Experimental evidence that induced defenses promote coexistence of zooplanktonic populations

Theoretical predictions suggest that adaptive phenotypic plasticity, and induced defenses in particular, exert a stabilizing effect on ecological systems and increase the likelihood of species coexistence. Nonetheless, up to now, there is little empirical support for this hypothesized mechanism of diversity preservation. We experimentally assessed the effects of induction of plastic morphological responses triggered by a predator kairomone, on patterns of co-occurrence of two herbivore populations of rotifers (Brachionus calyciflorus and B. havanaensis) sharing resources (Chlorella vulgaris) and predators (Asplanchna brightwelli). To our knowledge, this is the first experimental work conducted at the population level where non-induced and induced states of same prey species are obtained through manipulating the level of predator signal. Our objective was to assess the consequences of induced defenses on the mean population density, population variability, evenness and likelihood of persistence of competitor populations. Our results show that induced defenses promote species coexistence through increasing the likelihood of persistence and evenness of competing populations, over a gradient of resource availability.