Invasion of an Avian Nest Parasite, Philornis downsi, to the Galapagos Islands: Colonization History, Adaptations to Novel Ecosystems, and Conservation Challenges

During the establishment phase of a biological invasion, population dynamics are strongly influenced by Allee effects and stochastic dynamics, both of which may lead to extinction of low-density populations. Allee effects refer to a decline in population growth rate with a decline in abundance and can arise from various mechanisms. 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 spread phase of invasions results from the coupling of population growth with dispersal. Reaction-diffusion is the simplest form of spread, resulting in continuous expansion and asymptotically constant radial rates of spread. However, spread of most nonindigenous insects is characterized by occasional long-distance dispersal, which results in the formation of isolated colonies that grow, coalesce, and greatly increase spread. Allee effects also affect spread, generally in a negative fashion. Efforts to slow, stop, or reverse spread should incorporate the spread dynamics unique to the target species.