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7. Acacias and eucalypts

Summary

Analysis of the patterns of species richness, endemism and irreplacability of sites was undertaken for all species of Acacia and eucalypt (comprising the genera Angophora, Corymbia and Eucalyptus). This analysis used records held in Australian Herbaria.

For each subregion and bioregion of Australia, key attributes were collated:

• species richness (the total number of species);
• endemic species richness (the number of species that had more than 90% of all of their collection records in one region); and
• irreplacability (a measure of the degree to which the species in a region are represented in other regions).

Twenty-two bioregions have moderately high to high numbers of species (more than 150) of Acacia and eucalypt. There is a major centre of high species richness in the semi-arid south-west of Western Australia. Up to 222 species of Acacia and 160 species of eucalypt can be found in a single subregion. This includes the subregions of the Avon Wheatbelt, Mallee, Esperance Plains, Coolgardie and the Geraldton Sandplains bioregions.

The analysis reveals a number of other areas of significant diversity, including some desert bioregions of Western Australia (Murchison and Great Victoria Desert) and the Northern Territory (Tanami), and some of the bioregions of the tablelands and slopes of eastern Australia including Brigalow Belt South and the South Eastern Highlands.

Most subregions have some endemic species and a few subregions have high numbers of endemic species. For Acacia species, the subregions of high endemism are in the south-west of Western Australia (Avon Wheatbelt, Esperance Plains, Coolgardie and Mallee) and Northern Kimberley. For eucalypts, subregions with large numbers of endemic species also occur in the south-west of Western Australia (Coolgardie, Esperance Plains, Geraldton Sandplains, Mallee), in the Northern Kimberley, the Pilbara and the New South Wales North Coast.

Forty bioregions and 61 subregions have moderate to high irreplacability scores with concentrations occurring in the south-west of Western Australia, Great Victoria Desert, Pilbara, Kimberley area, and the coastal plains, ranges and western slopes of the Great Dividing Range from Cape York Peninsula to central Victoria.

The subregions and bioregions identified as important because of their endemic species and high irreplacability in south-west Western Australia and in the Murray-Darling Basin coincide with bioregions that are amongst the most extensively cleared, fragmented, and salinised in Australia based on the Landscape Health Assessment. This is of great concern for the on-going persistence of the Acacia and eucalypt species of special value in these regions and their associated ecological communities.

Introduction

Plants as a group are a significant component of the biological diversity of Australia. There are estimated to be 2,400 species of bryophytes (liverworts, hornworts and mosses), 2,500 species of lichens, and 15,000 species of flowering plants. About 80% of these are endemic to Australia. At the global scale, Australia has nine centres of species diversity, including two well-known regions of high plant diversity: south-west Western Australia and the Wet Tropics of north Queensland (Boden & Given 1995).

This assessment set out to more closely identify centres of species richness, endemism and irreplacability for Acacia and eucalypts. This analysis provides an example of flora biodiversity values that need to be fully recognised in conservation planning.

The Australia-wide analysis required plant groups that met the following criteria:

• comprehensively collected and studied across Australia;
• taxonomically resolved at species level;
• continent-wide distribution;
• consisting of large numbers of species; and
• available electronic database of records.

Only two major groups were able to fulfill these criteria - the acacias and eucalypts. These are two diverse groups of woody plants that are dominant in most Australian landscapes.

The results presented for acacias and eucalypts also provide reasonable surrogates for patterns in species richness for the communities of plants that are dominated by these woody sclerophyllous shrubs and trees. Changes recorded for the dominant plant species are likely to have major implications for many co-occurring species.

The Assessment

The analyses are based on the specimen records held by the major herbarium in each State or Territory of Australia. After extensive filtering of the herbarium records to remove duplicate, incorrect or imprecise records, data consisted of 95,685 records of 1,095 taxa of Acacia and 108,782 records of 1,090 taxa of eucalypt (comprising the genera Angophora, Corymbia and Eucalyptus).

The specimen point source location information was used to generate lists of species for each subregion to determine what proportion of all records of species occurred in particular subregions. This then allowed the following variables to be calculated:

• Species richness of each plant group in a subregion and bioregion.
• An Endemicity Score for each species in a subregion, using the proportion of all records for a species that came from a particular subregion.
• An Endemicity Index for a subregion, calculated as the sum of the squared endemicity values for each species in a subregion. The more species that are endemic (restricted) to a subregion, the higher the Endemicity Index for that subregion.
• An Irreplacability Score for a species in a subregion was calculated following Ferrier et al. (2000). A value from near 0 to 1 indicates how significant that subregion is for the species. If a species is found in only one subregion (i.e., it is endemic to that subregion) the irreplacability score is 1 because it can be found nowhere else, hence, the population of the species in that subregion is irreplaceable.
• An Irreplacability Index for a subregion, which sums the irreplacability scores for species, was calculated following Ferrier et al. (2000). The more species in a subregion that have restricted geographic ranges, that is, occur in one or a few subregions, the higher the Irreplacability Index for that subregion.
• Landscape stress classes from the Landscape Health Assessment (NLWRA 2001a) were used for those subregions with high irreplacability indices to determine key threatening processes.

