A metapopulation is a population composed of relatively isolated demes maintained by some degree of dispersal among suitable patches (Hanski and Simberloff 1997, Harrison and Taylor 1997, Levins 1970). Metapopulation structure can be identified at various scales (Massonnet et al. 2002), depending on the scale of distribution and the dispersal ability of the population (Fig. 7.7). For example, metapopulations of some sessile, host-specific insects, such as scale insects (Edmunds and Alstad 1978), can be distinguished among host plants at a local scale, although the insect occurs commonly over a wide geographic range. Local populations of black flies (Simuliidae) can be distinguished at the scale of isolated stream sections characterized by particular substrate, water velocity, temperature, proximity to lake outlets, etc., whereas many species occur over a broad geographic area (e.g., Adler and McCreadie 1997, Hirai et al. 1994). Many litter-feeding species occur throughout patches of a particular vegetation type, but that particular vegetation type and associated populations are fragmented at the landscape scale.
Metapopulation structure is most distinct where patches of suitable habitat or food resources are distinct and isolated as a result of natural environmental heterogeneity (e.g., desert or montane landscapes) or anthropogenic fragmentation. The spatial pattern of metapopulations reflects a number of interacting factors, including patch size, isolation, and quality (e.g., resource availability and disturbance frequency) and insect dispersal ability (Fleishman et al. 2002), and largely
Diagrammatic representation of different metapopulation models. Filled circles are occupied patches; open circles are unoccupied patches; dotted lines are boundaries of local populations; arrows represent dispersal. A: Classic (Levins) model of dispersal among demes. B: Island biogeography model with the mainland providing a source of colonists. C: A network of interacting demes. D: A nonequilibrium metapopulation with little capacity for recolonization of vacant patches. E: An intermediate case combining features of A-D. From Harrison and Taylor (1997).
determines gene flow; species viability; and, perhaps, evolution of life history strategies (e.g., Colegrave 1997). Hence, attention to spatially structured populations has increased rapidly in recent years.
Metapopulation structure can develop in a number of ways (see Fig. 7.7). One is through the colonization of distant resources and subsequent population development, which occurs during expansion of the source population (see earlier in this chapter). A second is through the isolation of population remnants during population decline. A third represents a stable population structure in a heterogeneous environment, in which vacant patches are colonized as local extinction occurs in other patches.
The colonization of new patches as dispersal increases during population growth is an important mechanism for initiating new demes and facilitating population persistence on the landscape. The large number of dispersants generated during rapid population growth maximizes the probability that suitable resources will be colonized over a considerable area and that more founders will infuse the new demes with greater genetic heterogeneity (Hedrick and Gilpin 1997). Species with ruderal life histories generally exhibit considerable dispersal capacity and often arrive at sites quite remote from their population sources (J. Edwards and Sugg 1990). Such species quickly find and colonize disturbed sites and represent a widely occurring "weedy" fauna. By contrast, species with competitive strategies show much slower rates of dispersal and may travel shorter distances consistent with their more stable population sizes and adaptation to more stable habitats (St. Pierre and Hendrix 2003). Such species can be threatened by rapid changes in environmental conditions that exterminate demes more rapidly than new demes are established (Hanski 1997, Hedrick and Gilpin 1997).
If conditions for population growth continue, the outlying demes may grow and coalesce with the expanding source population. This process contributes to more rapid expansion of growing populations than would occur only as diffusive spread at the fringes of the source population. A well-known example of this is seen in the pattern of gypsy moth, Lymantria dispar, population expansion during outbreaks in eastern North America. New demes appear first on ridgetops in the direction of the prevailing wind because of the wind-driven dispersal of ballooning larvae. These demes grow and spread downslope, merging in the valleys. Similarly, swarms of locusts may move great distances to initiate new demes beyond the current range of the population (Lockwood and DeBrey 1990).
As a population retreats during decline, subpopulations often persist in isolated refuges, establishing the postoutbreak metapopulation structure. Refuges are characterized by relatively lower population densities that escape the density-dependent decline of the surrounding population. These surviving demes may remain relatively isolated until the next episode of population growth. The existence and distribution of refuges is extremely important to population persistence. For example, bark beetle populations usually persist as scattered demes in isolated lightning-struck, diseased, or injured trees, which can be colonized by small numbers of beetles (Flamm et al. 1993). Such trees appear on the landscape with sufficient frequency and proximity to beetle refuges that endemic populations are maintained (Coulson et al. 1983). Croft and Slone (1997) and W. Strong et al. (1997) reported that predaceous mites quickly find colonies of spider mites. New leaves on expanding shoots provide important refuges for spider mite colonists by increasing their distance from predators associated with source colonies.
If suitable refuges are unavailable, too isolated, or of limited persistence, a population may decline to extinction. Under these conditions, the numbers and low heterozygosity of dispersants generated by remnant demes are insufficient to ensure viable colonization of available habitats (see Fig. 5.6). For most species, life history strategies represent successful adaptations that balance population processes with natural rates of patch dynamics (i.e., the rates of appearance and disappearance of suitable patches across the landscape). For example, Leisnham and Jamieson (2002) reported that immigration and emigration rates of the mountain stone weta, Hemideina maori, were equivalent (0.023 per capita). However, anthropogenic activities have dramatically altered natural rates and landscape pattern of patch turnover and put many species at risk of extinction (Fielding and Brusven 1993, Lockwood and DeBray 1990, Vitousek et al. 1997).
Lockwood and DeBray (1990) suggested that loss of critical refuges as a result of anthropogenically altered landscape structure led to the extinction of a previously widespread and periodically irruptive grasshopper species. The Rocky
Mountain grasshopper, Melanoplus spretus, occurred primarily in permanent breeding grounds in valleys of the northern Rocky Mountains but was considered to be one of the most serious agricultural pests in western North America prior to 1900. Large swarms periodically migrated throughout the western United States and Canada during the mid-1800s, destroying crops over areas as large as 330,000 km2 before declining precipitously. The frequency and severity of outbreaks declined during the 1880s, and the last living specimen was collected in 1902. Macroscale changes during this period (e.g., climate changes, reduced activity of Native Americans and bison, and introduction of livestock) do not seem adequate by themselves to explain this extinction. However, the population refuges for this species during the late 1800s were riparian habitats where agricultural activity (e.g., tillage, irrigation, trampling by cattle, introduction of non-native plants and birds) was concentrated. Hence, competition between humans and grasshoppers for refugia with suitable oviposition and nymphal development sites may have been the factor leading to extinction of M. spretus (Lockwood and DeBrey 1990).
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