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Ranges of migratory or mobile species can be very sensitive to climate when individuals show an immediate response in their migratory destinations. As with climatic data itself, one then needs long time series to distinguish year-to-year variation (noise) from long-term trends. Distributions of sedentary species have an inherent lag time stemming from limited dispersal abilities. Neither the numbers of populations nor the geographic location of the range limit may fluctuate strongly between adjacent years, and detectable shifts in species ranges may take decades or even centuries. In such cases, data often are not continuous through time, although data for a single year can be taken as representative of the state of the species during the surrounding multi-year period.
In addition to these shifts in species distributions, a suite of more subtle "plastic" responses allow organisms to adjust seasonally to natural variations of climate. Phenological changes—that is, shifts in the timing of events—can be assessed. These events include dates of budburst, flowering, seed set, fruit ripening, hibernation, breeding, and migration (Yoshino and Ono, 1996; Bradley et al., 1999; Menzel and Fabian, 1999). Changes in phenologies can be detected in a wide variety of organisms, but this requires studies conducted over several years, in which weekly or daily observations should be made before and during the target event (e.g., flowering). Remote-sensing data have the advantage that they can be analyzed for such effects years after the events, but they are limited to very general, community-wide questions such as dates when the ground begins to turn "green" from spring growth. They indicate trends only for the past 30 years because satellites with suitable detection equipment have been in place only since the early 1970s (Myneni et al., 1997). There are very long-term records (i.e., centuries) in a few unusual cases (Lauscher, 1978; Hameed, 1994; Sparks and Carey, 1995), but most monitoring data also are in the realm of the past 30 years (see Sections 5.4 and 19.2).
A different type of rapid response—probably nongenetic—is exemplified by changes in body size of small mammals and lizards (Sullivan and Best, 1997; Smith et al., 1998). Body size becomes smaller with general warming and larger with either cooling or increased variability of climate. This source of information has been studied with reference to historical climate (Morgan et al., 1995; Hadly, 1997; Badgley, 1998); it has been unexplored with respect to current trends and should be given greater attention.
Attribution of an observed biological trend to effects of climate change rests on several grounds (Easterling et al., 2000; Parmesan et al., 2000), namely:
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