Findings

Species Richness and Endemicity

Subregions of Australia support up to 222 species of Acacia and up to 160 species of eucalypt, with an average of 34 species of Acacia and 32 species of eucalypt per subregion, respectively (Figure 7.1). More than 80% of subregions have less than 50 species while a few subregions have 2 to 3 times this number.

Spatial patterns of species richness in subregions indicate similarities between acacias and eucalypts (Figure 7.2). There is a major centre of high species richness in the semi-arid south-west of Western Australia where up to 222 species of Acacia and 160 species of eucalypt are in a single subregion. This incorporates the subregions of the Avon Wheatbelt, Mallee, Esperance Plains and Coolgardie bioregions, for both groups, the Geraldton Sandplains for Acacia, and the Murchison subregion for eucalypts. Moderately species-rich bioregions occur throughout Western Australia, in the Kimberley, Arnhemland, Barkley Tableland, and the sub-tropical humid and temperate sub-coastal areas of eastern Australia.

When the species richness of acacias and eucalypts is combined (Figure 7.2) some of the centres of diversity coincide with those identified in other studies. For instance, south-western Western Australia, the Kimberley, the Top End and the Border Ranges were identified by Boden & Given (1995) using a wide range of plant taxa.

The analysis in this chapter, using just two plant groups, reveals a number of other areas of significant diversity. These include some desert bioregions of Western Australia (Murchison and Great Victoria Desert) and the Northern Territory (Tanami), and various bioregions of the tablelands and slopes of eastern Australia (e.g., Brigalow Belt South, South Eastern Highlands).

Species richness is one measure of conservation significance but it does not take into account the set of unique species found in each subregion. That is, a bioregion or subregion can harbour large numbers of species but these may be mostly of species that are widespread, common, and maybe of lesser conservation concern than rare, narrowly distributed or endemic species.

Conversely, a modest number of species may be found in a subregion, but this may include unique species that are found nowhere else. Consequently, the subregion could be of relatively high biological and conservation significance.

The differences between subregions and bioregions that appear to be significant in Figure 7.2 illustrates this limitation. At the subregional scale, there are many locations in eastern Australia where a moderately rich flora of acacias and eucalypts are found, but none with very high numbers of species. However, when these species lists are aggregated to the bioregional scale, the overall species count for the bioregion is in the highest class of significance. This is because the subregions comprising a bioregion have each contributed some unique species to the bioregional total. For the purposes of this analysis it is more relevant to examine patterns of species distribution in relation to where endemic species occur, and particularly, to define where threats from land use coincide with endemic species as an insight to conservation management needs.

While there is a general trend for subregions with large numbers of species to also have large numbers of endemic species (Figure 7.3), there is variation in this relationship. Therefore species richness is not a reliable measure for predicting the number of endemic species in a region. It cannot be assumed that knowing the number of species in a bioregion or subregion will indicate the significance of that bioregion or subregion in terms of special biological features such as endemic species.

Subregional and Bioregional Endemicity

Most subregions have only a few endemic species and a few subregions have high numbers of endemic species. For Acacia species, the subregions of high endemic richness are in the south-west of Western Australia (Avon Wheatbelt, Esperance Plains, Coolgardie and Mallee) and Northern Kimberley (Figure 7.4). For eucalypts, subregions with large numbers of endemic species also occur in the south-west of Western Australia (Coolgardie, Esperance Plains, Geraldton Sandplains, Mallee), in the Northern Kimberley, the Pilbara and the New South Wales North Coast.

Species richness and centres of endemism are two different measures of biodiversity and usually provide different levels of information. Comparing subregional data for Acacia and eucalypts shows that the centres of endemism occupy fewer adjacent subregions and are more restricted in area than the centres of species richness, although there is some overlap. This is to be expected since the number of species of restricted distribution is only a fraction of all the acacias and eucalypts in Australia.

The patterns of richness of endemic species for the combined Acacia and eucalypt data show that bioregions of high endemic richness are the Avon Wheatbelt, Esperance Plains, Coolgardie, Brigalow Belt South, and the Sydney Basin. Areas of moderately high endemic richness are the Geraldton Sandplains, Western Mallee, Pilbara, Northern Kimberley and New South Wales North Coast (Figure 7.4).

