Figure 1. Predicted relative abundance of chestnut-backed chickadee (CBCH) in the environmental conditions of grid cells across gradients of urban- and forest-related variables from six CBCH relative abundance models, three from allopatric locations (southern Vancouver Island, BC, Canada; northern CA, USA; and the San Francisco Bay Area, CA, USA) and three from sympatric locations (Metro Vancouver, BC, Canada; Seattle-Tacoma, WA, USA; and Portland, OR, USA).
Abstract
To understand the distribution of avian biodiversity, it is crucial to understand the interspecific interactions present in avian communities. The ‘natural removal experiment’, which explores patterns consistent with competition by comparing a species’ habitat relationships in sympatry and allopatry with those of a potential competitor, has held promise, but has thus far had limited general replicability. We offer an adaptation of this method applicable to many study systems and over broad geographic scales, without requiring a priori knowledge of an interaction. We use this method to ask whether the distribution of the chestnut-backed chickadee Poecile rufescens (CBCH) is consistent with competition with the black-capped chickadee Poecile atricapillus (BCCH) in urban areas in the Pacific Northwest. Using data from eBird, we compared relationships of CBCH relative abundance to urban- and forest-related variables in different local urban centres and across region-wide areas of allopatry and sympatry with the BCCH. As predicted under competition, we found that in allopatry, the CBCH adopted habitat relationships similar to the BCCH, inhabiting less forested, more urban habitats, both in local urban centres and at the region-wide scale. When examining the potential consequences of absence or ubiquity of BCCH across the region, we found when model predictions were made as if BCCH were ubiquitous, CBCH abundance was lower in urban areas, and that when predictions were made as if BCCH were absent CBCH abundance increased, consistent with competitive release. These lines of evidence suggest that the distribution of CBCH is consistent with that expected under competition with the BCCH in urban areas. As the BCCH expands its range further into the Pacific Northwest, our assessment foreshadows the eventual replacement of CBCH from urbanized areas. Expanding beyond this case study, we discuss considerations in the application of this method to offer researchers a broadly applicable tool to study species interactions.