The Action Plan for Australian Rodents
Environment Australia, April 1995
Introduction
The native mammalian land fauna of the Australasian Biogeographical Region is unique in several respects. It is the only fauna containing living representatives of the three sub-classes of extant mammals - Prototheria, Metatheria and Eutheria. It is the only fauna dominated by taxa with Gondwanan affinities, and it has experienced extinction rates over the past two hundred years which are probably unparalleled in recent mammalian history. For these reasons it warrants special attention.
In origin the Australasian mammals have two components - the monotremes (Prototheria) and marsupials (Metatheria) and possibly some bats (Eutherians), which have Gondwanan affinities and levels of structural diversity which reflect early radiations within the region, and the remaining eutherians, which are relatively recent invaders with Asian ties, and members of taxa whose members have recognised abilities to disperse along island chains. The early Tertiary association of the Australian Continental Plate with the remnants of Gondwana (South America and Antarctica), and its subsequent separation and progression northwards towards Asia, have facilitated these invasions.
Two taxa overwhelmingly dominate the eutherian component - the Chiroptera and the Rodentia. Both appear to have invaded the region more than once and both have experienced radiations within the Australasian region (McKay et al. 1989).
The historical decline of mammals in the region has been almost entirely restricted to marsupials and rodents (Kennedy 1992). Although the high extinction rates among Australian marsupials have received the greatest notoriety, and sometimes been used to support the argument that they are inferior to eutherians in a competitive situation, extinction rates among Australian rodents have been equally dramatic.
The conservation status of Australian rodents has previously been reviewed by Watts (1979), Watts and Aslin (1981) and Calaby and Lee (1989), but all their assessments were based upon subjective criteria. Burbidge and McKenzie (1989) provide a comprehensive review of the causes of historic declines in the Australian terrestrial vertebrate fauna. This Action Plan attempts to assess the status of Australian rodents using objective criteria, reviews the reasons for their decline and recommends actions that should arrest their decline and secure their future.
All rodents native to Australia belong to the family Muridae, and appear to have been derived from two invasions of the region (McKay et al. 1989). One of these probably occurred between the Miocene and early Pliocene and is represented today by the diverse Hydromyinae, sometimes referred to as the "old endemics". The second occurred during the late Pliocene or early Pleistocene and is represented by all of the endemic Rattus (Hand 1984).
The Hydromyinae have no known representative in south-east Asia (Watts and Aslin 1981). The oldest fossil rodents in the Australasian region have a maximum age of 4.5 million years (Hand 1984), but molecular data suggest that the Hydromyine radiation began much earlier, perhaps 15 to 20 million years ago. Watts and Aslin (1981) proposed that this occurred on the eastern Indonesian islands, and that the major Hydromyine groups, the Conilurini, Hydromyini and Uromyini, had differentiated before their invasion of New Guinea and Australia. Further radiation probably occurred 5 to 10 million years ago, among the Hydromyini and Uromyini in New Guinea, and among the Conilurini in Australia.
Flannery (1990, p. 173) describes the Uromyini (it is highly likely that the Uromyini is polyphyletic) as a "highly heterogeneous assemblage" of species which "are unique to, or have their distribution centred on, New Guinea, and are herbivorous". The only Uromyini genera represented outside Melanesia are Melomys (4 species), Uromys (2 species), and Pogonomys (1 species), all of which are found in Australia, primarily in the north.
The same claims of heterogeneity apply to the Conilurini or Australian "old endemics" (Lee et al. 1981), which occur almost entirely within Australia. Their greatest species-richness is in the arid centre of the continent and in the wet-dry tropics of the north. They are more catholic in diet; many species are clearly omnivorous but a proportion remain strictly herbivorous. Only two conilurines are found outside Australia, in New Guinea, Conilurus penicillatus and Pseudomys delicatulus, and both are restricted to the trans-Fly plains (Flannery 1990).
The Hydromyini of New Guinea and Australia has its greatest species-richness in the montane regions of New Guinea (Flannery 1990). Only two species are found in Australia, one of which, Xeromys myoides, may be endemic. They contrast with the Conilurini and Uromyini in that all hydromyines appear to be insectivorous or carnivorous.
