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

Abstract

Biodiversity decline causes a loss of functional diversity, which threatens ecosystems through a dangerous feedback loop: This loss may hamper ecosystems’ ability to buffer environmental changes, leading to further biodiversity losses. In this context, the increasing frequency of human‐induced excessive loading of nutrients causes major problems in aquatic systems. Previous studies investigating how functional diversity influences the response of food webs to disturbances have mainly considered systems with at most two functionally diverse trophic levels. We investigated the effects of functional diversity on the robustness, that is, resistance, resilience, and elasticity, using a tritrophic—and thus more realistic—plankton food web model. We compared a non‐adaptive food chain with no diversity within the individual trophic levels to a more diverse food web with three adaptive trophic levels. The species fitness differences were balanced through trade‐offs between defense/growth rate for prey and selectivity/half‐saturation constant for predators. We showed that the resistance, resilience, and elasticity of tritrophic food webs decreased with larger perturbation sizes and depended on the state of the system when the perturbation occurred. Importantly, we found that a more diverse food web was generally more resistant and resilient but its elasticity was context‐dependent. Particularly, functional diversity reduced the probability of a regime shift toward a non‐desirable alternative state. The basal‐intermediate interaction consistently determined the robustness against a nutrient pulse despite the complex influence of the shape and type of the dynamical attractors. This relationship was strongly influenced by the diversity present and the third trophic level. Overall, using a food web model of realistic complexity, this study confirms the destructive potential of the positive feedback loop between biodiversity loss and robustness, by uncovering mechanisms leading to a decrease in resistance, resilience, and potentially elasticity as functional diversity declines.

Functional diversity increases the resistance of a tritrophic food web to environmental changes

“…Therefore, the model is effectively four-dimensional, corresponding to three trophic levels and the nutrient pool. The comparison between a food chain and food webs with higher levels of functional diversity is nonetheless meaningful: when the trait values of all species at the same trophic level are identical, the system’s equations can be rewritten by introducing new state variables B, I , and T such that B = B u + B d , I = I us + I un + I ds + I dn , and T = T s + T n , which allow for the recovery of the equations for a four-dimensional food chain, as detailed in previous studies using the same model [Ceulemans et al, 2019, Wojcik et al, 2021] and justified by [Moisset de Espanés et al, 2021]. In addition, all calculations were performed at the trophic level (rather than the species level) to unify the comparison.…”

“…This so-called functional diversity may be quantified as the difference in functional traits (such as edibility or prey selectivity) between species at the same trophic level. A few theoretical studies have demonstrated that the ability of predator and prey species to adapt their respective levels of offence and defence greatly improved resistance to perturbations [Raatz et al, 2019, Wojcik et al, 2021].…”

“…However, such studies investigating the link between functional diversity and stability have rarely considered multitrophic systems with three adaptive trophic levels [Wojcik et al, 2021], particularly in the context of press perturbations. As it is well-established that the number of trophic levels influences food web dynamics, and tritrophic sysems can be expected to show richer dynamic behaviour than simple predator-prey systems, [Matsuno and Ono, 1996, Abdala-Roberts et al, 2019], we investigated the relationship between functional diversity and resistance to press perturbations in food webs with three adaptive trophic levels.…”

“…We used a tritrophic chemostat model developed by [Ceulemans et al, 2019], in which functional diversity (modelled as a combination of prey defence and selectivity of predators to undefended prey) could be systematically varied at all trophic levels. Functional diversity influenced several ecosystem functions in this model [Ceulemans et al, 2019], as well as its resistance to a single nutrient pulse [Wojcik et al, 2021]. In this paper, we studied the effect of increasing functional diversity on the system’s resistance to environmental change, modelled as press perturbations to the nutrient inflow concentration, N 0 , and to the dilution rate δ ; environmental parameters which are expected to more directly affect the bottom and the top trophic levels respectively.…”

“…Finally, we considered the transition time - the time taken for a system to transition to its new attractor after the onset of a perturbation. We hypothesised that, analogously to the increase seen in resistance to a nutrient pulse [Wojcik et al, 2021], increased functional diversity would increase the system’s resistance to press perturbations.…”