The Rain Forests of Latin America: Is There Hope for the Future?

Scott A. Mori
Director of the Institute of Systematic Botany and
Nathaniel Lord Britton Curator of Botany
The New York Botanical Garden

The following lecture, with a few modifications, was given as the Eleventh Annual Ellsworth Lecture at Johnson State College on April 19, 1994. Robert Alden Ellsworth taught Latin American history, American foreign policy, international relations, and Russian history at Johnson State college from 1957 to 1973.

Scott A. Mori collecting plants in the lowland forest of central French Guiana. Photo by Michael Rothman.

It is an honor to be at Johnson State College on this beautiful, nearly tropical, spring day to present the eleventh annual Robert Alden Ellsworth Lecture. I am pleased to be the first lecturer in this series to discuss an environmental problem, that of the rapid disappearance of rain forest in Latin America. It is especially moving that this lecture comes so shortly after the death of Robert Alden Ellsworth --- a man who so positively influenced all those with whom he came into contact.

The Tropical Rain Forest

The tropics is the region of the earth’s surface lying between the tropic of Cancer (23° 27’N) and the tropic of Capricorn (23° 27’S) which, respectively, mark the northern and southern limits of the passage of the sun. This band encompasses about 40% of the earth’s land surface and possesses an amazing diversity of vegetation types, ranging from the desert-like caatinga of northeastern Brazil to the wettest region of the world, the Chocó of the Pacific coast of Colombia where ca. 400 inches (10,000 mm) of rain fall each year. Moreover, within the tropics, mountains, such as those of the Andes, provide great altitudinal variation. At higher elevations, vegetation types such as paramo are completely different from those of lower elevations. In all, the wide variation in rainfall, altitude, soils, and geological history combine to make the New World tropics the richest repository of plant diversity in the world.

Neotropical forests are rich in species of animals as well as plants. Although the tropical forests of the world occupy only 7% of the earth’s surface, they may possess as much as two-thirds of the world’s species of plants and animals. Systematists (scientists, that, among other things, define and classify biodiversity) argue over the exact numbers --- but nobody disputes the claim that tropical forests are home to a disproportionate share of the biodiversity of our planet.

When I speak this evening of tropical rain forest, I include all closed forest in which most trees are never entirely leafless. Within this concept, ecologists recognize many different forest types – most of which are not considered rain forest in the strict sense. These distinctions, however, are not important to the concepts that I will develop this evening.

Neotropics versus Paleotropics

The tropics of the world are divided into two realms, those of the New World (the Neotropics) and those of the Old World (the Paleotropics). The latter, in turn, are found in two distinct regions, Africa and Southeast Asia. It is important to remember that the species of plants and animals of these different tropical regions are distinct from one another and that the sociological and conservation problems within each are unique. Nevertheless, the underlying biological principles that govern the evolution and ecology of the plants and animals of these tropical regions are essentially the same.

Since 1975 (except from 1978 to 1980 when I lived in Bahia, Brazil), I have had the privilege of working at The New York Botanical Garden as a tropical botanist. This great botanical research institution provides me with the opportunity to discover, describe, and communicate information about plants. I work in the Neotropics because it is there where the greatest number of species of plants are found as well as where there is still the greatest amount of systematic work to be done.

Botanists estimate that there are some 270,000 species of plants (mosses, and liverworts, ferns and allies, gymnosperms, and flowering plants) in the world. If two-thirds of these are tropical, then there are nearly 180,000 species of tropical plants. Half of these (90,000 species) are found only in the Neotropics while the other half is divided between the African and Asian tropics. Moreover, the flora of the Neotropics is so poorly known that the main attempt to inventory its plant species, a project called Flora Neotropica, would take over 300 years to complete at the current rate of publication! This evening I will limit my comments to rain forests of Latin America --- the part of the world in which I concentrate my botanical research.

Biodiversity

Growing up in Wisconsin, I grew to love trees. As I flew into Burlington, I was impressed by structural and compositional similarities between the Wisconsin and Vermont floras. Wisconsin has approximately 101 species of trees and Vermont, although only about 1/6th the size of Wisconsin, has 105 (numbers from Trees of North America by Tom Elias).

