An alien invasive plant is threatening to pulverise native vegetation across 21 islands in the Gulf of Mannar Biosphere Reserve (GoMBR), an avian distribution study has revealed. Prosopis chilensis, a drought-resistant plant native to the arid regions of four South American countries — Argentina, Bolivia, Chile, and Peru — is not the only trouble for these islands divided into the Tuticorin, Vembar, Kilakkarai, and Mandapam groups.
The coral reef has been destroyed in several places near these islands although coral quarrying for industrial purposes has been outlawed, the study published in the Journal of Threatened Taxa said. Human settlements, though not permanent, have also impacted the islands such as Poomarichan, Pullivasa, and Manoliputti in the Mandapam group, the study said, while recording 96 species of birds on these islands.
The authors of the study are H. Byju and S. Ravichandran of Annamalai University’s Centre of Advanced Study in Marine Biology, and N. Raveendran of the Madurai-based Iragukal Amritha Nature Trust. “Most of the islands have sand dunes along their coastlines with salt-dominant plant species. Some of the islands contain trees while the marshy sections of some of the islands are occupied by mangroves and allied species,” Dr. Byju told The Hindu.
“We found the Prosopis chilensis in seven or eight islands. Any invasive species on an island will slowly kill the native trees as well as the mangroves as we have seen some overgrown to the edge of the islands near the high tide areas,” he said. The scientists said they could not find any major studies on the invasiveness of this species or how it came to India, unlike the equally invasive Prosopis juliflora found in the Deccan Plateau, specifically Tamil Nadu, introduced by the British in 1877 to green the arid tracts.
While the native vegetation “has lost ground to” Prosopis chilensis on some islands closer to the coast, the study underlined the “indiscriminate destruction of marine life” continuing despite efforts to educate and monitor the fishing community. “Corals, seagrass, and mangroves are among the three unique ecosystems present on the islands,” the study said, pointing out that the coral reef was beyond conservation in several places.
The study cautioned about temporary settlements on the Mandapam group of islands primarily because of the use of the vegetation on the islands for cooking. Deployment of traditional fishing gear was also infrequently recorded, especially close to the mangrove fringes of many islands, which offer an ideal foraging ground for large wading birds.
The scientists did not find some of the earlier recorded species during their survey to update the distribution of the bird groups on the 21 islands from 2015 to 2022. Of the 96 species belonging to 34 families from 13 orders that they recorded, 58 were waterbirds while the others were terrestrial. Of the 29 shorebird species recorded, one is endangered and seven are marked near-threatened in the Red List of the International Union for Conservation of Nature.
The GoMBR, India’s first marine biosphere reserve, is one of the important habitats for coastal birds migrating as far as the Arctic Circle. The area is of particular significance as the 21 islands also serve as resting places for birds migrating to and from the nearby Sri Lankan islands.
The highest number of water bird species, inclusive of waders, ducks, terns, gulls, egrets, and herons,was recorded on Manoli island of the Kilakkarai group. But 19 species of shorebirds were recorded compared to 26 listed in a 1990 study. The Kentish plover, ruddy turnstone, lesser black-backed gull, greater crested tern, little cormorant, great egret, Indian pond heron, and Brahminy kite were among 12 species of birds found on all the islands. But many were recorded on only one island. These included the white-breasted tern and wood sandpiper (on Manoli island), pin-tailed snipe (on Musal island), greater thick-knee (on Shingle island), and Eurasian spoonbill, pied kingfisher and Indian roller (on Kurusadai island).