TRENDS IN ECOSYSTEM DIVERSITY
Biodiversity of Dry and Sub-Humid Land Ecosystems
What is a dryland?
Some 60% of the world's land surface may be considered as arid to some degree. In such areas shortage of available liquid water is a major constraint on living systems. Aridity may be defined, and measured, in a variety of ways. Probably the most useful at a global level is a measure of the ratio of precipitation - rain, snow, fog, dew etc - (P) to potential evapotranspiration (PET). The latter is essentially a function of temperature and number of daylight hours and represents the potential rate of loss of water from a unit area though evaporation from the soil and transpiration by plants. P/PET ratios are typically calculated on a monthly basis and then averaged over a year to produce a single P/PET ratio, known as an aridity index (AI). AI values greater than 0.65 are generally counted as humid.
All life requires water. On land the abundance and diversity of life as well as the kinds of life-form that exist in any given area are overwhelmingly influenced by two major factors: the amount of available moisture and the temperature. Variability in these factors, on daily, seasonal and inter-annual scales, is as important as long-term averages. Aridity is defined on the basis of the ratio of rainfall (and other precipitation) per unit area to the potential loss of water from that area through plant use and evaporation (see Box).
Humid areas, defined by an aridity index of 0.65 or above, extend over more than 39% of the Earth's land surface. The remaining non-humid 61% is made up of the cold regions, and the drylands. The former include polar and tundra areas, and certain high mountains and plateaus, together covering nearly 14% of the land. They differ ecologically from other non-humid areas, mainly by having temperatures below freezing for a period long enough to restrict or prevent plant growth, although they are `dry' in the sense that liquid water is unavailable for a significant part of the year. The true drylands extend over nearly half (47%) of the Earth's land surface, and can be subdivided into a number of zones on the basis of the aridity index (Table 1.11).
The hyperarid regions have very low biological productivity, and little or no opportunity for human occupation and human-induced land degradation. The less extreme and more productive arid, semiarid and dry subhumid areas are liable to become degraded by human activity, and are collectively known as the susceptible drylands
Drylands contain a very wide range of natural habitats, including barren desert with virtually no visible signs of life, semi-desert dominated by succulents and other xerophytic plants, grassland, savannah, and many different kinds of scrub- or shrub-land, woodlands and forests. However, in contrast to humid areas, where the dominant natural vegetation is usually forest, many drylands are characterised by sparse or absent tree cover. This may be because the climate is simply too dry to support closed forest ecosystems, or because the area is too severely affected by fire or grazing.
Table 1.11 World dryland areas
|% global land area
||True deserts. Rainfall irregular, may not rain at all for period of several years. The Sahara forms near 70% of the global hyperarid total.
||Annual average rainfall is almost invariably less than 200 mm, although there is considerable variation between years
||Highly seasonal rainfall regimes, with maximum average annual rainfall of 800 mm and considerable variation between years
||Typically have highly seasonal rainfall regimes with relatively little variation between years.
Dry and sub-humid ecosystems are the centres of origin of many major crops. Some of the most important categories are described briefly below and presented in Map 13.
Major types of dry and sub-humid lands
ecosystems are typified climatically by generally cool, wet winters and warm or hot dry summers. However, no single climatic or bioclimatic definition of a Mediterranean ecosystem has yet been established. Mediterranean ecosystems encompass a wide range of habitat types including forest, woodland and grassland, but are typified by a low, woody, fire-adapted sclerophyllous shrubland (variously known as maquis, chaparral, fynbos, mallee
) on relatively nutrient-poor soils. These systems occur in five distinct parts of the world: the Mediterranean basin, California (U.S.A.), central Chile, Cape Province (South Africa), and southwestern and south Australia. Each of these regions occurs on the west side of a continent and to the east of a cold ocean current that generates winter rainfall. They cover around 2.5 million km2
in total, or between 1% and 2% of the Earth's surface (according to definition), more than three-quarters of which is within the Mediterranean basin. They are disproportionately rich plant species compared with most other parts of the world. In most parts of the world where they are found, a proportion of ecosystems generally classified as Mediterranean occur within dryland areas according to the definition given above. Elsewhere, including those regions generally associated with the typical Mediterranean high diversity sclerophyllous vegetation, Mediterranean-type ecosystems occur in humid climatic zones and are therefore not classifiable as drylands.
ecosystems may be loosely defined as areas dominated by grasses (members of the family Gramineae excluding bamboos) or grass-like plants with few woody plants. Natural grassland ecosystems are typically characteristic of areas with three main features: periodic drought; fire; and grazing by large herbivores. In addition, they are often associated with soils of low fertility. The relative importance of different factors in maintaining grasslands varies locally and regionally.
are tropical ecosystems characterised by dominance at the ground layer of grasses and grass-like plants. They form a continuum from treeless plains through open woodlands to virtually closed-canopy woodland with a grassy understorey.