Lowland forest in central Amazonian Brazil. This forest can have as many as 285 species of trees greater than or equal to 10 cm diameter at breast height in an area slightly smaller than two and one-half football fields. Photo by Carol A. Gracie.

In contrast, the forests of Latin America are strikingly more diverse. For example, in a study my colleague, Brian Boom, and I made in central French Guiana, we found 295 species of trees in a sample of 800 trees --- a different species for every second or third tree we sampled, and then we had only looked at trees with diameters at breast height greater than or equal to 4 inches (10 cm)! The record number of species of trees of this size class is that reported by the late Dr. Al Gentry for a forest in Amazonian Peru. Al found 300 species of trees in a plot one hectare in size (a hectare is 100 x 100 m). In his study, every second tree represented a different species. In other words, the diversity of trees is so great in Latin America that nearly three times as many tree species as are found in the entire state of Vermont can be found in an area only slightly larger than two football fields in some forests of Latin America!

The same patterns of diversity exist in other organisms. For example, Vermont is visited by only a single species of hummingbird, the ruby-throated, whereas as many as 163 species of hummingbirds are found across the Amazon Basin. Bats number only 20 species in all of North America, and most of these are insectivorous. In contrast, in northeastern South America there are at least 120 species – comprising insectivores, nectar feeders, and vampires. The diversity of insects in tropical forests is astronomical. Terry Erwin of the Smithsonian Institution has estimated that there may be as many as 41,000 different species of insects in a single hectare of tropical forest, and, because of the great diversity of insects he has found in the canopy of tropical forests, Erwin calculates that there could be as many as 30 million species of insects in the world --- a figure, however, that is still the subject of great debate among the world’s systematists. It is clear that most groups of plants and animals increase in diversity from temperate to tropical regions.

Reasons for High Plant and Animal Diversity in the Neotropics

Why there are so many species of plants and animals in the Neotropics has been a question pondered by biologists since the earliest natural history explorers such as Aimé Bonpland and Alexander von Humboldt returned to Europe in the first years of the 19th century. The answer is not simple and probably is the result of a combination of many factors such as optimum growing conditions, great habit diversity caused by differential rainfall patterns and altitude, climatic changes in the Pleistocene, the geological history of Central and South America (e.g., uplift of the Andes and connection of North America and South America via the Isthmus of Panama), and the great number of plant/animal interactions in the tropics. Because I believe that an understanding of the coevolution of plants and animals is essential if there is to be any hope of conserving biodiversity in the Neotropics, I will emphasize this aspect of biodiversity conservation.

The great numbers of plants and animals in the Neotropics interact in such a way that the diversity of both plants and animals is enhanced. For example, plants that are pollinated by hummingbirds are often characterized by a syndrome of features that serve to attract hummingbirds and reward their visits. In return, the hummingbirds aid in the reproduction of the plants by transporting pollen from one plant to another. Hummingbird flowers are usually red in color, do not emit an aroma (since most birds have a poorly developed sense of smell), have their petals fused into a tube which serves as a container in which nectar accumulates, posses nectar with a dilute sugar concentration dominated by sucrose, and open their flowers during the day. Bat flowers on the other hand, are usually white in color, emit a musty aroma, may or may not have their petals fused into a tube, possess nectar with a dilute sugar concentration dominated by hexose, and open their flowers during the night. The interaction of these two groups of pollinators with plants has created species of plants which possess different syndromes of characters --- thereby resulting in an increase in biodiversity. As I mentioned earlier, the number of hummingbirds and bats in the tropics is much greater than in temperate regions. Therefore, the opportunity for speciation of plants via coevolution in the Neotropics far exceeds the opportunities provided by the more depauperate faunas of temperate regions.

Much of my research has been dedicated to understanding the evolution and ecology of species of the Brazil nut family (Lecythidaceae). I will use several examples from this family to illustrate how specific the coevolution of plants and animals can be.

Eschweilera pedicellata, a member of the Brazil nut family. These highly specialized flowers are pollinated only by large, robust bees. Photo by Scott A. Mori.

