dcsimg

published on 15 March 2008 in life

Biodiversity

Defining biodiversity
Much is said about biodiversity and of the need to protect it. But what is biodiversity? And why is it so important? Before answering these questions, it is important to explain that, in the course of hundreds of millions of years, life has spread all over the globe, conquering environments that range from the depths of the oceans to the tops of the mountains. In order to survive in such different places and for such long periods of time, living creatures have had to adapt, and in the course of this process they have adopted infinite forms, dimensions and other properties. So if we give a quick answer to the questions above, it would be helpful to define biodiversity as the wealth of life. A wealth that has multiplied and changed in time and space, and that can be seen today in the variety of organisms that populate the Earth, including Man.

Time and space
To understand biodiversity, it is important to strive to find order where there is much or little disorder. For example, the small differences that exist between the European and the American wolf – animals that are similar but not totally identical – have been brought about by geographical conditions, in other words, by living in places where they have had to adapt to different environmental conditions. In fact, they have a very ‘recent’ (on the geological time scale) common ancestor, that is, one that made its appearance a few million years ago, and therefore are still rather similar. On the contrary, the big gap that we can observe between a tree and a mouse has a much more ancient justification, because their common ancestor goes back to hundreds of millions of years ago. In actual fact, this extraordinary gap is the result of a series of differences that have intensified and built up in a much longer lapse of time. Having said that, in reality things do not always go in such a simple and clear way. In fact, often the two trends of biodiversity, in time and space, are undistinguishable.  It is the experience of biologists that will tell us whether it was the time factor rather than the space factor to have been more important in moulding the diversity of the living world.

Biodiversity and habitat
To speak about the spatial dimension of biodiversity means to speak about the biological variety that depends on different living habitats. Initially, in fact, it is the environment that determines which are the biological characteristics that the species has to be gifted with. With the word environment we refer to any space in which an organism has found it convenient to settle, therefore even a microscopic space: we must not forget that the majority of living organisms has dimensions that are imperceptible to the human eye and that their dimensions are measured in thousandths or hundredths of a millimetre. Hence, from a certain point of view, it is as if, to be able to stay in a particular environment, it were necessary to abide by certain rules that can vary greatly according to the ecological and geographical conditions. These rules are set by the type of physical environment, temperature, humidity, exposure to the sun, relations with other organisms and a variety of other factors. For example, animals that live in the sea always have a hydrodynamic shape, fin-shaped limbs and other biological systems that are efficacious for living, moving and feeding in an aquatic medium. The proof of this can be noted in the fact that sharks and dolphins, albeit very different in certain aspects (the former is a cartilaginous fish the latter is a mammal), however possess a similar appearance. These very reflections can be true in many cases, and as an exercise, it could be fun to carry out a research to suggest others, that are maybe even more interesting.

The big family
All the forms of life that have existed to this day have originated from a single type of organism that appeared over 3.5 billion years ago. Since this primordial creature does not exist any more, not only are we unable to get to know its characteristics accurately, but we cannot even draw sure conclusions on how it could have been generated. One can only suppose that, to a certain extent, it looked like a microbe that at a certain point, in the course of generations, started to transform until its characteristics were radically altered. The only fact that has been established is that from this creature all the living forms that have followed one another in the course of time on this planet have descended.  It follows that all organisms that constitute present-day biodiversity are related to one another, like people who belong to the same family, only that, in the case of biodiversity, this ‘family’ is infinitely big.
Similarities and relationships
The above observations have been a real brain teaser for scientists of the past. Their problem was the following one: how can one study the history of living forms to try and highlight a relationship between them? Or else, repeating one of the concepts mentioned above, how can we find order in life’s disorder? The solution emerged from researches carried out by various scholars, among which the great English biologist, Charles Darwin, who, in the second half of the 19th century wrote what is still considered today as the most important book on biodiversity: The Origin of the Species. His observations on the world’s natural environments lead him to elaborate the so-called Theory of Evolution. In particular, he discovered that to understand the history of the relationships between different types of organisms, in other words, the history of the progeny  of the species, one must concentrate on studying their similarities.
The tree of life
It must be made clear at once that, in biology, similarities are only a matter of degree of relationship. This implies that when between two species a high degree of similarity is noted, then there is a high probability that they are closely related: it is a bit like observing two brothers whose resemblance is always quite evident but never perfect (with the exception of monozygotic twins). On the other hand, when the degree of similarity is low, then the two species are probably not only different but also distantly related, as in the case of children who are cousins once or twice removed. As you can see, degrees of similarity and relationship move together: the greater the former, so much greater the latter. Following this reasoning, since Darwin,  biologists have begun to reconstruct biological evolution drawing the so-called ‘tree of life’, within whose roots we find  the oldest species in history, while in its tallest branches are the present-day species.

