200 Years. 200 Stories. Story 16: “Testing MacArthur-Wilson ”

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photo of Ruth Patrick
Ruth Patrick in a greenhouse housing artificial streams.

Testing MacArthur-Wilson

Eminent ecologists Robert MacArthur and E. O. Wilson presented their landmark paper “The Theory of Island Biogeography” in 1963. They proposed that the time at which the biodiversity of an island reaches equilibrium (a balance between the number of new species colonizing the island and those becoming extinct) is based on the distance of the island from the mainland and the size of the island itself. In other words, the bigger the island or the closer it is to the mainland, the greater the diversity of species that will inhabit the island. This theory would prove influential in the fields of conservation biology and protecting endangered species.

Dr. Ruth Patrick, founder of the Patrick Center for Environmental Research at the Academy of Natural Sciences, conducted one of the first independent verifications of this influential theory in 1964. Her three sets of experiments involved diatom communities in natural and artificial streams. (Diatoms are a type of microscopic algae that have proven useful in environmental monitoring.)

One experiment compared diatom communities colonizing glass slides in comparable streams located in Maryland and the Caribbean island of Dominica. As Dr. Patrick predicted based on the island’s isolation, diversity was substantially lower on the island. A second experiment tested the effects of “island size” (actually the size of glass slides) and the diversity of the upstream colonizing populations for two Pennsylvania streams. As expected, the diversity was higher per unit area on the larger slides and in the stream with the more diverse upstream colonizers. The third experiment used artificial streams to test the rate of colonization on equilibrium biodiversity. In this case, biodiversity was higher when more diatoms flowed through artificial streams.

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