Trophic Cascade of Shrimps, Ephemeropterans and Periphyton in a Neotropical Stream

Food web theory predicts that a top omnivore would not induce a trophic cascade through intermediate consumers and basal resources. We test this prediction by performing a perturbation experiment in a three trophic level stream assemblage with a top omnivorous shrimp, two primary consumers (ephemeropterans and chironomids) and periphyton as the basal resource. We used electricity at different intensities to produce (1) exclusion of shrimp and (2) exclusion of shrimp and ephemeropterans. Nine repetitions of the experiment, some with replication, were executed in the same area over 5 years and comprised different experimental configurations. We paired each exclusion treatment to its respective control treatment and analysed the before and after exclusion as an effect size and compared the exclusion with the control. Our results strongly suggest the existence of a trophic cascade of shrimp acting negatively on ephemeropterans that are grazers of periphyton. In the absence of shrimps, ephemeropterans increased in density on average to 3.1 times their density in controls, which led to a decrease of periphyton chlorophyll to 0.28 times that with shrimps and a decrease in periphyton dry mass to 0.5. With the exclusion of ephemeropteran grazers, periphyton chlorophyll and dry mass increased 3.5 and 3.7 times respectively. Chironomids did not consistently increase or decrease with exclusion treatments. The phenomenon appeared throughout our experiment repetitions at different times of the year and in different experimental designs, thus supporting the consistency of the trophic cascade and contradicting the original premise of an omnivore not producing a trophic cascade.