Chapter 2: How is Biodiversity Threatened by Human Activity?
Although species have gone extinct since life began, what distinguishes present-day extinctions from those that have occurred in the past is a distinctive human fingerprint. This chapter considers how human activity has resulted in environmental changes that are known to threaten species. Although each of these changes is discussed as if it were acting in isolation, the reality is that threatened species most often come under pressure from several environmental assaults at once. In some cases, these changes may even work synergistically, such that their combined impact is greater than the sum of their individual effects. They can also act in such a way that one insult sets the stage for another, as may have occurred with the demise of some species of harlequin frogs in Costa Rica, where climatic changes are thought to have predisposed them to chytrid fungal infections[
1] (see chapter 6, page 212).
Such a one-two punch, or one involving several human-caused factors acting together, may threaten many species on Earth. For example, research performed in northwestern Ontario lakes has shown that climate change and acid rain may act together to make water clearer and thus more easily penetrated by harmful ultraviolet (UV) radiation. Dissolved organic carbon, which consists of a variety of natural compounds that come from soils and plants, serves as an important UV radiation shield for aquatic life. More than twenty years of observation by David Schindler and his colleagues at the University of Alberta has demonstrated that the total amount of dissolved carbon in the lakes is lowered both by droughts associated with climate change (which result in a reduction of organic carbon flowing into the lakes from the surrounding land) and by acidification of the water by acid rain.[
2],[
3] As a result, some aquatic species, such as those whose young develop in shallow waters where dissolved organic carbon is most reduced, are put at risk from exposure to increased UV radiation, already at higher levels from stratospheric ozone depletion. UV radiation has also been shown to damage aquatic food webs, decreasing photosynthesis and growth in some aquatic algae[
4], and harming some aquatic invertebrates.[
5]
It may be difficult for some to read this chapter without having a sense of anger or despair, born out of recognizing how we humans are driving to extinction the very organisms that allow us to thrive on this planet. But before we can act effectively as individuals and as groups to reverse this self-destructive trend, guidelines for which are provided in chapter 10, we must first understand, as fully as we are able, the ways we threaten the survival of other species. That is the goal of this chapter, which, to our knowledge, is one of the most comprehensive reviews available on this subject. We start with habitat loss, which is currently the most serious threat to biodiversity.
Habitat Loss: On Land
Humans have already altered to varying degrees nearly half of Earth’s land surface, and in the next thirty years, this number will likely rise to about 70 percent.[
6] Given that the IUCN currently lists habitat loss as a contributor to the endangerment of nearly 50 percent of all threatened species, it is clear that habitat loss will remain a leading driver of species endangerment and extinction in coming decades. Some of the major factors resulting in habitat loss are considered below.
Deforestation
Forests range from hot, dripping rainforests to dry woodlands that merge into savannahs, from conifer forests in temperate regions to those that grade into tundras. They also include temperate deciduous forests (deciduous trees are those that lose their foliage, e.g., maples, oaks, and birches, for some part of the year). What qualifies as deforestation is equally diverse, ranging from absolute clear-cutting to selective logging to sustainable harvesting, and to the deforestation that accompanies damage by fires. Estimates of annual global deforestation tend to converge for tropical humid forests around the figure of 120,000 square kilometers (km
2), or about 46,300 square miles. Only about half the original 14 to 18 million km
2 (5.4 to 7 million square miles) of tropical humid forests remain, with much of the clearing having been done in the last fifty years. For tropical dry forests, annual rates of deforestation are around 40,000 km
2 (around 15,400 square miles), or about 1 percent of what remains today.[
7] These estimates are for forests that have been cleared. Estimates of tropical forests burned, selectively logged, or harmed by being near new forest edges are even larger than those for clear-cut areas. Some of these areas grow back, but most are left as almost useless land, capable of supporting only a fraction of their original diversity.
In other regions, such as in Asian Russia, up to 5,000 km
2 of forests are cut each year, most of which is clear-cutting. Although these logged areas are generally replanted or left to reforest naturally, such practices have led to widespread forest degradation in the region. Temperate forests are recovering in some places, such as the eastern United States, and some are encouraged by the appearance of tree plantations around the world. Globally, some 2 million km
2 (about 770,000 square miles) of these plantations have been planted, most of which are made up of pine or
Eucalyptus trees. While these plantations have the outward appearance of forests, they cannot be considered natural forest ecosystems any more than single-crop farm fields can be considered natural substitutes for the ecosystems that they have replaced, such as the prairies in the U.S. Great Plains or the pampas of Argentina. Both planted systems have markedly reduced levels of biodiversity compared to their natural forbears. The growing trend toward large tree plantations instead of sustainably managed natural forests, as has been occurring in such countries as Finland, where a great proportion of forests are now even-aged stands of Scots Pine and Norway Spruce, will endanger large numbers of forest species.[
8]
Population growth and migration into forested areas, government subsidies paid for forest clearance, corruption, improved harvesting technology, and, in some areas, human indifference to their destruction drive the overharvesting of tropical forests.
