Dung Beetle Ecologyby Ilkka Hanski
In many ecosystems dung beetles play a crucial role--both ecologically and economically--in the decomposition of large herbivore dung. Their activities provide scientists with an excellent opportunity to explore biological community dynamics. This collection of essays offers a concise account of the population and community ecology of dung beetles worldwide, with an… See more details below
In many ecosystems dung beetles play a crucial role--both ecologically and economically--in the decomposition of large herbivore dung. Their activities provide scientists with an excellent opportunity to explore biological community dynamics. This collection of essays offers a concise account of the population and community ecology of dung beetles worldwide, with an emphasis on comparisons between arctic, temperate, and tropical species assemblages. Useful insights arise from relating the vast differences in species' life histories to their population and community-level consequences. The authors also discuss changes in dung beetle faunas due to human-caused habitat alteration and examine the possible effects of introducing dung beetles to cattle-breeding areas that lack efficient native species. "With the expansion of cattle breeding areas, the ecology of dung beetles is a subject of great economic concern as well as one of intense theoretical interest. This excellent book represents an up-to-date ecological study covering important aspects of the dung beetle never before presented."--Gonzalo Halffter, Instituto de Ecologia, Mexico City
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Dung Beetle Ecology
By Ilkka Hanski, Yves Cambefort
PRINCETON UNIVERSITY PRESSCopyright © 1991 Princeton University Press
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The Dung Insect Community
Animal droppings are one of the best examples of what Charles Elton (1949) called minor habitats, "centres of action in which interspersion between populations tends to be complete and ecological dynamic relations at their strongest." In short, dung beetle ecology is about competitive exploitation of nutritionally rich resources in one such "minor habitat" by species with an elaborate breeding behavior. I do not wish to suggest that all dung beetles live in equally competitive communities, nor do all dung beetles exhibit equally intricate nesting behavior. The chapters in this volume will focus on a range of dung beetle populations and communities with respect to these and other ecological attributes. Nonetheless, my own interest in the ecology of dung beetles does stem from these elements: patchy and ephemeral habitat, severe competition, and complex behavior in many similar species living together. Though dung beetles may not comprise a model system for a large number of other communities, they do comprise one of the animal populations most worthy of the notion of community.
Dung beetles are often exceedingly abundant. Thousands of individuals and dozens of species may be attracted to single droppings in both temperate and tropical localities. Anderson and Coe (1974) counted 16,000 dung beetles arriving at a 1.5 kg heap of elephant dung in East Africa, and these beetles ate, buried, and rolled this "minor habitat" away in two hours! No wonder that much of the ecological interest in dung beetles has been generated by questions about coexistence of competitors using the same resource, in an apparent contradiction to the "principle of competitive exclusion," also known as Gause's law (Gause 1934; Hardin 1960; McIntosh 1985). The "competition paradigm" was established by the early work of Lotka, Volterra, Kostitzin, and Kolmogoroff in the 1920s and 1930s (Scudo and Ziegler 1978), and it was much elaborated in the 1960s and 1970s (MacArthur 1972; May 1973; Roughgarden 1979), when the principle of competitive exclusion was transformed into the theory of limiting similarity, that is, that competitors which are too similar cannot coexist. The competition-centered community ecology met stem criticism in the late 1970s and early 1980s (for example, see many chapters in Strong et al. 1984 and references therein), primarily due to a lack of rigorous demonstration of interspecific competition in natural species assemblages. It has subsequently been shown that competition occurs regularly in a wide variety of taxa (Schoener 1983; Connell 1983). These reviews do not report on competition in dung beetles. But as we have indicated above and shall abundantly demonstrate later in this volume, the reason is not that competition does not regularly occur in many dung beetle assemblages; the reason is the virtual lack of experimental studies on competition in dung beetles.
Species diversity of dung beetles is not all that great in comparison with many other groups of insects. The true dung beetles include the family Scarabaeidae, with some 5,000 species; the subfamily Aphodiinae of Aphodiidae, with about 1,850 species, most of which belong to a single genus, Aphodius; and the subfamily Geotrupinae of Geotrupidae, with about 150 species. Some species of Hybosoridae, Chironidae, Trogidae, and Cetoniidae also use dung at the adult or larval stage, and there are numerous small species of Hydrophilidae and Staphylinidae that use some components of the decomposing material or the microorganisms in dung pats (these latter species are not usually referred to as dung beetles). The relative scarcity of dung beetle species is in striking contrast with the often great abundance of individuals. One is tempted to speculate that competition limits the number of extant dung beetle species worldwide (Chapter 19).
