Short-term dynamics of processing chain systems

Abstract

A processing chain results when a single resource passes through a sequence of changes in condition and two (or more) consumer species specialize on resource in different stages of transformation. Each consumer species removes some resource from the total pool and may also accelerate processing of resource to the next condition in the sequence. As a result of the tradeoff between processing and consumption, interactions in processing chains may be either amensal or commensal at equilibrium. In this paper I examine short-term (transient) dynamics of processing chains, by considering population growth through time in a simple two-species processing chain model. I address interspecific interactions by comparing the performance of one species, to a given time horizon, in patches with and without the second. I examine the dependence of short-term interactions on various model parameters. The three parameters with the most influence on short-term interactions are those which largely determine interactions at equilibrium: the consumer-independent processing rate, upstream consumer sloppiness, and upstream resource loss rate. More generally, and regardless of the details of the model, short-term processing chain interactions are often commensal even when the equilibrium interaction in the same system is amensal. The tendency towards commensalism is most pronounced with the shortest time horizons: all processing chain interactions are commensal for at least an infinitesimal interval. This tendency to short-term commensalism, combined with the non-equilibrium nature of many ecological systems and the limited duration of many experiments, may explain why literature examples of processing chains are nearly all commensal. I discuss the importance of temporal scale in planning and interpreting field experiments with processing chain systems.