Migration, Patchiness, and Population Processes Illustrated by Two Migrant Pests

摘要

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 km2) 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.nnIn 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 implement migration and determine the fitness of the migrants (the “migration syndrome”), and (d) the genetic complex underlying the migration syndrome.nnWe 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.nnInsect 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 the Intertropical Convergence Zone (ITCZ) in West Africa, to breed in rivers that flow only in the wet season (Garms et al. 1979, Cheke and Garms 1983, Baker et al. 1990). Later, their descendents return south to repopulate perennial rivers during the dry season.nnThe ITCZ also determines the migrations of birds within Africa, where a variety of movement patterns is known. These patterns were summarized for Nigeria by Elgood and colleagues (1973), who recognized three main categories of intra-African migratory birds. The first category consists of transequatorial migrants such as the pennant-winged nightjar (Macrodipteryx vexillarius), which breeds in the southern tropics and winters north of the equator, and Abdim's stork (Ciconia abdimii), which winters in southern Africa but breeds in the Sahel during the rains. The second category includes migrants within the northern tropics with exclusive breeding and nonbreeding geographical ranges. For instance, the grey-headed kingfisher (Halcyon leucocephala) follows the same pattern as the blackflies, moving north with the rains in April and May, and retreating southward in October to breed in the winter. However, there are also birds such as the white-throated bee-eater (Merops albicollis) that do the opposite of the blackflies, moving south with the rains and returning north to breed in the dry season. The third category consists of species with overlapping breeding and nonbreeding ranges that concentrate in the south in the dry season (e.g., the variable sunbird [Cinnyris venustus]) and of species that concentrate in the north in the dry season (e.g., the cattle egret [Bubulcus ibis]). These examples from West African birds are just a few of the many and varied migration patterns known among animals, and differences between patterns are attributable to dietary and nesting requirements, population pressures, and environmental determinants. Synopses of migration systems in other continents have been provided for the Americas by Jahn and colleagues (2004), for Asia by Irwin and Irwin (2005), and for Australia by Griffioen and Clarke (2002). To reveal how this extensive variation in migration strategies has evolved, we need research linking the population dynamics and genetic compositions of subpopulations with data on the breeding success of individuals adopting different migratory strategies.nnIn some bird species (e.g., the starling [Sturnus vulgaris] and blackbird [Turdus merula]) and insects (e.g., the brown plant-hopper [Nilaparvata lugens]), some individuals migrate and others do not (e.g., in N. lugens, long-winged forms move but brachypterous morphs do not), a phenomenon often under density-dependent control. This “partial migration” (Lack 1943, 1968) is reported in 70% of South American migrant bird species (Stotz et al. 1996, Jahn et al. 2004) and in 60% of migrant bird species from Europe (Berthold 2001), while various degrees of “nomadism” are frequent in desert species (Dean 2004). Migrations vary enormously in terms of their spatial topologies and scales, periodicities, and timing, each of which can influence population processes.nnTable 1 summarizes spatiotemporal movements by birds in terms of their scale, varying from local movements to seasonal circuit migrations such as those of the barn swallow. Analyses of the proportions of all species of British and Irish birds, of different ages and sexes, that migrate, and many other aspects of the migration system, have been provided by Wernham and colleagues (2002). Such summaries emphasize how varied migration patterns are and how difficult it is to generalize about them. In this article we illustrate how new techniques and new concepts are helping to elucidate the forces affecting migration patterns, not least the role of environmental variation.nnEnvironmental conditions are paramount in controlling migration and population processes at various scales (Sæther et al. 2006). Evidence is mounting that migration patterns are altering with current changes in climate, and this has already been shown by phenological shifts (Cotton 2003, Jenni and Kéry 2003), such as the earlier spring migrations by Tringa sandpipers (Anthes 2004). Climatic conditions have been determinants of the evolution of many migration systems, and the processes that bring about seasonal changes are exploited by migrants to help them on their journeys. The atmospheric conditions in a particular hour, day, or week will influence the timing and duration of the individual movements. In some cases weather conditions will assist passage, but in others they may hamper it or prove to be fatal. Bird irruptions—sudden arrivals of many individuals of usually scarce fruit- and seed-eating species—are often due to cold-weather movements brought on by a combination of high population levels and a lack of available food in the source area. Cold-weather movements in general can be caused by the sudden freezing of water bodies, by falls of thick snow covering feeding areas, or by frost hardening the ground (table 1; Elkins 2005).