Eschweilera pedicellata (Brazil nut family), a species of tree widely distributed in northeastern South America, possesses a bilaterally symmetrical flower which provides a nectar reward located at the apex of an inwardly coiled, closed chamber. The pollinators are several species of robust bees which possess long tongues. These bees, known as euglossine or orchid bees, have the strength to force open the flower and, via their long tongues, the ability to extract nectar from the apex of the coil. The nectar serves as a source of nutrition to male and female bees alike. In exchange for the nectar reward, the bees inadvertently carry pollen from the flowers of one tree to those of another tree – ultimately resulting in fertilization and the production of seeds.

Male part of the flower (the androeciium) sectioned in medial section of Eschweilera pedicellata, a member of the Brazil nut family. Nectar, which is produced at the apex of the coil , is sought after by bees with long proboscises which then transfer the pollen that has been deposited on their heads and backs to the stigmas of the next flowers visited. Photo by Scott A. Mori

Males, and only males, of these same bees also visit certain species of orchids, gesneriads, and member of the jack-in-the-pulpit family. They gather floral aromas which they then sequester in specialized slits in their hind legs. While collecting the aromas, the bees transport the pollen from one plant to the other which, again, eventually leads to the formation of seeds. It appears that the male bees use the floral perfumes, either in the same form as collected or in some modified manner, in some aspect of their mating ritual.

The Brazil nut (Bertholletia excelsa) illustrates the relationship that often exists among seed dispersal agents and plants in tropical forests. The hollow, woody, cannon-ball like fruits of this species fall as far as 150 feet when they are ripe. Each fruit, weighing from one to five pounds, contains from 10 to 25 seeds. The seeds, however, remain trapped inside until they are removed, mostly by a Neotropical rodent called the agouti (Dasyprocta spp.) or, to a lesser extent by squirrels. These mammals consume some of the seeds and bury others for future consumption. However, some of the seeds that are stored for future use are forgotten, and it is these seeds that germinate up to a year later thereby establishing new generations of plants at some distance from the parent trees.

Eulaema bombiformis, a pollinator of Eschweilera pedicellata. Note the long proboscises. Photo by Scott A. Mori.

Bertholletia excelsa, the Brazil nut. Photo by Scott A. Mori

An eloquent essay on how the above kinds of interactions have resulted in increased biodiversity in the tropics has been presented by Louise Emmons (Emmons, L. H. 1989. Tropical rain forests: why they have so many species, and how we may lose this biodiversity without cutting a single tree. Orion 3:8-14). This tropical biologist makes several comparisons between tropical and temperate forests---only one of which I will share with you this evening.

Fruits in the crown of Bertholletia excelsa (Brazil nut). These fall to the ground with the seeds inside at maturity. Photo by Carol A. Gracie.

Emmons compared the number of bird species by dietary preference in a rain forest in Peru with those of a temperate forest in South Carolina. She found 25 vs. six species of carnivorous birds, 98 vs. 27 species of insectivorous birds, and 84 vs. seven species of birds that eat nectar or fruit in the Peruvian forest vs. the forest of South Carolina. In all cases, the Peruvian forest is more diverse, but the carnivores and insectivores are only about four times more diverse whereas the birds that depend on plants are nearly 11 times more diverse. Based on this and other observations, Emmons concludes that the diversity of tropical forests is greatly enhanced by the coevolution between plants and animals. Moreover, she points out that the elimination of animals, for example by overhunting, will eventually lead to the extinction of the plants that depend upon them for the pollination of their flowers or the dispersal of their seeds.

If there is to be hope for the preservation of tropical forests and their rich biodiversity, then the relationships between plants and animals must be understood, and measures must be taken to protect all of the species of plants and animals in this complex ecosystem.

The Economic Importance of Latin American Forests

Tropical forests are the home to many economically useful plants. Seeds of the Brazil nut tree, chocolate from the seeds of Theobroma, pineapples, palm hearts, palm fiber, cocaine, various timbers, and rubber are some of the important economic plants native to Latin American forests which are familiar to all of us.

The larva of the moth Pseudosphinx tetrio feeding on a tree of Himatanthus in the Apocynaceae. This tree produces a white, toxic latex which stops most other insects from preying upon it. This moth, however, has developed the mechanism to detoxify the latex. Photo by Carol A. Gracie.