Most, but by no means all, of the world's natural grasslands and savannas are found in dryland regions. Around 20% of the Earth's land surface (excluding Antarctica) supports grasslands of varying degrees of naturalness; temperate grasslands make up approximately one fourth of this area, and savannas the remainder.
Seasonally flooded grassland areas are found in many river basins and may be of considerable ecological and biotic importance. Such areas may be considered grassland ecosystems or inland water (wetland) ecosystems; there is no clear dividing line between the two.
Major values and uses of drylands
Drylands are home to over 2 billion people, or around 35% of the global population, a high proportion of whom are subsistence farmers or fishers. Major uses of dryland biological diversity include:
- Existing crops and livestock and their wild relatives. Drylands, grasslands and Mediterranean ecosystems are centres or origin of a significant proportion of the world's major crop plants. Populations of wild relatives of many existing crops are potentially very valuable genetic resources.
- Potential new crops for example salt-tolerant or halophytic species, such as Salicornia spp. and some Atriplex and Distichlis spp.
- Wild foods, particularly as famine foods.
- Aromatics and stimulants. Drylands have a high diversity of plants rich in secondary compounds such as terpenes, which may have aromatic properties (for example frankincense Boswellia sacra and myrrh Commiphora spp.) and may be of considerable economic importance.
- Ornamentals. Arid and semi-arid, and Mediterranean-type, ecosystems have proven important sources of ornamental plants, with many thousand species now in cultivation outside their natural ranges.
- Pastoralism. Grazing of domestic or semi-domestic livestock is a major land use in most of the world's drylands; extensive pastoral systems generally have negligible artificial inputs in the form of fertilisers or other chemicals, and are therefore reliant on natural ecosystem productivity and resilience.
- Soil stabilisation and prevention of erosion. Dryland soils are particularly prone to erosion, which is one of the major causes of land degradation. Natural vegetation cover plays a major part in reducing the erodibility of soils and in preventing or mitigating land degradation.
- Wildlife tourism. Grassland and savannah areas with major concentrations of large, wild mammals include many of the most important sites globally for wildlife-based tourism.
Major impacts on drylands
Humans have had enormous impact on dryland, grassland and Mediterranean ecosystems, often with major negative impacts on biological diversity (Map 14). These impacts are often complex and interrelated. The following are among the major categories of activities that have or can have an adverse impact:
- Conversion, particularly to cropland. Large-scale transformation to cropland in drylands is dependent on availability of freshwater, either as surface waters (lakes, rivers, reservoirs) or in aquifers. In many areas, conversion is followed by abandonment as soil becomes degraded, often through salinisation or waterlogging.
- Changing fire regimes. Many dryland ecosystems are naturally adapted to some degree to fire. However, in many areas fires caused by humans (often as deliberate burning to produce new growth for livestock) are far more frequent and extensive than natural fires, and have very different impact on ecosystems and their biodiversity.
- Impact of introduced herbivores, particularly livestock, through trampling, removal of plant biomass, introduction of pathogens and alteration of plant species composition through selective grazing and competition with native species.
- Introduction of non-native plants. Grassland ecosystems have been routinely modified by deliberate introduction of non-native species, particularly other grasses and leguminous plants.
- Water. By definition, water is a limiting resource in dryland ecosystems. Human use of existing water resources in these systems therefore has often a disproportionately extreme impact. Abstraction of water for irrigation from freshwater systems such as lakes and rivers has an often extreme impact on the biodiversity of these ecosystems. Depletion of groundwater resources may have a less obvious direct impact but is likely to affect natural ground-fed springs and deep-rooted vegetation where the aquifer is relatively near the surface. Creation of artificial water-sources for livestock leads to the creation of virtually barren “sacrifice zones” around the water source as a result of extremely heavy trampling by livestock.
- One of the most significant human impacts in natural or semi-natural dryland and savannah ecosystems in developing countries is through harvest of wood for fuel. Quantifying this, and assessing its long-term impact on ecosystems, has proved problematic, although in some areas the impact is undoubtedly severe.
- Overharvest of wild species. Excess hunting of wildlife and collection of plants, whether for subsistence use or national or international trade, can have severe impacts, in some cases driving species to extinction. Because dryland species tend to have relatively low population growth rates, and in the case of plants, individual growth rates, they may be particularly sensitive to overharvest.
- Chemical inputs. In many grassland ecosystems, highest biological diversity seems to be associated with poorer soils. Artificial enrichment of grasslands, particularly through application of nitrogenous fertiliser, generally leads to a very marked decrease in plant species diversity.
- Long-term impacts of climate change. The potential impacts on dryland ecosystems of human-induced climate change remain to be quantified but are likely to be significant.
Under the UN Convention to Combat Desertification, desertification is defined explicitly as “land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities”.
