Multiple factors may act synergistically to regulate populations. The rate of loss to predation may be greatest at medium prey density. Predator and prey populations influence each other. They may cycle together, cycle out of phase, or cycle independently. Communities may be closed or open.
Species interactions cascade through a community.
Community composition may be a continuum that follows environmental gradients. Few species are abundant. Many species are rare. The limiting resource is the resource in shortest supply relative to the organism's needs. The competitive exclusion principle says that two species cannot coexist indefinitely on the same limiting resource.