Diversity of Life

Introductory article Article Contents

Alessandro Minelli, University of Padua, Padua, Italy

. Why is Life so Diverse?

Diversity of life (or biodiversity) is the variety of existing organisms, including their diversity at the genetic level and the full range of ecological processes in which they take part and of ecosystems to which they belong.

. History of Life . Estimates of Current Diversity

doi: 10.1038/npg.els.0004120

Why is Life so Diverse? Article 2 of the Convention on Biological Diversity (the socalled Rio Convention, 1992) defines biodiversity as ‘the variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems’. According to this definition, three main levels of biological diversity can be identified: genetic diversity, species diversity and ecosystem diversity. See also: Conservation biology in action: case studies Genetic diversity is the heritable variation within a single species, including the differences among individuals in a local population. In an evolutionary perspective, this is the ultimate source of all kinds of diversity in the biosphere. On the other hand, ecosystem diversity is possibly the level of biodiversity most obvious to the lay observer, because of the immediate visual impact of the differences among aquatic and terrestrial landscapes or vegetation types, such as a pond and a seashore, a conifer forest and an alpine meadow. However, its measurement suffers from major problems of standardization. Most approaches to biological diversity, therefore, focus on species (or taxonomic) diversity; this operational choice will be followed in this article. See also: Variation, within species: introduction A first-level explanation of the diversity of life on Earth is the diversity of Earth itself. There are two major aspects of geographical diversity of the physical environment that allow living beings to become numerous. One aspect is habitat heterogeneity at local, regional, continental and even at global scale. Organisms successfully thriving in a wide spectrum of different habitats are rare, the bulk of living species being instead confined, more or less strictly, to a narrow set of environmental conditions. The physical heterogeneity of the planet’s surface, however, does not explain why similar habitats in different continents, and even in different regions within the same continent, are inhabited by widely dissimilar species. This is explained, instead, by history. Physical or ecological barriers between similar habitat patches may interrupt gene flow to such an extent as to bring about allopatric speciation. Similar habitats in individual islands within an archipelago or on individual peaks within a rugged mountain range are commonly inhabited by related but different species of sedentary animals, such as land snails or wingless beetles.

All these species, whose geographic range may be restricted to a few square kilometres, arose because of the physical or ecological barriers that interrupted the genetic flow between populations. See also: Biogeographical regions; Environmental heterogeneity: temporal and spatial; Isolating mechanisms; Speciation: allopatric In oceanic archipelagos, this condition of geographical isolation may affect the whole biota. For instance, the native fauna and flora of oceanic islands such as the Hawaiian chain is to a very large extent endemic: 89% for angiosperms and 99% for insects. Moreover, within this single island chain a large number of species are confined to one single island or even to a single district within one island, due to the habitat fragmentation caused by local topography or by recent lava flows. It has been estimated that more than 10 000 animal and plant species now inhabiting the Hawaiian archipelago evolved there from a few hundred successful colonizers, most of them of North American origin. See also: Geographical variation The importance of geographical isolation in determining high levels of species diversity is also apparent in a comparison between marine and freshwater fishes. Of all fish species described to date (some 25 000), those living in the sea are less than twice as numerous as those living in inland waters, although the total volume of oceanic waters is about 10 000 times greater than the volume of inland waters. The relatively enormous diversity of freshwater fish species is explained by the fact that inland waters are fragmented into thousands of more or less completely isolated basins, a condition largely facilitating allopatric speciation. See also: Speciation: introduction In more general terms, it has been estimated that only 15% of all living species described to date inhabit the sea. It is unlikely that future investigations will significantly alter this ratio. Probable explanations of this unbalanced distribution of diversity include the higher heterogeneity of continental environments and their higher structural (architectural) complexity with respect to the conditions prevailing in the oceans. At a higher taxonomic level, however, animal life is more diverse in the sea when compared with that on land. All animal phyla are represented in the sea and several phyla (e.g. echinoderms, ctenophores, sipunculans, brachiopods) are exclusively marine. This may be due, in part, to the fact that life originated in the sea and

