Some human activities result in habitat loss or degradation directly, such as cutting down tropical rainforests or draining wetlands for agricultural use. Others, such as those that emit greenhouse gases into the atmosphere, have indirect effects on habitats. Greenhouse gas emissions contribute to global climate warming. In turn, climate warming alters the habitats on the planet. As highlighted in the accompanying animation, climate change in the distant past, as well as more recent climate change, has altered habitats and species, shifting geographic ranges and the timing of life history events.
As part of global climate change, the average annual temperatures across North America are predicted to increase 2–5°C by the end of the 21st century. An average temperature found at any given location in North America this year could shift by then to be 500 to 800 kilometers north. How do species respond to climate change?
One way to study climate change is to look at events in the past, during which the climate had changed, to inform us about possible events in the future. Here is a study of plant communities in eastern North America. At present, these communities range from a southeast forest in the south, to a deciduous forest just north of it, followed by several other plant communities, with tundra at the very north.
A colder climate existed 18,000 years ago, and glaciers extended south as a result. Pollen preserved in sediments tells us what plant communities existed then. Note that the plant communities just mentioned had all been shifted south, and the southeast forest was not even found here. After the climate began to warm and glaciers started to retreat, the ranges of plant communities slowly shifted northward. Note that roughly 12,000 years ago, some novel, or what are termed "no analog" plant communities, formed under unique climate conditions that do not exist today.
One conclusion we may form from this study is that as climate rapidly changes in the future, unique combinations of species may come together to form novel communities. It is also important to see that because these plant communities provide habitats for many other organisms, shifts in range, habitat fragmentation, or a complete loss of a plant community could imperil the other organisms that live there.
Because of climate change, there is evidence that current species are responding to warming temperatures by shifting their distributions. As temperatures rise, the southern boundary of a species' range may become too warm as the northern boundary extends poleward into suitably warming temperatures. Studies have already found northward shifts in numerous bird species in Europe and butterfly species in North America.
As higher altitudes become warmer and more habitable, treelines are shifting up mountains. There is also evidence of plant communities shifting upward as temperatures increase at lower altitudes.
In addition to distributional shifts, there are shifts in the timing of life history events dependent on the changing of seasons. For instance, plants tend to flower at the same time their pollinators become active. As winter transitions to spring earlier, these activities may not shift in synchrony, leaving some plants without adequate pollination.
From all these data, it is clear that climate change results in changes to the geographic ranges and the timing of life history events in many species.
The global climate has varied widely over Earth's history and species have changed with it, either adapting, migrating to stay within their physiological and ecological ranges, or going extinct. This animation highlights one study of plant communities in eastern North America and how they have changed over time, from 18,000 years ago when the climate was much colder to present day. As the climate warmed and ice retreated, plant communities that were found in the south began to appear farther north. Additionally, novel communities of plants appeared during the transition, with unique combinations of species coming together. Other studies around the world note similar shifts during our current climate warming. In the Northern Hemisphere, many species have shifted their geographic ranges northward. Some plant and animal species have shifted to higher elevations. Climate warming is also correlated with changes in the timing of life history events, with some species migrating earlier, breeding earlier, or flowering earlier in the spring, which is warmer earlier.