The rescue effect was characterized by Brown and Kodric-Brown (1977) as a variation on the MacArthur-Wilson (M-W) equilibrium model of island biogeography. The premise of the M-W model is that there is an equilibrium between immigration and extinction on islands. The number of species remains constant, but species composition changes throughout time due to extinction and colonization. This rate of species turnover is dependent upon distance between the island and the mainland and island size. Species turnover rate is higher when islands are closer to the mainland and when islands are smaller in size. Islands that are closer to the mainland experience higher immigration rates because they are closer to the mainland, which is assumed to be the primary source of colonists in the model.
The rescue effect explains an increase in fitness of individuals on islands closer to the mainland through increased immigration rates of colonists. Local extinction rates of species on these islands are lower than on islands that are more isolated and farther away from the mainland (Gotelli, 2001). Due to the rescue effect, species turnover rates will be lower on islands closer to the mainland. This contradicts the M-W model, which states that species turnover rates and distance from island to mainland are inversely related, i.e. that species turnover will be higher on islands closer to the mainland (Brown and Kodric-Brown, 1977). If dispersal ability of a species is low, however, it is assumed that population dynamics will follow the M-W model.
References
Brown, J.H. and A. Kodric-Brown. 1977. Turnover rates in insular biogeography: effect of immigration on extinction. Ecology 58(2): 445-449.
Gotelli, N.J. 2001. A Primer of Ecology, 3rd ed. Sunderland, MA: Sinauer Associates, Inc. pp. 90-91.