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"Tropical" is sometimes used in a general sense for a tropical climate to mean warm to hot and moist year-round, often with the sense of lush vegetation.
The seasons in the tropics are dominated by the movement of the tropical rain belt (or ITCZ the intertropical convergence zone) which moves from the northern to the southern tropics and back over the course of a year, resulting in a dry season and a wet season rather than the various temperatures and day lengths indicative of the spring, summer, autumn and winter pattern found in areas outside tropics.
However, the starting dates of the seasons are related to the tropics, despite the fact that these dates only apply in the temperate and polar regions with only the winter solstice date applying in the tropics because the summer solstice occurs when the Sun is at the zenith, which occurs at different dates for different latitudes. Spring begins when the Sun is directly over the Equator (vernal equinox). Summer begins when the Sun is directly over the Tropic of Cancer in the north or when the Sun is directly over the Tropic of Capricorn in the south (summer solstice). Autumn begins when the Sun is again directly over the Equator (autumnal equinox). Winter begins when the Sun is directly over the Tropic of Capricorn in the north or when the Sun is directly over the Tropic of Cancer in the south (winter solstice).
Regions within the tropics may well not have a tropical climate. There are alpine tundra and snow-capped peaks, including Mauna Kea, Mount Kilimanjaro, and the Andes as far south as the northernmost parts of Chile and Argentina. Under the Köppen climate classification, much of the area within the geographical tropics is classed not as "tropical" but as "dry" (arid or semi-arid) including the Sahara Desert and Australian Outback.
Tropical plants and animals are those species native to the tropics. Tropical ecosystems may consist of rainforests, dry deciduous forests, spiny forests, desert and other habitat types. There are often significant areas of biodiversity, and species endemism present, particularly in rainforests and dry deciduous forests. Some examples of important biodiversity and/or high endemism ecosystems are: El Yunque National Forest in Puerto Rico, Costa Rican and Nicaraguan rainforests, Brazilian and Venezuelan Amazon Rainforest territories, Madagascar dry deciduous forests, Waterberg Biosphere of South Africa and eastern Madagascar rainforests. Often the soils of tropical forests are low in nutrient content making them quite vulnerable to slash-and-burn techniques, which are sometimes an element of shifting cultivation agricultural systems.
In biogeography, the tropics are divided into paleotropics (Africa, Asia and Australia) and neotropics (Central and South America). Together, they are sometimes referred to as the pantropics. The neotropic region should not be confused with the ecozone of the same name; in the Old World, this is unambiguous as the paleotropics correspond to the Afrotropical, Indomalayan, and partly the Australasian and Oceanic ecozones.
About 40 percent of the world's human population lives within the tropical zone (by 2008 statistics), and by 2060, 60% of the human population will be in the tropics due to high birth rates and migration.