Climate change presents a fundamental threat to the stability of global biodiversity, altering habitats faster than many life forms can adapt. There is no single “most affected” animal species, but rather a wide spectrum of vulnerability determined by where an animal lives and its inherent biology. The animals at greatest risk reside in rapidly changing environments while possessing biological characteristics that limit their ability to cope with new conditions. Understanding this complexity requires examining the scientific framework used to evaluate species at risk.
Defining Vulnerability to Climate Change
Scientists assess a species’ risk by evaluating three distinct components that combine to determine its overall vulnerability. The first component is Exposure, which quantifies the degree of climate change a species is likely to experience within its geographic range, such as temperature increase or sea-level rise. A species living in an environment projected to warm dramatically has a high exposure score.
The second factor is Sensitivity, which measures how much a species is negatively affected by a given climate change impact due to its unique physiological or ecological requirements. For example, a species dependent on a single food source would be highly sensitive to changes affecting that source. Sensitivity scores are based on intrinsic characteristics, like narrow thermal tolerances or specialized habitat needs.
The final component is Adaptive Capacity, which represents the species’ ability to cope with or adjust to climate change through changes in behavior, movement, or evolution. Species with high genetic diversity, short generation times, or the ability to migrate long distances have greater adaptive capacity. Overall vulnerability is a function of high exposure and high sensitivity paired with low adaptive capacity.
Ecosystems Under Extreme Stress
Vulnerability is acutely concentrated in geographic areas where the physical effects of climate change are amplified, leading to accelerated impacts on local wildlife. The Polar Regions, particularly the Arctic, are warming up to four times faster than the rest of the planet, making them zones of extreme exposure. This rapid temperature increase immediately threatens animals reliant on sea ice, which serves as a hunting platform, breeding ground, and travel corridor.
Tropical Coral Reefs face a dual threat: ocean warming and ocean acidification. Elevated sea temperatures cause thermal stress, leading corals to expel symbiotic algae, a phenomenon known as bleaching. Ocean acidification, caused by the ocean absorbing excess atmospheric carbon dioxide, reduces the availability of carbonate ions. This makes it difficult for corals to build and maintain their calcium carbonate skeletons.
Species living in High-Altitude Mountain Regions are experiencing “up-slope migration” or “escalator to extinction.” As temperatures rise, these animals are forced to move to higher, cooler elevations. However, the available habitat area often decreases rapidly as they approach mountain peaks. Models suggest that a \(2.2^{\circ} \text{C}\) warming could lead to a \(55\%\) loss of the upper alpine zone globally, pushing specialized species off the top of their mountains.
Biological Traits of High-Risk Species
Beyond geography, specific biological traits make certain species inherently more sensitive to environmental shifts. Ectotherms, or cold-blooded animals, are particularly vulnerable because their body temperature is regulated externally, making their metabolism and survival directly dependent on ambient conditions. A small, sustained temperature increase can push these species beyond their narrow physiological tolerance limits, leading to reproductive failure or death.
Species that exhibit high specialization are also at high risk because their survival hinges on a specific resource or habitat that may be easily disrupted. If an animal relies entirely on one type of plant for food or a single microclimate for shelter, it has little flexibility when conditions change. Similarly, species with low mobility or poor dispersal ability, such as flightless insects or slow-moving reptiles, cannot quickly shift their geographic range to track suitable climate conditions.
A unique vulnerability is seen in reptiles that use Temperature-Dependent Sex Determination (TDSD), including many sea turtles and crocodiles. In these species, the incubation temperature of the eggs determines the sex of the hatchlings. A slight increase in average nest temperature, sometimes less than \(2^{\circ} \text{C}\), can drastically skew the sex ratio of entire populations, potentially leading to the near-exclusive production of one sex. This imbalance severely limits reproductive potential and threatens the long-term viability of the population.
Case Studies of Highly Impacted Groups
Amphibians are widely recognized as one of the most threatened vertebrate classes due to their unique physiological profile. Their thin, highly permeable skin, which aids in respiration, makes them extremely susceptible to changes in moisture and temperature, leading to rapid dehydration during droughts and heat waves. Warmer, more fluctuating temperatures can also increase their susceptibility to emerging infectious diseases, such as the deadly chytrid fungus.
Pinnipeds and Polar Mammals, exemplified by the polar bear, are tightly linked to the fate of the rapidly melting Arctic sea ice. These animals primarily use the ice as a platform from which to hunt their preferred prey, like seals. Longer ice-free periods force polar bears to fast for extended durations or spend more time on land, leading to reduced body weight and lower cub survival rates.
In the marine food web, Pteropods, or “sea butterflies,” illustrate a foundational impact of ocean acidification. These tiny, free-swimming snails are a foundational food source for organisms ranging from small fish to whales. Pteropods construct their delicate shells from aragonite, a form of calcium carbonate that is highly soluble in increasingly acidic water. Exposure to corrosive conditions, already documented in the Gulf of Alaska, causes their shells to dissolve, compromising an entire marine food web.
Vulnerability to a changing climate is therefore a complex intersection of geography and biology. The animals most affected are those trapped in habitats experiencing rapid, extreme change, while lacking the biological capacity to adapt or move. These high-risk groups highlight the threat climate change poses to the interconnected health of entire ecosystems.