Stick two species of microorganisms in a Petri dish, and you won't see hand-to-hand combat with your naked eye. But under the microscope, a turf war is under way. If the organisms rely on the same food source and the same space to thrive, there will be intense competition to drive each other to extinction. Scientists studying how different species compete with one another have long relied on an assumption first made in Charles Darwin's 1859 book On the Origin of Species. The more closely related two species are, he wrote, the harder they'll compete. After all, closely related species are more likely to rely on the same food and habitat.
Ecologist Lin Jiang of the Georgia Institute of Technology in Atlanta has now turned to one-celled microorganisms to determine, once and for all, whether Darwin's competition hypothesis was correct. "You could look at a larger ecosystem," Jiang says, "but in the field, there's always confounding factors, and whether two species can coexist depends on so many things." A predator, for example, could enter the mix, knocking one of the species out of the equation. So he created a simple system in the lab using protists, one-celled organisms that naturally thrive in pond scum and feed on bacteria.
Jiang and his colleagues chose 10 species of protists—some closely related to each other and some more distant relatives—and paired them together into 45 possible combinations. They grew each pair in simple, artificial ecosystems called microcosms, essentially jars full of liquid and nutrients. Then they studied the microcosms for 10 weeks, taking samples each week to measure the prevalence of each protist species. After the 10 weeks, just over half the microcosms had only one surviving protist.
When Jiang's team combined the data on how each protist combination fared with how closely related they are, it found that Darwin's hypothesis held true. The more closely related each pair was, the more likely only one species would be left after 10 weeks of competition, the researchers report online this week in Ecology Letters. The more distantly related pairs were more likely to coexist in the microcosms and maintain stable populations until the end of the experiment.
Although ecologists have relied on Darwin's competition hypothesis for 150 years, Jiang's study is the first to test it in a controlled way, says ecologist Marc Cadotte of the University of Toronto, Scarborough, in Canada. "This paper finally supports one of the major assumptions of previous research." Understanding the ways species compete is important for learning how to restore habitats, avoid invasive species, maintain natural ecosystems, and increase the diversity of organisms in an area, he says.
"The next question should be, 'How much of this is true in other groups of organisms? Is it something particular about these microbes, or is it more generally true?' " Cadotte says. "My suspicion is that this is widely true, but that needs to be tested."