Analyses of studies of grasses, trees, and shrubs, for example, found that despite the obvious differences among these plant types, they all shifted toward less negative or more positive interactions.
"Typically, highly competitive species [e.g., grasses] have decreased competitive or neutral effects at high stress, whereas less competitive species [e.g., trees] have strong facilitative effects at high stress," the authors wrote.
Overall, the researchers found, studies with observations of greater degrees of stress increase "longer" stress gradients also observed greater degrees of shift toward positive interactions.
The hypothesis and the importance of positive interactions in ecology began to occur to Bertness in the 1970s and 1980s. As a junior faculty member at Brown, along the shores of Rhode Island, he noticed that seaweeds and barnacles would never survive the heat stress above the tides in isolation. They could only persist in groups, suggesting that with stress, organisms were better off together despite their competition than apart.
It's the same reason why sparsely planted gardens wilt in hot, dry conditions while more densely planted gardens survive. Mutually beneficial soil shading becomes more important than competition for that soil moisture when it becomes scarce.
Bertness published the Stress Gradient Hypothesis in Trends in Ecological Evolution with Ray Callaway, then a graduate student at the University of CaliforniaSanta Barbara. Callaway is now a professor at the University of Montana.
A shift in research?
Nearly two decades later with so much evidence now assembled, Bertness said, ecologists should feel confident enough in the Stress Gradient Hypothesis to employ it as a "rule of thumb." Rather than continuing to debate
|Contact: David Orenstein|