Nile crocodiles threatened by alien weed

June 18, 2001

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 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 with leading field scientists and researchers.

For information Dr. Leslie's continuing comparative study of Nile Crocodiles in Botswana's Okavango delta, see .

Earthwatch Institute

Related Crocodiles Articles from Brightsurf:

African crocodiles lived in Spain six million years ago
The crocodiles that inhabited the coasts of North Africa during the late Miocene period embarked on a journey to Europe across what is now the Mediterranean basin.

New study reveals how reptiles divided up the spoils in ancient seas
While dinosaurs ruled the land in the Mesozoic, the oceans were filled by predators such as crocodiles and giant lizards, but also entirely extinct groups such as ichthyosaurs and plesiosaurs.

Computational study of famous fossil reveals evolution of locomotion in 'ruling reptiles'
Scientists from the University of Bristol and the Royal Veterinary College (RVC) used three-dimensional computer modelling to investigate the hindlimb of Euparkeria capensis-a small reptile that lived in the Triassic Period 245 million years ago-and inferred that it had a ''mosaic'' of functions in locomotion.

Palaeontology: Ancient African skull sheds light on American crocodile origins
The extinct African crocodile species Crocodylus checchiai may be closely related to American crocodile species alive today, according to a study published in Scientific Reports.

Genetic differences between global American Crocodile populations identified in DNA analysis
A genetic analysis of the American crocodile (Crocodylus acutus) has re-established our understanding of its population structure, aiding its conservation.

High-tech CT reveals ancient evolutionary adaptation of extinct crocodylomorphs
New insights into the anatomy of the inner ear of prehistoric reptiles, the thalattosuchians, revealed details about the evolutionary adaption during the transition into the ocean after a long semiaquatic phase.

Ancient crocodiles walked on two legs like dinosaurs
An international research team has been stunned to discover that some species of ancient crocodiles walked on their two hind legs like dinosaurs and measured over three metres in length.

New discovery of giant bipedal crocodile footprints in the cretaceous of Korea
A new study has announced the surprising discovery of well-preserved footprints belonging to a large bipedal ancestor of modern-day crocodiles.

Palaeontology: Ancient footprints may belong to two-legged crocodile, not giant pterosaur
The discovery of large well-preserved footprints belonging to an ancestor of modern-day crocodiles from the Lower Cretaceous Jinju Formation of South Korea is reported this week in Scientific Reports.

Synchrotron X-ray sheds light on some of the world's oldest dinosaur eggs
An international team of scientists led by the University of the Witwatersrand in South Africa, has been able to reconstruct, in the smallest details, the skulls of some of the world's oldest known dinosaur embryos in 3D, using powerful and non-destructive synchrotron techniques at the ESRF, the European Synchrotron in France.

Read More: Crocodiles News and Crocodiles Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to