Diagnostic écologique du Parc National lchkeul (Tunisie) après la construction des barrages: cas des oiseaux d'eau

Abstract

Ecological diagnosis of the Ichkeul National Park (Tunisia) after dams building : the case of waterbirds. The Ichkeul National Park (INP) has been ratified as a National Park (1980), a Biosphere Reserve under UNESCO’s Man and the Biosphere Program (1977), and a Wetland of International Importance under the Ramsar Convention (1980), and subsequently has been inscribed on the World Heritage List (1980). However, this area is actually threatened by three dams which can retain up to 75 % of the inflow freshwater. According to this situation, fundamental modifications of different components of the INP were recognized. The modification of the hydrological parameters and the response of waterbird community to dam building have been specifically assessed in the present paper. Across 2001– 2004, the salinity of the lake ranged from an average of 21 g l−1 in winter to 37 g l−1 in summer with an annual average of 27 g l−1. This latest value was more than two times that recorded before the dams were built. Thus, the densities of Potamogeton and Scirpus populations were dramatically decreased, reducing the available food of herbivorous waterbirds, notably ducks and coots. They were restricted each year to within the mouths of rivers and covered a small area in September before disappearing by the end of October and November. Concerning waterbird communities, a total of 71 species were counted. The commonest species that accounted for more than 54 % of the total abundance of the waterbird communities were : Common Coot (21 %), Common Pochard (16 %), Eurasian Wigeon (12 %) and Greater Flamingo (5 %). The most abundant trophic groups were herbivores (11 species and 75 % of the total abundance of waterbird communities), filterers (3 species and 5 %), vertebrate consumers (14 species and 3 %) and insectivores (26 species and 9 %). Subordinate ones were invertebrates-vertebrates consumers (8 species and 1 %), invertebratesherbivores (6 species and 5 %) and omnivores (3 species and 2 %). The total species richness varied significantly among months. It ranged from 39 to 41 species, between November and December, and the minimum from 12 to 14 species, during June and July. Species abundance also varied significantly between months with a peak during November and December of about 50 000 individuals. The lowest species abundance values were recorded in July and August with an abundance of about 1500 to 3000 individuals. The INP constitutes a very important area during the autumn migration and wintering periods and provides a habitat for a large range of groups of wintering waterbirds, notably herbivorous species. Thus, the competition for resources may be higher in winter than in summer. The total number of species is still decreasing from 93 species (1966-1985) and 90 species (1982-1994) to 71 species in 2001-2004. This variation is most likely dependent on the variation of the number of sporadic species visiting the site than due to the local conditions. The annual density of all waterbird communities decreased dramatically of about 40 % after the building of dams. This condition was mainly caused by the decreasing number of herbivores. Indeed, they presented the lowest peak abundance estimations obtained since the beginning of waterbird counts in the park. There were about 120 000 birds in 1963-1983, 90 000 birds in 1984-1992, 121 000 birds in 1993-1994, but 60 000 in the present assessment. We support the hypothesis that freshwater diversion and increasing salinity caused severe depletion of Potamogeton and Scirpus, and therefore, a worrying reality of the feeding condition for this trophic group. Furthermore, the intensive pressure of feeding exercised by the herbivorous species on wetland production has been identified as having a negative local effect on the Potamogeton population dynamics mainly when the productivity of the Ichkeul National Park was low. From those results, it is suggested that dams built upstream of the Ichkeul National Park caused significant changes to the abundance of waterbird community.