Biocontrol, defined as the introduction of predatory species in areas where invasive species have proliferated, is a burgeoning method for managing invasive species.  The predator introduced is expected to control the invasive population and to decline in numbers relative to the invasive. Lake Victoria in Uganda has become home to the water hyacinth, a pervasive species that covers the shoreline and prevents fishermen’s ease of movement.  Biocontrol in the form of the weevil was implemented in 1995, and water hyacinth decline was apparent in 1999.

However, the occurrence of an El Nino in 1997-1998 clouded the explanation for water hyacinth decline. An El Nino event consists of weather pattern fluctuations usually resulting in an increase in natural phenomena like storms, and enacting global water-level changes.

Several scientific articles strove to explain water hyacinth population decline, namely Wilson et al. (2007) and Williams et al. (2007).  Wilson contrives to explain the decline based on biocontrol, stating that weevils brought to Lake Victoria were effective. Wilson asserts that water hyacinth populations did decrease in early 1998 during El Nino, but then rose again in late 1998, not nosediving again until 1999. Based off this time inconsistency, Wilson hypotheses that observed patterns of weevil control in other countries match this four-year gap between introduction and results (i.e.: water hyacinth declining).

Contrarily, Williams et al. (2007) states that biocontrol alone is not responsible for the water hyacinth reduction. Rather, the El Nino event in 1997-8 caused such flooding of tributaries into Lake Victoria as to upset the hyacinth and carry mats of it free from lakeside resting places. Williams states that while weevils impacted the decline, El Nino accelerated it—however future regulation should maintain weevil populations against the return of water hyacinth. Interestingly, both articles referenced the same data to support their different points, revealing the diversity in scientific evaluations that exists.

In lei of recent increases by water hyacinth in Lake Victoria (NASA Earth Observatory 2007) following the publication of the above referenced articles, biocontrol doesn’t seem like a viable long-term restraint for water hyacinth. Personally, I think that the revitalization of water hyacinth by 2006 (NASA Earth Observatory 2007) is proof that El Nino was likely a larger component to the 1999 reduction than weevils were, as Williams hypothesizes.  Broadly though, biocontrol necessitates a case-by-case determination and cannot be used to obtain the same results in each instance.

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.