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14 THREATS TO BIOLOGICAL DIVERSITY AS THE EARTH WARS Robert L. Peters This paper will describe how global warming I acting synergistically with habitat destruction and other human-caused threats, is likely to cause a wave of extinctions over the next 50 or so years. BIOLOGICAL DIVERSITY In the last 10 years a new concept has come to organize the thoughts of those concerned with the conservation of living things (see Soule, 1985~. This concept is that of biological diversity, which means the variety of biological organisms in the world, including animals, plants, fungi, and bacteria. Biological diversity includes not only the species themselves, like tigers, lions, and the greater Antillean nightjar (Caprimulgus cubanensis), but also the genetic variation within species that gives apples, for example, both green and red individuals. Addi- tionally, biological diversity includes the different ecological func- tions these organisms perform, from fixing nitrogen in the soil to cleansing the water we drink. The idea of biodiversity is the crystallization of an evolution in thought about how to conserve living things. Historically, protection measures focused on animal or plant species that were recognized as being important economic or sport resources--a species-focused approach to conservation. Although species efforts are still appropriate in a variety of circumstances, this view has since broadened as our under- standing of the importance of even apparently insignificant organisms has grown. Conservation now focuses primarily on conserving entire interde- pendent webs of species--an ecosystem approach. We now know that not only are all the biological pieces of the planetary jigsaw puzzle inter- dependent, but also that human society itself, even in an age of gene splicing, engineered food crops, and modern medicine, is vastly more dependent than most people realize upon the natural diversity of living organisms . The potential gifts hidden in the array of wild species are rich: There is the genetic variability found in wild strains of common food crops, essential for constant infusion of disease- and pest-resistant characteristics into global agriculture. There are hundreds of poten- tial new food crops, many superior to ones presently in widespread use. 139
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140 There are new medicines, insecticides, and materials for industry. In the varied fungi and bacteria are blueprints for genetic engineering. Possibly even more important, the existing natural ecosystems give us what are called ecosystem services: recycling of nutrients, restoration of tired soils, production of oxygen, and even the production of rain (Ehrlich and Mooney, 1983~. These are just some of the necessary func- tions supplied by the natural world. Their value in any one year is incalculable: In dollars, a single plant species may be worth billions to agriculture. In betterment of the human condition, a single obscure plant like the rosy periwinkle of Madagascar may save thousands of lives through the gift of medicine. Although our knowledge of most species is overshadowed by our ignor- ance--indeed, most of the possibly 30 million species of organisms have not even been named--by extrapolation from species that have been inves- tigated we can safely say that a large number are valuable. For example, of some 1500 previously uninvestigated plant species assayed for pharmacological activity, at least 15 percent promised medicinal importance (Caufield, 1985~. Further, although our knowledge of eco- logical interactions is limited, we do know that species, like symbiotic fungi, that at first glance seem insignificant may be vital to ecosystem functioning. Clearly humans would do well to save as many species as possible. Aldo Leopold said, "If the biota, in the course of aeons, has built something we like but do not understand, then who but a fool would discard seemingly useless parts?" (Leopold, 1953~. Valuable though they are, a large percentage of biological resources are being destroyed by the increasing demands of growing human popula- tions and economic development. Throughout the world, populations of most wild species are being decreased and fragmented, often to the degree that species of even present economic importance can no longer support viable industries, often to the degree that extinctions occur. For example, the blue pike (Stizostedion vitreum glaucum), a dominant fish of the U.S. Great Lakes, provided over 1 billion pounds of protein between 1888 and 1962, only to become extinct suddenly around 1965, most probably due to a combination of overfishing, pollution, and introduced species (Ono et al., 1983~. The best estimate of total world extinctions is still the estimate made by Lovejoy in 1980 for the Global 2000 report. He estimated, based on deforestation rates, that between 15 and 20 percent of all species on earth would become extinct by the year 2000 due to destruction of tropi- cal forests alone (Lovejoy, 1980~. This gives us a rough percentage rate, but to estimate absolute numbers of extinctions we need to know how many species exist on earth. Until the 1980s, estimates ranged between roughly 3 million and 10 million species. However, T. L. Erwin of the Smithsonian Institute has recently found that there are huge numbers of arthropod species, particularly beetles, high up in the Amazon rain forest canopy (1982, 1983~. This has led to an upward revision of the world total to perhaps 30 million species. If this estimate is correct and given Lovejoy's projections, it means that between 1.5 million and 6 million species will be lost due to habitat destruction alone by the end of this century. This estimate is probably somewhat low because deforestation rates have increased substantially since Lovejoy made his
