Two examples illustrate the impacts of climate change on species’ ranges. The first involves the distribution of a beautiful butterfly that lives in southwestern parts of the U.S., the Edith’s Checkerspot Butterfly (Euphydryas editha), a species that has been studied by many researchers for long periods of time. What has been found is that the butterfly has shifted its range northward and to higher altitudes over several decades. Despite their mobility, the butterflies are still at risk, because at the northern edge of their range they are being threatened by habitat loss from human activity; and at the southern edge, they are being wiped out by warming temperatures and drier conditions that have significantly reduced populations of the plants they feed upon, causing them to starve. They are caught in an extinction-vise, squeezed from the north by habitat loss and from the south by climate change.
The second involves vascular plants in the Alps (vascular plants are those that have specialized water-carrying tissues and include, among other groups, ferns, flowering plants, and gymnosperms like confers). For more than 90 years, European botanists have been studying these plants, so there was a record to compare to when scientists began to survey their present day ranges. In 1994, researchers from the University of Vienna demonstrated that at more than two thirds of the sites they studied, vascular plant species had moved their ranges up the mountains by an average of 4 meters (around 13 feet) per decade over the past 70-90 years, in response to an average regional warming on these mountains of 0.7°C (or about 1.3°F). The clear implication was that those species that had reached the summit would have no place to go if there were further warming, and if they could not adapt, they would become extinct.
However, things are far more complicated than the ability of an individual species to migrate or not. The impacts of climate change can involve cascades of events that, given our often very basic level of understanding of biological systems, are enormously difficult to anticipate. Species do not exist in isolation. They are parts of ecosystems in which they depend on a wide range of other species for their survival, for example, on those species that make up their food supply. They are also highly affected by the presence of predators, and of those species with which they compete. And, of course, these food species, predators, and competitors are themselves dependent on an enormous array of other species. As different species will have differing migration responses to climate change, there will be a consequent breakdown in relationships among species and in the functions of the ecosystems they comprise, in both the new and old environments, threatening many species with extinction.