During the 1990’s, the water hyacinth became a major problem in the world’s second largest lake located in Africa, Lake Victoria. Water hyacinth a fast spreading plant causes major problems in the places where it invades. It hinders efficient transportation of fishing boats, and its exponential growth can be detrimental to many local species. In order to combat this dangerous plant, weevils were introduced into Lake Victoria in 1995 as a means of biological control. Two years after the introduction of the weevils, a strong storm, El Nino, hit the Lake Victoria area, and the population of water hyacinth quickly dipped. However, after the storm passed the invader quickly grew and it wasn’t until 1999 that the water hyacinth population was greatly reduced and remained steady. Discussions on how to prevent another influx of water hyacinth has led to a disagreement between two groups of scientists.
The Wilson et al. (2007) group strongly believes that the biological control put in place with the introduction of the weevil was the dominant force in reducing the water hyacinth population in Lake Victoria. They claim that El Nino only had a temporary effect on the water hyacinth population, citing the resurgence of hyacinth after the storm as evidence that El Nino did not have a lasting impact on the invader. In the paper, the group also describes how weevils are able to eat away at the plant. The damaged plant gains water, and then sinks to the bottom of the water. Using this line of reasoning the group explains that when the hyacinth mats sunk during El Nino, weevils were brought down and killed with the hyacinth. Therefore, after El Nino passed, the water hyacinth flourished because of the low levels of weevils.
The Williams et al. (2005) group has a different take on the cause of the decline of the water hyacinth. In their paper, they state that the El Nino caused a lack of light, which is critical to the growth and reproduction of water hyacinth. They do not deny the fact that the biological control also had an impact on the reduction of hyacinth, but that cloud cover that El Nino brought weakened the plants and was the main factor in their control.
I lean more towards the side of the Williams et al. (2005) group because they have a more moderate explanation for the cause of the population decline. They do not rule out the weevils as a factor that helped to reduce the population. I think it is likely that the storm could have done great permanent damage to the hyacinth, but the weevil and storm synergistic effect was needed to control the population and immediately following the end of the storm, the weevil population was very low, which allowed the hyacinth to increase in number for a short period of time.
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. Katagaria, 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