Climate Change

Climate Change

1. Climate Change as a Driver of Biodiversity Loss

Within the broader decline of biological diversity, climate change is a relatively “new” but rapidly intensifying factor. Unlike many local stressors (such as overfishing in one sea or agricultural intensification in one region), climate change:

• Acts globally and simultaneously in many ecosystems.
• Alters basic physical conditions (temperature, precipitation, wind patterns, sea level).
• Interacts strongly with other pressures (habitat destruction, pollution, invasive species).

For many species, the speed and magnitude of current climate change exceed the pace at which they can adapt or migrate, resulting in increased extinction risk and loss of genetic and ecosystem diversity.

2. Key Physical Changes Relevant to Life

Climate change causes several physical changes that strongly affect organisms and ecosystems. Here we focus on those with direct consequences for biodiversity.

2.1 Rising Temperatures

Global mean surface temperature has already increased by more than $1\,^\circ\text{C}$ relative to pre-industrial levels, with:

• Uneven warming: land areas and high latitudes warm faster than oceans and tropics.
• More heat extremes: heat waves become more frequent, longer, and more intense.

Biological consequences include:

• Exceeding tolerance limits: Species adapted to narrow temperature ranges (e.g., coral reef organisms, alpine plants) rapidly approach or surpass their thermal limits.
• Metabolic acceleration: In ectothermic animals (e.g., insects, reptiles), higher temperatures speed up metabolism, growth, and reproduction—up to a point—potentially shifting community structures.
• Altered sex ratios in species with temperature-dependent sex determination (e.g., some reptiles): warmer incubation can skew populations toward one sex.

2.2 Changes in Precipitation and Hydrological Regimes

Climate change modifies:

• Total rainfall: some regions become drier, others wetter.
• Seasonality: timing and intensity of rainy and dry seasons shift.
• Extremes: more frequent heavy rainfall events and droughts.

Biological effects:

• Drought stress in plants: reduced growth, increased susceptibility to pests and diseases, higher risk of fire.
• Habitat drying: temporary ponds and wetlands important for amphibian reproduction may disappear or become unreliable.
• Flood disturbances: increased flooding can uproot vegetation and alter river courses and sediment loads, reshaping freshwater habitats.

2.3 Oceans: Warming, Acidification, and Deoxygenation

The oceans absorb a large fraction of excess heat and a significant share of anthropogenic $\text{CO}_2$.

• Ocean warming:
• Drives coral bleaching: corals expel their symbiotic algae (zooxanthellae) under thermal stress, turning white and often dying.
• Triggers species range shifts in marine communities (e.g., fish moving poleward or into deeper, cooler waters).
• Ocean acidification:
• Dissolved $\text{CO}_2$ forms carbonic acid, lowering pH.
• Reduces the availability of carbonate ions required for building shells and skeletons (e.g., corals, mollusks, many planktonic organisms).
• Weakens calcifying organisms and can disrupt food webs from the base.
• Deoxygenation:
• Warmer water holds less dissolved oxygen.
• Stronger stratification (layering) reduces vertical mixing.
• Results in expanding oxygen-poor “dead zones”, threatening fish and invertebrates.

2.4 Sea-Level Rise

Thermal expansion of seawater and melting land ice cause global sea-level rise, which:

• Floods coastal habitats: salt marshes, mangroves, beaches, and coastal freshwater wetlands.
• Increases saltwater intrusion into coastal aquifers and into estuarine and delta habitats, altering salinity regimes.
• Erodes coasts: losing nesting sites (e.g., for sea turtles and seabirds).

Not all coastal ecosystems can migrate landwards due to human infrastructure (seawalls, cities), resulting in a “coastal squeeze” and loss of habitat.

3. Biological Responses to Climate Change

Organisms react to changing climate conditions through plasticity, range shifts, and evolutionary adaptation. The success and limits of these responses determine biodiversity outcomes.

3.1 Phenological Shifts (Changes in Seasonal Timing)

Phenology is the timing of recurring biological events (e.g., leaf-out, flowering, migration, reproduction). Climate change often advances spring events and prolongs growing seasons.

Observed trends:

• Earlier budburst and flowering in many temperate plants.
• Earlier insect emergence (e.g., pollinators, pest insects).
• Shifted migration and breeding times in birds, amphibians, and other animals.

Consequences for biodiversity:

• Mismatches between interacting species:
• Plants may flower earlier, but if their specialized pollinators do not shift at the same rate, pollination success declines.
• Birds timed to feed chicks when caterpillars are abundant may now breed too late, reducing chick survival.
• Asymmetric advantages: Generalist species that can use a wide range of resources may benefit, while specialists suffer—contributing to community simplification.

