Abstract
Understanding the relationship between invasive species density and ecological impact is a pressing topic in ecology, with implications for environmental management and policy. Although it is widely assumed that invasive species impact will increase with density, theory suggests interspecific competition may diminish at high densities due to increased intraspecific interactions. To test this theory, we experimentally examined intra- and interspecific interactions between a globally invasive fish, round goby (Neogobius melanostomus), and three native species at different round goby densities in a tributary of the Laurentian Great Lakes. Eighteen 2.25 m2 enclosures were stocked with native fish species at natural abundances, while round gobies were stocked at three different densities: 0 m−2, 2.7 m−2, and 10.7 m−2. After 52 days, native fish growth rate was significantly reduced in the low density goby treatment, while growth in the high density goby treatment mirrored the goby-free treatment for two of three native species. Invertebrate density and gut content weight of native fishes did not differ among treatments. Conversely, gut content weight and growth of round gobies were lower in the high goby density treatment, suggesting interactions between round gobies and native fishes are mediated by interference competition amongst gobies. Our experiment provides evidence that invasive species effects may diminish at high densities, possibly due to increased intraspecific interactions. This is consistent with some ecological theory, and cautions against the assumption that invasive species at moderate densities have low impact.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-014-2899-5) contains supplementary material, which is available to authorized users.
Highlights
Species invasions have altered ecological communities worldwide, but impact can vary greatly among species and habitats (Williamson and Fitter 1996; Melbourne et al 2007)
Round goby density had a significant effect on the relative growth of round goby (F1,120 = 53.5, P < 0.001), Johnny darter (F2,49 = 4.1, P = 0.02), and white sucker (F2,47 = 6.3, P = 0.004) (Fig. 1) after accounting for significant negative correlations between length and relative growth rate for all species
Johnny darter and white sucker relative growth rates were lowest in the low-goby treatment (Fig. 1)
Summary
Species invasions have altered ecological communities worldwide, but impact can vary greatly among species and habitats (Williamson and Fitter 1996; Melbourne et al 2007). Species introductions are commonly associated with negative effects on native species through predation, or competition for food or habitat (Mack et al 2000; Mills et al 2004; Hermoso et al 2011). They can benefit native species by becoming an important prey source (King et al 2006) or through habitat modification or competitive or predatory release (Rodriguez 2006). Invasive species are not immune from self-regulating effects: invasive species sometimes follow a “boom–bust” population trajectory, where high initial abundance stabilizes at lower densities over the long term (Simberloff and Gibbons 2004; Strayer and Malcom 2006). Intraspecific interactions amongst invasive species could potentially mediate their ecological effects
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