Long-term trends in the breeding populations of waterbirds (1951-1985) at a sewage treatment plant

Long-term (1951-1985) data on the breeding waterbird populations and habita t modifications at Cope Fiats sewage treatment works (Strandfontein) and Ro ndevlei Bird Sanctuary (Rondevlei) were assessed to establish the main fact ors affecting the breeding species richness and nest abundance. Fifty-two ' water-associated' breeding species were recorded in the Zeekoe River catchm ent waterbodies, 46 (88.5%) at Strandfontein, 38 (73.1%) at Rondevlei Bird Sanctuary (Rondevlei), 33 (63.5%) at Zeekoevlei and ten (19.2%) at Princess Vlei. Thirteen (25.5%) of the species were unique to Strandfontein and fiv e (9.8%) to Rondevlei. Sorenson's similarity indices indicated high similar ity (C-S = 0.81) in species richness and low similarity in nest abundance p attern (C-N = 0.10) for the paired sites Strandfontein-Rondevlei;. The lowe st breeding species richness (11 species) was in winter (June) and the peak (42 species) was in spring (October). Twenty-four species joined the breed ing population of Strandfontein during 1959-1976 (during which time pond wa fer surface area increased 2372%). The mean breeding species richness incre ased significantly (chi (2)(2) = 14.42, P < 0.001) at Strandfontein during three distinctive periods: 1951-1958 (pre-construction), 1959-7976 (constru ction): and 1977-1985 (stabilization). At Rondevlei there were no significa nt differences in breeding species richness during the some periods. Howeve r, there were significant differences in the nest abundance pattern: betwee n these grouped years, both at Strandfontein (<chi>(2)(2), = 1472.2, P <0.0 01) and at Rondevlei (<chi>(2)(2) = 60.28, P <0.001). It is suggested that the same pool of species use the heterogeneous habitats at of the sites in the catchment according to their availability. The remarkable increase in s pecies richness and nest abundance at Strandfontein after the late 1970s is attributed to higher habitat structural complexity, brood habitat types, s tabilization and maturation of pond water and the subsequent increase in fo od diversity and availability. The decrease in Rondevlei nesting population s after the 1960s is probably related to fluctuations in water levels, cove r of mudbanks with vegetation and high density of alien fish. A multipurpos e management concept for conservation is proposed for the Zeekoe River catc hment area. Strandfontein provides the greatest diversity of aquatic habita ts not available elsewhere in the catchment (habitats for rare breeders suc h as Avocet Black-winged Stilt, Red-billed Teal and some plovers). Rondevle i has a complementary habitat spectrum (suitable for Great Crested Grebe, P odiceps cristatus, Little Bittern, Ixobrychus minutus, and reedbed species) . Zeekoevlei provides a large area of reedbed habitat for colonial breeding species (African. Spoonbill and herons) and Princess Vlei is suggested far attracting Laridae.