Cooperative Research Centre for Tropical Rainforest Ecology and ... Invariably, insects are overlooked when tropical forest management issues are discussed, ...
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POINTS OF VIEW
The conservation of saproxylic insects in tropical forests: a research agenda Simon J. Grove* and Nigel E. Stork Cooperative Research Centre for Tropical Rainforest Ecology and Management, James Cook University, PO Box 6811, Cairns, Queensland 4870, Australia Received: 3 November 1998 Accepted: 26 January 1999 Invariably, insects are overlooked when tropical forest management issues are discussed, because there are so many species, they are taxonomically intractable and so poorly known. Often people take the view that if you look after the vegetation and vertebrates, the insects will look after themselves. This may be true for some functional groups, but for saproxylic insects, this seems unlikely. Their study deserves high priority, since they are dependent on the very resource – wood – whose removal from the ecosystem is the usual object of forest management. Given the current international effort to develop ‘criteria and indicators’ to monitor sustainable forest management for biodiversity values, there is a window of opportunity for sound ecological research on saproxylic insects to influence the formulation of forest policy such that their needs can be taken into account. There is already a large body of knowledge on temperate and boreal region saproxylic insects, and on the effects that logging has on them, but knowledge of the tropical forest situation lags far behind. This paper proposes a research agenda to enable the needs of saproxylic insects to be taken into account in natural forest management in the tropics. Basic questions, such as whether logging has so far had an impact on tropical saproxylic insects, and whether there are workable sampling techniques to investigate this, still remain to be addressed and deserve high priority. The links between the responses of saproxylic insects and more ‘charismatic’ study species need to be investigated. We also need to know whether there is a correlation between the intensity of logging and the response of saproxylic insects, and, critically, whether we would be justified in measuring some surrogate aspects of forest structure (as potential habitat for saproxylic insects) rather than the saproxylic insects themselves, and modelling this to determine likely impacts of different management regimes. We consider such an ambitious research agenda as justified given the scale of impact that forest use and management is likely to have on tropical forest insects in the future. Keywords: saproxylic insects, indicators, rapid biodiversity assessment, tropical forests, logging
Introduction The sheer diversity of forest insects worldwide presents an enormous challenge to insect conservationists grappling with the major issues in forestry and conservation today (Bignell and Bignell, 1997). Probably less than 20% of all insect species have been described and a high proportion of these are found in tropical forests (Stork, 1997; Hammond, 1995; Stork, 1993; Erwin, 1982). Furthermore, the diversity of insects in any one small area of forest can be immense. For instance, Stork (1991) collected more than 4,000 species of insects and other arthropods from 10 trees in Borneo; Hammond (1990) recorded more than 4,000 beetle species from a hectare plot in Sulawesi; Hammond and 0wen (in press) collected more than 1000 species of beetles from Richmond Park in London. Lawton et al. 1366–638X © 1999 Kluwer Academic Publishers
(1998) showed that the task of inventorying just a small part of the insect fauna of rainforests is colossal. This diversity is frequently – and to some minds legitimately – seen as a reason to simply ignore insects when forest management issues are being discussed. Some conservationists accept that, if you look after the vegetation and the ‘charismatic megafauna’, the insects will look after themselves. In some such cases, it seems that species are chosen for study more for their visibility or their ability to attract research funding rather than for any particular characteristics that would make them vulnerable to logging (White and Tutin, 1998; Plumptre and Reynolds, 1994; Isabirye-Basuta and Kasenene, 1987). Whilst intuitively appealing, the view that the needs of insects will be catered for in managed forests is frequently wrong, especially in heavily managed temper-
