Tropical ecosystems

Tropical ecosystems are biological communities of interdependent flora and fauna situated between the Tropic of Cancer and the Tropic of Capricorn. While many types of ecosystems are found in this broad, diverse geographic area, tropical rainforests have a particularly high profile due to their biodiversity and importance to Earth’s planetary systems.

Multiple geographic and climate-based factors combine to broadly define conditions in tropical ecosystems. In tropical areas, relative amounts of daylight remain fairly constant throughout the year, and temperatures are consistently warm. The four distinct seasons seen at more northerly and southerly latitudes give way to two general weather periods known as the dry season and the rainy season (wet season). These factors combine to create rich ecological and biological diversity in tropical ecosystems, which are collectively home to a very large proportion of Earth’s known plant and animal species.

Background

The tropics are delineated by two latitude lines: the Tropic of Cancer (Northern Tropic) and the Tropic of Capricorn (Southern Tropic), with the Equator defining its center. Five of Earth’s six inhabited continents contain territory that lies partially or fully within the tropics, with Europe being the lone exception. By 2023, approximately 40 percent of the global population lived within the tropics, and the tropics accounted for 36 percent of Earth’s total landmass.

Weather conditions in the tropics remain warm and sunny throughout the year, with relatively little seasonal temperature variation. In most parts of the tropics, temperatures average about 77–82 degrees Fahrenheit (25–28 degrees Celsius) and rarely if ever fall below 64 degrees Fahrenheit (18 degrees Celsius). The tropics receive more sun exposure than other regions of Earth, and their central latitudes lead to very little variation in the relative number of daylight and twilight hours experienced each day throughout the year. The axial tilt that causes four distinct seasons in more northerly and southerly latitudes has a negligible effect in the tropics. Because of this, tropical destinations experience different seasons. Rather than spring, summer, autumn, and winter, tropical latitudes have only a dry season and a rainy or wet season. In the Northern Hemisphere, the dry season extends from roughly October to March and the rainy season extends from April to September. The dry and rainy seasons follow the opposite schedule in the Southern Hemisphere.

These geographic and climate factors combine to provide the source of the profuse biodiversity found in tropical ecosystems. The full set of flora and fauna native to the tropics has never been fully cataloged by scientists but estimates suggest that tropical ecosystems are home to nearly 80 percent of all plant and animal species on Earth. Approximately 91 percent of terrestrial birds are native to the tropics, with the figure rising if migratory bird species that spend part of the year in tropical regions are included. Biologists note that many plant and animal species that are native to the tropics are found nowhere else on Earth, and experts estimate that at least 150,000 tropical plant and animal species have yet to be discovered.

However, tropical ecosystems around the world are under pressure from resource exploitation. The Earth’s tropical regions house a wealth of natural resources, including many fruits and other foods, gums and resins, oils, fibers, plants, woods, and raw ingredients used in many medicines. They also support large marine fisheries. Scientists in many tropical regions have documented the growing overuse of these resources, which is mainly driven by the rapid population growth rates seen in many tropical countries and the accompanying need for ongoing economic development. Projections indicate that by 2050, more than half the world’s population will live in the tropics, a trend that suggests these pressures and stresses will intensify in the coming decades.

Deforestation is also a major problem in tropical rainforest ecosystems, particularly in Brazil and Indonesia. In Brazil, deforestation is heavily concentrated in the Amazon rainforest, which has been the focus of ongoing clearing efforts since the 1960s. Cleared areas are generally converted into farmlands, mainly for raising products including beef and soy. Indonesian tropical deforestation is mainly driven by palm oil production along with logging and mining. Tropical rainforests play a critical role in regulating Earth’s climate, as their dense tree cover filters enormous quantities of carbon out of the atmosphere. Tropical ecosystems also combine to represent an estimated 42 percent of Earth’s biomass carbon reserves. As such, slash-and-burn deforestation is considered doubly harmful to the environment, as it destroys carbon-filtering trees while also releasing large amounts of stored carbon into the atmosphere.

