Broad-scale environmental influences on the abundance of saltwater crocodiles (Crocodylus porosus) in Australia

Abstract

Saltwater crocodile (Crocodylus porosus) populations have recovered strongly across northern Australia over the 30 years since the species was protected from hunting. However, monitoring studies show large geographical variations in abundance across the Northern Territory, Queensland and Western Australia. The Northern Territory has considerably higher densities, raising questions about constraints on recovery in the other states. We examined broad-scale environmental influences on population abundance by modelling the species–environment relationships across northern Australia. The hypothesis-based models showed strong support for the linkage to (1) the ratio of total area of favourable wetland vegetation types (Melaleuca, grass and sedge) to total catchment area, (2) a measure of rainfall seasonality, namely the ratio of total precipitation in the coldest quarter to total precipitation in the warmest quarter of a year, and (3) the mean temperature in the coldest quarter of a year. On the other hand, we were unable to show any clear negative association with landscape modification, as indicated by the extent of high-impact land uses or human population density in catchments. We conclude that geographical variations in crocodile density are mostly attributable to differences in habitat quality rather than the management regimes adopted in the respective jurisdictions.

Long‐term monitoring reveals declines in an endemic predator following invasion by an exotic prey species

“…Fourth, freshwater crocodiles (but not saltwater crocodiles) were observed feeding on cane toads during surveys and dead individuals with no obvious signs of trauma were in these intermediate size classes. It is also worth noting that crocodile habitats in the study area changed little over the study period (Fukuda, Whitehead & Boggs, 2007) and are unlikely to explain our results.…”

Home Range Utilisation and Long-Range Movement of Estuarine Crocodiles during the Breeding and Nesting Season

“…Daily rate of movement (ROM) was included as the response variable, with days from 01 September (date) and body mass (extrapolated from SVL using the conversion factors in [34]) as covariates, sex as a factor, and crocodile ID as random effect. A second model assessed the relationship between the daily distance each individual was located from the centroid of its KUD 50%, with date and body mass as covariates, sex as a factor, and crocodile ID as random effect.…”

Abundance of Saltwater Crocodile Hatchlings is Related to Rainfall in the Preceding Wet Season in Northern Australia

“…This difference might result from highly variable habitat quality, especially for nesting, and thereby abundance of crocodiles among the monitored rivers. For example, the Adelaide, South Alligator and Wildman rivers had considerably larger freshwater habitats suitable for nesting (Fukuda et al, 2007), whereas the density of nonhatchlings in the Daly, East Alligator and Wildman rivers was generally higher than in the other rivers (Fukuda et al, 2011). Even within a single river, the density of hatchlings was correlated to only some months (Table 1).…”

“…Given that the typical incubation period of C. porosus eggs is approximately 90 d (Webb et al, 1984), extended raining over 7 or 8 mo might enable a nesting site to hold more than one clutch without temporally overlapping, resulting in higher numbers of eggs laid in a single season. Because the availability of suitable nesting sites is the most critical factor limiting the abundance of crocodiles across northern Australia (Fukuda et al, 2007), higher rainfall in these months is particularly important. Another positive effect of extended wet season is the enhancement of hatchling survival, which is density dependent on other hatchlings (Webb and Manolis, 1992).…”

“…Over the past century, habitat loss caused by an expanding human population and extensive hunting for commercial trade depleted many populations of C. porosus throughout its range (Webb and Manolis, 1989;Webb et al, 2010). Consequently, the species became protected in 1971 in the Northern Territory, where the greatest proportion of the Australian population of C. porosus occurs (Webb et al, 1984;Fukuda et al, 2007). Since protective legislation was established, some populations have demonstrated consistent recovery and today Australia supports one of the largest populations of Saltwater Crocodiles in the world (Webb et al, 2010).…”