Many different species

The exact number of species living on Earth is unknown. Some studies suggest there must be between 10 and 50 million living species and of these, only 1.5 million have so far been classed. Overall these figures are named biodiversity (diversity of life). The word biodiversity is used to describe the number and variety of organisms living on Earth. It is defined in terms of genetic inheritance (concerning the variation of genes between species), species (concerning the variety of species in a region) and ecosystems, which are the result of over 3 billion years’ evolution.
Even if the tropical forest is one of the places that has most been degraded by man in the 20th century, it still has one of the highest biodiversities. It has been estimated that over 50% of the biodiversity of the biosphere is located in this ecosystem, which covers just 7% of the Earth: 43 different species of ants have been observed on one tree of a Peruvian wildlife sanctuary, as many as all over Great Britain. 700 species of trees have been counted in 10 1-hectare forest areas, randomly selected in the Borneo, approximately the same number as all over North America. In an area as large as just one half of San Francisco, there are 545 species of birds, 100 species of dragonflies, 729 species of butterflies. This remarkable variety of animal species, such as birds, may be partly accounted for by the fact that each subspecies tends to live in a specific layer of the forest.
Over a few thousands of years, there have been 5 different mass extinctions, the most famous being certainly that of dinosaurs that took place approximately 65 million years ago. Today, we are seeing species disappear at a slow but constant rate because of man, whose pace is remarkably faster than the natural one.

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