Despite their recent arrival, the more homogeneous endemic Rattus have been surprisingly successful. There is at least one representative in most habitats in New Guinea and mesic coastal Australia, and two or three species are sympatric at many sites (Lee et al. 1981, Flannery 1990). In Australia, however, only two species, R. villosissimus and R. tunneyi, have penetrated the arid heart of the continent (Taylor and Horner 1973). Two species, R. leucopus and R. sordidus, occur in both Australia and New Guinea, and there is evidence that members of this group have dispersed between Australia and New Guinea on at least five occasions (Baverstock et al. 1977 a & b, Lee et al. 1981, Taylor et al. 1982). The endemic Rattus are all omnivorous or herbivorous.
Within Australia rodents make up approximately 25 percent of the native land mammals. All of the major groups of Australasian rodents are represented in this fauna but the tribe Conilurini (49 species) far overshadows the contributions of the endemic Rattus-group (7 species), and the tribes Uromyini (7 species) and Hydromyini (2 species).
Taxonomic Classification of Rodents Reviewed in this Action Plan
The classification and taxonomy of Australian rodents adopted in this Action Plan follows Baverstock (1984) and Mahoney and Richardson (1988), respectively. Pseudomys praeconis is included in the synonymy of P. fieldi (see Baynes 1990), and Kitchener's (1989) revision of the genus Zyzomys has been adopted. Sixty-two native rodent species extant at the time of European settlement are listed in Table 1.
Criteria for Assessing Conservation Status
Traditionally, classification of the conservation status of animal taxa (e.g. IUCN 1988) has been based upon systems of categories (endangered, vulnerable, rare etc.) which are defined subjectively. For those concerned with conserving species, these systems have a number of weaknesses. Because these categories bear no clear relationship to one another in time, they cannot be used to track taxa through the decline and recovery processes. Because they are usually applied independently to different taxa, they offer no means of objectively defining priorities for action across faunas.
To counter these and other weaknesses, Mace and Lande (1991) have redefined the categories previously used in IUCN Red Lists by adopting a set of objective criteria derived from theory of the extinction times for single populations. Taxa are classified according to their probable time to extinction. Ideally, species are assigned to a category on the basis of an accurate viability analysis. Where this is not possible, Mace and Lande provide a set of alternative, but theoretically consistent, criteria (see below).
We know of no probabilistic viability analysis for any Australasian rodent species and doubt whether knowledge of the population parameters of the majority of species are sufficient to perform such an analysis. Furthermore, because of our poor understanding of the biology of many Australian rodents, we have had difficulty in meeting the requirements of their alternate criteria. For most species, we have no knowledge of population size (N), number of extant populations and their sizes, or rate of change in the size of these populations. We nevertheless accept that the virtues of the system of categories defined by Mace and Lande (1991) outweigh the difficulties it poses, and importantly, define the information we need to judge the effectiveness of conservation action. For this reason and because the previously adopted subjective system of IUCN (IUCN 1988) is still widely used, we have identified the status of species according to both systems. A list of the categories and a description of the criteria for each is provided in Appendix 3.
Conservation Status of Australian Rodents
Our analysis of the conservation status of Australian rodents is summarised in Table 2 (table index on page 6), together with CITES listings, participation in the Coordinated Captive Breeding Program, and a previous assessment by ANZECC. Some qualifications to our decisions are provided in Table 3, and the criteria which provided the basis for decisions to categorise individual species as Critical, Endangered or Vulnerable under the system of Mace and Lande (1991) are given in Table 4.
Twelve percent of Australian rodent species extant at the time of European settlement are now extinct. A further 20 percent are categorised as either Critical, Endangered or Vulnerable, 18 percent require additional information before their status can be assessed, and 50 percent are considered secure. All of the extinct species and those listed as Critical, Endangered or Vulnerable are conilurines, whereas all of the endemic Rattus are considered secure.
Causes of Decline and Extinction of Australian Rodents
Australia accounts for about one third of the world's mammals that have become extinct in modern times (Kennedy 1992), and roughly one half of these are rodents. Seven, possibly eight Australian rodents extant at the time of European settlement are now extinct and another 11 rodent species have shown a decline in range of 50 percent or greater over the same time period (Table 2).
