Megaherbivores suppress precipitation-driven plant irruptions in a tropical savanna

Irruptions in plant and animal populations are not uncommon in plants and
animals. It has been suggested that these irruptions may be reduced by
predators or herbivores, but there is a paucity of controlled experimental
evidence. In addition, the factors underlying irruptions are rarely
explored quantitatively. Using data from the Kenya Long-term Exclosure
Experiment (KLEE), we show that populations of perennial Hibiscus spp.
(primarily H. flavifolius) show multiple short-term irruptions a year
after rainy periods, increasing in abundance in some cases by more than an
order of magnitude before declining in ensuing months and years. We
demonstrate that these irruptions are largely limited to experimental
plots from which large mammalian herbivores have been excluded,
particularly megaherbivores (elephants and giraffes). This represents the
only controlled replicated experimental demonstration of top-down
regulation of irruptions in either plants or animals. African elephants
and giraffes are often at greater risk of local extinction than other
large mammals, and their absence appears to destabilize this African
savanna ecosystem, providing additional support for their conservation.