Habitat area and environmental filters determine avian richness along an elevation gradient in mountain peatlands

Globally, relationships between avian richness and elevation in mountain
ecosystems typically reflect one of four well-documented patterns, but the
mechanisms responsible for these patterns are poorly understood. We
investigated which pattern best described bird species richness in
peatlands of the Upper Bow Basin of the Canadian Rocky Mountains (1300 to
2000 m a.s.l.) and used a model competition framework to investigate
possible mechanisms. Avian richness displayed a plateauing (cubic)
relationship in response to increasing elevation (AICc weight = 0.48). Log
richness was significantly positively related to log peatland area (R2 =
0.42, p = 0.001); however, once we accounted for the richness-area
relationship (area was not related to elevation (R2 = 0.13, p = 0.083)),
the richness-elevation relationship was best described by a negative
linear model rather than a cubic model (AICc weight = 0.69, R2 = 0.39).
Consequently, we reject the neutral model of the mid-domain effect and
conclude that peatland area and one or more environmental filters are
simultaneously driving relationships between avian richness and elevation
in Rocky Mountain peatlands. Multi-causality likely explains why
researchers in different geographies observe inconsistent patterns between
richness and elevation: drivers and interactions among drivers may vary
spatially. Importantly, Natural Subregion was a stronger predictor of
avian species richness than elevation per se (AICc weight = 0.96),
suggesting that the responsible environmental filter(s) is relatively
homogenous within ecological land classes (e.g., primary productivity)
rather than directly variable with elevation (e.g., temperature). The
results also lend insight into priorities for future research on
richness-elevation patterns in mountain birds.