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highest continuity of suitable conditions (often reflecting a long-term absence of intensive management). The consequent significance of saproxylic insects as habitat continuity indicators has come to be widely recognised over the last decade (Harding, 1995; Harding and Alexander, 1994; Speight, 1989), and the group as a whole is beginning to be adopted for ecological assessment (e.g. Fowles, 1997; Good, 1997; Grove, 1990). There is increasing agreement amongst conservationists and foresters that forestry and land management practices can be successfully adapted to cater for the requirements of many of these saproxylic species (Kaila et al., 1997; Reid and Kirby, 1996; Stubbs, 1972), often at low economic cost. All forests are probably capable of supporting some saproxylic species. However, more species-rich assemblages, including those with the most specialised or restricted niches, are only likely to develop on sites which have as close to the full range of saproxylic habitat features as exists (or would have existed), in regular and continuous supply, in natural old-growth forests. Chief amongst these are largediameter ‘overmature’ or ‘veteran’ trees. The snags and logs resulting from their natural death are also important (Økland et al. 1996; Hammond and Harding, 1991), but represent a more short-lived and essentially nonrenewable substrate for colonisation and thus tend to support fewer specialist species dependent on habitat continuity than the overmature trees themselves (Speight, 1989). Management prescriptions that favour the survival of such features – even if not throughout the whole forest stand – are likely to conserve a wider range of saproxylic species than those which only enable the survival of smaller-diameter trees and dead wood. Where the situation is more critical still, specific conservation interventions are sometimes required, such as deliberate premature senescence of trees (using ring-barking or even explosives) or artificially prolonging tree life through pollarding. This is frequently the case, for example, in the heavily managed and sparsely-wooded landscape of lowland England, (English Nature, 1998; Alexander et al., 1996; Read, 1996).
ate landscapes. But given the vast diversity and the taxonomic impediment for insects (New, 1996), where does one start to address this issue? Didham et al. (1996) suggested that a functional, process-driven approach rather than a straight taxonomic approach to the study of insects (and other organisms) in forests provides a more practical and useful insight into the relationships of animal communities in forests and the effects of human-induced disturbance on these communities. Saproxylic insects – insects associated with dead wood or with the fungi and microorganisms that decompose it (Speight, 1989) – comprise one such functionally-defined group. Note that the term embraces not only the wood-feeders and fungus-feeders (primary saproxylics) but also the predators and parasitoids (secondary saproxylics) and the commensals (tertiary saproxylics) dependent on these. They contain representatives from all the major insect orders but Coleoptera and Diptera are especially well represented. Other invertebrate groups, amongst them the mites, pseudoscorpions and onychophorans, also contain many saproxylic species. Saproxylic insect assemblages are functionally very dominant (Elton, 1966), and speciose (Siitonen, 1994), but include many naturally rare species (Stork and Hammond, 1997). If there is one group that warrants further study in the light of current human pressures on forests worldwide, it is the saproxylic insects, since they are by definition dependent on the very resource – wood – whose removal from the ecosystem is the usual object of forest management.
Saproxylic insects and natural forest management in temperate and boreal regions Studies in temperate and boreal areas, particularly in northern and western Europe, show that saproxylic insects are often extremely sensitive to forest management. There are now many examples in the literature concerning the impacts of logging or tree removal in temperate and boreal forests on saproxylic insects (e.g. Michaels and Bornemissza, 1999; Hanski and Hammond, 1995; Gutowski, 1986). There is general agreement that logging has both severe and long-term effects on the saproxylic fauna, especially over the time-scale of centuries or even millennia of management seen in Europe (Buckland and Dinnin, 1993). As a result many saproxylic species are threatened with extinction and gazetted in national red lists (Berg et al., 1995; Shirt, 1987). Many more are confined to forest relics with the
Saproxylic insects and natural forest management in the tropics In contrast to temperate forests, much less is known about saproxylic insects in tropical forests, and virtually nothing on the effects of human-induced disturbance on them. Termites are about the only exception to this (e.g. Eggleton et al., 1995; Collins, 1980). Insects barely feature in Grieser Johns’ (1997) seminal book reviewing impacts of logging on tropical forest wildlife, 68
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whether this ‘criteria and indicators’ approach will work sufficiently well to ensure the conservation of saproxylic insects in ‘sustainably’ managed natural forests.