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141 estimates (see below). Although population diminution or extinction of any particular species may hardly be noted, particularly if its contribution to human society is subtle or potential only, the cumulative effect of the pro- cesses of biological impoverishment may be easily seen in many parts of the world, from the arid Sahel region of Africa, where overgrazing and tree cutting have contributed to desertification and attendant human misery, to the United States, where a variety of species, including the California condor (GYmnogyps californianus), the largest bird of prey in North America, hover on the edge of extinction. The primary cause of species loss today is habitat destruction. At least 71,000 km2 Of primary tropical forest were being cleared yearly throughout the world at the time of the last definitive estimate in 1982 (Lanly, 19829. However this rate is increasing rapidly, as at least 77,000 km2 Of virgin forest were cleared and burned in 1977 in the Brazilian Amazon alone (Simons, 1988~. The result of such habitat destruction is to erase some species entirely and to leave others as remnant populations surviving in habitat fragments surrounded by cleared and hostile land. Understanding the effects of this fragmentation is key to under- standing how climate change will impact wild ecosystems in the disturbed landscape of the twentieth and twenty-first centuries. Fragmentation by definition makes populations smaller and isolated from each other, and small, isolated populations are at much greater risk from climate change than are larger ones. (Other factors that reduce populations, like pol- lution and hunting, will also make species more vulnerable to climate change, but habitat destruction will have the greatest effect.) To- gether, habitat destruction and global warming will act synergistically to cause many more extinctions than either will alone, as detailed below in the section on "Synergy of Habitat Destruction and Climate Change." THE NATURE OF THE ECOLOGICALLY SIGNIFICANT CAGES There is widespread consensus that ecologically significant global warming will occur during the next century. For example, Hansen et al. (1988a) have said, "we can confidently state that major greenhouse climate changes are a certainty." It is expected that within the next 40 years greenhouse trace gases in the atmosphere, including carbon dioxide (CO2), chlorofluorocarbons, and methane, will reach a concen- tration equivalent to double the preindustrial concentration of CO2. The National Academy of Sciences and others have estimated that this concen- tration of greenhouse gases will be sufficient to raise the earth's temperature by 3 + 1.5°C (Hansen et al., 1988b; NRC, 1983, 1987; Schneider and Londer, 1984; WHO, 1982~. More recent estimates suggest that warnings as high as 4.2 + 1.2°C (Schlesinger, 1989), or even 8 to 10°C (Lashof, 1989), cannot be ruled out. Because of a time lag caused by thermal inertia of the oceans, some of this warming will be delayed by 30 to 40 years beyond the time that a doubling equivalent of CO2 is reached (EPA, 1988), but substantial warming could occur soon--the Goddard Institute for Space Studies (GISS) model projects a 2°C rise by
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142 2020 A.D. (Rind, 1989). Such general circulation models have many un- certainties, but they provide the best estimates possible. As discussed below, this transitional warming would cause profound ecological change well before 3 or 4°C is reached--warming of less than 1°C would have substantial ecological effects. It should be stressed that although projections can be made about global averages, regional projections are much less certain (Schneider, 1988~. It is known that warming will not be even over the earth, with warming likely to be greater in the high latitudes, for example, than in the low latitudes (Hansen et al., 1988a). Regional and local peculiari- ties of typography and circulation will play a strong role in determin- ing local climates. For the purpose of discussion in this paper, I will take average global warming to be 3°C, since this is a commonly used benchmark, but it must be recognized that additional warming well beyond 3°C may be reached during the next century if the production of anthropogenic greenhouse gases continues. I will also make the conservative assump- tion that 3°C warming will not be reached until 2070 A.D. Additional warming or faster warming would cause additional biological disruption beyond that laid out in this paper. The threats to natural systems are serious for the following reasons. First, 3°C of