ENCYCLOPEDIA OF LIFE SCIENCES & 2005, John Wiley & Sons, Ltd. www.els.net

1

Diversity of Life

remained confined to this realm during much of its history; no less important, however, is the effect of the strict adaptations required for living in terrestrial (and, to a lesser degree, freshwater) environments, adaptations that cannot be met by animals with body designs similar to those of a sea urchin or a jellyfish. See also: Evolution of ecosystems: terrestrial; Origin of life A second major explanation of the diversity of life is found in the multiple adaptations developed by most living beings in relation to the other organisms with which they interact, be these competitors, prey, predators, hosts or symbionts. The relevance of these interactions for the evolution of biological diversity is particularly conspicuous when two species interact so closely that each of them represents a major selective agent in the evolution of the other, thus offering a case for coevolution. Interesting examples of coevolution are found in the relationships between flowering plants and their insect pollinators: in many instances, the two partners are so closely specialized that size and shape of the insect’s mouthparts, temporal flight schedules, etc. are strictly matched by the shape of the corolla, the location of the nectaries, the length and shape of the stamens and the timing of flower opening. Large plant families such as orchids (around 18 000 species) and legumes (around 16 500 species) and large genera such as Ficus (figs; around 800 species), owe much of their conspicuous species richness to their strict interactions with specialized pollinators. See also: Adaptation and natural selection: overview; Coevolution; Interspecific interaction Interspecific relationships are also crucial in explaining the astonishing diversity found in several groups of parasites. Most parasites attack a very restricted number of host species, sometimes just one. This explains, for example, the remarkable diversity found in Eimeria, a genus of sporozoan protists: more than 1000 species have been described to date and it has been estimated that in this genus there may exist some 35 000 species, each of them attacking a selected group (mostly a genus or even a single species) of vertebrate (rarely invertebrate) hosts. The same will possibly apply to other groups of parasites, e.g. to several families of nematodes. See also: Coevolution: host– parasite; Interspecific interaction A similar degree of host specificity is often found in small animals (many groups of insects and mites) and fungi (especially rusts, Puccinia and relatives) living on flowering plants. Adaptations to their hosts involve these parasites’ specializations to exploit only selected parts of the host plant, e.g. young leaves, mature leaves, stem, roots, seeds etc., so that many dozen parasite species may specialize for the same host. See also: Parasitism: the variety of parasites However, we must acknowledge that neither geographical isolation nor specific adaptations to other organisms can help explain these extraordinary instances of biological diversity that are commonly known as species flocks. These are groups of dozens and even hundreds of species, all clearly derived from a single common ancestor from which 2

they diversified without perceptible geographical isolation and now all living in the closest geographical proximity within a restricted geographical area. The most species-rich and best investigated species flocks are those of the African cichlid fishes living in three large freshwater basins, Lake Victoria, Lake Tanganyika and Lake Malawi. Each of these lakes hosts a few hundred different cichlid species, each of them with its strikingly different morphological, ecological and behavioural adaptations, living alongside its closest relatives inhabiting the same lake. In the case of Lake Victoria, the cichlid species flock developed from a single ancestor in no more than 100 000 years, a very short time span to account for the origin of some 300 species from a single ancestor. See also: Speciation: sympatric and parapatric As insects represent more than one half of the total biological diversity on Earth, it is sensible to ask the question, why are insects so numerous? A first explanation of their unique diversity is to be found in their size. Insects cannot be more than a few centimetres across, due to structural constraints such as the mechanical properties of their exoskeleton and the efficiency of gas diffusion in their tracheal system; on the other hand, their complex architecture cannot be easily accommodated in much less than 1 mm length. In fact, most insects are between 1 and 20 mm long. Insects not being individually too big, do not require large areas for populations to establish themselves, therefore, no long-distance displacements are generally necessary for either feeding or reproduction. On the other hand, most insects are either too heavy or too fragile for long-distance passive transport. These conditions facilitate the establishment of isolated populations, a prerequisite for allopatric speciation. A second major cause of insect diversity is their feeding specialization. This is true for phytophagous species as well as for those living as parasitoids of other arthropods. In both cases, high degrees of host specificity are common. See also: Ecological implications of body size; Insecta (insects)