a situation which he suggests needs redressing urgently. Recent studies (e.g. Hammond, 1990; Stork, 1987) indicate that saproxylic species form a high proportion of the tropical forest insect fauna. Given the much higher numbers of insect species in the tropics compared to higher latitudes, it would seem likely that many more tropical than temperate saproxylic insects are potentially under threat from long-term logging. At the same time, there is an increasing perception that biodiversity conservation and tropical forestry can and should be compatible (Grove, 1998). An encouraging recent trend is the move towards developing ‘criteria and indicators’ of sustainable natural forest management, which is gathering momentum at international, national and sub-national levels (MIG, 1997; Amazonian Cooperation Treaty, 1995; Helsinki Process, 1995; Montreal Process, 1995), as part of the global response to the Biodiversity Convention and Global Forests Agreement resulting from UNCED. There has probably never previously been such an opportunity for sound ecological research to influence the formulation of forest policy. In temperate areas, the development of criteria and indicators can be based on a good level of biological and ecological knowledge developed over many years (e.g. Stubbs, 1991; Dajoz, 1974; Fager, 1968; Paviour-Smith, 1959; Palm, 1959; Savely, 1939), but for the tropics this is not the case. The Centre for International Forestry Research (CIFOR), based in Indonesia, is taking the international lead in developing ‘criteria and indicators’ for sustainable management of natural tropical forests. Their biodiversity working group recognised that large-scale inventory-type approaches to rapid biodiversity assessment (Lawton et al., 1998; Oliver and Beattie, 1996a) were too time-consuming and required too high a level of skill and human resources to be suitable for widespread adoption in tropical countries. The working group hypothesised that if tropical forest ecosystem processes are ‘healthy’, (i.e. functioning as in an unmodified forest) and are maintained as such, then forest biodiversity should automatically be maintained (Stork et al., 1997a). The idea is that the condition of these processes might be easier to measure than biodiversity per se. The indicators that they have proposed have been chosen to reflect the integrity of critical ecological processes, including decomposition in which saproxylic insects probably play a key role. As yet these ideas need field testing, both to determine whether or not they are practical and to determine whether or not they are ecologically meaningful. Until that time, the hypothesised linkage between ecological processes and biodiversity is still a ‘leap of faith’. We do not yet know
Discussion: defining a research agenda A multidisciplinary programme of research on saproxylic insects and logging impacts in natural tropical forests is needed, to complement that which has been and continues to be carried out in higher latitudes. It should be carried out in such a way that its findings are readily applicable to forest managers and the development of natural forest management policy. The research should be framed to address the following key questions: 1. Can suitable techniques be found to effectively sample and enumerate the saproxylic insects of tropical forests? If so, could we use tropical saproxylic insects as indicators of ecological continuity, or is sampling likely to be so resource-demanding that it could only be attempted in exceptional circumstances, such as to investigate relationships between the saproxylic insects and ‘surrogate’ indicators such as key habitat variables (see point 5) or ‘charismatic megafauna’ (point 4)? Tried and tested sampling techniques for saproxylic insects include flight intercept trapping (Owen, 1993; Jessop and Hammond, 1993), bark-spraying with knockdown insecticides, emergence trapping (Owen, 1992), and hand collection (Upton, 1991), but will these be sufficient to sample the specialists of all the potential dead wood habitats? What about those that live permanently in log interiors, rotting roots underground, tree hollows, rot holes, sap runs or high in the canopy of overmature trees? Can new means of access to the canopy such as ‘canopy cranes’ (Stork et al., 1997b) be put to use to address some of these issues? How can techniques like hand collection be standardised? Tavakilian et al. (1997) took advantage of massive tree-felling operations preceding dam construction to investigate the cerambycid beetle fauna of rainforest trees in French Guyana. Are researchers in a position to make the most of similar opportunities should they arise? Alternatively, is it valid, effective or ethical to cut down a few overmature trees or saw up a few logs to search for cryptic species? 2. Do logged forests actually differ in their saproxylic insect assemblages from old-growth forests? Is their conservation really an issue in the tropics? We know this is the case in higher latitudes (Niemel¨a, 1997; 69
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6. Can forestry growth and yield models (e.g. Alder, 1995) be extended to model the long-term supply of overmature trees and dead wood resources under a range of silvicultural options? 7. Does any logging-induced change in saproxylic insect assemblages affect the normal breakdown of dead wood and recycling of nutrients? That is, is there a simple correlation between saproxylic community integrity and the speed and direction of wood decomposition pathways? Or is there a certain amount of ‘redundancy’ in the system such that a simplified subset of saproxylic species (such as might be expected to survive in logged forest) is sufficient to maintain the integrity of these processes, if not the original biodiversity (Franklin et al., 1989)?