This chapter begins with an overview of patchy and ephemeral microhabitats, that is, Elton's minor habitats, in order to place animal droppings in a wider ecological framework and to present two contrasting population dynamic models that define the range of possible competitive interactions in insects living in patchy habitats. Next comes a discussion of the physical characteristics and the chemical properties of droppings and the nutritive value of the resources available to insects in this microhabitat. Finally, we must keep in mind that dung beetles are seldom alone in droppings: they belong to a complex community comprising other beetles and flies as well as a multitude of other organisms, for example, mites and nematodes. I will outline the taxonomic composition and the structure of the entire dung insect community for the benefit of readers not familiar with these insects.
1.1. Patchy and Ephemeral Microhabitats: an Overview
The microhabitats we are concerned with here share several attributes: relatively small size, scattered spatial occurrence, and short existence or durational stability, generally not more than one insect generation. Many, though not all, of such microhabitats stand out in the matrix of the surrounding environment as "islands" of high-quality resources, which to a large extent explains the diverse assemblages of insects and other invertebrates that colonize the particular microhabitat (Chapter 2).
Density of patchy and ephemeral microhabitats in the surrounding environment varies enormously. At one extreme are fallen leaves that may carpet an entire forest floor and merge, for all practical purposes, into one contiguous habitat. The same thing almost happened to cattle pats in Australia before the introduction of foreign dung beetles (Chapter 15). Generally, droppings in pastures are so numerous that between-patch movement is not a great problem for dung beetles. The other extreme is exemplified by the fruiting bodies of rare macrofungi. Locating one of them would be time consuming, which surely contributes to the high degree of polyphagy observed in fungivorous insects (reviewed in Hanski 1989c). The droppings of larger mammals are a scarce microhabitat in many ecosystems, and we would not expect widespread specialization by many insects to any particular kind. The specialization that does exist occurs primarily among the major types of dung, in particular between omnivore and large herbivore dung (Chapters 9 and 18). Unlike perhaps some mushrooms and other living microhabitats, droppings do not "defend" themselves, though interactions may occur between the microbial populations and insects in droppings (Hanski 1987a and references therein).
Table 1.1 puts forward a hypothesis about a continuum of competitive interactions that I perceive in patchy and ephemeral microhabitats. This continuum is to a large extent correlated with the relative size of individual habitat (resource) patches. The key point is whether a small number of individuals is able to dominate a habitat patch to the exclusion of other individuals. In the ideal situation, of which there are examples, each habitat patch is dominated by a single individual or by a breeding pair of individuals. Such resource dominance is easier in the case of small than large microhabitats, though "smallness" and "largeness" naturally depend on the size of the consumers as much as on the size of the habitat patches themselves. Table 1.1 lists some examples. Cattle pats may be dominated by the largest tropical dung beetles but not by the mostly small northern temperate beetles. Small carcasses may be buried by Nicrophorus beetles (Silphidae), but large carcasses cannot be so dominated by any one insect species. Though some mushrooms are rapidly consumed by fungivorous fly larvae, perennial conks typically have a diverse community of insects that only slowly destroy the microhabitat. Considering host individuals as microhabitats for parasites (Price 1980). ectoparasites of plants and animals are not generally able to exclude others from host individuals. but parasitoids (insect parasites of other insects) provide good examples of dominance of habitat patches (Hassell 1978).
The mechanisms of resource dominance are varied (Hanski 1987a). In the most obvious case, exemplified by the burying beetles Nicrophorus and by the large Heliocopris dung beetles, a pair of beetles removes the entire habitat patch (a small carcass or a dung pat) to an underground nest, away from the reach of competitors, predators, and adverse climatic conditions. In other cases, habitat patches may be dominated by territoriality, as described for the carrion-breeding fly Dryomyza anilis by Otronen (1984a, 1984b), for the dung-breeding fly Scatophaga stercoraria by Borgia (1980, 1981) and for the fungivorous beetle Bolitotherus cornutus by Brown (1980). Some specialist species are able to exploit all the resources in a habitat patch extremely fast and thereby reduce the chances of reproduction by inferior competitors. A probable consequence of the race to resource preemption is the habit of depositing first-instar larvae instead of eggs by the carrion-breeding Sarcophaga (Denno and Cothran 1976) and Australian Calliphora flies (Levot et al. 1979; for a general discussion see Forsyth and Robertson 1975: Hanski 1987a). The dung beetles that construct a nest and provision it rapidly with food for their larvae achieve the same result even faster. In other species, rates of larval development appear to have been pushed to the physiological upper limit: Lucilia illustris, a carrion-breeding calliphorid fly (Hanski 1976), and Hydrotaea irritans, a dung-breeding muscid fly (Palmer et al. 1981). can complete their larval development in the amazingly short period of 50 hours under optimal conditions. The insect communities in which the use of resources in individual habitat patches is strongly dominated by a small number of individuals are here called "dominance communities" and are structured by lottery dynamics. The meaning of this concept will become clear in a moment.
