Interference versus exploitative competition in the regulation of size-structured populations.

Competition is a major regulatory factor in population and community dynamics. Its effects can be either direct in interference competition or indirect in exploitative competition. The impact of exploitative competition on population dynamics has been extensively studied from empirical and theoretical points of view, but the consequences of interference competition remain poorly understood. Here we study the effect of different levels of intraspecific interference competition on the dynamics of a size-structured population. We study a physiologically structured population model accounting for direct individual interactions, allowing for a gradient from exploitative competition to interference competition. We parameterize our model with data on experimental populations of the collembolan Folsomia candida. Our model predicts contrasting dynamics, depending on the level of interference competition. With low interference, our model predicts juvenile-driven generation cycles, but interference competition tends to dampen these cycles. With intermediate interference, giant individuals emerge and start dominating the population. Finally, strong interference competition causes a novel kind of adult-driven generation cycles referred to as interference-induced cycles. Our results shed new light on the interpretation of the size-structured dynamics of natural and experimental populations.