The number of endemic species in a subregion can be expressed by an index for endemicity. Bioregions and subregions that have higher values of this index have more species and more endemic species. By this method, those bioregions and subregions identified in Figure 7.4 remain highlighted and other bioregions and subregions emerge as significant (Figure 7.5 and Table 7.1) - the Jarrah Forests, Warren, Swan Coastal Plain, Carnarvon, Central Kimberley, Pine Creek, Arnhem Plateau, Cape York Peninsula, Einasleigh Uplands, Brigalow Belt North, South East Queensland, New England Tablelands, South East Coast and South Eastern Highlands, South East Coastal Plain, Victorian Midlands, Tasmania South East, Flinders Lofty Block, and Eyre Yorke Block. These bioregions are important because of the large numbers of endemic species they contain.

Table 7.1: Summary and comparison of different metrics used to examine the patterns of species richness, endemism and irreplacability for Acacia and eucalypts in bioregions.

BIOREGIONS	TOTAL NUMBER OF SPECIES	NUMBER OF ENDEMIC SPECIES	ENDEMICITY INDEX	IRREPLACABILITY INDEX
Arnhem Plateau			*	**
Avon Wheatbelt	**	*	**	**
Brigalow Belt North	*		*	**
Brigalow Belt South	**	**	**	**
Cape York Peninsula			*	**
Carnarvon			*	**
Central Kimberley			*	*
Central Ranges				*
Coolgardie	**	**	**	**
Dampierland				*
Desert Uplands	*
Einasleigh Uplands	*		*	**
Esperance Plains	**	**	**	**
Eyre Yorke Block			*	**
Flinders Lofty Block			*	*
Gascoyne				*
Geraldton Sandplains	**		**	**
Great Victoria Desert	*			**
Gulf Plains	*
Jarrah Forest			*	*
Little Sandy Desert				*
MacDonnell Ranges				*
Mallee	**	*	**	**
Mitchell Grass Downs	*
Murchison	*		*	*
Murray Darling Depression	*
Nandewar				*
New England Tableland	*			**
Northern Kimberley		*	**	**
N.S.W. North Coast	**	*	**	**
N.S.W. South Western Slopes	*
Ord Victoria Plain	*			*
Pilbara		*	*	**
Pine Creek			*	*
South East Coastal Plain			*	*
South Eastern Highlands	**		*	**
South East Corner	*		*	**
South Eastern Queensland	*		*	**
Swan Coastal Plain			*	**
Sydney Basin	**			**
Tasmanian South East			*	*
Victorian Midlands			*	**
Warren			*	*
Wet Tropics				*
Yalgoo				*
Note: ** indicate highest values for bioregions for the combined Acacia and eucalypt data * indicate moderately high values.

Irreplacability

Subregions with endemic species are important because they are the only places where these species are found. There are limitations with inferring conservation priorities from endemism. Importantly, endemicity is a scale-dependent attribute. For example, Figure 7.5 shows differences between areas of high endemism at the bioregional scale compared with the subregional scale. At the larger scales there will naturally be higher numbers of endemic species. To overcome this, an overall measure of irreplacability is assessed. This incorporates the significance of subregions in terms of both the number of species present and the proportion of those species that have restricted distributions.

The Irreplacability Index is a relative measure of the degree to which the species complement of a bioregion or subregion can be substituted for by another region or regions. Subregions with more endemic species will have a higher score of the index. Large numbers of species in a bioregion or subregion also contribute to a higher score. Both large numbers of species and the substitutability of the species are combined in the Irreplacability Index. This Index encapsulates both the importance of a location for irreplaceable species and for potentially high habitat diversity associated with large numbers of species.

Bioregions and subregions identified in Figures 7.4 and 7.5 all have a high Irreplacability Index. Additional subregions are highlighted (Figure 7.6, Table 7.1). For acacias, some subregions of the Murchison, Cape York Peninsula and the South East Corner become more prominent (Figures 7.4 and 7.6). For eucalypts, some subregions of the Murchison, Great Victoria Desert, Central Ranges, Ord Victoria Plain, Einasleigh Uplands, New South Wales South Western Slopes, and South East Corner become more prominent.

Comparison of the Irreplacability Index of the combined Acacia and eucalypt patterns at bioregional scale (Figure. 7.6) with the Endemicity Index (Figure. 7.5), shows that the same general patterns emerge (Table 7.1). The most significant bioregions are the Avon Wheatbelt, Esperance Plains, Coolgardie, Mallee, Geraldton Sandplains, Carnarvon, Pilbara, Northern Kimberley, Arnhem Plateau, Cape York Peninsula, Einasleigh Uplands, Brigalow Belt North, Brigalow Belt South, South Eastern Queensland, New South Wales North Coast, New England Tableland, Sydney Basin, South Eastern Highlands, Victorian Midlands, and the Eyre Yor ke Block. Areas of moderately high irreplacability are the Jarrah Forest, Warren, Yalgoo, Murchison, Gascoyne, Little Sandy Desert, Central Ranges, MacDonnell Ranges, Dampierland, Central Kimberley, Ord Victoria Plain, Pine Creek, Wet Tr opics, Nandewar, South East Coastal Plain, Tasmanian South East, and the Flinders Lofty Block.