Because the tropics lack the world’s greatest pesticide, the severe temperate winter, its plants often depend upon chemical warfare to protect themselves from predation. The bioactive compounds resulting from the interaction between plants and those animals seeking to eat them are often used by man in the production of medicines. For example, some Indians of the South American rain forests tip their blowgun darts with curare, a poison extracted from a woody rain forest vine. The poison works by relaxing the muscles of the animal shot in such a way that the muscles used in breathing are paralyzed and the animal suffocates. Curare is now part of the modern pharmacopoeia and is used in many different kinds of surgery as a muscle relaxant. The rain forest is a treasure house of medicines, most of which are still waiting to be discovered.

A Yaguas Indian from Peru showing a blowgun dart tipped with curare. Photo by Scott A. Mori.

Many of the products of the rain forest can be extracted without having a major negative effect on the ecology of the ecosystem. In the Amazon, Brazil nuts can be collected during the wet season and rubber trees tapped during the dry season thereby providing a year round source of income for Amazonian people. Their income can be supplemented by gathering other forest products such as medicinal plants, fruits, and fibers and even by extracting timber in a sustainable way. In this way, products of the rain forest can be harvested without destroying the biodiversity needed to maintain its integrity. Large areas, called extractive reserves, have been set aside in the Amazon in order to protect the cultures of the inhabitants as well as to protect the forest upon which they depend.

In contrast to extractive reserves, the substitution of rain forest with agricultural fields or pastures, if done in areas with poor soils, results in the abandonment of the former in about three years and the latter in around 10 years. It is noteworthy that most of the Amazon forest grows on poor soil --- only about 7% of the entire basin possess soil suitable for sustainable agriculture.

Nevertheless, too much reliance on extractive reserves as the answer to rain forest conservation is dangerous. In the first place, the people living in extractive reserves usually do not have an "ecological conscience" and therefore do not understand the limits of extractivism. These people hunt, cut forest for slash and burn agriculture, extract timber and other products in a nonsustainable way, and mine for various minerals, especially gold. John G. Robinson of The Wildlife Conservation Society has pointed out (Robinson, J. G. 1993. The limits to caring: sustainable living and the loss of biodiversity. Conservation Biology 7(1): 20-28) that "any use of a biological community will ultimately involve a loss of biological diversity." The activity of man in tropical forests, where so many plants and animals have coevolved, can have a profoundly negative effect on biodiversity.

In the second place, the world market for rain forest products might not be able to support much of an increase in the production of, for example, Brazil nuts. It is dangerous to place values on given areas of rain forest and then extrapolate these values to much larger areas. Moreover, such low intensity of land use is probably not capable of supporting human populations at the level needed to increase the standard of living being demanded by more and more people. John J. Ewel, of the University of Florida, has estimated that hunting-gathering and shifting agriculture can only support one person per five square kilometers and 1 person per square kilometer, respectively. If he is correct, then extractive reserves, which probably will support intermediate population densities, will do little to absorb population growth in Amazonian countries let alone contribute to national economies.

I believe extractive reserves are preferable to the replacement of rain forest by agricultural fields and cattle pastures. However, as in any system managed by man, overall biodiversity in extractive reserves will be negatively affected. Therefore, there is still a great need to place large tracts of representative vegetation types throughout Latin America in biological reserves where human activity is strictly controlled. As a step in this direction, The New York Botanical Garden, with support mostly from the W. Alton Jones Foundation, and working with IBAMA (the Brazilian Environmental Protection Agency), INPA (the Brazilian Center for Amazonian research), the Smithsonian Institution, Conservation International, and the Royal Botanic Gardens (Kew) organized a meeting in which over 100 specialists in all aspects of Amazonian biology made recommendations for priority areas for conservation. Based on their findings, a map was published in 1991 by IBAMA, INPA, and Conservation International and is now available to guide the nine Amazonian countries in their selection of Amazonian areas for biodiversity conservation.

Finally, it is important to keep in mind that tropical forests have value beyond that which is derived from the sale of rain forest products. Rain forests are the world’s greatest reservoirs of genetic diversity, they play an important role in the stability of the world’s atmospheric gases, and they help control hydrological cycles on the local, regional, and perhaps even global scale. The cost of all of these "indirect services" of rain forests must be considered in any plans for their use.

