Margaret Trani Griep, Regional Wildlife Ecologist, Southern Regional Office;
Patricia N. Manley, Institute of Pacific Islands Forestry, Pacific Southwest Research Station.
An archived version of this topic paper is available.
Biological diversity refers to the variation among living organisms and the ecological complexes of which they are a part. This includes the interrelated nature of genetics, species, and populations across the landscape (1). Biological diversity is essential to maintaining ecosystem processes and services; when loss occurs, ecosystem functionality is reduced (2, 3, 4). Losses of biological diversity over the past century have been unprecedented with environmental stressors such as land-use change, habitat degradation, landscape fragmentation, pollutants, and invasive species taking their toll.
Climate change has become an additional stress on species and communities, one that is expected to increase with time (5). Average temperatures in the United States have risen 2°F over the past half-century (6). The U. S. Global Change Research Program (7) reports that Alaska has warmed at twice the rate (3.4°F) during the same time period, causing reduced sea ice, glacier retreat, and permafrost warming. In the Southeast, fall precipitation has increased 30% and the number of freezing days has declined 4-7 days per year (7). Rising winter temperatures in the Northeast has resulted in longer growing seasons, less winter precipitation falling as snow, and earlier peak river flows. Heat waves, severe drought, and declining water resources are becoming issues in the Southwest and Great Plains (8). Higher temperatures (1.5°F - 4°F) in the Northwest have contributed to earlier snowmelt and reduced stream flows during the summer (9). Sea level rise, high water temperatures, and ocean acidification are concerns in coastal regions.
Questions exist about the challenges that these climate changes pose to biological diversity. Species respond to environmental conditions based on habitat needs and physiological tolerances (10), which in turn influences community composition, structure, and resilience (11). There may be shifts in the geographic range of many species, influencing seasonal movement, recruitment, and mortality (12). Changes in phenology (e.g., timing of resource availability, advances in flowering or nesting dates) may alter predator-prey, competitive interaction, and herbivore-vegetation dynamics (13, 1). Ecological niches may change at a pace slower than expectations for climate change (14); similarly, the pace of climate change will likely exceed the dispersal rate of several species (15). Existing communities may dissociate as species follow the range of suitable conditions, meaning that previously co-occurring species may move in divergent patterns (16, 17, 18, 19). Recolonization may be limited to areas similar to the range core (20).
Characteristics of species and communities at risk include those with restricted geographic ranges, fragmented distributions, and those that occur at the margins of their range. Other characteristics include limited dispersal ability, low genetic diversity, strong affinity to aquatic habitats, narrow physiological tolerance, and late maturation (18). Climate change may exacerbate these risks. For example, amphibians associated with cool, moist conditions may be subject to microclimates beyond their tolerance. Ephemeral streams and ponds may be especially vulnerable to drying with variable precipitation patterns. The small or disjunct populations that often characterize species of concern are likely to be impacted by stochastic climatic events and may not have the ability to adapt to a changing climate.
Climate change has been shown to affect the geographic range of species along elevational gradients (21, 22). Northern-temperate birds have shifted their ranges to higher latitudes, and tropical birds have shifted their breeding ranges to higher altitudes (11). These range shifts appear to have affected migration strategies, where success will depend on the rate of climate change relative to essential habitat needs and key community interactions. In the Southwest, small mammals have expanded their ranges upward in elevation while high-elevation species have contracted theirs, leading to changes in community composition (22). The elevation range shifts of butterfly species recorded in the Sierra Nevada Mountains may continue (23).
There are a number of other changes in biodiversity that are expected to result from climate change. Eastern tree species richness is projected to increase as temperatures warm (24), with the expansion of oak-hickory complex northward and contraction of aspen-birch habitat (25). Old-growth forests in the Northwest (26) and high-elevation forests (such as the spruce-fir complex) in the South (27) and elsewhere (25) appear particularly vulnerable. Rising temperatures may influence forest growth due to drought stress and declining soil moisture. This will increase the frequency of pine beetle and other insect attacks; milder winters may encourage the early emergence of other forest pests. Neotropical migratory birds that are sensitive to climate (i.e., climate associates) may change their migratory arrival in spring, as is being currently observed in the West (28).
Water-limited areas (e. g., weather-dependent, ephemeral) and aquatic systems are also expected to be vulnerable to change (29, 30, 31). Changes in water temperatures may result in reduced oxygen levels in streams and lakes, leading to declines in aquatic species diversity and stress on coldwater fisheries. Increased water temperatures in the Caribbean and Pacific Islands may continue to threaten coral reefs (32), shellfish, and other species. Barrier islands will be vulnerable to severe storm events, sea level rise, and saltwater intrusion (33), leading to declines in coastal wetlands and marshes (6). Communities along the Atlantic Coast and Gulf of Mexico supporting high concentrations of federally-listed species and migratory shorebirds will be especially vulnerable (27).
Options for Management
Climate change creates uncertainty about how best to design adaptation and mitigation strategies (34). Static management can no longer be assumed (35); the environment will change in a directional way rather than varying around a mean condition (36). The planning focus will be on spatial and temporal scales that are broader and longer than typically considered. Management for resilient forests and resistance to invasive species may become more focused in the future to account for changing climate, land use, and human population expansion. Difficult decisions on where to spend limited resources may favor some species over others. For example, restoration strategies may shift away from coastal areas under risk of sea level rise. As future impacts occur across large areas, the appropriate decision-making level may shift to cover landscape and regional scales.
Management options are further challenged by estimating the adaptive abilities of species where no current ecological analog exists. It will be critical to maintain conditions that allow for changes in species composition and migration while maintaining system function and process. The indirect effects on diversity created by shifting ranges and habitat associations are unknown (29, 37); predicting biogeographical shifts will be challenging.
Knowledge is evolving as researchers refine levels of uncertainty and contrast anthropogenic activities altering atmospheric composition with natural climate variability. Ecological response models (occupancy, vegetation, other) using downscaled climate data will play important roles. Management options that can help maintain biodiversity even where uncertainties exist include:
- Vulnerability assessments to identify species and communities at risk, including strategies to maximize persistence, dispersal, and ecosystem resilience (6).
- Ecological risk evaluation for areas of imminent change.
- Identification of barriers to migration and identification of mitigation measures to enhance landscape connectivity into future planning efforts.
- Long-term monitoring strategies to identify patterns in disturbance and phenology including the evaluation of current environmental indicators of biological diversity and resiliency.
- Adaptive restoration strategies based on predicted species range expansion and contraction, storm surge proximity, and seal level rise.
- Enhancement of genetic diversity to provide resilience against environmental stressors.
- Development of innovative tools for integrating climate change science into land management planning.