Condition and Trend of Bioregions and Subregions

Subregions with high levels of endemism and irreplacability were examined in the context of the condition of their landscapes using the landscape stress classes developed by the Audit (NLWRA 2001a). This approach was used in lieu of an examination of condition and trend for each endemic species in a bioregion or subregion as this level of information is not yet available. Further assessment is required to link endemic species to ecosytems and identify the status, threatening processes and management needs for each ecosystem. The top two categories of the Irreplacability Index (greater than 0.5 in Figure 7.6) are shown with their stress class values in Figure 7.7.

Many subregions of south-western Western Australia have a high level of irreplacability because they contain large numbers of Acacia and eucalypt species, and many endemic species of these taxa.

This region is also amongst the most degraded in Australia. There is a general lack of data describing the population trends and conservation status of species in these subregions and bioregions. Nevertheless it seems likely that they would be under moderate to high threat because of the pressures from clearing and fragmentation of native vegetation, increases in salinity and other threatening processes. This deficiency in knowledge should be addressed to assist their conservation.

These bioregions and subregions are important for preserving the variety of Acacia and eucalypt species. Forty-two species of Acacia and 29 species of eucalypt occur only in these highly degraded bioregions and subregions in south-western Australia. Parts of the New South Wales South Western Slopes, Brigalow Belt North and South, and Victoria also have a moderate rating of irreplacability and species there are likely to be threatened when compared with the overall assessment of landscape health.

Figure 7.7 Subregions with Irreplacability Indices greater than 0.5 for acacias (top left) and eucalypts (top right), and for combined data of acacias and eucalypts (bottom) in subregions showing landscape stress classes.

Conclusions

This assessment using two key plant groups, reveals a number of areas of significant diversity in addition to the regions of high diversity for a broad range of plant groups recognised in previous studies.

It confirms that species with restricted ranges are not randomly distributed across the landscape, but are aggregated in bioregional centres (Figure 7.4). The most likely explanation for these patterns is that over evolutionary timescale, variable climatic regimes and changing geological and geomorphological systems result in events which fragment species distributions and initiate speciation processes.

The analysis shows that species-rich regions are only one indicator of conservation priorities. Because this measure does not indicate how species are distributed across the Australian landscape, indices of endemism and irreplacability have been used in this analysis to identify bioregions and subregions that are important for the restricted types of species they support.

Areas (or parts thereof ) of south-west Western Australia, the Kimberley, the ranges in north-eastern New South Wales, south-eastern Queensland and the Sydney Sandstones were previously recognised as supporting significant numbers of endemic plants by Burbidge (1960) and more recently by Boden & Given (1995) and Crisp et al. (2001). The bioregions showing high levels of endemism for Acacia and eucalypts (Figure 7.5, Table 7.1) show some important differences from previous analyses.

Some bioregions that have not been recognised in the past, were found to support high species richness and endemicity of Acacia and eucalypts: the Pilbara, Carnarvon, Great Victoria Desert and Einasleigh Upland bioregions. The differences observed from previous analyses are most likely due to the wide range of plant taxa included in the datasets of the various analyses. Besides species of Acacia and eucalypt, rainforest taxa, rare and threatened species and other land cover groups have been included in the previous studies. The results of the current study also do not emphasise plant communities that are not dominated by Acacia or eucalypts, such as rainforests and heathlands. Hence, these results should not be seen as representing patterns for all Australian flora. Similar analyses should be repeated for other taxonomic groups as the information on these taxa becomes available.

Subregions of inland Australia that have high levels of irreplacability were found to have low landscape stress class values. This is not surprising as the stress class values are very strongly influenced by land clearing and fragmentation, primarily features of agricultural bioregions and subregions. In inland Australia, altered fire regimes and grazing by domestic stock are the primary variables driving species to the point of conservation concern, and these drivers of change are not fully delineated by stress class categories. Consequently, this analysis may understate the conservation issues occurring in inland Australia.

Indices of endemism and irreplacability have been used to identify bioregions and subregions that are significant for the numbers and types of Acacia and eucalypt species they support. These bioregions and subregions contain a complement of species that cannot be represented anywhere else on the continent and so are irreplaceable natural features.

Priorities for conservation need to also take into account the current status and threats to the species identified in this assessment. There is little specific information on the status and trends for acacias and eucalypts. This is also true for most other plant groups. Monitoring trends in key species and threatening processes in selected bioregions and subregions will build an understanding of the status of these elements of biodiversity at an Australia-wide scale, and greatly enhance the information required to support their conservation.

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