Anthropogenic activity has led to severe nutrient addition in freshwater and terrestrial ecosystems. This has led to an increase in productivity across these systems. To understand the impact of this productivity, a body of literature has tried to distinguish between top-down vs bottom up control of food webs in these ecosystems. Typically, these studies analyze the impact of the increased productivity on the equilibrium densities of the different trophic levels. However, they do not comment on the assembly and maintenance of species diversity within trophic levels in these communities. Since the assembly of the food web determines the diversity of these food webs, it also mediates the impact of increased productivity on these food webs.
Here, we study a theoretical freshwater food web module, where multiple phytoplanktonic species compete for two essential nutrients and are grazed upon by a shared zooplankton species. We explore the possible planktonic diversity (one to three) within this food web using contemporary niche theory. More precisely, we determine the impact of enrichment of each nutrient on the structure of the food web and the equilibrium densities of the species in the community, under various assumptions of trade-offs between the species.
Results/Conclusions
When assembling a food web module with a single phytoplankton species, we recover the well-known results of food web assembly and trophic cascade of a food chain. Interestingly however, the limiting factors for phytoplankton growth can switch between the two resources. The order of this switch depends on the manner in which the resource supplies are increased.
Next, we analyze whether a second phytoplankton species can coexist in this food web module. We find that either one or two species can coexist at high enrichment depending on the ratio of these nutrient supplies, and the trade-offs. For coexistence, a tradeoff is required between the competing phytoplankton species. There are three possibilities for a tradeoff – a tradeoff in the abilities to exploit the nutrients, a tradeoff in the ability to exploit any one nutrient and resistance to the grazer. We find that only one or two of these tradeoffs can be present in the model and presence of all three tradeoffs is not possible. These results highlight the conditions for assembling a realistic freshwater food web and the impact of increasing productivity in these systems.