Removing large herbivorous mammals from the African savanna can cause a dramatic shift in the relative abundance of species throughout the food chain, according to scientists from Stanford University, Princeton University and the University of California-Davis. Their findings were published in the Jan. 2 edition of Proceedings of the National Academy of Sciences (PNAS).
In the study, the research team used large electric fences to exclude cattle, elephants, zebras and other herbivorous mammals from experimental plots on a ranch in central Kenya from May 2004 to December 2005. During that time, the scientists monitored changes in the populations of trees, beetles, lizards and other plant and animal species.
"All of the species studied increased in abundance in the absence of large plant-eating mammals," said lead author Robert Pringle, a graduate student in the Department of Biological Sciences at Stanford. These results are examples of what ecologists call cascading effects, he added.
Although elephants and zebras do not interact directly with insects, they share plants as a food source, Pringle noted. Previous studies have shown that when elephants and zebras are experimentally removed or hunted out, plant matter accumulates and insect populations increase.
"With an increase in insects comes an increase in the insects' predators, such as lizards," Pringle said. "Thus, the actions of a few dominant species ripple throughout the ecosystem."
The authors also found that the strength of the cascading effects varied considerably across the landscape, and that it was possible to predict where the effects would be weak or strong in terms of "primary productivity"-the transformation of solar energy into plant tissue during photosynthesis. Plants in areas of high primary productivity grow faster, making more energy available throughout the food chain. The study revealed that cascading effects are weaker in places where producti
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