Gregarious Bamboo Flowering and Rodent Outbreaks – An Overview

Author(s): Sridhara, Shakunthala; Rajendran, T. P. | Abstract: At least 137 species of common bamboos of India and rest of Asia seed synchronously at long and supra-annual intervals. These include species belonging to the genera Phyllostachys, Arundinaria, Bambusa, Dendrocalamus, Gigantochloapus, Malocanna, Guadua, Nechouzeaua, Ochlandra, and Thyrostachys. The enormous quantity of seeds shed by these bamboos affect the population dynamics of several vertebrates, including mostly birds, rodents, and wild pigs. The rodents, as generalist seed predators, exhibit a very complex response to the sudden spurt in the availability of nutritious bamboo seeds. In North-east India, the popular saying and belief “when bamboo flowers, famine follows” refers to the phenomenon of gregarious bamboo flowering, massive seed fall, and irruptions in rodent populations, which, after exhausting the bamboo seeds, invade and devour the entire crop fields and stored foods, leading to famine. Such events have been occurring approximately every 40 - 45 years since 1880, the latest being 2005 - 2007 sans any famine. Although 14 species of rodents occur in North-east India, until now there is no strong evidence to identify the species exhibiting population outbreaks. The species suspected to irrupt are Rattus rattus, R. r. brunneusculus, R. bowersi, R. niviventer, and Canomys badius. Similar irruptions of rodent populations in South America has been documented since 1800. These eruptions, termed ratadas, were seen in Oligoryzomys longicaudatus and Abothrix longipilis consequent to flowering of Chusquea quilla. Comparable density increases are seen consequent to masting in trees other than bamboo. Although the mechanism of seed masting is attributed to several hypotheses, predation satiation is the most accepted theory. In this paper, rodent response to masting is discussed in relation to this theory. The non-occurrence of famine in North-east India consequent to 2005-2007 bamboo flowering is mainly due to the effective strategies formulated by the government of India to prevent population buildup of rodents. This seems to be a fine instance of prophylactic approach to rodent management in large areas encompassing several states.