New modeling work suggests why nature is more diverse than niche-based ecological theory predicts.

More than four decades ago, field ecologists set out to quantify the diversity of trees on a forested plot on Barro Colorado Island in Panama, one of the most intensively studied tracts of forest on the planet. They began counting every tree with a trunk wider than a centimeter.

They identified the species, measured the trunks and calculated the biomass of each individual. They put ladders up the trees, examined saplings and recorded it all in sprawling spreadsheets.

As they looked at the data accumulating year after year, they began to notice something odd in it. With more than 300 species, the tree diversity on the tiny 15-square-kilometer island was staggering. But the distribution of trees among those species was also heavily lopsided, with most of the trees belonging to only a few species.

Since those early studies, that overstuffed, highly uneven pattern has been seen repeatedly in ecosystems around the world, particularly in rainforests. The ecologist Stephen Hubbell of the University of California, Los Angeles, who was part of the team behind the Barro Colorado surveys, estimates that less than 2% of the tree species in the Amazon account for half of all the individual trees, meaning that 98% of the species are rare.

Such high biodiversity flies in the face of predictions made by a leading theory of ecology, which says that in a stable ecosystem, every niche or role should be occupied by one species. Niche theory suggests that there are not enough niches to enable all the species the ecologists saw to stably exist. Competition over niches between similar species should have sent the rarities into extinction.

A new ecological modeling paper in Nature by James O’Dwyer and Kenneth Jops of the University of Illinois, Urbana-Champaign explains at least part of this discrepancy. They found that species that should seemingly be head-to-head competitors can share an ecosystem if details of their life histories — such as how long they live and how many offspring they have — line up in the right way. Their work also helps to explain why one of the most successful ways to model ecologies often arrives at accurate results, even though it glosses over almost all we know about how organisms function.

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