Describe the major characteristics of low-latitude biomes and human impacts in each.

Low-latitude biomes are found almost entirely between 30 degrees north and south latitude. Excluding mountainous regions, low-latitude biomes lack any significant cold period. There are three low-latitude biomes: tropical rainforest, tropical seasonal forest, and tropical savanna.

The tropical rainforest biome, found in the humid lowland tropics, has the highest primary productivity, the highest biomass, and the highest bio-diversity of any biome (Figure 8.4). It occupies some 13% of Earth’s land surface and accounts for about 40% of the world’s biodiversity. A great variety of organisms in many different groups, including insects, birds, mammals, and amphibians, live in this biome. Worldwide, for example, there are about 250 species of primates, such as humans, gorillas, lemurs, and monkeys. All primates are mammals, and all have nails instead of claws, flexible hands and feet, good eyesight, and high intelligence. With the exception of humans, all primates live in tropical or subtropical areas. Most of them live in the tropical rainforests.

In the tropical rainforest biome, water is available in every month, and competition for light is a major limiting factor. As a result, the tropical rainforest biome develops a complex vertical structure. Its vegetation consists of multiple layers of broad-leaved evergreen plants (Figure 8.5). The topmost layer, called the emergent layer, consists of trees that protrude above the canopy. The shaded forest floor may receive as little as 1% of the sunlight that the canopy and emergent layers do. As a result, the forest floor is dark and damp. Fungi (mushrooms and molds) are common, and they rapidly decompose and recycle fallen plant material and dead organisms.

The strong vertical structure of the rainforest biome is driven by competition among the plants to reach sunlight. Many tropical rainforest trees have buttress roots to support tall growth to reach the light (Figure 8.6A). Lianas, which are woody climbing vines, are well adapted to tropical rainforest habitat. They grow quickly up trees to reach light in the canopy (see Figure 8.4A). Lianas also provide routes of travel for arboreal (tree-dwelling) animals such as monkeys. Epiphytes are plants that grow on the surfaces of other plants for access to light but do not take nutrients from those plants (Figure 8.6B).

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The “crowded jungle” perception of the tropical rainforest is misleading. A walk through the shaded floor of a healthy tropical rainforest would be clear and unobstructed by plants in many places. Crowded plant growth occurs only where there are high light levels, such as in the canopy above, in light gaps created where large trees have fallen, or along stream banks.

Mutualism (see Section 7.2) between plants and animals is common in the tropical rainforest biome. At midlatitudes, many plants rely on the persistent westerlies (see Section 4.3) to disperse their pollen, seeds, and fruits. In contrast, equatorial tropical rainforests are dominated by the windless doldrums. Instead of the wind, bats, birds, fish, and mammals pollinate flowers and disperse seeds and fruit. Many tropical plants produce nutritious and brightly colored fruits and seeds to attract animal dispersers. In some tropical rainforests, up to 90% of plant species are pollinated by animals.

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Where there is volcanic activity, such as throughout Indonesia, tropical rainforest soils are young and nutrient-rich because they are continually replenished by volcanic ash. However, most tropical rainforest soils (called oxisols, see Section 9.1) are poor in nutrients. Soil weathering (chemical disintegration) in the warm, humid climate and leaching by heavy rainfall move nutrients deep into the soil out of the reach of plant roots. Decaying organic material is quickly taken up by the shallow root systems of the forest vegetation, rather than entering the soils.

Soils in the Amazon rainforest are particularly nutrient-poor due to leaching. People living in the Amazon long ago modified the soils to support farming. Those modified soils are still being discovered today, as Picture This above discusses.

Human Footprint

Natural resources are like large bank accounts: Spend all the money in one shopping spree, and it disappears. Carefully manage it, live off the interest, and it can last generations. The same is true of rainforests and other natural resources. Currently, undisturbed tropical rainforest is quickly being lost through deforestation. Figure 8.7 shows the typical sequence of events that cause deforestation in the Amazon rainforest. The important issue of habitat loss and fragmentation is further explored in the Geographic Perspectives at the end of this chapter.

Each geographic region has unique factors that drive rainforest loss. While hardwood logging, cattle ranching, and soybean and sugarcane cultivation are driving rainforest loss in the Amazon, Indonesia is losing its rainforests to the production of palm oil. Picture This explores the Indonesian palm oil industry as a force of deforestation.

