South Africa and Namibia’s threatened African Oystercatcher is being helped by an invasive species of fast-growing mollusk that displaces native limpets.
By Rex Graham
The stunning bird with glossy black plumage, red bill and legs, and orange-red eye-ring feed on a variety of invertebrates in coastal inter-tidal zones and near-shore islands.
The birds can live up to 35 years, but human disruptions of the breeding grounds led to a steady population decline to a low point in 1980. Numbers then began to bounce back with the introduction of Mediterranean Mussels. Now, as many as 6,000 individual oystercatchers survive, thanks to conservation efforts, protected coastal areas, and their new food source.
Alien mollusk species
Africa’s coast is geologically and ecologically variable and the Mediterranean Mussel doesn’t prosper everywhere. For example, its abundance is significantly lower in the Kogelberg Biosphere Reserve and adjacent Betty’s Bay Marine Protected Area in the Western Cape of South Africa.
Human activities have spread non-native species of mollusks worldwide. The most rapidly reproducing species with few or natural predators in their new habitats have the potential to displace native mollusks and wreak ecological havoc in sometimes unpredictable ways.
For example, take 2 mollusks introduced into the Upper Paraná River floodplain in Brazil. Fifteen of the 27 native mollusk-eating fish readily eat the introduced, rapidly reproducing Golden Mussel (Limnoperna fortunei), which replaces native freshwater mussels. Its faeces add to the sediment, reducing the numbers of segmented worms on the bottom, but increasing the number of snails, slugs, leeches, caddisflies, and mayfly and midge larvae.
Only the Granulated Catfish in the Upper Paraná River ate the introduced Asiatic Clam (Corbicula fluminea), which also has been introduced to North America and Europe. The clam replaces threatened native clams and clogs the water-intake pipes of power plants.
Brazilian scientists said in a study in the Brazilian Journal of Biology that long-term studies are needed to fully understand the ecological impacts of both introduced mollusks.
In the rocky, mixed habitat along the shore of the De Hoop Nature Reserve in South Africa’s Western Cape Province, African Oystercatchers prefer the abundant and relatively large and nutritious Brown Mussel (Perna perna). Such large prey is often fed to oystercatcher chicks. The positive effects of ample supplies of large mussels “are likely to have been offset by increased [human] disturbance at the site,” South African and Namibian researchers reported in the journal Marine Ornithology.
African Black Oystercatcher Habitats
Shorebirds are under assault globally because of habitat loss, and the African Oystercatcher is no different. It has been pushed out of its normal range by habitat degradation and interference by tourists. Buffer zones around coastal nesting areas would help.
However, more adult birds of breeding age don’t necessarily form breeding pairs. “Floaters,” sexually mature birds that delay breeding, are found in areas with plentiful food. Time spent at a single location, rather than age alone may trigger breeding.
In some places, the highly territorial adults are defending smaller territories. This change is responsible for a rapid influx of previously excluded sexually mature birds.
Within a pair’s territory, differences in the bills of males and females can allow them to specialize on slightly different prey. Females are larger than males, and have significantly longer and more pointed bills. Still, both sexes eat mostly the same prey with a few local exceptions.
(Recordist: Lynette Rudman; Xeno-canto bird sounds)
For example, in Kenton on the southeast coast of South Africa, females feed preferentially on mussels while males used their shorter, blunter bills on limpets. Near Port Elizabeth, females preferred beached sea squirts, sac-like marine invertebrates.
The net result of all the ecological factors is positive for the oystercatchers. Researchers wrote in the Journal of African Ornithology that the status of the species could be upgraded from near threatened to least concern on the IUCN Red List.