Land degradation is further defined as “reduction or loss, in arid, semi-arid and dry sub-humid areas, of the biological or economic productivity and complexity of rainfed cropland, irrigated cropland, or range, pasture, forest and woodlands resulting from land uses or from a process or combination of processes, including processes arising from human activities and habitation patterns, such as:
- soil erosion caused by wind and/or water;
- deterioration of the physical, chemical and biological or economic properties of soil;
- long-term loss of natural vegetation”
These various impacts interact in a complex and sometimes unpredictable way. In drylands, the collective effect of factors, mainly of human origin, leading to land degradation make up a process often termed desertification
. According to the United Nations Environment Programme (UNEP), desertification directly affects some 36 million km2
of the world's drylands, i.e. about 70% of the total dryland area, and one-sixth of the world's people. The effects of desertification promote poverty among rural people, and by placing greater stress on natural resources, poverty tends to reinforce any existing trend toward desertification.
Status and trends in species in dry and sub-humid lands
Summary analysis of the habitat distribution of globally threatened mammals and birds shows clearly that while the majority of species occupy forest habitats (particularly lowland tropical moist forest), dryland, scrub and grasslands made up the second most important group of habitats (being somewhat more critical for mammals than for birds, in which group wetlands were significant). There are generally insufficient data overall at present to determine whether species in the ecosystem types under discussion are relatively more or less prone to extinction than those elsewhere - that is whether a higher or lower than average proportion of species in these ecosystems can be classified as threatened. However, for some groups and some ecosystems there is more detailed information.
With respect to mammals, a high percentage of continental (as opposed to island or marine) species believed or known to have become extinct since 1600 occurred in dryland ecosystems, most notably in Australia. In general, dryland mammals tend to be relatively wide-ranging but to occur at low population densities because of the low primary productivity of these areas. Larger species are also more conspicuous (and in the case of ungulates more gregarious) than forest dwelling species and thus more vulnerable to over-hunting. These factors have meant that a notable number of large dryland mammals are either highly endangered or extinct in the wild.
The Mediterranean-type ecosystems in general have a relatively high proportion of their species categorised as threatened. This is in part a consequence of human land-use development in agriculture, industry and housing, and, especially in Cape Province (South Africa) and in California, of the spread of non-native plant species (some 10% of the flora in parts of California consists of naturalised alien species, and some perennial grasslands have been replaced by annual alien-dominated grassland). The Cape flora, largely within a Mediterranean-type ecosystem, occupies only 4% of the land area of southern Africa, but accounts for nearly 70% of the region's threatened species. Around 10% of the California flora is considered threatened (equivalent to approximately one quarter of all threatened plants in the USA). The main threats are: urban coastal development, pollution, agriculture, tourism, water shortages and fire.
Forests and woodlands, and inland water ecosystems within drylands tend to be placed under disproportionate pressure. This is chiefly because they are inherently scarce resources but also, in the case of forests and woodlands, because they tend to be found in areas within drylands with somewhat more favourable conditions (microclimate and soil fertility) than normal. These areas are therefore most likely to be settled by people and suffer extensive habitat conversion. Mediterranean ecosystems in particular are noted for their amenable climates and therefore come under particular pressure for permanent settlement and tourism.
Notes on a selection of dryland species that are categorised as Critically Endangered, i.e. at highest risk of extinction, are provided in Table 1.12.
Table 1.12 Some Critically Endangered dryland species
|Formerly occurred through much of the Sahara. Reduced by uncontrolled hunting to a few remnant populations in inaccessible areas.
|Northern Hairy-nosed Wombat
|Restricted to a single colony of some 70 animals at Epping Forest Station, Queensland, Australia. Decline attributed to habitat loss and competition with introduced grazing animals.
|Bulo Burti Bush-shrike Laniarius liberatus
||Discovered during the late 1980s in fragmented acacia scrub of central Somalia. Despite searches, known only from a single individual.
|An extremely rare bird from central Australia, probably threatened by habitat degradation, predation by introduced species and loss of available water.
|Anegada Rock Iguana Cyclura pinguis
||Formerly present on several islands of the Puerto Rico Bank, now restricted to Anegada (British Virgin Islands). Threatened by predation and grazing pressure from introduced species.
|Bulgar Dagh Viper
|Restricted to open grassland and thorn habitats in the Cilician Taurus mountains of southern Turkey. Susceptible to habitat change and excess collection of specimens.
|Cachorrito de Mezquital Cyprinodon meeki
||A desert fish endemic to the upper Rio Mezquital drainage, Mexico. Threatened by pollution, introduction of exotic species and habitat modification.
|Prairie Sphinx Moth Euproserpinus wiesti
||A moth confined to high prairies of central USA threatened by insecticide use and perhaps collection.
|A coniferous tree known from a small population of fewer than 20 individuals in Sicily (Italy), most of which are non-reproductive.
||A tree-like succulent known from two or three populations in southern Madagascar, potentially threatened by habitat loss and collection.