Mechanisms driving component Allee effects during invasions: using a biological control agent as model invader.
A demographic Allee effect refers to increasing per capita population growth with increasing abundance. It arises from component Allee effects, which exist when some component of individual fitness has a positive relationship with density. Newly arrived populations tend to be small, and their establishment success is influenced by various factors, including demographic Allee effects. Identifying mechanisms driving the Allee effect is of relevance for understanding establishment failure, developing strategies for improving biological control and conservation, and for managing biological invasions. We utilised an invasive plant biocontrol agent, Neolema ogloblini, as model to study Allee effects experienced by invading populations. We investigated mating failure and predator satiation as two component Allee effects that could potentially drive a demographic Allee effect in N. ogloblini populations. We released unmated adults onto isolated host patches using five release sizes and evaluated the mating status of females after 3 weeks in the field. The probability of being mated increased as number of males recovered increased, suggesting the presence of a mating-failure component Allee effect in very small populations of N. ogloblini. Exclosures were used to evaluate survival of immatures of N. ogloblini developing in small group sizes, either in the presence or absence of generalist predators. Results revealed high levels of predation, but a positive relationship between larval survival and group size in the presence of generalist predators could not be verified. Our study identified relevant life-history traits contributing to establishment failure in N. ogloblini and demonstrates the value of biocontrol agents as models to study mechanisms of species invasion.