Climate is a primary constraint on species distribution and ecosystem function, and forecasting extinction risks, species range shifts, biome shifts, and altered disturbance regimes remains a central challenge. It is predicted that precipitation and temperature regimes will most likely be affected by future climate change, and that extreme events such as heat waves and resulting droughts and heavy precipitation associated with flooding will increase in both frequency and magnitude.
Indeed, global climate change is expected to drive particularly sharp declines in freshwaters, as resident species are less mobile and only a few taxonomic groups are capable to freely shift their distribution ranges. The impact of climate change on freshwater biodiversity is therefore of major concern for the EU Nature Directives, the Convention of Biological Diversity and the Ramsar Convention. The degree and direction of change, however, is likely to depend on latitude and altitude, as well as on ecosystem type and target organisms.
Aquatic biodiversity in mountain regions, for example, is expected to be especially vulnerable, as temperature directly influences life-cycle strategies and the distribution of biota. While many taxa may find suitable habitats at higher elevations, the most cold-adapted taxa may go extinct.
Changes in precipitation will favour some freshwater habitat types and their species-holding capacity, just as decreasing runoff and warming will negatively affect other habitats. For example, wetlands may be highly sensitive to changes in precipitation, while their particular species assemblages may be less affected because they already are adapted to fluctuating environmental conditions. Novel 21st-century climates may promote the formation of novel species assemblages with no adequate analogue in history and with unpredictable consequences.