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Wilson EO, Peter FM, editors. Biodiversity. Washington (DC): National Academies Press (US); 1988.
HAROLD A. MOONEY
Professor of Biological Sciences, Department of Biological Sciences, Stanford University, Stanford, California
Discussions on the loss of biological diversity are correctly focused on tropical regions because of the massive, rather recent alterations in the structure of these extensive biotic communities. The consequences of these alterations are many. There are of course no landscapes on Earth that have not been modified to some extent by the human species. Many of these landscapes have been totally altered from their prehuman configuration and functioning, and others appear less affected; however, none are protected from the types of global changes that are resulting from human-induced alterations of the Earth's atmosphere.
This section focuses on the nature and some of the consequences of alterations of nontropical biogeographic regions. The discussions are selective, concentrating on selected processes and organisms within a few systems. In Chapter 18, Franklin deals with temperate and boreal forests, which occupy 16% of Earth's land surface—an area equivalent to that covered by tropical forests (Waring and Schlesinger, 1985)—and which have provided to a large degree the timber and in part the fuel to support the growing human population. In the next chapter, Risser discusses the impact of humans on biological diversity in grasslands, the biome that has largely provided, either directly or indirectly, the food for the world's human population. Finally, in Chapter 20, Vitousek details the kinds of biotic changes that have resulted from human settlement on Hawaii and on oceanic islands in general—systems that have proven to be particularly susceptible to losses and additions of species.
Each chapter emphasizes somewhat different points. Franklin focuses on the consequences of structural diversity loss in forest ecosystems, drawing examples from the magnificent coniferous forests of the Pacific Northwest. Risser notes the low loss of species in the high-impact North American grasslands and the potentially high loss of ecotypes. He also discusses the variable consequences of different land-use patterns on species diversity. Vitousek relates the apparent devastating effects of species invaders on the endemics of the Hawaiian Islands, noting that although species diversity has actually increased, ecosystem types have been lost.
As an introduction to these chapters on threats to diversity in nontropical systems, I first compare the community diversity of tropical systems with those of temperate regions, providing plants as examples. I then focus more specifically on Mediterranean-climate (cool wet winter, dry summer climate) regions to balance the presentations on forests and grasslands. Mediterranean-climate regions, of which there are five in the world, are of special interest for two reasons: they rival tropical regions for their biological richness, and because they have had very different histories of human settlement, they serve as interesting comparison areas in studies to determine the human impact on biotic diversity.
Community Diversity in Tropical and Temperate Regions
The fact that tropical regions are biologically richer than temperate regions has been stated repeatedly: for example, Raven (1976) has noted that 65% of the world's 250,000 flowering plants are found there. Until recently, the tropics, particularly the lowland wet tropics, have remained one of the last areas that has not been subjected to extensive human exploitation. In temperate regions of the world, many of the natural ecosystems have been massively altered by human settlement and activities. By looking at some of these disturbed regions, we can assess the consequences of human activities on biological diversity and, to some extent, learn what we should expect in the tropics in the future. If we were to pick only one biome type to serve as a model of comparison, it should be the Mediterranean-climate regions of the world. These regions are remarkably diverse by any measure.
Gentry (1979) reported that the number of plant species he encountered in 0.1-hectare plots increased as he moved from dry tropical to wet tropical forests (Table 17-1). In his most diverse sites in Panama he encountered more than 150 species of woody plants thicker than 1 inch in diameter at breast height. In contrast, only 21 woody species were found in a temperate forest in Missouri. Data on total species counts in tropical forests have not been available. However, Whitmore (1986) reported the results of a survey in which 236 species of vascular plants were counted in a 0.01-hectare plot in Costa Rica; he estimated that ''one man decade would be required to enumerate one hectare" (Whitmore, 1986). Counts of all the vascular plants in sample plots in other climatic regions are available for comparison.
TABLE 17-1
Mean Numbers of Species per 0.1-Hectare Sample Area (Non-Mediterranean Sites Include Only Data for Woody Plants over 1 Inch in Diameter at Breast Height).
In the Mediterranean-climate region of Israel, Naveh and Whittaker (1979) found sites that included as many total species as woody species found by Gentry in Panama. The richest sites were those with some degree of current disturbance. Mediterranean-climate sites of the same size in other parts of the world also have relatively high species counts in comparison to counts of temperate-zone vegetation (Whittaker, 1977).
