Oriole niche evolution
The genus Icterus would seem to exist in many different environments, which in turn would suggest that its niches have diversified. Indeed, our analysis in which the non-equivalency of existing and fundamental niches was not controlled suggested quite a bit of change in the niche across the phylogeny. However, when we controlled for the availability of conditions across areas accessible to each species (i.e. bin-based coding incorporating uncertainty), we found little evidence of change in the fundamental ecological niche across the phylogenetic history of the genus, particularly when using maximum likelihood reconstruction. Icterus is a tropical genus that has “left” the Tropics only in ways that have kept lineages under tropical conditions, such as moving into northern areas of North America in the breeding season only. This niche conservatism has been termed “niche following” in previous work (Joseph and Stockwell 2000; Nakazawa et al. 2004).
The overall tendency observed in our analyses of oriole niche evolution across the history of the genus Icterus was one of remarkable niche stability. Particularly invariant was the upper end of the temperature tolerance spectrum (Fig. 3; Supplement 7 Fig. 2, Tables 3 and 4; Supplement 13). This observation coincides with recent results from Araújo et al. (2013), who presented a meta-analysis that concluded that heat tolerance was much more constrained over evolutionary history than cold tolerance. However, it is also true that our proposed framework for characterizing ecological niches and subsequent ancestral niche inference mayunder- estimate niche evolution because it only concludes niceh change when explicit evidence exists. Further detailed simulation study is needed to examine fully the sensitivity of our proposed methods to true niche evolution.
Specifically, we identified niche reductions for species that are relative habitat specialists within Icterus . Icterusorioles are a predominately lowland group, although some species also occur in foothills and low montane regions adjacent their core lowland ranges. However, for two species that do specialize in Mesoamerican montane habitats, I. abeillei and I. maculialatus , we identified corresponding reductions in high temperature tolerance. We also idenfied low temperature restriction for two strictly lowland tropical species, I. fuertesi and I. chrysocephalus.Icterus orioles occupy a variety of forest types across a variety of precipitation regimes. However, for two species that specialize in dry forest, I. auratus of the Yucatán Peninsula and I. graceannae of the Tubezian region, we identified niches corresponding to reduced precipitation.