Aquatic Invasive Species

By: David Lung

Bio-control has been an inconsistent method of controlling invasive species. Often it results in failure and may even exacerbate the situation. For controlling water hyacinth, the water hyacinth weevils were introduced to stop water hyacinth from disrupting Lake Victoria’s ecosystem any further.

Wilson et al. believed that the weevils were the greatest factor in reducing the water hyacinth population and the El Niño event being a contributing factor. They have stated that water hyacinth populations have not increased (after sufficient time allowed for weevil growth) since the introduction of the weevils in 1995 and also that the weevils’ actions were the main cause of the reduction and eventual control of the weed’s population and the El Niño’s weather patterns made the finishing blow. The weevils’ actions opportunistically allowed other factors to cause further damage to it. They also stated that the weevil “represents the only control method that was implemented across the whole of the lake…” The El Niño event, herbicides and other control measures were temporary or concentrated means of control on the lake.

Willams et al. focused on the El Niño event being the main cause of water hyacinth decline and the weevils being a later factor. They asserted El Niño event was a widespread factor whereas other factors were not. They stated that the El Niño event’s reduction of light slowed down plant growth, allowing other factors such as weather patterns and weevil activity to control the weeds’ population. After the El Niño event in 1998, there was a drastic population decrease in the weed’s population. Also, they noted that in 2000-2001, the water hyacinth population increased despite the weevils’ actions. Willams et al. also stated that the population density of the weevils was still low up to 2002, asserting they were merely a contributing factor to the El Niño event.

Both groups make valid statements on the major cause of the control of water hyacinth, but I agree with Wilson et al. They don’t counter Wilson et al.’s statement that significant plant growth can occur in low light and also neglects that there is a predator-prey sort of relationship between the weevil and weed and also the weevil’s long growth period in explaining the weevil’s fluctuating population. The satellite images show bio control works to an extent, but controlling the nutrient level and researching more into the issue is needed, which is supported by both groups.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/vi…. 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.

(400 words without references)