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Auteur Crawford, Robert J. M.
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Affiner la rechercheAn altered carrying capacity of the benguela upwelling ecosystem for African penguins (Spheniscus demersus) / Crawford, Robert J. M.
Titre : An altered carrying capacity of the benguela upwelling ecosystem for African penguins (Spheniscus demersus) Type de document : texte imprimé Auteurs : Crawford, Robert J. M., Auteur Importance : pp. 570-576 Langues : Anglais (eng) Index. décimale : 508 Voyages et relevés scientifiques. Histoire naturelle Résumé : The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5-3.0 million adult birds in the 1920s to just 10-20% of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruitment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability. An altered carrying capacity of the benguela upwelling ecosystem for African penguins (Spheniscus demersus) [texte imprimé] / Crawford, Robert J. M., Auteur . - [s.d.] . - pp. 570-576.
Langues : Anglais (eng)
Index. décimale : 508 Voyages et relevés scientifiques. Histoire naturelle Résumé : The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5-3.0 million adult birds in the 1920s to just 10-20% of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruitment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability. Exemplaires(0)
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Titre : Trends in numbers of cape gannets (Mows capensis), 1956/1957-2005/2006, with a consideration of the influence of food and other factors Type de document : texte imprimé Auteurs : Crawford, Robert J. M., Auteur Importance : pp. 169-177 Langues : Anglais (eng) Index. décimale : 508 Voyages et relevés scientifiques. Histoire naturelle Résumé : Cape gannecs (Mows capensis) breed at six colonies in Namibia and South Africa. Population size averaged about 250 000 pairs over the period 1956/1957-1968/1969 and about 150 000 pairs from 1978/1979 to 2005/2006. Over the whole 50-y period, numbers at the rhree Namibian colonies fell by 85-98%, with greater proportional decreases in the south. There were increases at two South African colonies between 1956/1957 and 2005/2006. The colony at Lambert's Bay increased between 1956/1957 and 2003/2004, but attacks by Cape fur seals (Arctocephaius pusillus) on birds at nests caused abandonment of the entire colony in 2005/2006. Long-term changes at colonies are thought to be largely attributable to an altered abundance and distribution of prey, especially sardine (Sardinops sagax) and anchovy (Engraulis encrasicolus). In both Namibia and South Africa, the numbers of Cape gannets breeding were significantly related to the biomass of epipelagic fish prey. Over the 50-y period, there was also a marked similarity in the proportions of gannets and epipelagic fish in the Benguela system, which were present in Namibia and South Africa. In the 2000s, there was an eastward shift in the distribution of sardine off South Africa and a large increase in the number of gannets breeding at South Africa's easternmost colony. When sardine were scarce off South Africa, gannets fed on anchovy, but off Namibia anchovy only temporarily and partially replaced sardine. Ecosystem management measures that might improve the conservation status of Cape gannets are considered. Trends in numbers of cape gannets (Mows capensis), 1956/1957-2005/2006, with a consideration of the influence of food and other factors [texte imprimé] / Crawford, Robert J. M., Auteur . - [s.d.] . - pp. 169-177.
Langues : Anglais (eng)
Index. décimale : 508 Voyages et relevés scientifiques. Histoire naturelle Résumé : Cape gannecs (Mows capensis) breed at six colonies in Namibia and South Africa. Population size averaged about 250 000 pairs over the period 1956/1957-1968/1969 and about 150 000 pairs from 1978/1979 to 2005/2006. Over the whole 50-y period, numbers at the rhree Namibian colonies fell by 85-98%, with greater proportional decreases in the south. There were increases at two South African colonies between 1956/1957 and 2005/2006. The colony at Lambert's Bay increased between 1956/1957 and 2003/2004, but attacks by Cape fur seals (Arctocephaius pusillus) on birds at nests caused abandonment of the entire colony in 2005/2006. Long-term changes at colonies are thought to be largely attributable to an altered abundance and distribution of prey, especially sardine (Sardinops sagax) and anchovy (Engraulis encrasicolus). In both Namibia and South Africa, the numbers of Cape gannets breeding were significantly related to the biomass of epipelagic fish prey. Over the 50-y period, there was also a marked similarity in the proportions of gannets and epipelagic fish in the Benguela system, which were present in Namibia and South Africa. In the 2000s, there was an eastward shift in the distribution of sardine off South Africa and a large increase in the number of gannets breeding at South Africa's easternmost colony. When sardine were scarce off South Africa, gannets fed on anchovy, but off Namibia anchovy only temporarily and partially replaced sardine. Ecosystem management measures that might improve the conservation status of Cape gannets are considered. Exemplaires(0)
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