warming would present natural systems with a warmer world than has been experienced in the past 100,000 years (Schneider and Lander, 19841. An increase of 4°C would make the earth its warmest since the Eocene, 40 million years ago (Barron, 1985; see Webb, 1990~. This warming would not only be large compared to recent natural fluctuations, but it would also occur very rapidly, perhaps 15 to 40 times faster than past natural changes (Gleick et al., 1990~. For reasons discussed below, such a rate of change may exceed the ability of many species to adapt. Even widespread species are likely to have drastically curtailed ranges, at least in the short term. Moreover, human encroachment and habitat destruction will make wild populations of many species small and vulnerable to local climate changes. Second, ecological stress would not be caused by temperature rise alone. Changes in global temperature patterns would trigger widespread alterations in rainfall patterns (Hansen et al., 1981; Kellogg and Schware, 1981; Manabe et al., 1981), and we know that for many species precipitation is a more important determinant of survival than is temperature per se. Indeed, except at the tree line, rainfall is the primary determinant of vegetation structure, trees occurring only where annual precipitation is in excess of 300 mm (Woodward, 1990~. Because of global warming, some regions would see dramatic increases in rain- fall, and others would lose their present vegetation because of drought. For example, the U.S. Environmental Protection Agency (EPA, 1988) concluded, based on several studies, that a long-term drying trend is likely in the midlatitude, interior continental regions during the summer. Specifically, based on rainfall patterns during past warming periods, Kellogg and Schware (1981) projected that substantial decreases in rainfall in North America's Great Plains are possible--perhaps as much as 40 percent by the early decades of the next century. Other environmental factors important in determining vegetation type
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143 and health would change because of global warming: Soil chemistry would change (Kellison and Weir, 1987), as, for example, changes in storm patterns alter leaching and erosion rates (Harte et al., 1990~. In- creased CO2 concentrations may accelerate the growth of some plants at the expense of others (NRC, 1983; Strain and Bazzaz, 1983), possibly destabilizing natural ecosystems. And rises in sea level may inundate coastal biological communities (NRC, 1983; Hansen et al., 1981; Hoffman et al., 1983; Titus et al., 1984~. As mentioned, it is generally concluded from a variety of computer projections that warming will be relatively greater at higher latitudes (Hansen et al., 19879. This suggests that, although tropical systems may be more diverse and are currently under great threat because of habitat destruction, temperate zone and arctic species may ultimately be in greater jeopardy from climate change. Arctic vegetation would experience widespread changes (Edlund, 1987~. A recent attempt to map climate-induced changes in world biotic communities projected that high- latitude communities would be particularly stressed (Emanuel et al., 1985), and boreal forest, for example, was projected to decrease by 37 percent in response to global warming of 3°C. A final point, important in understanding species response to climate change, is that weather is variable, and extreme events, like droughts, floods, blizzards, and hot or cold spells, may have greater effects on species distributions than does average climate per se (e.g., Knopf and Sedgwick, 1987~. For example, in northwestern forests global warming is expected to increase the frequency of fires, leading to rapid alteration of forest character (Franklin, 1990~. SPECIES' RANGES SHIFT IN RESPONSE TO CLIMATE CHANGE We know that when temperature and rainfall patterns change, species' ranges change. Not surprisingly, species tend to track their climatic optima, retracting their ranges where conditions become unsuitable while expanding them where conditions improve (Peters and Darling, 1985; Ford, 19829. Even very small temperature changes of less than 1°C within this century have been observed to cause substantial range changes. For example, the white admiral butterfly (Ladoga camilla) and the comma butterfly (Polygonia c-album) greatly expanded their ranges in the British Isles during the past century as the climate warmed approxi- mately 0.5°C (see Ford, 1982~. The birch (Betula pubescens) responded rapidly to warming during the first half of this century by expanding its range north into the Swedish tundra (Kullman, 1983~. On a larger ecological and temporal scale, entire vegetation types have shifted in response to past temperature changes no larger than those that may occur during the next 100 years or less (Baker 1983; Bernabo and Webb, 1977; Butzer, 1980; Flohn, 1979; Muller, 1979; Van Devender and Spaulding, 1979~. As the earth warms, species tend to shift to higher latitudes and altitudes. From a simplified point of view, rising temperatures have caused species to colonize new habitats toward the poles, often while their ranges contracted away from the equator as conditions there became unsuitable.
