Application of island biogeographic principles to the selection and management of Tasmanian dry sclerophyll reserves

Throughout the world today the areas occupied by many natural habitats are undergoing two types of change. Firstly, the total area occupied by natural habitats is shrinking and secondly, formerly continuous natural habitats are being fragmented into disjunctive pieces. Both these processes have important consequences for the future of the world's plant and animal species. To preserve natural diversity it is critical, therefore, that the present methods of conserving wildlife be reviewed and that careful planning of reserves be undertaken, based on appropriate ecological principles. A comparison may be drawn between a nature reserve and an oceanic island from the point of view that each is surrounded by a zone of inhospitable habitat which acts as a barrier to dispersal for many species. Considerable progress has been made over the last decade towards an understanding of the functioning of oceanic island systems, particularly as regards the maximum number of species that an island may support without threat of extinction to any species. Application of this knowledge may provide a basis for understanding what to expect from reserves and, most importantly, may offer a method for determining the minimum size of a reserve needed to maintain the diversity of plant and animal species characteristic of that habitat type. To date, studies in the field of island biogeography have concentrated on natural islands with little attention given to the habitat islands created by man in clearing land for agricultural and commercial activities. The aim of the present study is to investigate the species-area relationship for a series of islands of dry sclerophyll forest on the Tasmanian mainland and to compare the findings of this study with previous studies undertaken on oceanic islands. The implications of establishing a similarity between the two types of islands are far-reaching. It is hoped that this and other similar studies will serve to emphasize the special features of habitat islands and that, in future, more attention will be given to the planning of reserves in accordance with island biogeographical principles. Thus determination of the most suitable size and shape of a reserve, as well as planning for an integrated system of reserves which functions in the most effective way possible, may be achieved. Apart from being of general significance the study is also important at the local level. In Tasmania, as in other places of the world, the natural environment is being fragmented as land is cleared to make way for urban and rural development. In the past the dry sclerophyll forest has been the habitat type most heavily exploited for these purposes and, more recently, has been subject to further pressure from the woodchip industry. Furthermore, whilst provision has been made for the conservation of other vegetation types within the State adequate reservation of areas of dry sclerophyll forest has failed to occur. For these reasons it is imperative that the present situation regarding the dry solerophyll reserve system in Tasmania be examined and the need to establish additional reserves be investigated as soon as possible. The desirability of having large-sized reserves to ensure maintenance of species diversity has been emphasized by several authors recently (Willis 1974; Diamond 1975; Moore and Hooper 1975; Sullivan and Shaffer 1975; Terborgh 1975). Few authors, however, have actually suggested dimensions. Unfortunately, it is extremely difficult to demonstrate experimentally the concept of minimum reserve size; nevertheless there have been some attempts, using a variety of approaches, to determine the size of a reserve required to maintain a particular taxon or group of species. These studies will be discussed further in the following chapter but it is important to emphasize that none of the authors have adopted the approach of the present study, in making use of the island nature of reserves and applying the corresponding biogeographic principles. In order to understand why the field study was conducted and how the results of the study could be used to determine the area requirements for a dry sclerophyll reserve, it is necessary to trace the development of the theory of island biogeography and to examine the arguments which have been advanced in applying these principles to the design and management of nature reserves. This review introduces the field study where the planning and executing of the study are described in detail. To develop a method for determining the size requirements of a reserve it was not necessary to conduct lengthy studies and, accordingly, all field work could be completed within a year by three workers. The apparent brevity of this study sets it apart from other more detailed pieces of scientific work but does not detract from the validity of the methods used or from the results obtained. The results of this study are analysed and a minimum area, which would preserve the diversity of dry sclerophyll species,is suggested. The exploitation of the forest, both past and present, is then examined and the existing reserve system is also assessed. The need for augmenting this system is emphasized and some areas, which in biological terms may be appropriate for reservation, are discussed. It should be pointed out, however, that there are many other aspects which need to be considered before reserving an area but which are essentially beyond the scope of this report. The responsibility of dealing with these factors lies with those agencies which are involved in the planning Of reserves at the government level.