Not attackable or not crackable—How pre‐ and post‐attack defenses with different competition costs affect prey coexistence and population dynamics

Abstract

It is well‐known that prey species often face trade‐offs between defense against predation and competitiveness, enabling predator‐mediated coexistence. However, we lack an understanding of how the large variety of different defense traits with different competition costs affects coexistence and population dynamics. Our study focusses on two general defense mechanisms, that is, pre‐attack (e.g., camouflage) and post‐attack defenses (e.g., weaponry) that act at different phases of the predator—prey interaction. We consider a food web model with one predator, two prey types and one resource. One prey type is undefended, while the other one is pre‐ or post‐attack defended paying costs either by a higher half‐saturation constant for resource uptake or a lower maximum growth rate. We show that post‐attack defenses promote prey coexistence and stabilize the population dynamics more strongly than pre‐attack defenses by interfering with the predator's functional response: Because the predator spends time handling “noncrackable” prey, the undefended prey is indirectly facilitated. A high half‐saturation constant as defense costs promotes coexistence more and stabilizes the dynamics less than a low maximum growth rate. The former imposes high costs at low resource concentrations but allows for temporally high growth rates at predator‐induced resource peaks preventing the extinction of the defended prey. We evaluate the effects of the different defense mechanisms and costs on coexistence under different enrichment levels in order to vary the importance of bottom‐up and top‐down control of the prey community.

Competition–defense tradeoff increases the diversity of microbial plankton communities and dampens trophic cascades

“…in eutrophic conditions, eutrophic conditions allow the existence of better defended organisms (Våge et al 2014(Våge et al , 2018, and co-existence is (slightly) more pronounced when the costs are in terms of reduced affinities rather than reduced growth (Ehrlich and 2018). In addition to the increase in diversity along the competition-defense dimension as explored by the simpler models, we here also find changes in diversity in sizes as well as in trophic strategies (i.e.…”

“…While earlier theoretical studies have examined the effects of defense in simpler food web structures (e.g. 'diamond' or 'pentagon' food webs model) and in constant environments (Våge et al 2014, Ehrlich and Gaedke 2018 we have here examined a more complex -and realistic -size based food web with multiple trophic levels and including trophic strategies. While the resulting patterns in our simulation appear much richer than those emerging in simpler models, they do in fact reproduce some of the key findings from previous studies.…”

“…Previous modelling exercises have demonstrated that both the shape of the tradeoff (Våge et al 2014) and how the costs are paid (Ehrlich and Gaedke 2018) have implications to the outcome of competition. Here, we extend the framework proposed by Våge et al (2014) by investigating the influence of both the shape of the tradeoff and the way the costs are paid for on the community structure in a size-based food web model of unicellular plankton that also considers trophic strategies (auto-, hetero-and mixotrophy) (Chakraborty et al 2017).…”

“…Size distribution, trophic strategies, defense strategies and food web structure are all emergent properties of the model, not prescribed. The plankton community we explore is therefore more realistic and complex than the simple food webs examined by Våge et al (2014Våge et al ( , 2018 and Ehrlich and Gaedke (2018) (Fig. 1c), but its structure is governed by simple rules and few parameters.…”

Diatom defence: Grazer induction and cost of shell‐thickening

“…Predator–prey interactions are key in shaping ecosystem structure and function (Belgrad & Griffen, 2016; Creel, 2018; Lima, 1998). In the oceans, zooplankton predation is believed to be an important mechanism maintaining phytoplankton species diversity by allowing the coexistence of defence‐ and competition‐specialists, as demonstrated both theoretically (Cadier, Andersen, Visser, & Kiørboe, 2019; Ehrlich & Gaedke, 2018; Våge et al., 2018) and experimentally (Leibold, Hall, Smith, & Lytle, 2017; McCauley & Briand, 1979). Indeed, phytoplankton have evolved a large range of defence mechanisms, ranging from morphological to biochemical or behavioural (Pančić & Kiørboe, 2018; van Donk, Ianora, & Vos, 2011).…”