PROVIDENCE, R.I. [Brown University] Ecology is rife with predation, competition, and other dramatic "negative interactions," but those alone do not determine the course life on Earth. Organisms sometimes benefit each other, too, and according to the Stress Gradient Hypothesis, their "positive interactions" become measurably more influential when ecosystems become threatened by conditions such as drought. Ecologists have argued about the hypothesis ever since Brown University ecologist Mark Bertness co-proposed it in 1994; Bertness says a large new global meta-analysis he co-authored in Ecology Letters definitively shows that it is true.
The evidence, principally analyzed by former Brown visiting graduate student Qiang He of Shanghai Jiao Tong University in China, comes from 206 studies of 727 shifts of plant interactions amid varying degrees, or gradients, of stress on six continents. Examining the data from each paper and contacting original authors when necessary, He determined the overall trends across the many experiments.
In the vast majority of studies, as stress increased, the significance of interactions shifted toward mutual support in that positive interactions, such as those that promoted neighbors' survival, strengthened in influence, and negative interactions, such as those that hindered neighbors' growth, weakened. In some studies, stress did not change interactions, but negative interactions never increased as stress did, no matter what kinds of plants were involved, what kinds of conditions they were under, or where they were.
"Our results show that plant interactions generally change with increased environmental stress and always in the direction of an outright shift to facilitation (typical for survival responses) or a reduction in competition (typical from growth responses)," the authors wrote in the paper published online. "We never observed an increase in competition at higher stress. These findings wer
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