Most clearing of tropical rainforests (perhaps as much as 70 percent according to United Nations figures from the 1990s) occurs to make way for agriculture, including livestock grazing. While the newly established farms can provide habitat to support some species that had once lived in the rainforests, most cannot survive in their new homes. In addition, because many rainforest organisms can live only in limited areas of the forest that meet their peculiar needs for temperature, water, and food, and nowhere else (see discussion below on endemic species), when those areas are cut or burned down, those organisms are lost. Indeed, according to the IUCN’s 2006 Red List, forest clearing for crops and livestock is a threat to more than 20 percent of terrestrial species.
Other Threats to Species on Land
Cities may impinge upon and pollute habitats, making them less suitable for their flora and fauna. Presently home to about half the world’s population, cities are growing by 2 percent each year, so that urban populations, according to the U.N. population division, will grow to 60 percent of the world’s total by the year 2030, with even greater proportions in the developing world.[
9] Also, the building of dams, irrigation projects, and other water development activities can disrupt the integrity of habitat and threaten species. While each of these activities brings great benefits to humanity, they all come with significant costs to species and ecosystems (see chapter 3 for a more detailed discussion of threats to ecosystem services).
Discerning the role of habitat loss among other drivers of species extinctions, such as introduced species and hunting, can sometimes be difficult. For example, some bird species such as the Passenger Pigeon (
Ectopistes migratorius), the Carolina Parakeet (
Conuropsis carolinensis), and Bachman’s Warbler (
Vermivora bachmanii) became extinct in the forests of eastern North America following massive deforestation in the nineteenth century. But for the Passenger Pigeon, it was overhunting that ultimately led to its demise, with forest losses contributing both directly and indirectly, by serving to concentrate the birds into smaller and smaller areas, thereby making them more vulnerable to hunters. (See section on overexploitation on page 42 for further discussion of the Passenger Pigeon.)[
10]
Other parts of the world, notably Europe, have not become centers for species extinctions despite extensive land transformations. Habitat destruction undeniably causes different numbers of extinctions in different places. Why should this be so?
Endemic Species and “Hotspots”
Another way to ask this question is: What are the features common to centers of human-caused extinctions? Each of the areas in the three case studies of extinction presented in chapter 1—Hawai’i, the Cape Floristic region, and Australia—and several others not mentioned, holds a high proportion of species found nowhere else. Scientists call such species
endemics. Remote islands are rich in endemics. For example, endemics constitute 90 percent of Hawaiian plants and 100 percent of Hawaiian land birds. But continental areas can also be rich in endemics. About 70 percent of the plants in the Cape Floristic region, 74 percent of Australian mammals, more than 90 percent of North American fish, and the great majority of North American freshwater mollusks are endemic to those regions. In contrast, only about 1 percent of Britain’s birds and plants are endemics, and all of eastern North America in recent times has had only about thirty-five endemic birds (including the three mentioned above that are now extinct).
Past extinctions are so concentrated in small, endemic-rich areas that an analysis of global extinction is effectively a study of extinctions in only a few extinction centers. Why is this the case? Consider some simple models of extinction. The simplest one supposes only that some species groups are more vulnerable than others. This model does a poor job of predicting global patterns for the following reasons. First, the model predicts that the more species that are present, the more there will be to lose. Yet the number of species an area contains is not a good predictor of the number of extinctions. Relative to continents, islands have few species, yet they can suffer many extinctions. Second, if island birds were intrinsically vulnerable to extinction, then Hawai’i and Britain, with roughly the same number of breeding land birds and both with widespread habitat modification, would have suffered equally. Hawai’i had more than 100 extinctions; Britain, only three.[
11]
All the Hawaiian species were found only on the islands; none of the British species was. This suggests another model of extinction, the so-called “cookie-cutter” model, where something destroys, or “cuts out,” a randomly selected area. Species that were in this cut-out area but that were also found elsewhere survive, for they can recolonize. Only some of the endemics go extinct, the proportion depending on the extent of the destruction. In this model, where habitat destruction “cuts out” areas, the number of extinctions correlates weakly with the area’s total number of species but strongly with the number of its endemics. And this seems to be the case. Small endemic-rich areas, called centers of endemism, contribute disproportionately to the total number of extinctions, with endemic species that have small, geographically concentrated ranges being the most at risk. The localization of endemics, then, is the key variable in understanding global patterns of recent—and future—extinctions.
