What is biodiversity?

The impressive diversity of life forms on the planet Earth and their distribution over the globe have always aroused the wonder and curiosity of many scientists and enthusiast naturalists. The term biodiversity, or biological diversity, indicates precisely all of these life forms. The history of biodiversity coincides with the history of life on Earth, life that originated in all probability about 3.7 billion years ago, certainly in the water, perhaps in deep hot springs (see Fig. 1). We must not forget that the earth's crust and atmosphere, as we know them now, have been moulded by the evolving organisms. The first living beings inhabited anoxic environments (originally there was virtually no oxygen in both water and the atmosphere). The evolution of organisms capable of performing photosynthesis allowed the accumulation of oxygen and the development of an atmosphere like the one we experience today.

Figure 1: A diagrammatic clock which summarizes the various eras of life on Earth (Des Marais, 2005).

From the very moment life appeared on our planet until now new organisms have been evolving thus producing an incredible branching and diversification of life forms as demonstrated by systems of taxonomic classification (Fig. 2). In his book “What is life? ”, the Nobel Prize for Physics Erwin Schrödinger (1944) had predicted that the key part of a living cell was to be an aperiodic crystal. His prophecy, that in some way was a source of inspiration for James Watson (at least according to what he states, Watson and Berry, 2003), was confirmed by the discovery of the structure of DNA and other nucleic acids a few years later as a result of the work of Crick and the same Watson, who were however inspired by the discoveries of the late Rosalind Franklin. Schrödinger pointed out that the genome must be on the one hand very stable (after all, one of the main characteristics of the organisms is that they can make copies of themselves), but on the other hand quite flexible, i.e., capable of development and evolution. In fact, the structure of the DNA possesses exactly these remarkable characteristics.

Figure 2: The branching and diversity of life as described by the classification into 5 kingdoms of Whittaker (1969). This system is based on three levels of organization: prokaryotes (kingdom of Monera), unicellular eukaryotic organisms (kingdom of protists) and multicellular eukaryotic organisms (kingdoms of plants, fungi and animals). The last three kingdoms are distinguished essentially by the differences in the acquisition of vital resources, as indicated symbolically in the lower right corner. Some phyla are difficult to classify: Rhodophyta (red algae) and Phaeophyta (brown algae) are traditionally classified as protists, Myxomycota and Acrasiomycota (mucilaginous fungi) and Labyrinthulomycota as protists, but Oomycota (water molds) as fungi, Porifera (sponges) and Mesozoa (worm-like parasites of marine invertebrates) as animals.

Without knowing anything about genetics and molecular biology, one of the giants of modern scientific thinking, Charles Darwin, had already understood the main mechanisms of this fantastic ramification (Darwin, 1859). As we will see later, the phenomenon of genetic mutation is the main driver of the diversification of life, but if mutation, which is basically random, were the sole cause of diversification, we would observe no organized and coherent structures in the terrestrial biosphere. Instead, the organization of life is set up by the process of natural selection. In fact, in the first place, mutations are very rare, because of the genome stability, and many mutations are also deleterious. Of those not detrimental to the survival of organisms only a few manage to pass through the filter of natural selection, which favours those mutants that have a demographic advantage. Selection is very strict, but the filter operates continuously on a myriad of organisms; therefore it is very effective in the long run and produces organisms adapted to their environment. In small populations, neutral mutations may also establish due to a random process known as genetic drift (this fact was pointed out in more recent timesby Kimura and Crow, 1964). Also, we must not forget that during the branching of life the evolution of the various organisms has been constrained by the presence of other organisms with which they have been interacting ecologically: therefore, evolution is in fact co-evolution and adaptation is indeed co-adaptation.

Figure 3: Hierarchies of biodiversity: genetic diversity, taxonomic diversity, ecosystem diversity.

But genetics and taxonomy are not sufficient to fully define biodiversity. Actually, we can distinguish hierarchies of biological diversity (Fig. 3). There exists diversity of genomes within a species, diversity of species or other taxonomic classes within an ecosystem and diversity of ecosystems within a landscape or biome. Traditionally, the diversity of species within an ecosystem (also called $\alpha$-diversity) is the concept most commonly used, but there is a growing consensus among biologists that diversity of the functions performed by species within an ecosystem and diversity of habitats in a landscape play a very important role and deserve much more attention than the one which has been paid to them until now (to this purpose, it is possible to introduce other dimensions of ecological diversity, such as the $\beta$ and $\gamma$-diversity (Whittaker, 1960)). If we consider a single ecosystem, we can, for example, pay attention to its trophic structure distinguishing primary producers (plants), herbivores, carnivores and decomposers. Very often, it is reasonable to study the diversity of species within each trophic level or of each functional group of organisms that play a similar role within the ecosystem. However, the overall design of the ecosystem diversity can be captured only by coupling these studies with the analysis of the food web structure and of the flows of energy and matter within that network.