Trade-offs among Mediterranean shrubland plants between direct herbivore defense and indirect defense by birds
Insectivorous predators, plant-eating insects (herbivores), and host plants constitute a multi-trophic interaction of fundamental importance that shapes ecosystem function. The top-down effect of bird predation has been shown to reduce herbivore and benefit plant growth and fitness. The strength of indirect defense by birds may in turn be mediated by plant traits. Direct plant defenses that influence herbivore abundance may affect bird recruitment if foraging is density dependent, while structural plant traits can affect bird recruitment and foraging behavior. We assessed plant traits and bird foraging within the coastal Mediterranean shrublands of southern California. We predicted that (1) plants with lower direct resistance will receive stronger indirect defense by birds and (2) other plant structural traits will mediate bird foraging. We measured plant traits host plant resistance to a generalist herbivore (Spodoptera exigua) and host plant structural complexity and used bird point counts to quantify relative foraging effort between plant species.
Our bioassay of host plant resistance (the inverse of generalist herbivore performance) showed high variation in caterpillar weight after one week on the plant. Caterpillars feeding on the lowest resistance plants weighed 40 times that of those feeding on the highest resistance plants. Plant resistance was in turn negatively associated with bird density (consistent with prediction 1), and bird density was negatively associated with host plant complexity (consistent with prediction 2). Furthermore, host plant quality and complexity trade-off, such that more structurally complex and branching shrubs were of higher resistance to herbivores. Accordingly, these plants occupied a trade-off spectrum, ranging from species with high direct resistance, high structural complexity and low indirect defense by birds, to species with low direct resistance, low structural complexity, and high indirect defense by birds. The findings suggest that evolutionarily convergent trade-offs between direct and indirect defense may drive covariance among plant traits in this ecosystem.