Competitive release in island Song Sparrow populations

Abstract

Song Sparrow territory sizes at 30 mainland and island sites in the Pacific Northwest and one site in Wyoming were measured, and mean values were obtained for each site. Differences in territory sizes were found even though the food supply at each site was similar or varied independently. Song Sparrow territory size was also independent of vegetation structure. A direct correlation was found between Song Sparrow territory size and the number of co-occurring potential competitor species. As the number of competitors decreased, the Song Sparrow territory size decreased. The mechanism through which this decrease occurs is that of competitive release, as realized niche approaches fundamental niche with reduced competitor diversity. Theoretical predictions of Song Sparrow territory size utilizing various forms of α-values were made; results supported the conclusion that competitive release had occurred and that Song Sparrows had expanded ecologically into niches vacated by absent competitors.