2.4 Impacts Of Climate Change On Ecosystems

Changing Conditions For Life On Earth

Climate change alters the basic environmental conditions that ecosystems depend on. Temperature, rainfall, wind patterns, and the chemistry of air and water are all shifting. Ecosystems are not static, but the current speed and scale of change are faster than many species and habitats can adapt to. This leads to visible and less visible impacts, from shifting seasons to ecosystem collapse.

In this chapter the focus is on how climate change affects ecosystems themselves, not on the causes of climate change or the detailed human consequences, which are covered elsewhere in the course.

Species Ranges On The Move

Many plants and animals live within a certain climate envelope, defined by temperature, moisture, and other conditions they can tolerate. As the planet warms, this envelope shifts. Species respond by trying to track suitable climate zones. They may move uphill to higher, cooler altitudes, toward the poles to cooler latitudes, or into deeper, colder waters in the ocean.

In mountain regions, plants and insects are already observed at higher elevations than in the past. Birds, butterflies, and forest trees in temperate zones are extending their ranges northward in the Northern Hemisphere and southward in the Southern Hemisphere. Marine species, such as fish and plankton, are moving toward cooler waters, which can profoundly change local food webs.

Not all species can move. Some plants depend on specific soil types, mutualistic fungi, or pollinators that are not shifting in the same way. Freshwater species in isolated lakes and streams, and plants confined to mountaintops or small islands, can find themselves trapped. When suitable conditions disappear from the limited area they occupy, these species face a high risk of local or total extinction.

Changes In Timing And Seasonal Rhythms

Ecosystems are tuned to the seasons. Many species rely on recurring patterns of temperature and daylight, known as phenology, to trigger flowering, breeding, migration, and hibernation. Climate change disrupts these rhythms, because temperatures are changing faster than the length of day.

Spring is arriving earlier in many regions, and autumn can be delayed. Plants may flower and leaf out earlier in the year, insects may emerge sooner, and some bird species are migrating or breeding earlier. However, different species respond at different speeds and in different ways. This can break the synchrony that many ecological interactions depend on.

A classic example is the relationship between plants, insect herbivores, and insect eating birds. If plants flower early and insect populations peak sooner, but birds do not adjust their breeding period enough, chicks can hatch when insect food is already past its peak. Similarly, pollinators may emerge when few flowers are available, or flowers may bloom when their specialized pollinator insects are not yet active.

This phenomenon, known as phenological mismatch, can reduce survival and reproductive success. Over time it can weaken populations, simplify food webs, and reduce the stability of entire ecosystems.

Stress On Terrestrial Ecosystems

On land, climate change alters water availability, temperature extremes, and the frequency and intensity of disturbances such as drought, heat waves, storms, and wildfires. Ecosystems that evolved under relatively stable climates must now cope with more frequent and severe stresses.

Forest ecosystems are particularly affected. Warmer temperatures combined with drought stress trees and make them more vulnerable to pests and diseases. Bark beetle outbreaks in some conifer forests, for instance, are amplified by mild winters that no longer kill the insects efficiently. This can lead to large areas of tree mortality and transform the structure of forests.

In drylands and grasslands, climate change influences the balance between grasses, shrubs, and trees. Prolonged droughts and heat stress can cause vegetation dieback, soil degradation, and loss of ground cover. Once vegetation is lost, soils become more vulnerable to erosion by wind and water, which further reduces the capacity of these ecosystems to support diverse life.

Tundra and boreal ecosystems, located in high latitudes, are warming rapidly. Permafrost soils are thawing, which destabilizes the ground, changes hydrology, and can release previously frozen organic matter. Vegetation in these regions is shifting toward more shrubs and sometimes trees, altering habitat for specialized cold adapted species like certain lichens, mosses, and migratory animals that depend on open tundra.

Ocean Warming, Acidification, And Marine Life

Marine ecosystems are strongly influenced by changes in ocean temperature and chemistry. The ocean absorbs most of the excess heat from global warming, which raises sea surface temperatures and can alter currents and stratification, the layering of water with different temperatures and densities.

Warmer waters can stress marine organisms, particularly those that are already near their upper temperature limits. Coral reefs are especially sensitive. Corals form a close relationship with microscopic algae that live in their tissues and provide much of their energy. When water becomes too warm or experiences rapid temperature swings, corals can expel these algae in a process known as coral bleaching. Bleached corals appear white and, if stressful conditions persist, large areas of reefs can die.

