Once one species goes extinct, there will likely be other extinctions or even an avalanche of them. Some cases of these secondary extinctions are simple to understand—e.g., for every bird or mammal that goes extinct, one or several species of parasite also will likely disappear. From well-studied species, it is known that bird and mammal species tend to have parasites on or inside them that can live on no other host.

Other extinctions cause changes that can be quite complicated. Species are bound together in ecological communities to form a food web (see food chain) of species interactions. Once a species is lost, those species that fed on it, were fed on by it, or were otherwise benefited or harmed by that species will all be affected, for better or worse. These species, in turn, will affect yet other species. Ecological theory suggests that the patterns of secondary extinction are quite complicated and thus may be difficult to demonstrate.

The most easily recognizable secondary extinctions should be seen in species that depend closely on each other. In the Hawaiian Islands, anecdotal evidence for secondary extinctions comes from a consideration of nectar-feeding birds. Before modern human activity on the islands, there were three nectar-feeding Hawaiian honeycreepers—the mamo (Drepanis pacifica), the black mamo (Drepanis funerea), and the iiwi (Vestiaria coccinea)—that had long decurved (downward-curving) beaks, the kind adapted to inserting into appropriately long and curved flowers. The first two birds are extinct, whereas the third is extinct on two islands, is very rare on a third, and has declined on others.

Extensive habitat destruction is likely the cause of many species of Hawaiian birds, as previously discussed. In addition, native Hawaiians hunted some species for their feathers. In the case of the three honeycreepers described above, however, their extinctions may have followed the destruction of important nectar-producing plants by introduced goats and pigs. Many of the native lobelias, such as those of the genera Trematolobelia and Clermontia, have clearly evolved to be pollinated by the three honeycreepers, and the plants were once important components of the forest’s understory. About a quarter of these plant species are now extinct, a rate that plainly exceeds those for the rest of the flora, perhaps because they were so vulnerable to introduced mammalian herbivores. It is not certain, however, whether the plants disappeared first and then their bird pollinators or vice versa.

Similarly, some surviving Hawaiian birds seem to be unusually specialized feeders and to be threatened as a consequence of the loss of their food sources. For example, another rare honeycreeper, the akiapolaau (Hemignathus munroi), is an insectivore that feeds on insects mainly on large koa (Acacia koa) trees (see acacia). Today, however, few koa forests remain, because the trees have been overharvested for their attractive wood. Yet another Hawaiian honeycreeper, a seed-eating species called the palila (Loxioides bailleui), is endangered because it depends almost exclusively on the seeds of one tree, the mamane (Sophora chrysophylla), which is grazed by introduced goats and sheep.

Stories of secondary extinctions are nearly always unsatisfactory anecdotes because of the difficulty of teasing apart the various explanations for what happened in the past. There is abundant evidence from small-scale ecological experiments that a change in one species’ numbers (including its complete elimination) will cause cascading effects in the abundance of other species. The stories are plausible enough that particular attention should be paid to the future consequences of contemporary extinctions.

Global warming

The global effects of flooding the atmosphere with carbon dioxide and other greenhouse gases created as by-products of human activity are many and complex. Global warming, an increase in global average surface temperature, is but one of them. Both the land and the oceans are increasing in temperature at different rates in different places. The Arctic and Antarctic seem to be heating up the most, while temperature changes in the tropics are more modest. In addition, the heating is melting glaciers and ice sheets, causing sea levels to rise, and it may be increasing the frequency of the most intense hurricanes and the intensity of other storms and extreme weather, with consequences to the flow of rivers. Some areas and their ecosystems are becoming wetter, while others are becoming drier and hotter and thus likely to suffer more wildfires. Scientific knowledge is constantly increasing and changing what is known about the way Earth works, and knowledge of global warming and its effects on species is expanding particularly quickly.

Since the turn of the 21st century, the relationship between changes to Earth’s physics and chemistry and biodiversity has been clarified significantly. Although the precise effects of global warming on species extinction rates are still uncertain, they almost certainly will be large.

It is now clear that most species are shifting their geographic ranges toward cooler places and are starting important events such as breeding, migration, and flowering earlier in the year. One study, published in the early 21st century, found that more than 80 percent of nearly 1,500 species of animals and plants from a wide variety of habitats worldwide were changing in the direction expected from global warming. Another study, published about the same time, found consistent northward movement of the northern boundaries of animal and plant species in Europe and North America. Such geographic shifts of species alter important ecological interactions with their prey, predators, competitors, and diseases. Some species can benefit, but others can be harmed—for example, when migratory insect-eating birds arrive too late to exploit the emergence of moth larvae that they typically feed to their young.

Even if scientists know that species likely will move in the direction of cooler habitats—toward the poles or up mountainsides—they will find the exact changes difficult to predict because so many different factors are involved. For example, one contemporary study examined the changes in the ranges and feeding ecology of butterflies in Great Britain in the last decades of the 20th century. Two species were found to have increased in range, as expected, but by unexpected amounts. One species lived in isolated habitat patches and could not fly the distance to some of the patches that were suitable for populating. Rising temperatures increased the number and density of suitable patches, allowing the butterfly to reach the distant patches by making use of intervening newly available ones as “stepping-stones.” The second butterfly species was able to exploit a previously unused food plant that grew in shady places that formerly had been too cool. With that change in diet, the butterfly was able to greatly expand its range. In both cases, the species benefited from the warming climate, at least at the northern edges of their ranges. It takes little imagination to foresee the consequences if, for example, the mosquito Aedes aegypti, which carries dengue hemorrhagic fever, were to expand its range to an unexpected degree across the southern United States, which is its present northern limit.

Can anything useful be inferred about the likely consequences of climate change to species extinction? Assuming that the change has simple effects, scientists can predict where a species should be in the future if it is to live in the same range of climatic conditions that it does now. The real concern is that this range of suitable conditions, or the species’ climate envelope, may shrink to nothing as conditions change—i.e., there may be no suitable conditions for a species in the future.

Other things being equal, one would expect that species with small geographic ranges will more likely be affected than species with large ranges. For a species with a large range, global warming may cause the species to disappear from the south and appear farther north, but the change in the size of the range may be quite small. The species may be present in many of the same areas both before and after global warming. The American robin, whose large range size is discussed above, could be expected to behave this way.

Of the species that have very small ranges, many live in mountainous areas, as can be seen in the map. To date, human activity has had relatively small effects on such species because of the impracticalities of cultivating land in mountains, especially on steep slopes. But it is these species—living on cool mountaintops that are now becoming too warm for them—that have nowhere else to go. Such arguments lead scientists to believe that extinctions caused by rising temperatures will be additional to those caused by land-use changes such as deforestation. Rough calculations suggest that rising temperatures may threaten about a quarter of the species in hot spots.