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144 ; Orl91nal A t ~ FIGURE 14.1 (a) Initial distribution of two species, A and B. whose ranges largely overlap. (b) In response to climate changes latitudinal shifting occurs at species-specific rates, and the ranges disassociate. During several Pleistocene interglacials the temperature in North America was apparently 2 to 3°C higher than it is now. Sweet gum trees (Liquidambar) grew in southern Ontario (Wright, 19J1~; Osage oranges (Maclura) and papaws (Asimina) grew near Toronto, several hundred kilometers north-of their present distributions; manatees swam in New Jersey; and tapirs and peccaries foraged in North;Carolina (Dorf, 1976~. During the last of these interglacials, which ended more than 100,000 years ago, vegetation in northwestern Europe, which is now boreal, was predominantly temperate (in Critchfield, 1980~. Other significant changes in species' ranges have been caused by altered precipitation accompanying past global warming, including expansion of prairie in the American Midwest during a global warming episode approximately 7,000 years ago (Bernabo and Webb, 1977~. It should not be imagined, because species tend to shift in the same general direction, that existing biological communities move in syn- chrony. Conversely, because species shift at different rates in response to climate change, communities often disassociate into their component species (Figure 14.1~. Recent studies of fossil packrat (Neotoma spp.) middens in the southwestern United States show that during the wetter, moderate climate of 22,000 to 12,000 years ago, there was not a concerted shift of plant communities. Instead, species responded individually to climatic change, forming stable but, by present-day standards, unusual assemblages of plants and animals (Van Devender and Spaulding, 1979~. In eastern North America, too, post- glacial communities were often ephemeral associations of species, changing as individual ranges changed (Davis, 1983; Graham, 1986~. A final aspect of species response is that species may shift aptitudinally as well as latitudinally. When climate warms, species shift upward. Generally, a short climb in altitude corresponds to a major shift in latitude: The 3°C cooling of a 500-m rise in elevation equals roughly the cooling achieved by a 250-km northward shift in latitude (MacArthur, 1972~. Thus, during the middle Holocene, when temperatures in eastern North America were 2°C warmer than they are at
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500m ~B~ \ ton —~ | FIGURE 14.2 (a) Present attitudinal distribution of three species, A, B. and C. (b) Species distribution after a 500-m shift in altitude in response to a 3°C rise in temperature (based on Hopkin's bioclimatic law; MacArthur, l972!~ . Species A becomes locally extinct. Species B shifts upward and the total area it occupies decreases. Species C becomes fragmented and restricted to a smaller area, while species D successfully colonizes the lowest-altitude habitats. present, hemlock (Tsuga canadensis) and white pine (Pinus strobus) were found 350 m higher on mountains than they are today (Davis, 1983~. Because mountain peaks are smaller than bases, as species shift upward in response to warming, they typically occupy smaller and smaller areas, have smaller populations, and may thus become more w lnerable to genetic and environmental pressures (see Murphy and Weiss, 1990~. Species originally situated near mountaintops may have no habitat to move up to and may be entirely replaced by the relatively thermophilous species moving up from below (Figure 14.2~. Examples of past extinc- tions attributed to upward shifting include alpine plants once living on mountains in Central and South America, where vegetation zones have shifted upward by 1000 to 1500 m since the last glacial maximum (Flen- ley, 1979; Heusser, 1974) MAGNITUDE OF PROJECTED LATITUDINAL SHIFTS If the proposed CO2-induced warming occurs, species shifts similar to those in the Pleistocene will occur, and vegetation belts will move hundreds of kilometers toward the poles (Davis and Zabinski, 1990; Frye, 1983; Peters and Darling, 1985~. A 300-km shift in the temperate zone is a reasonable minimum estimate for a 3°C warming, based on the
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146 positions of vegetation zones during analogous warming periods in the past (Dorf, 1976; Furley et al., 1983~. Additional confirmation that shifts of this magnitude or greater may occur comes from attempts to project future range shifts for some species by looking at their ecological requirements. For example, the forest industry is concerned about the future of commercially valuable North American species, like the loblolly pine (Pinus taeda Lob. This species is limited on its southern border by moisture stress on seed- lings. Based on its physiological requirements for temperature and moisture, Miller et al. (1987) projected that the southern range limit of the species would shift approximately 350 km northward in response to a global warming of 3°C. Davis and Zabinski (1990) have projected possible northward range withdrawals among several North American tree, species, including sugar maple (Acer saccharum) and beech (Fagus grandi- folia), of from 600 km to possibly as much as 2000 km in response to the warming caused by a doubled CO2 concentration. Beech would be most responsive, withdrawing