophic cascade,” seemingly dependent on predator complementarity, even when there is no significant classic trophic cascade altering the central tendency of biomass. Therefore predator diversity, especially if correlated with diversity of prey use, could play a role in regulating ecosystem stability. This link between predator diversity and producer stability has implications for conservation and for potential biological control methods to improve crop yield reliability.

This collection of files consists of freshwater plankton biomass data from a laboratory microcosm experiment and an accompanying field mesocosm experiment in which we manipulated the presence of two heteropteran predators. In the laboratory experiment, we incubated 20 large microcosms with phytoplankton and zooplankton, fully crossing a 1x vs. 2x zooplankton density treatment with presence or absence of a single Notonecta undulata adult. Two of these microcosms were lost, resulting in data for 18 of these large microcosms. Concurrently, we incubated 20 small microcosms with a smaller amount of the same plankton mix, which similarly were fully crossed with the 1x vs. 2x zooplankton density treatment and presence or absence of a single Neoplea striola adult. After five days we collected the zooplankton remaining in each microcosm for identification and biomass estimation. For the field experiment, we established 20 mesocosms (cattle tanks) with phytoplankton and zooplankton collected from the same sources, and added six Notonecta adults to five mesocosms, 90 Neoplea adults to another five, and three Notonecta and 45 Neoplea to another five, with the remaining five mesocosms acting as no-predator controls. We sampled both the phytoplankton and zooplankton once per week for six weeks for identification and biomass estimation.

Trophic cascades, or indirect effects of predators on non-adjacent lower trophic levels, have become paradigmatic in ecology, though they are thought to be stronger in aquatic ecosystems. Most research on freshwater trophic cascades focused on temperate lakes, where fish are present and where Daphnia tend to dominate the zooplankton community. These studies identified that Daphnia often play a key role in facilitating trophic cascades by linking fish to algae with strong food web interactions. However, Daphnia are rare or absent in most tropical and subtropical lowland freshwaters, and fish are absent from small and temporary water bodies, where invertebrates fill the role of top predator. While invertebrate predators are ubiquitous in freshwater systems, most have received little attention in food web research. Therefore, we aimed to test whether trophic cascades are possible in small warmwater ponds where small invertebrates are the top predators and Daphnia are absent. We collected naturally occurring plankton communities from small fishless water bodies in central Texas and propagated them in replicate pond mesocosms. We removed zooplankton from some mesocosms, left the plankton community intact in others, and added one of two densities of the predaceous insect Neoplea striola to others. Following an incubation period we then compared biomasses of plankton groups to assess food web effects between the trophic levels including whether Neoplea caused a trophic cascade by reducing zooplankton. The zooplankton community became dominated by copepods which prefer large phytoplankton and exhibit a fast escape response. Perhaps due to these qualities of the copepods and perhaps due to slow consumption rates by Neoplea on key grazers, no evidence for food web effects were found other than somewhat weak evidence for zooplankton reducing large phytoplankton. More research is needed to understand the behavior and ecology of Neoplea, but trophic cascades may generally be weak or absent in fishless low-latitude lowland water bodies where Daphnia are rare.

Hundreds of studies that have explored how biodiversity affects the productivity and stability of ecosystems have produced a consensus that communities composed of more species tend to have higher biomass that is more stable through time. However, the majority of this work stems from studies performed using highly simplified food webs, often composed of just primary producers competing for inorganic resources in the absence of trophic interactions. When studies have incorporated trophic interactions, diversity-function relationships have been more variable, leaving open the question of how biodiversity affects the functioning of ecosystems with more trophic levels. Here we report the results of a laboratory experiment that used freshwater microcosms to test for effects of algal diversity (one or four species) on community biomass and temporal variability in the presence and absence of two different herbivore species (cladocerans Ceriodaphnia dubia and Daphnia pulex). When no herbivores were present, we found the classic pattern observed in hundreds of other studies – as species richness of algae increased, algal biomass increased, and the temporal variation in biomass decreased. This pattern was retained when one of the herbivores (C. dubia) was present. Ceriodaphnia dubia exhibited weak and non-selective grazing on the focal algae, leaving the effect of diversity on biomass and variability essentially intact. In contrast, D. pulex exhibited strong and selective grazing in algal polycultures that qualitatively altered both diversity–function relationships. As algal richness increased, total algal biomass decreased and variation through time increased. These changes were coupled with larger and less variable populations of D. pulex. Our results show that herbivory leads to a richer array of diversity–function relationships than often observed in studies focused on just one trophic level, and suggests trophic interactions should be given more attention in work that seeks to determine how biodiversity impacts the functioning of ecosystems.

Hundreds of studies that have explored how biodiversity affects the productivity and stability of ecosystems have produced a consensus that communities composed of more species tend to have higher biomass that is more stable through time. However, the majority of this work stems from studies performed using highly simplified food webs, often composed of just primary producers competing for inorganic resources in the absence of trophic interactions. When studies have incorporated trophic interactions, diversity-function relationships have been more variable, leaving open the question of how biodiversity affects the functioning of ecosystems with more trophic levels. Here we report the results of a laboratory experiment that used freshwater microcosms to test for effects of algal diversity (one or four species) on community biomass and temporal variability in the presence and absence of two different herbivore species (cladocerans Ceriodaphnia dubia and Daphnia pulex). When no herbivores were present, we found the classic pattern observed in hundreds of other studies – as species richness of algae increased, algal biomass increased, and the temporal variation in biomass decreased. This pattern was retained when one of the herbivores (C. dubia) was present. Ceriodaphnia dubia exhibited weak and non-selective grazing on the focal algae, leaving the effect of diversity on biomass and variability essentially intact. In contrast, D. pulex exhibited strong and selective grazing in algal polycultures that qualitatively altered both diversity–function relationships. As algal richness increased, total algal biomass decreased and variation through time increased. These changes were coupled with larger and less variable populations of D. pulex. Our results show that herbivory leads to a richer array of diversity–function relationships than often observed in studies focused on just one trophic level, and suggests trophic interactions should be given more attention in work that seeks to determine how biodiversity impacts the functioning of ecosystems.

Evolution of life history traits can occur rapidly and has the potential to influence ecological processes, which can also be shaped by abiotic and biotic factors. Few studies have shown that life history phenotype can affect ecological processes as much as commonly studied biotic ecological variables, but currently we do not know how the ecological effects of life history phenotype compare in size to the effects of abiotic factors, or whether the ecological effects of phenotypes are sensitive to variability in abiotic conditions. Using a factorial mesocosm experiment we compared the ecosystem effects of guppy Poecilia reticulata life history phenotypes in two light treatments representing a four-fold difference in light levels, which was comparable to upstream downstream differences in light availability in Trinidadian streams. Light and phenotype had significant effects on similar aspects of ecosystem function. Whereas light had a stronger effect on ecosystem structure (algal and invertebrate stocks) than phenotype, phenotype and light had nearly equal effects on many ecosystem processes (nutrient recycling, nutrient fluxes, ecosystem metabolism and leaf litter decomposition). Light had a stronger effect on most guppy life history traits and guppy fitness than differences between phenotypes. The effect of light on these traits was consistent with higher availability of food resources in the high light treatments. Interactions between light and phenotype were weak for the majority of response variables suggesting that abiotic variability did not alter the mechanisms by which phenotypes affect ecosystem function. We conclude that subtle phenotypic differences in consumers can affect ecosystem processes as much as meaningful variability in abiotic factors which until recently were thought to be the primary drivers of ecosystem function in nature. However, despite its effects on traits and the ecosystem, light did not alter the effect of guppy phenotype on ecosystem function.