Migration, Patchiness, and Population Processes Illustrated by Two Migrant Pests
Articles I n this article we consider how migration, environment, and population processes interact within the "migration system" outlined by Dingle and Drake (2007) . This system is an elaboration of the proposition by Drake and colleagues (1995) of a conceptual migration model incorporating aspects of (a) the environment in which migration occurs (the "migration arena"), (b) the spatiotemporal population demography that results from migration (the "population trajectory"), (c) the traits that
... (c) the traits that implement migration and determine the fitness of the migrants (the "migration syndrome"), and (d) the genetic complex underlying the migration syndrome. We concentrate on the migration arena and population trajectory when discussing the migration systems of terrestrial birds and insects and how some of these organisms' movements interact with environmental variability. In doing so, we briefly describe some novel approaches (connectivity, carryover effects, and metapopulations) and techniques (satellite telemetry, stable isotopes, and molecular methods) that are being used to improve understanding of migration. Much of such current research is focused on the Americas or on Palearctic-Afrotropical migrants in their European breeding quarters. To help redress this imbalance, we highlight work on the movements of two migrant species: the red-billed quelea (Quelea quelea), which is a major agricultural pest in sub-Saharan Africa, and the desert locust (Schistocerca gregaria), which devastates crops in Africa, the Middle East, and Asia. Q. quelea and S. gregaria have yet to be studied using satellite telemetry, and little is known about carry-over effects among them; nonetheless, we can illustrate recent advances in understanding their migrations by using a metapopulation approach to study their movements and by presenting the results of molecular and connectivity analyses. Insect migrations differ from bird migrations in that insects seldom perform seasonal circuit migrations in the way that, for example, barn swallows (Hirundo rustica) do. These birds, and many other Palaearctic migrants, breed in Europe and spend their winters in Africa, returning to the same locations in each continent year after year. This migratory pattern is not typical of insects, but some insects, such as the monarch butterfly (Danaus plexippus L.), do perform a form of seasonal circuit migration, although more than one generation may be involved in a round trip (Dingle et al. 2005 , as modeled by Yakubu et al. 2004 ). Blackflies such as savanna cytospecies of the Simulium damnosum species complex, which are vectors of onchocerciasis, or "river blindness," migrate up to 500 km. They travel north with the advancing rain fronts of Robert A. Cheke New technologies are improving scientists' understanding of the links between sources and destinations of subpopulations of migrants within populations as a whole (metapopulations). Such links and the importance of environmental patchiness are illustrated by migrations of two major pests, the red-billed quelea (Quelea quelea) and the desert locust (Schistocerca gregaria). The spatiotemporal distribution of rainfall determines where and when Quelea can breed, as shown for Quelea populations in southern Africa. Numbers and distributions of swarms of desert locusts in four different regions of their huge invasion area (29,000,000 km 2 ) were analyzed as local populations of a metapopulation. Lagged cross-correlations of seasonally adjusted monthly data demonstrate links between the local populations, which vary in significance according to the pairings of regions analyzed and the lengths of the lags, illustrating the strength of the connectivity between them. Understanding such relationships is essential for predictions concerning future climate change scenarios.