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Sherley R.B.,University of Cape Town | Barham P.J.,University of Cape Town | Barham P.J.,University of Bristol | Barham B.J.,Penguin Datasystems | And 6 more authors.
Population Ecology | Year: 2014

Colonial breeding is characteristic of seabirds but nesting at high density has both advantages and disadvantages and may reduce survival and fecundity. African penguins (Spheniscus demersus) initiated breeding at Robben Island, South Africa in 1983. The breeding population on the island increased in the late 1990s and early 2000s before decreasing rapidly until 2010. Before the number breeding peaked, local nest density in the areas where the colony was initiated plateaued, suggesting that preferred nests sites were mostly occupied, and the area used by breeding birds expanded. However, it did not contract again as the population decreased, so that nesting density varied substantially. Breeding success was related positively to the prey available to the breeding birds and negatively to local nest density, particularly during the chick-rearing period, suggesting a density-dependence operating through social interactions in the colony, possibly exacerbated by poor prey availability when the breeding population was large. Although nest density at Robben Island was not high, nesting burrows, which probably reduce the incidence of aggressive encounters in the colony, are scarce and our results suggest that habitat alteration has modified the strength of density-dependent relationships for African penguins. Gaining a better understanding of how density dependence affects fecundity and population growth rates in colonial breeders is important for informing conservation management of the African penguin and other threatened taxa. © 2013 The Society of Population Ecology and Springer Japan. Source

De Villiers M.S.,University of Cape Town | Mecenero S.,University of Cape Town | Sherley R.B.,University of Bristol | Heinze E.,Friedrich - Schiller University of Jena | And 7 more authors.
South African Journal of Wildlife Research | Year: 2010

The management objectives of Robben Island, a World Heritage Site, include the maintenance of the island's ecosystem. Yet little is known about the size and impacts of the island's population of introduced European rabbits (Oryctolagus cuniculus). We present the first quantitative assessment of the Robben Island rabbit population. Population estimates should be treated cautiously because of methodological biases, but were useful for identifying trends. Rabbit numbers fluctuated seasonally but were relatively stable between October 2003 and November 2005. Following a programme in 2006 to eradicate feral domestic cats (Felis catus), there was an apparent increase in the rabbit population and the highest estimate of rabbits was made in November 2008. By February 2009, rabbit numbers had decreased considerably and this was probably due to reduced food availability following the 2008 population explosion. Nevertheless, during 2009 rabbit numbers remained higher than they had ever been before November 2008. Rabbits usually preferred open-range habitat, but switched to transitional habitat when numbers were high. Single-species eradication programmes could have devastating impacts on the island's ecosystem. It is thus recommended that a thorough risk assessment be carried out and a holistic management strategy, rather than a single-species approach, be formulated. Source

Sherley R.B.,University of Bristol | Sherley R.B.,University of Cape Town | Barham B.J.,Penguin Datasystems | Barham P.J.,University of Cape Town | And 3 more authors.
Emu | Year: 2012

Loss of nesting habitat threatens many cavity nesting birds worldwide and has contributed to the decline of several species of burrow-nesting seabirds. Replacing lost habitat with artificial nesting structures is considered to be a useful conservation intervention. Here we report on an investigation into the effectiveness of such a strategy providing artificial nests for the endangered African Penguin (Spheniscus demersus) at a colony on Robben Island, South Africa. The re-colonisation of Robben Island by breeding African Penguins in the 1980s was partly attributed to the availability of shaded nesting habitat under introduced vegetation. However, the suitability of this habitat had not been tested empirically. In addition, artificial nests have been present at Robben Island since 2001, but whether they were a viable means of providing improved nesting habitat was not known. The reproductive output of African Penguins was monitored on Robben Island from 2001 to 2010. Breeding success varied between years but, overall, was within the range of figures previously reported for the species. Relative to pairs in nests under vegetation, birds occupying artificial nests and nests in abandoned buildings had increased nesting survival during chick-rearing, with 9 and 13% more chicks fledged per egg hatched over the study period. These artificial structures seem to offer the advantages of shelter from the weather and protection from predators, without the risks of collapse associated with natural burrows in non-guano substrates. This study supports findings from Namibia, and also supports the continued use of artificial nests as a conservation tool throughout the range of the species. © 2012 BirdLife Australia. Source

Sherley R.B.,Marine Research Institute | Sherley R.B.,University of Bristol | Underhill L.G.,Marine Research Institute | Underhill L.G.,University of Cape Town | And 8 more authors.
Marine Ecology Progress Series | Year: 2013

Population trends of African penguins Spheniscus demersus in the Western Cape, South Africa, and their breeding success have been linked to the abundance of their main prey, sardine Sardinops sagax and anchovy Engraulis encrasicolus. During the late 1990s and early 2000s, both fish species increased markedly in abundance, but after 2004, sardine biomass decreased to below average levels. In addition, adults of both stocks were principally located to the east of Cape Agulhas from 2001 to 2009 and were thus distant from seabird colonies on South Africa's West Coast. The number of African penguin pairs counted at Robben Island from 2001 to 2009 and the fledging period of chicks from successful nests increased and decreased in apparent response to the biomass of sardine prior to each breeding season, possibly linked through adult condition at the onset of breeding. Breeding success and chick-fledging rates increased during the study period and showed positive relationships with local food availability, indexed through the annual industrial catch of anchovy made within 56 km (30 nautical miles) of the colony. In addition, chick-fledging rates were depressed in 2-chick broods during years when anchovy contributed <75% by mass to the diet of breeding birds. Previously reported relationships between the overall abundance of forage fish in South Africa and penguin breeding success were not supported. Taken together, these results highlight the combined importance of ensuring adequate local food availability for seabirds during the reproductive cycle and safeguarding regional prey abundance during the non-breeding season. © Inter-Research 2013. Source

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