Hotspots are centers of endemism that have unusually high levels of habitat destruction. Currently hotspots make up only about 1.4 percent of Earth’s total land surface (their original area had been almost ten times greater than it is now), yet they contain more than a third of all known mammals, birds, reptiles, and amphibians. Only slightly more than a third of hotspot habitat is presently protected in any way. Sixteen of the twenty-five areas are forests, with most of these being tropical forests. Even for the three that are relatively undisturbed—in the Amazon, the Congo, and New Guinea—only about half the original tropical forest remains. As a consequence of high levels of habitat loss, these twenty-five hotspots are where the majority of threatened and recently extinct species are to be found.[
7]
Habitat Loss: In the Oceans
Athough scientists are uncertain about the extent of marine biodiversity, they have no doubts about the growing impact that humanity is having on the oceans. More than 50 percent of the world’s population lives within 100 miles (60 kilometers) of the coast, and the figure could rise to 75 percent by the year 2020.[
12] It is hardly surprising, then, that coastal waters are becoming increasingly polluted and are suffering large-scale losses of wetland habitat. Moreover, some 95 percent of marine fish catches come from continental shelf regions, and these fisheries end up consuming a quarter to a third of all the primary production in these areas. (Primary production, the total amount of organic compounds produced by photosynthetic organisms harvesting energy from the Sun, constitutes the base of the food web, with herbivores consuming these organisms, called primary producers or autotrophs, and being consumed, in turn, by carnivores.)[
13] It is in these same coastal waters that the majority of known marine biodiversity resides.[
14]
As on land, the peak of marine biodiversity lies in the tropics, particularly in coral reefs. Coral reefs are home to almost 100,000 marine species (although, as stated in chapter 1, the total of coral reef species may be more than nine times that number), including an estimated 4,000 to 5,000 species of fish, which comprise almost 40 percent of the world’s known marine fishes.[
15] Though their combined area is just 0.2 percent of the ocean surface, coral reefs fringe approximately one-sixth of the world’s shorelines.[
16]
The global center of marine biodiversity lies in the Southeast Asian archipelago, encompassing the Philippine and Indonesian islands. This region, sometimes referred to as the Coral Triangle, supports the greatest concentrations of known marine species found anywhere on the planet, both in the coral reefs and in the vast expanses of its mangroves and seagrass beds. In the Atlantic Ocean, the Caribbean holds the greatest biodiversity. As on land, those reef areas that are the most threatened, including those in Southeast Asia and the Caribbean, are the same ones that hold the greatest number of endemic species. And, as on land, these marine hotspots are the most in danger.[
15]
An estimated 20 percent of the planet’s reefs have already been destroyed by human activities, and an additional 50 percent are threatened and at risk of collapse.[
17] Threats to reefs come from overfishing and damaging fishing practices, such as by using cyanide or dynamite; coastal development and pollution by sewage, agricultural runoff, and toxic substances; land erosion and silting; direct physical damage; the harvesting of coral for limestone, jewelry production, and for other industries; and the acidification of seawater by higher atmospheric carbon dioxide dissolving in water (impeding formation of the coral skeleton). But, most of all, reefs are threatened by warming sea surface temperatures from global warming that can cause coral bleaching and lead to various lethal infectious diseases (see section on cone snails in chapter 6, page 257). It is believed that such widespread impacts on coral reefs will translate into large numbers of extinctions, especially for coral reef species that have limited ranges.
Marine Habitat Loss from Fishing
In addition to coral reefs, several other marine ecosystems have been imperiled by human activity. Overfishing is widely regarded as the single greatest threat to marine biodiversity (this topic is also covered in the section on overexploitation below). Some fishing practices are particularly destructive. Bottom trawling, for example, in which weighted nets are dragged across the sea floor, destroys critical ocean floor habitat for developing organisms, undermining the marine food web. Dragging heavy trawling gear across the seabed has been likened to the clear-cutting of forests on land, but the scale of destruction in the case of bottom trawling is significantly greater. Trawls are estimated to scour nearly 1.5 billion hectares (~5,800,000 square miles) of continental shelf habitat each year, an area roughly one-tenth the size of the entire land surface of Earth, and 1,000 times the area of forests that are lost each year.[
18] Much of that area has been hit by trawls before, sometimes many times, resulting in less diverse and less structurally complex habitats than those that existed before the onset of trawling. Trawling has had such a negative impact on shallow-water seafood stocks that in recent years trawlers have had to move to the deep oceans. Some 40 percent of trawling now occurs at depths beyond the continental shelves. Today’s trawlers can penetrate to depths as great as 2 kilometers (1.24 miles), and their deep-water catch can be found in supermarkets worldwide.[
19] In a story that parallels what occurred in shallow waters, the trawlers that first fished these deep and virgin grounds brought up as much coral as fish. Yet, after only a few years, trawling these same areas yielded relatively little coral bycatch because the seafloor habitats had already been stripped bare.
