2. BIODIVERSITY IN FRESHWATERS

SPECIES NUMBERS IN FRESHWATERS

At high taxonomic levels the diversity of freshwater organisms is much narrower than on land or in the sea: no extant phyla or classes, and few orders, are restricted to freshwater habitats. The number of species overall (species richness) is low compared with marine and terrestrial groups. However, species richness in relation to habitat extent is extremely high in many freshwater groups.

For example, about 10,000 (40%) of the 25,000 known fish species are freshwater forms. Given the distribution of water on the Earth's surface this is equivalent to one fish species for every 100,000 km³ of sea water, compared with one species for every 15 km³ in freshwaters. This high diversity of freshwater fishes relative to habitat extent is probably promoted by the extent of isolation between freshwater systems. Many lineages of fishes and invertebrates have evolved high diversity in certain water systems, and in some cases, species richness and endemism tend to be positively correlated between different taxonomic groups (eg. Watters, 1992).

At global level, species richness increases strongly toward the equator; ie. in most groups of organisms, there are many more species in the tropics than in temperate regions. The same applies to freshwater fishes in general and to other groups in freshwaters, although certain groups, eg. freshwater crayfish, are much less diverse in the tropics than temperate regions. The number of fish species present in rivers is highly correlated with the area and annual discharge of the drainage basin (especially the latter).

Table 7 provides an outline of the major groups of plants and animals present in freshwaters. There are more than 600 species of freshwater fungi known, currently more from temperate regions than from the tropics, although probably only a small fraction of existing species have been described, and the tropics have been little sampled (Goh and Hyde, 1996). There is no group of plants as diverse and species-rich in freshwater habitats as fishes, bivalve molluscs, dragonflies and other animal groups. Although important along water margins, and sometimes on the water surface, plants other than microscopic forms are in general nowhere as prominent in freshwater ecosystems as animals.

Among the 'lower' (non-vascular) plants, the mosses and liverworts are virtually all terrestrial, although flourishing only in moist environments; but the larger algae are primarily aquatic. The larger algae comprise some 5,000 species in three major groups (the green, brown and red algae), the great majority of which are marine or brackish water forms ('seaweeds'). The green algae Chlorophyta include one order of around 80 species (Ulotrichales) that is mainly freshwater. However, one major group sometimes associated with the green algae - the stoneworts (Charophyta) - is almost entirely freshwater. The stoneworts include some 440 species, most of which are endemic at continent level or below; they tend to be very sensitive to nutrient enrichment and have declined in many areas (Tittley, 1992).

The higher (vascular) plants include ferns and allies (pteridophytes), conifers and allies (gymnosperms) and flowering plants (angiosperms). It has been estimated that at most 2 percent of pteridophytes and 1 percent of angiosperms, ie. up to 250 and 2,500 species respectively, are aquatic forms (Sculthorpe, 1967). These groups together comprise around 400 families; only some 33 of these include aquatic species, and most of these are not rich in species. Most of these aquatic species occur in freshwaters but some 50 species of seagrass grow in shallow coastal marine habitats. Table 8 includes basic information on a small selection of the approximately 33 plant families that include freshwater species.

Note: This table includes basic information on a small selection of the few plant families that include freshwater species.

Source: based on data in Sculthorpe (1967) and Mabberley (1997).

Several species of aquatic plant, particularly free-floating species able to spread rapidly by vegetative growth, but also other forms, have dispersed widely over the globe and become major pest species. They block drainage channels, sluices and hydro-electric installations, impede boat traffic, and hinder fishing. In recent decades the question of how best to control or eradicate pest species has been the foremost issue in conservation and management of aquatic plants.

Animal species are considerably more diverse and numerous in inland waters than plants. Most of the major groups include terrestrial or marine species as well as freshwater forms. Apart from fishes, important groups with inland water species include crustacea (crabs, crayfishes and many smaller organisms), molluscs (including mussels and snails), insects (including stoneflies Plecoptera, caddisflies Trichoptera, mayflies Ephemoptera), sponges, flatworms, polychaete worms, oligochaete worms, numerous parasitic species in various groups, and numerous microscopic forms. Palmer et al. (1997) provide a review of diversity and the ecological rôle of microorganisms and microinvertebrates in freshwater sediments.

