How successful are insects? (Insect biodiversity)

We generally consider vertebrates as higher animals, believing that they are more highly evolved than other animal forms. This notion has basis on the prejudice that they are the dominant forms in the living fauna, and the “reigning” species, Homo sapiens, is a member of the group of vertebrates. But, while holding this opinion, one aspect is ignored, namely that largeness of body size and development of intelligence are not the only criteria to measure progress in evolution. The following
discussion aims at bringing out the spectacular progress made by insects, almost comparable to, if not more than, the evolutionary advancement of vertebrates, though in a different direction. One of the achievements of evolution is that the members of a group of organisms tend to occupy the various niches in their environment by adapting themselves to those niches, and in this process they come
to present increased biodiversity. While man has come to invade different environmental conditions and niches through his cultural and technological evolution, insects have done this through the basic
or “classical” organic evolution, and as a result they have come to present much greater biodiversity than vertebrates. We have to admire their evolutionary potential, while we appreciate the cultural and
technological prowess of man.
How many insect species are known to us? As one of us (Jolivet, 1991) has pointed out that more than a million and a half species of living animals are known, and groupwise breakup of this figure:

Protozoa 50,000

Arthropoda (Crustacea, Insecta, Arachnida, Myriapoda) 1,250,000

The various remaining invertebrate groups 150,000

Chordates 44,000

That figure was good during the nineties. Since then, the estimates have varied continuously, and, in reality, no one knows how many Arthropoda are there on the earth, and how many living beings exist. Only for vertebrates and flowering plants we have a rather fair estimation.

Arthropoda obviously present much greater biodiversity than other groups of animals. E. O. Wilson (1987), a well known myrmecologist and a protagonist of invertebrate conservation, said., “I estimate that a total of 41,000 vertebrate species have been described, of which 5800 are reptiles, 9040 are birds, and 4000 are mammals. In contrast 990,000 species of invertebrates have been described, of which 290,000 alone are beetles – seven times the number of all the vertebrates together.” In 1988, Wilson estimated the described living organisms at 1,392,485, as compiled from diverse sources. May (1988) gave a total estimate of 3,193,800 living beings. But, in his later paper (May, 1992), he does not give any estimate. Not only don’t we know the number of living species, but we don’t even know the exact number of described species. Wilson too does not give any close estimate in a later

publication (Wilson, 1992), but mentions only a rough estimate; he thinks that 3 to 5 million could be the total number of the living species. Of course, in the past the extinct organisms were numbering by millions and that could not be evaluated, as fossils known at present are only a very small part of what are there in nature (Labandeira and Sepkoski, 1993). Flowering plants, for instance, were much more numerous than today. They were quickly increasing in diversity after the Cretaceous (Burger, 1988).

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Most models suggest that increasing plant diversity was accompanied by increasing animal diversity, and later they both simultaneously regressed. Insects are small bodied. Though vertebrates are large bodied, their biodiversity could not be accurately decided. In 2004 the number of known species of fishes was 28,500, but we are aware of this that a large number still remain to be discovered and described, specially in the depth of oceans and in the Amazon region. There is a similar situation with

Amphibia. We believe that our knowledge of the biodiversity of birds and mammals is most complete. Even then 2 or 3 new species of birds are being described every year, and a few small species of rodents or monkeys are being recorded at times. In contrast beetles among insects are very

small, specially members of Staphylinidae and Curculionidae. Nanosella (Ptiliidae) is only 0.25 mm long; even then its “architecture” includes a remarkable stridulatory or sound producing organ (Sörensson, 1997). Many beetles, belonging to the family Chrysomelidae are almost 1.0 mm

long. In case of beetles the principle that small organisms are usually more diverse than large ones (Dial and Marzluff, 1988) holds well. The famous scientist Haldane once said to an anglican priest, “God has show an inordinate fondness for beetles”. While we are so awe-struck with the biodiversity of beetles, 70-95% of all beetle species remain to be discovered and described (Grove and Stork, 2000). In spite of the body size difference between vertebrates and invertebrates including beetles, at

present 900,000 of invertebrates have been described, amongst which 300,000 are beetles alone, and this number of beetles is seven times the number of all vertebrates together.

