Figure 1.

Diagram of an idealized metapopulation. Each numbered circle represents a distinct population (or colony), with circle size proportional to population size. Most populations are connected by periodic exchange of individuals, which may be unilateral (single-headed arrows) or bilateral (doubleheaded arrows), and which can occur frequently (wide arrows) or infrequently (narrow arrows). Movements between some populations may be effectively blocked or limited by physical barriers, such as rivers or major highways