TROPHIC RELATIONSHIPS AND MECHANISMS OF ECOLOGICAL SEGREGATION AMONG HERON SPECIES IN THE PARANA RIVER FLOODPLAIN (BIRDS: ARDEIDAE)
Authors: Adolfo H. Beltzer, Juan A. Schnack, Martín A. Quiroga, María de la Paz Ducommun , Ana Laura Ronchi Virgolini and Viviana alessio
Abstract: Herons are one of the best represented families in the floodplain of the Paraná River. The fact that interspecific competition constitutes the most significant factor in resources distribution is a prevailing idea in the ecological theory. According to recent studies, even though competition is important, the modeling of the community’s structure results from the combined action of other factors which operate independently from the interspecific interaction. The distribution of resources is closely related to the ecological niche concept, this being the quantitative description of the organic unit requirements. It is hypothesized in our work that the studied species: Ardea cocoi (White-necked Heron), Butotides striatus (Striated Heron), Bubulcus ibis (Cattle Egret), Ardea alba (Great Egret), Egretta thula (Snowy Egret), Syrigma sibilatrix (Whistling Heron), Nycticorax nycticorax (Black-crowned Night Heron), Tigrisoma lineatum (Fasciated Tiger-Heron) and Ixobrychus involucris (Stripe-backed Bittern), despite of the observed sympatry, have developed adaptative mechanisms of ecological segregation. This allows these species to use the resources in such a way that their diet composition (trophic sub niche) and other parameters of their ecological requirements (temporal sub niche and spatial sub niche) are differentiated. The index of relative importance was applied to calculate the contribution of each food category to the diet of each species. The trophic overlapping of the alimentary spectra, accumulated trophic diversity, alimentary efficiency, dietary selectivity, trophic spread of the niche, spatial sub niche and habitat preference were estimated. As regards trophic spectrum, even though fishes were found to be the basic diet for all four species and insects the second food category, slight differences exist between them which would establish mechanisms at the catches level. This is reinforced by the low overlapping values obtained and the lack of significance in the selectivity values. Variations concerning temporal and spatial sub niches were also obtained. Summarizing, the coexistence is mainly based on the differential utilization of the resources as basic isolation mechanisms and less subtly on space and time. Without leaving aside the usefulness of further research, we think these results provide valuable data for the understanding of the Paraná complex system dynamics.