3.2 Range Shifts and Habitat Tracking

Many species attempt to stay within suitable climate conditions by moving:

• Poleward shifts: distribution areas move toward higher latitudes.
• Altitudinal shifts: species climb to higher elevations in mountains.

Effects:

• Invasion into new communities:
• Warm-adapted species can colonize previously cooler regions, sometimes outcompeting native cold-adapted species.
• Novel species interactions emerge (new predator–prey and host–parasite relationships).
• “Escalator to extinction” for alpine and polar organisms:
• Species already at the top of mountains or near the poles have nowhere colder to go.
• Their available habitat shrinks and can eventually disappear.

Human-made barriers (cities, roads, intensively used farmland) and fragmented landscapes often prevent effective range shifts, especially for less mobile species and plants.

3.3 Evolutionary Adaptation and Its Limits

Populations can respond to climate change through microevolution (changes in gene frequencies across generations):

• Shifts in tolerance ranges (e.g., higher heat tolerance).
• Changes in life-history traits (e.g., earlier reproduction, altered growth rates).
• Altered physiological processes (e.g., enzyme variants adapted to higher temperatures).

However, adaptation is constrained by:

• Rate of change: Current warming is extremely rapid on evolutionary timescales.
• Population size and genetic diversity: Small, already stressed populations have limited adaptive potential.
• Multiple, interacting stressors: Combined effects of climate change, pollution, habitat loss, and exploitation reduce survival and reproduction.

For many species, especially long-lived organisms (e.g., trees, large mammals), evolutionary adaptation is too slow to keep pace with rapid climate shifts.

4. Ecosystem-Level Impacts

Climate change does not just affect individual species; it alters entire ecosystems and their functioning.

4.1 Community Composition and Structure

When species differ in their sensitivity and response speed:

• Community reshuffling occurs:
• Some species decline or disappear locally.
• Others expand or newly colonize.
• Loss of keystone species or ecosystem engineers (e.g., corals, large trees, beavers) can trigger cascading effects, affecting many other organisms.

Examples of structural changes:

• Forest transitions:
• Drought and heat favor more drought-tolerant tree species or shrubs.
• Increased fires convert forests to savannas or grasslands in some regions.
• Coral reef degradation:
• After repeated bleaching events, structurally complex coral reefs can turn into algae-dominated, flatter systems with fewer niches.

4.2 Disturbance Regimes: Fires, Storms, and Extreme Events

Climate change often modifies the frequency, intensity, and spatial pattern of disturbances:

• More wildfires in many dry or seasonally dry ecosystems:
• Extended droughts dry out biomass and increase ignition risk.
• Fire-adapted species may temporarily benefit, but more intense or frequent fires can exceed their resilience, leading to soil degradation and loss of long-lived species.
• More intense storms and hurricanes:
• Damage forests, coral reefs, and coastal ecosystems.
• Repeated severe disturbances can prevent recovery and favor fast-growing opportunistic species.
• Heat waves and marine heat waves:
• Cause mass mortality in aquatic organisms, coral bleaching, and die-offs in terrestrial animals (e.g., bats, birds) when temperature exceeds lethal thresholds.

These disturbances can be natural parts of ecosystem dynamics, but climate change often pushes them beyond historically experienced levels, undermining ecosystem stability.

4.3 Ecosystem Services and Feedbacks

Biodiversity underpins ecosystem services (e.g., pollination, water purification, climate regulation). Climate-driven biodiversity loss affects:

• Pollination services: loss or decline of pollinators reduces crop yields and wild plant reproduction.
• Carbon storage:
• Forest dieback, peatland drying, and permafrost thawing release $\text{CO}_2$ and methane ($\text{CH}_4$), reinforcing climate warming (positive feedback).
• Degradation of marine ecosystems (e.g., seagrass beds, mangroves) reduces coastal carbon sinks (“blue carbon”).
• Protection from hazards:
• Loss of mangroves, coral reefs, and coastal wetlands reduces natural protection against storms and erosion.

Such feedbacks create a vicious circle: climate change reduces biodiversity, weakened ecosystems store and buffer less carbon, and further climate change ensues.

5. Vulnerable Systems and Species

Some ecosystems and types of organisms are particularly sensitive to climate change as a driver of biodiversity decline.

5.1 Polar and High-Mountain Ecosystems

• Arctic and Antarctic:
• Rapid warming leads to loss of sea ice, changing habitat availability for ice-dependent species (e.g., polar bears, seals, some penguins).
• Altered ocean circulation and productivity affect entire food webs.
• Alpine zones:
• Alpine plants and animals are squeezed into ever-smaller high-elevation refuges.
• Upslope shifts of lower-elevation species bring new competitors and predators.

5.2 Coral Reefs and Tropical Marine Systems

• Coral bleaching from thermal stress and acidification is a leading climate-related cause of mass biodiversity loss in the sea.
• Reef loss reduces habitat for countless fish and invertebrate species, undermines local fisheries, and removes coastal protection.

