Herbivore control of plant populations is most effective in aquatic systems and least effective in forests. Multiple factors may act synergistically to regulate populations. The rate of loss to predation may be greatest at medium prey density. Selection may be stabilizing, directional, or disruptive. Nature selects mutation rates that balance the need for genetic novelty against the adverse effects of bad mutations.
Intraspecific competition is more intense than interspecific competiton.
Predation or disease on a stronger competitor may enable a weaker competitor to coexist. Competition may result in character displacement. Prey organisms have evolved different means of escaping predation. Mutualisms may be obligate or facultative. The limiting resource is the resource in shortest supply relative to the organism's needs.