Nile crocodiles threatened by alien weed

Nile crocodiles (Crocodylus niloticus) have no natural enemies. These efficient predators grow to a length of six meters and eat everything from fish to antelopes in wetlands across sub-Saharan Africa as they have for millennia. But recent findings in South Africa by Earthwatch Institute-supported researchers suggest that these ancient reptiles may have met their match. Nile crocodiles could face local extinction due to a perennial shrub.

The alien plant, known as locally as trifid weed (Chromolaena odorata), has invaded critical shoreline nesting habitat for crocodiles at Greater St. Lucia Wetland Park, a World Heritage Site in northern Kwazulu-Natal and the largest estuarine system in Africa. The fibrous roots of the weed cause some nesting crocodiles to abandon their nest sites, while shade from the plant alters the temperature of other nests, making them non-viable or resulting in all female hatchlings.

"This problem is definitely not limited to St. Lucia," said Dr. Alison Leslie, Department of Conservation Ecology at the University of Stellenbosch and chair of the Herpetological Association of Africa. "If ignored it will significantly affect crocodile populations in South Africa." Dr. Leslie and Dr. James Spotila of Drexel University reported their results, which were supported by volunteers on Earthwatch's Nile Crocodile project, in a recent issue of Biological Conservation (98 (2001), pp. 347-355).

As in several other reptiles, the sex of crocodiles is determined by the temperature of the nest while they are still in the egg, so that the sex ratio of hatchlings can be skewed by nest sites that are too warm or too cold. Female crocodiles at St. Lucia and elsewhere tend to return to the same nesting site year after year, usually open sandy areas close to the water. But when these nest sites are shaded by the invasive trifid weed, the hatchlings are like to be entirely female, threatening the viability of the population.

"We found a three-to-one

Contact: Dr. Alison Leslie
Earthwatch Institute

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