Other reports in this collection
With the concentration of population on the coasts, various development constraints and environmental regulations have been enacted to protect areas of coast as wildlife preserves and for harvesting of shellfish. Coastal ecosystems clearly are vulnerable to change associated with eventual sea-level rise. Coastal and marine biota also are vulnerable to changes in upwelling, current dynamics, freshwater inflow, salinity, water temperatures, and other processes that affect food webs and nursery areas (Boesch et al., 2000). Moreover, long-term studies of estuaries such as San Francisco Bay have indicated that natural cycles of ecosystem processes, such as phytoplankton blooms, are being altered on a global scale by human activities. This includes manipulation of river flows, input of toxic contaminants and nutrients, and invasion of exotic species (Cloern, 1996). Thus, in the nearer term, estuaries and coastal ecosystems may be most vulnerable to hydrological changes in rivers and groundwater flows from shifts in inland precipitation, evapotranspiration, and river ecosystem dynamics.
The increased frequency and geographical range of incidents of hazardous blooms in estuaries on the Gulf, Atlantic, and Pacific coasts has caused serious economic impacts for numerous fisheries and poses a great public health challenge in closing beaches to shellfish. Consumption of toxic algae by shellfish causes the shellfish to contain concentrations of toxins that are high enough to cause paralysis and death of humans who consume the shellfish. This situation has raised concerns that increased nitrogen loading to watersheds has led not only to general coastal eutrophication but also to a greater probability of circumstances that are conducive to these blooms. Factors that appear to contribute to harmful algal bloom occurrence are warmer temperatures and high runoff from watersheds that feed the estuaries, although much remains to be learned and predictive capability has yet to be achieved. Thus, indirect effects of climate change, which exacerbate the hazardous algal bloom problem, could be significant and difficult to identify until better understanding is gained (Anderson, 1997; see Chapter 6).
The Endangered Species Act in the United States and other regulatory efforts
preserve and manage wildlife populations in many regions of North America. These
efforts have begun to involve habitat protection and ecosystem management rather
than taking a strict population focus. Changes in habitats driven by climate
change could further restrict wildlife populations. One process would be causing
habitats to be less interconnected, restricting migration of individuals among
different populations and causing loss of genetic diversity within more isolated
populations. This process is a concern for aquatic and terrestrial wildlife.
Fish populations and other aquatic resources are likely to be affected by warmer
water temperatures, changes in seasonal flow regimes, total flows, lake levels,
and water quality. These changes will affect the health of aquatic ecosystems,
with impacts on productivity, species diversity, and species distribution (Arnell
et al., 1996). For example, warming of lakes and deepening of thermoclines will
cause a loss of habitat for coldwater fish in areas such as Wisconsin and Minnesota,
and decreases in summer flow and increased temperatures will cause loss and
fractionation of riverine habitat for coolwater fish species in the Rocky Mountain
region (Rahel et al., 1996; Cushing, 1997).
Wetlands and dependent wildlife resources may be adversely affected by general
increases in evapotranspiration and reduced summer soil moisture, which may
reduce the extent of semi-permanent and seasonal wetlands, particularly in the
prairie regions of North America (Poiani et al., 1995).
The state of terrestrial wildlife in North America varies geographically, by
taxa, and by habitat association. In general, biodiversity increases from north
to south, and species that are associated with rare habitats are most likely
to be at high risk of extinction (Dobson et al., 1997; Ricketts et al., 1999).
Many factors can cause a species to be at risk of extinction, but the most common
causes include loss of habitat, pressures from introduced species or hunting,
and reduced fitness as a result of chemical contaminants (Wilson, 1992; Meffe
and Carroll, 1994). A minimum estimate of the number of species at risk comes
from data for the United States, for which a recent summary suggests that 42
mammal species, 56 bird species, 28 reptile species, and 25 amphibian species
are considered at least vulnerable to extinction (UNEP, 2000). Key additional
pressures on wildlife associated with global climate change include changes
in temperature and precipitation, changes in sea level, and changes in the frequency
of extreme weather events (Peters, 1992; Parmesan et al., 2000).
Climate-related pressures can act directly on wildlife through physiological effects (i.e., changes in growth rates, food demands, abilities to reproduce and survive) or indirectly through effects on other plant and animal species (e.g., Payette, 1987; Lewis, 1993; Post and Stenseth, 1999). These physiological effects can lead to changes in the range and abundance of North American species; recent studies suggest that we already are seeing climate-linked changes in butterflies (Parmesan, 1996) and desert-associated species (Brown et al., 1997b; Smith et al., 1998a). A key potential indirect effect of climate change on wildlife is loss of total habitat available as a result of changes in the distribution of a particular habitat type. An obvious example is loss of coastal habitat as a result of sea-level rise; in many places, coastal habitats will not be able to shift inland because adjacent lands already are developed (Harris and Cropper Jr., 1992; Daniels et al., 1993). Similarly, potential shifts in the ranges of species in northern habitats are bounded by the Arctic Ocean (Kerr and Packer, 1998). A second key impact of climate-change related pressures on wildlife relate to how species interact with other species on which they depend. Because temperature and precipitation can be triggers for many wildlife behaviors, changes in these factors may differentially impact species in the same location. We already have evidence that the timing of bird migrations (Bradley et al., 1999; Inouye et al., 1999), bird breeding (Brown and Li, 1996; Brown et al., 1999; Dunn and Winkler, 1999), and emergence of hibernating mammals (Inouye et al., 1999) is becoming earlier. Depending on how food sources and other related species respond to changes, wildlife may become decoupled from the many ecological relationships of which they are a part.
|
Other reports in this collection |
|