Invasive ferns have made some freshwater ponds look like lawns, threatening other life in the bodies of water.

Mark Outerbridge, the senior biodiversity officer with the Department of Environment and Natural Resources, said in the summer edition of the Envirotalk newsletter that dense mats of salvinia had covered Cloverdale Pond in Smith’s and Seymour’s Pond in Southampton.

Dr Outerbridge said two species of the aquatic fern family, the giant salvinia and common salvinia, had grown in the ponds, making them look like a “grassy lawn” when seen from a distance.

“Nathaniel Britton reported salvinia growing in the ditches of Pembroke Marsh more than 100 years ago. However, these days it can also be found growing in Cloverdale Pond, the Edmund Gibbons Nature Reserve and in Seymour’s Pond,” he said.

“Introductions into the wild occur as intentional releases or via hitch-hiking on waterfowl and waterbirds. Flooding also distributes fragments that can create new colonies.

“Salvinia can only grow in freshwater wetlands where the salinity is less than seven parts per thousand. Anything greater and the plants weaken and die.”

Dr Outerbridge said the plants were listed among the top 100 most invasive species on the planet and could reproduce prolifically by forming new branches, which break off to form new plants.

Under ideal conditions, the species can double its biomass in two days and reach a density of 30,000 small plants per square metre.

“Dense mats of salvinia cause numerous ecological problems,” he said. “They prevent atmospheric oxygen and sunlight from entering the water, seriously affecting the survival of aquatic flora and fauna.

“Also, the ferns create a large amount of organic matter, which settles and rots on the bottom. This rotting matter depletes oxygen in the water, causing fish kills and harming other aquatic wildlife.

“Waterbirds have a difficult time accessing food resources below the mats and migratory birds are much more likely to overlook our wetlands as they pass by.”

Outside Bermuda, salvinia have been known to clog waterways, cause problems for agricultural irrigation and restrict boat activities.

Dr Outerbridge said that four different approaches had been adopted to remove the plants, including physical removal, the use of herbicides and the introduction of biological controls such as the salvinia stem-borer moth or giant gourami fish.

“None of these species live on Bermuda so risk assessments would be required before any could be imported for use a biocontrol agent,” he said.

The fourth approach is to increase salinity in the ponds, which can occur naturally during a long drought or hurricane.

“Mechanical methods alone have proven to be the least effective because any plant fragments left behind will grow and quickly recolonise the treated area,” Dr Outerbridge said.

“Combining two or more methods increases the chances of success and repeating the control activities is necessary to deal with residual plants.”