Dassen Island (7163)
South Africa, Africa
Site overview
KBA status: confirmed
Global KBA criteria: A1a, A1c, B1, B2, D1a
Year of last assessment: 1998
National site name: Dassen Island
Central coordinates: Latitude: -33.4167, Longitude: 18.0833
System: marine, terrestrial
Elevation (m): 0 to 19
Area of KBA (km2): 7.36915
Protected area coverage (%): 99.98
KBA classification: Global
Legacy site: Yes
Site details
Site description: Dassen Island, South Africa’s second largest coastal island, lies 9 km from the mainland between Saldanha Bay and Cape Town. This island reaches 19.2 m at its highest point, and is generally flat or gently sloping, with extensive sandy areas and a few patches of exposed rock. It is richly covered with vegetation in winter. Several buildings occur in the north-east, as does a large manned lighthouse in the south-east. The island is partially enclosed by a low solid concrete wall. Hedges of non-native manitoka Myoporum occur near the buildings.
Rationale for qualifying as KBA: This site qualifies as a Key Biodiversity Area of international significance that meets the thresholds for at least one criterion described in the Global Standard for the Identification of KBAs.
Additional biodiversity: See Box for key species. Owing to its proximity to the mainland, comparative isolation, and suitable cover, the island offers sanctuary to a variety of land and seabirds. The most important resident is Spheniscus demersus. Numbers have been stable since 1989, following a 26% decrease during the late 1970s. Dassen Island also holds up to 4.6% of the global population of Haematopus moquini—the largest island population in South Africa. Dassen Island and Lake St Lucia (IBA ZA044) in KwaZulu-Natal are the only two sites in South Africa supporting breeding Pelecanus onocrotalus. Unlike most pelican populations, the Western Cape population has increased substantially during the twentieth century. The numbers on Dassen Island’s Boom Point have increased from less than 100 pairs in the mid-1970s to c.550 pairs in 1996. The island also supports healthy breeding populations of Phalacrocorax coronatus, P. capensis, Larus dominicanus, L. hartlaubii and Sterna bergii, as well as supporting many Arenaria interpres and other migratory waders during summer. Phalacrocorax neglectus, which used to breed in large numbers, has decreased dramatically over the last five years. Oceanodroma leucorhoa breeds on this island in very small numbers. Non-bird biodiversity: Scelotes gronovii (LR/nt), a reptile endemic to the west coast, occurs on the island.
Delineation rationale: Type 1 marine IBA: suitable for the seaward extension approach. Type 2 marine IBA: non-breeding (coastal) congregations.
Habitats
| IUCN Habitat | Coverage % | Habitat detail |
|---|---|---|
| Marine Intertidal | 50 | |
| Marine Coastal/Supratidal | 50 |
Threats
Summary of threats to biodiversity at KBA: Dassen Island was proclaimed a Provincial Nature Reserve in 1988, and an island warden who enforces regulations currently mans it. Many non-native plants have been introduced, as have mice Mus musculus, cats Felis catus and rabbits Oryctolagus cuniculus. The cats, which eat mostly rabbits, are also known to prey on Haematopus moquini and Sterna hirundo, and in 1983 cats were estimated to take some 2,000 nestlings of Spheniscus demersus each year. Cat numbers have been reduced considerably since 1983 and now only a handful remain. These should be eliminated from Dassen Island as soon as possible. Cat predation on Dassen probably results in lowered breeding success compared to cat-free islands. Certain problems are known to affect seabirds throughout their ranges. Competition with commercial fisheries, especially purse-seining for surface-shoaling fish such as anchovy Engraulis capensis and pilchard Sardinops sagax, has been implicated as the most significant factor causing seabird population declines, especially in populations of Spheniscus demersus and Phalacrocorax capensis. A recommendation has been made that marine reserves with a radius of 25 km be created around important breeding islands. Commercial fishing should be banned or restricted within these zones. An unpredictable threat, which is difficult to control, is chronic pollution by crude oil or other pollutants that spill into the ocean when tankers break open, wash their tanks, dump cargo or pump bilge. Spheniscus demersus is particularly susceptible to these events and a single oil disaster could severely affect populations. The South African National Foundation for the Conservation of Coastal Birds (ZANCCOB) cleaned, rehabilitated, and returned some 3,000 penguins to the wild between 1981 and 1991.
| Threat level 1 | Threat level 2 | Threat level 3 | Timing |
|---|---|---|---|
| Biological resource use | Hunting & collecting terrestrial animals | Unintentional effects (species being assessed is not the target) | Ongoing |
| Biological resource use | Fishing & harvesting aquatic resources | Unintentional effects: subsistence/small scale (species being assessed is not the target) [harvest] | Ongoing |
| Biological resource use | Fishing & harvesting aquatic resources | Unintentional effects: large scale (species being assessed is not the target) [harvest] | Ongoing |
| Invasive & other problematic species, genes & diseases | Invasive non-native/alien species/diseases | Named species | Ongoing |
| Invasive & other problematic species, genes & diseases | Problematic native species/diseases | Named species | Ongoing |
| Pollution | Industrial & military effluents | Oil spills | Only in the future |
| Residential & commercial development | Housing & urban areas | Ongoing | |
| Energy production & mining | Renewable energy | Only in the future | |
| Transportation & service corridors | Utility & service lines | Ongoing | |
| Transportation & service corridors | Shipping lanes | Ongoing | |
| Transportation & service corridors | Flight paths | Ongoing | |
| Human intrusions & disturbance | Recreational activities | Ongoing | |
| Human intrusions & disturbance | Work & other activities | Ongoing | |
| Natural system modifications | Other ecosystem modifications | Ongoing | |
| Pollution | Garbage & solid waste | Ongoing | |
| Climate change & severe weather | Habitat shifting & alteration | Ongoing | |
| Climate change & severe weather | Storms & flooding | Ongoing |
Additional information
References: Adams (1991), Berruti (1986), Branch (1991), Brooke and Prins (1986), Cooper (1981), Cooper and Berruti (1989), Cooper and Brooke (1986), Cooper et al. (1983, 1984), Crawford (1995), Crawford and Dyer (1995), Crawford and Shelton (1978, 1981), Crawford et al. (1982a,b, 1983, 1989, 1990, 1994, 1995b,c), Frost et al. (1976), Furness and Cooper (1982), Hockey (1983), Hockey and Hallinan (1981), Morant et al. (1981), Randall et al. (1980), Shelton et al. (1982), Siegfried (1982), Summers and Cooper (1977), Underhill (1992), Williams et al. (1990), Wilson et al. (1988).