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Water hyacinth Eichhornia crassipes is a highly invasive weed which is considered to pose a threat to the many aquatic ecosystems it grows in Navarro Phiri, 2000. It is a fast-developing and -reproducing plant which is able to quickly overtake a body of water such as a lake or river. The problem is exacerbated in slow-moving water bodies. Because of the density of the mats it forms, it has the ability to out-compete other native aquatic plant life and block sunlight from entering the water, thus causing the death of submerged flora as they are unable to photosynthesise. This in turn depletes oxygen in the water and kills native fish and other organisms in the ecosystem (Navarro & Phiri, 2000). The effect of water hyacinth on another species of free-floating aquatic plant, Azolla foliculoides, will be investigated. Establishing this is useful as every type of organism affected by a water hyacinth invasion will in turn affect another element of the ecosystem. By studying the potential for destruction by water hyacinth in an environment, other species can be considered in order of the threat posed to them when infestations are controlled. In this way, more threatened species can be better protected by having a higher priority placed on their protection. By examining other research in this area, information about what has already been discovered with regard to this investigation can be gathered, and areas to focus on can be narrowed.
Azolla foliculoides is also an invasive weed, though it does not pose the same problems as water hyacinth. The main hazard of Azolla is that it covers smaller areas of water and provides ideal habitats for mosquitos, posing a malaria risk to communities living near these infested regions (Bromilow, 2001). Both water hyacinth and Azolla are native to South America and are free-floating aquatic plants. Bromilow’s literature is a reliable source as he has a degree in applied biology and ecology and is known in his field for his work with invasive plants in South Africa. His material provides an overview of weeds and their impact on the environment. His book also has the support of AVCASA (Crop Protection and Animal Health Association).
Within a body of water, plant diversity can be greatly threatened by water hyacinth infestations as the weed competes with other plants for nutrients, growing space and water. In the Dianchi Lake in China, the number of plant species was greatly reduced from 16 to 3 following water hyacinth establishment in the lake in the 1960s (Jianqing et al, 2001). Water hyacinth also has the ability to form thick mats which cover a large surface area of a body of water. This means that sunlight is unable to reach immersed plants and they are unable to photosynthesise, resulting in their death (Ramey, 2001). These mats grow exponentially with the potential to develop to twice their original size in just 7 days (Navarro & Phiri, 2000). Examining these results is important to establish the potential threats of water hyacinth on an ecosystem, in particular to native plants growing around and beneath water hyacinth mats.
Information obtained from Navarro’s book is reliable as he is a respected researcher and senior lecturer in the Department of Biology at the University of Vigo. He has conducted much research into plant conservation and animal-plant relations which has been published in peer-reviewed journals. Because he has conducted so much research, he is well informed on both water hyacinth and other invasive plants and his book is written with a deep understanding of the subject. Jianqing compiled his research using some results from other water hyacinth investigations. He is an established researcher with many publications in academic journals. He has a BSc in plant protection and a PhD in entomology. This was secondary research which used results from an investigation done by Wu in 1993 and is reliable as it was edited by other recognised scientists in the field such as M. Julien. It is useful as it gives specific effects of water hyacinth in an uncontrolled situation, allowing us to see the potential of wild water hyacinth to destroy other plants.
It was observed that having any competition causes water hyacinth growth rates to slow (Katagira et al, 2011), though there is no evidence that the effect of the water hyacinth on the competing plants was recorded in this investigation. Mass and height values of water hyacinth plants were recorded to be higher when grown alone than when grown with other plants such as hippograss (V.cupsidata), dayflower (Commelina sp) and water-willow (Justicia sp). There is no evidence that the growth in mass and length of the competing species was recorded, and the water hyacinth was by no means outcompeted. In another investigation, the effect of alligatorweed on the growth of water hyacinth was found to be minimal (Wundrow et al, 2012). It did, however, show that the presence of alligatorweed greatly decreased water hyacinth establishment in a body of water. In this investigation, the effect of the introduction of water hyacinth on an alligatorweed population was also recorded. In contrast, water hyacinth greatly impacted the growth of alligatorweed and a decrease in density was noted (Wundrow et al, 2012). This shows that while water hyacinth establishment may be prevented by competing species, once it has been established it is capable of outcompeting and negatively impacting the growth of other aquatic plants. Wundrow’s investigation is more reliable as ratios of plant species were monitored and controlled, while Katagira merely used “a weed mixture containing at least 20% of the competing species” (Katagira et al, 2011). Discrepancies may have arisen from unfixed variables; in this way the results are not reliable. Dr Katagira works for the Ministry of Agriculture in Tanzania. She conducts much research on plants and environmental health which is published in peer-reviewed journals. Wundrow conducts research for the Department of Ecology and Evolutionary Biology at Rice University in Texas. Wundrow’s research was not published in an academic journal, though the method is more reliable than Katagira’s.
