Complex interactions between topographic heterogeneity and steep gradients in climate and environmental conditions are commonly assumed to promote biotic diversification. Using tropical frogs as a model, I investigate the nature of these interactions that disrupt migration between populations, causing genetic divergence and speciation. I determine the role of several putative factors that affect gene flow (Euclidean distances, Least Cost Path (LCP) distances, topographic complexity, and elevation difference) and promote genetic structure (FST) between populations of three tropical Andean frog species. Moreover, I investigate, from an intraspecific perspective, whether montane frog species display on average larger genetic distances per kilometer relative to lowland species. Finally, I test if recent genetic divergence caused by topographic barriers to gene flow is paralleled by independent character systems such as acoustics and morphological traits in the high Andean frog Dendropsophus labialis. Even though the effect of geographic features on migration (and conversely, FST) was species-specific, LCP and Euclidean distances had the strongest effect on migration rate. Topographic complexity also reduced migration rate whereas elevation difference did not have an effect. I found that indeed highland species show larger genetic distances per kilometer between haplotypes than do lowland species. Also, genetic divergence is strongly associated with topographic heterogeneity, which is an intrinsic characteristic of montane regions. Finally, I found that acoustic variation in D. labialis diverges according to genealogical history, but external morphology does not follow this relationship. Stochastic processes due to genetic drift appear to be a better explanatory mechanism for the divergence in calls than adaptive variation. The strong and congruent divergence observed in acoustic and genetic characters indicates that these two groups correspond to morphologically cryptic parapatric species. Overall, the results of this study suggest some of the mechanisms that allow tropical mountains to promote intraspecific genetic divergence. The combined effect of ridges (promoting allopatric differentiation) and environmental gradients across elevation (promoting parapatric differentiation) are effective forces that are present mostly in highland biomes. Unfortunately, such biomes are critically threatened by habitat destruction and climate change, possibly more than any other biome on earth. / text

The stratification of bird species along elevational gradients is widely reported, with montane bird communities typically characterized by distinctive species occurring in relatively small and isolated populations; as such, these species are the subject of considerable interest to ecologists and conservationists. The stratification of species along elevation is largely attributed to compressed climatic zonation. Recent evidence that bird species are shifting up in elevation has fueled speculation that these species are tracking their climactic niches in response to climate change. However, there is also evidence plant communities are shifting in elevation, presenting a potential additional mechanism explaining changes observed in the bird community. Uncertainty as to the degree to which climate directly influences bird abundance versus the degree to which climate indirectly influences bird abundance via habitat composition and structure represents a key impediment to understanding the ecology of these species in montane environments. To address this question I measured species abundance, habitat characteristics, and temperature at 150 survey points located along 15 elevational transects in the Presidential Mountains of New Hampshire in the summers of 2014 and 2015. I used N-mixture models to correct for imperfect detection of species and structural equation models, incorporating abundance, habitat, temperature, and precipitation derived from a downscaled regional dataset to assign variation to the direct and indirect effects of climate upon birds. Analysis of 21 species revealed species-specific patterns on how climate exerts direct effects and indirect effects mediated by forest composition and structure on bird abundance. This work represents an important contribution to the ecological understanding of the pathways by which climate influences bird abundance. Finding that 62% of species experience both direct and indirect effects of climate, with 62% experiencing stronger direct than indirect effects, these results underscore the vulnerability of these species to climate change. With 81% of species found to experience indirect effects of climate via forests, these findings indicate great conservation value of maintaining forest habitat amidst climate change. Overall, this information will facilitate the refinement of predictive models of the abundance of montane bird species and represents an approach that will advance future investigations of climate effects in the Northern Appalachians and other systems.