5.3 Mediterranean-Type and Dryland Ecosystems

Regions with seasonal drought and high climate variability are sensitive to incremental warming and rainfall shifts:

• Increased drought length and severity promote desertification.
• Plant communities change toward more drought-tolerant and often less diverse assemblages.
• Wildlife dependent on seasonal water points and ephemeral vegetation is particularly at risk.

5.4 Freshwater Systems

Rivers, lakes, and wetlands respond quickly to climate-related changes:

• Altered flow regimes (e.g., earlier snowmelt peaks, longer low-flow periods).
• Warming reduces oxygen and changes species composition in lakes.
• Shrinking wetlands reduce habitat for amphibians, migratory birds, and numerous invertebrates.

Because freshwater habitats are already heavily pressured by extraction, pollution, and damming, climate change adds a strong additional stressor.

6. Climate Change and Other Threats to Biodiversity

Climate change rarely acts alone. It interacts with other factors causing biodiversity decline.

6.1 Synergies with Habitat Destruction and Fragmentation

• Fragmented landscapes obstruct species’ ability to track shifting climates.
• Small, isolated populations are more vulnerable to extreme climate events and have lower adaptive capacity.
• Conversion of natural habitats to agriculture, infrastructure, or urban areas removes potential climate refugia (areas that remain relatively stable despite regional climate change).

6.2 Interaction with Overexploitation, Pollution, and Invasive Species

• Overfished or overhunted populations have reduced resilience to climate stress.
• Pollutants (e.g., pesticides, heavy metals, plastics) and nutrient enrichment (eutrophication) weaken organisms and ecosystems, lowering resistance and recovery after climate-related disturbances like heat waves.
• Invasive species often benefit from disturbed and warming environments, while native species may lose ground, increasing homogenization of biotas.

These combined pressures accelerate biodiversity loss beyond what climate change would cause alone.

7. Strategies to Limit Climate-Driven Biodiversity Loss

Measures to protect biodiversity in a changing climate involve both mitigating climate change itself and adapting conservation to new conditions.

7.1 Climate Mitigation: Reducing the Root Cause

Because climate change is a global driver, protecting biodiversity increasingly depends on:

• Reducing greenhouse gas emissions from energy, transport, agriculture, and industry.
• Protecting and restoring natural carbon sinks:
• Forests, peatlands, wetlands, mangroves, seagrass meadows, and healthy soils.
• Avoiding maladaptive measures:
• For instance, large-scale bioenergy plantations that destroy high-biodiversity habitats.

These actions belong to broader climate policy but are crucial context for any biodiversity conservation strategy.

7.2 Climate-Smart Conservation and Protected Areas

Conservation planning must account for shifting climates:

• Expanding and connecting protected areas:
• Creating ecological corridors and networks to enable species migrations (latitudinal and altitudinal).
• Identifying and safeguarding climate refugia:
• Places with relatively stable microclimates (e.g., north-facing slopes, deep ravines, groundwater-fed wetlands) that can harbor species under future climate scenarios.
• Dynamic management:
• Allowing protected area boundaries or management objectives to adapt as species distributions and community compositions change.

7.3 Supporting Ecosystem Resilience

Resilient ecosystems are better able to absorb shocks and reorganize without losing core functions:

• Maintaining or restoring structural complexity:
• Mixed, multi-age forests instead of monocultures.
• Diverse river habitats (side channels, floodplains) instead of heavily straightened courses.
• Reducing non-climatic stressors:
• Lowering pollution, overuse, and habitat destruction improves the capacity to cope with climate extremes.
• Assisted migration and genetic management (case-dependent):
• In some situations, humans may actively move species or genotypes to suitable climates or manage genetic diversity.
• These interventions are controversial and require careful risk–benefit assessment.

7.4 Monitoring and Research

To respond effectively, we need:

• Long-term monitoring of climate, species distributions, and ecosystem states.
• Modeling of potential future ranges and community shifts.
• Experimental studies on tolerance limits and adaptive capacity of key species.

Such information helps anticipate and prevent biodiversity losses rather than reacting after critical thresholds have been crossed.

8. Climate Change, Biodiversity, and Human Societies

Declining biodiversity due to climate change is not only a biological issue; it has direct consequences for humans:

• Food security (through impacts on wild fisheries, pollination, and crop resilience).
• Water quality and availability.
• Protection against natural hazards (floods, erosion, storms).
• Cultural values and identities tied to specific landscapes and species.

At the same time, intact, diverse ecosystems play a central role in mitigating climate change and buffering its impacts. Conserving biodiversity and stabilizing climate are therefore mutually reinforcing goals, and effective environmental and nature protection requires addressing both together.