Let us dwell further on the existing biodiversity. So, how many arthropods are there on the earth? Erwin estimated them to be 30 millions, by calculating the biodiversity of the trees in the tropics (about 50,000 species) (Erwin, 1983). Erwin speculated only about the phytophagous forms, but the number of invertebrates living in the soil, including mites, must also be enormous. Stork estimated once the whole fauna to be between 20 to 80 millions, to come back to a more reasonable figure later on (vide infra). The sad truth is that no one knows, after 250 years of systematic research, a more accurate number (May 1992). PJ asked Basset in 2002, in Panama about this number, and he was not able to answer. There is no answer also in his recent book (Basset et al., 2003), and the present authors believe that global estimates of biodiversity cannot be based on a handful of tree canopy studies. As a matter of comparison, there are a little less than 300,000 flowering plants on the earth. Like the insects, many species of plants also are slowly going extinct on the planet.

Stork (1988) gave later on a figure of 1.8 million, a very underestimated figure, as the number of all the living animal species, about 70% are arthropods, nearly 60% insects, while other invertebrates and vertebrates together would contribute to only about 30% of the total biodiversity, presented by the Animal Kingdom. This biodiversity is much more in tropical forests than in other regions. E. O. Wilson discovered in the trees in forests of Peru 43 species of ants, belonging to 26 genera, which figures are more than the numbers for all the British Isles put together. Stork (1993) said that estimates of global diversity range from about 2 to 50 millons, but 5 to 15 millions seem reasonable. Figures given are totally different from one author to another: Ehrenfeld (1986): 30 to 40 millions; Adis (1990): fewer than 30 millions; Gaston (1991): less than 10 millions; Hammond (1992): 12 millions, including 8 million insects. So the figure of 5 millions, given by Wilson (1992), seems reasonable, but probably underevaluated.

Every year hundreds of new insect species are being recorded. What would be the number of insect species, when all the species have been discovered and described? Studies on tropical forest canopies are yielding particularly large number of new insect species. Terry Erwin of the Smithsonian Institution, Washington has studied the tropical forest insects of Brazil, Panama and Peru; Nigel Stork of the British Museum has recorded insects in the forests of Borneo and Queensland, and Yves Basset the forests of Australia, New Guinea and Panama. All these eminent workers have attempted to estimate the total number of insect species, recorded and unrecorded. Their estimates are, however, widely different. In fact making such an estimate is groping in the dark. Besides, while new species are being described, many species are going extinct. Rate of extinction of organisms is going up every year through human interference.

So let us not venture into estimating the total number of insect species. One situation is obvious, that insects present much greater biodiversity than vertebrates. Evidently it is because the former could adapt themselves to a much wider range of niches than the latter.

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There is another parameter, other than biodiversity, which speaks of great evolutionary success for insects; it is biomass. By biomass is meant total weight or mass of living matter in a certain group of

organisms in a certain area. In spite of their much smaller body size, insects present a much greater biomass than vertebrates. This situation is well brought out by E. O. Wilson (1987), “….in tropical rain forest near Manaus, in the Brazilian Amazon, each hectare (or 2.5 acres) contains a few dozen birds and mammals but well over I billion invertebrates of which the vast majority are….mites and springtails. There are about 200 kilograms dry weight of animal tissue in a hectare, of which 93 percent consists of invertebrates”. This greater biomass of insects and other invertebrates speaks of their better adaptation to their niches and much higher fecundity.

Insects, mites and other invertebrates, because of their habits, occupy significant positions in the food chain or food pyramid in an ecosystem. Food synthesizing organisms or plants occupy a position near the base of a food pyramid. Above them is the tier occupied by herbivores. Further upward tiers are for primary, secondary, tertiary predators etc., and at the top stands man in a pyramid which includes him. The biomass in a tier goes on declining as we move upward; hence the pyramidal shape of such a theoretical visualization. The organic discharges (excreta, dead bodies, fallen leaves etc.) from the different tiers reach soil or water (depending on whether it is a terrestrial or aquatic ecosystem), where they are decomposed by decomposers or reducers, which include insects, mites, other invertebrates, bacteria and fungi. Through combined action of the reducers, inorganic nourishment is

released from the organic waste falling into soil/water. This nourishment is needed by the producers or plants. The decomposers constitute the lowest most tier in the food pyramid. If man disappears or goes extinct, most of the pyramid will survive. But, if the decomposers and the pollinating insects are lost, the whole pyramid will collapse, and the earth will be covered with garbage. That is why E. O. Wilson has said, “The truth is we need invertebrates but they don’t need us.”

“Gaia, Gaia, don’t go away!” Gaia, as per Lovelock, is the auto-supporting blue planet, which has maintained oxygen level in its atmosphere for the past 300 million years. All plant and animal components are contributors this equilibrium. That is why an undisturbed biodiversity is our absolute need.