Temperature and Species Geographic Ranges

To date, most predictions regarding the potential effects of climate change on wildlife and ecosystems have focused on shifts in species' geographic ranges. Species are expected to respond to warming temperatures by gradually shifting to higher latitudes or altitudes to remain within their thermal tolerance ranges (Breeman 1990, Davis and Zabinski 1992, Gates 1993, Lubchenco et al. 1993). Several recent studies in both marine and terrestrial systems report long-term changes in species distributions that are consistent with these predictions (Barry et al. 1995, Parmesan 1996, Pounds et al. 1999, Sagarin et al. 1999, Parmesan et al. 1999, Thomas and Lennon 1999, Reed, this volume, Sagarin, this volume).

Projected range shifts are based on the long-standing hypothesis that temperature sets the geographic range limits of most species (e.g., Orton 1920, Hutchins 1947). Although there are only a handful of studies that establish a direct physiological or experimental connection between temperature and geographic range boundaries (Sundene 1962, Nobel 1980, references in Breeman 1990), there are many strong correlations between temperature isotherms and species' range limits that suggest such links may be common (e.g., Woodward 1987, Root 1988).

These patterns imply a general model of range limits based on the relationship between temperature and physiological tolerance. An organism's performance or fitness is expected to vary as a roughly bell-shaped function of temperature (Fig. 4.1A), driven by the relationship between temperature and the efficiency of underlying physiological processes (Shelford 1913, Huey and Stevenson

1979, Cossins and Bowler 1987, Huey and Kingsolver 1989). Optimal performance occurs in the middle of this temperature range. At high and low temperatures there are critical limits, beyond which an organism is no longer able to reproduce successfully (i.e., the organism may be unable to produce offspring, or alternatively, the organism's offspring may be unable to survive; Orton 1920, Hutchins 1947, Bhaud et al. 1995). These upper and lower critical limits are thought to set the geographic boundaries of a species' range (Fig. 4.1A).

Given this model, a species' range contracts when temperature change eliminates populations living at the margin of the range. For example, a population living near its southern boundary may disappear when a small temperature increase pushes individuals beyond their upper critical limit (Fig. 4.1B).This model implies that populations living near their range limits are most vulnerable to a slight temperature change (Davis and Zabinski 1992), whereas populations living near the middle of their geographic range (i.e., close to their physiological optimum) should be relatively unaffected by a slight temperature change.

Considerable evidence suggests that physiological optima and critical limits are likely to vary among species within the same community. For example, different species of plants living within the same habitat frequently vary in their physiological responses to temperature and other environmental factors (such as light, CO2, and soil nutrients; Pacala and Hurtt 1993). A similar pattern may apply to animals. For example, a recent study found that seven congeneric species of darkling beetles living in a short-grass prairie in Colorado showed peak activity at different ambient temperatures (Whicker and Tracy 1987). In the laboratory, beetles selected these same, species-specific temperatures from a thermal gradient. These and similar studies (e.g., Leveque 1997, Tomanek and Somero 1999) suggest that species within a community are likely to respond differently to temperature change.

This idea is also supported by the fossil record in North America, which indicates that species' ranges shifted independently from one another in response to past changes in climate (Foster et al. 1990). For example, during the last North American glacial cycle (14,000-10,000 years ago), individual species of small mammals showed shifts in geographic range marked by different directions and rates (Graham 1992). Similarly, pollen records suggest that the

Temperature

Figure 4.1. A simple model relating temperature and the geographic range of a hypothetical sedentary species in California and Oregon. (A) Individual performance varies as a species-specific function of temperature. Geographic range limits are set by a critical minimum (Cmin) and critical maximum (Cmax) temperature, beyond which the organism is unable to successfully reproduce (shaded regions under curve). (B) Dynamics of a hypothetical range contraction. A small temperature increase occurs throughout the geographic range. Individuals near their southern range limit formerly experienced temperature "a," but now experience a higher temperature "b" within the species' upper critical range. Individuals are unable to successfully reproduce, and southern populations ultimately disappear (cross-hatched region of geographic range). Analogous processes may expand the northern range limit, leading to a northward shift of the entire range.

Temperature fc.

low high

Figure 4.1. A simple model relating temperature and the geographic range of a hypothetical sedentary species in California and Oregon. (A) Individual performance varies as a species-specific function of temperature. Geographic range limits are set by a critical minimum (Cmin) and critical maximum (Cmax) temperature, beyond which the organism is unable to successfully reproduce (shaded regions under curve). (B) Dynamics of a hypothetical range contraction. A small temperature increase occurs throughout the geographic range. Individuals near their southern range limit formerly experienced temperature "a," but now experience a higher temperature "b" within the species' upper critical range. Individuals are unable to successfully reproduce, and southern populations ultimately disappear (cross-hatched region of geographic range). Analogous processes may expand the northern range limit, leading to a northward shift of the entire range.

distributions of tree species responded individualistically to climatic changes over the past 18,000 years (Webb 1992).To the extent that species' ranges reflect temperature tolerances, these individualistic responses suggest that communities are impermanent associations of species with different underlying physiologies.

Assuming these individualistic responses apply broadly to most species, ongoing climatic warming is likely to shift species' ranges independently of one another, and perhaps disassemble present-day communities, leading to the formation of new species associations. As a result, scientists considering climatic impacts have generally treated species as independent units, rather than considering the influence of temperature on communities and community processes. Most studies have focused on predicting range shifts of single species and have used a three-part strategy (summarized by Harte et al. 1992): (1) field-derived correlations and physiological information from laboratory studies are used to estimate a species' thermal tolerance, (2) atmospheric models are used to predict temperature changes, and (3) these data are combined to determine whether a species would need to move in order to remain in a suitable climate. This strategy has been used to project range shifts for a variety of species of marine algae, trees, insects, fish, and mammals (e.g., Breeman 1990, Scott and Poynter 1991, Davis and Zabinski 1992, Rogers and Randolph 1993, Johnston and Schmitz 1997).

This approach is conceptually simple and appealing, and predicting range shifts is a useful first step in evaluating potential impacts of climate change. However, the near exclusive focus on range shifts implies that: (1) impacts on communities can be evaluated solely by assessing the response of individual species to the direct effects of temperature change, (2) changes will be manifested mainly through lethal effects on individuals, and (3) populations near geographic range limits will be most vulnerable to temperature change, whereas those near the central portion of a range will be relatively unaffected.

0 0

Post a comment