Metapopulation Structure

The irregular distribution of many populations across landscapes creates a pattern of relatively distinct (often isolated) local demes (aggregations) that compose the greater metapopulation (Hanski and Gilpin 1997). Insect species characterizing discrete habitat types often are dispersed as relatively distinct local demes as a result of environmental gradients or disturbances that affect the distribution of habitat types across the landscape. Obvious examples include insects associated with lotic or high-elevation ecosystems. Populations of insects associated with ponds or lakes show a dispersion pattern reflecting dispersion of their habitat units. Demes of lotic species are more isolated in desert ecosystems than in mesic ecosystems. Populations of western spruce budworm, Choristoneura occidentals, and fir engraver beetle, Scolytus ventralis, historically occurred in western North America in relatively isolated high elevation and riparian fir forests separated by more xeric patches of pine forest (Wickman 1992).

Metapopulations usually are composed of demes of various sizes, reflecting the size or quality, or both, of habitat patches. For example, Leisnham and Jamieson (2002) found that demes of mountain stone weta, Hemideina maori, which shelter under rocks on isolated rock outcrops (tor) in alpine habitats in southern New Zealand, ranged in size from 0 to 6 adults on tors with 1-12 rocks and from 15 to 40 adults on tors with 30-40 rocks. Small tors were more likely to experience extinction events (4 of 14 small tors experienced at least 1 extinction during the 3-year study) than were large tors (no extinction events during the study).

Population structure among suitable patches is influenced strongly by the matrix of patch types. Haynes and Cronin (2003) studied the distribution of plant-hoppers, Prokelisia crocea, among discrete patches of prairie cordgrass, Spartina pectinata, as affected by surrounding mudflat, native nonhost grasses, or exotic smooth brome (Bromus inermis). Planthoppers were released into experimental cordgrass patches constructed to be identical in size (about 24 x 24 cm), isolation (>25 m from natural cordgrass patches), and host plant quality. Within patches, planthopper density was higher against mudflat edges, relative to patch interior, but not against nonhost patches. Among patches, density increased with increasing proportion of surrounding matrix composed of mudflat. The influence of matrix composition was equal to the influence of patch size and isolation in explaining planthopper distribution.

Population distribution and degree of isolation among local demes affect gene structure and viability of the metapopulation. If local demes become too isolated, they become inbred and may lose their ability to recolonize habitable patches following local extinction (Hedrick and Gilpin 1997). As human activities increasingly fragment natural ecosystems, local demes become isolated more rapidly than greater dispersal ability can evolve, and species extinction becomes more likely. These effects of fragmentation could be exacerbated by climate change. For example, a warming climate will push high-elevation ecosystems into smaller areas on mountaintops, and some mountaintop ecosystems will disappear (Fig. 5.2) (Franklin et al. 1992, D.Williams and Liebhold 2002). Rubenstein (1992) showed that individual tolerances to temperature changes could affect range changes by insects under warming climate scenarios. A species with a linear response to temperature could extend its range to higher latitudes (provided that expansion is not limited by habitat fragmentation) without reducing its current habitat. Conversely, a species with a dome-shaped response to temperature could extend into higher latitudes but would be forced to retreat from lower latitudes that become too warm. If the pathway for range adjustment for this species was blocked by unsuitable habitat, it would face extinction. Metapopulation dynamics are discussed in more detail in Chapter 7.

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