The bases for the high diversities among the different Mediterranean-type vegetations differ. In Israel, the diversity is accounted for mostly by herbaceous species, principally annuals, and is the result of human-driven "relatively rapid evolution under stress by drought, fire, grazing and cutting" (Naveh and Whittaker, 1979). In contrast, the high diversity of the South African fynbos (Mediterranean-climate scrubland) vegetation consists of woody species, of which there are few annuals. This type of vegetation has not been subject to a long history of human disturbance.
The data thus indicate that tropical systems are probably among the world's richest in terms of local, or alpha, diversity, but that the vegetation of Mediterranean-climate regions is also quite rich. In Mediterranean-climate regions the basis for the localized diversity can differ with the pattern of disturbance. In some systems with a long history of association with human activities, diversity has actually increased (Naveh and Whittaker, 1979).
Data on diversity at a given site indicate its structural dynamics as related to both evolutionary history and pattern of disturbance. We are just now beginning to appreciate the role of both natural disturbances and the impacts of humans in controlling community structure, including its diversity (Bazzaz, 1983). Such knowledge is essential for understanding and hence managing a given level of diversity.
Mediterranean-Climate Floristic Diversity
Data on local diversity are an indication of disturbance pattern and evolutionary history leading to niche diversification. Another view of the biotic richness of an area is the degree of endemism of the biota. Data on species numbers and degree of endemism for Mediterranean-climate regions form the basis for identifying them as critical sites for conservation. An indication of the diversity and uniqueness of Mediterranean-climate plant life is given below for South Africa, California, and the Mediterranean basin—areas that share unusually high biotic diversities but have dissimilar histories of human impact. For example, South Africa has large tracts of land dominated by the original species-rich shrubland, and the Mediterranean basin contains predominantly herb or shrub degradation forms of the original vegetation. The diversity of South Africa is threatened by development and the invasion of alien species; the Mediterranean basin diversity, by changes in land-use patterns.
South Africa
The Mediterranean-climate region (fynbos biome) of South Africa covers 75,000 square kilometers. This area includes 8,550 vascular plants (Macdonald and Jarman, 1984), three-quarters of which are endemic (Jarman, 1986). According to estimates by Hall (1978), the flora indigenous to the South African Cape, which is found in an area of 46,000 square kilometers, contains at least 6,000 higher plant species—a species richness three times that found in tropical regions of similar areas. This subregion has been considered one of the world's six distinctive floristic regions.
In the fynbos biome, 1,585 plant species are considered rare and threatened (Macdonald and Jarman, 1984), and 39 have recently become extinct (Jarman, 1986). Although the fynbos region occupies less than 1% of southern Africa, it contains 65% of the threatened plant species (Hall, 1979).
Much of the vegetation in this region has been destroyed by human activities, but not to the extent it has occurred in other Mediterranean-climate areas. In the lowland regions, only about 30% of the original vegetation remains, whereas in the mountains, approximately 80% of the vegetation remains intact. Overall, about 67% of the natural fynbos vegetation remains (Jarman, 1986). One threat to the native flora is the presence of alien, generally woody species, which have invaded about one-fourth of the native vegetation (Jarman, 1986). Of 70 critically threatened or recently extinct taxa, 23% are threatened by invading acacias, 8% by pines, and 2% by hakeas (Hall, 1979).
In summary, the South African Mediterranean-climate vegetation is as rich as any found on Earth. This richness is being threatened by human development, as everywhere, but also by a rather remarkable invasion of woody plants that are altering the basic functioning of these systems (Macdonald and Jarman, 1984).
California
There is rather complete information describing the biotic richness of the State of California, most of which falls within a Mediterranean-type climate. Although not as rich as South Africa in plant species, it certainly is one of the world's most biotically diverse areas. In an area of 411,000 square kilometers, there are more than 5,046 native vascular plant species, 30% of which are endemic. (In comparison, there are about 20,000 vascular plant species in the continental United States.) About one-tenth of the flora in these regions of California has recently become extinct or endangered. This represents 25% of all the extinct and endangered species of the United States as a whole (Raven and Axelrod, 1978).
California has suffered great losses of natural communities through human development of agriculture, industry, and housing, especially in coastal and valley regions. Entire ecosystems have evidently been irrevocably lost. One of the most spectacular examples of this is the native perennial grassland of the Central Valley and north coastal regions, which has been replaced by an annual grassland dominated by species mostly inadvertently introduced from the Mediterranean basin (Burcham, 1957). Raven and Axelrod (1978) estimate that more than 10% of the flora in these regions is now composed of naturalized aliens. Thus California, like other Mediterranean-climate regions, has an unusually diverse biota that is being threatened by human activities. But to a greater extent than in other regions, substantial areas of the state have been set aside as parks and preserves.
