Aquatic Invasive Species

An experiment on biological and mechanical control across an entire lake showed that taken together, the two types of control can effectively reduce rusty crayfish populations.

Catherine Hein and her colleagues from the Department of Limnology at University of Wisconsin explored potential for rusty crayfish control by trapping adult crayfish and limiting fishing to increase bass populations in Sparkling Lake. They found that the combined control method significantly reduced the invasive crayfish population, removing an estimated 1,212,148 specimens in a three year period from 2001 to 2003.

From the results of the study, Hein et al. concluded that taken together, predation and trapping can effectively remove and control established populations of the invasive species Orconectes rusticus, and should continue to be considered as a method of management.

Can. J. Fish. Aquat. Sci. 63: 383–39, 2006.

The Brazilian peppertree, an invasive species from southern South America, intruded on Florida before the 1900s and spread to Texas and Hawaii soon after. Specifically in Florida, this weed plagues pine land, hammock, and mangrove dwellings as well as the Everglades National Park. In the 1980s, a search for Brazilian pepper’s natural enemies for use as biological controls began and several candidate agents were unearthed. The TAG advocated the release of P. ichini in May 2007. First examined in Brazil, these thrips proved effective in reducing the strength and reproductive rate of Brazilian pepper in both their larval and adult stages. Using snap cap vial quarantine experiments, researchers developed a mass-breeding technique to verify that ample healthy insects could be released as control agents. These trials proved successful and allowed the Entymology and Nematology Department to gain approval for conducting field releases of P. ichini against the Brazilian pepper in locations throughout Florida.

Cuda, J. P., Gillmore, J. L., Medal, J. C., & Macedo, J. H. P. (2008). Mass rearing of pseudophilothrips ichini (thysanoptera: Phlaeothripidae), an approved biological control agent for brazilian peppertree, schinus terebinthifolius (sapindales: Anacardiaceae). The Florida Entomologist, 91(2), 338. Retrieved from http://search.proquest.com/docview/219330109?accountid=10598

Bio-control is an easy concept to oversimplify: if an invasive species is succeeding because of a lack of predators, then bring in a predator.  The truth is that a delicate balance is needed; in many cases the predator itself becomes invasive, and in some it even fails to affect its target.  The water hyacinth infestation at Lake Victoria is a good example.  Neochetina weevils were introduced in the mid 90’s, and in 2000 the hyacinth population plummeted.  But in 2006 the weed suddenly returned en masse, raising doubts that the weevils were a successful control method.

Prior to the resurgence, two teams of scientists offered different views on this issue.  Wilson et al. believed that the weevils behaved exactly as expected, and would continue to prevent massive outbreaks of hyacinth.  Williams et al. differed, proposing that the strange climactic effects of a particularly strong El Nino in 1997-1998 caused most of the damage to the hyacinth beds and that the weevils might not be able to prevent any resurgence.  Both teams’ reports were based off the same three sources: satellite data estimating the amount of hyacinth, studies of the effect of lighting on hyacinth, and a collection of other research papers about previous hyacinth-weevil cases.  As such, it is difficult to say which view is correct.

The main battleground between the two sides is whether the El Nino weather damaged the hyacinth significantly.  It is a central point to either argument, as no other events at the time were likely to have caused such a drop in the hyacinth numbers.  Both teams use the same area-of-infection data, but graph it in different ways to bolster their points.  Wilson shows the total hyacinth-covered area of the lake from 1996 to 2002, with markers for El Nino and the introduction of the weevils.  He then points out the small drop after El Nino and the larger one over a year later, which he claims fits the three to five year lag phase of Neochetina bio-control.  Williams instead splits the hyacinth count along the three sections of the lake.  From this view it is clear that the Ugandan and Tanzanian portions of Victoria were nearly cleared of hyacinth by El Nino, while the Kenyan portion actually rose for the time.  Williams argues that the strong winds blew much of the dead or weakened Ugandan and Tanzanian water hyacinth into the more protected Kenyan section, and that these weakened crops served as a jump start for the weevils.  The graphs’ scales are somewhat misleading, however.  While the Kenyan chart is the same size as the others, the area’s hyacinth population was always much larger, and the amount it increased directly after El Nino was more than the total decrease of the other sections combined.  Moreover, none of the three populations increased significantly in the years after the El Nino, as would be expected if that was the main cause of their destruction.  Judging simply from the satellite data, Wilson’s view seems more persuasive.

As neither point can truly be proven with just population info, the groups turn to previous research on hyacinth bio-control.  Williams had experimented with hyacinth plants, and determined that they cannot grow as quickly in dim lighting.  He believes that the darker sky during El Nino prevented the plants growth, making them more vulnerable to other attacks.  Wilson claims that this research was conducted on small numbers of hyacinth and does not indicate the actions of larger weed growths.  Likewise, Wilson uses previous successful hyacinth-weevil cases as examples, and Williams dismisses them as small-time infestations unrelated to the massive problem at Lake Victoria.  It is clear that neither side had a decisive point to cinch the argument.

