Invading species can stabilize simple trophic systems

Bifurcation analysis is presented for the dynamics of a nutrient-two-prey-p redator bi-trophic food web in a chemostat. A simple model where food is co nverted into energy with a fixed efficiency is used. The available energy i s used for maintenance and the rest for growth. The Holling type II functio nal response is used to model the ingestion rate of the prey consuming the nutrient as well as the predator consuming the two-prey species. The invasi on of a competitor of the prey into a nutrient-prey-predator bi-trophic foo d chain is evaluated using bifurcation diagrams. We will show that an invad ing competitor prey can stabilize an oscillatory nutrient-prey-predator sys tem. It is well known that in the absence of the predator generally one pre y can invade and establish itself while the other is eliminated: this is ca lled competitive: exclusion. We will show that the presence of a predator c an allow coexistence of two competing prey populations. This illustrates a top-down effect, where one species (predator) affects the interaction of tw o other species (competing prey). Bifurcation analysis results are here app lied to community assembly theory; regions of invasibility where obtained u sing available computer packages based on the bifurcation theory with the i mplementation of continuation techniques. Regions with multiple interior at tractors can be distinguished systematically. We discuss the applicability of the proposed. technique for large-scale food webs. (C) 2000 Elsevier Sci ence B.V. All rights reserved.