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Alternative stable states explain unpredictable biological control of Salvinia molesta in Kakadu


Suppression of the invasive plant Salvinia molesta by the salvinia weevil is an iconic example of successful biological control. However, in the billabongs (oxbow lakes) of Kakadu National Park, Australia, control is fitful and incomplete. By fitting a process-based nonlinear model to thirteen-year data sets from four billabongs, here we show that incomplete control can be explained by alternative stable states1,2,3,4—one state in which salvinia is suppressed and the other in which salvinia escapes weevil control. The shifts between states are associated with annual flooding events. In some years, high water flow reduces weevil populations, allowing the shift from a controlled to an uncontrolled state; in other years, benign conditions for weevils promote the return shift to the controlled state. In most described ecological examples, transitions between alternative stable states are relatively rare, facilitated by slow-moving environmental changes, such as accumulated nutrient loading or climate change5,6. The billabongs of Kakadu give a different manifestation of alternative stable states that generate complex and seemingly unpredictable dynamics. Because shifts between alternative stable states are stochastic, they present a potential management strategy to maximize effective biological control: when the domain of attraction to the state of salvinia control is approached, augmentation of the weevil population or reduction of the salvinia biomass may allow the lower state to trap the system.

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Figure 1: Log biomass of salvinia in four Kakadu billabongs.
Figure 2: Fitted model to log salvinia biomass (black) and logit weevil damage (grey) for four billabongs.
Figure 3: Phase portraits of logit weevil damage against log salvinia biomass for the four billabongs.


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We thank S. Winderlich (Kakadu National Park) for facilitating data processing, and S. Cruickshank and G. Willis of the Northern Territory Department of Natural Resources, Environment, the Arts and Sport for providing gauging station data. M. Storrs, C. Murakami, F. Hunter, M. Hatt and F. Baird provided field assistance. A. Bourne assisted with data interpretation, and P. Milewski provided value insight into the mathematical analyses. S.S.S. received funding from the Australian Department of Agriculture, Fisheries and Forestry and A.R.I. received funding from a CSIRO McMaster’s Fellowship and United States National Science Foundation funds through the North Temperate Lakes Long Term Ecological Research Network and individual grants. S. R. Carpenter, Y. Buckley, and R. van Klinken provided comments on earlier versions of this manuscript.

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M.H.J. developed the initial hypothesis explaining the partial success of salvinia biological control and designed the CSIRO sampling program at Kakadu. B.S. managed the Kakadu sampling program, and S.S.S. and B.S. assembled the data set. S.S.S. and A.R.I. designed the overall concept of the project reported here, and A.R.I. performed the analyses.

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Correspondence to Anthony R. Ives.

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The authors declare no competing financial interests.

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Schooler, S., Salau, B., Julien, M. et al. Alternative stable states explain unpredictable biological control of Salvinia molesta in Kakadu. Nature 470, 86–89 (2011).

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