Species track their climatic optima, and retract their ranges where conditions are unsuitable. These shifts are the sum of many local pressures on extinction and colonization due to factors such as physiology and interactions with other species. For plants, increases in heat and decreases in moisture can have direct effects on survival and reproduction. It is unlikely that all species will be able to evolve new tolerances with sudden changes in climate, leading to shifts in community composition and impacts on ecosystem dynamics. 

Specifically, changes in climate have an impact on the phenology and distribution of species, along with community composition and ecosystem dynamics. For each of these impacts, I discuss the expected changes with climate change along with changes that have been observed. While these impacts can occur in other systems, I am focusing on the terrestrial system.
 
 

Phenology

Phenology is the timing of seasonal activities and life cycle events. Warming temperatures are expected to impact phenological events, such as flowering and fruiting in plants or larval stages in insects. 

There have been observed changes in phenology, the timing of seasonal activities. Diurnal temperature ranges have decreased, leading to a lengthening of the freeze-free period in many regions. Studies in Europe and North America show a progressive increase in the timing of spring activities since the 1960s. Evidence of phenological changes include:

• Plant species are flowering 1.4 to 2.1 days per decade earlier in Europe over the last 30 to 48 years. 
• There has also been earlier breeding observed among amphibians in the UK over the last 25 years. 
• Studies on aphids in the UK have shown an increase in flight time from 3 to 6 days over the last 25 years.

Distribution

Distributions of plants and animals are heavily influenced by temperature and moisture patterns. Studies of fossil and pollen distribution show that species are very sensitive to climate changes. During several of the Pleistocene interglacial periods, the temperature was 2 to 3 degrees higher and vegetation was drastically different. When the climate changes, species often die out in their present areas and colonize new areas. Therefore, as the climate changes in the future, there will be disruption of natural communities and extinction of populations and species. Some species will be able to disperse to new areas but many will not be able to colonize new areas in sufficient time. An rapid increase of 3 degrees may exceed the ability of many species to adapt. 

Based on similar shifts in the past, it is reasonable to expect a 300 km shift in the temperate zone with a 3 degree increase. Species may also shift attitudinally as well as latitudinal: A 3 degree cooling of 500 meters in elevation is equal to 250 kilometers in latitude. As species move up mountains, they occupy smaller areas, have smaller populations and become vulnerable to genetic pressures. Species at the mountaintops may have nowhere to move to. 

Studies of sedentary species have shown a poleward and upward shift in species ranges across different taxonomic groups and geographic locations. However, due to the difference in dispersal abilities, there are different rates of range shifts among and within species. Specific examples of observed range shifts are:

• An increase in the treeline in Europe, with elevation shifts of 1 to 4 meters per decade. Plant species richness on European Alps have shown increases when compared to historical records.
• A study of butterflies in Europe showed that the ranges of 22 butterfly species have shifted north by 35 to 240 kilometers in this century, while only 2 species shifted south. The Edith's Checkerspot butterfly has been shown to shift 124 meters upward and 92 kilometers northward in the last 100 years. 
• In Britain, there has been a 18.9 kilometer average shift northward among 12 bird species over the last 20 years.

Community Composition

Community compositions are expected to shift with changes in climatic variables due to the changes in physiology, phenology, and distributions.. As species distributions change, the composition of communities changes as well. For example, in the diagram below, species A and B initially overlap. After changes in climate, the distribution of species A has moved northward while the distribution of species B did not change. This leads to a reduction in the area where both species A and species B are found.

Changes in patterns of vegetation types are determined by temperature and precipitation. In addition, plant communities are sensitive to a rise in carbon dioxide concentrations, and increases in concentrations may lead to changes in vegetation patterns.

Observed changes in community composition include:

• A community shift has been observed in the Sonoran desert where there have been increases in woody shrubs due to regional climatic changes.
• In the shortgrass steppe of northern Colorado, there has been a significant decline in the net primary productivity of the dominant native grass, Bouteloua gracilis as the temperatures have risen. Broadleaf plants in this area have shown a increase in abundance in this area. 

Ecosystem Dynamics

There may be complex dynamics from a changing climate as a result of different responses from interacting species. Species that closely interact or compete may have different responses to climate change, influencing the outcome of their interactions.

Examples of ecosystem changes include:

• Warmer spring weather led to disruption of synchrony between the winter moth hatching and the oak bud burst in Europe. This led to a change in the peak of insect availability, conflicting with the food demands on the great tit nestlings.
• Studies of large mammals show that juvenile survival is influenced by climatic extremes. Warm weather has incluenced the fecundity of red deer adn Soay sheep in Norway and the UK. These changes may create impacts on future population dynamics when the juveniles reach reproductive maturity. 
• Winter warming in Britain has led to breeding season changes among amphibians. These changes have led changes in temporal niche overlaps, leading to consequences for trophic interactions.

Sources: Walther et al. 2002, Hughes 2000, and McCarty 2001.
 
 

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