from its present southern extent along the Gulf Coast and retreating into Canada. MECHANISMS UNDERLYING RANGE SHIFTS The range shifts described above are the sum of many local processes of extinction and colonization that occur in response to climate-caused changes in suitability of habitats. These changes in habitat suitabil- ity are determined by both direct climate effects on physiology, includ- ing temperature and precipitation, and indirect effects secondarily caused by other species, themselves affected by temperature. There are numerous examples of climate directly influencing survival and thereby distribution. In animals, the direct range-limiting effects of excessive warmth include lethality, as in corals (Glynn, 1984), and interference with reproduction, as in the large blue butterfly, Macu- linea arion (Ford 1982~. In plants, excessive heat and associated decreases in soil moisture may decrease survival and reproduction. Coniferous seedlings, for example, are injured by soil temperatures over 45°C, although other types of plants can tolerate much higher tempera- tures (see Daubenmire, 1962~. Many plants have their northern limits determined by minimum temperature isotherms below which some key physio- logical process does not occur. For instance, the grey hair grass (Corynephorus canescens) is largely unsuccessful at germinating seeds below 15°C and is bounded to the north by the 15°C July mean isotherm (Marshall, 1978~. Moisture extremes exceeding physiological tolerances also determine species' distributions. Thus, the European range of the beech tree (Fagus sylvatica) ends to the south where rainfall is less than 600 mm annually (Seddon, 1971), and dog's mercury (Mercurialis perennis), an herb restricted to well-drained sites in Britain, cannot survive in soil where the water table reaches as high as 10 cm below the soil surface (see Ford, 1982~. The physiological adaptations of most species to climate are conser- vative, and it is unlikely that most species could evolve significantly new tolerances in the time allotted to them by the coming warming trend.
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147 Indeed, the evolutionary conservatism in thermal tolerance of many plant and animal species--beetles, for example (Coope, 1977~--is the underlying assumption that allows us to infer past climates from faunal and plant assemblages. Interspecific interactions altered by climate change will have a major role in determining new species distributions. Temperature can influence predation rates (Rand, 1964), parasitism (Aho et al., 1976), and competitive interactions (Beauchamp and Ullyott, 1932~. Changes in the ranges of tree pathogens and parasites may be important in determin- ing future tree distributions (Winget, 1988~. Soil moisture is a criti- cal factor in mediating competitive interactions among plants, as is the case where the dog's mercury (Mercurialis perennis) excludes oxlip (Pri- mula elation) from dry sites (Ford, 1982~. Given the new associations of species that occur as climate changes, many species will face ''exotic'' competitors for the first time. Local extinctions may occur as climate change causes increased frequencies of droughts and fires, favoring invading species. One species that might spread, given such conditions, is Melaleuca quinquenervia, a bamboo-like Australian eucalypt. This species has already invaded the Florida Everglades, forming dense monotypic stands where drainage and frequent fires have dried the natural marsh community (Courtenay, 1978; Myers, 1983). The preceding effects, both direct and indirect, may act in synergy, as when drought makes a tree more susceptible to insect parasitism. DISPERSAL RATES AND BARRIERS The ability of species to adapt to changing conditions will depend to a large extent on their ability to track shifting climatic optima by dispersing colonists. In the case of warming, a North American species, for example, would most likely need to establish colonies to the north or at higher elevations. Survival of plant and animal species would therefore depend either on long-distance dispersal of colonists, such as seeds or migrating animals, or on rapid iterative colonization of nearby habitat until long-distance shifting results. A plant's intrinsic ability to colonize will depend on its ecological characteristics, including fecundity, viability and growth characteristics of seeds, nature of the dispersal mechanism, and ability to tolerate selfing and inbreeding upon colonization. If a species' intrinsic colonization ability is low, or if barriers to dispersal are present, extinction may result if all of its present habitat becomes unsuitable. There are many cases where complete or local extinction has occurred because species were unable to disperse rapidly enough when the climate changed. For example, a large, diverse group of plant genera, including water-shield (Brassenia), sweet gum (Liquidambar), tulip tree (Lirioden- dron), magnolia (Magnolia), moonseed (Menispermum), hemlock (Tsuga), arbor vitae (ThuJa), and white cedar (Chamaecyparis), had a circumpolar distribution in the Tertiary (Tralau, 1973~. But during the Pleistocene ice ages, all became extinct in Europe while surviving in North America. Presumably, the east-west orientation of such barriers as the Pyrenees,
