Tropical jungles in Central America host hundreds of different plants while Montana’s forests only support dozens. Isn’t that just because jungles get more rain and sun?
What sounds like a simple answer overlooks some really complicated science. University of Montana fire ecologist Andrew Larson was part of a 50-author study to sharpen that understanding. And the result shows plant communities depend on good enemies as much as good resources.
“This is the most difficult study I’ve tried to explain,” Larson said last week after the project was published in Science magazine. “We can’t pick up everything we know from the tropics and use it to explain forests here. We couldn’t take a tropical biologist and turn them loose in the Rattlesnake, because they couldn’t fully explain what they see any more than someone from Montana going down to Costa Rica. We’re going to have to learn some new things to explain and understand what’s going on.”
The research also builds support for an old idea, broached in the 1970s by ecologists Daniel Janzen and Joseph Connell. They suggested jungles teemed with rare species not just because there was lots of fertile soil and water and sun, but because it was easier to be rare than common in a competitive environment.
Imagine growing up in a farming town like Malta or a big city like Seattle. Children of a Malta farmer can successfully find work growing wheat for the family or on another farm. But the children of a baker in Seattle would probably fail if they opened another bakery next to Mom and Dad. They’d have more success with a furniture repair shop or a bookstore — occupations that would struggle in a small farming town.
Similar patterns play out in forests. Tropical jungles in Central America display dozens or hundreds of species not only because there’s more water and heat near the equator, but because the rarity of each individual species makes it harder for a specialized bug or disease to find victims. That’s known as “rare species advantage.”
Conversely, offspring of tropical species struggle in the shadow of their parents because the mature plants hog the few resources they’ve specialized in consuming. This falls under the mouthful term “conspecific negative density dependence.”
Move toward the harsher environments of the temperate zones (like North America), and the pattern changes. Plant species may team up with allies like underground fungi to make the best of limited resources. We wind up with extensive forests of lodgepole pine or prairies covered with a few species of grass.
“The 50 authors from 12 countries who contributed to this finding asked an enormously important question that has been on the minds of scientists since Darwin,” said David J. Skorton, secretary of the Smithsonian Institution. “Until now, there was no satisfying answer to the question of why there are so many species of trees in tropical forests.”
This study, convened by the Smithsonian Forest Global Earth Observatory network, used 24 research plots 25 to 60 acres in size. Each had a team of ecologists individually tagging, measuring, and identifying everything bigger than half an inch in diameter — every year. Former UM wildlife biology graduate Joe LaManna led the project as a post-doctoral student at Washington University.
“It took thousands of hours to generate all that data,” Larson said. “You can learn a lot from a satellite, but you can’t do this. You have to get out in the field and track performance and demography.”
Larson was responsible for three of the sites, in Yosemite National Park, southwestern Washington and Utah. As the project progresses — it will continue after publication of the study — he hopes to add a Montana plot to the inventory.
One thing the study shows is how the relationships with friends and enemies changes as species move from the tropics to the temperate zones. In the jungle, the kaleidoscope of rare plants jumbled together makes it hard for a predator to concentrate on the weaknesses of a particular species. The same principle keeps one species from becoming dominant because it would support an equally dominant predator that would overwhelm it in a boom-and-bust cycle.
But the pattern changes as landscapes get closer to the poles. As the resources get limited, specialization pays off even as it encourages specialized enemies. That may stem from the need to take advantage of whatever the tougher habitat offers. But it also might mean that changes in climate, like more seasonal rains or heat waves, could have amplified impacts on how temperate forests look.
“Papers like this one are exciting, because we’ve documented a strong global pattern,” Larson said. “We’ve speculated about this, but no one had the huge amount of data needed to show it.”