George C. McGavin

Macroecology of local insect communities

The inter-relationships between animal body weight, range size, species richness and abundance are currently the basis of macroecology. Using 41 099 insects sampled from 31 Acacia tree canopies in north-east Tanzania, we first documented the basic macroecological patterns. The relationship between body weight and both species richness and abundance was polygonal with the highest insect species richness and abundance occurring at intermediate body weights. Across individual tree communities, the most statistically robust relationships were found between range size, abundance and species richness and they were all linear. In a second part, we focused on the positive abundance-range size relationship and we could test predictions of six of the eight proposed hypotheses to explain this widely documented pattern of community structure. The relationship is most likely explained by the metapopulation hypothesis stating that with more patches being occupied, local abundance in a given patch increases due to a higher rate of immigration from nearby patches. In addition, we found high slopes for the species-area relationship, typical of island systems and thus it seems reasonable to characterise Acacia trees in the savannah as habitat islands for insects.

The insect fauna of Acacia species in Mkomazi Game Reserve, north‐east Tanzania

The last 15 years have seen an immense increase in knowledge of insect communities in tropical forest tree canopies (Erwin, 1982, 1983; Moran & Southwood, 1982; Adis et al., 1984; Stork, 1987a,b, 1991; Morset al., 1988; Basset, 1991a,b, 1996; Basset & Kitching, 1991). However, insect communities in tropical savannah tree canopies are still comparatively poorly studied. No large-scale study has yet been conducted anywhere in an African savannah habitat (Erwin, 1995), although savannahs cover over 40% of the surface of the tropics (Cole, 1986; Solbrig, 1996). Consequently, there are virtually no estimates of insect diversity in this habitat (Lewinsohn & Price, 1996). To assess habitats for their relevance for conservation, ecological inventories provide an essential tool for environmental management (Campbell, 1993) and insects are a major component in every terrestrial habitat. However, studying arthropod communities in savannah habitats is difficult because of a lack of clear habitat boundaries (Lewinsohn & Price, 1996). In contrast, trees provide an excellent framework for insect community research, because they can be considered a discrete ecological unit (Southwood & Kennedy, 1983). Trees also have great niche diversification because of structural complexity (Lawton, 1978, 1986; Lawton & Price, 1979), are a stable resource (Southwood, 1978) and their inhabitants are all more or less trophically interlinked (Moran & Southwood, 1982). As Moran & Southwood (1982) conclude, ‘in almost all respects it is easier to sample arboreal communities more completely, more widely and more accurately, [. . .], than it is to sample other very complex communities’. This is especially true for savannah tree canopies where the overlap to neighbouring trees is lower than in forest habitats. This study analyses the insect community associated with thirty-one trees of sixAcacia species in Mkomazi Game Reserve, north-east Tanzania using a mist-blowing technique. It is the biggest insect sample ever taken from a tropical savannah habitat. Sampling effort and its effect on species richness as well as abundance patterns and the distribution of diversity, abundance and biomass among insect taxa are analysed.

Predator–prey ratio and guild constancy in a tropical insect community

An analysis of predator-prey ratio and guild constancy was made for 41 099 insects from 31 Acacia tree canopies in north-east Tanzania. A strong proportionality between predator-prey species richness, abundance and biomass was found, with reduced major axis slopes not statistically different from one. Predictions of five existing hypotheses to account for predator-prey ratios were tested: the energy ratio and common determinants of diversity hypotheses were well supported and most likely to explain the pattern reported here. Since predator-prey ratios can be considered a special case of the general guild constancy problem, it was also investigated whether species and biomass shares of guilds differed between six tree species and nine sampling localities. For most guilds no significant differences were found and both energy ratio and common determinants of diversity hypotheses might explain the observed general pattern of resource division in this ecological community.

Variation in populations of mine- and gall-forming Diptera and the growth form of their host plant, bracken (Pteridium aquilinum (L.) Kuhn)

Eight species of Diptera belonging to the families Anthomyiidae, Cecidomyiidae and Agromyzidae utilize bracken in different ways and show preferences for different types of bracken habitat. The annual and local variation found in populations of these mining and gall-forming herbivores in different sites in Southeast England over a four-year period are described. Possible reasons for the differences in distribution patterns are discussed, in terms of the growth and phenology of the host plant, resource availability and natural enemies.

N EW , T. R. 1998. Invertebrate surveys for conservation. Oxford University Press: Oxford. ISBN 0-19-850012-2 (hard cover), 0-19-85011-4 (soft cover).

The world we know now is very different from that of 100 years ago, and 100 years or even only 50 years from now, it is likely to be even more different. The existence of more human beings dependent for their survival on the same resources and the probability of a warmer and drier climate, will lead to complicated and unpredictable consequences. The loss of soils, natural habitats and species may well have progressed to a point where their combined effects are felt not just locally but over large areas of the globe. Conservation must take centre stage now but how to go about it? Furry and feathery species are, like us, relatively big animals and our inherent vertebrocentric bias is evident everywhere from Primary Schools to Universities and at all levels of decision and policy making. Pandas, tigers and the like may be attractive and cuddly but they are really not that important. The critical fauna is made up of invertebrates, mini-beasts, creepy-crawlies, call them what you like. It is these small creatures that make the world go round, or at least keep its terrestrial and marine ecosystems functioning. Working out just how they do this and what will happen if they stop doing their fundamental jobs will keep biologists busy for the foreseeable future. People working in conservation – managers, students, land owners, grant-givers – have simply got to realise the simple truth that the total impact of invertebrate species is orders of magnitude more significant that that of vertebrates. Management plans that are based on vertebrate data are nowhere near adequate. It is high time we changed our frame of reference and looked at the world around us in much closer detail. It is true there are problems – invertebrates are many, varied and as most of them are small or very small, they are difficult to sample effectively and identify accurately. Before you can do any conservation work you need to know what species are present and in what quantities. With macrofauna this is simple enough but for most invertebrates the estimation of biodiversity from local sampling can be a complex and demanding job. Leaving aside the problem of the ever shrinking number of trained taxonomists who can actually identify invertebrates at the specific level, there are right and wrong ways of sampling, preserving material and interpreting survey results. It is a sad truth that a great deal of well-intentioned field work has been wasted over recent years either through the use of the wrong techniques, insufficient sampling or poor preservation. Most museums house the results of innumerable surveys that have never been studied properly, save a cursory listing of the major taxa present. Anyone wanting to do it right must read Tim New’s excellent book before they start, for it contains all you need to know in a concise and readable form. The journalist John Junor once said ‘an ounce of emotion is equal to a ton of facts. Well, we’ve had quite enough emotion as far as the environment and conservation issues are concerned – we need as many facts as we can get. Invertebrate surveys for conservation provides the tools to do the job. Is the job too big for us? That’s a different question.