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148 the Alps, and the Mediterranean, which blocked southward migration, was partly responsible for their extinction (Tralau, 1973~. Other species of plants and animals thrived in Europe during the cold periods but could not survive conditions in postglacial forests. One such previous! y widespread dung beetle, Aphodius hodereri, is now extinct throughout the world except in the high Tibetan plateau where conditions remain cold enough for its survival (Cox and Moore, 198S). Other species, like the Norwegian mugwort (Artemisia novegica) and the springtail Tetracanthella arctica, now live primarily in the boreal zone but also survive in a few cold, mountaintop refugia in temperate Europe (Cox and Moore, 1985~. These natural changes were slow compared to changes predicted for the near future. The change to warmer conditions at the end of the las~- ice age spanned several thousand years yet is considered rapid by geologic standards (Davis, 1983~. We can deduce that, if such a slow change was too fast to allow many species to adapt, the projected warming--possibly 40 times faster--will have more severe consequences. For widespread, abundant species, like the loblolly pine (modeled by Miller et al., 1987), even substantial range retraction might not threaten extinction; but rare, localized species, whose entire ranges might become unsuitable, would be threatened unless dispersal and colonization were successful. Even for widespread species, major loss of important ecotypes and associated germplasm is likely (see Davis and Zabinski, 1990~. A key question is whether the dispersal capabilities of most species prepare them to cope with the coming rapid warming. If the climatic optima of temperate zone species do shift hundreds of kilometers toward the poles within the next 100 years, then these species will have to colonize new areas rapidly. To survive, a localized species whose present range becomes unsuitable might have to shift poleward at several hundred kilometers or faster per century. Although some species, such as plants propagated by spores or "dust" seeds, may be able to match these rates (Perring, 1965), many species could not disperse quickly enough to compensate for the expected climatic change without human assistance (see Rapoport, 1982), particularly given the presence of dispersal barriers. Even wind-assisted dispersal may fall short of the mark for many species. In the case of the Engelmann spruce (Picea engelmanni~) a tree with light wind-dispersed seeds fewer than 5 per- , , , cent of seeds travel even 200 m downwind, leading to an estimated migra- tion rate of 1 to 20 km per century (Seddon, 1971~; this reconciles well with rates derived from fossil evidence for North American trees of between 10 and 45 km per century (Davis and Zabinski, 1990; Roberts, 19899. As described in the next section, many migration routes will likely be blocked by the cities, roads, and fields replacing natural habitat. Although many animals may be, in theory, highly mobile , the dis - tribution of some is limited by the distributions of particular plants, i.e., suitable habitat; their dispersal rates may therefore be deter- mined largely by those of co-occurring plants. Behavior may also re- strict dispersal even of animals physically capable of large movements. Dispersal rates below 2.0 km per year have been measured for several
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149 species of deer (Rapoport, 1982), and many tropical deep-forest birds simply do not cross even very small unforested areas (Diamond, 1975~. On the other hand, some highly mobile animals may shift rapidly, as have some European birds (see Edgell, 1984~. Even if animals can disperse efficiently, suitable habitat may be reduced under changing climatic conditions. For example, it has been suggested that tundra nesting habitat for migratory shore birds might be reduced by high-arctic warming (Myers, 1988~. SYNERGY OF HABITAT DESTRUCTION AND CLIMATE CHANGE We know that even slow, natural climate change caused species to become extinct. What is likely to happen given the environmental con- ditions of the coming century? Some clear implications for conservation follow from the preceding discussion of dispersal rates. Any factor that would decrease the prob- ability that a species could successfully colonize new habitat would increase the probability of extinction. Thus, as previously described, species are more likely to become extinct if there are physical barriers to colonization, such as oceans, mountains, and cities. Further, species are more likely to become extinct if their remaining populations are small. Smaller populations mean that fewer colonists can be sent out and that the probability of successful colonization is smaller. Species are more likely to become extinct if they occupy a small geographic range, which is less likely than a larger range to offer some suitable remaining refuge when climate changes. Also, if a species has lost much of its range because of some other factor, like clearing of the richer and moister soils for agriculture, it is possible that re- maining populations are located in poor habitat and are therefore more susceptible to new stresses. For many species, all of these conditions will be met by human- caused habitat destruction, which increasingly confines the natural biota to small patches of original habitat, patches isolated by vast areas of human-dominated urban or agricultural lands. Habitat destruction in conjunction with climate change sets the stage for an even larger wave of extinction than that previously imagined, based on consideration of human encroachment alone. Small, remnant populations of most species, surrounded by cities, roads, reservoirs, and farmland, will have little chance of reaching new habitat if climate change makes the old unsuitable. Few animals or plants would be able to cross Los Angeles on the way to new habitat. Figure 14.3 illustrates the combined effects of habitat loss and warming on a hypothetical reserve. AMELIORATION AND MITIGATION Because of the difficulty of predicting regional and local changes, conservationists and reserve managers must deal with increased uncer- tainty in making long-range plans. However, even given imprecise
