A supply/demand perspective of species invasions and coexistence: applications to biological control

Using a physiologically-based model with a supply/demand driven functional response, the dynamics of a species invading a tritrophic food chain are analyzed leading to four conclusions. First, it is shown that a trophic stack may establish via a sequence of invasions, approach a stable steady-state and collapse following a perturbation that significantly alters the ratio of biomass between adjacent trophic levels. Second, when two consumers compete for a common prey, the outcome is determined by a simple rule: the competitor with the lowest metabolic compensation point (the ratio of prey biomass to total consumer demand at which prey assimilation exactly compensates for metabolic costs) displaces the other. Third, intraguild predation can mediate coexistence only if the intraguild preyʹs metabolic compensation point is lower than the intraguild predatorʹs metabolic compensation point. Fourth, when two consumers share a prey and a predator, a simple rule determines dominance: the consumer with the lowest ecological compensation point excludes the other. The ecological compensation point of a consumer is defined to be the ratio of prey biomass to consumer demand when the prey–consumer–predator food chain is at equilibrium. To illustrate the utility of the analysis, several intensively studied biological control programs are examined by parameterizing the model using life table statistics, field observations and mass scaling laws. Using our invasion criteria, we construct assembly diagrams that describe how invasions result in a transition from one persistent subsystem to another. In all cases, there exist paths within these theoretical diagrams that correspond to the observed case histories.