by Christopher Dick
Christopher Dick is an assistant professor in the Department of Ecology and Evolutionary Biology at the University of Michigan, and a research associate with the Smithsonian Tropical Research Institute.
The immense diversity of our planet’s tropical forests cannot be overstated. To put it in perspective, in all of North America, from the US-Mexican border to the North Pole, there are perhaps only 700 species of trees in all. Compare that to the species-rich forests of the Amazon basin or the great biodiversity in the Indonesian jungles of Borneo, where one finds up to 300 tree species per hectare (a hectare covers slightly less area than two football fields). In the Yasuní Park of Ecuador, researchers from the Catholic University of Quito, in collaboration with the Smithsonian Tropical Research Institute, have spent the past 10 years attempting to document the tree diversity in a 25 hectare plot (approximately 60 acres). After years of struggle, these researchers have been able to document over 1100 tree species. For these forests there are no handy field guides, and the key taxonomic characters for identification – flowers – are not available every year for each species. Moreover, many of these species are completely new to science. As a result, of the 1100 tree species, only about 600 have been identified to the level of species (Valencia et al., 2004).
It is quite remarkable that a small tract of Amazon forest contains 1.5 times the number of tree species found in the entire continent of North America. It is disturbing to consider the depth of our ignorance of these species. Not only do we not have proper scientific names for many of them, we also do not have knowledge of their geographic ranges or their pollinators and seed dispersers. Perhaps most significantly, we lack information about the chemicals contained in their leaves that defend against pathogens and herbivores, which often form the basis for many human medicines.
Tropical forests differ ecologically from the forests of the northern United States. In order to pack so many tree species in such small areas, tropical trees tend to occur in low population densities, often with less than one reproducing adult per hectare. In order to reproduce, these trees cannot rely on wind pollination, as do the pines, oaks, maples and aspens of temperate zone forests, because most of the pollen would land on flowers from the wrong species or get tangled in lush foliage. With few exceptions, tropical trees rely on animals for pollination. The list of pollinators include bats, hummingbirds, butterflies, moths, bees, wasps, and even flies and cockroaches, with many species in each category. While wind does play a role in the seed dispersal of many tropical trees, animals such as toucans, large rodents and monkeys prove far more efficient and are thus the vehicle of choice for most tropical species. (Bawa, 1990).
This raises the question of how tropical trees will respond to the fragmentation of forests that accompanies agricultural expansion and development. In the Atlantic rainforests of southeast Brazil, near Rio de Janeiro, rainforests have been reduced to 1% of their historical area, and most of the remaining forests are less than 1 hectare in area. That means that these forest fragments are likely to include only one large reproductive tree of a given species. For some species, such small numbers of reproductive individuals may not be sufficient to maintain breeding populations.
The second process that accompanies fragmentation is known as an “edge effect.” Tropical forests typically maintain a moist and relatively cool understory beneath the trees. When tropical forests are fragmented, their borders are exposed to dry air, winds, intense sunlight, and incursions from exotic insects and animals, including people. The microclimatic changes at rainforest edges cause reduced soil moisture, high mortality of mature trees due to drought and wind-throw, and dense growth of fast-growing “pioneer species”. The “edge effect” extends 50 to 100 meters into a forest. In a one-hectare forest fragment (100 by 100 meters in area), the entire remnant forest is exposed to edge effects.
The initial deforestation combined with edge effects may severely reduce the breeding populations of tree species in tropical forests. Are the spatially isolated trees able to reproduce across long distances and modified habitats? Several recent studies, including some of my own research (Dick, 2001), have shown that tropical trees are able to reproduce in low-density populations in fragmented, logged, or semi-agricultural landscapes (Lowe et al., 2005). The same pollinators that forage over long distances in undisturbed forest also fly over long distances in disturbed or fragmented forests (= 1000 meters), as long as there is sufficient remnant forest in which pollinators can nest and reproduce. In the case of some large legume trees left behind in pasture, exotic honeybees have taken over the pollination and pollinate trees up to 3 km apart (Figure 1). This is good news for the remnant trees, perhaps until the introduced honeybees are wiped out by disease, as is presently the case of honeybees in the U.S., or unless the spatial isolation exceeds the distances traveled by pollinators.
