Reciprocal subsidies between freshwater and terrestrial ecosystems structure consumer resource dynamics

Cross-ecosystem movements of material and energy, particularly reciprocal resource fluxes across the freshwater-land interface, have received major attention. Freshwater ecosystems may receive higher amounts of subsidies (i.e., resources produced outside the focal ecosystem) than terrestrial ecosystems, potentially leading to increased secondary production in freshwaters. Here we used a meta-analytic approach to quantify the magnitude and direction of subsidy inputs across the freshwater-land interface and to determine subsequent responses in recipient animals. Terrestrial and freshwater ecosystems differed in the magnitude of subsidies they received, with aquatic ecosystems generally receiving higher subsidies than terrestrial ecosystems. Surprisingly, and despite the large discrepancy in magnitude, the contribution of these subsidies to animal carbon inferred from stable isotope composition did not differ between freshwater and terrestrial ecosystems, likely due to the differences in subsidy quality. The contribution of allochthonous subsidies was highest to primary consumers and predators, suggesting that bottom-up and top-down effects may be affected considerably by the input of allochthonous resources. Future work on subsidies will profit from a food web dynamic approach including indirect trophic interactions and propagating effects.     

Multiple functions increase the importance of biodiversity for overall ecosystem functioning

Biodiversity is proposed to be important for the rate of ecosystem functions. Most biodiversity-ecosystem function studies, however, consider only one response variable at a time, and even when multiple variables are examined they are analyzed separately. This means that a very important aspect of biodiversity is overlooked: the possibility for different species to carry out different functions at any one time. We propose a conceptual model to explore the effects of species loss on overall ecosystem functioning, where overall functioning is defined as the joint effect of many ecosystem functions. We show that, due to multifunctional complementarity among species, overall functioning is more susceptible to species loss than are single functions. Modeled relationships between species richness and overall ecosystem functioning using five empirical data sets on monocultures reflected the range of effects of species loss on multiple functions predicted by the model. Furthermore, an exploration of the correlations across functions and the degree of redundancy within functions revealed that multifunctional redundancy was generally lower than single-function redundancy in these empirical data sets. We suggest that by shifting the focus to the variety of functions maintained by a diversity of species, the full importance of biodiversity for the functioning of ecosystems can be uncovered. Our results are thus important for conservation and management of biota and ecosystem services.     

Grazing regulates the spatial variability of periphyton biomass

The presence of consumers not only alters the mean biomass of the prey assemblage, but also affects the spatial heterogeneity of biomass distribution. Whereas the mean prey biomass is generally reduced by consumer presence, the effect on spatial heterogeneity is less clear-cut. A meta-analysis of almost 600 field experiments manipulating the presence of benthic invertebrate or vertebrate grazers was conducted to analyze the effect of grazers on both the absolute spatial variability of periphyton biomass and the relative variability, which was standardized to the mean. Effects on absolute variability were measured as the log response ratio of the standard deviation of biomass (LR-SD), whereas effects on relative variability were measured as the log response ratio of the coefficient of variation of biomass (LR-CV). The overall magnitude and range of LR-SD and LR-CV indicated that grazers not only reduced periphyton biomass, but also substantially altered their spatial distribution. However, grazer effects differed strongly for absolute and relative variability. On average, grazers reduced the absolute spatial variability in prey biomass by 50% (average LR-SD = -0.68) but increased the relative variability by 24% (average LR-CV = 0.22). The magnitude of LR-SD strongly depended on the efficiency of grazing, with strong biomass removal leading to strong homogenization. Moreover, LR-CV and LR-SD were significantly affected by habitat type (freshwater vs. coastal) and substrata. Given the importance of spatial heterogeneity for resource uptake, competition and the maintenance of diversity, grazer presence has potentially strong indirect effects on the interactions within prey assemblages.