Division within the North American boreal forest: Ecological niche divergence between the Bicknell's Thrush (Catharus bicknelli) and Gray‐cheeked Thrush (C. minimus)

Abstract

Sister species that diverged in allopatry in similar environments are expected to exhibit niche conservatism. Using ecological niche modeling and a multivariate analysis of climate and habitat data, I test the hypothesis that the Bicknell's Thrush (Catharus bicknelli) and Gray-cheeked Thrush (C. mimimus), sister species that breed in the North American boreal forest, show niche conservatism. Three tree species that are important components of breeding territories of both thrush species were combined with climatic variables to create niche models consisting of abiotic and biotic components. Abiotic-only, abiotic+biotic, and biotic-only models were evaluated using the area under the curve (AUC) criterion.Abiotic+biotic models had higher AUC scores and did not over-project thrush distributions compared to abiotic-only or biotic-only models. From the abiotic+biotic models, I tested for niche conservatism or divergence by accounting for the differences in the availability of niche components by calculating (1) niche overlap from ecological niche models and (2) mean niche differences of environmental values at occurrence points.Niche background similarity tests revealed significant niche divergence in 10 of 12 comparisons, and multivariate tests revealed niche divergence along 2 of 3 niche axes. The Bicknell's Thrush breeds in warmer and wetter regions with a high abundance of balsam fir (Abies balsamea), whereas Gray-cheeked Thrush often co-occurs with black spruce (Picea mariana). Niche divergence, rather than conservatism, was the predominant pattern for these species, suggesting that ecological divergence has played a role in the speciation of the Bicknell's Thrush and Gray-cheeked Thrush. Furthermore, because niche models were improved by the incorporation of biotic variables, this study validates the inclusion of relevant biotic factors in ecological niche modeling to increase model accuracy. K E Y W O R D Sboreal forest, Catharus thrushes, ecological niche divergence, ecological speciation, niche modeling, sister species | INTRODUCTIONWhether speciation results in niches that are conserved or divergent remains a highly contested question within evolutionary biology.Sister taxa show niche conservatism if niche characteristics were ancestrally inherited and if speciation occurs in similar environments with no ecological selection (Peterson, Soberón, & Sánchez-Cordero, 1999;Warren, Glor, & Turelli, 2008;Wiens & Graham, 2005;Wooten & Gibbs, 2012). For example, Peterson et al. (1999) showed that most of 37 sister taxon pairs of birds, mammals, and butterflies in 5286 | FITZGERALD southern Mexico had similar ecological characteristics, but ecological niches were less similar at the familial level. Similarly, closely related North American wood-warblers (Parulidae) are more likely to have similar niches than more genetically distant warbler pairs (Lovette & Hochachka, 2006). McCormack, Zellmer, and Knowles (2009) found that although allopatric Mexican Jay (Aphelocoma ultramarina) subspecies show niche differen...

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