I believe Wilson’s arguments made more sense than William’s, as they were based more on research and data, while William’s seemed to be more hypothetical.  Particularly, the satellite info, the most significant data, supports Wilson more.  However, the intent of both papers was to determine whether bio-control had eliminated the threat of hyacinth overgrowth, and the recent resurgence proves otherwise.  Though I think the weevils caused the drop in numbers, William’s point that “bio-control should not be taken for granted” is very true.

Water hyacinth has been a menace to all who depend on Lake Victoria since its proliferation in the early 1990s. The aquatic plant forms a dense mat over the surface of the water, blocking sunlight and deoxygenating water, to the detriment of all native species. The vegetation stills water flow, which creates an ideal breeding ground for disease-carrying mosquitoes. (NASA Earth Observatory, 2007) Indeed, there has been a significant increase in the incidences of malaria, dysentery, schistosomiasis, and other diseases, according to the 2005 documentary series “Strange Days on Planet Earth.” Water hyacinth is also a dire concern for subsistence fishermen on the lake because the mats hinder boating. Finally, the dense growth has widespread consequences in the surrounding regions because it clogs intake valves in hydroelectric plants and fouls drinking water. Clearly, it was necessary for the people who depend on Lake Victoria to take action towards containment, if not eradication, of the water hyacinth.

Responding to the desperate situation, scientists implemented classical biocontrol methods in an effort to weaken and kill the population of water hyacinth. In biocontrol, scientists import natural enemies, in this case the weevils Neochetina bruchi and Neochetina eichhorniae, to harm the invasive plant, which usually lacks predators in its new location. Adult weevils damage water hyacinth by consuming its leaves, while larvae tunnel through its roots and stems. The combination opens the plants to secondary infection and causes them to sit lower in the water, according to Wilson et al. (2007), who claim the 2000 reduction in water hyacinth population was caused mainly by the weevils.

There has been recent controversy over whether the reduction of water hyacinth was due to biocontrol or natural weather events, such as the El Niño of 1997-1998. Wilson et al. (2007) argue that, while El Niño contributed to the water hyacinth decline, it can mostly be attributed to the weevils because they weakened the plant. They claim that the low light levels during El Niño did not harm the plants severely enough to cause their rapid decline. Williams et al. (2007) dispute this claim, saying prolonged suboptimal light would weaken the plants and make them vulnerable to other weather-related factors, such as wind, waves, and water quality. They also claim that the weevil population was too unstable after El Niño to cause a lake-wide reduction in the water hyacinth population, arguing that the severe weather patterns played a larger role in the water hyacinth decline. Both teams of scientists agree that both biological control and weather factors played a role in reducing water hyacinth prevalence, but they disagree as to their respective magnitude.

In my opinion, Williams et al. (2007) provide the more convincing argument. Wilson et al. (2007) maintain that the use of weevils in biological control usually takes 3-5 years on large bodies of water. The reduction of the water hyacinth population 3-4 years after the weevil was introduced would make sense in the absence of the El Niño event. However, they state that the weevil population was decimated when the large mats sank as a result of wind and wave action. It is doubtful the weevil population could recover fully enough after such a reduction in population size to eradicate the plant on their predicted schedule. Water hyacinth control could be more believably attributed to the weevil if the massive population reduction had occurred several years later. Furthermore, photos taken by NASA in 2007 showing a resurgence of water hyacinth may point to environmental factors. If the population of weevil had been large enough to cause the massive water hyacinth decline, they would have been able to maintain control. However, if environmental factors were the primary cause, the end of the weather event could allow a second invasion.

In reality, the initial decline of the water hyacinth population on Lake Victoria would have been less likely without both the weevil and the El Niño. For a permanent reduction in the water hyacinth population to take place, we cannot rely on unpredictable weather patterns for control. Biological control methods are necessary to keep the population in check. However, for biocontrol to be effective, especially in a body of water as large as Lake Victoria, all populations of weevils must be highly monitored and kept in good health.

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria.  http://earthobservatory.nasa.gov/IOTD/view.php?i=7426. Viewed 24 Jan 2011
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 is an invasive plant that was first introduced to Lake Victoria in 1989 ( Wilson et al. 2007). The species spread quickly, infesting about 77 square miles of the lake  by 1998 (NASA Earth Observatory 2007). This has become a huge problem, threatening the biodiversity of the lake, contaminating drinking water, and hampering transportation. In order to remove the aggressive species, biological control agents were released into different parts of the lake in 1995. There was also an El Niño weather pattern during 1997/1998, around the same time water hyacinth populations started to decline. Recently, there has been some debate over the cause of the decline of water hyacinth on Lake Victoria. Some researchers argue that without the presence of weevils, the control agents that eat water hyacinth, the population crash would not have occurred. Others disagree, saying that the wet and cloudy weather of el Niño played a major part by accelerating the decline through direct effects.

