Habitat conservation

Organisms have evolved to live in certain habitats. It stands to reason, then, that the most severe problem in protecting global biodiversity concerns habitat destruction and fragmentation. Habitats are generally defined by myriad physical parameters like temperature, rainfall, elevation, topography, salinity, soil type and many others. For tuna, habitat might be generally defined as the pelagic ocean within a certain temperature range. For tiny tardigrades, or moss bears, habitat may be forest moss, lichen, beach sand or arctic tundra. For gnathostomulids, the interstices between silt-sized sand grains in the deep sea comprise their habitat. Flightless birds and many herptiles (reptiles and amphibians) may not require much physical space but may be highly specific regarding the space they can inhabit because of such factors as predators, food availability, or breeding sites. An organism's habitat is thus defined by a combination of physical and biological factors.

Biodiversity is not equally distributed but varies from ocean to tidepool, entire rainforest to one strangler fig tree, from temperate zones to tropical. Ecologists and conservationists understand that biodiversity is unrelated to aesthetics or the sweeping vistas of national parks. In fact, habitats with some of the highest numbers of species ("species-rich") include such places as streams, wetlands, coastal mangroves, rainforests, sloughs and estuaries—places often targeted by farmers and developers.

Hotspots are found in areas where the habitats of several rare species overlap. Because so much diversity is concentrated within relatively small areas they are, in fact, hot spots for extinction. The tropics have an inordinate number of hotspots because both species diversity and endemism increase as one travels from the poles to the equator. By way of example, the southeastern Appalachian mountains and all of southern California have many endangered species, but the vast majority of endangered species in the United States occur in the much smaller area of the Hawaiian Islands.

As of 2003, habitat has been already greatly reduced within biodiversity hotspots. This fact is of great concern to scientists. Worldwide, approximately two-thirds of all eukaryotic (multicellular) species occur in humid tropical forests. As the twenty-first century begins, these same forests are being cleared at a rate of approximately 386,100.5 mi2 (1 million km2) every 5-10 years. Logging and burning account for several times that loss. From the time a species' habitat begins to decline to the time when the population of that species also declines there will usually be a lag. Thus, the loss of habitat may initially cause only a few extinctions as small pockets of habitat remain; then, however, as habitat is lost entirely or so fragmented that some species cannot survive, individuals die out and the number of extinctions rises. The concern here in this "fewer extinctions now, more later" scenario is that by the time scientists have quantified the loss, it will be too late to stop it.

Consequently, designating and protecting hotspots may be a logical first step in conservation. Myers and colleagues calculate that a staggering 46% of all plant diversity involves endemics. They also estimate that 30-40% of all terrestrial vertebrates could be protected in 24 hotspots. Moreover, those 24 hotspots would span only about 2% of the earth's surface. Protecting hotspots makes economic sense as well. Although the expense of conservation per unit area varies hugely, from less than a cent in United States currency to over a million dollars per 0.4 mi2 (1 km2), costs are generally lower in less developed countries. More importantly, these same countries often have the most left to conserve. Furthermore, as habitat becomes fragmented through development or exploitation, the costs of mitigation and conservation rise. Pimm and Raven agree that the selective protection of hotspots is necessary but caution that it is insufficient for long-term conservation of biodiversity. Usually conservation biologists favor increasing the size of protected areas as a means of including more organisms and as safeguarding against some of the random causes of extinction discussed previously. In some cases less diverse but cheaper habitats can be purchased, ultimately conserving the same number of species on larger or more numerous tracts of land.

Marine conservation and shifting baselines

The conservation strategies outlined above are based largely on terrestrial research. This bias may result in part from the fact that the marine environment was long considered less at risk for environmental degradation because of its sheer immensity and the "unlimited bounty" of the seas. Many marine invertebrates and vertebrates have planktonic larvae capable of spreading over great distances in ocean currents; such species would be less likely to suffer from genetic bottlenecks, inbreeding, or the risk associated with endemic status. Yet in fact the ocean's bounty is limited. Many scientists are concerned that we may be seeing those limits breached.

Overfishing refers to fishing practiced unsustainably. Overfished species are species that have had their numbers so depleted that stocks may never recover. In most coastal ecosystems, manatees, dugongs, sea cows, monk seals, crocodiles, swordfish, codfish, sharks, and rays are functionally or formally extinct. The data compiled by these authors are impressive and cause for foreboding. In order to detect ecological trends a baseline for comparison is necessary, as is a distinction between natural and human-caused changes. As a starting point for such a baseline, Jackson and Kirby gathered paleoecological data, beginning about 125,000 years ago, together with archeological data from human coastal settlements from about 100,000 years ago. To augment these data, they made use of historical records from documents, charts and journals from the fifteenth century. Ecological records from scientific literature spanning the twentieth century completed their data set. Based on these data the authors found that from the onset of overfishing, lag times of only decades to centuries preceded large-scale changes in ecological communities.

