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News Article

Why are there more tree species in tropical forests?

Study suggests predators may help keep tree populations at sustainable levels 

In the early 1970’s, two ecologists called Daniel Janzen and Joseph Connell suggested that host-specific natural enemies that kill seeds and seedlings clumped near parent trees could keep locally common species from dominating a forest and give locally rare species a space to flourish.  The Janzen-Connell hypothesis is now nearly 50 years old, but it has been difficult to evaluate, particularly on a global scale.  Very few studies had looked explicitly at the relationship between self suppression and species diversity and no studies had explored this link across temperate and tropical latitudes, until now.

Last year, Jonathan Myers, Assistant Professor of Biology at Washington University and Joe LaManna, a post-doctoral Research Associate at Washington’s Tyson Research Centre, proposed a test of the hypothesis to the investigators of the Smithsonian Center for Tropical Forest Science-Forest Global Earth Observatory (CTFS-ForestGEO), an international network of long-term forest dynamics research sites.

LaManna analysed data from 24 research plots, which, when combined, were home to more than 3,000 tree species and approximately 2.4 million trees.  This research provided the first evidence that the Janzen-Connell effect contributes to the biodiversity gradient across tropical and temperate latitudes. 

“This is the first time we’ve had the data to do this kind of in-depth analysis and to look across temperate and tropical latitudes,” said LaManna.

The analysis also made an unexpected discovery - that the plant predators that kill rare species could also prevent them from going extinct.  “When species get too rare, their enemies also thin out and they have what is known as a rare species advantage,” said Myers. 

“We were able to show for the first time that this stabilising effect may be stronger for rare species in the tropics; this may explain why rainforests harbour so many rare trees,” added LaManna.

The researchers emphasise that the Janzen-Connell hypothesis doesn’t point a finger at particular plant enemies. However, it requires that the enemies be host specific and not generalists.  “That means the FBI (fungus, bacteria and invertebrates), the very tiny organisms that are evolving rapidly and are more likely to be host-specific,” said LaManna.

The main observation on which the Janzen-Connell hypothesis is based on, is that seedfall is heaviest under a parent tree, but that the young seedlings usually do better away from the parent. 

According to the researchers, the effect of pathogens on trees is similar to its effect on people.  For example, we’re more likely to suffer from a disease if we’re living in a multi-occupancy building, than we are if we’re living in rural isolation, which means that we are self suppressing, even without modern medicine.  The study also highlighted two areas of self suppression.  “We find that self suppression is on average stronger in a tropical forest than a temperate one and that it changes systematically with latitude.  This is big result number one,” said Myers.

Two aspects of the CTFS-ForestGEO network gave the data significance, according to Myers.  Firstly, the forest plots were large enough to contain a number of rare species and secondly, all of the trees in the plots had been tagged, mapped, measured and identified.  “No other globally coordinated forest plot network combines large plots with censuses that go down to 1 centimeter in diameter,” he said.

The second result is that self suppression in the tropics is stronger for rare species than for more common ones.  Rare and common species are equally affected in the temperate zone, but in some instances self suppression goes the other way and is stronger for the common species rather than the rare ones.

For the researchers, this is an exciting result because it could explain a puzzling characteristic of tropical forests: their diversity is not related to large numbers of species in general, but to large numbers of rare species.

"How can you pack more than a thousand species in a 50-hectare plot in the tropics if the rare species are being negatively impacted by these specialised enemies?" Myers asks. "You'd think that if these species are rare they'd be more likely to go extinct, so what maintains them in the system?"

"Paradoxically, enemies can be beneficial," said LaManna. "Yes, they kill, but by killing they prevent population booms and busts. If you have no enemies, you're going to have exponential population growth followed by a crash. If you add an enemy that tracks abundance, over time the population stabilises. It's never going to become large, but the flip side is it's never going to crash. And so these enemies are a stabilising force."

Journal Reference

LaManna, J.A., Mangan, et al. Plant diversity increases with the strength of negative density dependence at the global scaleScience, 2017 DOI: 10.1126/science.aam5678

Article details

  • Author(s)
  • Stephanie Cole
  • Date
  • 10 July 2017
  • Source
  • Washington University in St. Louis
  • Subject(s)
  • Environment
  • Forest trees