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 bottom-up and top-down effects cascade simultaneously throughout the food web, intimately linking the effects of functional diversity of any trophic level to the amount of diversity of other trophic levels, which may explain the difficulty in unifying results from previous studies. Strikingly, only with high diversity throughout the whole food web, different interactions synergize to ensure efficient exploitation of the available nutrients and efficient biomass transfer to higher trophic levels, 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 predator diversity and food web parameters affecting the top trophic level were of highest importance to determine the biomass and temporal variability of any trophic level. Overall, our study reveals that the mechanisms by which diversity influences ecosystem functioning are affected by every part of the food web, hampering the extrapolation of insights from simple monotrophic or bitrophic systems to complex natural food webs.

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

“…This corresponds to previous theoretical (Rall et al, 2008) and experimental findings (Weithoff et al, 2000) in bitrophic food webs, where the authors underlined the importance of the top-down control 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 1: A2), because of its decisive influence on the biomasses of the two lower trophic levels (Ceulemans et al, 2021;Wollrab et al, 2012).…”

“…The increase in the biomass of the intermediate trophic level led subsequently to an increase in the biomass of the top trophic level, which, in turn, led 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 1 : A2; Ceulemans et al ( 2019 ) and Ceulemans et al ( 2021 )). Thus, the web exhibited stronger top‐down regulatory processes resulting in a higher resistance to a nutrient pulse.…”

“…We used this method consistently for all the parameters varying with functional diversity. For a detailed description of how the trait values varied with , see Ceulemans et al ( 2019 ).…”

“…In addition, since the Hill exponent values in this study are close to 1 ( ), the difference is small even for the highest Hill exponent : and . A precise mathematical argument of this equivalency is presented in the supplementary information of Ceulemans et al ( 2019 ).

FIGURE A1 Comparison of two systems with no functional diversity ( ): (a) a food chain with 3 species, and (b) a food web with 8 species where all species at each trophic level have identical traits, biomasses, metabolic rates, and fluxes.

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“…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 (Abdala‐Roberts et al, 2019 ; Matsuno & Nobuaki, 1996 ), and top predators can have a large influence on ecosystem functioning and dynamics (Ceulemans et al, 2021 ; Estes et al, 2011 ).…”