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The World in a Water Drop

March 15, 2005

Astrobiology Magazine — Ecologists know that when it comes to habitats, size matters, and now a new study finds that contrary to earlier beliefs, that maxim holds true right down to the tiny plants at the bottom of many oceanic and freshwater food chains.

The study, conducted by University of Florida, University of Kansas and University of Texas researchers, is important because it shows that tiny microbes follow the same diversity patterns as larger organisms, said Robert D. Holt, a UF professor of zoology and one of the study’s authors. The study appears in the current issue of the Proceedings of the National Academy of Sciences.

In addition, species diversity is “a reflection of the richness of life,” Holt said. Though there are few rigorous mathematical laws in ecology, that relationship between the size of a habitat and the range of species in it has been observed for nearly all organisms.

“Understanding the diversity of life itself is a basic scientific question, in the same way as understanding the causes for the diversity of human civilizations around the world, or understanding questions like the origin of the universe,” Holt said.

Earlier studies have suggested that the diversity-habitat size rule may not hold true for one of the most ubiquitous species in the world: phytoplankton, a group of microscopic algae afloat in the world’s oceans, seas and lakes.

In contrast, the new study demonstrates that the range of phytoplankton species in an ecosystem increases with ecosystem size according to a general mathematical rule. The study also finds that this holds true whether the system is a laboratory test tube or an ocean.

“A skeptic might say, I cannot conceive of how a bottle in the lab that holds as much as a can of beer can tell me anything about Lake Okeechobee,’” said Val Smith, professor of ecology and evolutionary biology at the University of Kansas and the lead author on the study.

But for at least this one principle, that connection exists, Smith said. “Species diversity scales upward with size,” he said.

The researchers analyzed pre-existing scientific data on phytoplankton species in 142 oceans, lakes and natural ponds worldwide, ranging from a few square inches to millions of square miles, and from 239 laboratory test tubes, flasks and beakers.

They found that not only did a correlation between species diversity and habitat area exist for natural water bodies like lakes and oceans, but that an identical pattern exists for experimental ecosystems such as test tubes and laboratory tanks.

“Interestingly, that must say something about the mechanism that controls species diversity,” Smith said. “If it works for (habitats ranging from) little tiny flasks on the desktop to the Arctic Ocean, that says something about how size really matters. We need to figure out which elements of size are the dominant factors that cause this pattern.”

Some of the factors known to influence species variety within a habitat are simply size-dependent, Holt said. For example, some species, such as sharks, simply require more space to survive. “There’s not enough for them to eat in a small lake,” he said.

Bigger ecosystems also will tend to have a greater variety of mini-habitats within them. A large lake contains both shallow and deep areas and can therefore accommodate a greater variety of organisms than a shallow pond, he said. And with the larger total population sizes found in a larger habitat, overall species extinction rates are likely to be lower.

Other possible causes are related to interactions between ecosystems. Larger habitats are bigger targets for the wind or for migrating birds that can transfer algal cells. And larger freshwater bodies tend to have higher river inflows, which can also transport more algae.

For the phytoplankton, the most important factors influencing species diversity are not yet certain, Holt said. However, because the experimental ecosystems were cut off from the surrounding area but the diversity trend was nearly identical to that of the open natural systems, internal factors may play a stronger role, he said. “You’re restricting the processes that could be acting on them, but you’re still seeing this scaling effect,” he said.

Previous studies have looked for a species-area relationship in phytoplankton but found none, possibly because their sample sizes were too small, Smith said. The current study used data from samples spanning more than 15 orders of magnitude, compared with three or four in previous studies.

The results are encouraging for scientists who want to get a handle on scientific questions in hard-to-study, ocean-scale systems, Holt said.

“By studying these smaller systems we may be getting insight into much larger systems where it’s impossible to do experiments and where it’s really hard even to get the raw data,” he said.

Smith agreed. “The fact that lab-scale systems behave similarly to larger natural systems gives us hope that if we ask questions at the lab scale, properly phrased, we can get reasonable answers.”

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