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.