The Mediterranean Basin
The entire Mediterranean basin encompasses more than 2 million square kilometers and may include as many as 25,000 higher plant species, about half of which are endemic (Quezel, 1985). Of 2,879 species endemic to individual Mediterranean countries (excluding Syria, Lebanon, Turkey, and the Atlantic islands), 1,529 are rare (1,262) or threatened, and 300 are not categorized. If the Atlantic islands (Azores, Madeira, and the Canaries) are included, these figures increase to 3,583 endemics and 1,968 rare or threatened plant species (Leon et al., 1985)
In contrast to California and South Africa, where large areas of climax vegetation remain, much of the Mediterranean basin has been completely transformed from its native state. Naveh and Dan (1973, p. 387) reported that the region as a whole "is composed of innumerable variants of different degradation and regeneration stages." Since the impact of humans in this region has been so extensive for a long time, it is believed that the Mediterranean endemic has evolved under conditions of frequent disturbance or in depauperate microsites, such as rock outcrops (Gomez-Campo, 1985). Greuter (1979, p. 90) observed that "the rare threatened taxa are seldom members of the characteristic vegetation units as defined by the plant sociologists: they are marginal creatures living on the borderline of biota…." This general viewpoint has led to the following conclusion of Ruiz de la Torre (1985, p. 197): "Unlike the tropical rain forest, where most of the indigenous species can be conserved with climax formations under conditions of maximum stability, the Mediterranean region has been severely influenced by man and various other factors and is still very rich in species. Very few of these species are known to be part of Mediterranean climax vegetation. Most of them correspond to successional stages affected by either natural or artificial exploitation, and they should be conserved under the prevailing conditions of relative instability."
Increasing Biotic Diversity—the Invaders
As indicated above, plant diversity in Mediterranean-climate regions is among the world's richest in terms of numbers of species, but there have been losses of species and continuing threats of extinction to many others. However, there have also been additions of new species to these and other regions of the world. As shown in Table 17-2, the floras of certain islands, ranging from subarctic to tropical, have been enriched half again by species from other biographic regions. In mainland Mediterranean-climate regions such as California, and even to a greater extent in South Australia, there are also substantial numbers of invading species that have become naturalized, many maintaining large and dominating populations. In these regions, as elsewhere, these invading species are not distributed uniformly in the landscape but are generally associated with ecosystems that have experienced human impact. Organisms other than plants are also being enriched by the addition of species in these climates. In California, for example, 49 species have been added to the 132 indigenous inland fishes (Moyle, 1976).
TABLE 17-2
The Plant Invaders.
Thus in some cases, human disturbance can actually enrich biotic diversity. However, species counts in a given area give us little understanding of ecosystem functioning and how the invasions affect it. Some invaders may become the dominant species in the host-region ecosystem. Examples of this include a species of oat (Avena fatua) in the grasslands of California (Burcham, 1957) and brome grass ( Bromus tectorum) in the intermountain West (Mack, 1986). Many of the invaders are pest species of one sort or another and may cause economic havoc. These species of course receive considerable attention, and their biology and community role is generally well known. However, we generally know little about the effects of most invaders on the ecosystem or, for that matter, the effects of most species on natural communities.
Are these invaders enriching biotic diversity? They are when considered in absolute numbers of species. In many cases, however, they are impoverishing the biota by leading to species exclusions (Race, 1982) or even to extinctions. The invaders are generally symptoms of an abused landscape, one that has been disturbed and has generally lost some of its original productive capacity. The successful introduction of exotic mammals has often resulted in greatly perturbed ecosystem function and losses of indigenous species. In general, new community types are being added to the original ones that in turn are being reduced in extent. The landscapes are becoming more complex. Yet, when viewed on a more global scale, the biota is becoming less interesting because of homogenization. For example, geographically separate and distinctive biological regions are often invaded by the very same weedy species. As a result, regions such as parts of California and Chile, which once had only a few plant species in common, now share hundreds.
The maintenance of a diverse landscape, rich in community types and species, requires knowledge of the dynamics of ecosystems as well as the ecology of individual species. Since this information is generally lacking, attempts to conserve individual species or populations are still filled with surprises, even in preserves.
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