History of Life A few major events punctuated the history of life on Earth. Some of these events were more or less particularly instrumental for the resulting biological diversity. For a couple of billion years (roughly speaking, 3000–1000 million years ago (Ma)), life was represented only by prokaryotic, mostly unicellular forms, later accompanied by the first, still unicellular, eukaryotes. Within this long time span, two evolutionary transitions proved to be of fundamental importance for the subsequent history of biological diversity: the origin of sex, with which it becomes meaningful to speak of biological species, and, later, the origin of multicellularity, a prerequisite for the evolution of complex and potentially diverse body plans such as those of animals

Diversity of Life

Table 1 The chronology of Earth’s history, according to the International Commission on Stratigraphy (basic table, 2002) Eon

Era

Period

Ma

Phanerozoic

Cenozoic

Neogene Palaeogene

Mesozoic

Cretaceous Jurassic Triassic Permian Carboniferous Devonian Silurian Ordovician Cambrian

} –23.8 23.8–65.5 65.5–146.0 146.0–200 200–251 251–299 299–359 359–416 416–444 444–488 488–542 542–1000 1000–1600 1600–2500 2500–2800 2800–3200 3200–3600 3600–3860 3800–3850 3850–3950 3950–4150 4150–

Palaeozoic

Proterozoic

Archean

Hadean

Neoproterozoic Mesoproterozoic Palaeoproterozoic Neoarchean Mesoarchean Palaeoarchean Eoarchean Early Imbrian Nectarian Basin Groups Cryptic

and plants. The few multicellular organisms found in rocks older than 1 billion years are simple algal threads composed of chain-linked single cells. See also: Diversity of life through time; Eukaryotes and multicells: origin; Geological times: principles; Sex: advantage; Universal tree of life The first unequivocal metazoan-type fossils are those of the Vendian or Ediacaran age (late Neoproterozoic, see Table 1), c. 620–550 Ma Their genealogical relations to modern phyla, however, are much disputed. According to some palaeontologists they represent an early, independent experiment in multicellularity, not belonging to the ancestry of the true metazoans. These true metazoans suddenly appear at the base of the Cambrian strata, c. 550 Ma, in what has been described as the Cambrian explosion of animal life. Whether this stratigraphic evidence actually records an abrupt increase in biodiversity or merely the consequence of the development of the first mineralized (fossilizable) skeletons is still a matter of dispute. However, this Cambrian event is the single most significant event in the history of biodiversity documented in the fossil record. See also: Cambrian radiation; Fossil record The Cambrian explosion was soon followed by the diversification of the marine biota into four main components: the infauna living within soft substrates, the epifauna living at the surface of soft and especially hard substrates, the plankton and, somehow later, the necton, which is the complex of actively swimming animals, many

of them predators, including fishes and large arthropods such as the eurypterids. Most recent animal phyla were already present in the Cambrian, some of them (e.g. arthropods) with a great number of different species and body plans. A further crucial event in the history of life was the invasion of land by plants (Middle Silurian), arthropods (Upper Silurian) and vertebrates (Upper Devonian). Plants colonized terrestrial habitats by developing rigid stalks bearing photosynthetic leaves and reproductive organs, a root system to anchor the stem and a vascular system to conduct water and minerals; terrestrial animals modified body surface and respiratory organs to keep water loss to a minimum. The limited availability of water also caused both plants and animals to adopt new reproductive strategies. See also: Terrestrialization (Precambrian–Devonian); Tiering on land – trees and forests (late Palaeozoic) Animals were obliged to abandon external fertilization and to adopt spermatophores or to evolve internal fertilization. The susceptibility to desiccation of eggs and embryos was prevented either by laying the eggs in water (thus retaining or developing anew, an amphibian life style) or otherwise. Many insects developed ovipositors to lay eggs in living plant tissue, but the two definitive answers to the danger of desiccation were found later, either in viviparity or in the production of better encased eggs, such as those of 3

