Plants have evolved a myriad of strategies to protect themselves against herbivores, including both direct resistance traits (e.g. chemical and physical) and indirect defense incurred through trophic interactions. Such defenses are often constrained by trade-offs in which investment in anti-herbivore strategies are negatively associated with critical plant functions like growth and reproduction, or contrasting defensive traits that are redundant or costly. In this sense, it has been predicted that direct and indirect plant defenses should trade-off.
We hypothesized that density-dependent foraging by predators should result in a trade-off between direct defense and indirect defense by modifying the functional response of predators. In particular, insectivorous birds are generalist feeders that exhibit prey-switching behaviors, allowing them to respond to abundant prey resources. Simultaneously, the top-down effects of predators may be attenuated by other plant traits that can directly affect predator foraging, like morphology, or indirectly by mediating herbivore traits.
Using bird exclusion, we quantify the relative top-down effects of bird predators across nine shrub species characteristic of the coastal sage scrub ecosystem. We examine whether indirect defense from bird predators is related to herbivore resistance, and to what extent plant quality and structure explain variation in defensive strategy.
Results/Conclusions
Interspecific variation in herbivore resistance was negatively correlated with indirect defense by birds (raw, r = -0.72, P = 0.044; PIC, r = -0.43, P = 0.095), supporting a trade-off between direct and indirect defense. Across shrub species, herbivore resistance was negatively correlated with host plant quality, such that low quality shrub species also tended to have lower herbivore densities (raw, r = -0.68, P = 0.048; PIC, r = -0.74, P = 0.036). However, there was no relationship between indirect defense and host plant quality (raw, P = 0.70; PIC, P = 0.71), indicating that herbivore densities influence bird foraging independent of herbivore quality. Plant structural complexity was positively correlated with herbivore resistance (raw, r = 0.78, P = 0.012; PIC, r = 0.72, P = 0.039) but showed no relationship with host plant quality (raw, r = -0.68, P = 0.048; PIC, P = 0.42) and a marginally significant negative association with indirect defense (raw, r = -0.59, P = 0.089; PIC, r = -0.74, P = 0.33). We conclude that density-dependent foraging patterns result in a trade-off between indirect defense and direct herbivore resistance, which is further shaped by plant traits.