The Effects of Enrichment on the Dynamics of Apparent Competitive Interactions in Stage‐Structured Systems

Abstract

In the absence of other limiting factors, assemblages in which species share a common, effective natural enemy are not ex- pected to persist. Although a variety of mechanisms have been pos- tulated to explain the coexistence of species that share natural en- emies, the role of productivity gradients has not been explored in detail. Here, we examine how enrichment can affect the outcome of apparent competition. We develop a structured resource/consumer/ natural enemy model in which the prey are exposed to attacks during a vulnerable life phase, the length of which depends on resource availability. With a single prey species, the model exhibits the "par- adox of enrichment," with unstable dynamics at high levels of re- source productivity. We extend this model to consider two prey spe- cies linked by a shared predator, each with their own distinct resource base. We derive invasion and stability conditions and examine how enrichment influences prey species exclusion and coexistence. Con- trary to expectations from simpler, prey-dependent models, apparent competition is not necessarily strong at high productivity, and prey species coexistence may thus be more likely in enriched environ- ments. Further, the coexistence of apparent competitors may be fa- cilitated by unstable dynamics. These results contrast with the stan- dard theory that apparent competition in productive environments leads to nonpersistent interactions and that coexistence of multi- species interactions is more likely under equilibrial conditions.