Climate Impacts on Ecosystems
Climate Impacts on Ecosystems
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- Climate change can alter where species live and how they interact, which could fundamentally transform current ecosystems.
- Impacts on one species can ripple through the food web and affect many organisms in an ecosystem.
- Mountain and arctic ecosystems and species are particularly sensitive to climate change.
- Projected warming could greatly increase the rate of species extinctions, especially in sensitive regions.
- IPCC Working Group II
- NRC Ecological Impacts of Climate Change
- USGCRP Synthesis Assessment Product 4.3: The effects of climate change on agriculture, land resources, water resources, and biodiversity in the United States
- UNEP Review of the Literature on the Links between Biodiversity and Climate Change
- U.S. Fish and Wildlife Service: Landscape Conservation Cooperatives
An ecosystem refers to the animals, plants, and microorganisms that live in one place, as well as the environmental conditions that support them.
Ecosystem services include the products and services provided by ecosystems, such as food, fuel, timber, water, clean air, and medicines. It also includes less material benefits, such as regulation of local climate conditions and aesthetic value or cultural identity. 
An ecological threshold is the point at which there is an abrupt change in an ecosystem quality, property, or phenomenon, or where small changes in one or more external conditions produce large and persistent responses in an ecosystem. 
A food web is made up of predators and prey that interact in a habitat or ecosystem.
A stressor is a factor that reduces the health or productivity of an ecosystem (i.e., causes stress).
The American pika is a small, hamster-like animal that is a relative of the rabbit. The species is found in the western United States in cold areas near mountaintops. The warming climate is causing pika populations to die off at elevations below 7,000 feet. Of 25 pika populations studied in the Great Basin between the Rocky Mountains and the Sierra Nevada, more than one third has disappeared in the past few decades. 
Climate change may be the leading factor decreasing the populations of the American pika (Ochotona princeps). Source: National Parks Service (2012)
Penguins and Climate Change: A Case of "Winners" and "Losers"
Even within a single ecosystem, there can be winners and losers from climate change. The Adélie and Chinstrap penguins in Antarctica provide a good example. Over the past 25 years, the population of Adélie penguins decreased by 22%, while the population of Chinstrap penguin increased by an estimated 400%. The two species depend on different habitats for survival: Adélies inhabit the winter ice pack, whereas Chinstraps remain in open water. During the past 50 years, a 7-9°F increase in midwinter temperatures on the western Antarctic Peninsula has led to a loss of sea ice and a shrinking habitat for Adélie penguins. 
Climate is an important environmental influence on ecosystems. Climate changes and the impacts of climate change affect ecosystems in a variety of ways. For instance, warming could force species to migrate to higher latitudes or higher elevations where temperatures are more conducive to their survival. Similarly, as sea level rises, saltwater intrusion into a freshwater system may force some key species to relocate or die, thus removing predators or prey that were critical in the existing food chain.
Climate change not only affects ecosystems and species directly, it also interacts with other human stressors such as development. Although some stressors cause only minor impacts when acting alone, their cumulative impact may lead to dramatic ecological changes.  For instance, climate change may exacerbate the stress that land development places on fragile coastal areas. Additionally, recently logged forested areas may become vulnerable to erosion if climate change leads to increases in heavy rain storms.
Changes in the Timing of Seasonal Life-Cycle Events
For many species, the climate where they live or spend part of the year influences key stages of their annual life cycle, such as migration, blooming, and mating. As the climate has warmed in recent decades, the timing of these events has changed in some parts of the country. Some examples are:
- Warmer springs have led to earlier nesting for 28 migratory bird species on the East Coast of the United States. 
- Northeastern birds that winter in the southern United States are returning north in the spring 13 days earlier than they did in the early 20th century. 
- In a California study, 16 out of 23 butterfly species shifted their migration timing and arrived earlier. 
Changes like these can lead to mismatches in the timing of migration, breeding, and food availability. Growth and survival are reduced when migrants arrive at a location before or after food sources are present. 
As temperatures increase, the habitat ranges of many North American species are moving northward in latitude and upward in elevation. While this means a range expansion for some species, for others it means a range reduction or a movement into less hospitable habitat or increased competition. Some species have nowhere to go because they are already at the northern or upper limit of their habitat.
For example, boreal forests are invading tundra, reducing habitat for the many unique species that depend on the tundra ecosystem, such as caribou, arctic fox, and snowy owl. Other observed changes in the United States include expanding oak-hickory forests, contracting maple-beech forests, and disappearing spruce-fir forests. As rivers and streams warm, warmwater fish are expanding into areas previously inhabited by coldwater species.  Coldwater fish, including many highly valued trout species, are losing their habitats. As waters warm, the area of feasible, cooler habitats to which species can migrate is reduced.  Range shifts disturb the current state of the ecosystem and can limit opportunities for fishing and hunting.
See the Agriculture and Food Supply Impacts page for information about how habitats of marine species have shifted northward as waters have warmed.
Food Web Disruptions
The impact of climate change on a particular species can ripple through a food web and affect a wide range of other organisms. For example, the figure shows the complex nature of the food web for polar bears. Declines in the duration and extent of sea ice in the Arctic leads to declines in the abundance of ice algae, which thrive in nutrient-rich pockets in the ice. These algae are eaten by zooplankton, which are in turn eaten by Arctic cod, an important food source for many marine mammals, including seals. Seals are eaten by polar bears. Hence, declines in ice algae can contribute to declines in polar bear populations.   
In some cases, ecosystem change occurs rapidly and irreversibly because a threshold, or "tipping point," is passed.
One area of concern for thresholds is the Prairie Pothole Region in the north-central part of the United States. This ecosystem is a vast area of small, shallow lakes, known as "prairie potholes" or "playa lakes." These wetlands provide essential breeding habitat for most North American waterfowl species. The pothole region has experienced temporary droughts in the past. However, a permanently warmer, drier future may lead to a threshold change - a dramatic drop in the prairie potholes that host waterfowl populations and provide highly valued hunting and wildlife viewing opportunities. 
Similarly, when coral reefs become stressed, they expel microorganisms that live within their tissues and are essential to their health. This is known as coral bleaching. As ocean temperatures warm and the acidity of the ocean increases, bleaching and coral die-offs are likely to become more frequent. Chronically stressed coral reefs are less likely to recover.
Pathogens, Parasites, and Disease
Climate change and shifts in ecological conditions could support the spread of pathogens, parasites, and diseases, with potentially serious effects on human health, agriculture, and fisheries. For example, the oyster parasite, Perkinsus marinus, is capable of causing large oyster die-offs. This parasite has extended its range northward from Chesapeake Bay to Maine, a 310-mile expansion tied to above-average winter temperatures.  For more information about climate change impacts on agriculture, visit the Agriculture and Food Supply Impacts page. To learn more about climate change impacts on human health, visit the Health Impacts page.
Climate change, along with habitat destruction and pollution, is one of the important stressors that can contribute to species extinction. The IPCC estimates that 20-30% of the plant and animal species evaluated so far in climate change studies are at risk of extinction if temperatures reach levels projected to occur by the end of this century.  Projected rates of species extinctions are 10 times greater than recently observed global average rates and 10,000 times greater than rates observed in the distant past (as recorded in fossils).  Examples of species that are particularly climate sensitive and could be at risk of significant losses include animals that are adapted to mountain environments, such as the pika, animals that are dependent on sea ice habitats, such as ringed seals, and cold-water fish, such as salmon in the Pacific Northwest. 
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