L’impact écologique de Quelea quelea (L.) sur les savanes sahéliennes. Raisons du pullulement de ce plocéide

Abstract

The ecological impact of a nesting colony of the social weaver Quelea quelea upon a sahelian savanna of southern Mauritania has been estimated. The average density of nests was 12,400 per hectare, each nest beeing used by one breeding pair and 3 fledglings. The breeding season lasts a month and the nestling period 18 days. During that short time 186 kg/ha of grasses (dry weight) were used for building the nests, and 1845 kg/ha of seeds plus 214 kg/ha of insects were eaten by adults and young. In the area studied, the grass biomass averaged 700 kg/ha and the insect biomass (grass and shrub layers) only 0,70 kg/ha. The ecological impact of a breeding colony of Qaelea upon the available resources (nest material plus food) of the savanna is therefore great and the birds have to collect their plant and animal food up to 10 kilometers from the colony. In very dense colonies reaching a density of 100,000 nests per hectare the limited amount of fresh grass blades required for «weaving» the nests may be a more important limiting factor than food. The probable reasons of the success of Quelea quelea in african savannas are discussed, The more important are the abundance of their stapple food, their excellent flight performances, their ability to live far from free water, to support high environmental temperatures and to breed in large colonies. Breeding is adjusted to take place at the time of the year when the standing crops of plants and insects reach their peak values. Furthemore Quelea is a very catholic species, quick to adapt itself to new situations.