In some regions of the continent subfossil deposits allow us to reconstruct the pre-European faunas, providing opportunity for a more detailed assessment of historical trends. Twenty-one rodent species occurred in Victoria in the late Pleistocene and Holocene (Wakefield 1966, 1974; Cockburn 1979). Two of these (Pseudomys higginsi and Melomys cervinipes) were probably extinct in Victoria prior to European settlement (Wakefield 1972; Hope 1974), ten have become extinct since the arrival of Europeans, and four have suffered significant range reductions (Cockburn 1979). Thus not all of the extinctions and range reductions detected by these comparisons can be attributed to impacts following European settlement. Nevertheless, the attrition since 1800 has been catastrophic. Similar declines have occurred elsewhere in Australia. Five of 16 species of rodents recorded in subfossil deposits from the south and western coasts of Western Australia are now extinct and a further nine have reduced ranges (Cockburn 1979). A similar, perhaps even greater attrition has occurred among the rodents of semi-arid and arid Australia (Aitken 1970; Newsome and Corbett 1975, Burbidge and McKenzie 1989, Bennett et al. 1989, Morton 1990, Dickman 1993).
In their comprehensive review of the terrestrial vertebrate fauna of Western Australia, Burbidge and McKenzie (1989) found that modern declines were virtually confined to non-flying mammals with an average body weight in the range 35 to 4200g. This they referred to as the Critical Weight Range, or CWR. Within this group of mammals, extinctions and declines were greatest among omnivorous and herbivorous ground-dwelling species in arid and semi-arid areas. Marsupials and eutherians (rodents) were similarly affected. These conclusions held when the Australian non-flying mammal fauna as a whole was examined, although the upper limit of the CWR was extended to 5500g.
Burbidge and McKenzie (1989) dismissed explanations for these declines which were based on single factors such as disease, altered fire regimes, climatic change or the impact of introduced grazers etc. Instead they attributed these declines to environmental changes since European settlement which have emulated an increase in aridity by reducing the environmental productivity available to vertebrates. Such changes include the diversion of environmental resources to humans and introduced species, a reduction in vegetation cover by exotic herbivores and altered fire regimes. Exotic predators have exacerbated this attrition. They argued that CWR non-flying mammals are particularly susceptible because of their limited mobility and high daily energy requirements. Based on this argument there should be no lower size limit to the CWR. Indeed, one of the Australian rodents which has experienced a substantial reduction in range has an average body weight substantially less than 35g (Table 5), although there is no question that range reductions have occurred more frequently in medium-sized mammals.
Morton (1990) has extended Burbidge and McKenzie's general hypothesis, viewing arid Australia as a resource-poor landscape through which were scattered resource-rich, highly productive patches. Morton concluded that many of the arid zone mammals were dependent upon these resource-rich patches, particularly in times of drought, and that resources in these patches have been depleted by human intervention (e.g. grazing by introduced herbivores, altered fire regimes etc.). In addressing the influence of body size on susceptibility, Morton argued that medium-sized non-flying mammals, by virtue of metabolic constraints, are confined to smaller, richer patches of habitat than larger species. Their lack of mobility also restricts their ability to recolonise patches. At the lower end of the size scale, small non-flying mammals require less energy per individual and so can persist in smaller refuge patches. Thus, both small and large mammals have advantages which do not accrue to medium-sized mammals. Morton, like Burbidge and McKenzie (1989), argued that a number of factors operating in concert have caused declines and extinctions of the medium-sized mammals, particularly those factors contributing to habitat destruction in resource-rich patches in the arid zone.
Our assessment of the conservation status of Australian rodents shows patterns largely consistent with their conclusions. Extinctions and substantial range contractions have been most frequent among rodents from arid regions of Australia, and among rodents whose range extended into those regions (Table 5). Most have occurred among rodents in the CWR. Where it has been possible to infer the cause of the decline in abundance or range of a species, several factors have been implicated which, acting in concert, result in destruction of vegetation cover (Table 6). We also note that declines have been largely confined to the Conilurini, which unlike the Australian Hydromyini, Uromyini and Murinae, are well represented in the arid regions of Australia.
While this analysis suggests where conservation management should be directed (e.g. towards conserving conilurine rodents in the CWR from arid Australia), and how it should be directed (e.g. preventing habitat destruction particularly in resource-rich patches), a number of the species whose status is of concern (Critical, Endangered or Vulnerable) occur outside the arid zone and should not be overlooked.