Modelling Koala Populations and Management Strategies

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Modelling Koala Populations and Management Strategies

In Australia we have many species that we consider pests. Most people don’t consider the koala (Phascolarctus cinereus) to be one of them. Unfortunately in the late 1800’s and the early 1900’s millions of koalas were killed for their thick reddish brown to grey fur making them extinct in the state of South Australia and endangered across the rest of the country. In a bid to reverse this disturbing trend and conserve this clearly vulnerable animal a group of 18 koalas and a number of young were introduced onto Kangaroo Island in South Australia. These koalas had no natural predators and humans were forbidden from hunting the animals for their fur (Masters etal, 2004).

Unfortunately with none of these pressures that mainland koalas experienced the population quickly rocketed to unsustainable numbers reaching well over 20,000 individuals by 2001. Koalas selective breeding habits have seen significant negative impacts on many habitats that contain their preferred trees. Since 1997 a management program which involves sterilisation of female koalas and translocation of koalas to mainland habitat has been in place to reduce the amount of koalas on the island (Masters etal, 2004).

Unfortunately the number of koalas present on the island was underestimated and therefore management targets were set to low. It is for this reason that we have been employed by the South Australian government to provide an indication of what management is needed to continue to reduce the koala population to a sustainable level. The management strategies available to us as pest management experts are sterilisation of koalas and release back onto the island or translocation of sterilised individuals(Masters etal, 2004).

In order to complete this task and set a management strategy for the area in question we will be using a logistical growth model. This is simply a mathematical model that will provide an indication of the population growth rates under different management strategies and environmental conditions. These mathematical models have many limitations however they can give a basic idea of how the population will respond to the changes in there management (Tsoularis & Wallace, 2002).

The main objectives that we wish to achieve throughout this study are to evaluate the effectiveness of removal and fertility control strategies on Kangaroo Islands population of koalas and find out which one is the optimum strategy. The other objective is to identify the strengths and weaknesses of mathematical models such as the one that we are using for assessing pest animal management strategies.

In our scenario we have been given an area of 5,000 hectares to manage in which we have 10,000 individual koalas present. Our aim is to reduce this population to a sustainable population of 3,750 koalas. To reduce the population to 3,750 individuals we can use either translocation and sterilisation or a combination of both. We have been given a number of different strategies and each of these must be tested through the logistical growth model to see which strategy will give us our desired result of maintaining the population at our target rate.

The first major part of this task was to assess the given management scenarios to see which one gave us our desired results of a constant population of 3,750 individual koalas. This simply involved typing the listed figures into our mathematical model and viewing the graph supplied. After testing all the scenarios supplied in figure 1, scenario four and five gave results showing the population reaching the sustainable population target of 3,750 individuals and maintaining this target for many years. I believe scenario four provided the best results when translocation of five hundred individuals was completed every five years and fertilisation of 50% of females was completed every two years.

Scenario four (below) was not logistically feasible due to the...
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