The Instability of Tropical Forests

A gap in the lowland forest of central French Guiana. Tropical forests may be pictured as mosaics of gaps at different stages of succesion. Photo by Carol A. Gracie.

Tropical trees are characterized by very shallow root systems and are therefore prone to falling as a result of natural disasters such as unusually strong winds. When a tree falls, nearby trees may also fall because trees in tropical forests are often bridged by woody vines called lianas. The opening in the canopy caused by trees falling is called a gap and may be small or very large in area. Small gaps are caused by the fall of large limbs or individual trees. Large gaps, on the other hand, result from major disturbance such as those caused by landslides, hurricanes, localized but strong winds, and unusually prolonged flooding. Gap formation is a natural and frequent occurrence in tropical forests --- so common in fact that any tree in a given patch of tropical forest can be expected to fall on the average once every 125 years. This is called the turnover time of forests, and the study of associated ecological and evolutionary aspects of gaps is called gap dynamics. Tropical forests may be viewed as a mosaic of gaps at different stages of plant succession.

Because gaps are so frequent in tropical forest, many species of trees have evolved in response to them. For example, rain forest trees have a great capacity for sprouting such that many of them survive snapped trunks by sprouting from below the damaged point of the trunk. However, fire, a tool often used by man in tropical agricultural, usually destroys the capacity of damaged trees to resprout. In addition, different species of trees occur in gaps that are found in more mature forests. Some species may be adapted to small gaps, others to large gaps, and others may not be gap dependent. There is also some evidence that different species may adapt to the different zones created in gaps---i.e., the upturned root area, the area along the trunk, and the area around the crown. Gaps, therefore, may actually promote diversity of plant species in tropical forests.

The bat Sturnia lilium approaching the fruits of a member of the tomato family Solanum rugosum. Bats are essential for the regeneration of tropical forests after the formation of gaps. Photo by Merlin Tuttle of Bat Conservation International.

Bats are essential for the recolonization of plants into large gap areas because the seeds they disperse are often from plants adapted for growth in large, disturbed areas. The fruits of these early colonizers are usually fleshy and contain numerous, small seeds. The bats ingest the fruits, digest the pulp surrounding the seeds, and then defecate the seeds. Seed retention time within bats is often less than 20 minutes and the bats often defecate the seeds while in flight. The seeds of such plants as Cecropia, Solanum, and Vismia are adapted for dispersal by bats and are often the first plants to colonize large open areas. A plastic sheet placed in the middle of one of these fields and checked for seeds periodically reveals that scarcely any seeds arrive during the day whereas there is a steady "seed rain" during the night. Moreover, observation with a night vision scope reveals that bats actively consume the fruits of those species of plants that first invade large gaps. Once the plants dispersed by bats are in place, the conditions then become favorable for other seed dispersers such as birds and mammals to bring the seeds of plants they transport into the area. After about 100 years, a large disturbed area will regenerate to the point that it is structually indistinguishable from pristine areas. It will, however, take a much longer time for the reestablishment of the high species diversity found in areas not recently affected by major disturbance. Remove bats from the ecosystem and regeneration of tropical forests after large-scale disturbance will take much longer.

Man in Tropical Forests

Small, man-made gaps in the lowland forest of French Guiana. A limited number of small-scale, man-made gaps mimics the natural mosaic of tropical lowland forests and therefore does not have a negative impact on biodiversity. Photo by Carol A. Gracie.

Man at low population densities imitates the natural gap formation of tropical forests. Traditional slash and burn agriculture of the tropics is essentially the man-made creation of gaps followed by fire. Fire is utilized because most of the nutrients found in tropical forests are located in the biomass, i.e., in the roots, wood, stems, leaves, flowers, and fruits. Burning rapidly releases nutrients that can then be utilized as a fertilizer to grow crops. In most tropical soils, these nutrients are used up in about three years after which the fields are abandoned and left to undergo plant succession in much the same way that succession follows normal gap formation. These areas can be left fallow for at least 10-20 years and then, after the nutrients have been reestablished in the biomass; the process can be repeated.