Aquatic Invasive Species

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.

Sources:

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

Laura Hubbard

The water hyacinth is an invasive plant that grows in mats on the edge of lakes, blocking light from reaching the bottom of the lake, which eventually drives out or kills native plant species. Water hyacinth was introduced to Lake Victoria in 1989 and within a few years the plant had taken over the lake, and thousands of hectares of the lake were covered in the alien species (Wilson, et al. 2007).  In the late 1990s, scientists introduced the plant’s native herbivores, weevils of the genus Neochetina, in an attempt to reduce the population through biocontrol. The hyacinth population did decline, on average four years after the introduction of the weevil species to an area. Debate raged over the ultimate reason for the plant’s retreat. While researchers agreed on the influence of the weevils, different studies disagreed on the extent of their influence. Williams et al. argue that the majority of the decline was due to reduced light levels because of El Nino weather patterns, while a study released the same year written by Wilson et al. posits that the decline was mainly due to the introduction of the weevils.

Wilson et al. and Williams et al. use much of the same data analyzed in different ways to come to similar but slightly conflicting results. Wilson et al. contend that any affect El Nino had on hyacinth was minimal compared to the effect of the weevils, and that El Nino did not change the cloud cover and light availability in the Lake Victoria region significantly enough to cause any change in water hyacinth growth. The plants, they argue, thrive in otherwise shady regions whose light availability is not much different from the cloud cover caused by El Nino. Most of the effect of El Nino came through the movement of water hyacinth, which both spread the weevils and caused the amount of hyacinth in different parts of the lake to differ and confusing the study of the plants’ reduction. Conversely, Williams et al. state that El Nino had more of an impact than the weevils: the low light prevented growth, while variable waves caused masses of hyacinth to sink to the bottom after they had been attacked by the weevils. While the study does not deny the importance of the weevils, they do argue that to state that a single variable is responsible for the change or to represent the system with a single draft is “an over simplification of the special complexity” (Williams, et al, 2007) of the lake and the problem. Instead, they posit that the decline of the water hyacinth is due to many interconnected factors, including the weevils.

Wilson et al. and Williams et al. also disagree on the probability of the water hyacinth returning and on the stability of the weevil population. Both sets of authors agreed that the weevils had some importance to controlling the population. Williams et al. argued that since the host population for the weevils was variable and declining, this could increase the variability of the weevil population, and hyacinth could die before the weevils had a chance to reproduce successfully and infect other hyacinth. Wilson et al, however, argues that there is no reason that the weevil population should be inherently unstable, and that there are no signs of this instability. Also, since the two studies argue on the reason for the decline, they also disagree on the probability of the hyacinth reestablishing itself: Wilson, et al argues that this won’t happen since biocontrol has taken effect and the weevils are controlling the hyacinth population, but Williams et al states that biocontrol cannot be completely effective since the hyacinth coming in from the river has not been infected with weevils, and with the weevil population in an unsteady place, there is a possibility that the hyacinth will reestablish itself.

While Wilson et al’s argument is compelling, the argument put up in the Williams et al article is more realistic. As Williams et al state, nature is complicated, and it is dangerous to oversimplify such a complex system. The return of the hyacinth to the area further support their claims: it is possible that without the el Nino system and with a possibly unstable weevil population, the hyacinth reestablished itself.