|Resource use by the Eastern Grey Kangaroo and the Black Wallaby in a managed remnant woodland community|
|Fred G. de Munk|
|School of Ecology and Environment
This work studies the relationship that exists between the resource utilisation of two large macropodid species and its impact on their distribution and abundance in a managed remnant woodland environment in Victoria, Australia.
Managed remnant woodlands offer conservationists many strategic decision making opportunities. The arrival of European agriculture, its accompanying suite of introduced species, structural changes to the vegetation and the concomitant disappearance of ancient, indigenous practices has brought into focus ecologically based disturbances in natural processes and regimes which challenge current practices.
The Black Wallaby (Wallabia bicolor Desmarest) (also known as the Swamp Wallaby) and the Eastern Grey Kangaroo (Macropus giganteus Shaw) (Marsupialia, Macropodidae) are two species of large macropodid that occupy many such habitats in south-eastern Australia. The management of these macropodids has often been problematic. This thesis provides a closer understanding of the way in which these species interact to allow them to occupy similar habitats. It also provides information managers can use to assist them in providing solutions which arise when the species are present in confined, remnant woodland habitat patches.
A managed remnant woodland community in eastern Victoria was the study site. Faecal pellets from Eastern Grey Kangaroos and Black Wallabies were collected from ten different communities on three occasions over one calendar year. The spatial and temporal distribution of the animals over the seasons was identified by analysing the frequency of occurrence of fresh faecal pellets in the various communities. These distribution patterns were then compared with the botanical composition. Hence a floristic analysis of these communities was also conducted during this field work. This consisted of plant biomass measurements, estimates of abundance and cover determinations. The utilisation of various plant associations and communities by the two macropodid was then measured.
A reference herbarium of the stem, abaxial and adaxial leaf epidermis of 233 species of plants was accumulated to assist in determining the foods taken by the two macropodid species. The epidermal specimens were prepared for, and examined using Confocal Scanning Laser Microscopy (CSLM). The information gained was enhanced and stored digitally and a digital image herbarium was thus amassed. Diagnostic information critical for the identification of these epidermal fragments was assembled into a computer database which was then used to assist in the recognition of unknown epidermal fragments in macropodid faeces.
These epidermal plant recognition techniques enabled a comprehensive list of the cuticular contents of Eastern Grey Kangaroo and Black Wallaby faeces during the sampling periods in the individual communities to be accumulated. By correlating the abundance of dietary plants in the field to their frequency of occurrence in the faeces a measurement of macropodid dietary selection was made.
For Eastern Grey Kangaroos, diets demonstrated small variations over the seasons but remained constant in any one season over the communities studied. Similarly diet selection was shown to be constant over the year and throughout the different communities studied. The occupation of habitat by Eastern Grey Kangaroos could be demonstrated to be a function of floristic structure, and kangaroos appear to be less able to utilise diverse food species than are wallabies.
Black Wallabies showed considerable differences in their diets over both the seasons and the communities at any one time. They were able to utilise diverse community structures and plant associations, including food niche used by Eastern Grey Kangaroos when it was available. Black Wallabies were shown to be generalist feeders.
Historically the two macropodid species have been considered able to co-exist because Eastern Grey Kangaroos were thought to utilise graze whilst Black Wallabies were considered browsers. The findings of this work indicate that there is significant overlap in dietary components of the two species. If Black Wallabies utilise Eastern Grey Kangaroo feeding habitat then competition for forage may occur. Similarly, the diet of Black Wallabies was found to be more diverse than that of Eastern Grey Kangaroos and it is concluded that Black Wallabies are more capable of utilising a variety of food resources than Eastern Grey Kangaroos, which were found to be more specialist feeders. Black Wallabies are therefore considered to be largely responsible for decreases in plant diversity which may be observed when populations of the two macropodid species increase in size in shared habitat in the absence of significant predation.
The use of faecal pellet density as a measure of habitat utilisation by macropodids is examined and it is concluded that although the method has merit great care must be used in its application. The rate of faecal pellet production and the number of scats produced changes with the seasons and the habitat utilised. The method must therefore be temporally and spatially standardised. It was shown that for the two macropodid species studied at the Coranderrk Reserve, faecal pellet counts were satisfactory measures of animal densities and that it was not necessary to measure their biomass. Black Wallabies used habitat in all communities in the study site and were able to utilise resources in a variety of habitats. Eastern Grey Kangaroos selected habitat more narrowly, limiting themselves to grassy and more open ecosystems. Significant resource partitioning with respect to diet and habitat selection was demonstrated.
The implications of these findings for current wildlife management practices involving populations of these two species is discussed. It is recommended that both species need to be targeted in control strategies. A common current practice is to reduce the numbers of Eastern Grey Kangaroos only. It is argued in this work that such monospecific population reduction techniques will not address the observed decreases in plant diversity which accompany population increases. It is concluded that Black Wallaby herbivory also impacts significantly on ecosystem integrity and that management strategies should be implemented which establish sustainable populations of both species.
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