NUMERICAL VARIATION IN 3 COEXISTING MACRODETRITIVORE SPECIES (PROASELLUS-COXALIS, PLANORBARIUS-CORNEUS AND BITHYNIA-TENTACULATA) UNDER DIFFERENT LEVELS OF VERTICAL FORCES IN AQUATIC MICROCOSMS

Laboratory experiments were conducted to assess the impact of both top-down and bottom-up forces on simplified macrodetritivore guilds. Three macrodetritivore species of Lake Albano (central Italy) were used: Proasellus coxalis, an isopod, and Planorbarius corneus and Bithynia tentaculata, two snails. They were stocked at the ratio of 32:160:164 individuals in each of 24 aquaria. The simplified guilds were governed by different vertical pressures. Three types of top-down (predation) pressures, viz. (i) absence of predators, (ii) high density of the predator Erpobdella octoculata, a leech, (iii) high density of Erpobdella and Gambusia affinis, a fish, and two types of bottom-up pressures, viz. high and low leaf-detritus availability, were employed. The effects of vertical forces were determined by the numerical variation in the survivors of each species and the ratio between the survivors of the three species of macrodetritivores. A quantitative index of the daily top-down force (TDF) was computed. The results showed that the three coexisting macrodetritivore species were sensitive to vertical pressures. When increasing the top-down pressure in aquaria with high detritus availability, the guild response was non-linear: (i) in absence of predators, the percentage of the survivors of the three macrodetritivore species was the highest; (ii) the presence of Erpobdella reduced the macrodetritivore abundance and (iii) the set of aquaria with coexisting Erpobdella and Gambusia significantly increased the number of survivors of the three macrodetritivore species when compared top aquaria with Erpobdella. On the contrary, in aquaria with low detritus availability, the increase of the top-down pressure caused no significant variation in the number of surviving species.