In some cases, such time lags existed because other species filled in the gap left by the overfished species. For example, sea otters were all but eliminated from the northern Pacific by fur traders in the 1800s. Because the voracious appetite of sea otters kept sea urchin populations in check, sea urchin numbers increased and decimated kelp forests. Sea otters off the southern California coast were similarly wiped out at approximately the same time as those in the northern Pacific; however, the California kelp forests did not begin to disappear until the 1950s. Diversity across trophic levels was the reason for the lag time between overharvest and threshold response. Predatory fishes like sheephead also ate sea urchins, and spiny lobsters and abalone competed with urchins for kelp. Together these animals effectively "shifted over" to occupy a portion of the ecological niche formerly occupied by the sea otters. Since sea urchins have become a popular fishery, some well-developed kelp forests have returned. Unfortunately, they now have only a vestige of their former complexity. Kelp forests off southern California now lack the trophic diversity of sea otters and such predatory fishes as sheephead, black seabass, and white seabass that they once possessed. Unregulated fisheries exist for echinoids like sea cucumbers, for crabs, and for small snails. The numbers of abalone—greens, reds, whites, and blacks—have dwindled from overharvest and disease. The once diverse southern California kelp forest community has been reduced to a community of primary producers. Thus overfishing affects not only the target species, but dramatically alters ecosystem diversity when "keystone" species are removed.

Nor are coastal ecosystems the only habitats affected. Pelagic longlines are the most widespread fishing gear used in the ocean and threaten many open ocean species. Some species form legitimate fisheries; such others as sea turtles constitute by-catch. Because many pelagic animals have such extensive habitat and evolved to swim great distances often at high speeds, they can be extremely difficult to study. Nevertheless, we know that billfishes, tuna, and sea turtles are in need of conservation because they have been subjected to such intense exploitation. Relatively less is known about open ocean sharks, but with the exception of makos their numbers are estimated to have declined by at least half in less than two decades. Such large animals as these generally bear fewer young, reproduce less often, and do so at an older age. In ecological terms, they have a low intrinsic rate of increase. This low rate means that their ability to rebound if fishing pressures are decreased is slim, and that conservation efforts will have to be long-term if their numbers are to increase.

Jellyfish, medusae, ctenophores and siphonophores are invertebrate predators that typically feed on the same prey as larval and adult fishes do. Concern exists that jellies may be sliding over to fill the void left by declining pelagic predators. Although Carr and his colleagues studied fishes, their field experiments provide insight as to how such "cascading negative consequences" occur. They found that by removing important predators (groupers or jacks) and other highly competitive fish (territorial damselfishes), as is often done by fishermen as well as by the aquarium trade, species interactions changed. As species interactions decreased, population fluctuations increased. As fish populations grew more unstable, the likelihood of extinction increased locally and regionally. In part, jellies are able to shift and fill the ecological "holes" left by declining fish stocks because they can reproduce quickly and in great numbers. Mnemiopsis leidyi, a tiny comb jelly, was introduced into the Black Sea, probably when a grain ship pumped out her ballast. The Mnemiopsis population was able to take advantage of habitat conditions that were unfavorable to potential competitors; as a result, M. leidyi populations peaked in the late 1980s and 1990s. Over this same time Mnemiopsis consumed most of the zooplankton production that had previously

A diver with a camera looking at a coral reef near Lizard Island in the Great Barrier Reef. (Photo by A. Flowers & L. Newman. Reproduced by permission.)

been taken up by commercial fisheries. Consequently commercial fisheries in the Black Sea went virtually extinct themselves. This example illustrates both the shift in keystone species as well as the devastating effect that introduced species can have at the level of an entire ecosystem.

In addition, fisheries suffer because the fishing industry is a powerful political lobby. Ample scientific evidence exists that illustrates the need for change in fisheries management. The industry itself, however, continually clamors for additional proof, unable or unwilling to listen to the nails being driven into its own coffin. If a dearth of evidence exists, it is any that can demonstrate that fisheries are harvested sustain-ably. In part, fisheries may be slow to acknowledge problems because, as the numbers of fish decrease, technological development has advanced in the fishing industry—such as satellite imagery that can be downloaded to computers; sensitive sonar that can locate fish schools; large fleets of fast boats— disguising diminishing stocks by more efficiently harvesting what remains.

Aquaculture is commonly believed to relieve pressure on fisheries. It is true that between 1986 and 1996 the global production of fishes by aquaculture more than doubled. Aqua-

Mangroves Keys
The calm mangroves of the Florida Keys are a protective network of canals for manatees and a diverse range of marine flora and fauna. (Photo by A. Flowers & L. Newman. Reproduced by permission.)

decisions and use their buying power to encourage conservation and improved management of fisheries.

