The functional structure of beetles populations in the marsh estuarine of Tafna (Algeria)

An ecological study of Coleoptera communities was conducted from October 2009 to September 2010 in the salt marsh estuary of the Tafna river. Pitfall trapping in five vegetal formations provided 3833 specimens distributed in 140 species, comprising 59 phytophagous species, 69 predators, 5 polyphagous species, 3 scavengers and 4 coprophagous species. Confrontation of species richness with two fundamental ecological parameters -salinity and humidity - showed a specialization of trophic groups which seemed to increase with decreasing gradients of humidity and, especially, salinity. Thus, in the lowest parts of the Arthrocnemum glaucum formation (G.I) with highest values of salinity and humidity, the fauna is arranged in two functional groups only. Conversely, in the highest parts of Typha angustifolia (G.IV) and Suaeda fruticosa (G.V) formations with lowest values of salinity and humidity, five functional groups are identified, with a clear domination of dung beetles in the G.V formation. Juncus maritimus (G.III) and Salicornia fruticosa (G.II) have four and three trophic groups respectively. Analysis of the species richness in each trophic group included in the 5 vegetal formations showed the highest diversity of predators in hydro-halophilous and hydrophilous formations G.I, G.II and G.III. Conversely, phytophagous species were more abundant in formation G.V, with 45 species. Polyphagous species (two species only) appeared in formation G.II only, followed by necrophagous and coprophagous species from formations G.III and GIV, with a very low species richness. Specialization of Coleoptera plays an important role in the functioning of the communities. The rise in trophic group numbers was correlated with a decrease of salinity; it seemed to be directly related also with increasing of anthropic pressure in the highest parts of the Suaeda fruticosa formation. In the lowest parts of the Arthrocnemum glaucum formation, the nocturnal ground-beetles (Carabidae) are dominant. The specialization in the food regime of these predatory beetles is correlated with their body size: small diurnal species such as Microlestes abeillei, Microlestes corticalis, Syntomus fuscomaculatus and Amblystomus algirinus are less specialized and have a wider trophic range. Moreover, a polyphagous regime enables large species to offset the rarity of prey in environments considered extreme and hostile for most of organisms, especially for Coleoptera. This may account for the occurrence of polyphagous species in the hydro-halophilous and hydrophilous formations. The lack of coprophagous species in the lowest levels may be explained by a semi-permanent submersion, but also by a very compact clayey and silty soil, preventing the beetles from burrowing below the excrement to construct their pedotrophic nests (e.g. Onthophagus). The increased plant diversity in the more xeric formation with Suaeda fruticosa induced a rise in the diversity of phytophagous species and their associated predators. Analysis of the functional structures of beetle communities enabled us to show the importance of some environmental variables for the functioning of this wetland, which appeared to be in equilibrium, despite habitat fragmentation caused by the construction of an aquaculture farm in the lowest part of the marsh, and by the expansion of crops in the highest part of the marsh.