Heliovaara ¨ & V¨ais¨anen, 1984), but in the tropics logging may be selective (or inefficient) enough to retain sufficient overmature trees, and to maintain a permanent supply of a range of suitable dead wood substrates; or if not then the areas under forest may be sufficient to allow ready recolonisation; or insect mobility may be greater given the more benign climatic conditions. 3. If there is an effect of logging, is this apparent in the short term (say within a decade of the first logging event, which is the timescale over which many impact studies are carried out) (e.g. Hill et al., 1995; Johns, 1989), or does it take several logging cycles to develop? In the short term, dead wood (at least in some of its forms) may become superabundant (Dykstra, 1992), but over progressive logging cycles the abundance of large-diameter trees (and logs) may diminish (Plumptre, 1996; Nicholson et al., 1988). Or is it too early to say for sure, given the relatively short history of tropical forestry compared to that of temperate Europe? 4. If logging is sufficiently benign to enable the survival of the tropical ‘charismatic megafauna’, is that sufficient to sustain tropical saproxylic insects, or is there no simple correlation? Do the areas that naturally support important populations of charismatic megafauna (and which tend to be selected as protected areas) necessarily coincide with those supporting important populations of saproxylic insects? Prendergast et al. (1993) showed that ‘hot-spots’ for different taxa do not necessarily coincide and that many rare species do not occur in the most speciesrich areas. 5. Can researchers get a good enough handle on what are the key dead wood habitats in tropical forests to say which habitats and associated insects are likely to be under particular threat from logging, as is increasingly the case in temperate areas (Michaels, 1996; Økland, 1994)? If so, can simple surrogates be found for the diversity of saproxylic insects in tropical forests, to act as indicators that are easy to measure? Even for temperate forest regions, there are very few taxonomists who can readily identify many of the critical saproxylic insects, particularly the rare and threatened species. For tropical forests the degree of difficulty is inevitably much greater. Therefore we may need to identify simple measures, such as the amount of standing or fallen dead wood or an index of tree diameter-class distributions, which can be used with some confidence as surrogates for the diversity or integrity of the saproxylic insect fauna.
Acquiring this level of knowledge and understanding is surely justified given the scale of impact that forest management is likely to have on tropical insects over the coming decades and, indeed, centuries. But how achievable are these research goals? Point one is an obvious place to start: can tropical saproxylic insects be adequately sampled and enumerated? Some progress could be made simply by reinterpreting existing data-sets derived from more general tropical forest insect biodiversity surveys, especially those that have already been classified into feeding guilds (e.g. Hammond, 1990). It would be possible, for instance, to assign a ‘saproxylicity probability score’ to taxa according to their likelihood of having saproxylic associations, based on existing knowledge of these or related taxa (but see comments at the end of this paragraph). It would then be possible to determine the approximate proportion of saproxylic taxa in such a data-set. However, more targeted sampling programmes are undoubtedly needed too. Investigating the saproxylic insect fauna of living, overmature trees should be as much a priority as that of dead wood on the forest floor. In any study like this, the taxonomic impediment is vast, but sorting to morphospecies can help here (Oliver and Beattie, 1996b). For beetles, it is generally known which are the main saproxylic groups, and into which functional groups they fall. This is one reason why we are focusing on beetles in our current research in tropical Australia. But for other insect orders, this is less the case. Is it valid to infer lifehistory information from related groups from higher latitudes, or to make judgements based on adult or larval morphology? A few autecological studies of candidate tropical saproxylic taxa could help clear up this question. Given that point one can be addressed, then points 70
