Scholars debate the main factor of the significant decrease of water hyacinth on Lake Victoria around 1999-2000 in Wilson et al. (2007) and Williams et al. (2007). First, Wilson et al. (2007) replies to a previously published article by Williams et al. (2005) which claims that although weevils played a role in the eventual decrease of water hyacinth on Lake Victoria, the invasive species’ population was ultimately and predominately the result of the 1997/1998 El Nino. Williams et al. (2005) cites the condition of “low light availability” from El Nino and its subsequent effect on water hyacinth’s growth as the main contributor to the weed’s destruction.
Wilson et al. (2007) counters the referenced article by stating first, Neochetina bruchi and Neochetina eichhorniae (collectively Neochetina spp.) were the primary destructive agents to water hyacinth; second, El Nino caused waves and strong currents on Lake Victoria which dispersed its water hyacinth and made the weed easier to destroy by weevils; and third, water hyacinth will not reemerge in Lake Victoria unless its Neochetina spp. populations are disturbed. The authors reevaluated the light conditions around the time of El Nino, but found that by mid-1998, water hyacinth was already rebounding on Lake Victoria. Wilson et al. (2007) acknowledged that weevils took nearly four years to take effect against the invasive species in 1999 (weevils were released into the lake in 1994), but this timetable was expected and is congruent with weevil versus water hyacinth time frames from other countries with similar climates.
Lastly, in Williams et al. (2007)’s rebuttal to Wilson et al. (2007), the authors restate their aforementioned claim that weevils contributed to the reduction of water hyacinth around 1999-2000, but the invasive species would certainly still be growing strong in the absence of the 1997/1998 El Nino. Williams et al. (2007) believe Wilson et al (2007)’s arguments are oversimplified and thus erroneous because Lake Victoria is simply too vast to be considered on an individual graph of experimental results. Williams et al. (2007) maintains the diminution of water hyacinth was a result of El Nino’s flooding because it transpired “synchronously…during the second quarter of 1998”. The authors claim the hyacinth’s first reduction occurred because the floodwaters dislodged the mats that held the weed to the lake floor, and the hyacinth simply washed away into the lake. While Wilson et al. (2007) stated that Williams et al. (2005) believed low light levels caused the reduction, Williams et al. (2007) stressed that the plant mortality was due to prolonged low light, not intermittent glares, which then caused minimal growth in the plants and weak mats.
Ultimately, I believe Wilson et al. (2007) had the soundest argument, which claimed that weevils played the most significant role in reducing the population of water hyacinth on Lake Victoria. While both articles acknowledged the “lesser” cause (whether it was weevils or the effects of El Nino), I felt that Williams et al. (2007) was especially narrow-minded and barely accredited weevils as a destructive force to weevils, when they were clearly a great contributor to the hyacinth’s periodical demise. However, the fact that the water hyacinth continues to make reappearances on Lake Victoria suggests tha solely biocontrol as a method of eradication is insufficient and ultimately non-cost effective. There needs to be a more radical and long-lasting approach to ridding Lake Victoria from the ruthless water hyacinth.
NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 10 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.
Water hyacinth first invaded Africa’s Lake Victoria in 1989, quickly spreading and forming thick mats around the shoreline. In an attempt to control it, a weevil, Neochetina spp. was introduced in 1995. In 1997 and 1998 the water hyacinth began to decrease drastically, seeming to clearly indicate a success for the weevil. But around this same time, El Nino weather patterns occurred over the Pacific Ocean with repercussions that could be felt over Lake Victoria. From these facts has sprung a debate over whether the decline of the water hyacinth was due to the weevils or the El Nino-related weather.
The main point of disagreement between the two opinions about water hyacinth’s decline is the meaning of the time between the introduction of the Neochetina spp. and the noticeable decline in water hyacinth. Wilson et al. (2007) states that although the El Nino-associated weather, namely increased wind and wave patterns on the lake and decreased light availability, added further stress for the water hyacinth to deal with, they were just a mercy stroke for already doomed plants that were fatally damaged by the weevil. They believe that the time difference was a normal occurrence for a biocontrol agent, no more than a “lag time” while it became established and that it was coincidence that it really began to take effect at the same time as the El Nino event. Williams et al. (2007) on the other hand assert that although the weevils did their part, the decline of water hyacinth on Lake Victoria would not have occurred without the timely El Nino event. They say that while the weevil did some damage to the water hyacinth, it would not have declined as it did without the El Nino weather patterns.
