Heterogeneity in avian richness-environment relationships along the Pacific Crest Trail

Abstract

Predictions of the responses of montane bird communities to climate change generally presuppose that species and assemblages hold constant relationships to temperature across large study regions. However, comparative studies of avian communities exploring the factors that currently shape species richness patterns rarely analyze relationships across neighboring ecological regions of the same mountain chain. Evaluations of the intrinsic regional differences in species-environment relationships are needed to better inform expectations of how bird communities may be affected by future climate change. In this study, we evaluated the relative importance of three environmental factors (temperature, precipitation, and net primary productivity) in structuring avian richness patterns along a continuous mega-transect. We followed the route of the Pacific Crest Trail (PCT) (32.58° N to 42.00° N, ranging in elevation from 365 to 4020 m) on the California cordillera and completed avian point counts on 3578 systematically established survey plots. We divided this mega-transect into five sections, which corresponded to distinct ecological regions along the mountain chain. Regions differed both for elevation-richness patterns, exhibiting linear and unimodal trends, and for model-supported environmental drivers of patterns, with some richness-environment correlations changing sign across adjacent regions. These results were robust to sampling bias, regional species availability, and spatial autocorrelation. Although seasonal variation in avian movements may have limited influence on our results, we conclude that intrinsic regional environments affect bird species richness differently in each of these sections on the PCT, thus creating region-specific species-environment relationships. Appreciation of regional environmental heterogeneity will only increase in light of forecasted climate change, where regional predictions often diverge greatly from global trends, necessitating a site-specific approach to climate adaptation rather than 'one size fits all' strategies.

Keywords

• birds
• California
• elevation gradient
• mega-transect
• net primary productivity
• precipitation
• species richness
• temperature