Diversity

The best recent count of the total number of named insect species in the world is 1,111,225 (Arnett 1983), although many textbooks still place the figure at 650,000 to 700,000. Plausible arguments are offered by authors for many more, 3 to 5 million or 10 million (May 1988) existing species. By liberal extrapolation of some data, there are indications of even as many as 30 million actual living species of tropical forest arthropods alone (Erwin 1982) or possibly several times that amount (Stork 1988). Whatever figure is accurate, most of the total could be expected to occupy Latin America, because of the region's very rich flora, large area, and complexity of physical and biotic habitats. An educated guess of 40 percent of the total world's biota could be made for the number of species ultimately discoverable in the region, based on available insect host diversity and land habitat area present compared to that of the other zoogeo-graphic regions. Most of these species occur in lowland forests.

The reasons for the especially high diversity in the tropical portions are not fully understood, although various theories have been proposed (Dobzhansky 1950, Pianka 1966). No doubt, the inclusion of vast areas of wet lowland forest that provide abundant niches and a stable climate, at least in refuges, has much to do with it. There is little doubt that there are more species of insects of most major groups in the tropics than in temperate areas, with the possible exception of some parasitic Hymenoptera and bees (Wolda 1983), aphids (Dixon et al. 1987), the Plecoptera, and a few others. According to Janzen (1976), the explanation lies chiefly in the amount here of "harvestable productivity" arranged in a sufficiently heterogeneous manner. This allows utilization by a vast complex of small and very adaptable reducing animals, most fittingly, the insects.

Heterogeneity is provided by climatic and geographic variation over wide areas for long periods of time. The best example is provided by the Amazon Basin, where insect diversity is very high as a result of habitat disturbances—from a constantly changing vegetation, dissection of the landscape by rivers, alternation of inundated— dry forest types, and possibly other phenomena—for at least the duration of the Pliocene—Pleistocene era (Erwin and Adis 1982).

The stability of the wet lowland tropics gives the insects that live there a head start on speciation and allows them to accumulate, but because of competition, no single area becomes greatly enriched. The fact that there are a great number of species but relatively few individuals is thought by some to be largely due to the high productivity and availability of resources in such forests and the reduced seasonality that makes it possible for insects to occupy marginal niches (MacArthur 1969).

References

Arnett,Jr, R. H. 1983. Status of the taxonomy of the insects of America north of Mexico: A preliminary report prepared for the subcommittee for the insect fauna of North America Project. For Stand. Comm. System Res., Entomol. Soc. America. Dixon, A. F. G., P. Kindlmaivn, L. Leps, and J. Holman. 1987. Why there are so few species of aphids, especially in the tropics. Amer. Nat. 129: 580-592. Dobzhansky, T. 1950. Evolution in the tropics.

Amer. Sci. 38: 209-221. Erwin, T. L. 1982. Tropical forests: Their richness in Coleoptera and other arthropod species. Coleop. Bull. 36: 74-75. Erwin, T. L., and j. Adis. 1982. Amazonian inundation forests: Their role as short-term refuges and generators of species richness and taxon pulses. In G. T. Prance, ed., Biological diversification in the tropics. Columbia Univ. Press, New York. Pp. 358-371. Janzen, D. H. 1976. Why are there so many species of insects? 15th Int. Congr. Entomol. Proc. Pp. 84-94. MacArthur, R. H. 1969. Patterns of communi ties in the tropics. Biol. J. Linnean Soc. 1: 19— 30.

May, R. M. 1988. How many species are there on earth? Science 341: 1441-1449.

Pianka, E. R. 1966. Latitudinal gradients in species diversity: A review of concepts. Amer. Nat. 100: 33-44.

Stork, N. E. 1988. Insect diversity: Facts, fiction and speculation, Biol. J. Linnean Soc. 35: 321-337.

Wolda, H. 1983. Diversity, diversity indices and tropical cockroaches. Oecologia 58: 290-298.

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