In an Aquatic Botany article, Wilson et al. (2007) claims that, although el Niño hastened the destruction of water hyacinth, the presence of weevils was the major contributor to the rapid decrease. Satellite images of water coverage were analyzed and then summarized on a graph, showing the trend of how lake-wide water hyacinth populations changed over time. They agree that increased wind and wave action from El Niño may have been a major stress to plants, but their data shows that the major turning point was in 1999, when the weevils finally became effective.

Williams et. al (2007) combats this opinion in another Aquatic Botany article, stating that el Niño was the major cause of the water hyacinth destruction. Wilson’s article is criticized for condensing the system into an “oversimplification of the spatial complexity”(Williams et. al 2007). They argue that the collapse could not be fully attributed to the weevils because they were introduced 4 years prior to the rapid reduction. There was also a concern of resurgence of the plant because weevils have never become established within the river systems around Lake Victoria. An infestation of weevils must occur before biocontrol can take affect.

Little did they know that in December 2006, satellite images revealed that water hyacinth was back (NASA Earth Observatory 2007). For Wilson, this must have been a hard thing to comprehend. The resurgence of water hyacinth, in my opinion, supports the Williams article. I also agree that Wilson oversimplified years worth of data onto one chart; I was skeptical that the analysis was accurate. Also, their approach was not restricted to dates where the whole lake was clear from cloud cover. Therefore, the Williams article provided the most convincing argument with the data.

In light of the MODIS satellite images that showed images of growing water hyacinth populations, it can be said that biocontrol in general does not always work, and can sometimes turn out for the worse. In this case, the weevils were not acclimated to the size and environment of Lake Victoria, and therefore did not thrive as well. In order for biocontrol to be effective, a large scale infestation must occur; this could take years. The biocontrol must also be regulated frequently; as water hyacinth starts to die off and rot, the weevils also die from rotting plants or lack of food. On broad terms, invasive species pose major threats to our society and environment, and biocontrol should not be taken for granted.

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria.  http://earthobservatory.nasa.gov/IOTD/view.php?i=7426. Viewed 24 Jan 2011
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

Eichhornia crassipes, otherwise known as Water Hyacinth, is among one of the most tenacious invasive water plants known to man. It was first introduced to Lake Victoria around 1979, and has since then almost taken over the worlds second largest lake. The plant was thought to have spread because people imported them for their attractive lavender colored flowers and large floating leaves, and later they escaped into the wild. Now the plant is causing a myriad of problems for animal inhabitants of Lake Victoria as well as people who live around the lake and depend on it to make their living.

Water Hyacinth blocks the shorline of large sections of Lake Victoria, and because they are thick and dense, local fishermen are unable to get their boats out to the open water to catch the fish that they make their living off of. The thick mats of leaves also block sunlight from reaching other plant species that thrive beneath the lake’s surface. In some cases, there may be such large concentrations of Hyacinth that they use up all the oxygen in that area, which creates a “dead zone,” where there is not enough oxygen for any species of plants or animals to survive. The plants also float around and can clog irrigation canals and water supply pipes.

Since the decline of the plants on Lake Victoria before 2006, there have been a few prominent theories over the cause of their decline. The main method employed by humans against the invasive plant is the use of biological control through imported Neochetina weevils, which feed on the plant in its native habitats in South America. The use of weevils is thought by some scientists to be the main cause of the decline of Water Hyacinth on Lake Victoria in the early 21st century. However, some scientists have concluded that the decline of the plant was more due to stress from the El Nino weather pattern, which may have reduced the amount of light that the Water Hyacinth requires to below a tolerable level.

John R.U. Wilson et. al, published an article in volume 87 of Aquatic Botany journal, concludes that the reduction in Hyacinth populations was due to successful biological control by the weevil. This result was determined through summing up the coverage of Water Hyacinth on different parts of Lake Victoria through the use of satellite images. Wilson et. al. acknowledges that El Nino did in fact cause an initial reduction in the population of Water Hyacinth, but the bulk of the control was due to employment of the weevils.

On the other hand, some scientists have concluded that the decline in Hyacinth populations have been more due to stress from the El Nino weather patterns. Adrian E. Williams et. al. acknowledge in a later edition of Aquatic Botany that the weevils did play a role in the desruption of the spreading of Water Hyacinth, but they remained convinced that the decline of Water Hyacinth was due more so to El Nino weather patterns. El Nino produces uncommonly strong storm systems, and Williams et. al. states that these storm systems dislodged the Hycinth and moved them around the lake. The floating mats of plants then provided a perfect opportunity for native plants to grow on top of them. This in turn led to the decline of Water Hyacinth Populations.

I think the methods employed by Wilson et. al. bring about a more plausible conclusion. It is hard to state for certain that changing weather patterns can disrupt the development of a non-native species when they have already been so successful at adapting to their new environment. However, the reinsurgence of the Hyacinth makes it doubtful that the biological control methods will continue to work. If the weevils had done their job, then the Hyacinth would not be making a comeback.

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria.  http://earthobservatory.nasa.gov/IOTD/view.php?i=7426. Viewed 24 Jan 2011
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