Competition-driven Allee effects in the highly invasive Argentine ant
Allee effects or the phenomenon of inverse density dependence may cause negative growth rate in small populations leading to their further decline, and eventual extinction. This has important implications in applied ecology, especially in disciplines such as invasion ecology, which deal with species management and control. Could the spread of an invasive species be contained by pushing it below the Allee threshold via competition with a native species? Empirical evidence to suggest such a competition-driven Allee effect is currently lacking. To investigate whether competitive relationships between native and invasive ants can interact with the founding populations and trigger Allee effects, we paired the highly invasive Argentine ant, Linepithema humile, with the native Mediterranean odorous ant, Tapinoma nigerrimum. Each experiment consisted of different initial worker abundance (0, 30 or 300 workers), and the same initial queen abundance (three queens) of the two species. We recorded survival, productivity and behaviour of both species.
We found significant negative effect of interaction between initial worker abundance of L. humile and presence of the native competitor T. nigerrimum, on productivity of L. humile, but no effect on its survival. Our study provides the first evidence of competition-driven component Allee effect. A component Allee effect caused by reduced productivity may imply that range expansion of L. humile could be halted using native competitors. Whether this component Allee effect has demographic consequences in L. humile needs to be tested. Given the worldwide invasion success of L. humile, it is likely that reduction in productivity due to competition is being counteracted by other fitness parameters. Our study highlights the need to identify, conserve and strengthen native species which could at least halt the process of biological invasions, if not completely render it inoperative.