Biologists remember landmark theory

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In 1967, MacArthur and Wilson published ‘The Theory of Island Biogeography’

Forty years ago, Edward O. Wilson and Robert H. MacArthur described how size and isolation determine how many species an island can support. Last week, biologists gathered to mark the theory’s anniversary, calling it a “pivotal point” in ecology’s relatively short history.

Professor Lord Robert May of Oxford University said the word “ecology” — which describes the interaction between an organism and its environment — was coined just a little more than a century ago. By the 1960s, he said, the science of ecology was still mainly a descriptive one, lacking theories to tie together the observations by scientists in the field.

In 1967, Wilson and MacArthur changed all that. In a book titled “The Theory of Island Biogeography,” they provided a theoretical framework to explain the variation biologists found in the numbers of species on oceanic islands.

The theory said the number of species an island can maintain is a function of both the island’s size and its distance from the mainland as a colonizing point. Together, those factors influence how often new species arrive at the island and how many it can maintain.

“You look at an ecology book today and it’s entirely different from years ago. … It’s absolutely clear that a pivotal point in it was MacArthur and Wilson and ‘Island Biogeography,’” May said.

The field has changed considerably in the past four decades, May said. It has become more crowded and is grappling with ecological questions of planetwide import, such as global warming.

“There’re probably more people in the audience today than the total number of active research ecologists in 1967,” May said, addressing a packed Geological Lecture Hall. “It’s a more crowded world, and just as well it is because the problems we face are huge.”

May was one speaker at a two-day conference organized to mark the 40th anniversary of the publication of “The Theory of Island Biogeography.”

May, who delivered opening remarks on Oct. 5, was among more than 20 speakers who discussed applications of the theory, past and present. Among them were Peter and Rosemary Grant, whose work on the Galapagos helped illuminate evolution among Darwin’s finches; and Harvard Professor of Biology Brian Farrell; Jonathan Losos, the Monique and Philip Lehner Professor for the Study of Latin America; Professor of Earth and Planetary Sciences and Professor of Environmental Science and Engineering Daniel Schrag, who also is director of the Harvard University Center for the Environment; and Wilson, Pellegrino University Professor Emeritus.

The conference, “The Theory of Island Biogeography at 40: Impacts and Prospects,” was sponsored by the Harvard University Center for the Environment and the Harvard Museum of Comparative Zoology.

Wilson reflected on the events that led to the theory’s publication. His studies of the ants on the islands of Melanesia illuminated for him relationships between the spread of species across island chains and the development of new species on islands. Species would move from the richness of the rainforest to marginal habitats, such as savannah and beachfronts. They’d move from there to an island and from the island’s shore to its richer interior where niches were vacant. Over time, they’d evolve new forms to take advantage of the niches and then begin the process all over again.

“One day, it really happened, like a eureka moment. I was guided to see a relation between the spread of individual species across archipelagos and evolution within island species,” Wilson said.

After Wilson met MacArthur, the two toiled to create a theoretical framework that would mirror the “intellectual rigor” found in other biological fields. The two eventually prepared a 1963 paper together and the 1967 book. MacArthur died of cancer in 1972.

“MacArthur and I discussed between us how our old-fashioned subject could achieve the intellectual rigor of other fields. We agreed that population biology would do it,” Wilson said. “We began a series of discussions.”

Wilson said islands were an ideal subject for their discussions because they were essentially natural experiments that were ongoing all the time.

“There were a huge sample of experiments, already performed, awaiting examination,” Wilson said.

Wilson and Daniel Simberloff, today the Nancy Gore Hunger Professor of Environmental Studies at the University of Tennessee, then studied tiny mangrove islands off of Florida to validate the theory. The tiny islands — barely larger than a single tree — were good study subjects because they were small enough that their entire insect population could be examined.

They censused the islands’ insects and then fumigated the islands to remove all their insects. Then they monitored the islands, observing insect immigration, the establishment of new populations, and their shifting makeup over time. The results confirmed the theory that showed that, though the makeup of species shifted over time, the islands supported a roughly constant number of species that rose back to pre-fumigation levels.

Other presentations during the two-day conference examined the theory as it relates to the birds of the Solomon Islands, microevolution, colonization dynamics, colonization and extinction, and natural selection, among others.

Wilson said he couldn’t have imagined the flowering that his and MacArthur’s work would lead to.

“You have gone further than I could possibly have imagined 40 years ago,” Wilson said.