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A Neotropical Companion
An Introduction to the Animals, Plants, and Ecosystems of the New World Tropics
By John Kricher, William E. Davis Jr.
PRINCETON UNIVERSITY PRESS Copyright © 1997 Princeton University Press
All rights reserved.
Tropical Climates and Ecosystems
NEVER does nature seem more bountiful than in the tropics. Anyone with a passion for natural history must try and visit the tropics and experience Earth's most diverse ecosystems firsthand. This is a book about the New World or Neotropics. Alexander von Humboldt, Henry Walter Bates, Charles Darwin, Alfred Russel Wallace, Louis Agassiz, Thomas Belt, Charles Waterton, William Beebe, Frank M. Chapman, and other eminent naturalists have each been profoundly influenced in their beliefs about natural history by visits to the Neotropics. Their spirits of adventure and investigation are no less fervent today. Thousands of tourists annually travel to Neotropical jungles and rainforests in hopes of seeing some of the myriad bird species, colorful butterflies and other diverse insects, noisy monkey troops, and numerous other attractions of these majestic ecosystems. Students and professional researchers by the dozens are patiently and painstakingly unraveling perhaps the most complex Gordian knot in ecology, the multitudes of interactions among plants, animals, and microbes resulting in the vast biodiversity of tropical forests.
There is an urgency about the science of tropical ecology: tropical forests, which occupy approximately 7% of Earth's surface but may harbor as much as 50% of the world's biodiversity (Myers 1988; Wilson 1988), are being cleared at alarming rates (Repetto 1990). Cattle ranches and soybean fields are replacing rainforests. Though tropical rainforests also exist in Africa and Asia, approximately 57% of all rainforests remaining on Earth are in the Neotropics, with 30% in Brazil alone. Many of these are being cut: already only 12% of Brazil's unique Atlantic coastal forest remains (Brown and Brown 1992), and in 1987 alone some 20 million acres of Brazilian rainforest were cut and burned (Miller and Tangley 1991). Other Neotropical areas in danger and judged to require immediate conservation attention include the Colombian Choco, forests of western Ecuador, and the uplands of western Amazonia (Wilson 1992). At the current rate of deforestation, within 177 years all tropical rainforests on Earth could be gone. Right now, less than 5% of the world's tropical forests are protected within national parks or reserves. Though some encouraging data suggest a slowing of Amazonian rainforest clearance (Bonalume 1991), concerns remain about the long-term future of these rainforests as well as other tropical ecosystems, not just in the Neotropics but globally. Obviously, the ecosystems comprising the main subject of this book are potentially endangered. These ecosystems deserve better. Alexander von Humboldt, one of the first of the great naturalists to learn from the tropics, captured the sense of wonder one receives upon seeing rainforest for the first time:
An enormous wood spread out at our feet that reached down to the ocean; the tree-tops, hung about with lianas, and crowned with great bushes of flowers, spread out like a great carpet, the dark green of which seemed to gleam in contrast to the light. We were all the more impressed by this sight because it was the first time that we had come across a mass of tropical vegetation.... But more beautiful still than all the wonders individually is the impression conveyed by the whole of this vigorous, luxuriant and yet light, cheering and mild nature in its entirety. I can tell that I shall be very happy here and that such impressions will often cheer me in the future. (Quoted in Meyer-Abich 1969.)
Most people who have never been to equatorial regions assume them to be continuous rainforest, much as described by Humboldt. Tropical rainforest is, indeed, a principal ecosystem throughout much of the area and is the major focus of this book. Other kinds of ecosystems, however, also characterize the tropics (Beard 1944; Holdridge 1967; Walter 1971). Climate is generally warm and wet but is by no means uniform, and both seasonality and topography have marked effects on the characteristics of various tropical ecosystems. In this chapter I will present an overview of the tropical climate, seasonality, and major ecosystem types occurring in the Neotropics.
Definition of the "Tropics"
Should you decide to move to Manaus, Brazil, or perhaps to Iquitos, Peru, both well within the Amazon Basin, you should expect at least 130 days of rain per year, and in some places up to 250 days. Temperature will be consistently warm, often hot (highs of about 31°C [88°F], nighttime low of about 22°C [72°F]), and relative humidity will never be less than 80% (Meggars 1988). Though it can rain on any given day, rainfall, in most places, will be seasonal. That, in a nutshell, is what it's like in the tropics. In the Amazon Basin, the very heart of the Neotropics, climate is permanently hot and humid, with the temperature averaging 27.9°C (82°F) during dry season and 25.8°C (78.5°F) in rainy season. In the tropics, daily temperature fluctuation exceeds average annual seasonal fluctuation (see below) and air humidity is quite high, about 88% in rainy season and 77% in dry season (Junk and Furch 1985).
