PARASITES BENEFIT HOSTS WITH SURVIVAL INSURANCE
Donald A. Windsor
Parasites benefit their hosts at the species level by insuring that they survive, thus ensuring their own survival. Parasites also help ecosystems survive by preventing monocultures.
Insurance provides benefits, but paying the premiums is a burden. Parasites act as insurance agents for their hosts and indeed, these agents certainly burden their hosts with payments. Parasites enable their host species to survive catastrophes that otherwise could have rendered them extinct. The result of this insurance is the enormous array of biodiversity that our biosphere exhibits.
Catastrophes are not common, but when they do occur, they can be fatal. Insurance is not used to compensate for the routine day-to-day hassles, just for serious calamities. Parasites have to confer benefits only when their hosts are at risk of extinction.
Individual organisms do not receive benefits from parasites. Species receive the benefits. The reason is that individuals do not evolve. Species evolve. Individuals die, but their species lives on, until it goes extinct. Individual parasites harm their individual hosts, by definition. By sharp contrast, at the species level, parasites benefit their hosts.
Insurance agents dole out money, but they also make money. Parasites insure hosts, but the parasites also get benefits. When a host species goes extinct, the species parasitizing that host also risk extinction. Parasitic species persist over long time periods by either keeping their host species extant or by switching to other host species. Parasitic species that do both increase their chances for survival.
Hosts and their parasites do not dwell in isolation. They live in ecosystems and they interact with other organisms. When their resident ecosystems collapse, they risk dying out. Consequently, species within ecosystems must cooperate to insure the survival of their ecosystems. Each species must fit in with the other species.
They fit in by not forming monocultures. Parasites prevent monocultures. When competition and predation are not able to retard rampant population growth, disease breaks out and monocultures are prevented. Our biosphere is biodiverse because of this process. Diseases are caused by pathogenic parasites attacking hosts already weakened by their normal parasites.
A good conceptual model is provided by financial systems. Unregulated capitalism results in wealth accumulation by a few people while most people wallow in poverty. Government regulation, taxes, black markets, and illegal schemes act as parasites on the capitalists, redistributing the wealth. Parasites fulfill this role in ecosystems.
The end result is that parasites insure that biodiversity prospers in our biosphere.