Cohort dynamics give rise to alternative stable community states

Background/Question/Methods

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. Consequently, the presence of ASS may prevent recovery of populations and ecosystem services regardless of restored environmental conditions. Identifying the conditions under which ASS can occur is therefore crucial, not only for our ability to understand ecosystem function but also for ecosystem based management. Here we have used the framework of physiologically structured population models, applied to competing populations, to study mechanisms causing ASS. Specifically, we investigate the significance of cyclic population dynamics driven by cohort interactions, for different levels of size-dependent mortality, as a mechanism causing ASS based on priority effects. Our model represents a mixed continuous-discrete time system of two size-structured competitor species, representing Baltic Sea sprat and herring, and three resources. Growth, survival, consumption and resource production are continuous processes but reproduction occurs as a discrete process. The model formulation consists of a mathematical description of how individual growth, survival and reproduction depend on individual physiology and food densities.

Results/Conclusions

We show how population cycles driven by size-based competitive asymmetries may result in priority effects leading to ASS. Several ASS, including one-species states and a state where sprat and herring could coexist, were identified across a productivity gradient. We also observed how successful invasion could be followed by extinction of the former invaders as result of an attractor switch of the resident population. Both phenomena were critically dependent on size- and food-dependent development and a difference in size at maturation. We argue that the prerequisites for the occurrence of ASS in our model system, communities characterized by competing populations exhibiting cohort cycles and variation in size at maturation, are common in ecological systems. In conclusion, we have identified a novel mechanism causing ASS in competition systems, suggesting that variation in cycle period among populations may have profound influence on community composition.