Current insect extinctions

Abstract

The question 'how many species inhabit the earth?' is not just the result of the inherent property of human mind for continuous intellectual search. It is the urgent need to know the biodiversity of our planet, to predict its fate and increase our understanding of various biological species, their mutual dependencies and their abiotic requirements. Insects constitute the dominant taxonomic group of the earth. Many authorities summing both the taxonomically described and undescribed species, estimate the total to be from 1.8 to 50 millions (May 1988, 1990; Stork 1993). In an attempt to assess the ratio of the described to undescribed living taxa Hammond (1990, 1992) showed that insects are the richest taxonomic group in both categories. Even the described species surpass all the other groups of organisms in all five kingdoms. In this respect it is expected that among the organisms that are going to become extinct in the next few decades insects will constitute a large proportion (Stork 1993). Pimm and Raven (2000) estimate that among living species one-tenth may become extinct in the next fifty years. This is expected because of extensive habitat loss, which in a wider sense includes habitat fragmentation and inbreeding suppression among the remaining populations. Despite the fact that insects, especially the strict specialists, are among the first organisms liable to become extinct our knowledge on their taxonomy, distribution and ecological role is very limited. What we need is time and money, for the documentation of insect extinctions (Dunn 2005). In this context the monitoring of insect extinctions is different from that of mammals and birds. Dunn (2005) states that the IUCN Red List (2002) records 129 bird species extinctions within the last 600 years, which constitute 1.3% of all bird species. By analogy we may expect through the world some 44,000 of the estimated total of 3.4 million insect species to become extinct. However, only 70 insect species extinctions have been documented so far. This great discrepancy is largely the result of our limited study of insects. If we accept the arguments of other authors in biodiversity calculus and the estimation of insect species this discrepancy becomes even larger. The 'guestimates' of world insect species derived by Stork (1993) is 5.3 million species. Gaston (1992) estimates range between 6.88 and 8.75 million species. Hodkinson and Casson (1991) extensively collected tropical Hemiptera species sampled with all known devices from Sulawesi, Borneo estimated the number of insect species of the world to be 3.0 - 4.0 millions. On the same line of arguments by accepting the working figure of 10,000 beetle species in the collection area and taking species accumulation curves the estimates rise to 5.0 - 6.0 million species. Other much higher numbers, though not so convincing are provided by Erwin (1992) who estimated the world insect fauna as 30.0 million, while on the basis of distribution models relating species abundance and body size of various taxa inferred global estimates of insect fauna between 10.0 - 50.0 million (May 1988, 1990). Surprisingly, the number of studies predicting the richness of the world insect fauna is not balanced with studies predicting the number of insect species to be affected by modern extinctions either on a global or a regional scale. One explanation lies probably in the fact that science is dominated by western schools of thought and associated cultures in which insects and arthropods in general are primarily considered as pests that should be controlled (Kim 1993a, b). An immediate consequence is the exclusion of insects from almost all conservation plans, except possibly those for butterflies and some dragonflies. In the current extinction event, which is induced by humans, the need to separate background extinctions from exceptional extinctions is urgent since it leads to the quantification of local extinctions and the organization of a global network of databases and experts. It is well known to perceptive field entomologists that many insect populations are disappearing from their area of collection and that other species may invade the area establish new populations or even a network of metapopulations depending on the fragmentation status of the habitat. Nevertheless, it is not known if local extinctions are part of a global mass extinction or they are merely above-background events. Paleontologists used to face the problem by interpreting the scarcity or disappearance of insect families as extinctions in excavation layers corresponding to geologic epochs (Labandeira 2005), a fact that reflects the efforts and the cost rather than scientific reality. In this chapter I have attempted to present the extent of current insect extinctions and relate them to possible taxonomic bias. Also, causes of extinctions and similarities with previously recorded mass extinctions are discussed. Current theories about colonization of an area by insects and the applicability of these to current insect extinctions and their detection is examined. The necessity for extensive taxonomic studies is obvious and the ways that this can be achieved and function in the estimation of species loss are presented. © 2008 Springer-Verlag Berlin Heidelberg.