Studies that relate observed changes in natural biota to climatic changes are necessarily correlational. It is not possible to address this question through a standard experimental approach, so direct cause-and-effect relationships cannot be established. However, the level of uncertainty can be reduced until it is highly unlikely that any force other than climate change could be the cause of the observed biotic changes. Studies can reduce uncertainty in three ways:
Statistical power is gained by using:
Confounding factors can be addressed in a correlational study. Biologists know that many nonclimatic anthropogenic forces affect population dynamics, community stability, and species distributions. These forces fall largely under the main headings of land-use change, hydrological changes, pollution, and invasive species. The term "land-use change" comprises a suite of human interventions that eliminate or degrade natural habitats, leading to loss of species that are dependent on those habitats. Habitat loss can be overt destruction, as occurs with urbanization, conversion to agriculture, or clear-cut logging. Habitat degradation is more subtle; it usually results from changes in land management, such as changes in grazing intensity/timing, changes in fire intensity/frequency, or changes in forestry practices (coppicing, logging methods, reforestation strategies), as well as irrigation dams and associated flood control. Loss of habitat by either means not only causes extinctions at that site but endangers surrounding good habitat patches by increased fragmentation. As good habitat patches become smaller and more isolated from other good patches, the populations on those patches are more likely to become permanently extinct.
The main airborne pollutants that are likely to affect distributions and compositions of natural biotic systems are sulfates, which lead to acid rain; nitrates, which fertilize the soil; and CO2, which affects basic plant physiology (particularly the carbon/ nitrogen ratio). Urban areas, in addition to having locally high amounts of sulfates and nitrates, are artificial sources of heat. Aquatic and coastal marine systems suffer from runoff of fertilizers and pesticides from agricultural areas and improperly treated sewage, as well as fragmentation from dams and degradation resulting from trawling, dredging, and silting.
These confounding factors cannot be completely eliminated, but their influences can be minimized by (Parmesan, 1996, 2001; Parmesan et al., 1999):
If a particular confounding factor cannot be greatly minimized, it should be measured and analyzed alongside climatic variables to assess their relative effects.
Ideal target species, communities, or systems in which to look for biotic responses to climate change meet the following criteria (DeGroot et al., 1995; Parmesan, 2001):
Use of indicator species or communities is crucial for defining the level of climate change that is important to natural systems and for giving baseline data on impacts. However, caution is advisable when extending these results to predictive scenarios because the indicators often are chosen specifically to pinpoint simple responses to climate change. Thus, these studies purposefully minimize known complexities of multiple interacting factors, such as:
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