Coordinated approaches to rehabilitating a river ecosystem invaded by alien plants and fish

Impson, N. Dean ; Van Wilgen, Brian W. ; Weyl, Olaf L. F. (2013-11-27)

CITATION: Impson, N. D., Van Wilgen, B. W. & Weyl, O. L. F. 2013. Coordinated approaches to rehabilitating a river ecosystem invaded by alien plants and fish. South African Journal of Science, 109(11/12), Art. #a0041, doi: 10.1590/sajs.2013/a0041.

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Large sums have been spent on controlling invasive alien species in South Africa over the past two decades, but documented accounts of successes are almost non-existent. This brief account describes progress with a coordinated pioneering project to simultaneously clear invasive alien trees and predatory alien fish from a degraded but ecologically important river ecosystem in the Cederberg region of the Western Cape Province, South Africa. Dense infestations of invasive Australian Acacia and Eucalyptus species were cleared over 2 years (2010–2012) from the riparian zone of the lower reaches of the Rondegat River. This clearance was followed by the local eradication of an alien fish, smallmouth bass Micropterus dolomieu, from the lower reaches of the river in February 2012 and March 2013 using the piscicide rotenone, so that native threatened fish species could re-colonise from the upper reaches of the river. Overall costs of the clearance and eradication amounted to nearly R4.5 million, and early indications are that the native riparian vegetation and fish are recovering well. The project illustrates several aspects of good practice: careful planning; close and enthusiastic collaboration between affected state and private landowners; public participation to address concerns; the simultaneous and coordinated application of mechanical, chemical and biological control of alien plants and chemical control of alien fish; and direction by qualified ecologists. Although it is too early to assess the long-term success, the exercise provides useful lessons for such projects elsewhere, and paves the way for the more widespread use of similar approaches in selected areas that are a priority for conservation.

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