Introduction: A longstanding goal of ecological research is to understand the mechanisms that allow many species of organisms to co-exist in the same habitat. Ecological theory states that, in order to co-exist at the same site, species must differ in traits of ecological importance. If not, a few species would emerge as winners. A second major goal is to understand the mechanisms by which populations of a species evolve and diverge in their traits and, eventually, form new species. In tropical rainforests, these ecological and evolutionary questions may have related answers.

Objectives: As a model for Amazon tree diversification, we are analyzing adaptive responses to abiotic and biotic stresses within and between species in a diverse genus of trees, Inga (Fabaceae) in Amazonian Ecuador.

Methods: Using a set of focal species that are a) habitat generalists, b) sub-populations with abiotic habitat preferences and/or divergent defensive traits, and c) sister species with local adaptations, we are extensively quantifying defensive and resource acquisition traits that vary across this range of phylogenetic divergence. Herbivores feeding on leaves are recorded to determine host choice. Reciprocal transplant gardens are assessing whether trait differences are genetic or environmentally determined.

Results: Preliminary results show that in general, Inga pairs are divergent in defensive traits, even for pairs that are occurring in the same habitat. For pairs that occur in different habitats, traits related to abiotic challenges are divergent. Insect herbivores show high specialization and pairs with more similar defenses, share more similar herbivore communities. Reciprocal transplants in periodically flooded vs terra firme habitats, show that periodically flooded specialist populations grew faster than terra firme specialists in both habitats when protected from herbivores. However, all saplings of the terra firme population died when transplanted to the periodically flooded forest, suggesting a lack of adaptation to flooding stress. Saplings of the flooded population, on the other hand, only survived in the terra firme sites if they were protected from herbivores suggesting that biotic pressures could be a key filter.

Conclusions: The comparisons of Inga within and across habitats, and across a range of potential divergence times is allowing us to determine the extent to which traits contribute to initial stages of divergence. This unique comparison of both defenses and abiotic adaptations will test for synergy between herbivory and habitat heterogeneity in shaping niche evolution and local adaptation in plants in tropical forests.