Global warming

Aside from being responsible for causing the most species extinction worldwide, tropical deforestation creates 20-30 percent of global carbon emissions, with the burning of fossil fuels accounting for most of the remainder. Although it is commonly thought that carbon dioxide emissions are to blame for the rapid warming trends observed in recent decades, such other noncarbon greenhouse gases as chlorofluorocarbons and nitrous oxides contribute as well. Aerosols are tiny particles emitted into the air as pollution, which we see as visible smog or haze. Aerosols alter the brightness of clouds and increase solar heating in the atmosphere. These changes serve in turn to weaken the earth's hydrologic cycle, reducing rainfall and fresh water supplies.

Biological indicators for global warming abound. Species from butterflies to marine invertebrates show a tendency to migrate northward; by so doing, they act as indicators of climate change. Like extinction, global warming predates human influence; however, the global effects that humans can and do have on the earth's climate are facts that cannot be culture alone, however, is not an answer; by reducing wild fish supplies for seedstock collection or for feed, aquaculture can in many ways be detrimental. Hundreds of thousands of Atlantic salmon raised in pens in the Pacific have escaped to locations where they can hybridize and genetically weaken native stocks. Atlantic salmon escaped from Atlantic pens can also interbreed with wild stocks and interfere with the latter's ability to find their spawning grounds, which is a trait that is passed on genetically. Other problems posed by aquaculture include the spread of diseases amongst pens and to wild stocks as well as the discharge of untreated effluent and nitrogenous wastes that result in eutrophication.

Marine reserves are controlled-take areas that have been helpful in restoring depleted fish and invertebrate populations. Because open ocean species as well as fishing fleets move around, however, the effectiveness of reserves to help species living in the open ocean is equivocal. Reserves, although helpful, are not enough; effective conservation will require intelligent consumer choices. The Monterey Bay Aquarium is making an attempt to educate the public regarding sustainable harvesting of fishes. Toward this end the aquarium offers a free "Seafood Watch Card" that takes into account the sustainability of the fishery as well as the ecology of the species; then rates that species accordingly. Informed consumers can make more intelligent purchasing dismissed. As with concerns over extinction, global warming is not an unfounded notion propounded by aging hippies or ecoterrorists. Rather, it is a measurable phenomenon that concerns scientists around the world. Warming has been shown to increase the spread of infectious diseases; and in concert with the overharvesting of resources terrestrial, freshwater or marine, temperature change can have synergistic affects that lead to more extinctions and loss of biodiversity. Warming causes bleaching in coral reefs as well as some sea anemones. Coral reefs fringe no less than a sixth of the world's coastlines. They are species-rich and more biologically diverse than any other shallow-water marine ecosystem.

An obvious way to decrease human contribution to global warming is to restrict the release of greenhouse gases. Now, as we contemplate using the deep sea to store excess carbon dioxide, scientists are asked to assess the risks. Certainly sequestering carbon in the deep sea seems a logical way to decrease atmospheric input and concomitant warming. This approach, however, is only a bandage solution because it fails to address causative agents. Moreover, many of the lower metazoans discussed in this chapter live in the deep sea. Deep sea organisms generally have slow metabolism and difficulty dealing with even minor changes in the acidity or alkalinity of sea water. Dumping carbon dioxide into the deep sea causes pH changes of a little to a lot depending on proximity. It im-

pairs the metabolism of deep sea animals and weakens their exoskeletons, either of which causes increased mortality. As a pool of carbon dioxide forms in the deep sea, such destructive changes are not limited to the benthic fauna living on top but extend to the benthic infauna as well—the habitat occupied by gnathostomulids, loriciferans and their metazoan kin.

Humans claim to cherish our natural environment, yet each year we lose between 14,000 and 40,000 species from tropical forests alone. Between one-third to one-half of the land surface has been transformed by our species; we use more than half of the accessible freshwater. Gone with those species may be life-saving medicines, models for research, or services to the ecosystem that sustain our quality of life. Not that of future generations, but our own. Through it all we must remember the importance of biodiversity and conservation. Conservation is not a luxury; rather, it has been a luxury for humankind to progress as far as the twenty-first century without putting proper emphasis on conservation. Designating reserves can no longer be an opportunistic action performed at the whim of politicians with financial ties to business and industry. Can conservation be a priority for the twenty-first century? Humans are too knowledgeable for excuses and too skilled to do nothing. What is biodiversity worth? What price conservation? Are these questions that science can answer? When it does, are we willing to listen?

Resources

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Other

Global Warming: Early Warning Signs. <http//www .climatehotmap.org>

IUCN Red List of Threatened Species. <http://www .redlist.org>

Seafood Watch Regional Card. Monterey Bay Aquarium. http://www.mbayaq.org/cr/cr_seafoodwatch/sfw_regional .asp>

Union of Concerned Scientists. <http://www.ucsusa.org>

Organizations

IUCN Species Survival Commission. Rue Mauverney 28, Gland, 1196 Switzerland. Phone: +41 (22) 999 0000. Fax: +41 (22) 999 0002. E-mail: [email protected] Web site: <http:// www.iucn.org>

Rob Sherlock, PhD

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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