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Broad scale influences on contemporary abundance of saltwater crocodiles (Crocodylus porosus) in Northern Australia

Fukuda, Yusuke (2004). Broad scale influences on contemporary abundance of saltwater crocodiles (Crocodylus porosus) in Northern Australia. Honours Thesis, Charles Darwin University.

Document type: Thesis
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Author Fukuda, Yusuke
Title Broad scale influences on contemporary abundance of saltwater crocodiles (Crocodylus porosus) in Northern Australia
Institution Charles Darwin University
Publication Date 2004
Thesis Type Honours
Abstract Populations of the Saltwater crocodile Crocodylus porosus in northern Australia were severely depleted due to uncontrolled commercial hunting from the 1940s to 1970s. Since legal protection in the 1970s, recovery of the species has been carefully monitored across north Australia. Population data accumulated from these monitoring surveys consistently show that the Northern Territory has achieved much greater increases in population number than in Queensland and Western Australia. The difference in population abundance between the Northern Territory and the other States has been linked to a number of different aspects of the environment including natural feature and human impacts. However, the nature of the relationship between population abundance and the quality of the environment remains unclear.

To examine potential species-environment relationships, I proposed that the status of contemporary crocodile populations is most influenced by the environment, rather than the residual effects of past hunting. I first examined this hypothesis by comparing density estimates for contemporary and pre-hunting populations in a range of rivers in the Northern Territory. The comparison showed a reasonable match between the contemporary and pre-hunting populations, suggesting that the populations have recovered to levels broadly comparable with status prior to commercial hunting.

To further test this hypothesis, I then examined trends in population growth of the rivers since their protection by fitting a range of population growth models using information theoretic approach. For the most frequently surveyed major river systems, the information theoretic criteria showed strong support for the logistic model rather than the linear and exponential models. The predicted carrying capacities were close to the observed contemporary densities, which suggests that the recovery of populations is well advanced in most river systems of the Northern Territory and the rate of population increase is now being limited by the quality of the environment rather than the impact from past hunting.

Having found support for this hypothesis, I then developed hypothesis-driven models about relationships between population abundance and the environment based on the species’ biology and ecology. The fit of these models was also examined using the information theoretic procedures. The results showed strong evidence for a model with natural environmental variables namely ‘the mean temperature in the coldest quarter of a year’, ‘the ratio of total area of freshwater wetland dominated by the Melaleuca species and seasonal floodplain dominated by the grass and sedge to total area of catchment or subcatchment’, and ‘the ratio of total precipitation in the warmest quarter of a year to total precipitation in the coldest quarter of a year’. The results also showed no strong support for models incorporating human-impact variables, suggesting that contemporary population abundance of the species appears to be primarily influenced by the quality of the natural environment rather than the contemporary impact of human activities. However, further examination of these questions using different datasets at finer spatial scales is desirable.

The natural environmental factors selected as best describing the variation in population abundance of C. porous in northern Australia are all likely affected by increase in temperature, precipitation, and sea level due to global warming. Maintaining favourable vegetation communities in freshwater wetlands and seasonal floodplains seems the primary management issue for maintaining the species abundance and its large contribution to the environment and economy of northern Australia.

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