Abstract
Food web structure and species richness are both subject to biotic (e.g. predation pressure and resource limitation) and abiotic stress (e.g. environmental change). We investigated the combined effects of both types of stress on richness and connectance, and on their relationship, in a predator-prey system. To this end, we developed a mathematical two trophic level food-web model to investigate the effects of biotic and abiotic stress on food web connectance and species richness. We found negative effects of top-down and bottom-up control on prey and predator richness, respectively. Effects of top-down and bottom-up control were stronger when initial connectance was high and low, respectively. Bottom-up control could either aggravate or buffer negative effects of top-down control. Abiotic stress affecting predator richness had positive indirect effects on prey richness, but only when initial connectance was low. However, no indirect effects on predator richness were observed following direct effects on prey richness. Top-down and bottom-up control selected for weakly connected prey and highly connected predators, thereby decreasing and increasing connectance, respectively. Our simulations suggest a broad range of negative and positive richness-connectance relationships, thereby revisiting the often found negative relationship between richness and connectance in food webs. Our results suggest that (1) initial food-web connectance strongly influences the effects of biotic stress on richness and the occurrence of indirect effects on richness; and (2) the shape of the richness-connectance relationship depends on the type of biotic stress.
Highlights
Food web structure and species richness are both subject to biotic and abiotic stress [1,2]
In the absence of abiotic stress, prey richness under bottom-up control can be considered a reference situation, since prey do not experience any biotic stress under bottom-up control
Effects of biotic stress on prey richness were more severe under high than under low initial connectance, because prey were by definition–on average–connected to more predators under high initial connectance
Summary
Food web structure and species richness are both subject to biotic and abiotic stress [1,2]. Biotic stress can occur through predation (top-down control on prey), resource limitation (bottom-up control on predators), or through a combination of both (mixed control [3]). Biotic stress can affect richness, e.g. when high predator richness leads to a reduction of prey diversity [4]. Species loss can lead to secondary extinctions, and to changes in food web structure and connectance [5,6]. Abiotic stress occurs when environmental gradients exceed tolerance limits, as such impacting food web structure and connectance [7,8,9,10,11,12,13].
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