Both articles present valid arguments and it seems clear that the both the weevils and the El Nino weather had a part in the decline of the water hyacinth. In light of the new satellite images from NASA’s Earth Observatory, depicting a resurgence of water hyacinth, it is clear that the biocontrol was a short term solution. Williams et al. provides the best explanation for this resurgence of water hyacinth: “unstable host populations may well lead to unstable controlling herbivore populations,” that is, once the water hyacinth had declined by a significant amount, the weevils lost their food source and their population dropped too low to effectively control the invasive species.
NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 11 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 biologial control? A Reply. Aquatic Botany 87: 94-96.
Wilson, J. R. U., O. Anjuonu, T. D. Center, M. P. Hill, M. H. 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.
When water hyacinth first became rampant on Lake Victoria in 1989 (Wilson et al., 2007), the general consensus among the scientific community involved the need to eradicate the invader by any means necessary. To expunge the weed, biological control agents Neochetina bruchi and Neochetina eichhorniae (weevils) were set loose into the water hyacinth populations – what had already become a vast mass of floating vegetation – in 1995; from 1997 to 1998, however, the strongest El Niño in a century (Williams et al., 2007) took place. The current debate centers on the comparative impacts of Neochetina and the El Niño phenomenon on the water hyacinth population.
One argument claims that the reduction in water hyacinth was effected predominantly by the biocontrol of the Neochetina colonies (Wilson et al., 2007). The hungry Neochetina larvae “tunnel the petioles and the root-stock” of the water hyacinth, thereby allowing damaging bacteria and fungi to invade the plant; these tunnels, due to their obvious hollowness, then become flooded, reducing the plant’s buoyancy and causing it to sink (Wilson et al., 2007). Also, because similar weevils species have led to clear repressions in plant population densities in other tropical locales, effective weevil biocontrol would be an appropriate assumption (Wilson et al., 2007).
A counter argument claims that El Niño played the major part by accelerating water hyacinth decline through direct – wind/wave action – and indirect – reduced incident light – effects (Williams et al., 2007). The flooding, in addition to breaking up mats, would have submersed the water hyacinth, thereby reducing the amount of light they receive (and food they produce). According to this view, El Niño was responsible for the sharp initial decrease in hyacinth, and the weevils were simply responsible for sustaining these low levels.
Wilson et al. presents the more compelling argument, in that, while the El Niño weather would account for the temporary 1998 decrease, it is illogical to claim a correlation between it and the decrease that took place in 1999-2000. The reason for the hyacinth population rebound after the 1998 crash is most likely because the El Niño floods would have destroyed a significant portion of the weevil population as well, slowing their effectiveness. The past successes of weevil biocontrol agents strengthen the case of Wilson et al., and the fact that water hyacinth growth was suppressed from 1999 to 2005 indicates that the weevil populations were able to find a niche in the lake area (Williams specifically states that “weevil populations although present are likely unstable”). Unstable biocontrol agents are not effective ones. The general and consistent decline in water hyacinth density on Lake Victoria after El Niño, which, according to MODIS satellite images, spanned from the year 2000 to the year 2005, is in no way attributable to El Niño phenomenon, due to the gap in time; this proves that the weevils were effective at hyacinth repression (NASA Earth Observatory 2007). But with no food readily available, most of the weevils would have perished. Therefore seeds left behind by the dying water hyacinth would have slowly germinated and flourished under a condition devoid of the bugs but rich in nutrients from both the decomposing sunken water hyacinth mats and agriculturalrunoff due to heavy rains. The result is a renewed blooming of the weed.
As stated by Wilson et al., one of the basic principles of biological control of plants is that it is “sustainable through population regulation.” But because insect populations change in response to changes in the host plant population, effective biocontrol is a mutually-destructive process. This is the main reason why greater human regulation of biocontrol is necessary – to ensure there are always enough weevils to do the job. However, if the weevil colonies establish themselves again like the water hyacinth, the successful precedent they set in 1999-2000 will repeat itself.