Ebook Perspectives in ecological theory and integrated pest management: Part 2

Continued part 1, part 2 of ebook Perspectives in ecological theory and integrated pest management provide readers with content about: conservation, biodiversity, and integrated pest management; ecological risks of biological control agents: impacts on IPM; ecology of natural enemies and genetically engineered host plants; modeling the dynamics of tritrophic population interactions;...

7 Conservation, biodiversity, and integrated pest management s d wratten, d f hochuli, g m gurr, j tylianakis and s l scarratt 7.1 Introduction Conservation biology has been described as a ‘‘mission oriented discipline’’ (Soule and Wilcox, 1980), while Samways (1994) goes further, describing it as a ‘‘crisis science’’, in recognition of the immediate and adverse impacts facing our biosphere Similarly, the development of integrated pest management (IPM) over the past 50 years has been driven by ‘‘real world’’ pressures, in this case, the failure of unilateral pesticidal management to provide effective control of pests A further similarity between the disciplines of conservation biology – the science of preserving biodiversity (Pullin, 2002) – and IPM is that both are ecological disciplines Despite this common ground, however, the two broad disciplines have developed in relative isolation from each other One reflection of this is that conservation biologists have tended to use ecological theory to a considerably greater extent than have those involved in IPM Indeed, the growth of the discipline of conservation biology and the conceptual framework that has developed around it is one of the most prominent advances in ecology in recent years (Caughley, 1994; Dobson et al., 1997) Recognizing the theoretical maturity of conservation biology (at least compared with IPM), one aim of this chapter is to explore the ‘‘common ground’’ of these disciplines, with the intention of identifying research directions that may further the discipline of IPM Associated with this objective, we aim to avoid the reductionist tendency of scientific exploration and consider the extent to which the objectives of IPM and conservation biology may be compatible in agricultural landscapes The need for such compatibility is acute Not only pests continue to cause severe losses to agricultural production 223 224 S D Wratten et al (Oerke et al., 1994), but so much of the Earth’s land surface is used for agriculture – approximately 35% (Gerard, 1995) – that conservation cannot be confined to reserves Increasingly it will need to be integrated with agriculture on farmlands ‘‘The struggle to maintain biodiversity is going to be won or lost in agricultural ecosystems’’ (McIntyre et al., 1992) Despite the background outlined above, the goals of conservation biology are sometimes seen as incompatible with many modern agricultural practices Hill et al (1995) tabulated 20 such practices with clearly identifiable ecological impacts Attempts to manage pests are responsible for some of these effects, including on- and off-farm impact of pesticides on non-target species, the sometimes over-use of tillage to control soil pests, as well as habitat destruction and fragmentation Thus, advances in IPM that reduce such impacts offer scope to make agroecosystems, and the habitats affected by them, better able to contribute to the conservation of biodiversity The objective of ecological sustainability has been incorporated into planning and management by many farmers and agencies (see Robertson and Harwood, 2001) This objective is not, however, always easy given the pressure to produce cheap, temporally reliable food (Passioura, 1999) This is reflected also in the ‘‘pesticide paradox’’ (Gurr et al., 1996), whereby consumers demand cosmetically perfect produce yet abhor the use of pesticides which, in many cases, are used to prevent such damage Reducing reliance on pesticides by widespread implementation of IPM is a critical objective for twenty-first century agriculture This chapter will explore the extent to which that objective is compatible with conservation of biodiversity in farming landscapes and also the lessons that can be drawn from the field of conservation biology By integrating issues in conservation biology, biodiversity, and IPM, we aim to identify some new directions for IPM research These will need to recognize the increasing significance of farmlands as the setting for conservation of animals and plants, many of which play no direct role in agricultural production but need to be conserved there because of loss of habitat elsewhere In doing this we address a gap in current research, linking ecological theory to managed systems (Brown et al., 2001), suggesting applications of conservation biology to the implementation of IPM We also identify the many parallels between applied ecological research contributing to conservation biology and that supporting IPM, arguing that practical integration of their conceptual frameworks and ultimate goals is an inevitable consequence of the pressures for conservation outcomes and ecologically sustainable production Conservation, biodiversity, and integrated pest management 7.2 Conservation biology and biodiversity The status of conservation biology among ecologists has grown immensely