Irruptions in plant and animal populations are not uncommon but the factors underlying irruptions are rarely explored quantitatively. In addition, it has been suggested that these irruptions may be reduced by predators or herbivores, but there is a paucity of controlled experimental evidence. 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 a rare controlled replicated experimental demonstration of top-down regulation of irruptions. 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.