The Water Hyacinth is an invasive species that has proliferated since its introduction to Lake Victoria in the 1980′s. The lack of natural predators has allowed the plant to form dense mats over the lake, stagnating surface waters.  Lentic water provides breeding grounds for mosquitos and other disease causing organisms. The plant also robs sunlight, nutrients, and oxygen from the native species, hinders boating, and interrupts subsistence fishing. One of the more recent cleanup efforts involves the use of Neochetina, a weevil that predates on the Water Hyacinth. This form of biocontrol’s degree of effectiveness is a subject being debated.

El Nino weather patterns in 1997/1998 Lake Victoria were stormy, and water hyacinth numbers dipped around same time. Wilson et al. (2007) concluded that the incidental weather was only supplemental to the pressure imposed by the weevils. Low sunlight levels, they argue, present little trouble to the flourishing hyacinths in West Africa and Papua New Guinea and is not a major threat to the plants’ survival.  The study also points out that Lake Victoria’s time frame between weevil introduction and hyacinth decline is consistent with the maximum four years shown in other countries. Being the only control method implemented across the whole lake, weevil biocontrol is likely the main source of the drop. Wilson et al. however acknowledges that the wind and wave action can be major stressors to the perforated, weevil-ravaged plants.

Williams et al. (2007) address the arguments presented by Wilson et al. (2007) by clarifying that though poor lighting may not kill the plants, it is a stressor that compounds with other weather related factors such as water level, wave action, water quality, temperature and humidity. The study also calls attention to the 1998 “crash” that came prior to the larger hyacinth drop. Williams et al. (2007) believe that early deterioration of plant quality led to instability and weevil decline; plants sink, “taking with them weevil eggs, larvae, and pupae.” Williams et al. (2007) agrees that weevils are integral to the reduction of the water hyacinth, but notes that weevil densities remained low up to 2002 and likely did not play the largest role in hyacinth damage.

The authors of both studies seek to better monitor the hyacinth and weevil populations.

Hyacinth populations have rebounded since the two studies were written; nutrient laden runoff is suspected (NASA Earth Observatory, 2007).

I think the studies complement each other very well because each presents new scenarios and variables to be considered. I believe Williams et al. (2007) explanation regarding unstable bug populations is correct because weevils are very weak swimmers and heavily dependent on their hosts.

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.