The Impact of Climate Change on Biodiversity

Climate change is affecting biodiversity at all its levels and at different spatial and temporal scales. It has a potential impact not only on the individual and species level but also on the ecosystem level affecting ecosystem functionality and services. For a systematic classification, see Geyer et al.¹¹⁶¹

Impact on Individuals, Populations, and Habitats

Probably the most studied impacts of climate change can be found on the individual and population levels of animals and plants. These include changes in their behavior and physiology comprising their metabolism, growth rates, life cycling, immune functions, and also their population dynamics such as changes in gene pools, dispersal abilities, population growth rates, or sex ratios.¹¹⁷¹ Climate change is also often having an impact on species distributions through changes in habitat quality. This may result in the spatial shift of species distributions if new suitable areas are available and if the species is able to reach them. However, in many cases it is more likely that species will not be able to adapt, and populations or even whole species may become extinct as conditions turn unsuitable.¹¹⁸¹ For example, corals are suffering from increasing water temperatures (see Table 3.1).

Species in some ecosystem types are more vulnerable than others as, e.g., at mountain tops where no cooler refuges are available when temperature rise drives species uphill.¹¹⁹¹ But also other types of physical boundaries such as roads, dams, or fragmented habitats in general may restrict species to escape unsuitable conditions.^|20|Therefore, habitat connectivity and landscape permeability are critical to mitigate those negative effects. Fragmentation may also cause the loss of genetic diversity, as species populations are becoming increasingly isolated and genetic exchange is delimited. This leads to a decrease in genetic or intraspecific diversity.¹²¹¹ Reduced levels of intraspecific diversity are also relevant for the adaptive capacity of species under changing environmental conditions since higher genetic diversity

TABLE 3.1 Coral Bleaching

Coral reefs are described as the rainforests of the sea as they represent the most diverse marine habitat. They are, however, very vulnerable to climate change since water temperature increase is causing large-scale diebacks of coral reefs through a process called coral bleaching. Corals live in symbiosis with algae (zooxanthellae) that grow in corals, give them their specific color, and provide them with nutrients. When the temperature increases, zooxanthellae start to produce toxins leading to the release of the algae from the coral and the loss of the corals’ color. Corals are able to survive a limited time without nutrients from the symbiotic algae but a general increase of water temperature and temperature fluctuations during the last years coupled with other stressing factors such as overfishing, water pollution, acidification, and mechanical destruction has already caused the loss of large proportions of coral reefs. About 19% of the original coral reefs have already been lost and about 35% are threatened. Predictions estimate that about 50% of the world’s human population will live along coasts by 2015.¹²³¹ Altogether, these stressors are likely to lead to ever increasing pressures on coral reefs and the loss of essential ecosystem services they provide.

increases the probability that some individuals are able to survive and reproduce under the changing conditions.¹²²¹ Lower genetic diversity on the other hand is likely to negatively affect the persistence of species under changing conditions.

Impacts on the Interactions between Species and the Community

Climate change impacts species interactions and community structure in many different ways. Often, this is a consequence of temporal or spatial decoupling between the interacting species, resulting in changes or loss of tropical relationships and other interactions.¹²⁴¹ A popular example is the temporal decoupling between the earlier flowering times of plants well before the awakening from hibernation of bees as their pollinators.¹²⁵! At the same time, new interactions may become apparent under climate change, e.g., through the spread of invasive species.¹²⁶! “Winners” of climate change may extend their distribution ranges into areas where there are fewer competitors or predators and can reach high densities. All these changes may lead to general alterations in species composition and community structure having an impact in turn on other biotic and abiotic factors of whole ecosystems.

Impact on Ecosystems and Ecosystem Services

Climate change has a direct impact on abiotic conditions such as humidity, evaporation, wind patterns, microclimatic conditions, snow loads, water bodies, and the condition of soils. Impacts on ecosystem structure, processes, and dynamics include changes in photosynthetic activities, and the frequency and intensity of disturbances, e.g., from floods or fire events, nutrient cycling, and succession¹²⁷¹ Finally, not only species are likely to shift their distribution under altered climatic conditions but also ecosystem types.¹²⁸’²⁹¹ The melting of the permafrost soil leading to a spatial increase of the Taiga vegetation or the drying of the Mediterranean region are just two examples of climate change impacts on ecosystem-type distributions.

Changes in ecosystem functions, processes, and distributions will also affect ecosystem services.¹³⁰¹ In some areas of the world, extreme events such as droughts and floods will have tremendous effects on the availability of food, fiber, timber, and water. Climate change will also influence several supporting services, such as water cycling, primary production, and nutrient cycling as well as different regulating services like pest regulation, seed dispersal, and water purification. Finally, climate change is having a general impact on the appearance of ecosystems and therefore on their spiritual, religious, or aesthetical value. Altogether, these impacts on ecosystems may directly or indirectly influence human society and feed conflicts about scarce resources, food, and clean water.

System Resilience to Climate Change

Climate change impacts are not uniformly distributed and different biodiversity elements will suffer from climate change to varying degrees. The vulnerability of biodiversity, e.g., an ecosystem or a certain species, is a function of its exposure and sensitivity toward climate change as well as its ability to adapt to these changes.¹¹¹³¹¹ This means that an element of biodiversity that has a high exposure towards climate change can still be less vulnerable than another element with low exposure as long as it is either less sensitive or has the ability to adapt to these changes, e.g., through the spatial shift of a species distribution. This is connected to the related concept of resilience. Traditionally, resilience has been described as the ability of a system or a species to return to its original state after disturbance. However, recent trends in ecological studies have highlighted the dynamic behavior of ecosystems, therefore giving rise to a different definition where resilience means merely the ability of a system to return to a stable state that could be different from its original one.¹³²¹ According to this concept, systems can have different stable states and it is possible that an ecosystem can transform to another without losing its functionality. The potential nonlinear impact of climate change on ecosystems and biodiversity has also been described

FIGURE 3.1 Map of global tipping elements.

Source: Adapted from Schellnhuber & Held,¹⁵⁵¹ Lenton et al.,^|M| by permission of Oxford University Press.

TABLE 3.2 Tipping Points

with the concept of tipping points (see Figure 3.1 and Table 3.2). These are “points of no return” beyond which it is impossible to stop or even reverse the negative impacts from climate change.¹³³¹ Different definitions for all of these concepts can be found in literature and a discussion on the relation between them and their differentiation is still ongoing.