The Delta's Tiny Troublemaker
"There is always one troublemaker!" I hear teachers exclaim this quite often, but this time the statement was coming from a group of scientists. The troublemaker in this instance is not a rowdy student, but invasive plant species Salvinia molesta, commonly known as Kariba Weed.
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AquaRAP scientists examine salvinia at Paradise
Pools. |
Salvinia is a small, free-floating fern consisting of two hairy leaves and a trailing root system. These plants are asexual and reproduce vegetatively, simply by fragmenting. When nutrients are present, and temperatures and light conditions are favorable, salvinia can double its mass in as little as three days. Animal movements and hydrological flows also enhance the movement of weeds through the wetland.
Salvinia has been a major concern for Botswana's Department of Water Affairs since it was first reported in Moremi Game Reserve in 1986. The negative impacts of the weed had been witnessed before throughout Africa, so controlling it became an immediate concern. Without control, these plants form thick mats or floating islands, which have the potential of greatly altering an ecosystem.
Not only does salvinia wipe out indigenous plants submerged in the water, but it removes nutrients, decreases oxygen content (destroying the fish life), and creates excess sediment along river beds as the plant's leaves die and fall to the bottom.
| At first glance, the collected material looked like a cloud of mud and algae. |
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Salvinia is indigenous to Latin America, where it is naturally controlled by a weevil species known as Cyrtobagous salviniae. These weevils attack the xylem and phloem-the life system of the plant-making it difficult for salvinia to function properly and reproduce.
The method of control salvinia using weevils was introduced into Moremi Game Reserve under the management of Pete Smith of the Department of Water Affairs. Because this species of weevil only feeds on the S. molesta plant, it doesn't pose a threat to the indigenous plants in the Delta.
Jane Nengu of AquaRAP's water quality team states, "We've tried to introduce weevils wherever salvinia is found because it is the most sustainable biological tool." Though the biological control method has been effective in Moremi, monthly monitoring is still necessary.
The AquaRap team was interested in determining what effects salvinia might be having on the presence of other species in an infested area. So we drove to an
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Scientist Fred Ellery speculates
on the role of nutrients in
supporting salvinia growth. |
infested area in the Paradise Pools region to take a closer look. The invertebrate team went straight to work, shaking the roots and leaves of the salvinia in a shallow pool. At first glance, the collected material looked like a cloud of mud and algae.
However, upon closer examination, it quickly came to life with tiny organisms which were making the salvinia roots their home. Among the species collected, all smaller than 2 mm, were: nematodes; mosquito, mayfly, and dragonfly larvae; mites; beetles; and a few undetermined species that will need to be analyzed under a microscope.
The fish team found the Snake catfish (Clarias theodorae) adapted to live in the mud; and an Aplocheilichthyes sp. that has not been scientifically described. Both of these fishes are commonly found in this type of pool.
Fred Ellery, team leader of the plant group, noticed a strong smell in the organic matter. After some discussion with the rest of the scientists, it was determined that the odor is most likely being caused by local hippos importing nutrients into the pool. Since turnover of water in the pool is low, nutrient enrichment increases, creating an ideal habitat for the growth of salvinia. Most likely, Fred speculates, the density of salvinia would not have been so high without the input of these nutrients.
Since water in most of the Delta system is nutrient poor, salvinia hasn't had a major impact. However, in nutrient-rich local pools, the species must continue to be monitored.
– Reported by Sharon Safran
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