Deep-water trawling has especially devastating eff ects on slow-growing marine organisms and their habitats, in particular, those found in deep-sea or “cold-water” coral reefs. These reefs, essentially unknown and unexplored until the 1990s, are found off the coast of all the world’s continents, in waters as deep as 1,000 meters (3280 feet). Nowhere is the analogy of clear-cutting forests more appropriate than it is for the tops of deep-water seamounts and steep continental slopes where deep-sea corals can be found. Dense and diverse communities of invertebrates that have taken thousands of years to develop are being cleared to bare rock in these habitats in the space of only a few years. Some of the largest sea fans brought up in trawls are hundreds to thousands of years old.[
19] Not only are these deep and unseen habitats being destroyed, but along with them, there are almost certainly widespread extinctions, with many species highly vulnerable to habitat loss likely to be disappearing far more quickly than we can identify them.[
20]
It has often been assumed that marine species are resilient to extinction, because they are believed to produce abundant off spring that disperse widely over large geographic ranges. But this is not so. Many marine species produce relatively few young and have limited dispersal, and a significant fraction have highly restricted geographic ranges.
Notes
[
1] Pounds, J.A., et al., Widespread amphibian extinctions from epidemic disease driven by global warming.
Nature, 2006;439(7073):161–167.
[
2] Schindler, D.W., et al., Consequences of climate warming and lake acidification for UV-B penetration in North American boreal lakes.
Nature, 1996;379(6567):705–708.
[
3] Schindler, D.W., The cumulative effects of climate warming and other human stresses on Canadian freshwaters in the new millennium.
Canadian Journal of Fisheries and Aquatic Sciences, 2001;58(1):18–29.
[
4] Kohler, J., et al., Effects of UV on carbon assimilation of phytoplankton in a mixed water column.
Aquatic Sciences, 2001;63(3):294–309.
[
5] Tank, S.E., D.W. Schindler, and M.T. Arts, Direct and indirect effects of UV radiation on benthic communities: Epilithic food quality and invertebrate growth in four montane lakes.
Oikos, 2003;103(3):651–667.
[
6] U.N. Environment Programme,
Global Environmental Outlook—3. Earthscan, London, 2002.
[
7] Pimm, S.L.,
The World According to Pimm: A Scientist Audits the Earth. McGraw Hill, New York, 2001.
[
8] Heinrich, B.,
The Trees in My Forest. HarperCollins, New York, 1997.
[
9] UN Population Division,
World Population Prospects: The 2005 Revision. United Nations, New York, 2005.
[
10] Pimm, S.L., et al., The future of biodiversity.
Science, 1995;269(5222):347–350.
[
11] Pimm, S.L., and P. Raven, Biodiversity—extinction by numbers.
Nature, 2000;403(6772):843–845.
[
12] Small, C., and R.J. Nicholls, A global analysis of human settlement in coastal zones.
Journal of Coastal Research, 2003;19(3):584–599.
[
13] Pauly, D., and V. Christensen, Primary production required to sustain global fisheries.
Nature, 1995;374(6519):255–257.
[
14] Ray, G., et al., Effects of global warming on the biodiversity of coastal-marine zones, in
Global Warming and Biological Diversity, R. Peters and T. Lovejoy (editors). Yale University, New Haven, CT, 1992, 91–104.
[
15] Roberts, C.M., et al., Marine biodiversity hotspots and conservation priorities for tropical reefs.
Science, 2002;295(5558):1280–1284.
[
16] Birkeland, C.,
Life and Death of Coral Reefs. Springer, New York, 1997, 536.
[
17] Wilkinson, C. (editor),
Status of Coral Reefs of the World: 2004, Vol. 1. Australian Institute of Marine Science, Townsville, Queensland, Australia, 2004.
[
18] Watling, L., and E.A. Norse, Disturbance of the seabed by mobile fishing gear: A comparison to forest clearcutting.
Conservation Biology, 1998;12(6):1180–1197.
[
19] Roberts, C.M., Deep impact: The rising toll of fishing in the deep sea.
Trends in Ecology and Evolution, 2002;17(5):242–245.
[
20] Roberts, J.M., A.J. Wheeler, and A. Freiwald, Reefs of the deep: The biology and geology of cold-water coral ecosystems.
Science, 2006;312(5773):543–547.
Excerpted from
Sustaining Life by Nobel Laureate Eric Chivian with Aaron Bernstein. Copyright © Oxford University Press, 2008. All rights reserved.
