A full version of this synthesis, along with references and links, is available from the Climate Change Resource Center.
Climate change is expected to impact most parts of an ecosystem, and mammals are no exception. Some mammals have very specific climatic adaptations, such as requirements for snow, sea ice, or temperatures within a narrow range for hibernation. Some have distributions that are dependent on climate. Most mammals will not be able to avoid the effects of climate change, with both positive and negative effects possible. Mammals generally utilize a variety of often disjunct resources. They need places to hide, eat, drink, and breed, and in many cases, these places are distinct and may change seasonally. Thus, there are many opportunities for climate change to disrupt mammalian life histories. In general, they will not be able to effectively hide in microhabitats; in contrast many plants can persist as rare endemics long after the climate has changed. Most mammals are also highly mobile and, compared to perennial plants, have relatively short (generally < 20 yrs) life spans. Thus, if climates become unsuitable, mammalian response can be expected to be rapid.
Mammals play dominant roles in many systems. They make up most of the terrestrial large-bodied predators in North America, and these large, high-trophic mammals have significant impacts on the ecosystems they inhabit. For example, gray wolf introduction in Yellowstone National Park has reduced the northern elk herd to approximately 1/3 of its former size (1, Figure 1), which has initiated a cascade of ecological effects (2, 3).
Rodents and lagomorphs (hares, pikas, and rabbits) are the primary prey for many mammalian, avian, and reptilian predators and rodents can affect the composition of vegetative communities through seed predation (4). Small terrestrial mammals, including rodents and insectivores (shrews), typically comprise the largest and most diverse group of mammals in many ecosystems (e.g., 5); thus, most of the changes in mammal abundances and distributions resulting from climate change are expected to be in this group (e.g., 6).
Additionally rodents host many diseases that can affect us. Many tick- and flea-borne diseases that infect humans, including Lyme disease, Rocky Mountain spotted tick fever, Tularemia, plague, and Ehrlichiosis are carried by rodents or rodents serve as the reservoir hosts (7); the prevalence of these diseases is often strongly linked to population size (e.g., 8, 9). Thus, changes in mammal communities will have profound impacts on ecosystems and may directly affect human societies.
The effects of climate changes on mammals can sometimes be ascertained directly through the study of their biology and physiology. For example, once it is understood that wolverines require snow to den (10, 11) and persistent spring snow defines the southern extent of their range (11), the likely effects of reduced snowpack are straightforward (12). There is no evidence that wolverines will be able to persist in areas that lose their snow as a consequence of climate change. For most mammals, however, climate broadly defines their ecological niche. Therefore, for many species, future distributions are estimated by correlating climatic factors with their current ranges and projecting these models into the future (e.g. 13). Here we examine three species that illustrate some of the myriad of ways that climate is likely to affect mammals.
The Indiana bat (encounters with diseases)
The Indiana bat (Myotis sodalis) presents an example of how changing climates and introduction of exotic organisms can together threaten the persistence of native mammal species. The Indiana bat, (listed as federally endangered in 1967), ranges throughout the northeastern, midwestern, and parts of the southeastern U.S. During winter, Indiana bats hibernate in caves and mines within a narrow thermal range (3.0-7.2°C; 14).
The most important hibernation sites occur primarily in the Midwest and Southeast (15). Currently, most of these important sites are exterior to the area affected by white nose syndrome (WNS), a lethal disease caused by an imported European fungus (16).
Climate change predictions suggest the summer range of Indiana Bats may move north and east (17). This would move them closer to the areas most affected by WNS; the disease has resulted in the death of approximately 72% of the Indiana bats that hibernate in the Northeast since 2006-2007 (18).
The snowshoe hare (snow dependency)
The snowshoe hare (Lepus americanus) is a small lagomorph well-adapted to seasonally snowy environments. Its range includes most of Alaska and Canada, with montane extensions as far south as California and New Mexico in the western U.S. and West Virginia in the eastern U.S. It has oversized rear paws leading to low foot loadings (19, Figure 4.3), and seasonally changes its pelage from mostly brown to white.
