Anthropogenic Effects On Spatial Dynamics

The disappearance of M. spretus indicates the vulnerability to extinction of even cyclically abundant species when populations decline to near or below their extinction thresholds (see Chapter 6). Populations always have been vulnerable to local extinctions as a result of disturbances or habitat loss during environmental changes. Species persist to the extent that dispersal capabilities are adapted to the frequency and scale of these changes. Species adapted to relatively unstable habitats usually have higher reproductive rates and greater dispersal capabilities than do species adapted to more stable habitats.

Human activities affect spatial distribution of populations in several ways. Climate changes eventually will force many species to shift their geographic ranges or face extinction as changing temperatures and humidities exceed tolerance ranges or alter energy balance in their current ranges (Franklin et al. 1992, Kozar 1991, Rubenstein 1992) (see Fig. 5.2). Changing conditions may favor range expansion for other species. D. Williams and Liebhold (2002) projected that southern pine beetle distribution would shift northward and expand in area with warming climate, whereas mountain pine beetle, Dendroctonus ponderosae, distribution would move to higher elevations with shrinking area. D. Williams and Liebhold (1995) found that some climate-change scenarios predicted larger areas of defoliation by gypsy moth, whereas other scenarios predicted smaller areas of defoliation (Fig. 7.8).

Fragmentation of terrestrial ecosystems, alteration and pollution of aquatic ecosystems, and redistribution of species arguably are the most serious and immediate threats to ecosystems worldwide (Samways 1995). Patch scale, distribution, and abruptness of edges have been altered as a result of habitat fragmentation. This has been particularly evident for wetlands and grasslands. Wetlands historically occupied large portions of floodplains but have been virtually eliminated as a result of draining, filling, and stream channelization for

| Potential outbreak areas of gypsy moth in Pennsylvania under climate change scenarios. A: Current temperature and precipitation; B: a 2°C increase; C: a 2°C increase and 0.5 mm d-1 precipitation increase; D: a 2°C increase and 0.5 mm d-1 precipitation decrease; E: GISS model; and F: GFDL model. From D. Williams and Liebhold (1995) with permission from the Entomological Society of America.

| Potential outbreak areas of gypsy moth in Pennsylvania under climate change scenarios. A: Current temperature and precipitation; B: a 2°C increase; C: a 2°C increase and 0.5 mm d-1 precipitation increase; D: a 2°C increase and 0.5 mm d-1 precipitation decrease; E: GISS model; and F: GFDL model. From D. Williams and Liebhold (1995) with permission from the Entomological Society of America.

urban and agricultural developments. Grasslands have been fragmented severely worldwide because of their suitability for agricultural uses. Reservoirs have altered drainage characteristics and reduced the distances between lake ecosystems. Industrial and agricultural pollution threatens many aquatic species. A large number of vagrant species (including various crops and "weeds," rodents, and livestock, as well as insects and pathogens) have been transported, intentionally and unintentionally, far beyond their natural ranges by human activities. These exotic species have significantly altered the structure and function of their new ecosystems.

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