Each living thing is unique, different in details from all others. However, individuals may be grouped into kinds or species, which are defined as assemblies of organisms that in nature are capable of interbreeding freely but are unable to breed with any other species. The exact number of species of plants and animals is unknown, for more are being discovered all the time. To date, over a million of species of animals and a quarter of a million species of plants have been described and named. Estimates vary as to how many remain to be recognized. A few researchers suggest up to 30 million, but most biologists believe that around eight million are so far unknown. The figure may be raised by recent findings that the deep sea, hitherto thought to be sparsely in habited, could contain up to two million new species.
This huge diversity of living things has come about because each within its environment occupies a niche that is different from all of the others. The differences may often be slight but no two creatures or plants can make identical demands on their habitat. Recent work has shown that in a species rich environment the use of the Sun’s energy is more efficient than in a species poor habitat. Within an environment, species are largely interdependent. If a species should be lost rapidly, there is likely to be a knock on effect as other plants and animals feel the consequences. The pattern of life in the whole are is changed and probably impoverished.