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volumes and size-class distributions of logs and snags? (i.e. is saproxylic insect community composition correlated with forest log and snag characteristics?). Or, given that many saproxylic species may require substrates other than dead wood on the ground, would we need to go one stage further, and record the frequency of microhabitats such as rot-holes, cavities, fissures and dead wood on living trees? Point seven really falls within the ‘nice-to-know’ category rather than the ‘need-to-know’, at least in regard to insect conservation. However, all these points (and probably more) will need to be addressed before we can predict with any confidence what the long-term consequences of logging in tropical forests will be for this important and neglected group of organisms. In our own research in the Daintree lowlands, in the Wet Tropics of North-East Queensland, Australia, we have adopted two main approaches. First, a comparison of the saproxylic beetle fauna and stand structural and dead wood attributes of a series of old-growth, repeatedly logged and regrowth sudy sites; and second, a comparison of the saproxylic beetle fauna of logs and snags of different size-classes in a single old-growth study site. Sampling insects for the former approach has involved ground-level flight intercept traps, while for the latter we have used log emergence traps, dead trunk-mounted flight intercept traps, knockdown spraying and hand collection. These latter techniques have the advantage of being able to confirm the saproxylic associations of some of the species being caught in flight intercept traps. We anticipate that the resulting data-set (400 saproxylic beetle species so far) will enable us to investigate many of the issues on the research agenda outlined in this paper. We further intend to develop a sampling protocol for testing the validity of surrogate indicators, which will be tested under CIFOR’s research programme on criteria and indicators, in the lowland rainforests of Kalimantan, Indonesia, within the coming year.
two and three merit early consideration. A series of case studies, in which intensive sampling of saproxylic insects is carried out in old-growth and nearby comparable logged tropical forest, would give an early indication of whether the rest of the agenda was worth the effort. Studies should choose forest with as long a history of logging as possible, rather than a single recent logging event. This will increase the chances of detecting any differences, and will more closely reflect the management conditions in temperate regions that are known to have resulted in measurable changes to the saproxylic insect community. We assume that the same fate (repeated logging over many cycles) eventually awaits many natural tropical forests. We are currently conducting one such study in Australia, but further case studies will be needed elsewhere. One approach would be to choose forests differing in their intensity of management, from selective logging through various silvicultural ‘refinement’ practices to clear-felling (or the regrowth developing therefrom). Points four to six are critical, and in line with current thinking about the role of indicators in monitoring sustainable natural forest management (IUFRO, 1998). Given that there will be few situations in which the saproxylic insect fauna of a managed tropical forest will ever be known adequately, how can their requirements still be catered for? For instance, not only do we need to determine whether managing for ‘charismatic megafauna’ would automatically cater for saproxylic insects, but we also need to address the question of whether the absence of charismatic megafauna implies that a forest is no longer important for the conservation of saproxylic insects. This is certainly not the case in temperate countries such as Britain which have lost much of their original megafauna but retain forests with significant assemblages of saproxylic insects (Warren and Key, 1991). A more pertinent question, for the majority of production forests, is whether existing criteria and indicators of sustainable forest management can be adopted to cater for the needs of saproxylic insects. For instance, enumerating tree stand structure (e.g. diameter-class distributions) is already a standard foresters’ technique for monitoring the effects of silvicultural management, and forms the basis of standard growth and yield models. Research is needed to determine whether this could be sufficient for evaluating the condition of a forest with regard to saproxylic insects (i.e. does a permanent supply of overmature trees really matter for saproxylic insects, and if so, how many?). Or might we need information on other easyto-record surrogate measures that could be adopted for forestry monitoring and included in models, such as
Acknowledgements The authors gratefully acknowledge the valuable comments made on earlier drafts of this paper by M.C.D. Speight, J. Niemel¨a, M. Samways, T. New, R. Key, A. Fowles and an anonymous referee.
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