Biological diversity or biodiversity, in ecology, is the variety of living organisms in their different forms, and in their respective ecosystems. This term indicates both all species present in the ecosystem of the globe, and the differences that characterize a single individual within the same species, and the presence, within the biological communities that occupy a particular habitat, of various species that adapt to each other, forming niches and associations.

It includes the entire biological variability of genes, species, ecological niches and ecosystems with genetic resources considered the determining component of biodiversity within a single species. The total number of species described by science is about 1.74 million, while the value of those estimated ranges from 3.63 to over 111 million; however, these estimates are incomplete as new species are discovered and added to the overall total. Species extinction is the main threat to biodiversity.

There are many initiatives, on a global scale, aimed at mitigating biodiversity loss and protecting and conserving living species, especially if native or endangered. In recent years, the new technologies of genomic sequencing are also used to try to understand and limit the loss of biodiversity. The use of these technologies in biodiversity conservation has made possible the development of a new subject: conservation genomics, where this knowledge and techniques are condensed and are combined to help protect endangered species.

The term biodiversity comes from the Greek bios meaning life, and the Latin diversitas meaning difference or diversity. As an alternative translation we could propose biovariety or variety of life on the planet. The term biodiversity is now well established and is commonly used in different scientific and cultural fields.

Levels of biodiversity

We consider three distinct levels of biodiversity:

  • genetic diversity, the sum total of living things that inhabit the planet;
  • species diversity, which indicates the abundance and taxonomic diversity of species present on the earth;
  • ecosystem diversity, which indicates the set of all natural environments present on our planet.

Biodiversity is not a fixed and stable value, for example in a given environment the biodiversity of the species present may increase or decrease over time due to various factors that may be natural and/or anthropogenic.

Historical notes

Biological diversity, or biodiversity, is the result of the evolutionary process that has generated through natural selection, over the millennia, the great variety of living species of animals and plants; the events that have determined the current biodiversity are difficult to distinguish from each other, because often more than one cause has contributed to the definition of a current species; however we must remember that the origin of new species lies in the evolution of any difference that prevents the production of fertile hybrids by two populations; these differences are caused, in turn, by the emergence of geographical barriers, when for example a river or a mountain range cause the physical separation of two populations, or physiological, when there is a functional incompatibility in mating, or behavioral, when for example the rituals of courtship of a population are not recognized as such by the other, or to polyploidy, ie the variation of the number of chromosomes in the germ cell line, the cell line intended for the production of gametes, which makes a cross not fertile.

Of fundamental importance for the evolutionary success of a species is, in addition to the ability to adapt to the environment well defined by Darwin’s evolutionary theory, also the number of individuals that compose it: the greater the number of individuals of the same species within a certain habitat, the greater the possibility of crossing and consequently the intraspecific variability will be greater; small communities instead tend to be constituted by blood relatives who cross each other, and consequently the genetic variability of single individuals is limited.

The development of individual species is allowed by natural selection, while the number of individuals of the same species plays a key role in making the community more or less stable in the face of environmental stresses and disasters. Moreover, in any biological environment, natural selection determines the development of differentiated structures for each individual species, so that each can coexist with the other without competing, as each will exploit different potentials offered by the environment.

Therefore analyzing any natural environment, it is possible to verify that, if there are no external destabilizing factors, the various species that live there reach a state of equilibrium, called climax, characterized by the presence of a small number of dominant species and a greater number of species represented by a small group of individuals. In this condition, the equilibrium reached is due to interactions so complex as to admit neither the addition nor the subtraction of any species, nor any environmental variation.

In some environments, in fact, the presence of continuous variations originates communities that never reach the equilibrium in which each species can become dominant over another. In the last six hundred million years this extraordinary event of differentiation has been interrupted five times by enormous natural catastrophes (meteorite falls, climatic changes, huge volcanic explosions), each of which has determined an impoverishment of the diversity of living species.

Five are, in fact, the great mass extinctions occurred during the last half billion years: in the Ordovician, 440 million years ago; in the Devonian, 365 million years ago; in the Permian, 245 million years ago; in the Triassic, 210 million years ago; finally in the Cretaceous, 66 million years ago. Minor events also occurred between the five catastrophes, but they still played a role in evolutionary selection. Each time, however, life has been able to re-establish, albeit in new forms, the original levels of biodiversity; this could happen through long periods of time, on a geological scale.

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