Why Would a Good God Create Parasites?

Why Would a Good God Create Parasites?

Some skeptics base their rejection of the Christian faith on bugs-specifically parasites. They argue that the existence of parasites is incompatible with belief in the all-loving, all-powerful God of Christianity. While human beings will never know all God’s reasons for creating parasites, the following true story1 illustrates how their existence may be considered a good thing, rather than an excuse for rejecting God.

In the nineteenth century, a famous Harvard anatomy professor, Dr. Etienne Leopold Trouvelot, took up the hobby of studying exotic insects. One afternoon in 1868, a few gypsy moths he had obtained from Europe escaped from his home laboratory. Unchecked by any predators or parasites, the moths multiplied to pandemic proportions. Within several years, deciduous forests all over the eastern United States were stripped bare as the moths in their larval state consumed every leaf. When this happened, not only were the forests devastated, but so were hundreds of species dependent on those forests, including the gypsy moths.

For some time, the only significant control on the gypsy moth population (and its effects) was the episodic deterioration (quality and quantity) of forest foliage. Following each devastation, the forests would take decades to recover, and when they did, the moths would multiply again-leading to another cycle of widespread destruction. What’s worse, each cycle yielded a progressively weakened gene pool for all species involved.

Eventually, local carnivores, primarily birds and mice, adapted to the new source of prey. Epidemics became less catastrophic but were still very destructive.

A turnaround began with the introduction of a virus (from Europe) that attacked only gypsy moths. It hastened the collapse of the gypsy moth population but did not stop the epidemics.

Major help arrived in 1989. A second parasite, a fungal pathogen, was introduced to the gypsy moths. Finally scientists saw evidence that the cycle of epidemics could be broken and balance restored. They found that multiple predator species and at least two different parasite species must feed on the moths to prevent epidemics and ensure that North American forests remain healthy-healthy enough to sustain hundreds of different species, including gypsy moths, with an optimal quality of life.

As this story demonstrates, without the right parasites, everybody loses. But, with an adequate number and diversity of parasites, all species in the ecosystem thrive, even the species the parasites attack. In this one simple case, where researchers gained a fairly complete understanding of the relevant ecological data, the existence of parasites proves clearly compatible with an all-loving, all-powerful Creator. And while the data for more complex creatures (than moths) and their ecosystems prove vastly more complicated, this one case might encourage a skeptic to reconsider his position before charging God with weakness or malice.

  1. Greg Dwyer, Jonathon Dushoff, and Susan Harrell Yee, “The Combined Effects of Pathogens and Predators on Insect Outbreaks,” Nature 430 (2004): 341-45; Lewi Stone, “A Three-Player Solution,” Nature 430 (2004): 299-300.