Geographically, the tropics is an equatorial region, the area between the Tropic of Cancer (23° 27'N) and the Tropic of Capricorn (23° 27'S), an approximately 50-degree band of latitude that, at either extreme, is subtropical rather than tropical. The Tropic of Cancer passes through central Mexico and extreme southern Florida. The Tropic of Capricorn passes through northern Chile, central Paraguay, and southeastern Brazil, almost directly through the Brazilian city of Sao Paulo. The Neotropics thus include extreme southern North America, all of Central America, and much of South America. You can visit the Neotropics by traveling to southern Mexico, Guatemala, Belize, El Salvador, Honduras, Nicaragua, Costa Rica, Panama, Venezuela, Colombia, Guyana, Surinam, French Guiana, Ecuador, Peru, Brazil, and parts of Bolivia and Paraguay. In the Caribbean Sea, the Greater and Lesser Antilles are within the Neotropics.
The tropics are warm and generally wet because the sun's radiation falls most directly and most constantly upon the equator, thus warming Earth more in the tropics than at other latitudes. As one travels either north or south from the equator, Earth's axial tilt of 23° 27' results in part of the year being such that the sun's rays fall quite obliquely and for much shorter periods of time, thus the well-known cycles of day length associated with the changing seasons of temperate and polar regions. At the equator, heat builds up and thus the air rises, carrying the warmth. Water is evaporated so water vapor rises as well. The warm, moist air is cooled as it rises, condensing the water, which then falls as precipitation, accounting for the rainy aspect of tropical climates. The normal flow of warm, moisture-laden air is from equatorial to more northern and southern latitudes. As the air cools it not only loses its moisture to precipitation, but also becomes more dense and falls, creating a backward flow toward the equator. At the equator, two major air masses, one from the north and one from the south, along with major ocean currents, form the Intertropical Convergence (ITC), the major climatic heat engine on the planet.
In the Amazon Basin, precipitation ranges between 1,500 (59 in) and 3,000 mm (118 in) annually, averaging around 2,000 mm (79 in) in central Amazonia (Salati and Vose 1984). About half of the total precipitation is brought to the basin by eastern trade winds blowing in from the Atlantic Ocean, while the other half is the result of evapotranspiration from the vast forest that covers the basin (Salati and Vose 1984; Junk and Furch 1985). Up to 75% of the rain falling within a central Amazonian rainforest may come directly from evapotranspiration (Junk and Furch 1985), an obviously tight recycling of water, and a recycling system that clearly demonstrates the importance of intact forest to the cycling of water. This vast precipitation and water-recycling system is essentially in maintaining equilibrium, though large-scale deforestation could significantly upset the balance (Salati and Vose 1984; see also chapter 14).
Tropical areas fall within the trade-wind belts (so named because winds were favorable for sailing ships trading their goods) except near the equator, an area known as the intertropical convergence or doldrums, where winds are usually light, often becalming sailing ships. From the equator to 30°N, the eastern trade winds blow steadily from the northeast, a direction determined because of the constant rotation of Earth from west to east. South of the equator to 30°S, the eastern trades blow from the southeast, again due to the rotational motion of the planet. As Earth, tilted at about 23.5° on its axis, moves in its orbit around the sun, its direct angle to the sun's radiation varies with latitude, causing seasonal change, manifested in the tropics by changing heat patterns of air masses around the intertropical convergence that result in seasonal rainfall. In the Western Hemisphere, from July throughout October, severe wind- and rainstorms called hurricanes can occur in parts of the Neotropics. Similar kinds of storms are referred to as monsoons in the Old World tropics.
Seasonal variations in day length are not nearly as dramatic in the tropics as in the temperate zone. At the equator, a day lasts exactly twelve hours throughout the year. North of the equator, days become a little longer in the northern summer and shorter in winter, but this only means that summer sunset is at 6:15 or 6:20 rather than 6:00 P.M. Temperature fluctuates relatively little in the tropics. Typically, daytime temperature is somewhere around 29°C (85°F), though in many areas it may be 32–37°C (in the 90s), with surprisingly little seasonal fluctuation. In general, there is no more than a 5°C difference between the mean temperatures of the warmest and coldest months. For example, at La Selva Biological Station in Costa Rica, August is the month with the highest mean temperature, 27.1°C (80.5°F), while January has the lowest mean temperature, 24.7°C (77°F) (Sanford et al. 1994). Relative humidity, as noted above, is generally high in the tropics, especially in lowland rainforests where humidities ranging from 90% to 95% at ground level are common. Humidity is less in the rainforest canopy, usually no higher than 70%.
