science

How far is the end of the rainbow? In “The Wizard of Oz,” you could solve that problem by following the yellow brick road. For people who study the dispersal of plant species, it’s a bit trickier.

Ran Nathan, a professor from the Hebrew University of Jerusalem’s Department of Evolution, Systematics and Ecology, has spent a long time thinking about long-distance species dispersal. At a January 12 lecture at the Smithsonian Tropical Research Institute’s Tupper Auditorium, he first considered a marine animal, the hairy triton. These gastropods don’t move very much when they are adults living in their shells, but their larvae get carried far and wide on ocean currents and thus the species is widespread.

But what about trees? They also don’t move, and most of their seeds tend to fall near the parent plant, but long-distance dispersal can also be a factor.

Species dispersal, Nathan noted, happens in both time and space. The most important dimension for his studies is the spatial one. He did his PhD studies on the dispersal of Aleppo pines on Mount Carmel and his further research has taken him to places like North Carolina’s Duke Forest. One thing that he has found is that while short-range dispersal of tree seeds is quite predictable, long distance dispersal is not. For one thing, in addition to the usual natural forces dispersing tree seeds, a range of human activities including the marketing of fruit also play a factor.

To study dispersal of plants that are already established, DNA comparisons of the same species to determine the relationships of individuals in widely separated spaces is one valuable method. Biogeography and mathematical models also have their useful applications to the problem. The dispersal of vertebrates has been studied by telemetry to track individuals --- radio collars on wild animals, for example --- and new miniaturized techniques could make this type of study applicable to plant seeds as well.

Then scientists can do what was done at Surtsey, a volcanic island that rose from the sea about 30 miles south of Iceland in 1963. Within nine years 48 different plant and animal species had arrived there, 10 of which had set up reproducing colonies. With the plants, the morphology of the seeds wasn’t at all predictive of their arrival on the island, but slightly predictive of which plants would colonize. Based on studies of Surtsey, many people in his field concluded that it’s not possible to predict long distance dispersal.

However, by setting up towers with nets and meteorological instruments in the Duke Forest near Durham, North Carolina, and by mapping the locations of trees there and experimentally releasing marked seeds and recording where they go, it was determined that under certain conditions the wind will uplift a seed and send it far away. While these may be relatively rare events, he believes that with the right data about a given tree and the winds that affect it, it is possible to construct a mathematical model by which long distance seed dispersal by the wind can be predicted.

Then the problem becomes the germination and growth of the seeds once they get carried where their possibly predictable fate will have them go. There are “big differences among species,” Nathan noted.

The Israeli scientist said that further work in the field will need to be interdisciplinary, with joint work among meteorologists and biologists just to look at wind dispersal, and another different range of specialities to look at dispersal by animals.

“I don’t think simple models will be able to show long distance dispersal,” Nathan concluded, but he’s not ready to give up on the problem just yet.

Given that part of Nathan’s work is happening in North Carolina, an obvious question arose in the audience question period. What about the hurricanes that sweep across there from time to time? “Very complicated,” he replied. “But such an event doesn’t have to happen very often” to affect a tree’s natural range, the questioner pointed out.



Also in this section:
Genome pioneer J. Craig Venter speaks in Panama
Putting the causes of Amazonia's deforestation in perspective
Long distance dispersal of species
Mussel glue
Gene-spliced crops use more pesticides

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