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The roles of dispersal and population dynamics in determining species' range boundaries recently have received theoretical attention but little empirical work. Here we provide data on survival, reproduction, and movement for a Virginia opossum (Didelphis virginiana) population at a local distributional edge in central Massachusetts (USA). Most juvenile females that apparently exploited anthropogenic resources survived their first winter, whereas those using adjacent natural resources died of starvation. In spring, adult females recolonized natural areas. A life-table model suggests that a population exploiting anthropogenic resources may grow, acting as source to a geographically interlaced sink of opossums using only natural resources, and also providing emigrants for further range expansion to new human-dominated landscapes. In a geographical model, this source-sink dynamic is consistent with the local distribution identified through road-kill surveys. The Virginia opossum's exploitation of human resources likely ameliorates energetically restrictive winters and may explain both their local distribution and their northward expansion in unsuitable natural climatic regimes. Landscape heterogeneity, such as created by urbanization, may result in source-sink dynamics at highly localized scales. Differential fitness and individual dispersal movements within local populations are key to generating regional distributions, and thus species ranges, that exceed expectations.

Optimum temperatures for growth and temperature pret•rence were estimated and compared for young striped bass Morone saxatilis, white perch Morone am•icana, white catfish lctalurus catus, and spottail shiner Notropis hudsonius to determine how closely behavioral thermoregulation corresponded to optimal growth temperatures. Difibrences between the final prefbrenda and the optimum growth temperatures were less than 2 C. The percentage of prefkrred temperatures that were within the temperature range corresponding to 75-100% of maximum growth was 76-100%. Only a small percentage (0-17%) of prefbrred temperatures were greater than the upper li•nit of the range supporting 75% or more of maximran growth, indicating that fish were avoiding temperatures that were suboptimal for prolonged exposures. Numerous studies on the eIfbcts of temper-ature on fish behavior and physiology have been conducted since the mid-1960s, mostly in con-.junction with the assessment of environmental effects of thermal effluents. Examination of this extensive literature has led some researchers to conclude that optimum temperatures for various physiological t•nctions, particularly growth, coincide generally with final thermal preferenda. Brett (1971) demonstrated such a relationship for juvenile sockeye salmon Oncorhynchus nerka, and Kellogg (1982) found that the final preferendum of young alewives A losa pseudoharengus coincided with the temperature at which net biomass gain was maximum. Other evidence in support of this thesis results from comparisons between fiual preferenda and thermal optima determined in separate studies and with difibrent stocks of fish and sometimes different ages of fish (Beitinger and Fitzpatrick 1979; Jobling 1981; McCauley and Casselman 1981). This apparent relationship between behavioral thermoregulation and thermal optima prompted McCauley and Casselman (1981) to propose that fish culturists could use age-specific temperature preference information as a i Present address: North Carolina State University,

9The purpose of this chapter is to present an indicator of how changes in pesticide use in agriculture have changed the potential for risk to human health and the environment from pesticide contamination of water leaving farm fields. Environmental indicators are designed to be relative estimates of potential risk that are based on pesticide use and on the factors that are known to be important determinants of pesticide loss from farm fields, such as the intrinsic potential of soils to leach or runoff pesticides, the chemical properties of the pesticides, annual rainfall and its relationship to leaching and runoff, and changes in cropping patterns. The analytical framework consists of about 4,700 resource polygons representing the intersection of 48 states, 280 watersheds at the 6-digit Hydrologic Unit level, and 1,400 combinations of climate and soil groups. Twelve crops are included in the analysis: com, soybeans, wheat, cotton, sorghum, barley, rice, potatoes, oats, sugarbeets, tobacco, and peanuts. Model estimates of pounds of pesticides applied, mass loss, and annual concentrations leaving the farm field (edge of field and bottom of root zone) were obtained for pesticides used on each of the 12 crops in each of the resource polygons for each year from 1960 through 1997. Indicators of potential risk are constructed from estimates of annual concentrations that exceed "safe" thresholds for chronic exposure to four target groups -humans, fish, crustaceans, and algae. It is expected that temporal and spatial trends of these indicators will closely track the change in potential risk to human health and the environment from agricultural use of pesticides.