Snowshoe hares are typically killed by predators (as opposed to dying of starvation, old age, etc.; 20). Thus, predator avoidance is a critical aspect of snowshoe-hare behavior (21). Hares may be particularly vulnerable when their coloration does not match the background-a white hare on a brown background is more visible to predators (22).
The period when the landscape is predictably snow covered is extremely sensitive to climate (23, 24). With climate change, spring snowmelt is predicted to occur earlier (23, 22) and snow cover lost sooner (25) in areas inhabited by the snowshoe hare.
Hare pelage change has limited plasticity in the rate of the spring white-to-brown molt, but both initiation dates of color change and the rate of the fall brown-to-white molt are fixed; unless the timing of coat color change can be modified by natural selection, the reduced snowpack will increase periods of mismatch by 3 - 8 times (22).
The American pika (sensitivity to temperature and water-balance stresses, perhaps mediated by vegetation)
The pika (Ochotona princeps) is a small lagomorph that often inhabits alpine areas in the western U. S. In the Great Basin, pika habitat occurs as small islands near the tops of mountains. Pikas appear dependent both on moist and cool summer conditions and winter snow (26).
Historically and during the 20th century, pikas have reacted to climate changes by moving upslope or becoming locally extirpated when the weather becomes hot and dry (26). In the Great Basin, sites of pika extirpation (1994-2008; 27) were generally hotter in the summer and more frequently very cold in the winter than sites where pika persisted - warming led to the removal of insulating snow causing nighttime winter temperatures to plummet (27). Winter snowpack not only insulates pikas during cold snaps, but also may provide water longer into the summer season, when plant senescence at drier sites occurs earlier in the year, which eliminates water availability for pika.
Analyses suggest that both chronic stresses (average temperature during all of summer, snowpack, and growing-season precipitation), acute (hot and cold) temperature stresses, and vegetation productivity may all be playing a role in pika declines in the Basin over the last decades.
Managing lands or species in the face of climate change requires an acknowledgement of both the range of different effects and the high levels of uncertainty involved in local projections. Additionally, projections are for climate, not weather. As such, they produce long-term averages. Organisms, including mammals, generally respond to weather events that deviate from the average such as droughts, extreme or unusual cold or heat, and storms. Thus, the weather that organisms actually respond to will be inferred from relatively broad-scale climate projections.
Additional uncertainties exist in predicting mammalian responses to changing climate. Unforeseen opportunities and stressors can be expected-white nose syndrome, for example, was discovered in 2006 (16). However, these uncertainties do not preclude active management to conserve mammals; they simply change the nature of that management. Many approaches are generically beneficial for native species, and are particularly beneficial given climate change. These include:
Maintain and if possible improve landscape connectivity.
Reduce stresses on current populations and habitats.
Maintain or improve current habitat for specific species.
Manage to maintain landscape diversity.
A connected landscape allows mammals to seek appropriate habitats and prevents the negative consequences of small isolated populations, such as increased extinction risks and lower fitness. Negative effects of climate change can be ameliorated by reducing other human-caused stressors (e.g. invasive species, development, overharvest). With higher fitness across habitats, organisms may be able to persist in what was formerly "sink" habitat*, both increasing overall population size and range and increasing adaptive potential.
Similarly, habitat improvement will help maintain a large, healthy population, which may improve its likelihood of persistence. Diverse landscapes increase overall resilience and provide opportunities for adaptation.
Lastly, because climate change will lead to many unexpected ecological effects, systems must be in place to rapidly identify and monitor these effects and facilitate appropriate management responses.
* Sink habitat =habitat that is currently occupied, but where populations cannot persist without external subsidies from organisms that emigrate from better quality areas.
McKelvey, K.S.; Perry, R.W.; Mills, L.S. 2013. The Effects of Climate Change on Mammals. U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. www.fs.usda.gov/ccrc/topics/wildlife/mammals