Freshwater lineages that originated within continental water systems may show general patterns of distribution similar to terrestrial groups, corresponding more or less to broad biogeographic realms. Lineages of marine origin may remain restricted to peripheral systems corresponding to the area where the ancestral forms moved into freshwater.

Unlike many terrestrial species, that can disperse widely in suitable habitat, the spatial extent of the range of strictly freshwater species tends to correspond to present or formerly continuous river basins or lakes; these species include fishes and most molluscs and crustaceans. Watersheds between river basins are the principal barriers to their dispersal between systems, and their ranges are extended mainly by physical changes to the drainage pattern (eg. river capture following erosion or uplift can allow species formerly restricted to one system to move into another), or by accidental transport of eggs by waterbirds, or by flooding.

In many instances, the range within a system will also be restricted by particular habitat requirements (variations in water turbulence or speed, shelter, substrate, etc). These frequently differ between different stages in the life cycle (eg. in fishes, different conditions and different sites are often required for egg deposition and development, for early growth of fry, and for feeding and breeding of adults).

Many cave or subterranean freshwater aquatic species (eg. of fishes, amphibians and crustaceans) have very restricted ranges, perhaps consisting of a single cave or aquifer, and very limited opportunities for dispersal, depending on the surrounding geology and the consequent morphology of the water system occupied.

Insects with an aquatic larval phase but a winged adult phase are often restricted to particular river basins (even if adults disperse widely, they may not find suitable habitat), but in general are much less restricted in this way than entirely aquatic species. A relatively large number of species, particularly of crustaceans, occupy temporary pools and have a desiccation-resistant stage that can undergo long-range passive dispersal between drainage basins; some such species are thus widely distributed.

Vascular plants are essentially terrestrial forms, and existing aquatic species are derived from terrestrial ancestors; several different lineages include aquatic species and this transition has therefore occurred several times. Most inland water plant species are relatively widespread, ranging over more than one continental land mass; many are cosmopolitan, occurring around the world and on remote islands. Of the widespread forms, some are essentially northern temperate species extending to a great or lesser extent into the tropics; some are mainly tropical. Tropical regions of Asia, Africa and South America appear to be most rich in species restricted to a single continent, or to a single country or smaller area (Sculthorpe, 1967).

The Podostemaceae is particularly noteworthy for its many monotypic genera, and a large number of narrowly endemic species, in at least one instance with several forms restricted to different stretches of a single river; tropical South America, Madagascar, Sri Lanka, India, Myanmar, and Indonesia hold such localised species (Willis, in Sculthorpe, 1967).

A large number of species have been spread by intentional or accidental human introductions to areas beyond their native range, and in many cases have occupied vast areas and had serious ecological impacts.

The general goal of biodiversity conservation is to minimise loss of irreplaceable biodiversity. This not only includes individual species, but also habitats and ecosystems where these are known to be unlikely to regenerate naturally or difficult to restore artificially. The first step in planning for biodiversity conservation at any geographic scale is to assess the diversity of natural resources present and identify those which are most important, or in this context, most irreplaceable.

The species diversity of an area can be evaluated in different terms:

• an area with a large number of species (high species richness) can be described as more diverse than an area with fewer;

• an area with more species restricted to it (ie. more endemics), includes more diversity than one with few or none,

• an area with representatives of more higher taxa (ie. groups higher than species level, such as genera, families, or classes), is more diverse than one with fewer
• an area with more phylogenetically 'primitive' taxa (eg. lungfishes or sturgeon among fishes) is more diverse than areas with representatives only of more recent radiations.

A collation and analysis of expert opinion on the location of areas of special importance for inland water biodiversity, assessed in terms of species richness and endemism, is provided in Chapter 4 of this document.

Species diversity is not the sole criterion to be used in prioritising species or sites for conservation action. Human attitudes and values also determine the significance attached to elements of biodiversity: areas supporting species threatened with extinction, or which are consumed or which yield important commodities, may with justification be rated more highly than areas not supporting such species.