  We developed mathematical models that simulate community dynamics derived
from a series of perturbation experiments. These experiments were performed in
an Atlantic Forest stream. The three trophic level community was submitted to
two combinations of press perturbation experiment. In the first, the top
trophic level was removed by electrical exclusion. In the second configuration,
the top trophic level was still excluded plus a group of species from the
second trophic level was inhibited, also by electrical pulses. Experiments were
repeated at different times to increase confidence in the observed mechanisms.
Community responses indicated the existence of cascading interactions similar
to a classic trophic cascade. The community was composed of Macrobrachium
shrimps at the top trophic level, Ephemeroptera and Chironomidae larval insects
at the second trophic level and periphyton (= epilithon = biofilm) at the first
trophic level. The shrimp exclusion caused an increase in ephemeropterans and
chironomids and led the periphyton to decrease in the exclusion zone. When
ephemeropterans were inhibited together with the exclusion of shrimps, the
chironomids and the periphyton increased. We modelled the community
interactions by means of differential equation systems, simulating the three
configurations: 1. natural condition, 2. shrimp exclusion condition and 3.
shrimp-exclusion-plus-ephemeropteran-inhibition condition. All parameters were
calculated via experimental results and the stability of the models was
determined by their matrix eigenvalues. The models successfully reproduced the
qualitative responses of the community. They proved to have attraction points,
which enables us to predict that the system has some ability to return to its
previous condition after local perturbation.
