Definitions - Realms and Bioregions
Regionalisations of biota can occur at many spatial scales and species diversity is intimately a function of spatial scale. Realms represent the highest level of regional structuring and are defined at the scale of ocean-basin geographic processes affecting species diversity. At these scales, the processes affecting diversity are (Ricklefs and Schluter, 1993): biotal exchanges, mass-extinctions, evolutionary novelties and adaptive radiation. Australasia, consisting of mainland Australia, New Zealand and Papua New Guinea form one of the six super-regions of the world or realms. Realms are groups of regional proximity which have evolved according to one or more of the biogeographic processes noted above.
Bioregions have been defined as 'assemblages of flora, fauna and the supporting geophysical environment contained within distinct but dynamic spatial boundaries' (Welsh, 1994). The delineation and comprehension of these boundaries and functioning of bioregions are essential for enlightened ecological management (Welsh, 1994). While species distributions are determined by an almost inextricably complex network of environmental and biological variables, occurrences are closely linked to particular environmental parameters (Miller, 1994). Distributional boundaries are manifestations of the responses of organisms to shifts in environmental factors (US GLOBEC, 1994); hence studies of species distributions may lead to a more comprehensive understanding of the dynamics of marine regions.
One view of bioregions is that they are defined by geophysical and climatic limits that constrain the natural communities of organisms in space and time through interactions with the physiological and behavioural needs and capabilities of these organisms (Welsh, 1994). Provided the physiological and behavioural constraints can be qualified, or quantified, in terms of the geophysical and climatic attributes, these attributes can be used as surrogates defining the regional and temporal constraints of the organisms. An equally valid view, and alternate definition, is that the distributions of the organisms by themselves implicitly define their regional and temporal constraints. As with the first approach, the difficulty here is in defining the assemblage of species representing, or comprising, each bioregion. The first approach is generally advocated because available data are usually better for geophysical attributes than taxonomic distributions. However, a drawback with this approach is the lack of understanding of which biological attributes are relevant and how they are to be weighted. The bioclassification approach produces a more direct regionalisation, but is mainly limited by a lack of data - hence the necessity for RAP procedures as employed in this project.
Related to bioregions is the concept introduced by Sherman and Alexander (1989) of Large Marine Ecosystems (LMEs). These are large geographical regions, often over 200 000 km2, that have unique bathymetry, hydrography and productivity. Within LMEs, populations of plants and animals are assumed to have adapted reproductive, growth and feeding strategies, and the close linking of physical conditions, biological communities and fish stocks indicate that the areas can be managed as single units. LMEs include upwellings, semi-enclosed seas, shallow shelf ecosystems on western ocean boundaries, coral reefs, and ocean shelf-deltaic-riverine interactive systems. The number and extent of these systems globally has not yet been established but a program is underway to map them (Kelleher et al., 1995).
In the terrestrial environment, plants have generally been considered more useful than animals in describing bioregions because of their lesser agility (Udvardy, 1975; Welsh, 1994). However, it has long been recognised that terrestrial biogeographic methods are unlikely to produce useful results in the marine environment (Pielou, 1979; Hayden et al., 1984; Williams and Gaston, 1994). It seems likely that the greater contiguity and three-dimensionality of marine systems, which complicates distributions, provides high levels of environmental stability and facilitates species dispersion, will have profound effects on species distribution patterns relative to those studied on land (Upton, 1992; Kelleher et al., 1995; Norse, 1995). Furthermore, the boundaries of estuarine, coastal, shelf, slope and offshore waters are not necessarily geographically coincident (e.g. Walls, 1995). Because of this complexity marine bioregions have been treated by various physical and biological means, with no one method dominating. Some attributes used in classifications include oceanographic features (Dunbar, 1951, 1972; Dietrich, 1963; Markov, 1964); climatic traits (Holdgate, 1964); or biological characteristics (distributions of littoral biota (Knox, 1960), island vegetation (Wace, 1965) and birds (Watson et al., 1971)). This disparity of classification methods has led to inconsistencies in resulting bioregions and interpretations of results, as well as conflicts over their adequacy for different conservation needs (Thackway, 1995). Ideally, physical and biotic properties should be considered in an integrated way so that the classifications are at least compatible (Hayden et al., 1984).
Biomes are defined as 'major regional ecological communities of plants and animals extending over large natural areas' (Abercrombie et al., 1951). Marine biomes, as applied to primary ecosystems, have been classified in many ways (Lagler et al., 1962) and the breakdown of spatial components is often inconsistent (Christy and Scott, 1965). Presumably for these reasons, few regional biogeographic studies have attempted to discriminate between the distributional structure of different biomes. Clearly, evolutionary events and contemporary physical conditions are not homogenous throughout the oceans, even within a region. For example, factors affecting the biogeographic structure of abyssal fishes are unlikely to be the same as those that effect fishes in the estuaries or the coastal zone. Thus an analysis of the biota of each biome is needed to gain a full understanding of the biogeographic structure of the region.
