Typically, in any ecological community, most of the species are relatively uncommon or rare and there are only a few that are abundant, and threatened species are normally amongst the least abundant. It is often the rare or uncommon species that government environmental regulators will indicate that require management and are of conservation interest. So what is ‘rare’ and ‘common’ in the context of an ecological community? Although rare means in low abundance, a review of the literature indicates that the notion of low abundance must be considered in a broad context. There is no absolute definition of rarity; rather it is at one end of the continuum from common to rare. Sampling effort and methodology in fauna surveys also often defines what is rare, as we seldom actually know the structure of a fauna community. Cunningham and Lindenmayer (2005) provided the following useful table to assist with defining what is common and rare.
Table 1. What is rare and common (taken from Cunningham and Lindenmayer, 2005, p. 1136)
Abundance of species within a community |
Habitat specifcity |
Geographic range |
Description |
Common | Common | Widespread |
Widespread, occurs in a wide range of habitats |
Common | Common | Restricted/localized |
Highly localised distribution but occurs across a range of habitats and is abundant in places where it occurs |
Common | Rare/specialised | Widespread |
Widespread, but occurs in few habitats and is common in places where it occurs |
Rare | Rare/specialised | Restricted/localized |
Highly localised distribution and occurs in a few habitats, but is common in places where it occurs |
Rare | Common | Widespread |
Widespread and occurs across a range of habitats but is scarce in places where it occurs |
Rare | Common | Restricted/localized |
Highly localised, occurs across a range of habitats but is scarce in places where it occurs |
Rare | Rare/specialised | Widespread |
Widespread but occurs in few habitats and is scarce in places where it occurs |
Rare | Rare/specialised | Restricted/localized |
Highly localised distribution, occurs in few habitats and is scarsce in places where it occurs |
Another way of looking at rare is species that are under direct threat or are vulnerable to extinction. If this is what we mean, then we should be focussed on threatened and be less concerned about rare species in an environmental impact assessment (EIA). However, Possingham et al. (2002) suggested that threatened species lists are designed to provide a qualitative indication of the risk of extinction, specifically to assess potential adverse impacts, help inform conservation priorities and as a component of State of the Environment reports. They went on to argue that these lists are now being used for the following four purposes, but were never designed to be used in this way:
Possingham et al (2002) went on to indicate the following three issues with the use of these lists to constrain development or exploration:
The last of these being the most serious, as it can lead to land managers destroying important habitat, denying the presence of the species in the area and thus avoiding implementing mitigation programs, and denying access to the area for research purposes.
The creation of a ranked abundance curve or a species abundance curve from fauna trapping data is always a downward sloping curve, with an abundance of a few species and a lot of species that are infrequently caught (Figure 1). This is also known as a Whittaker plot.
If the ‘Y’ axis is log transformed, then the curve is transformed to a near linear line (Figure 2) and the slope of the line can be used to describe the structure of the community.
In EIA, consultants typically focus on species listed as threatened under the EPBC Act or the Wildlife Conservation Act, whereas the WA EPA (2002) Position Statement No 3 indicates that it is important to clarify that the intrinsic value of a species should not be judged only by its rarity or how threatened it is, and these species should also be considered in the context of their geographical distribution or their contribution to the sustainability of the ecosystem. So in this context, which of the categories shown in Table 1 should be the focus of an EIA assessment?
When mitigating potential impacts, should we be particularly concerned about and throwing a lot of money at:
Or are there more fruitful ways in which these funds could be spent for the betterment of Australia’s threatened fauna?
References
Cunningham, R., B. and D. B. Lindenmayer. 2005. Modeling count data of rare species: some statistical issues. Ecology 86:1135-1142.
Environmental Protection Authority. 2002. Terrestrial Biological Surveys as an Element of Biodiversity Protection: Position Statement No. 3. Environment Protection Authority, Perth.
Possingham, H. P., S. J. Andelman, M. A. Burgman, R. A. Medellin, L. L. Master, and D. A. Keith. 2002. Limits to the use of threatened species lists. Trends in Ecology and Evolution 17:503-507.
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