Aquatic Invasive Species

DURHAM, N.C. – Invasive species have become a large problem worldwide. They cause environmental as well as economic problems. Scientists have been studying numerous invasive species looking at their breeding patterns, suitable habitats for them, and many other aspects of their existence. However, researchers are still having difficulty discovering effective methods to control their current spread as well as to prevent their future spread. Every invasive species is different, and rarely is any control method completely effective.

There are several current methods in use by scientists including chemical, physical, and genetic methods (that prevent reproduction). One popular method in use since the late 19^th century is biological control. Biological control is the intentional use of one organism to control the population size of another organism. Biological control has had many disasters throughout its practice. The problem with biological control is that many times, organisms introduced to control one alien species problem, actually cause further negative environmental impacts.

The important questions about biological control is what do you think? What do your friends and neighbors think? Depending on what you say, biological control may soon be coming to your nearest invasive species if you and your neighbors say it is socially acceptable. Jeremy Joven, a student at Duke University, is searching for the answers to these questions that many researchers have ignored for years.

Scientists do not always fully explain scientific processes such as biological control and ask for the average layman’s opinion. However, Joven is about to change all that. Joven is in the process of researching invasive species control methods and their popularity in random samples of people across all dimensions of knowledge in society including representatives of aquatic conservation groups, local, state, and national aquatic managers, international scientists, and average citizens like you and me. He says the current methods of control that are publicly acceptable are largely ineffective and cost inefficient. He speaks of rare success stories with all methods including biological control.

Knowing that the government is unlikely to fund programs and projects containing control methods unpopular with taxpayers, Joven’s goal is to find the “perfect solution” to alien species being both effective as well as socially acceptable. He wants to educate people and differentiate between the actual effectiveness of biological control and its social acceptability. He does not want a repeat of the public relations’ disasters of recent years regarding inhumane killing methods of invasive species.

Joven believes that biological control will be deemed socially unacceptable because of its low success rate and high costs. People want consistent positive results, and biological control is just too unreliable for people to accept as the best method for widespread use. However, Joven believes that his research will lead to further research into determining a publicly approved program with better results. Most importantly, Joven believes his work will lead to an established order for the use of the various control methods on invasive species in the future and bring greater success to invasive species prevention.

Sarah S. Roley and Raymond M. Newman of the University of Minnesota conducted a study in order to examine the differences in resistivity to milfoil weevil of three different strains of milfoil: the native northern, invasive Eurasian, and (Eurasian X northern) hybrid. The milfoil weevil is a well known biological control agent for Eurasian milfoil, but concern has been raised regarding the hybrid’s potential for higher resistivity. Roley and Newman tested the resistance of these three strains by adding weevils to an isolated milfoil plant in in vitro aquatic conditions, and after 24 hours of exposure determining the weevil survival rate on the given host. As expected, Eurasian milfoil was the least resistant to the weevil, but the hybrid, having a resistivity in between that of the Eurasian and northern strains, was not as resistant as was predicted. Roley and Newman believe future studies should be focused on the hybrid’s growth and spread abilities to better determine the threat of this strain of milfoil.

Environmental Entomology. Developmental Performance of the Milfoil Weevil, Euhrychiopsis lecontei (Coleoptera: Curculionidae), on Northern Watermilfoil, Eurasian Watermilfoil, and
Hybrid (Northern X Eurasian) Watermilfoil. 2006.

Ben Berg

Chemoecology 8.10.2009

Since their introduction to mainland Australia in 1935, the cane toad (Bufo marinus) has significantly decreased biodiversity amongst reptiles in Australia.  Scientists are now growing desperate in their attempts to limit the growth of this species.  Researcher Mattias Hagman conducted experiments involving biological control mechanisms to fight the toad invasion.

Working in conjunction with Team Bufo (a team of cane toad researchers) and receiving funding from the Australian government, Hagman tested lab results that suggested that a pheromone produced by larval toads lowered survival rates.  Toad larvae were placed into two groups in outdoor ponds, and the treatment group was exposed to the pheromone.  The pheromone, which is emitted by injured larvae to alert other individuals, caused the tadpoles to undergo accelerated metamorphosis, causing a smaller body size and reducing survivorship by 50%.  While this significantly effected cane toad populations, it appeared to have no effect on native species.  The authors have called for follow-up research on the topic.