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The tropical rainforest biome is home to approximately 40% of all species on Earth. So this biome is central to efforts to estimate the global extinction rate. The global extinction rate is in large part a reflection of the rate of deforestation in this biome. A recent assessment published in Science in 2013 states that there are anywhere from 5 million to 8 million species worldwide. About 1% to 5% of those species are lost each decade, mostly because of habitat loss, and mostly in tropical rainforests. Crunch the Numbers asks you to use these figures to estimate about how many species are being lost each year.

Consumers of many of the products that are driving rainforest loss—such as palm oil, soybeans, beef, and hardwood lumber—are becoming increasingly aware of the connection between these products and the forests from which they come. For example, a growing number of governments and companies require that imports of palm oil are sourced from growers that meet stringent sustainability requirements, including the use of farming methods that do not cause deforestation. Roughly 45 million metric tons of palm oil are produced each year, and about 13% (6 million metric tons) is certified as sustainable. This percentage, though small, is growing.

The tropical seasonal forest biome is less well known than the tropical rainforest. In the summer wet season, it is often mistaken for tropical rainforest, but it is distinguished from that biome by its winter dry season. The tropical seasonal forest biome is found in the warm lowland tropics, where it borders tropical rainforest (Figure 8.8). The tropical seasonal forest is sometimes considered an ecotone (a transition between two biomes) for the tropical rainforest and the tropical savanna. Many trees in this biome are broad-leaved and deciduous. A deciduous tree or shrub sheds its leaves, leaving bare branches. In the tropical seasonal forest biome, many trees shed their leaves in response to winter drought.

The most distinctive characteristic that separates the tropical seasonal forest from the tropical rainforest is a dry season in the winter that lasts weeks to months. Throughout South Asia, summer precipitation in the tropical seasonal forest comes from the summer monsoon. There are only three dominant layers in most tropical seasonal forests: canopy, shrubs, and forest floor. The canopy is lower and more open than that of the tropical rainforest, so more light reaches the forest floor. The diversity of insects, birds, mammals, reptiles, and amphibians is, in many cases, nearly as high as that of the tropical rainforest.

Human Footprint

As a result of the dry season, the tropical seasonal forest is extremely vulnerable to fire. It is easy to burn the forest during the dry season. Increasing pressure from growing human populations, coupled with poverty, force people into the forest for subsistence farming. The soils of the tropical seasonal dry forest are better suited for crops and grazing than the soils of tropical rainforests.

Tropical seasonal forest grades into tropical savanna, which is centered at about 25 degrees north and south latitude (Figure 8.9). Tropical savanna is characterized by wet summers and dry winters. Tropical savanna is a woodland biome, characterized by widely spaced trees with a continuous cover of grass. The winter dry period lasts much longer in tropical savanna than in tropical seasonal forest, as long as seven months in some locations. In the drier portions of tropical savanna, where the winter dry season lasts for six months or more, a type of tropical savanna called thorn woodland is found. In thorn woodland, the vegetation is dominated by shrubs and small trees that are tough and thorny. This growth form is a response to frequent fires, grazing by animals, and prolonged drought.

The tropical savanna has low biomass and low biodiversity. Three factors strongly influence its vegetation structure:

Human Footprint

One prevalent threat to the tropical savanna biome is overgrazing by livestock, particularly cattle, sheep, and goats. Increased burning by people to stimulate grass growth for livestock is also degrading savanna woodlands and threatening the habitat of native grazing ungulates. In addition, populations of these large animals have declined greatly because of hunting for trophies, elephant ivory, and rhinoceros horn (Figure 8.11). An international boycott of ivory has helped to protect elephants from illegal poaching. Many large game preserves and parks have been established throughout Africa to protect its large animals. These parks are also generating significant revenue from travelers called ecotourists, who seek to visit natural places (see Section 10.2).

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Increasingly, conflicts between African elephants and people have arisen in parts of Africa. In some cases, wildlife conservation measures have allowed elephant populations to increase. More often, however, as a result of human population growth, people have expanded their farms and villages into elephant habitat, escalating the conflicts. As a result, it is sometimes necessary to manage the elephant populations and control their numbers, as detailed in Figure 8.12.