Applying metapopulation theory to conservation of migratory birds

Metapopulation theory has proven useful for understanding the population st ructure and dynamics of many species of conservation concern. The metapopul ation concept has been applied almost exclusively to nonmigratory species, however, for which subpopulation demographic independence-a requirement for a classically defined metapopulation-is explicitly related to geographic d istribution and dispersal probabilities. Defining the degree of demographic independence among subpopulations of migratory animals, and thus the appli cability of metapopulation theory as a conceptual framework for understandi ng population dynamics, is much more difficult. Unlike nonmigratory species , subpopulations of migratory animals cannot be defined as synonymous with geographic areas. Groups of migratory birds that are geographically separat e at one part of the annual cycle may occur together at others, but co-occu rrence in time and space does not preclude the demographic independence of subpopulations. I suggest that metapopulation theory can be applied to migr atory species but that understanding the degree of subpopulation independen ce may require information about both spatial distribution throughout the a nnual cycle and behavioral mechanisms that may lead to subpopulation demogr aphic independence. The key for applying metapopulation theory to migratory animals lies in identifying demographically independent subpopulations, ev en as they move during the annual cycle and potentially co-occur with other subpopulations. Using examples of migratory bird species, I demonstrate th at spatial and temporal modes of subpopulation independence can interact wi th behavioral mechanisms to create demographically independent subpopulatio ns, including cases in which subpopulations are not spatially distinct in s ome parts of the annual cycle.