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150 Range Limit b :~ < INFORMER \ ~RESERVE,, ~ old RL ~—..—I t —_~_ FIGURE 14.3 Climatic warming may cause species within biological reserves to disappear. Hatching indicates: (a) species distribution before either human habitation or climate change (range limit, RL, indicates southern limit of species range); (b) fragmented species distribution after human habitation but before climate change; (c) species distribution after human habitation and climate change. regional projections ? informed guesses can be made at least about the general direction of change, specifically that most areas will tend to be hotter and that continental interiors in particular are likely to experience decreased soil moisture. How might the threats posed by climatic change to natural commu- nities be mitigated? One basic truth is that the less populations are reduced by development now, the more resilient they will be to climate change. Thus, sound conservation now, in which we try to conserve more than just the minimum number of individuals of a species necessary for present survival, would be an excellent way to start planning for climate change. In terms of responses specifically directed at the effects of climate change, the most environmentally conservative response would be
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151 to halt or slow global warming by cutting back on production of fossil fuels, methane, and chlorofluorocarbons. Extensive planting of trees to capture CO2 could help slow the rise in CO2 concentrations (Sedjo, 1989~. Nonetheless, even were the production of all greenhouse gases stopped today, it is very likely that there are now high enough concen- trations in the air to cause ecologically significant warming after a brief lag period (Rind, 1989~. Therefore, those concerned with the conservation of biological diversity must begin to plan mitigation activities. To make intelligent plans for siting and managing reserves, we need more knowledge. We must refine our ability to predict future conditions in reserves. We also need to know more about how temperature, precipi- tation, CO2 concentrations, and interspecific interactions determine range limits (e.g., Picton, 1984; Randall, 1982) and, most important, how they can cause local extinctions. Reserves that suffer from the stresses of altered climatic regimes will require carefully planned and increasingly intensive management to minimize species loss. For example, modifying conditions within re- serves may be necessary to preserve some species; depending on changes in moisture patterns, irrigation or drainage may be needed. Because of changes in interspecific interactions, competitors and predators may need to be controlled and invading species weeded out. The goal would be to maintain suitable conditions for desired species or species assem- blages, much as the habitat of Kirtland's warbler is periodically burned to maintain pine woods (Leopold, 1978~. In attempting to understand how climatically stressed communities may respond and how they might be managed to prevent the gradual depau- perization of their constituents, restoration studies, or more properly, "community creation" experiments can help. Communities may be created outside their normal climatic ranges to mimic the effects of climate change. One such "out-of-place" community is the Leopold Pines at the University of Wisconsin Arboretum in Madison, where there is periodi- cally less rainfall than in the normal pine range several hundred kilometers to the north (Jordan, 1988~. Researchers have found that, although the pines themselves do fairly well once established at the Madison site, many of the other species that would normally occur in a pine forest, especially the various herbs and small shrubs, have not flourished, despite several attempts to introduce them (Anderson et al., 1969~. If management measures are unsuccessful and old reserves do not retain necessary thermal or moisture characteristics, individuals of disappearing species might be transferred to new reserves. For example, warmth-intolerant ecotypes or subspecies might be transplanted to re- serves nearer the poles. Other species may have to be periodically reintroduced in reserves that experience occasional climate extremes severe enough to cause extinction but where the climate would ordinarily allow the species to survive with minimal management. Such transplanta- tions and reintroductions, particularly involving complexes of species, will often be difficult, but some applicable technologies are being developed (Botkin, 1977; Lovejoy, 1985~.