El Niño—Southern OsaUatiion
South American and, indeed, global climates are periodically and sometimes dramatically affected by a still poorly understood climatic event called El Niño ('The Child"), or the Southern Oscillation. Originally named because it tends to begin around Christmas, El Niño causes sufficient short-term climate change to produce major disruptions to ecosystems, especially marine ecosystems (Glynn 1988). An El Niño event involves the unpredictable warming of eastern Pacific Ocean surface waters around the equator.
El Niño occurs periodically, approximately every two to seven years, when a high-pressure weather system that is normally stable over the eastern Pacific Ocean breaks down, destroying the pattern of the westward-blowing trade winds. Trade winds thus weaken severely, sometimes reversing from their normal westward direction. Warm water from the western Pacific flows eastward, enhancing the Equatorial Counter Current and causing an influx of abnormally warm water to the western coast of South America. Instead of winds pushing water from the west coast of South America, creating an upwelling of deeper, colder, nutrient-rich water (page 15), the trade winds quit. When that happens, warm waters flow along the normally cold South American coast, global heat patterns vary, and weather systems change, causing floods in some regions and droughts where there should be rainfall, effects that can be anywhere from mildly stressful to disastrous to plant and animal populations. For example, some parts of South America experience abnormally heavy downpours while other areas, particularly in Central America, become drought-stricken. Droughts can also occur in places such as Australia, Indonesia, and southern Africa.
There have been eight major El Niño events since 1945, and at least twenty during this century. In 1982–83 an El Niño considered up to then to be the most powerful of this century caused an estimated $8.65 billion worth of damage worldwide. An even more severe El Niño occurred in 1986–87. A comparable El Niño occurred in the winter of 1994–95. The California coast was inundated by rain, resulting in extensive flooding and mudslides from Los Angeles to the Russian River area north of San Francisco, while New England experienced far less winter precipitation than usual. Satellite data indicated that the northern Pacific Ocean was nearly eight inches higher than normal, due to the influx of warm surface waters. The causal factors responsible for the periodicity of El Niños are thus far unknown (Canby 1984; Graham and White 1988), but it is clear that the Intertropical Convergence, a complex system of oceanic and air currents, migrates to a lower latitude, raising sea surface temperatures and destroying the normal upwelling pattern along the west coast of South America. The cessation of an El Niño occurs then the ITC returns northward to its normal position (hence the alternate term for El Niño, the Southern Oscillation). Tropical ecosystems, already sensitive to seasonal variation (see next section), can be anywhere from moderately to severely affected by changes caused by El Niño (Glynn 1988; also see Foster 1982b, below). Indeed, El Niño of 1986–87 has been suggested to have contributed strongly to the apparent extinction of two amphibian species, the golden toad (Bufo periglenes) and the harlequin frog (Atelopus vanus), from Monteverde Cloud Forest Preserve in Costa Rica (Pounds and Crump 1994).
The Importance of Seasonality
Rainfall in tropical latitudes varies seasonally, often dramatically. Because of warm air throughout the year, precipitation is in the form of rain (except atop high mountains such as the Andes, where snow occurs, even at the equator), but the amount of rain varies considerably from month to month and from one location to another. Overall, precipitation is highest in the central Amazon Basin and the eastern Andean slopes and lowlands, and less to the north or to the south, varying from about 6,000 mm (eastern Andean slopes) to 1,500 mm (236–59 in) (extreme north or south) (Junk and Furch 1985). Even within the central Amazon Basin, seasonal rainfall is variable from place to place. For example, Iquitos, Peru, along the Amazon River, receives an average of 2,623 mm (103 in) of rainfall annually, while Manaus, Brazil, also on the Amazon River, receives an average of 1,771 mm (70 in) and experiences a strong dry season. As a more extreme example, Andagoya, in western Colombia, receives 7,089 mm annually (approximately 280 in). (The area that receives the most rainfall on Earth is not in the Neotropics but in the United States! It is Mount Waialeale, Hawaii, averaging 11,981.18 mm [471.7 in] annually.)
Excerpted from A Neotropical Companion by John Kricher, William E. Davis Jr.. Copyright © 1997 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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