  We investigate how perturbations propagate up and down a food chain with and
without self-interaction and omnivory. A source of perturbation is a shift in
death rate of a trophic level, and the measure of perturbation is the
difference between the perturbed and unperturbed steady state populations. For
Lotka-Volterra food chains with linear functional response, we show
analytically that both intraspecific competition and intraguild predation can
either dampen or enhance the propagation of perturbations, thus stabilizing or
destabilizing the food web. The direction of the effect depend on the position
of the source of perturbation, as well as on the position of the additional
competitive and predatory links . These conclusions are confirmed numerically
for a food chain with more realistic Type-II functional response. Our results
support the positions of both sides in the long-standing debate on the effect
of intraspecific competition and omnivory on the stability of trophic systems.

Populations and Evolution