Unifying Species Distributions, Community Science and the “Natural
Removal Experiment” to Investigate Species Interactions at Broad
Geographic Scales
Abstract
As species interactions influence species distributions, attempts to
work backwards from observed species distributions to infer the effect
of a potential interaction have been enticing. The “natural removal
experiment,” an approach that tests for patterns consistent with
competition by comparing a species’ habitat relationships in sympatry
and allopatry with a potential competitor, has held promise, but has
often been limited by the scales of data required. The recent expansion
of community science-based datasets invites renewed opportunity for
investigation using this approach. We revitalize the natural removal
experiment by applying it 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. Using data from the community science project eBird, we
compared relationships of CBCH relative abundance to urban- and
forest-related variables in individual urban-centres and across regions
of allopatry and sympatry with the BCCH. As predicted under competition,
we found that in allopatry, the CBCH adopted habitat relationships
similar to the less forested, more urban habitats of the BCCH at both
the urban-centre and regional scale. By applying predictive modeling to
“imagine” either the absence or ubiquity of BCCH across the studied
range of CBCH, we found that sympatry with the BCCH suppressed CBCH
abundance in urban areas, and that in their absence CBCH abundance
increased, consistent with competitive release. These lines of evidence
suggest that the observed distribution of CBCH is consistent with that
expected under competition with the BCCH in urban areas. Expanding
beyond this case study, we discuss important considerations in the
application of this approach, which presents a significant opportunity
for researchers to harness community science to conduct more powerful
investigations of species interactions extending to broad spatial
scales.