The Secret to Biodiversity is in the Soil

Image: Photoshop Top Secret, via Flickr

Mankind has long been fascinated by biodiversity, even imagining species beyond the natural realm. We have learned to appreciate the value of biodiversity by enjoying the variety in nature, and by benefiting from products derived from many different species. But did you ever wonder why there is so much diversity on our planet? If "survival of the fittest" is the rule, is the planet naturally trending towards a time when just a few of the fittest survive?

You probably know that some adult animals eat their own offspring. But did you know that older trees threaten their young? In fact, scientists have know for a long time that seedlings grow better in the shade of foreign species than in the neighborhood of their own kind. Now, a well-designed scientific experiment published in the journal Nature explains why: the secret is in the soil. And the secret suggests that biodiversity does not happen by accident.

The hypothesis that negative interactions between trees and their seedlings may be behind the biodiversity of natural forests -- the Janzen-Connell Hypothesis -- was formulated over thirty years ago. By creating a disadvantage for the spread of forest monoculture, this phenomenon effectively results in biodiversity of plants and naturally leads to biodiversity in insects and animals up the ecological chain.

Scientists have been trying to understand the mechanisms underlying the lower survival rates of seedlings growing near trees of the same species. Suspected causes include competition for resources and shared natural enemies (enemies of the adult species have a greater presence in the neighborhood of members of that species, and these enemies repress growth of the seeds that land too near others of their kind). But the complexity of the variables such as fungus, insects, root competition, sun, rain, and animals make it difficult to pinpoint the exact causes.

Scott Mangan -- of the University of Wisconsin, Milwaukee, and the Smithsonian Tropical Research Institute in Panama -- has succeeded to isolate soil as the cause with a clever experiment conducted in parallel in the native environment and the greenhouse. Mangan planted five species of seedlings in the forest and also collected dirt nearby for growing seedlings in a greenhouse in soils that match the soil of the trees planted in nature. The similarity of the results in the greenhouse and results in the natural environment provide strong evidence that the relevant enemies are in the soil.

Mangan's results correlate well with another study just published in Science. In that research, a team led by Liza Comita, also of the Smithsonian Tropical Research Institute, studied over 30,000 seedlings of 180 species on Panama's Barro Colorado Island. Comita's team showed that the species most vulnerable to negative feedback from others of their own species are also the rarest species.

The findings of these two scientists may help to improve methods for promoting survival of the species most threatened with extinction. They also demonstrate that biodiversity is promoted by natural mechanisms, and that abundance (or rarity) of species is not random, but is clearly linked to traits specific to the abundant (or rare) species -- dealing a potential death blow to the Neutral Theory of Biodiversity.

More on Biodiversity:
Photographer David Liittschwager Documents Amazing Biodiversity Found in 'One Cubic Foot'
Shade-Grown Coffee Protects Tree Biodiversity
Crop Biodiversity A Cure for Ocean Dead Zones?
Rice Biodiversity Techniques Remain Intact in Rural Thailand