In tropical forests, pests and pathogens exert significant pressure on plant communities. Insect herbivory in particular accounts for up to 70% of annual leaf consumption in tropical forests, and plants invest in defense at a significant cost to their growth. Adaptation to herbivory is also hypothesized as a driver of high tropical diversity, such that plants and their herbivores promote their mutual diversification. This may be particularly prevalent across environmental gradients that cause turnover in herbivore composition and abundance. For example, the pest pressure gradient hypothesis suggests that pest pressure will increase in wetter forests due to relaxed constraints on pest abundance. Thus, local adaptation in defense may commonly occur along such rainfall gradients, but this idea has not previously been explored. To assess evidence for local adaptation to herbivory, as well as quantify herbivore damage along rainfall gradients, we tracked foliar damage on transplanted seedlings of 13 tree species across three sites spanning a rainfall gradient in Panama (1500 to 3000 mm/year across 65 km). Seed source was controlled such that all populations of each species were represented at every site across its range, allowing us to compare herbivore damage on local vs non-local seedlings of the same species.
Results/Conclusions
When we compared herbivory between populations of the same species, seedlings from local populations were less likely to be attacked than those from other populations, but only on the wetter end of the rainfall gradient. The strongest local population advantage was apparent at our wettest site, where local seedlings also exhibited less standing herbivory. We interpret these findings as evidence for local adaptation to herbivory, with stronger pressure to adapt to local conditions in wetter forest. At the same time, overall standing herbivory was lowest at our wettest site. Together, these results suggest that high herbivory in drier vs wetter forests likely results from generalist herbivores, while specialized herbivores, which would drive local adaptation, are more prevalent in wetter forests. Alternatively, higher understory light levels in the drier forest may have caused seedlings to invest more in growth than defense. Ultimately, our findings support the idea that herbivory drives intraspecific diversification in defense in the tropics.