Diversity of Life

amniote vertebrates. In parallel, flagellated male gametes requiring water to travel to the female gametes were abandoned by the evolutionary line leading to the flowering plants. The transition to land opened an enormous scope for a new diversification of life, because of the physical discontinuities so widespread on the land masses and the speciation facilitating opportunities (insect–host plant or insect–parasitoid relationships) as mentioned in the previous section. See also: Reproductive strategies Arachnids and myriapods were already present in the Upper Silurian, whereas the oldest record for insects only dates from the Lower Devonian, and the other major group of nonmarine invertebrates, the pulmonate snails, is only known from the Carboniferous. More or less at the same time (end of early Carboniferous) insects had developed flight ability. Vertebrates came a bit later on the scene than arthropods. The earliest known land-dwelling vertebrates or tetrapods date from the Upper Devonian and the earliest flying vertebrates did not evolve before the late Triassic. These were pterosaurs, later to be joined by birds, in the late Jurassic, whereas the first flying mammals (bats) did not evolve before the Eocene. By that time, however, most recent orders of mammals had already differentiated from mammal origins in the late Triassic. See also: Mammalia (mammals); Transitions between major classes: vertebrates; Vertebrata (vertebrates) A latecomer to the evolutionary scene are the angiosperms, the earliest fossil evidence for this group only dating from the early Cretaceous. After this late appearance, however, the flowering plants experienced a rapid burst of differentiation, largely triggered by the simultaneous explosion of insect diversity. The Palaeogene was the time of origin of grasses (Gramineae), a plant family whose enormous success is largely due to their habit of continuous growth. Finally, the Neogene was the age of herbaceous plants with the explosion of families such as the Compositae (daisies and sunflowers), but also of the passerine birds, whose diversification is probably related, on one hand, to the diversification of seed-bearing plants and, on the other hand, to their frequent specialization to chasing flying insects. Other groups that gently increased in diversity during the Neogene include frogs and snakes. See also: Evolution of ecosystems: terrestrial; Neogene time scale The history of biological diversity, however, is not just one of uninterrupted increase, it was also punctuated by some major critical events known as mass extinctions. See also: Extinction One of the major mass extinctions happened towards the end of the Devonian. Apparently, it did not affect the vascular plants that had already placed their foot on land, but in the sea it had catastrophic consequences on the reef communities and affected with particular severity trilobites, ammonoids, brachiopods and placoderms. See also: Extinction: late Devonian mass extinction 4

The most severe of all mass extinctions, however, was probably the event that marks the end of the Permian (and of the Palaeozoic era), when 70–90% of marine invertebrate species became extinct within a short time span. Previously successful groups such as trilobites, tabulate and rugose corals and fusulinid foraminifera disappeared completely; and others, such as brachiopods, bryozoans, ammonoids and the stalked echinoderms, were severely affected. See also: Extinction: end-Permian mass extinction; Post-Permian radiation The next major event was the K–T extinction, at the boundary between the Cretaceous and the Tertiary. This is the most widely investigated extinction, marked by the disappearance of two well known and very diverse groups, the ammonoids and dinosaurs. Other groups that went extinct by the end of the Cretaceous include two groups of large aquatic reptiles, the plesiosaurs and the mosasaurs, and the rudists, a family of large reef-building bivalves with heavy, odd-shaped shells. The K–T extinction also severely affected the planktonic realm, especially foraminifera, radiolarians and coccolithophores. See also: Extinction: K– T mass extinction

Estimates of Current Diversity The single most commonly used descriptor of species diversity is species number. This number is correlated, indeed, with some measures of ecological diversity, such as the complexity of food webs or topographic diversity. However, species richness is, at best, just a measure of one aspect of the global diversity of life. See also: Conservation of populations and species To improve the information content of biodiversity estimates, it has been suggested that we need to incorporate measures of phylogenetic relatedness among the species present in a given area, so as to approach a more informative description of ‘character richness’ in the sample. But even estimating species diversity on Earth is not easy. This is due only in part to the incompleteness of our current inventory of biodiversity. There are serious problems, indeed, even with that part of biological diversity that has been already described and named. A first problem arises because of the lack of comprehensive and reliable monographs for many, if not most, of the major groups of living beings. For example, there is no recent world catalogue for popular groups such as the Coleoptera (beetles) or the Lepidoptera (butterflies and moths): that means that the present estimates of 400 000 species described in the first group and 150 000 in the second group may well be some 20% wrong. The major difficulty is not so much with retrieving all existing species names from a very scattered literature, but identifying all synonymies; that is, all cases where two or more different names apply to one and the same species. Synonymization requires a critical appraisal

