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King of the jungle: Biodiversity

Q: We’re having a raging debate at home right now. The question is — which has more biodiversity, rainforests or the ocean? My husband and kids insist: the rainforest. I am quite certain that the oceans contain more different forms of life than any place on land. Can you help us? (Dia, Washington DC)

A: It looks like your husband and kids win if by "biodiversity" you mean "the number of species found in any particular area or environment." That’s the usual definition. I got an inkling today at the zoo when I wandered into the zoo jungle house and its humid air closed around me.

High in a jungle tree, a three-twoed sloth is safe from most attacks — except for the harpy eagle. [Passport to Knowledge ©, used with permission]

Birds swoop through the miniature canopy. A mouse scurries for cover and a sign reads, "The rainforest is the most biodiverse of any ecological niche in the world."

But I checked with experts just to be sure.

"The rainforests win on numbers of described species (mainly a lot of insects), but the oceans win on the numbers of different kinds of animal groups (phyla). If undescribed species, particularly microbes, are considered, it could be a tossup," emails biologist Larry Madin of Woods Hole Oceanographic Institution.

"To be frank, we’re not entirely sure whether rainforests or the oceans have the greatest biodiversity, as we’re a very long way from discovering all their species, emails staff scientist Bill Laurance of the Smithsonian Tropical Research Institute. "If I were a betting man, I’d go with rainforests. Acre for acre, they’re unquestionably the most diverse ecosystems on Earth."

"There are only about 4000 marine-plant species compared with an estimated 250,000 species of terrestrial green plants," says marine biologist Tony Rice of Fathom (a consortium of the world’s leading universities). "Similarly, of the 1.7 million-or-so known animal species, only about one-tenth are from the oceans, with most of these living on or close to the bottom in relatively shallow water."

That nails it. The rainforests have far greater diversity of plant and animal species than the ocean does. A surprising situation, really, considering that life originated in the ocean and its waters cover 75% of our planet whereas rainforests cover less than 1% of the planet.

The reason that the ocean has so few species is its size and open range. It’s a big place with few barriers behind which animal can carve out niches. Many ocean species roam the entire ocean. As a result, a group doesn’t get separated into isolated branches and evolve into different species. At a given depth of sea, especially in mid-water, it’s much the same environment so animals don’t specialize much.

Each solitary, fragile, glassy 4-inch (10-cm) individual of Salpa thompsoni can produce hundreds of identical offspring when his/her/its food is abundant. A salp’s life cycle alternates between sexual and asexual. Salps are filter feeders, each one a tireless vacuum continuously clearing phytoplankton (minute free-floating plants) from the sea by filtering water through a mucus net as it swims. [Larry Madin ©, Woods Hole Oceanographic Institution]

Thus, the rainforest wins and your kids and husband are right if by biodiversity you mean the number of species. Your question, however, also says the number of different life forms. As Madin mentions, the situation changes for the major animal groups.

Major groups are called phyla and consist of animals that share the same body plan. For example, the group, chordates, is a phylum and contains animals (e.g., vertebrates, hagfishes, and sea squirts) that have a rod body plan. The rod (through the entire body) stiffens the body and supports it during locomotion. The rod for vertebrates is a spine.

The ocean abounds in body plans since life began in the ocean and mostly stayed there for the first 3 billion years of our planet’s 4.5 billion-year existence. As a result, the ocean has 28 phyla versus a paltry 11 on land, says Rice. So, looking at it this way, you win — the ocean has more life forms. Sounds like you all win. Evolution is tricky.

Further Reading:

Passport to Knowledge (& NASA, NSF): Rainforests

Fathom: Vertical life zones and biodiversity

University of Michigan Museum of Zoology, Animal diversity web: Chordates

Australian Government, Antarctic Division: Salps

(Answered July 1, 2005)