Dynamic modelling of a freshwater stream community from experimental field data

We developed mathematical models that simulate community dynamics derived from a series of perturbation 13 experiments. These experiments were performed in an Atlantic Forest stream. The three trophic level community 14 was submitted to two combinations of press perturbation experiment. In the first, the top trophic level was 15 removed by electrical exclusion. In the second configuration, the top trophic level was still excluded plus a 16 group of species from the second trophic level was inhibited, also by electrical pulses. Experiments were 17 repeated at different times to increase confidence in the observed mechanisms. Community responses indicated 18 the existence of cascading interactions similar to a classic trophic cascade. The community was composed of 19 Macrobrachium shrimps at the top trophic level, Ephemeroptera and Chironomidae larval insects at the second 20 trophic level and periphyton (= epilithon = biofilm) at the first trophic level. The shrimp exclusion caused an 21 increase in ephemeropterans and chironomids and led the periphyton to decrease. When ephemeropterans were 22 inhibited together with the exclusion of shrimps, the chironomids and the periphyton increased. Although the 23 insects-periphyton interactions were of a trophic nature, the shrimps-insects interactions were not. We modelled 24 the community interactions by means of differential equation systems, simulating the three configurations: 1. 25 natural condition, 2. shrimp exclusion condition and 3. shrimp-exclusion-plus-ephemeropteran-inhibition 26 condition. All parameters were calculated via experimental results and the stability of the models was 27 determined by their matrix eigenvalues. The models successfully reproduced the qualitative responses of the 28 community. They proved to have attraction points, which enables us to predict that the system has some ability 29 to return to its previous condition after local perturbation.