The connection between species richness and area occupied, recognized by biologists for more than a hundred years as a fundamental ecological relationship in plant and in animal communities, has been discerned for the first time at the microbial level.
A pair of papers in the Dec. 9 issue of the journal Nature, one focused on bacteria and another on a microbial fungi, shows that the number of species present – the diversity – increases as the area they occupy increases.
"The results suggest that this relationship may be a universal law common to all domains of life," say Claire Horner-Devine, University of Washington assistant professor of aquatic and fishery sciences and lead author of the paper concerning bacteria. Jessica Green, University of California, Merced, assistant professor of natural sciences, is lead author of the other.
If true of other microbes, the work will give ecologists new ways of understanding the ecology and biodiversity of these tiny organisms, most of which are too small to see even with microscopes, Horner-Devine and Green say. Bacteria and fungi may well comprise the bulk of species on Earth and, despite their small size, play roles in everything from global climate change to water purification to recycling of dead plants, animals and other matter.
"Bacteria, for example, decompose organic material that, among other things, provides the majority of nitrogen needed by the plants we eat," Horner-Devine says. "So understanding the distribution and basic ecology of one of the most abundant and diverse groups of organisms on Earth is crucial."
The idea that the number of species increases as the area increases -– referred to as the "species-area relationship" -– may seem obvious to anyone who has, say, compared a garden-size patch of wildflowers to an entire meadow and realized how many more kinds of flowers there are in the latter, Horner-Devine says. Still, some scientists thought microbes might be different.
"There has been a long standing idea that microbes are so abundant and so small that all the different types of bacteria are mixed up all the time and are, therefore, randomly distributed," says Jennifer Hughes of Brown University, a co-author on the bacteria paper, along with Melissa Lage of Brown and Brendan Bohannan of Stanford University. Horner-Devine was a graduate student at Stanford before joining the University of Washington this fall.
The groups took advantage of existing distance-decay formulas: mathematical formulas previously developed for plant and animal communities that describe how many more types, or species, can be expected to be shared from two samples taken far apart – say at opposite ends of a field or a lake – than from two samples taken close together.
The researchers were the first to couple this ecological thinking with information about microbes found using molecular tools developed in just the last 10 years, Horner-Devine says. Because it is so difficult to determine exact species of microorganisms, the researchers looked for DNA, or pieces of DNA, and compared that to tell different "types" apart.
It's a different taxonomic resolution than species, Green says, but it is a consistent measure of community composition. Green and her co-authors sampled the microbial fungi Ascomycota in desert soils of a 62-square-mile national park in Australia. The Horner-Devine paper was based on bacteria sampled across a half-acre in a New England salt marsh.
The studies span different microbial taxa, habitats, continents, molecular techniques and spatial scales, leading Green to say, "Our data firmly establishes that like plants and animals, microbes are not randomly distributed but rather exhibit spatially predictable, aggregated patterns at multiple spatial scales."
This result has big implications, says Hughes. "If the composition of bacteria is different in different places, then they might be performing these functions differently. For example, a salt marsh in Rhode Island may behave differently in terms of how it buffers Narragansett Bay from nitrogen pollution than a similar looking marsh in San Francisco Bay."
Both studies received funding from the National Science Foundation. The bacteria work also was supported the American Association of University Women, and the fungi work by the Australian Research Council and the New South Wales Resource and Conservation Assessment Council.
"The search for generalities has been especially challenging in ecology," says Stanford's Bohannan. "This work supports the idea that the species-area relationship is a truly general pattern, applying to elephants and bacteria and everything in between."
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