Diversity of Life

of old and new evidence and, as such, requires the timeconsuming work of many dedicated specialists. Even for a well-researched group such as the flowering plants, 5 20% of the currently recognized species have been treated in genus- or family-level monographs during the twentieth century. A more subtle but far from trivial problem derives from the uncertainties in the definition of the basic unit of biodiversity. The circumscription of species may be very different, indeed, if one adopts a biological or a phylogenetic species concept. This is well exemplified by a 1992 study of the birds-of-paradise, a well-known group where traditional classifications, based on the biological species concept, acknowledge the existence of 40–42 species, whereas not less than 90 phylogenetic species may be distinguished in the same group. Still worse is the case of organisms with uniparental reproduction, where the biological species concept simply does not apply, by definition. Examples are offered, in the northern temperate regions, by the brambles (Rubus), the hawthorns (Crataegus) and the dandelions (Taraxacum). In each of these three genera, hundreds of species names have been given to slightly different morphotypes, which are perhaps morphologically distinct, but often, occurring together in the same spot, do not behave as different ecological units within the local community. For the strict advocates of the biological species concept, these plants simply demonstrate that not all living beings are part of a biological species; this would imply that describing biodiversity only in terms of species counts is, in principle, unsatisfactory. An estimate of the number of species named to date is given in Table 2. From a geographical point of view, there are some prominent hot spots of biological diversity. For example, the four areas of highest diversity for higher plants are Latin America, where one-third of the world flora is at

Table 2 Estimated number of species named to date

Bacteria Fungi ‘Protozoa’ ‘Algae’ Land plants Nematodes Crustaceans Arachnids Insects Molluscs Chordates Others Total

Described (  103)

Existing (working figure) (  103)

4 75 40 45 270 25 45 80 1000 100 50 130 1900

1000 1000 300 400 300 500 150 750 10 000 200 55 300 15 000

home with some 85 000 species thus far recorded, China (30 000 species, some 12% of the world total), Mexico (26 000) and Indonesia (20 000). A latitudinal gradient of biodiversity, with species number decreasing from the Equator to the Poles, is broadly observable despite the existence of many plant and animal groups whose distribution is centred in the temperate areas, such as the Rosaceae, the Cruciferae and the aphids. These latitudinal differences in species diversity are observed at the local as well as at the regional level. For instance, on 1 ha of tropical forest in Ecuador there may be as many as 473 tree species, whereas in a temperate forest a mere handful of tree species (if not just one species, as in many forest stands in cold temperate areas) may cover hundreds of square kilometres. Historical factors as well as present-day conditions concur to the explanation of the higher species diversity in the tropics. For instance, it has been suggested that during the Pleistocene the Amazonian forest became fragmented into a large number of small areas that acted as refugia for the forest fauna and provided opportunity for intensive allopatric speciation. Later on, when these forest fragments joined together again to form the present-day forest, species that had differentiated in separate refugia had a chance to expand and to become sympatric. How much their present coexistence depends on the high complexity of the ecosystem or on its high productivity is still matter for dispute. The present level of knowledge varies greatly between different groups. In the case of birds, if we disregard the problems arising from adopting different species concepts, we can reasonably expect that no more than a few dozen species remain to be described. In the case of mammals, however, recent descriptions of previously unknown species are not limited to small inconspicuous species of rodents, shrews and bats, but also include, unexpectedly, some large animals such as the bilkis gazelle (Gazella bilkis) from Yemen, described in 1985, Madagascar’s golden lemur (Hapalemur aureus), described in 1986, and four large ruminants from the Vietnamese forests, three of them representing completely new genera, first described in the 1990s: the saola or spindlehorn (Pseudoryx nghetinghensis), the linh duong (Pseudonovibos spiralis), a giant muntjac deer (Megamuntiacus vuquangensis) and another muntjac (Muntiacus truongsonensis). It is for invertebrates, however, that a truly significant increase in species description has taken place in the last few decades. An impressive example is provided by arachnids and crustaceans: in these groups, the number of new species described between 1960 and 1970 equals the total number of species described in the same groups during the previous two centuries. This example easily suggests that a large percentage of existing species have not yet been described. See also: Chelicerata (arachnids, including spiders, mites and scorpions); Crustacea (crustaceans) Presently, at least 15 000 species of living organisms are annually described as new. 5

