Cohort dynamics give rise to alternative stable community states.

Many ecological systems can exhibit alternative stable states (ASS), which implies that ecological communities may diverge depending on their initial state, despite identical environmental conditions. Here we present a new mechanism that can cause ASS in competition systems. Using a physiologically structured model of competing populations, representing Baltic Sea sprat and herring and their resources, we show how cohort-driven population cycles may result in priority effects leading to ASS. Similar mechanisms could, depending on mortality level, also result in a "resident strikes back" phenomenon. We argue that the prerequisites for the occurrence of ASS in our model system, that is, communities with competing populations exhibiting cohort cycles and variation in size at maturation, may be common in ecological systems.