Giant Carnivores Threatened By Alien Weed

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 ratio of females to males in our sample of the adult population," said Leslie. "We are not sure what happens to nesting females that abandon overgrown nest sites. They possibly 'dump' their eggs in the water or continue looking for a suitable site, eventually nesting in 'secondary' nesting sites due to lack of suitable ones."

Leslie and Spotila found that nests shaded by trifid weed were 5 to 6 degrees Centigrade cooler, on average, than nests in preferred unshaded habitat, ensuring that the hatchlings were heavily skewed toward females, if they were viable at all. They also conducted trifid-weed removal experiments, and found that nesting females made use of this reclaimed nesting habitat.

Trifid weed was originally introduced into South Africa in the form of seeds in packaging material on board a ship in Durban Harbor in the last 60 years. It has since spread rapidly throughout southern South Africa, and has recently been reported at Hluhluwe and Umfolozi Game Parks as well. The full extent of this problem is not known, but the threat is clear and the authors hope to raise awareness and encourage management measures throughout the Nile crocodile's range.

"A management plan was put into action at St. Lucia 1997, involving the manual removal of C. odorata from identified nesting sites every year," said Leslie. The plan was based on the promising removal experiments conducted by Leslie and Spotila. "The management plan is being carried out. However, with an incredibly limited budget I dare not say how efficiently."

Nile crocodiles bring a bounty of ecological benefits to African wetlands, from nutrient recycling to keeping fish populations in check, and the loss of these predators would take a tragic toll on the ecosystem. Dr. Leslie is continuing her research on Nile crocodiles in Botswana's renowned Okavango Delta, along with Dr. Hannes Van Wyk of the University of Stellenbosch and Earthwatch volunteers on Crocodiles of the Okavango.

Earthwatch Institute is an international nonprofit organization which supports scientific field research worldwide by offering members of the public unique opportunities to work alongside leading field scientists and researchers. The Institute's mission is to promote sustainable conservation of our natural resources and cultural heritage by creating partnerships between scientists, educators, and the general public.