Demographic consequences of invasion by a native, controphic competitor to an insular bird population

K M Johnson, R R Germain, C E Tarwater, J M Reid, P Arcese (Corresponding Author)

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)


Species invasions and range shifts can lead to novel competitive interactions between historically resident and colonizing species, but the demographic consequences of such interactions remain controversial. We present results from field experiments and 45 years of demographic monitoring to test the hypothesis that the colonization of Mandarte Is., BC, Canada, by fox sparrows (Passerella iliaca) caused the long-term decline of the resident population of song sparrows (Melospiza melodia). Several lines of evidence indicate that competition with fox sparrows for winter food reduced over-winter survival in juvenile song sparrows by 48% from 1960 to 2015, enforcing population decline despite an increase in annual reproductive rate in song sparrows over the same period. Preference for locally abundant seeds presented at experimental arenas suggested complete overlap in diet in song and fox sparrows, and observations at arenas baited with commercial seed showed that fox sparrows displaced song sparrows in 91-100% of interactions in two periods during winter. In contrast, we found no evidence of interspecific competition for resources during the breeding season. Our results indicate that in the absence of marked shifts in niche dimension, range expansions by dominant competitors have the potential to cause the extirpation of historically resident species when competitive interactions between them are strong and resources not equitably partitioned.

Original languageEnglish
Pages (from-to)155-165
Number of pages11
Issue number1
Early online date2 Mar 2018
Publication statusPublished - 31 May 2018

Bibliographical note

Funding Our work was supported by the University of British Columbia, W. and H. Hesse, the American Ornithologists’ Union, and Natural Sciences and Engineering Research Council of Canada


  • Journal Article
  • interspecifc competition
  • competitive exclusion
  • invasive species 
  • range shifts
  • Melospiza melodia
  • Passerella iliaca


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