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

Water hyacinth is an invasive species that harms the ecosystem of Lake Victoria in Africa by creating mats that block sunlight from reaching native species in the water (NASA Earth Observatory 2007). Insects called weevils were introduced as predators of the invasive weed in 1995 in an attempt to solve the problem through biological control (Wilson et al. 2007). A sharp decrease in water hyacinth population was seen by 2000, but the scientific community was not convinced that the weevils caused this change. Williams et al. (2007) released a subsequent paper suggesting that while weevil presence did help reduce hyacinth in Lake Victoria, el niño weather conditions at the time, were more significant factors. They alleged that wave conditions and limited sunlight had more significant effects, also stating that they believed the weevil population was unstable, and monitoring the population would be necessary in preventing a resurgence.
The arguments presented by Williams et al. (2007) seem to be more convincing. A major point made in the article is the synchronous nature of the disappearance of hyacinth. The population decreased uniformly across the lake, suggesting that a larger factor such as weather was responsible for the changes. While Wilson et al. (2007) states that reduced sunlight due to el niño would not kill the hyacinth, Williams et al. (2007) argues that while not fatal, such conditions would seriously impede growth. The Williams et al. article also appears to be more convincing in its logical claim that an unstable host population creates instability in the weevil population, and thus the weevils must be monitored to prevent hyacinth resurgence.
To me, the argument contained within the papers seems to be a moot point. Both sides agree that both factors have some level of influence, and the conclusion of Williams et al. (2007) supports continues use of weevils, fearing their disappearance at the hands of unstable conditions. Since we as humans are not presented with the ability to choose el niño conditions over the use of weevils, we should do what we can and use weevils to try and keep hyacinth populations low. Unfortunately, this did not happen. According to the NASA Earth Observatory (2007), hyacinth populations have returned in full force, once again covering Lake Victoria. Perhaps instead of arguing, both groups should have focused on maintaining the somewhat effective use of weevils and finding a more effective and proven solution.

NASA Earth Observatory. 2007. Water Hyacinth Re-Invades Lake Victoria.
http://earthobservatory.nasa.gov/IOTD/veiw.php?id=7426. Viewed 27 January 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. 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

The recent resurgence on water hyacinth in Lake Victoria has brought into the light the debate regarding the role of bio-control in the elimination of invasive species. Water hyacinth, a weed native to South America, caused significant damage to Lake Victoria through the late 1990’s followed by a decline in 1997. Several factors have been considered in determining the cause of this decline. In December of 1995, weevils were steadily released into Lake Victoria to reduce the massive mats of the weed that covered Lake Victoria (Williams et. al). During the 1997 decline in water hyacinth, an El Nino weather pattern occurred (Williams et. al.). Until the water hyacinth population began to regain lost ground in 2007 (NASA Earth Observatory 2007) bio-control methods, namely the weevils, were considered the explanation for water hyacinth population decline. The El Nino weather pattern was recently suggested as a significant contributing factor to previous water hyacinth decline (Williams et. al.). Arguments made in favor of each explanation must be critically evaluated as both presentations were published before the resurgence of water hyacinth was recorded.

Williams et. al. proposes that the El Nino weather pattern, not the weevil introduction, is responsible for the reduction in water hyacinth during 1997/1998. Wave action and reduced light levels, a result of the wet and cloudy weather, are suggested as facilitating weed decline (Williams et. al.) Wilson et. al. counters this argument suggesting that bio-control remains the primary explanation for weed decline. Instead of directly causing plant decline, the effects of El Nino only exacerbated the damage caused to the weevil-weakened water hyacinth population. Citing the basic tenant of bio-control, “it is sustainable through population regulation,” (Wilson et. al.) a convincing argument is made in favor of bio-control’s successful implementation in Lake Victoria.

Clearly, neither presentation can be considered conclusive as both were written prior to the 2007 water hyacinth resurgence. However, the evidence reviewed in both articles clearly indicates that bio-control is the main action responsible for water hyacinth decline. The El Nino weather pattern is demonstrated as exacerbating weed death; however, Williams et. al. fails to demonstrate El Nino as the sole cause of weed decline. Thus, I believe bio-control is the most suitable and successful method of water hyacinth control. A new strategy for eliminating new water hyacinth growth is imperative, but bio-control must be a component of this plan.

References:

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