2.2.1.5. Data and Response Types
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:
- Known fundamental mechanistic links between thermal/ precipitation tolerances
and species in the studies
- A large body of theory that links known regional climate changes to observed
biotic changes
- Direct observations of climate effects in some studies.
|