Living Within Limits
Every species lives within a range of conditions it can tolerate: temperature, moisture, salinity, acidity, season length. Ecosystems are collections of species whose ranges overlap and whose lives are linked through eating, competing and cooperating. When the climate shifts, the conditions shift under all of them at once, but each species responds differently. That uneven response is the main reason climate change disrupts ecosystems and not merely moves them.
The average global temperature has risen by roughly 1 degree Celsius or a little more since the late 1800s, and the changes in rainfall, snow cover and ocean chemistry that accompany it are already affecting living systems. Some effects are well documented; others are projections that carry uncertainty.
Species on the Move
One widely observed response is movement. Many plants, insects, birds, fish and other animals have shifted their ranges toward the poles or to higher elevations, tracking the climate conditions they prefer. Studies of marine species often find shifts faster than those on land, partly because ocean animals can swim to cooler water more easily than a plant can spread seeds uphill.
Movement is not simple. Species can only move if there is suitable habitat to go to, and cities, farms and roads can block the way. Species on mountaintops, on islands or at the edge of a continent have nowhere cooler to go. Different species also move at different speeds, so communities may be reshuffled into combinations that have never existed before. New neighbors compete or prey on each other, and diseases and pests can spread into new areas.
Timing Mismatches
Many organisms rely on seasonal cues, such as warmth, day length or rainfall, to time events like flowering, migration, hatching and breeding. Because temperature and daylight do not change together, climate change can pull these cues apart. Spring may arrive earlier, for example, while a migrating bird, which responds to day length at its wintering site, arrives on its usual schedule. If the insects the birds feed to their chicks now peak earlier, the chicks may hatch after the peak food supply has passed. Researchers call this a phenological mismatch, and documented cases exist in several systems. Not all species are affected equally, and some adjust well.
Oceans: Warming and Acidification
Oceans absorb most of the extra heat trapped by greenhouse gases and a substantial share of the carbon dioxide released by human activity. This has two big consequences.
Warming water. Marine heatwaves can stress or kill organisms. Because of the properties of water and dissolved gases, warmer water also holds less oxygen, and stronger layering of the water can limit the mixing that brings nutrients to the surface. Changes to circulation, including ocean currents, can alter where fish and plankton thrive.
Acidification. When carbon dioxide dissolves in seawater it forms carbonic acid, lowering the pH and reducing the availability of carbonate ions that many animals use to build shells and skeletons. Oysters, mussels, some plankton and corals are among the groups that may be affected, with larvae often more sensitive than adults. The ocean is not becoming acidic in the everyday sense; it is becoming somewhat less alkaline, but the change is large relative to what marine life has experienced over recent geologic history.
Coral reefs. Corals live in partnership with algae that give them food and color. When water becomes too warm, corals expel the algae, a process called bleaching. Corals can recover if conditions improve, but repeated or prolonged events cause widespread death. Reefs shelter a large number of species relative to the area they cover, which is why their decline attracts so much concern.
Rainfall, Drought and Fire
Shifting rainfall patterns change freshwater and land ecosystems. Wetter conditions in some places and longer dry periods in others affect lakes, rivers, wetlands and forests. Drought can weaken trees and make them more vulnerable to insects and disease. Warm, dry conditions also increase fire risk in many regions. Some ecosystems depend on periodic fire, but changes in frequency or intensity can overwhelm their ability to recover and in some cases convert forests to shrubland or grassland.
In polar and mountain regions, glacier and ice loss alters habitat directly. Animals that depend on sea ice for hunting or breeding face shrinking platforms, and glacier-fed streams may change in temperature and flow.
Why Food Webs Can Unravel
Because species are linked, damage can spread. If a small, abundant prey species such as krill, certain plankton or a key insect declines, the predators that feed on it may follow. If a predator that controls a grazer disappears, the grazer may overeat its food plants. If a pollinator is mismatched with its flowers, plant reproduction may drop, affecting seed-eating animals.
Ecologists talk about trophic cascades for this kind of chain reaction, and about tipping points where a system shifts rapidly into a different state. Predicting exactly when and where those occur is difficult. Greater species diversity can provide a cushion, since multiple species may perform similar roles, but sustained stress can still erode that cushion. The same principles apply to any ecosystems and environment, from ponds to oceans.
Not Only Losers
Some species may benefit, at least temporarily, from warmer conditions or longer growing seasons, and invasive species, pests and disease carriers may be among them. The overall picture is of reorganization with many losses, some gains and a lot of uncertainty about the net result. Climate change also combines with other pressures such as habitat loss, pollution, overharvesting and invasive species, and it is often hard to separate one cause from another.
What Can Be Done
Responses include cutting emissions to slow the pace of change, protecting large, connected areas so that species can move, restoring habitats, reducing other pressures such as pollution and overfishing, and monitoring changes so that managers can respond. Some conservation programs consider assisted movement of species, which is controversial because it carries its own risks. The more quickly the climate changes, the harder it is for natural systems to keep up, and that is why the pace of change is a central factor in how serious ecosystem disruption becomes.