Victory!? But Still No Consensus

Water hyacinth is a fast growing South American plant which spread along the shores of Africa’s Lake Victoria, forming a blanket that suffocated native species and the fish industry.  To combat this plant, weevils were introduced and several years later the water hyacinth population began to decline.  The biologists responsible for this action have claimed victory; however these claims have been challenged by other scientists who point to atypical weather phenomena as playing a role in the water hyacinth’s decline.

Wilson et al.  (2007) argues that the implementation of biological control by introducing the weevils is primarily responsible for the subsequent decline in the water hyacinth.  They support this conclusion with estimates of total water hyacinth coverage of Lake Victoria extrapolated from satellite data that show a marked decline approximately 3 years after the weevils were introduced.  Also, they note that this result is similar to that of other nations who have enacted this form of biological control and that the introduction of weevils is the only management technique common to all parts of the lake.

The argument that weather phenomena, specifically the El Nino event of 1997/1998 contributed significantly to the decline of water hyacinth on Lake Victoria is expressed by Williams et al. (2007).  They maintain that the lake-wide summaries produced by Wilson et al. (2007) do not respect the diversity and size of Lake Victoria.  Thus, they examine each of the lake’s main sections separately and noticed that the water hyacinth began to decline in all three sections roughly simultaneously after the El Nino event whereas the weevils had been introduced to each region at different times.  Their argument therefore is that the El Nino event weakened the water hyacinth population and left it susceptible to destruction by both the weevils and other factors.

I agree with Williams et al. (1997) that the water hyacinth population of Lake Victoria cannot be accurately modeled as a single graph.  The simultaneous decline of water hyacinth suggests a cause that is universal, not susceptible to regional variation like the weevil population.  The importance of regional considerations was recently highlighted by a report from the NASA Earth Observatory (2007) which found that rain runoff had sparked a rapid resurgence of water hyacinth.  Efforts to control this new growth and prevent future growth should place an emphasis on regional attributes such as agriculture that make a region susceptible to water hyacinth invasion.

Eli Wilber

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 20 Jan 2010.

Williams, A. E., R. E. Hecky, and H. C. Duthie. 2007. Water hyacinth decline across Lake Victoria – Was it caused by climatic perturbation or biological control? A reply. Aquatic Botany 87:94-96.

Wilson, J. R. U., O. Ajuonu, T. D. Center, M. P. Hill, M. H. Julien, F. F. Katagira, P. Neuenschwander, S. W. Njoka, J. Ogwang, R. H. Reeder, and T. Van. 2007. The decline of water hyacinth on Lake Victoria was due to biological control by Neochetina spp. Aquatic Botany 87:90-93.



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