Tallying the Tropics

Abstract

C asual hikers probably don't notice the faded orange and green ribbons lying half buried in the leaf litter of the tropical forest on Barro Colorado Island (BCI) in Panama. Likewise, visitors to this island reserve probably don't miss the birds that no longer live here or the herds of whitelipped peccaries or the big cats such as puma and jaguar. Those animals seem to have disappeared after this 1,500-hectare (3,700-acre) hilltop was cut off from surrounding land by engineers creating Lake Gatun, part of the Panama Canal. Only because this reserve has served as a mecca for tropical biologists for the last 70 years do the data exist to reveal the changes in species likely brought about by isolation. And even if one of the ribbons did catch a watchful eye, how could that hiker guess its purpose? Two ecologists began tying ribbons around tree trunks no bigger than their thumbs more than 12 years ago. At the same time, they tacked metal plaques onto larger trees. Each plant so decorated received a code number. Stephen P Hubbell, now at Princeton University, and Robin B. Foster, now at Chicago's Field Museum of Natural History, surveyed more than 238,000 trees in this 1 kilometer by 0.5 km rectangle, placing each within a grid marked every 20 meters and again every 5 m. The name, size, and location of each became part of a computer database. Then considered an ambitious if not outrageous undertaking, the 'Tropical Forest Dynamics Plot now serves as a model for similar study sites being established across the globe (see sidebar). At first, the ribbon-tying scientists just wanted to monitor species diversity: They planned to count the kinds of trees present at regular intervals. But before long, many more people started trekking up to the plot, an hour's walk from BCI's buildings. For more than a decade, senior scientists, students, even amateur researchers have been watching these trees grow and produce fruit and have monitored the seedlings in order to learn what controls the organization of the forest. Hubbell and Foster have led two more censuses on the 50-hectare plot, one in 1985 and another in 1990. Meanwhile, other investigators have ventured elsewhere on the island and to nearby peninsulas, tracking fauna as well as flora. Bit by bit, the countless hours of note taking are building a new body of knowledge. ecology has gotten to the point where a lot of the easy things have been done, says Gregory S. Gilbert, a scientist at the Smithsonian Tropical Research Institute (STRI), which runs the island reserve. Now we must get to the underlying mechanisms.