Alien plant threatens Nile crocodile (Crocodylus niloticus) breeding in Lake St. Lucia, South Africa.

We observed that the majority of Lake St. Lucia's nesting Nile crocodiles (Crocodylus niloticus) selected open, sunny, sandy areas in which to deposit their eggs. Nests were only found in shaded sites in the Mpate River breeding area and these nests were shaded primarily by an alien plant Chromolaena odorata. Soil temperatures of shaded sites at 25-cm depth, were on average 5.0-6.0 degreesC cooler than in sunny sites at the same depth. They were well below the pivotal temperature for nests of St. Lucia's Nile crocodiles, i.e. they probably produced a female-biased sex ratio, and may have prevented embryonic development altogether. Many females abandoned nesting sites when they encountered the fibrous root mats of Chromolaena odorata while digging egg chambers. When additional nesting sites were experimentally created, the percent of sites utilized increased, indicating that suitable nesting sites were in short supply. This alien plant is posing a very serious threat to the continued survival of the Nile crocodile in Greater St. Lucia Wetland Park, and unless immediate action is taken, a female-biased sex ratio will result in eventual extirpation of the species from this recently acclaimed Word Heritage Site. (C) 2001 Elsevier Science Ltd. All rights reserved.