Possibly the most widely accepted classification of ecosystems of the sea is that of Hedgepeth (1957). Several authors have either closely followed this scheme (Last et al., 1983) or closely adapted versions (Lagler et al., 1962). The sea is divided into neritic and oceanic zones which include ecosystems of the continental shelf and slope respectively. The functional ecotonal boundary between these two zones is deemed to approximate the 200m isobath.
The neritic zone contains four primary biomes: estuarine, coastal marine, demersal shelf and pelagic shelf ecosystems.
Estuaries have been classified in many ways (Barnes, 1974) based on hydrological themes (e.g. Ketchum, 1951; Moore, 1958; Perkins, 1974), geomorphological themes (e.g. Emery and Stevenson, 1957) or a combination of both (e.g. Odum, 1959; Cameron and Pritchard, 1963; Pritchard, 1967; Clark, 1977). The various inadequacies of each methodology have been clearly discussed by Day (1981) who proposed a more complete definition based on an amended version of Pritchard (1967).
The coastal marine biome includes habitats within the nearshore zone to the circalittoral fringe (taken as a depth of 40m) but excludes estuaries. The demersal shelf and pelagic biomes lie beyond this depth between the 40m and 200m isobaths. Animals occupying demersal habitats live close to the bottom either in the substrate, on the bottom, or in the associated water column immediately above. Pelagic habitats are those of the open water column above.
The oceanic zone also consists of four biomes: continental slope, abyssal, epipelagic and meso/bathypelagic ecosystems. The continental slope and abyss contain demersal habitats that are usually separated by the 2000m isobath although other schemes exist. The upper 200m of the ocean is defined as the epipelagic biome. The deeper pelagic ocean below consists of mesopelagic (200 - 1000m) and bathypelagic habitats (deeper than 1000m) that comprise the meso/bathypelagic biome.
Biomes are partly ecological entities and partly groups of convenience as they are not exclusive disjunctions of biota. While some species are mostly restricted to a particular biome others occur more broadly and may occur in more than one biome. This is especially the case for vertical migrators which spend daylight hours at mesopelagic depths and much of the night in the epipelagic zone. However, generally species (or a particular life history stage of a species) have their core distributions within a biome.
The term 'biodiversity', which is most often used to refer to species diversity, applies at several differing ecological and biogeographical levels. Biodiversity can be considered to operate on three levels (eg. Beattie, 1995): genetic (intraspecific), organismal (species) and ecological (communities). In reality, higher biotic levels, such as realms and provinces, are also expressions of biological diversity. We prefer to recognise marine biodiversity of Australia according to the descending hierarchy of standard biogeo-ecological units: provinces, subprovinces, ecosystems, communities, species, populations, and genes. A subprovince may be defined as an assemblage of biological communities that are linked by a particular suite of environmental conditions.
Biological units need to be represented geographically for management purposes. Higher levels, such as provinces and subprovinces, can be identified as mesoscale bioregions. At mid levels, ecosystems and communities have a finer scale spatial structure that is still definable.
From a management standpoint, we need to understand regional biodiversity at a functional level, being mindful that the basic unit is at the species level. Generally, our knowledge of species and their dynamics is inadequate. Consequently, management of diversity in the sea needs to be undertaken at a more appropriate working level up the hierarchy. We argue that this work should be conducted at the habitat level but information at higher levels is first needed as a baseline.
The Australian realm has one of the world's most diverse marine biotas. Its extant biota comprises the products of evolution bounded by contemporary environmental features. Biogeographic subdivisions of a region termed provinces comprise meso-scale units with their associated fauna. These units, each with its unique subset of species, provide a basis for focused subregional evaluation based on their ecosystem and community diversity. As communities in the form of biocoenoses are often difficult to define, a more useful working platform is an environmental surrogate - the habitat or biotope.
A prevailing contemporary paradigm is that habitat conservation is the most effective way of conserving biodiversity. If so, a major post-regionalisation research objective should be to identify and describe major habitat types and to estimate their spatial coverage within bioregions. Until this is done, rare or unusual communities and habitats will remain undetected and their biota may be left open to threatening processes. Similarly, a knowledge of habitat diversity will provide a framework for establishing future research priorities that do not presently exist.
Some have claimed that biotopes cannot always be used to define communities. However, we maintain that as a management baseline in the sea, where both the physical structure of habitats and the components of communities are less visible than in terrestrial environments and the geographic scale of the Australian region is so large, an approximate technique will provide benefits.
Next Chapter: 6. Review of Regionalisation Strategies