Scale of Investigation and the Detection of Competition: An Example from the House Sparrow and House Finch Introductions in North America

I present an example that illustrates an evaluation of interspecific competition by combining investigations conducted on different scales. Results from investigations on different scales are interpreted with an assessment of the scales on which the competitors perceive habitat and each other. Interference competition between the introduced house sparrow (Passer domesticus) and native house finch (Carpodacus mexicanus) observed at backyard nest boxes for 4 yr in the western United States may have resulted in habitat segregation on a larger scale along a rural-urban habitat gradient. Competition may be occurring in the eastern states since the introduction of C. mexicanus in 1940. Christmas Bird Count data were examined for patterns in species abundance predicted by competition theory, using three scales of investigation: local (area 24 km in diameter), regional (area 250 km in diameter), and continental (between areas of sympatry and allopatry). The major findings suggest that competition has not influenced patterns of changes in the abundances of the competitors on a local scale in urban and rural habitats in the eastern United States or on a regional scale in the eastern and western United States. On a continental scale, the presence of C. mexicanus may have accelerated widespread declines in the density of P. domesticus in areas of sympatry across the United States, possibly because of the energetic demands of interference competition. The dissimilar behavior and foraging strategies of the competitors may permit coexistence in all habitats, such that the niche of P. domesticus is included in the niche of C. mexicanus. Differences in the perception of habitat by the competitors may preclude the development of habitat segregation along a rural-urban habitat gradient. A multi-scale approach to evaluating interspecific competition suggests that investigations on any single scale (e.g., a field experiment) may overemphasize or miss the importance of competition.