Diversity of Life

Different approaches have been followed to obtain estimates of the number of existing species. These methods generally focus either on less intensively investigated taxonomic groups, such as bacteria, fungi, nematodes, mites and insects or on some exceptionally species-rich habitats. The two aspects, however, are closely interrelated. For example, insects are the main component of species diversity in the tropical forest canopy, as nematodes are in the deep sea floor. See also: Ecological methods A sample of arthropods collected on just 10 trees in Borneo included 24 000 specimens, belonging to more than 2800 species. One of the most abundant and diverse groups were the tiny parasitic chalcid wasps: among the 1455 specimens belonging to this group, 739 different species could be counted, 437 of these being represented by just one specimen each. These and similar data have led to an estimation of the total number of arthropod species in the tropical forests worldwide at somewhere between 10 and 80 million. Much more conservative estimates, however, have been obtained following different approaches. It may be sensible, for instance, to compare the number of described and undescribed species collected by prolonged sampling efforts in biologically rich and hitherto less investigated areas. Thus, in a very extensive collection of Hemiptera from a topographically diverse area of tropical rainforest in Sulawesi (Indonesia), the described species amounted to more than one-third of the total. If a similar proportion applies to insects worldwide this would suggest that the total number of extant species of insects would only be around 2.5 million, i.e. less than three times the figure for the species described thus far. See also: Arthropoda (arthropods) Besides the tropical forest canopy, possibly the richest reservoir of uncharted biodiversity is the deep-sea, despite the relatively low amount of energy flowing through it.

6

Estimating this aspect of biodiversity, however, is even more problematic than for tropical insects. Global estimates of existing biodiversity are thus uncertain. Figures ranging from 5 to 130 million species have been recently offered for the gross total. Those given in Table 1, although subjective, are only slightly higher than the working figures most often offered in the literature. Species counts are the simplest but not the only possible way of describing biodiversity at either local or regional level. Interesting comparisons between ecosystems may be obtained, for instance, by considering the local distributions of species in terms of the average size of adult individuals or in terms of relative or absolute abundance of the species.

Further Reading Briggs DEG and Crowther PR (eds) (2001) Paleobiology II. Oxford: Blackwell Scientific Publications. Forey PL Humphries CJ and Vane-Wright PJ (eds) (1997) Systematics and Conservation Evaluation. Oxford: Oxford University Press. Gaston KJ and Spicer JI (2004) Biodiversity: an Introduction. Oxford: Blackwell. Groombridge B (ed.) (1992) Biodiversity: Status of the Earth’s Living Resources. London: Chapman & Hall. Harper JL and Hawksworth DL (1994) Biodiversity: measurement and estimation. Philosophical Transactions of the Royal Society of London B 354: 5–12. Heywood VH and Watson RT (eds) (1995) Global Biodiversity Assessment. Cambridge, UK: Cambridge University Press. Maynard SJ and Szathma´ry E (1995) The Major Transitions in Evolution. New York: Freeman. Novacek MJ and Futter EV (eds) (2001) The Biodiversity Crisis: Losing What Counts. New York: The New Press. Reaka-Kudla ML Wilson DE and Wilson EO (eds) (1997) Biodiversity II. Washington DC: Joseph Henry Press. Wilson EO (1999) The Diversity of Life. New York: W.W. Norton.