Measuring biodiversity
Biodiversity in a determined geographical area can be measured taking into account the number of species that live within it: from the microscopic ones, at times unicellular like species of bacteria to those of bigger dimensions, formed by billions of cells, such as many species of mammals and plants. Therefore the different species are, in a certain sense, the units of measurement of biodiversity. One must be careful however not to make improper use of this concept. The number of species in fact is not sufficient to give a reliable measurement of biodiversity. To be able to evaluate the latter and express it correctly it is also essential to know how many individuals make up each species. If for example we want to express the biodiversity of two small city woods and we only say that in both the biodiversity is equal to ten species, the statement would be incomplete. The problem is that the number of individuals belonging to each of the ten species can vary greatly. In fact, if nine out of the ten species are made up of only five individuals each, while the tenth has fifty individuals, we can conclude that the biodiversity value is low. If, on the other hand, each of the ten species is made up of more than fifty individuals, then the biodiversity value is high. In the case taken into consideration, a greater biodiversity value will be found in the wood that not only has a higher number of species but also a greater number of individuals in each species. To make estimations easier, and taking the above-mentioned principles into account, biologists have compiled biodiversity indices that enable us to evaluate the biological diversity of the habitats taken into consideration. Thanks to the biodiversity indices, eight geographical areas of the world have been identified, mainly concentrated in the tropical and equatorial belts, that are called hotspots: these are the ‘spots’ where the Earth’s biodiversity is highest.

Biodiversity in numbers
A delicate topic for biodiversity scholars regards the total number of species living at present. Today just under 1.8 millions are known, but some statistical estimates suggest that the actual number is comprised between 10 millions and 100 millions. However you see it, it is clear that there are more species we are unaware of respect to those that we know. In addition to this, it must be said that the majority of species that have existed in the course of the life of our planet became extinct in past; those present today represent an infinitely small fraction of those that lived in remote times. If we could represent the total number of known and living species on a pie chart, an enormous slice would be taken up by insects, that include nearly a million species (more than a half of the total known species). On the contrary, the slice that represents mammals, that includes Man, would be very thin, an indication that numerically mammals give a very modest contribution to biodiversity – only 5,000 species. From all this we can infer that biologists are only at the beginning of the count and of the description of the living species.  This causes some difficulties. For example, since all the species present on Earth have not been identified, we cannot know how many of those whose existence we ignore are about to become extinct or have died out in the last years due to Man or natural causes.

More and more
Whatever is the actual number of species existing at present, it is very important to know that they are essential to our survival. If their number decreases, in fact, even the possibility of working the land to produce food will decrease; the availability of raw materials from which clothes, medicines, paper and other products used daily are obtained will diminish; furthermore, the quality of the air, of water and of the land will be undermined, and these conditions must be good, as they are essential for our own health. To get to know biodiversity, however, is not important only for our survival but it is also a way of understanding how fascinating Nature is and of discovering the origins of our existence. It is for all these reasons put together that biodiversity is important and must be protected.

Written by Carlo Modonesi

With the sponsorship of the Italian Ministry of Education, Universities and Research
 
Eni S.p.A. - P.IVA 00905811006