Abstract
BackgroundIntroduced organisms can alter ecosystems by disrupting natural ecological relationships. For example, red imported fire ants (Solenopsis invicta) have disrupted native arthropod communities throughout much of their introduced range. By competing for many of the same food resources as insectivorous vertebrates, fire ants also have the potential to disrupt vertebrate communities.Methodology/Principal FindingsTo explore the effects of fire ants on a native insectivorous vertebrate, we compared the reproductive success and strategies of eastern bluebirds (Sialia sialis) inhabiting territories with different abundances of fire ants. We also created experimental dyads of adjacent territories comprised of one territory with artificially reduced fire ant abundance (treated) and one territory that was unmanipulated (control). We found that more bluebird young fledged from treated territories than from adjacent control territories. Fire ant abundance also explained significant variation in two measures of reproductive success across the study population: number of fledglings and hatching success of second clutches. Furthermore, the likelihood of bluebird parents re-nesting in the same territory was negatively influenced by the abundance of foraging fire ants, and parents nesting in territories with experimentally reduced abundances of fire ants produced male-biased broods relative to pairs in adjacent control territories.Conclusions/SignificanceIntroduced fire ants altered both the reproductive success (number of fledglings, hatching success) and strategies (decision to renest, offspring sex-ratio) of eastern bluebirds. These results illustrate the negative effects that invasive species can have on native biota, including species from taxonomically distant groups.
Highlights
Since Darwin [1], ecologists have often assumed that competition for food resources is most intense among closely related species [2]
Fire Ant Treatment The abundance of foraging fire ants, measured as the mean number of ants per collection vial, was significantly lower in territories treated with the hydramethylnon ant bait (x = 38.9 6 SE 11.8, range 0.0–142.2) compared to adjacent control territories (x = 128.8627.9, range 6.67–352.25; paired t-test, n = 15, t = 3.2, p = 0.006)
The abundance of foraging fire ants had a marked effect on the breeding success and reproductive strategies of eastern bluebirds
Summary
Since Darwin [1], ecologists have often assumed that competition for food resources is most intense among closely related species [2]. In desert habitats of the American Southwest, ants, diurnal birds, and nocturnal mammals all compete for the same seeds (reviewed in [6]), and the abundance of each species in the guild affects the abundance of the others. Ants and birds are among the most abundant consumers in many ecosystems [7,8] and they frequently compete for resources. Competitive relationships between ants and birds can influence the distribution [9], habitat-use [10], foraging behavior [11,12], and offspring quality [13] of birds. By competing for many of the same food resources as insectivorous vertebrates, fire ants have the potential to disrupt vertebrate communities
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