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Exploiting the Achilles heels of pest invasions: allee effects, stratified dispersal and management of forest insect establishment and spread
Given the increasing problem of invasions by forest insects, there is an urgent need to develop effective strategies for managing them. Trends of escalating globalisation impede ongoing efforts to limit the arrival of new species. Consequently, either preventing establishment (through eradication) or limiting the spread of alien species are likely to play increasingly important roles. Here we argue that two traits common to many invading species can be exploited in the design of eradication and containment strategies. The first trait is the Allee effect, in which per capita growth rates decline with decreasing abundance. Allee effects can arise from several different mechanisms and are capable of driving low-density populations to extinction. Strategies to eradicate newly established populations should focus on either enhancing Allee effects or suppressing populations below Allee thresholds such that extinction proceeds without further intervention. The second trait is stratified dispersal, in which occasional long-distance dispersal results in the formation of isolated colonies ahead of the continuously infested range boundary. These colonies grow, coalesce and greatly increase spread rates. An efficient approach to containing the spread of invading species focuses on locating and eradicating these isolated colonies. Thus, Allee effects and stratified dispersal both represent 'weak links' in the invasion process that can be exploited in invasion management strategies.