At the same time, the ocean absorbs a significant fraction of human emitted carbon dioxide, which reacts with seawater and makes it more acidic. This process, ocean acidification, lowers the availability of carbonate ions that many marine organisms need to build shells and skeletons from calcium carbonate. Shell forming plankton, corals, mollusks such as oysters and mussels, and some algae are especially affected.

Weaker shells and skeletons, combined with warming and deoxygenation in some regions, disrupt marine food webs. Since small plankton and shellfish are often at the base of the food chain, these changes can propagate upward, affecting fish populations, marine mammals, and seabirds. Fisheries that depend on stable marine ecosystems can therefore be indirectly affected by these ecological shifts.

Freshwater Systems Under Pressure

Rivers, lakes, wetlands, and groundwater dependent ecosystems are highly sensitive to changes in precipitation, snow and ice melt, and temperature. Climate change modifies both the amount and timing of water flow, as well as water quality.

In mountain regions, glaciers and snowpack act as natural water storage. Warming temperatures cause glaciers to shrink and cause snow to melt earlier in the year. Initially this can increase river flows in spring and early summer, but over time, as glaciers lose mass, dry season flows can decrease. Aquatic species that depend on cool, oxygen rich waters may become stressed as streams warm and flows become more variable.

In many regions, more intense rainfall events combined with longer dry spells can increase flooding and drought. Flood pulses are important for some floodplain ecosystems, but extreme floods can erode riverbanks, wash away habitats, and carry pollutants into waterways. Droughts, on the other hand, can shrink lakes and wetlands, concentrating pollutants and harming fish, amphibians, and water birds.

Higher water temperatures reduce dissolved oxygen levels, which can lead to fish kills and favor species that tolerate low oxygen conditions. Harmful algal blooms can become more frequent when warm temperatures and nutrient rich runoff coincide, especially in lakes and slow moving waters. These blooms can produce toxins and alter the composition of aquatic communities.

Wetlands, which are crucial habitats for many species and also store carbon, can dry out or convert to different habitat types if changes in rainfall and evaporation are strong and persistent. This leads to a loss of specialized wetland species and can release previously stored carbon from soils.

Polar Regions And Cryosphere Ecosystems

Polar regions and cryosphere associated ecosystems, which include glaciers, sea ice, snow cover, and permafrost, are experiencing climate change more intensely than many other parts of the world. These regions are warming at a rate faster than the global average, and the ecological consequences are profound.

Sea ice in the Arctic is shrinking in extent and becoming thinner. Many species, such as polar bears, seals, and walruses, use sea ice as a platform for hunting, breeding, or resting. As ice becomes less reliable, their access to food and safe breeding grounds decreases. Some species try to adjust their behavior, but the overall carrying capacity of the Arctic marine ecosystem can decline as the ice associated food web is disrupted.

On land, warmer temperatures and changing snow conditions influence the timing and availability of food for herbivores like caribou and reindeer. If snow becomes crusted due to freeze thaw cycles, or if rain falls on snow and then freezes, these animals may struggle to access the vegetation beneath. Predators and scavengers are also affected indirectly by changes in the abundance and health of their prey.

Glacier retreat reshapes mountain ecosystems as new terrain becomes ice free. Pioneer plants and microbes colonize these areas, followed by more complex communities. However, species that are highly adapted to cold glacial conditions lose habitat. Stream ecosystems that depend on steady glacier melt may experience more extreme flows and warmer temperatures, making them less suitable for cold adapted aquatic life.

Extreme Events And Disturbance Regimes

Ecosystems naturally experience disturbances such as fires, storms, and insect outbreaks. Many have evolved to rely on a certain pattern of disturbance for renewal and diversity. Climate change, however, can alter the frequency, intensity, and spatial extent of these events, creating new disturbance regimes that surpass what ecosystems have experienced in the recent past.

Heat waves combined with droughts increase the likelihood of large, intense wildfires in forests, shrublands, and grasslands. Some fire adapted ecosystems can recover from occasional fires, but more frequent or severe fires can kill seed sources, alter soil properties, and favor invasive or fire tolerant species at the expense of native ones. Over time, this can transform forests into more open woodlands or grasslands, with different species assemblages and carbon storage capacities.