Overview

The Earth’s tropical regions have many types of ecosystems, as defined by the natural habitats in which they operate. These habitats include tropical rainforests, cloud forests, wetlands, dry deciduous forests, spiny forests, and deserts. Some areas of the tropics also have alpine habitats and tall mountains.

Tropical Rainforests

Tropical rainforests arise in tropical regions where there is no distinction between dry and rainy seasons due to close equatorial proximity. As a result, these luxuriant forests receive large amounts of year-round rainfall as well as extensive sunlight, which drives their rich, thick growth. They are mainly comprised of tall, dense collections of coniferous trees and contain four distinct layers. From bottom to top, these layers include a forest floor, an understory, a canopy, and an emergent layer. Each layer has its own unique set of defining features, but all four layers combine to create unified and interdependent ecosystems.

Experts recognize that tropical rainforests are the world’s most biodiverse ecosystems. For example, the Amazon rainforest is home to 2.5 million different insects and an estimated 40,000 plant species, 3,000 fish species, 1,300 bird species, and more than 400 mammal species.

Cloud Forests

Cloud forests, also known as montane rainforests, are vegetation-rich belts of land found in tropical highland regions. They form from high rainfall and a constant process of atmospheric condensation, which causes cooled and humid air to collect along the mountainside and contribute to the growth of thick tree cover. Clouds and mist typically hang over montane rainforests at their canopy level, which is why they are sometimes called “cloud forest” derives.

To form, cloud forests require a precise set of geographic and climate conditions. Thus, they are uncommon and exclusive to mountainous regions within the tropics.

Wetlands

The wetland ecosystems found in tropical regions mainly consist of floodplains, swamps, and forested peatlands. Floodplains are relatively flat areas adjacent to nearby rivers or streams that contain dense concentrations of river sediment. They are prone to flooding during periods of heavy or sustained precipitation. Tropical swamps are wooded areas with shallow waters that mainly form on the margins of lakes and rivers and in low-lying areas. Forested peatlands develop in areas with dense vegetation cover and waterlogged conditions that prevent plants from achieving a complete state of decomposition.

Dry Deciduous Forests

Dry deciduous forests, also known as tropical dry forests and monsoon forests, form in regions of the tropics that experience long, low-precipitation dry seasons and extended wet seasons that bring heavy rainfall. These sensitive and relatively uncommon ecosystems display higher levels of seasonal variation than most other tropical landforms, with deciduous trees losing their leaves when the dry season begins and growing new leaves at the outset of the rainy season.

Rich in biodiversity, dry deciduous forests often border on rainforests, and many animal species migrate freely between the two landforms. Tropical dry forests have been heavily impacted by deforestation, which is mainly carried out using slash-and-burn techniques.

Spiny Forests

Spiny forests are uniquely found on the island of Madagascar, off the southeast coast of mainland Africa. Deriving their name from the large quantities of spiked vegetation they contain, spiny forests rank among the most singular ecoregions on Earth. Almost every species living in spiny forests is found only in Madagascar, and spiny forests themselves display a great deal of variation among the areas and regions they contain due to wide variations in soil characteristics and rainfall patterns.

Threatened by climate change and resource exploitation, Madagascar’s spiny forests have become destabilized. They are currently the focus of an urgent international preservation effort.

Deserts

Deserts are rare in tropical regions but can form on the leeward sides of mountains and in interior regions subject to year-round high atmospheric pressure. Most of the world’s tropical deserts are found in North America, including Baja California and the Mexican interior.

Hot temperatures and very low humidity levels make tropical deserts generally inhospitable environments. Relative to other tropical geographies, deserts host closed ecosystems with relatively little biodiversity.

Alpine Habitats

Climate regions with alpine characteristics exist in some highly localized regions of the tropics. These regions, which are concentrated in the Andes Mountains of South America, the highlands of Africa's Afroalpine zone, and New Guinea, host unique ecosystems noted for their relatively low levels of genetic diversity. These insular biological communities host hardy plant and animal species capable of withstanding the cold temperatures that occur at high elevations.

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