Mapping habitat for biodiversity

Vegetation type and condition are widely used as surrogates of ‘biodiversity’ such as in Australia’s headline sustainability indicators 192 and State of Environment reporting 193. Native vegetation is also used in a number of State-based tools for assessing biodiversity, such the Biodiversity Benefits Index (developed as part of the NSW Environmental Services Scheme 194).

Vegetation maps are perhaps the most frequently used surrogates for biodiversity since these may be the only useful information available for a catchment or region where few or no biological surveys have been undertaken. It is also relatively easy to map vegetation at the catchment scale. In this approach it is typically assumed that protection of a proportion of each vegetation type will protect sufficient proportions of the populations of other organisms such as mammal and bird species. Vegetation maps may fail as surrogates in cases where sets of species are dependent on particular successional stages within a vegetation community (e.g. the old growth stage of a particular type of forest), or when species respond to environmental variables to which the vascular flora are insensitive 195.

A common example of the use of native vegetation as a surrogate for biodiversity is where a target (e.g. 10 to 20 per cent of area of remaining vegetation type) is set in a catchment to protect a vegetation ecosystem and its biodiversity. This approach may be adequate if the species found in the overall vegetation ecosystem are well represented in the portion of this ecosystem chosen for protection, and if the habitat requirements of these species can be met by the portion setaside. Often, however, these requirements may not be met for well-founded ecological reasons 196. Hence, this approach to biodiversity management is best served by basing decisions on a more diverse range of biological information about the catchment and the vegetation systems, and ensuring that appropriate monitoring is in place to determine how well the approach actually works.

Managers should be aware that the extent of native vegetation is unlikely to be a suitable surrogate when implementing strategies to conserve all biodiversity. Research across a range of vegetation types demonstrates that vegetation type and condition only capture a subset of the diversity of plants and animals that occur in a region 197,  198. For example, vegetation is often used synonymously with habitat, but they are not the same 199. Habitat is an area with a combination of resources (like food, cover and water) and environmental conditions (temperature, precipitation, presence or absence of predators and competitors) that allows a given species or population to survive and reproduce.

Recent research demonstrates only moderate congruence between native vegetation type and different species groups, highlighting some of the limitations of using vegetation type as the principal surrogate for the habitat of species of interest. Other research has observed that interactions between habitat and environmental variables differentially affect groups such as arboreal and ground-dwelling mammals 200. Rocks, cracks in the soil, access to water and other factors are also important habitat for native plants and animals, and can be the key factors determining their distribution. Work undertaken in the rangeland regions, and reported at the Biodiversity Monitoring Workshop demonstrates that native vegetation is a poor surrogate for patterns of invertebrate biodiversity and that these must be monitored directly 201.

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	Access and learn more about the concept of native vegetation as a surrogate for biological diversity at the catchment level by reading recent reviews.
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Caution has been recommended about relying too heavily on remotely-sensed data on surrogates for biodiversity such as vegetation type 202. Three broad ways have been put forward to alleviate potential risks when using biological data. These are:

• attempt to more closely integrate biological and environmental data through predictive modelling;
• make more use of remotely mapped surrogates in conservation planning by incorporating knowledge of heterogeneity within land-classes; and
• use relatively data-rich regions to test the performance of surrogates before they are used in data-poor regions.

When using GIS layers to create generalised notions of habitat, it is important that the suite of assumptions used is explicit and terms are clearly defined.

Identifying focal species

An approach at regional level that has gained considerable attention is the use of focal species to specify conservation targets and determine the actions required to meet those targets. This technique also uses a surrogate approach, and is based on detailed knowledge of the fauna of a particular study area as well as the processes that can put species at risk in that area if no management action takes place 203. Attention is given to identifying factors that limit the distribution and abundance of target species, such as the minimum patch sizes needed and the maximum distance between patches which permits species to disperse. Species that are limited by remnant size or isolation, or insufficient resources, will require reconstruction of habitats in the landscape.

For example, in the Wallatin Creek Catchment in south-west Western Australia it was recommended that the minimum size for habitat patches should be 25 ha, the distance between remnants should not exceed 2 km and vegetation that links habitats occupied by dispersal limited birds should be approximately 50 m wide 204. Importantly, the nature of revegetation was also specified with a recommendation for revegetation to contain clumps of dense understorey vegetation for some invertebrates and small mammals, and the use of plant species that produce nectar over the summer-autumn period. For species limited by factors such as predators, stock grazing or inappropriate fire regimes, improved management of threatening processes was recommended as the main action required. Several other projects 205,  206,  207 have used the focal species approach, as outlined in Native Vegetation and Property Management 208.

Recently there has been considerable debate among scientists about the strengths and weaknesses of the focal species approach 209,  210. One of the strong points is that it provides a pragmatic and explicit approach to landscape design that managers can use now. It is also underpinned by a number of assumptions, the key one being that designing landscapes to meet the spatial needs of the most sensitive species will meet the requirements of other less sensitive species. While the intention of the focal species approach is to examine all species and all threats so that the most sensitive species can be identified and managed, to date applications of the focal species approach have relied on bird survey data and on the size, shape and proximity of remnant vegetation patches as the focus for management. Testing Approaches to Landscape Design in Cropping Lands 211 suggests that birds only act as ‘focal species’ for some of the needs of some species other than birds, but the evidence is weak. There is stronger evidence that birds are a poor indicator of the amount of native vegetation needed by mammals and reptiles sensitive to habitat loss. This supports other research that suggests that using one species (which is usually an animal) as an indicator for remnant quality or as an umbrella for other plants and animals may have limited application 212.