The other extreme of the continuum in Table 1.1 is found in microhabitats where resource patches cannot be dominated by a small number of individuals, usually because the patches provide such a large amount of resource compared with the size of the insect consumers that resource dominance by burial or by other means is not possible. In this case a large assemblage of many species may accumulate in the same patches. The cattle pat community in Europe is a case in point, with up to one hundred species of insects occasionally crowding in a single pat. The opposite extreme to the dominance community is a "mixed community" (of many species) structured by variance-covariance dynamics. We shall now turn to the two population-dynamic models corresponding to the end points of this continuum.
The first papers that explicitly dealt with lottery competitive dynamics were written by Sale (1977, 1979), who worked with coral reef fishes, which defend interspecific territories and hence compete for space. Deaths of adult fishes create empty lots available for colonization by young individuals from the planktonic pool. The lottery mechanism operates at this stage: all individuals of all species are assumed to have an equal chance of obtaining a vacant territory. Sale (1977) assumed that there is no density dependence in recruitment—in other words, that both locally rare and common species have equal probabilities of securing a territory. This assumption is not essential for lottery dynamics, and it is difficult to accept unless the recruits arrive from a larger area than the "community" under study, in which case the apparent density independence is an artefact of scale.
Sale (1977, 1979) originally conjectured that the lottery mechanism would allow coexistence of a large number of equal competitors—the coral reef fishes in his example. But this is not correct (Chesson and Warner 1981; Chesson 1985; Hanski 1987b). Assuming that the competitors are identical in every respect, no one species will deterministically replace the others, but nothing prevents the relative frequencies of the species from entering a process of random walk: by chance, one species does better than the others in a particular generation, increasing its frequency in the community. These changes are often reversed, as every species has the same chance of increasing; but given sufficient time, one species after another is lost from the community. The greater the spatial scale and the number of habitat patches, the slower the process of species elimination, but the point is that no mechanism exists in the lottery dynamics itself that would tend to maintain species richness.
Chesson and Warner (1981) showed that in iteroparous species, which may reproduce several times in their lifetime, environmental stochasticity can promote regional coexistence in lottery-competitive systems. For such species, even grand failures in some lotteries (breeding seasons) are not catastrophic, because the lost opportunities to increase in frequency can be made up by successful reproduction in subsequent breeding seasons; the great longevity of the species buffers their populations against severe declines (Chesson's "storage effect"). For this mechanism to work, different species must not have complete temporal correlation in reproductive success (Chesson and Huntly 1988). (There is no storage effect or only a slight one in dung beetles [see Chapter 17], hence the lottery dynamics is not expected to facilitate coexistence in dung beetles even with environmental stochasticity.)
To go back to insects colonizing patchy and ephemeral microhabitats, lottery dynamics in the purest form occurs when the first individual (female), or a pair of breeding individuals, that locates a habitat patch is able, in one way or another, to exclude any later-arriving individuals. Locating the habitat patches is the lottery, at which some species may be better than others but which nonetheless involves an essential component of randomness. The resource owners reproduce, and the newly emerged individuals take part in the next round of the lottery.
Dynamics in the burying beetles Nicrophorus and in some large dung beetles (for example, Heliocopris) closely approximate the lottery model, as one pair of beetles may completely dominate a resource patch, a carcass (Nicrophorus), or a dung pat (Heliocopris). In other cases, droppings are so large compared to the individual beetles and their resource requirements that one dropping has resources for more than one individual or a pair of beetles. However, due to their nesting behavior (Chapter 3), Scarabaeidae dung beetles are able to rapidly remove a portion of the resources in a dropping for their exclusive use, essentially on a first arrived-first served basis, until the whole resource is used up. The lottery dynamics applies to this situation, but now the contested resource units correspond to the chunks of resource required by individual beetles, or by pairs of beetles, rather than to the entire resource patches themselves.
Not all dung beetle communities have dynamics that even approximately fit the lottery model. We need another model for communities in which species do not rapidly preempt resources in habitat patches but stay and possibly compete for a prolonged period of time. An essential feature of most such species assemblages is extensive variation in the numbers of individuals between habitat patches. Atkinson and Shorrocks (1981) and Hanski (1981) originally developed the idea of incorporating the spatial variances and covariances of populations in competition models (see also Lloyd and White 1980). Further work on these and related models can be found in Ives and May (1985), Ives (1988a, 1988b, 1990), Shorrocks and Rosewell (1986, 1987), and Hanski (1987b).
Excerpted from Dung Beetle Ecology by Ilkka Hanski, Yves Cambefort. Copyright © 1991 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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