Defense-competition trade-offs shape prey eco-evolutionary dynamics across environmental gradients.

Predators can affect ecological communities not only through direct consumption but also by indirectly triggering costly defensive responses in prey that cascade through food webs. Understanding how prey navigate the competing demands of predation pressure and resource exploitation is thus central to predicting ecological (i.e., persistence) and evolutionary (i.e., adaptation) dynamics. Drawing on niche theory and the influence of keystone predators, this study investigates how variation in resource availability drives the persistence and evolution of prey with diverse trait combinations balancing defensive and competitive traits. At the heart of this ecological puzzle lies a fundamental constraint: Defended prey inherently have lower competitive abilities, generating a trade-off that governs fitness success along environmental gradients. Using a mathematical model, we show that the shape of the defense-competitiveness trade-off (i.e., the magnitude of change of competitiveness costs with increasing defense) influences the persistence of prey populations and the evolution of prey competitive and defensive traits. Strong trade-offs (i.e., when defenses strongly reduce competitiveness) or neutral trade-offs (i.e., when gains in defense and losses in competitiveness change proportionally) favor competitiveness, while weak trade-offs (i.e., when defenses weakly affect competitiveness) favor defensive traits, especially in resource-rich environments. In resource-poor environments, however, competitive prey are predominantly favored. Coexistence among prey individuals with different resource allocation strategies is evolutionarily unstable, as our model revealed that evolutionary processes eroded the persistence of multiple prey strategies. By linking resource availability to prey trait evolution, this study highlights the environmental dependence of eco-evolutionary dynamics in food webs, offering insights into the complex interactions that drive the prevalence and evolution of defense strategies.