Public release date: 25-Jun-2010
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Contact: Beth King
Smithsonian Tropical Research Institute
Biodiversity's holy grail is in the soil
Why are tropical forests so biologically rich? Smithsonian researchers
have new evidence that the answer to one of life's great unsolved
mysteries lies underground, according to a study published in the
"We've known for a long time that tree seedlings do not grow and survive
well under their mothers or other adult trees of the same species," said
Scott Mangan, postdoctoral fellow at the University of
Wisconsin‹Milwaukee and the Smithsonian Tropical Research Institute in
Panama. "One explanation for the maintenance of the diversity of
tropical trees is that adult trees harbor pests and diseases that harm
seedlings of their own species more than they do seedlings of other
species." The experiments show that underground organisms are key to the
maintenance of species diversity and patterns of tree-species relative
abundance. The detrimental effects of soil organisms from adult trees
not only explain seedling growth and survival patterns, but moreover
that these effects are much more severe for seedlings of rare species
than for seedlings of common species.
Mangan planted seedlings of five species under adults of each species in
the forest and coupled that experiment with a greenhouse experiment in
which he grew seedlings of each species in soil collected around the
base of each of the other species. Consistent across experiments, Mangan
and colleagues found that the ability of seedlings of a species to
survive when grown in soil from the same species actually predicted how
common or rare they are as adults.
Their result closely mirrors results presented in Science magazine this
week by Liza Comita and colleagues, based on a survey of survival of
30,000 tree seedlings‹part of a major effort to understand forest
dynamics worldwide sponsored, in part, by the HSBC Climate Partnership.
"It's been more than 30 years since the idea that negative interactions
between adults and seedlings of the same species may be driving
diversity‹the Janzen-Connell Hypothesis‹was first proposed, and only now
is this story really coming together," said co-author Allen Herre, staff
scientist at STRI. "Two completely different approaches‹analysis of
long-term forest dynamics observations and direct experiments on
Panama's Barro Colorado Island‹are telling us to look for the answer
under the ground. Scott's experiments provide a direct comparison across
species of how much their seedlings suffer from a sort of 'self
inhibition' mediated by these soil organisms."
Biologists refer to soil as a "black box" because it is notoriously
difficult to study a tangle of roots, bacteria, fungi, tiny insects and
other creatures without isolating or changing them. Very similar results
in the greenhouse and in the field reveal that plant interactions with
soil biota alone‹not nutrients, insects, mammals or above-ground
diseases‹are sufficiently powerful and specific to explain why multiple
species co-exist and importantly the strength of those interactions can
be measured and plant species that are most abundant are least
influenced by the soil biota around their parents.
"We have dealt yet another blow to the ailing Neutral Theory of
Biodiversity, which is premised on the idea that all species are the
same," said Herre. "These two publications provide strong evidence that
there are stabilizing mechanisms that maintain diversity, and thus that
neutral dynamics do not explain plant species diversity and abundance."
This study received funding from the Smithsonian Tropical Research
Institute, the University of Wisconsin-Milwaukee and the U.S. National
STRI, headquartered in Panama City, Panama, is a unit of the Smithsonian
Institution. The institute furthers the understanding of tropical nature
and its importance to human welfare, trains students to conduct research
in the tropics and promotes conservation by increasing public awareness
of the beauty and importance of tropical ecosystems. Website:
Ref. Scott A. Mangan, Stefan A. Schnitzer, Edward A. Herre, Keenan M.L.
Mack, Mariana C. Valencia, Evelyn I. Sanchez and James D. Bever. 2010.
Negative plant-soil feedbacks predict relative species abundance in a
tropical forest. Nature.
Authors and afiliations
Scott A. Mangan and Stefan A. Schnitzer, University of
Wisconsin-Milwaukee, Smithsonian Tropical Research Institute
Edward A. Herre, Evelyn I. Sanchez, Smithsonian Tropical Research
Keenan M.L. Mack, Indiana University, Bloomington
Mariana C. Valencia, University of Illinois-Chicago
James D. Bever, Indiana University, Bloomington
Bill S. Jersey USA zone 5 shade garden
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