Parasite populations and communities as non-equilibrium systems

Crucial for understanding ecological systems is whether species live under equilibrium or non-equilibrium conditions. Rohde (2005) has discussed in detail the concepts of balance and non-equilibrium in nature, history of the concepts and evidence for non-equilibrium conditions in many ecological systems, including parasites. The main arguments in favour of equilibrium conditions in nature are that many studies have indeed shown that ecological systems and their components are in dynamic equilibrium; that production and respiration in communities 'must ultimately balance' that even non-climax communities which have not reached a steady state, probably have some sort of equilibrium, determined by the frequency and pattern of disturbances that in most communities rate of energy influx exactly balances outflow in each trophic level; that on islands, immigration and extinction of species is balanced; that birth and death rates populations are equal over long periods; and that prey-predator and similar pairs must be 'in some sort of ecological and evolutionary balance to coexist with one another over any period of time' (Pianka 1974). Also, interspecific competition is believed to play a major role in shaping ecological communities and ecosystems.