9.7. Environmental monitoring using grounddwelling hexapods
Human activities such as agriculture, forestry, and pastoralism have resulted in the simplification of many terrestrial ecosystems. Attempts to quantify the effects of such practices — for the purposes of conservation assessment, classification of land-types, and monitoring of impacts — have tended to be phytosociological, emphasizing the use of vegetational mapping data. More recently, data on vertebrate distributions and communities have been incorporated into surveys for conservation purposes.
Although arthropod diversity is estimated to be very great (section 1.3), it is rare for data derived from this group to be available routinely in conservation and monitoring. There are several reasons for this neglect. Firstly, when “flagship” species elicit public reaction to a conservation issue, such as loss of a particular habitat, these organisms are predominantly furry mammals, such as pandas and koalas, or birds; rarely are they insects. Excepting perhaps some butterflies, insects often lack the necessary charisma in the public perception.
Secondly, insects generally are difficult to sample in a comparable manner within and between sites. Abundance and diversity fluctuate on a relatively short time-scale, in response to factors that may be little understood. In contrast, vegetation often shows less temporal variation; and with knowledge of mammal seasonality and of the migration habits of birds, the seasonal variations of vertebrate populations can be taken into account.
Thirdly, arthropods often are more difficult to identify accurately, because of the numbers of taxa and some deficiencies in taxonomic knowledge (alluded to for insects in Chapter 8 and discussed more fully in Chapter 17). Whereas competent mammalogists, ornithologists, or field botanists might expect to identify to species level, respectively, all mammals, birds, and plants of a geographically restricted area (outside the tropical rainforests), no entomologist could aspire to do so.
Nonetheless, aquatic biologists routinely sample and identify all macroinvertebrates (mostly insects) in regularly surveyed aquatic ecosystems, for purposes including monitoring of deleterious change in environmental quality (section 10.5). Comparable studies of terrestrial systems, with objectives such as establishment of rationales for conservation and the detection of pollution-induced changes, are undertaken in some countries. The problems outlined above have been addressed in the following ways.
Some charismatic insect species have been high-lighted, usually under “endangered-species” legislation designed with vertebrate conservation in mind. These species predominantly have been lepidopterans and much has been learnt of the biology of selected species. However, from the perspective of site classification for conservation purposes, the structure of selected soil and litter communities has greater realized and potential value than any single-species study. Sampling problems are alleviated by using a single collection method, often that of pit-fall trapping, but including the extraction of arthropods from litter samples by a variety of means (see section 17.1.2). Pitfall traps collect mobile terrestrial arthropods by capturing them in containers filled with preserving fluid and sunken level with the substrate. Traps can be aligned along a transect, or dispersed according to a standard quadrat-based sampling regime. According to the sample size required, they can be left in situ for several days or for up to a few weeks. Depending on the sites surveyed, arthropod collections may be dominated by Collembola, Formicidae, and Coleoptera, particularly ground beetles (Carabidae), Tenebrionidae, Scarabaeidae, and Staphylinidae, with some terrestrial representatives of many other orders.
Taxonomic difficulties often are alleviated by selecting (from amongst the organisms collected) one or more higher taxonomic groups for species-level identification. The carabids are often selected for study because of the diversity of species sampled, the preexisting ecological knowledge, and availability of taxonomic keys to species level, although these are largely restricted to temperate northern hemisphere taxa.
Studies to date are ambivalent concerning correlates between species diversity (including taxon richness) established from vegetational survey and those from terrestrial insect trapping. Evidence from the well-documented British biota suggests that vegetational diversity does not predict insect diversity. However, a study in more natural, less human-affected environments in southern Norway showed congruence between carabid faunal indices and those obtained by vegetational and bird surveys. Further studies are required into the nature of any relationships between terrestrial insect richness and diversity data obtained by conventional biological survey of selected plants and vertebrates.