Top predators govern multitrophic diversity effects in tritrophic food webs

Abstract

It is well known that functional diversity strongly affects ecosystem functioning. However, even in rather simple model communities consisting of only two or, at best, three trophic levels, the relationship between multitrophic functional diversity and ecosystem functioning appears difficult to generalize, due to its high contextuality. In this study, we considered several differently structured tritrophic food webs, in which the amount of functional diversity was varied independently on each trophic level. To achieve generalizable results, largely independent of parametrization, we examined the outcomes of 128,000 parameter combinations sampled from ecologically plausible intervals, with each tested for 200 randomly sampled initial conditions. Analysis of our data was done by training a Random Forest model. This method enables the identification of complex patterns in the data through partial dependence graphs, and the comparison of the relative influence of model parameters, including the degree of diversity, on food web properties. We found that the effects of functional diversity on any trophic level are intimately linked to the amount of diversity on other trophic levels, which may explain the difficulty in unifying results from previous studies. Strikingly, with high diversity throughout the whole food web, different interactions synergize to ensure efficient exploitation of the available nutrients and efficient biomass transfer, ultimately leading to a high biomass and production on the top level. The temporal variation of biomass showed a more complex pattern with increasing multitrophic diversity: while the system initially became less variable, eventually the temporal variation rose again due to the increasingly complex dynamical patterns. Importantly, top diversity and food web parameters affecting the top level were of highest importance to determine the biomass and temporal variability of any trophic level. Therefore, given its high ecological importance and vulnerability to global change, it is essential to preserve diversity on the top trophic level.

Top predators govern multitrophic diversity effects in tritrophic food webs

“…Subsequent analysis of the food web data was performed in Python 3.6 using NumPy (Van Der Walt et al, 2011), pandas (McKinney, 2010, and Matplotlib (Hunter, 2007). Further details on our computational procedure, as well as the code itself and the data required to produce Figures 3-6 and various Appendix Figures can be found in Data S1 (Ceulemans et al, 2020).…”

“…The code and data (Ceulemans et al, 2020) required to produce and/or analyze the results are available on Zenodo: https://doi.org/10.5281/zenodo.4672681…”

Functional diversity alters the effects of a pulse perturbation on the dynamics of tritrophic food webs

“…This corresponds to previous theoretical (Rall, Guill, and Brose, 2008) and experimental findings (Weithoff, Lorke, and Walz, 2000) in a bitrophic food web, where the authors underline the importance of the top-down 14 pressure to explain the response of systems against a nutrient perturbation. Our tritrophic study also highlights the importance of top-down processes to dampen the effects of a nutrient pulse, and reveals that the top level may strongly affect the response of the food web as a whole (see Appendix A2), because of its decisive influence on the biomasses of the two lower trophic levels (Wollrab, Diehl, and De Roos, 2012;Ceulemans, Guill, and Gaedke, 2020).…”

“…The increase in the biomass of the intermediate trophic level leads subsequently to an increase in the biomass of the top trophic level, which in turn, leads to the intermediate level being under strict top-down control and thus unable to exploit the high basal biomass following the nutrient pulse (cf. Appendix A2, Ceulemans et al (2019) and Ceulemans, Guill, and Gaedke (2020)). Thus, the web exhibits stronger top-down regulatory processes leading to a higher resistance to nutrient pulses.…”

“…These studies underline how functional diversity at one or two trophic levels generally enhances the ecosystem’s ability to buffer against perturbations, and that the effects of functional diversity can be modulated at different trophic levels. However, tritrophic systems with at least two functionally diverse trophic levels are more realistic, since strictly bitrophic interactions are rare in nature (Pimm et al, 2014; Matsuno and Nobuaki, 1996; Abdala-Roberts et al, 2019), and top predators can have a large influence on ecosystem functioning and dynamics (Estes et al, 2011; Brose et al, 2019; Ceulemans, Guill, and Gaedke, 2020).…”