How can a plant threaten crocodiles? By shading and thus cooling the river banks where they nest, the plants skew the temperature-dependent sex balance toward females. New research suggests this may be happening in Lake St. Lucia, where a non-native plant is taking over one of South Africa’s last remaining Nile crocodile (Crocodylus niloticus) nesting habitats.
“Unless immediate action is taken, a female-biased sex ratio in all nesting areas will result in eventual extirpation of the Nile crocodile from the Lake St. Lucia ecosystem,” say A.J. Leslie of Stellenbosch University in South Africa and J.R. Spotila of Drexel University in Philadelphia, Pennsylvania in a paper published in the April 2001 issue of Biological Conservation.
While the Nile crocodile is generally widespread along waterways south of the Sahara Desert, the species is threatened in South Africa where only about 4,500 remain. About 800 of the crocodiles live in the Greater St. Lucia Wetland Park, a World Heritage Site that is the largest estuary in Africa. Like other protected areas on South Africa’s east coast, the park has been invaded by paraffin bush (Chromolaena odorata), a neotropical plant that can grow more than 12 feet tall and produce more than a million wind-dispersed seeds. Notably, paraffin bush has overgrown many of the sunny, sandy areas where Nile crocodiles prefer to nest.
To see how this invasive plant affects the crocodiles, Leslie and Spotila studied 16 nesting sites in the Lake St. Lucia ecosystem: five existing sites that were sunny, five existing sites that had been overgrown by paraffin bush but were cleared, and six new sites dug in areas shaded by dense stands of paraffin bush. The researchers monitored the soil temperatures about 10 inches below the surface (the average midpoint of nest depth) and about 30 feet from water’s edge throughout the November-February breeding season.
Leslie and Spotila found that the shaded nest sites were about 5º C cooler than the sunny sites (an average of 26-28 vs. 31-33º C). Because the pivotal temperature for determining sex in Nile crocodiles is 31.7º C, the shaded nests presumably produce too many females. Moreover, some of the nests are so cool that the embryos don’t even develop.
Over the four years of the study, about 40 percent of the sunny nesting sites were lost to paraffin bush invasion. This meant that many females returning to the same nest sites ended up laying eggs in shaded or partly shaded sites.
To help protect Nile crocodiles in the Lake St. Lucia ecosystem, managers have adopted the short-term solution of manually clearing paraffin bush from their nesting sites each summer. Other researchers are testing biocontrol agents in hopes of finding a long-term solution to controlling paraffin bush.
Leslie, A.J. and J.R. Spotila. 2001. Alien plant species threatens Nile crocodile (Crocodylus niloticus) breeding in Lake St. Lucia, South Africa. Biological Conservation 98:347-355.