BCI Plants and Their Pests
By Fran Zak
After hiking on Barro Colorado Island (BCI) for two full days, we met Bert Leigh. Bert is the "senior scientist" of this amazing research island. BCI became an island less than 100 years ago when the Americans dammed the Chagres River to create a lake that supplies all of the necessary water for the running of the locks of the Panama Canal. Shortly after the creation of this rainforest island, the Smithsonian Institute decided that BCI would make an ideal tropical rainforest research island and so the Smithsonian protected and preserved it for scientists to study wildlife and enjoy the beauty of a natural, protected rainforest. Get more information about the Smithsonian Barro Colorado Island Station.
Bert knows what every scientist on BCI is studying and keeps tabs on most of the research projects on BCI. The scientific coordinator, Oris Acevedo, also knows what the scientists are doing and provides assistance through the staff and guards that report to her. The guards protect the island's animals from poachers.
Bert gave a talk to us on the ecology of tropical rainforests. He compared BCI and other rainforest ecosystems to other biomes; and he related such facts as amount of rainfall, amount of diversity of species, amount of fruit and leaf production, how seeds are dispersed, flowering and fruiting cycles, and most importantly, how specialized (vs. generalized) plants and their pests co-exist in the rainforest compared to the temperate forest.
As I listened to Bert's talk (it was a welcome change of pace since the room was air conditioned and it was the only time I was cool and comfortable during my stay on BCI), I thought about the forest ecosystem. I remembered a passage from Tapir's Morning Bath by Elizabeth Royte where a young researcher named Bret was making an analogy about generalists and specialists and their survival in different ecosystems. Bret said, "Imagine two travelers in the temperate zone. One, dresses in heavy winter clothing and carries a sleeping bag, tent, and stove. The other wears running shorts and carries no gear. In the summer, the light traveler will get to food sources far quicker than his encumbered competitor, and he'll possibly spread around more genes as a result. But, come winter, the gear-laden competitor will easily outpace his scantily clad cohort-who may even my now, be dead. Put the two competitors in a warm, stable climate", he continued, "and the speed specialist will crush the ponderous, ready-for-anything generalist. Unless, of course, the generalist finds a way to shed his unnecessary gear or exploit some empty niche with no competitors in it. In other words, unless the generalist learns to specialize." From this analogy, it seems reasonable that evolutionary pressures have selected for specialists in this consistently warm environment of the Panama rainforest. This would also mean that species of pests would co-evolve into specialists as their host species evolved in a similar fashion.
So, now lets return to Bert's talk about tropical rainforest ecology, specifically on BCI, where he has spent many years gathering ecological information.
Here are some staggering figures Bert shared with us:
- It rains approximately 2.6 meters (about 8 feet) per year
- The average annual temperature is 27 C, with a range of only 8 C from warmest to coolest temperature during any single day
- Only about 1% of the sunlight coming in from above the canopy reaches the forest floor
- BCI produces: o 7 tons of leaves per year
- 5 tons of wood per year
- 1 ton of fruit (dry weight)
- 3 tons of twigs
- half of the pest insects' total weight is eaten by birds on BCI, so the birds are keeping the pests under control. In turn, the pests are keeping the trees under control.
BCI shows much less tree diversity than other tropical rainforest biomes, such as those in southeast Asia or in the Amazon in South America. This seems to be because there are sharp dry seasons in Panama, not the relatively evenly wet climates in some other forests. Even though BCI shows less diversity than some other rainforests, it still shows much more diversity in tree species than a typical temperate forest. This seems to support the quote by Bret in the beginning of this article. The tropical rainforest is a warm stable climate where the specialist (the runner in shorts with no gear) can thrive and reproduce. The temperate forest has much greater fluctuations in temperature and amount of sunlight during the four seasons in any year and so the generalists (in Bret's story, these would be the travelers carrying warmer clothes and more gear) would survive better in this more variable climate. So, the theory says, temperate forests have more generalist plants and animals and less diversity in types of species than tropical forests.
Pests can grow all year around in the warm rainforest, so they also keep their host trees under control. As a result, not very many of one species can thrive in a given area. Young leaves of plants from tropical rainforests could not grow at all without defenses against pests, so many young leaves are more toxic than mature leaves. Plants produce toxins to kill or damage pests that would eat their leaves. In the temperate forest, mature leaves produce more toxins than young leaves. Trees in a temperate forest produce toxins effective in deterring several types of pests, whereas trees in a tropical forest typically make toxin specific to their specialist pest. The production of toxins costs energy, so a tree needs to be able to limit this expensive production of toxins.
The pests in the tropics will become specialized and each pest will be specific for a certain species of tree. A generalist pest in a temperate forest is able to feed on several species of hosts, but only in the warm months. In the tropics, specialized pests attack only one species of tree, and the individual trees are spread out in the forest among many other species. These specialist pests search hard for their host tree, and may fail to find another, dying before they reproduce. So the greater diversity that is seen in a tropical rainforest is a natural defense against specialist pests, spreading the trees far apart from others of their own species. Sometimes they are far enough to thwart their pests from reaching them from other "sick" trees.
So, comparing different biomes, the greater diversity in plant species seen in a tropical rainforest as compared to a temperate forest can be explained using the concept of specialization vs. generalization. The more constant temperatures of the rainforest allow specialists not only to survive but to thrive and reproduce rapidly. Specialist plants in the temperate forest would not be able to survive and reproduce as well since the temperature variations during the seasons varies much more drastically in a temperate forest. So, rainforest trees have had evolutionary pressures to co-evolve along with their ever-specializing pests, causing these tropical organisms to become specialized for their specialized partners.
Leigh, E.G. Jr. Lecture on the Ecology of Barro Colorado Island. July 25,2002.
Royte, Elizabeth. Tapirâ€™s Morning Bath. 2001:New York, Houghton Mifflin Co.
Windsor, D.M. 1990. Climate and Moisture Variability in a Tropical Forest: Long-term Records from Barro Colorado Island, Panama. Smithsonian Contributions to the Earth Sciences 29: 1-145.
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