Some additional research may change our optimistic view of the sustainability of tropical tree species in forest fragments. First, the studies of reproduction and regeneration of trees in agricultural landscapes have focused on just a handful of prominent tree species, which represent a small fraction of the species found in a diverse tropical forest. Little is known of the resilience of rare species, understory trees, and plants with relatively specialized pollinators. Moreover, many of these species were exterminated from fragmented tropical landscapes before biologists could even begin to study them.
Thankfully, there is some good news for some Amazon forests. Because the old soils underlying many tropical forests are not optimal for European-style agriculture, many of the Amazon’s vast and wasteful farms have been abandoned and are being reclaimed by vegetation from the remnant forests. In parts of the Brazil, for example, much of the area deforested in the 1990’s is now cloaked in dense secondary forest. This forest has the potential to return to its initial high levels of diversity, provided there are enough remnant forests in the vicinity to act as seed sources.
Study is needed on the interaction of forest fragmentation with the effects of global climate change. Recent studies indicate that the woody vines (lianas) are responding to the global increase in carbon dioxide. Lianas are growing more quickly now than they did twenty years ago; they are increasing in biomass, and contributing to more frequent tree falls in tropical forest (Phillips et al., 2002). Edge effects are now covering whole regions. For example, upper elevation cloud forests in Costa Rica appear to by drying out (Still et al., 1999) due to regional deforestation. This will not only devastate the plants of the forest, but also effect many endemic cloud forest-dwelling animals, such as frogs.
With the effects of regional and global climate change, even the few protected forests in National Parks may be ephemeral habitats. The long-term survival of some species in parks and forest fragments will depend on their ability to migrate across human dominated agricultural and urban landscapes to reach optimal local climates. Conservationists will have to take this kind of ecological change into consideration, and develop parks that serve as migration corridors as well as refuges.
Tropical forests provide people worldwide with clean air, water, and new medicines. In this era of habitat destruction, climate change, and ecological disruption, humans must learn more and take care to ensure the preservation of this planet’s valuable biodiversity and the valuable resources they provide.
Bawa, K. S. (1990) Plant-pollinator interactions in tropical rainforests. Annual Review of Ecology and Systematics, 21, 399-422.
Dick, C. W. (2001) Genetic Rescue of Remnant Tropical Trees by an Alien Pollinator. Proceedings of the Royal Society of London Series B-Biological Sciences, 268, 2391-2396.
Lowe, A. J., Boshier, D., Ward, M., Bacles, C. F. E. & Navarro, C. (2005) Genetic resource impacts of habitat loss and degradation; reconciling empirical evidence and predicted theory for neotropical trees. Heredity, 95, 255-273.
Phillips, O. L., Martinez, R. V., Arroyo, L., Baker, T. R., Killeen, T., Lewis, S. L., Malhi, Y., Mendoza, A. M., Neill, D., Vargas, P. N., Alexiades, M., Ceron, C., Di Fiore, A., Erwin, T., Jardim, A., Palacios, W., Saldias, M. & Vinceti, B. (2002) Increasing dominance of large lianas in Amazonian forests. Nature, 418, 770-774.
Still, C. J., Foster, P. N. & Schneider, S. H. (1999) Simulating the effects of climate change on tropical montane cloud forests. Nature, 398, 608-610.
Valencia, R., Foster, R. B., Villa, G., Condit, R., Svenning, J. C., Hernández, C., Romoleroux, K., Losos, E., Magård, E. & Balslev, H. (2004) Tree species distributions and local habitat variation in the Amazon: large forest plot in eastern Ecuador. Journal of Ecology, 92, 214-229.
Originally posted in “On Eagles’ Wings” May 16th 2007