Investigations showed that the competitive effect of water hyacinth on other aquatic plants is greatly reduced by the presence of the mirid, Eccritotarsus catarinensis (Coetzee et al, 2005). Water hyacinth greatly outcompeted water lettuce when no biological control agent was present but this effect was reduced with a biological control agent – it proved originally “23 times more competitive than waterlettuce, but only 10 times more competitive when exposed to mirid feeding” (Coetzee et al, 2005). This shows that destructive ecological effects of water hyacinth can be diminished if the water hyacinth population is controlled by a biological control agent. A similar effect was observed when water hyacinth was exposed to the grasshopper Cornops aquaticum. Water hyacinth’s competitive ability was more than halved in the presence of the control agent (Bownes et al, 2010). This research shows that despite the threat that water hyacinth poses to an aquatic environment, biological control of the invasive alien plant is a valid means of protecting non-invasive and indigenous aquatic plant species. Both Coetzee and Bownes are affiliated with Rhodes University. Coetzee holds a PhD from the University of Witwatersrand and Bownes a MSc in entomology. The methods of both these experiments were reliable and valid, with all constant variables controlled (such as volume of water in each tub, water sources used and number and size of plants). Plants were also properly monitored to ensure fair testing throughout the investigations.
A fair amount of research has been done on relationships between water hyacinth and plant life in their surrounding environment. Valid research has been conducted in very specific areas such as the changing competitive effects of water hyacinth when controlled and when not, as well as effects of other plants on water hyacinth for the purposes of control. Very little research is readily accessible on the effect of water hyacinth on specific plants and indigenous species. It is valuable research because isolating detrimental effects of the weed is important in treating a body of water which has been infested by water hyacinth. It can also be useful when rehabilitating an ecosystem to know the damage which may have been caused. It is an area in which more research can be done to gain a broader perspective on the effect on different types of plants and varying conditions in the ecosystem.
- Using a permanent marker, number twelve tubs: each labelled A, B, or C with the numbers 1 through 4.
- Using a marked bucket, fill twelve tubs with 60l of water each.
- Using an electronic balance, measure out twelve 24g portions of 7:1:3 fertiliser and mix it into each tub.
- Measure out twelve 8g portions of iron chelates with an electronic balance and distribute it into each tub.
- Place 200g of fresh Azolla foliculoides into each tub.
- Place 300g of water hyacinth into experiment 2 tubs, 500g of water hyacinth into experiment 3 tubs and 1000g of water hyacinth into experiment 4 tubs. Leave experiment 1 tubs as the control.
- Mark the water level of the tubs once they have been filled .
- Replenish the tubs with water every week up to the original water level.
- Using a bucket, collect the Azolla from each tub individually, measure its mass on a scale and record every two weeks.
Data Collection Plan
Every two weeks, the wet mass of the Azolla from each tub will be measured and recorded over a period of ten weeks. Each tub’s Azolla will be placed individually in a bucket and measured using an electronic scale. At each recording session, twelve samples of Azolla will be measured. Overall, data will be collected six times including at the commencement of the experiment. By collecting data in this way, the growth or deterioration of the Azolla can be closely monitored and any abnormal external factors which could cause outliers in the experiment can be detected and corrected. It allows a trend of growth over the ten weeks to be measured, recorded and observed. A table will be used to record the data such as the one below:
A table showing the effect of varying masses of water hyacinth on the growth of Azolla foliculoides over a period of ten weeks
Mass of Azolla (g)
Mass of water hyacinth (g)
Once all of the data has been collected, it will be represented on a line graph where time (weeks) is plotted on the x-axis and mass of Azolla foliculoides (g) is plotted on the y-axis. Four lines corresponding to each mass of water hyacinth used will be drawn and represented using a key. The graph will enable the viewer to easily compare the effect of different masses of water hyacinth on the Azolla in a competitive environment. Comparative conclusions can be drawn based on the trends observed with each mass.
Every two weeks when the mass of the Azolla is recorded, photographs will be taken documenting the state of the Azolla in each tub, accompanied by written observations. Following the investigation, this data will be combined as a descriptive summary of the experiment to enable more conclusive observations and conclusions to be drawn.
Following the conclusion of the experiment, water (containing fertiliser) will be carefully emptied from the tubs to minimise risk of excessive fertiliser seepage into groundwater and storm drains.
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