The loss of biodiversity

Figure 6: A simplified scheme of the great extinctions that occurred over geological time (revised after Primack, 2000). The width of the gray area indicates the relative abundance of living taxa, the name of the period or geological era in which the mass extinction occurred is indicated by horizontal arrows, to the right of which are listed the major taxonomic groups that suffered mass extinctions. The numerical labels shown on the vertical axis on the left indicate the distance in time of the extinction occurrence from the present era (in millions of years).

The problem of present and future biodiversity loss is often confused by some people (even by some scientists) with the “natural” decreases of biodiversity that occurred on evolutionary time scales. Our planet has always experienced losses of biodiversity from the very beginning of life on earth. To be precise, on a geological scale there have been five periods of mass extinction, termed according to the corresponding geological era, followed by radiation, namely the appearance of new species. Fig. 6 shows the change of the number of taxonomic classes in the last 500 million years (revised after Primack, 2000). It is apparent that the number of taxonomic classes has been increasing over time, even if the increase has not been regular. Currently, we are probably at the beginning of a sixth extinction, that of the Holocene. The causes of past extinctions are not very well-known. Instead, the cause of the forthcoming sixth extinction is quite well-known because it is the action of man. That's why Paul Crutzen (2002) has termed the present era the Anthropocene.

Figure 7: Rates of species extinctions that occurred since 1600, reworked after Smith et al. (1993).

When skeptics argue that species have always become extinct and that we should not worry about this phenomenon, they make a very common mistake: they do not pay attention to the time and space scales. In fact, while the geological average lifetime of a species was of the order of a few million years, the current average residual time of many species is estimated to be of the order of a few tens of thousands of years. Fig. 7 shows the changes over time of the extinction rates of mammals and birds, as estimated by Smith et al. (1993). Note that these two taxa are indeed at risk, because their the average lifetime is reduced to a few hundred years (Lawton and May, 1995). The fact that the extinction rates have been very low in very recent years (see again Fig. 7) is not due, as one might erroneously think, to the positive impact of conservation policies. Rather, it is the criteria that are now adopted to declare a species as extinct that have become stricter. Also, there are now technologies that allow the recovery and monitoring of even a very small number of individuals of a declining species. This explains a recent decrease of the number of species that are actually declared extinct.

As results from Tab. 1, islands are particularly vulnerable environments. Remember, in this regard, the relationship between number of species and area illustrated in the theory of island biogeography (MacArthur and Wilson, 1967). For example, of the many species of birds currently hosted in Hawaii there are very few that are indigenous because most of them have become extinct or are on the brink of extinction. Of the existing 272 species found in Hawaii, 54 are alien, 50 species are indigenous residents, 155 are migratory species that do not reproduce in Hawaii and 13 are migratory species that breed in the archipelago. It is estimated that before the arrival of man (about 2,000 years ago) there were at least 128 species of native nesting birds (Union, 1983). Usually, when we talk about extinct or endangered species we think of charismatic exotic species of unquestioned charm such as the black rhino, the African elephant, the giant panda, the dodo. However, even in Italy there exist many species that are under threat or have been in the past. Among these it is worth remembering the best-known cases:the wolf, the lynx, the brown bear, the ibex, the Sardinian deer, the monk seal, the otter, the golden eagle, the bearded and the griffon vultures, the capercaillie, the partridge. Other examples of some lesser known species that are definitively extinct in Italy are shown in Fig. 8.

Table 1: Total number of extinctions recorded from 1600 to present time (fifth column) divided by taxon (first column) and habitat (from the second to the fourth column). The last column shows the percentage of the taxon that have become extinct with respect to the estimated total, while the sixth column shows the percentage of extinctions that have occurred on islands. Data from Primack (2000).

	NUMBER OF EXTINCTIONS IN
Taxa	islands	land	ocean	total	% islands	% total
Mammals	51	30	4	85	60	2.1
Birds	92	21	0	113	81	1.3
Reptiles	20	1	0	21	95	0.3
Amphibians	0	2	0	0	2	0:05
Fish	1	22	0	21	4	0.1
Molluscans	151	40	0	191	79
Invertebrates	48	49	1	98	49	00:01
Flowering plants	139	245	0	384	36	0.2

Figure 8: Some of the animal and plant species that are extinct in Italy.

It is important to remark that there are different levels of endangerment or extinction threat, levels that are now internationally recognized by the IUCN (International Union for Conservation of Nature and Natural Resources, http://www.iucnredlist.org/). This institution regularly produces red lists, i.e. lists of endangered animal and plant species. A species is considered extinct when all over the world there no longer exists (as far as humans know) even a single living individual of that species. For example (see Fig. 9) Bachman's warbler Vermivora bachmanii is considered extinct. If a species is still represented by a few individuals living in a zoo or in a botanical garden, or more in general in captivity, then the species is termed extinct in the wild. For example (Fig. 9) the Franklin tree (Franklinia alatamaha) belongs to this category because it grows only in botanical gardens. In both cases described above, the extinction refers to the whole biosphere (these species are thus globally extinct). In some cases, however, the extinction did not occur throughout the world, but only in some regions. In this case the species are termed locally extinct . For example (see again Fig. 9) the American burying beetle (Nicrophorus americanus) was widespread in the eastern and western United States, while today only three populations of this species persist, two in central US and one in Long Island, New York. Even the white-headed duck (Oxyura leucocephala, see Fig. 8), a small diving duck, is in these same conditions: extinct in all of Eastern Europe, in Italy and Corsica, it still existed in the 1970's in Spain, where a small population was still thriving, was fortunately placed under protection and is now recovering. In some cases, unfortunately, these small local populations are almost certainly doomed to extinction. For example, in Somalia there exist only two trees of Moringa pygmaea, real “Living Dead” (see again Fig. 9): when they die the species will become extinct. In the international literature species of this sort are termed dead-living species. Finally, some ecologists call ecologically extinct the species that are able to persist even with so small populations that their ecological role is practically negligible. An example of ecologically extinct species is the Asian tiger, whose effect on prey populations is now basically immaterial.

Figure 9: Examples of different extinction levels. (A) A print depicting two individuals of Bachman's warbler (Vermivora bachmanii a globally extinct species because of deforestation) on a tree of a species which is extinct in the wild (Franklinia alatamaha, still existing in botanical gardens). (B) One of the two specimens found in Somalia (circled in green) in 2001 of the species Moringa pygmaea, which is considered a “living dead”. (C) Once widespread throughout the eastern and central United States, the American burying beetle (Nicrophorus americanus) is confined to a very small territory, being locally extinct in many areas it occupied.