The billabongs of Australia's Kakadu National Park are the site of a revealing ecological phenomenon reported today in Nature by Anthony Ives and colleagues. Kariba weed, an invasive fern, arrived in the lakes in 1983, but has been kept somewhat in check through the deliberate introduction of the silvinia weevil. The weevils eat kariba's buds, slowing their normally rapid growth.
Weevil control has had mixed results, though. In some years the plants are kept other control. In other years, they escape control and form mats up to a meter thick. The authors show that these two conditions are alternate stable states. When plant populations are low, the plants consist mostly of tasty buds, which weevils eat to grow and keep eating more buds. However, when the plant populations get large, they consist mostly of tough to eat tissue and float high above the reach of weevils, and the weevils' number fall.
Both these conditions are stable, but regular flooding resets the ecosystem and can flip the lakes between weevil- and kariba-dominated states. What's interesting is how regularly this happens. Jumps between alternative stable states have often been thought of as rare, catastrophic events that drastically re-organize ecosystems. In this case, the switches are common. The authors write:
Ecological systems...are subjected to stochastic and cyclic perturbations, and if alternative states are weakly stable and perturbations are large enough, then shifts between states may be routine. Alternative stable states may thus generate underlying forces that govern the stochastic dynamics of the system, leading to complex and seemingly irregular, eruptive behaviour. In fact, it may be difficult to identify the alternative stable states, yet at the same time be difficult to understand the dynamics of the system without first identifying that alternative stable states exist.
It will be interesting to see where else this phenomenon pops up.
Schooler, S., Salau, B., Julien, M., & Ives, A. (2011). Alternative stable states explain unpredictable biological control of Salvinia molesta in Kakadu Nature, 470 (7332), 86-89 DOI: 10.1038/nature09735