In 1989, the invasive species known as water hyacinth was first reported on Lake Victoria. In 1998, an outbreak of the plant covered as much as 17,231 hectares (NASA Earth Observatory,  2007) of the lake.  Weevils, insects that eat the plant, were introduced into the lake in 1995 (Williams et al., 2007) and by 2000, an almost complete eradication of water hyacinth was observed in Lake Victoria (NASA Earth Observatory,  2007). Scientists are debating over the cause of the decreasing population of the water hyacinth. The question is: Was the decrease in water hyacinth on Lake Victoria caused by the introduction of weevils or a weather phenomenon El Niño?

Wilson et al. (2007) argues that the decline in water hyacinth on Lake Victoria was due to biological control by Neochetina weevils. They cite how the reduction of water hyacinth occurred four years after the introduction of the weevils, and how this time frame has also been observed in other parts of the world where weevils have been introduced to reduce water hyacinth populations. Wilson et al. (2007) shows that only a few years after the initial introduction of the Neochetina weevils the water hyacinth populations crashed.

Contrastingly, Williams et al. (2007) contributes the majority of the water hyacinth decrease to the natural weather phenomenon El Niño. Williams et al. (2007) claim that the low light conditions caused by El Niño “will reduce growth and reproduction rates and relatively increase the effect of other debilitating influences such as other weather related factors, e.g. water level, wave action, water quality, temperature and humidity as well as weevil herbivory and phytopathogenic attack”.

The low populations of water hyacinth Lake Victoria did not last. By December of 2007 the water hyacinth was back in the lake (NASA Earth Observatory, 2007). NASA determined this using a satellite Moderate Resolution Imaging Spectroradiometer (MODIS). NASA contributes the rise in water hyacinth populations to “unusually heavy rains that flooded the rivers … and swept agricultural run-off and nutrient rich sediment into the water”. This new data may mean that the biocontrol in Lake Victoria was ineffective.

I believe that the Neochetina weevils undoubtedly contributed to the initial drop in the water hyacinth (Wilson et al., 2007), however, Wilson et al. (2007) presented a more convincing argument that the majority of the decline in water hyacinth was caused by El Niño. This is apparent when looking at the graphs presented in Wilson et al. (2007), they clearly show that there is a more direct connection to the decline in water hyacinth with El Niño than there is with the introduction of weevils. The problem of water hyacinth needs to be solved and hopefully scientists can find affective methods to keep the growth of the plant controled.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 12 Sept 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.