Warming shifts top-down and bottom-up control of pond food web structure and function

Abstract

The effects of global and local environmental changes are transmitted through networks of interacting organisms to shape the structure of communities and the dynamics of ecosystems. We tested the impact of elevated temperature on the top-down and bottom-up forces structuring experimental freshwater pond food webs in western Canada over 16 months. Experimental warming was crossed with treatments manipulating the presence of planktivorous fish and eutrophication through enhanced nutrient supply. We found that higher temperatures produced top-heavy food webs with lower biomass of benthic and pelagic producers, equivalent biomass of zooplankton, zoobenthos and pelagic bacteria, and more pelagic viruses. Eutrophication increased the biomass of all organisms studied, while fish had cascading positive effects on periphyton, phytoplankton and bacteria, and reduced biomass of invertebrates. Surprisingly, virus biomass was reduced in the presence of fish, suggesting the possibility for complex mechanisms of top-down control of the lytic cycle. Warming reduced the effects of eutrophication on periphyton, and magnified the already strong effects of fish on phytoplankton and bacteria. Warming, fish and nutrients all increased whole-system rates of net production despite their distinct impacts on the distribution of biomass between producers and consumers, plankton and benthos, and microbes and macrobes. Our results indicate that warming exerts a host of indirect effects on aquatic food webs mediated through shifts in the magnitudes of top-down and bottom-up forcing.

Effects of patch connectivity and heterogeneity on metacommunity structure of planktonic bacteria and viruses

“…Our results provide no support for this prediction as responses of viral and bacterial composition to the experimental treatments differed qualitatively (for example, response to nutrient addition in bacteria but not in viruses) and Mantel tests provided no evidence for overall compositional associations. Similarly, Shurin et al (2012) found that the response of viruses to experimental treatments of warming, fish predators and eutrophication was distinct from those of bacteria. Weak associations between bacteria and viruses can be expected if a large fraction of the viruses infect other organisms, such as phytoplankton cells.…”

The effect of temperature on herbivory by the omnivorous ectotherm snail Lymnaea stagnalis

“…An increasing strength of plantherbivore interactions has been found to coincide with rising temperatures (Barton et al, 2009;O'Connor, 2009;Shurin et al, 2012). Compared to the amount of studies currently investigating the effects of global warming on trophic interactions, there seems to be only relatively few studies investigating the role of omnivores in this situation (e.g., Boersma et al (2016)).…”

Effects of warming on Potamogeton crispus growth and tissue stoichiometry in the growing season

“…alterations of trophic interactions (O'Connor, 2009), which may have cascading effects throughout the food web (Shurin et al, 2012) due to a mismatch in timing (Winder and Schindler, 2004) or change in food quality. Plant nutrient concentrations and the C:nutrient stoichiometry have been shown to be strong predictors of plant consumption rates across ecosystems (Elser et al, 2000a;Sterner and Elser, 2002).…”