Tropical cyclones, storms, and heavy rainfall can damage coastal and inland ecosystems. Mangroves, coral reefs, and coastal marshes can buffer coasts from storm impacts, but stronger storms and sea level rise can exceed their protective capacity and damage them directly. Inland, intense storms can knock down trees, fragment forests, and reshape habitats. While some species benefit from the new gaps and light conditions, others decline due to habitat loss and fragmentation.

When multiple stressors combine, for example drought, heat, and pests in forests, or warming, acidification, and overfishing in marine systems, ecosystems may reach tipping points. Beyond such a threshold, they can shift abruptly from one state to another, for example from a clear water lake with abundant plants to a turbid, algae dominated lake, or from a diverse coral reef to an algae covered rubble field. Such shifts can be hard or impossible to reverse within human time scales.

Biodiversity Loss And Extinction Risks

As climate change alters habitats, food webs, and disturbance regimes, it contributes to a global loss of biodiversity. Species may decline in abundance, disappear from parts of their range, or go extinct entirely. While exact numbers are uncertain, the combination of climate change with habitat destruction, pollution, overexploitation, and invasive species creates strong pressures on many forms of life.

Some species have traits that help them adapt, such as broad tolerance to environmental conditions, rapid reproduction, or the ability to move long distances. Others are highly specialized, slow reproducing, or restricted to narrow habitat types. These more vulnerable species are at higher risk. Mountain endemics, island species, and polar organisms are prominent examples of groups with elevated climate related risks.

Loss of species is not only about rare or charismatic animals. Many small, inconspicuous species such as soil microbes, insects, and fungi play crucial roles in nutrient cycling, pollination, decomposition, and soil formation. Their decline can weaken ecosystem functions even when larger species still appear present. The overall result is a reduction in the resilience and productivity of ecosystems.

Ecosystem Services At Risk

Ecosystems provide services that support human societies, such as clean water, fertile soils, pollination of crops, natural pest control, climate regulation, and cultural values. When ecosystems are degraded by climate change, these services are altered or reduced.

Forests that are stressed or burned more frequently may store less carbon and regulate water cycles less effectively. Pollinator declines linked to climate driven mismatches and habitat changes can reduce crop yields and affect wild plant reproduction. Coral reef degradation reduces coastal protection and fish habitat, affecting food security and livelihoods in many tropical regions.

Freshwater ecosystems that experience frequent algal blooms or low oxygen events become less suitable for drinking water supplies, recreation, and fisheries. Wetlands that dry out store less carbon and support fewer water bird and fish species. Mountain ecosystems that lose glacier and snow based water storage provide less reliable water to downstream areas.

In many cases, climate change interacts with unsustainable land use, pollution, and resource extraction to amplify these impacts. The resulting changes in ecosystem services feed back into social and economic systems, creating additional challenges that are examined in more detail in the chapter on human impacts.

Adaptation, Limits, And The Need For Protection

Ecosystems are not entirely passive. Species can adjust behavior, distribution, and in some cases even evolve in response to new conditions. Ecosystems can reorganize around different sets of species and still provide some functions. However, there are limits to adaptation when changes are faster or more extreme than what organisms have experienced historically.

Conservation and restoration efforts can help ecosystems cope by reducing non climatic stresses, protecting climate refugia where conditions remain relatively stable, and maintaining or re establishing connectivity so species can move. For example, linking habitat patches through ecological corridors can enable species to track shifting climate zones, and protecting intact forests can preserve microclimates that buffer species from extreme temperatures.

Yet, there are thresholds beyond which even well managed ecosystems may not retain their current character. Coral reefs are an example of ecosystems with relatively narrow climatic tolerances. Even with reduced local pollution and overfishing, frequent and severe marine heat waves can cause widespread bleaching and mortality. Similarly, some cold adapted species may simply run out of suitable habitat as conditions warm.

These realities underline how essential it is to limit the magnitude of climate change in order to keep ecosystems within ranges they can adapt to. Efforts to protect and restore ecosystems also support climate mitigation, because healthy ecosystems, especially forests, wetlands, and oceans, play crucial roles in absorbing and storing carbon. The close connection between ecosystem health and climate stability is a core theme that runs throughout this course and links ecological impacts with broader discussions of sustainability and climate action.