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152 To the extent that we can still establish reserves, pertinent in- formation about changing climate and subsequent ecological response should be used in deciding how to design and locate them to minimize the effects of changing temperature and moisture. o The existence of multiple reserves for a given species or com- munity type increases the probability that, if one reserve becomes un- suitable for climatic reasons, the organisms may still be represented in another reserve. o Reserves should be heterogeneous with respect to topography and soil types, so that even given climatic change, remnant populations may be able to survive in suitable microclimatic areas. Species may survive better in reserves with wide variations in altitude, since, from a cli- , matic point of view, a small attitudinal shift corresponds to a large latitudinal one. Thus, to compensate for a 2°C rise in temperature, a northern hemisphere species can achieve almost the same result by in- creasing its altitude only some 500 m as it would by moving 300 km to the north (MacArthur, 1972~. O Corridors between reserves, important for other conservation reasons, would allow some natural migration of species to track climate shifting. Corridors along attitudinal gradients are likely to be most practical because they can be relatively short compared with the longer distances necessary to accommodate latitudinal shifting. o As climatic models become more refined, pertinent information should be taken into consideration in making decisions about where to site reserves in order to minimize the effects of temperature and moisture changes. In the northern hemisphere, for example, where a northward shift in climatic zones is likely, it makes sense to locate reserves as near the northern limit of a species' or community's range as possible, rather than farther south, where conditions are likely to become unsuitable more rapidly. o Maximizing the size of reserves will increase long-term persistence of species by increasing the probability that suitable microclimates exist, by increasing the probability of attitudinal variation, and by increasing the latitudinal distance available to shifting populations. o Flexible zoning around reserves may allow us to actually move reserves in the future to track climatic optima, as, for example, by trading present rangeland for reserve land. The success of this strategy, however, would depend on a highly developed restoration technology capable of guaranteeing, in effect, the portability of species and whole communities. CONCLUSION The best solutions to the ecological upheaval resulting from climatic change are not yet clear. In fact, little attention has been paid to the problem. What is clear, however, is that these climatological changes would have tremendous impact on communities and populations isolated by development and by the middle of the next
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153 century might dwarf any other consideration in planning for reserve management. The problem may seem overwhelming. One thing, however, is worth keeping in mind: The more fragmented and the smaller populations Of species are, the less resilient they will be to the new stresses brought about by climate change. Thus, one of the best things that can be done in the short-term is to minimize further encroachment of development on existing natural ecosystems. Further, we must refine our climatological predictions and increase our understanding of how climate affects species, both individually and in their interactions with each other. Such studies may allow us to identify those areas where communities will be most stressed, as well as alternate areas where they might best be saved. Meanwhile, efforts to improve techniques for managing communities and ecosystems under stress, and also for restoring them when necessary, must be carried forward energetically. ACKNOWLEDGMENTS The bulk of the text on global warming was previously published as an article in Forest Ecology and Management, in a special 1989 volume containing the proceedings of the symposium on Conservation of Diversity in Forest Ecosystems, University of California at Davis, July 25, 1988. It draws heavily on other previously published versions, including those in Endangered Species Update (Vol. 5, No. 7, pp. 1-8) and Preparing for Climate Change: Proceedings of the First North American Conference on Preparing for Climate Change: A Cooperative Approach (J.C. Topping, Jr., ea., Government Institutes, Washington, D.C.~. Many of the ideas and the three figures derive from Peters and Darling (1985~; please see that pa- per also for a complete list of acknowledgments for help with this work. REFERENCES Aho, J.M., J.W. Gibbons, G.W. Esch. 1976. Relationship between thermal loading and parasitism in the mosquitofish. Pp. 213-218 in G.W. Esch and R.W. McFarlane, eds. Thermal Ecology II. Technical Information Center, Energy Research and Development Administration, Springfield, Va. Anderson, R.C., O.L. Loucks, and A.M. Swain. 1969. Herbaceous response to canopy cover, light intensity and throughfall precipitation in coniferous forests. Ecology 50:255-263. Baker, R.G. 1983. Holocene vegetational history of the western United States. Pp. 109-125 in H.E. Wright, Jr., ed. Late-Quaternary Environments of the United States. Volume 2. The Holocene. University of Minnesota Press, Minneapolis. Barron, E.J. 1985. Explanations of the Tertiary global cooling trend. Palaeogeography, Palaeoclimatology, Palaeoecology 50:17-40. Beauchamp, R.S.A., and P. Ullyott. 1932. Competitive relationships between certain species of fresh-water triclads. J. Ecol. 20:200-208.
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Representative terms from entire chapter: