Memoirs of the Museum of Victoria 56(2):411-419 (1997) 28 February1997 https://doi.org/10.24199/j.mmv.1997.56.32 POPULATION BIOLOGY OF SOCIAL INSECT CONSERVATION PEKKA PAM1Lo1.2 AND Ross H. CROZIER' 1 School of Genetics and Human Variation. La Trobe University, Bundoora 3083, Australia, and Victorian Centre for Conservation Genetics 2 Program of Conservation Biology, Department of Genetics, Uppsala University. Box 7003, 750 07 Uppsala. Sweden Abstract Pamilo, P. and Crozier. R.H., 1997. Population biology of social insect conservation. Memoirs of the Museum of Victoria 56: 411-4 I 9. Social insects, especially ants and termites, are keystone species in most non-arctic ter­ restrial ecosystems, but their special features have received little attention in conservation discussions. Both plants and other animals arc affected by social insects as keystone species. Despite the abundance of species it may be that rather fewin any one location are critical to understanding the ecosystem, because of the mosaic nature of the distributions of the dominant species. Social insects present a wide array of life patterns within the broad sway of sociality, varying in having colonies which are annual or perennial, multi- or single-nested, and multi- or single-queened. Populations of social insects may often be much more vul­ nerable than they appear on the basis of numbers of individuals: the reproductive division of labor means that the effective population size may be small for a species of apparently reasonable abundance. Factors increasing effective population size occur in many species, especially rare ones, but it is uncertain these represent adaptations to rareness. In eusocial Hymenoptera, many species. probably most, show inbreeding depression of a special kind due to the production of sterile males caused by homozygosity at the sex locus. In many termites, on the other hand, colonies pass naturally through cycles of inbreeding and out­ breeding. Socially parasitic ants which habitually inbreed appear to have evolved a different means of sex-determination and do not show inbreeding depression. Differences in the mode of colony formation between species also lead to difference in the longevity of col­ onies. dispersal abilities, and the robustness of populations to disturbance. The greatest threats to social insects, apart from humans, are other social insects, and this applies also to invading ant pest species, which tend to have particularly strong impacts on native species. Social insects thus form a vital part of ecosystems, but also impel the need to accept long­ term studies because of the slow pace at which their populations change. Introduction area can be small and comparable to that of many vertebrates. Yet, the non-reproductive Invertebrates generally differ from vertebrates workers constitute a large biomass which affects in the conservation problems they are prone to both the food requirements of the population because of the scale on which they live. Even a and the impact of the population on its environ­ small area can contain a large population, so that ment. Second, the life of social insects normally considerations such as the area of habitat centres around a nest, which makes them seden­ required and problems ofa small population size tary and can restrict dispersal. In that respect (leading, at least potentially, to inbreeding their population biology can resemble that of depression) can be less important and more plants rather than of other insects. Third, most easily solved. For such populations, habitat loss social insects (ants, bees and wasps) belong to can remove whole populations without an inter­ the order Hymenoptera and have a male­ vening stage of fragility. haploid sex-determination system. This genetic Social insects, however, differ in several mechanism affects some genetic population important aspects from other invertebrates. We characteristics, particularly the effective popu­ can recognize three major features that charac­ lation size and the genetic load resulting from terize social insects and are important in their the sex-determination mechanism. population biology and in the population bio­ logical aspects of their conservation. First, the Our aim here is to describe how these features, ratio of population size to biomass is small. Only which are characteristic of social insects, affect a small number ofindividuals reproduce and the their populations and in which ways they can effectivepopulation size maintained in a given influence their conservation. 411 412 P. PAMILO AND R.H. CROZIER The role of social insects in ecosystems pletely on the host ants. There have been several national extinctions of Maculinea species in One criterion that can be used when setting western Europe (five species live in Europe). The priorities for conservation is to what extent conservation of the butterfly populations other species depend on the species in question requires simultaneous attention on the host ant (Soule, 1987). Without focusing on any single populations, and active management on these species, we can state that social insects in general lines has been carried out to reintroduce the form an important group both in ecosystems in large blue (M. arion) in Britain (Elmes and general and in having important mutual Thomas, 1992; Thomas, 1995). Similar situa- relationships with other organisms, plants as tions are expected in Australia, where the lycae- well as animals. nid butterflies include species depending on the The social life pattern has made social insects care provided by ants (New, 1993). evolutionarily very successful, not so much in As regards the plants, the main roles of social species diversity but in biomass. Social insects are in protecting them against herbivores Hymenoptera make only about 10% of all (which has led to the evolution of special doma- described hymenopteran species, but the esti- tia, extrafloral nectaries and food bodies by mates from South-American tropical rainforests some plants (Beattie, 1 985)), in spreading seeds, show that social insects make more than a quar- and as pollinators. For example, Handel et al. ter of all animal biomass, and c. 80% of all insect (1981) counted that 13 of 45 herbaceous plant biomass (reviewed by Wilson, 1990). It is species in a mesic forest had seeds dispersed by reasonable to assume that they have a key role in ants, and they comprised ca 40% of the above- the energy budgets of many other ecosystems, ground herbaceous biomass. Ants contribute although the ant biomass in temperate grassland significantly to the population dynamics of such areas is only 1-15% of all invertebrates (Pis- myrmechochores and shape the structure of the arski, 1978). The diversities of social insect whole plant community. They are also import- species arc highest in the tropics, except of ant for the viability of some rare and endangered bumblebees that exist mainly in temperate plant populations. Although bees are major pol- areas. Of the major groups of social insects, ter- linators of angiosperms, social bees have gener- mites and stingless bees occur mainly in tropics ally a less important role. They can, however be and subtropics. The major centres of social locally important. Some bees visit only a few wasps are partly in south-east Asia (vespine closely related species of plants, and the protec- wasps) and in neotropics (polybine wasps). tion of such oligolectic bees relies on the preser- Interspecific competition among ants has led vation of these plants. At the same time special- to dominance hierarchies between species, and ist pollinators can be important for the success the dominant species can largely influence the of the plant populations. Such a close relation- composition of the whole ant community. This ship exists between Aconitum septentrionaie has led to so called ant mosaics (Leston, 1978: and its pollinator, the bumblebee Bombus conso- Majer, 1993). found particularly in tropical brinus, both of them being rare and endangered forests. Replacement of the dominant species in northernmost Europe (Pekkarinen, 1979). can lead to a large change in the assemblage not The relatively low species diversity of of only ant species but also of other arthropods social insects, at least when compared to many other (perhaps even vertebrates) as well. In fact, insects, makes it feasible to use the phylogenetic Gilbert (1980) suggested as one of the research approach for defining priority areas priorities for neotropical conservation biology to protect the worldwide species diversity. 'autecology of link species, keystone mutualists Bumblebees of the Bombus sibiricus-gwup comprise one of the and dominant ant species'. Majer (1993) notes first taxonomic groups where this approach that ant mosaics become rarer as one moves has been applied, although away from the tropics. the authors note that conservation of bumblebees is not yet generally Social insects affect their environment also by considered to have a high priority building nests and by manipulating the foraging (Williams et al., 1993). There are, however, reports areas. These activities create niches for many indi- cating that bees are generally declining other organisms, and indeed, the nests of social (OToole. 1993). insects harbour many different guests — and parasites. Some of the best studied examples come from the Maculinea butterflies, the big Social insect life patterns blues, whose caterpillars live in colonies of the Social insect life patterns vary enormously, Myrmica ants and w hose life cycles depend com- and their societies can be classified on several bases. POPULATION BIOLOGY OF SOCIAL INSECT CONSERVATION 413 We present here some basic features (Crozier nest and/or foraging area against competing and Pamilo, 1 996), all of these adding important colonies. Such single-nest colonies are termed aspects also to conservation, mainly because monodomous. It is, however, common particu- they affect either the effective population size or larly in some ants that neighbouring nests are dispersal. connected to each other and exchange individ- Annual vs perennial colonies. Bumble bees, uals, brood and food.