Research on relative species abundance provides new theoretical foundation

A paper in this week’s journal Nature, building on radically new ecological theory by University of Georgia professor Stephen Hubbell, challenges half-century-old ideas about how natural plant and animal communities are put together.

The paper in Nature includes research by physicists Jayanth Banavar and Igor Volkov of Penn State University and Amos Maritan of the International School for Advanced Studies in Trieste, Italy, along with Hubbell.

Conventional ecological theory says that species coexist with one another by being different and the best competitors in their own ecological niches (functional roles) in the community. Hubbell challenged this theory in a widely acclaimed but controversial 2001 book called “The Unified Neutral Theory of Biodiversity and Biogeography.” In it he argued that many of the ecological patterns we see can be more simply and often better explained if competing species are treated as if they were essentially identical.

“This theory flies in the face of 50 years of research that has celebrated the uniqueness of all species in nature,” said Hubbell, a professor of plant biology at UGA. “And while I certainly do believe that many aspects of species are unique, this theory is, more and more, fitting the patterns we see in nature.”

Banavar, Volkov and Maritan were intrigued by the theory, and out of their reading of Hubbell’s book sprang an intense collaboration, the first fruit of which is the paper in this week’s Nature. The paper provides Hubbell’s theory with a mathematically stronger and more general theoretical framework, which allowed the authors to solve one of the oldest and most celebrated theoretical problems in ecology.

“Sixty years ago, the great geneticist and statistician, Ronald Fisher, discovered a mathematical distribution describing patterns of relative species abundance – the pattern of commonness and rarity in species – in ecological communities,” said Hubbell. “Fisher had no biological idea at all why it worked so well, but now we do. In this paper, we show that Fisher’s distribution falls right out of the theory into your lap and explains what it means biologically.”

The Nature paper also shows how Fisher’s distribution changes when the immigration of species into an ecological community is restricted. This is a famous unsolved problem in the Theory of Island Biogeography published by renowned biologists E. O. Wilson and Robert MacArthur 36 years ago.

For the past half century, the field of ecology has operated inside a Darwinian paradigm that assumes species in nature are “niche-differentiated” – that is, they have largely separate roles to play in biological communities. During that time, scientists have tried to measure the characteristics of species to predict their distribution and abundance in nature by measuring their “niche characteristics.”

Ecological nature is, many scientists assert, fundamentally asymmetric because of the inherent uniqueness of species in ecological communities. This is called the Niche Assembly Theory. According to this theory, species coexist in closed assemblages that are in equilibrium or near equilibrium.

“These niche assembly models have not really worked as well as we hoped, though we have learned a lot,” said Hubbell. As it turns out, there is a great deal scientists do not yet know about how species form ecological communities. Predictive ecological theories that provide real-world answers are needed for issues ranging from protecting biological diversity to improving land management laws.

Starting in 1979, Hubbell began to develop a theory that grouped similar species instead of assuming all species are different and act differently. Over time, he began to discover that many patterns that had heretofore been explained by “niches” might have a simpler explanation.

Hubbell began to formulate what is now generally called “Neutral Theory.” He believes that Symmetric Neutral Theory, as it is now called, is consistent with a number of major ecological pattern regularities at large geographic spatial scales as well as evolutionary time scales, many of which have resisted explanation by asymmetric niche-assembly theory.

“One of my personal lessons from this interdisciplinary collaboration is that physics may provide fresh approaches to some old problems in ecology,” said Hubbell. “Ecologists often start with an already complex hypothesis and then add even more complexity. Physicists tend to start with the simplest hypothesis they can think of and then add complexity only when they’re forced to by the data. Maybe this is why they jumped on my Neutral Theory.”

Hubbell admits that his Neutral Theory will probably continue to be controversial among many of his colleagues for some time to come, and he expects the new paper in Nature to draw considerable scrutiny. The paper rebuts some of the firestorm of criticism kicked off by Hubbell’s 2001 book, which brought both supporters and critics into a major scientific debate over the book’s validity.