Climate change will continue to have ecological consequences

What will the projected 1.1°C-4.8°C change in average global temperature over the next 80 years mean for biological communities? We can get a sense of what such a temperature change might mean by comparing it with the climate variation associated with elevation in mountains.

Climate-biome correlations, such as those described in Concepts 3.1 and 4.1, are useful as a demonstration of what could happen with climate change, but it would be naive to use them to predict what will actually happen to biological communities. We know that biological composition is influenced by a multitude of factors, including climate—particularly climate extremes—as well as species interactions, the dynamics of succession, dispersal ability, and barriers to dispersal (as described in Unit 5). Because the ongoing climate change will continue to be rapid relative to the climate changes that have shaped current biological communities, it is unlikely that the same assemblages of organisms will form the communities of the future.

Paleoecological records reinforce the suggestion that novel communities may emerge with climate change by showing that some plant communities of the past were quite different from modern plant communities.

FIGURE 25.16 Past Changes in Plant Communities Vegetation types in eastern North America have changed since the last glacial maximum, 18,000 years ago (ka = thousand years ago). Vegetation composition was determined from pollen preserved in sediments.

What factors may have led to the development of vegetation types different from those found in North America today following retreat of the continental glacier?

(From J. T. Overpeck et al. 1992. Geology 20: 1071-1074.) View larger image

The rate of climate change will require rapid evolutionary change or the ability to disperse to new environments. Climate Change Connection 6.1 presents evidence that organisms with rapid life cycles have undergone evolutionary change in response to climate change. For more long-lived species, evolutionary responses are less likely, and thus for those species dispersal may be the only way to avoid extinction. Organisms’ dispersal abilities, and barriers to their dispersal associated with anthropogenic habitat fragmentation, will be important constraints on their responses to climate change. Plant dispersal rates are, on average, much slower than the predicted rate of climate change. In order to track the projected change in climate over the twenty-first century, plant species populations will need to move 5 to 10 km per year.

For most animals, mobility is not a problem, but their habitat and food requirements are intimately associated with the presence of specific vegetation types. In addition, barriers to dispersal may prevent organisms of all kinds from migrating in response to climate change. Dams, for example, may prevent fish from moving to water with more suitable temperatures. Fragmentation of habitat by human development may pose significant barriers to dispersal for some species (see Concept 24.2). Without habitat corridors through which they can disperse, species face a greater probability of local extinction in the face of climate change. Projections of the risk of extinction due to climate change, based on multiple published studies, indicate as many as 17% of Earth's species could be lost (Urban 2015).

In addition to affecting the geographic ranges of species, climate change will affect ecosystem processes, such as NPP, decomposition, and nutrient cycling and retention. Both photosynthesis and respiration are sensitive to temperature, and because their balance determines NPP, the direct effects of climate warming on NPP may be relatively minor. As indicated in Concept 20.2, however, variation in NPP is related to water and nutrient availability and vegetation type, all of which may be affected by climate change. Changes in precipitation patterns and evapotranspiration rates resulting from climate change may strongly influence both water and nutrient availability. Because of the heterogeneity of climate change, and of the resulting changes in vegetation types, both increases and decreases in NPP may occur.

Biological indicators of global climate change are diverse, and they are increasing over time. Experiments, modeling, and comparisons with historical and paleoecological records provide clues to how Earth's biota will respond to climate change. Substantial uncertainties in predicting the effects of climate change still exist, however, many of which are associated with other environmental changes that are occurring at the same time. In the next section, we'll look at two such anthropogenic changes that are having profound effects on ecosystems: emissions of sulfur and nitrogen into the atmosphere.