over the past decade (Caughley and Gunn, 1996; Burgman and Lindenmayer, 1998; New, 2000), despite a broadly perceived crisis in its rigor and direction in its formative years (Caughley, 1994) Goals for conservation biology are often set at multiple spatial, temporal, and biological scales, with the latter imposing a conceptual breadth through contributions from biologists from diverse backgrounds Conceptual advances attributed to the discipline revolve mainly around research on landscapes and genes (Burgman and Lindenmayer, 1998), although advocates for species-centered research and its effectiveness make a compelling case for not abandoning that approach (Caughley and Gunn, 1996) The goal of conservation biology is essentially simple: to preserve, through effective management, biodiversity (at all its scales, see below) while ensuring its long-term viability The emergence of conservation biology as a discipline has been driven by a recognition of the need for targeted research of highly specific applied questions, identified and often funded by those responsible for managing conservation issues Considerable effort has recently been directed at reconciling the gamut of system- and speciesspecific literature in conservation biology so that general themes can be identified, despite the difficulties of doing so unequivocally (Beck, 1997; Lawton, 1999) Generalizations in conservation biology focus on responses to perceived threats and impacts, appropriate mechanisms for the management of landscapes, species, and populations as well as methods for prioritizing conservation actions at all of its relevant scales (Burgman and Lindenmayer, 1998) The ongoing and active discourse about the strength and validity of the generalizations outlined below highlights the youth of the discipline, as well as the difficulty of making generalizations at multiple biological and spatial scales Nevertheless, the inherent scope of conservation biology dictates that general principles must operate across these scales to be effective Much current research focuses on the development of a greater understanding of mechanisms to aid in conservation of biodiversity at multiple scales and the testing of different tools to this Among the many arguments used to make the case for conserving biodiversity, the search for an understanding of the ecological roles of biodiversity and their significance in natural systems is one of the most prominent (Johnson et al., 1996; Chapin et al., 2000) This chapter differs from such studies in addressing agricultural systems 225 226 S D Wratten et al 7.3 Tenets and themes of conservation biology and biodiversity The uncertainty pervading attempts to generalize in conservation biology reinforces the need for unambiguous operational definitions in any critical assessment of conservation biology and biodiversity (Simberloff, 1988; Peters, 1995; Ghilarov, 1996; Lawton, 1999) In all cases below, the generalizations are scale-dependent and goal-dependent; the themes and conclusions presented reflect consensus despite considerable debate about the strength with which they are held 7.3.1 The theory of island biogeography One of the key pieces of theory to have influenced conservation biology is the theory of island biogeography (MacArthur and Wilson, 1967) This predicts that the numbers of species found on a given island is the product of the rates of immigration and extinction Thus, the numbers of species on small, remote islands are low because relatively few colonizers arrive and rates of extinction among established species are high Conversely, larger islands sited closer to large landmasses receive more colonizers and these are able to establish large populations, which by virtue of the types of factors outlined below, are less likely to become extinct within a given time period The theory of island biogeography was extended to conservation biology by Diamond (1975), who considered optimal-sized and shaped designs for nature reserves This exercise took the pragmatic view that even if nature reserves may, at the time of their creation, be distinct from the surrounding habitat only in terms of their declared anthropogenic status, they tend to become floristically and faunistically distinct over time as adjoining areas are subject to changes in use, such as conversion to farmland or housing Thus, nature reserves mimic islands, being defined by their size and level of remoteness from other areas with which they may exchange organisms They do, however, differ from islands in being subject to relatively severe edge effects arising from adverse impacts of neighboring land use practices, such as the agricultural ones outlined above One potential advantage of such reserves over true islands is that the level of isolation may be mitigated by connectivity (in the form of a vegetated corridor) or small ‘‘stepping stones’’ of habitat which, though sub-optimal, may suffice to allow movement of organisms between reserves separated by otherwise inhospitable habitats 7.3.2 Connectivity and metapopulations The importance of connectivity to invertebrates has been demonstrated in an eloquent study in which the moss covering stones was Conservation, biodiversity, and integrated pest management manipulated into patterns with differing levels of connectivity (Gilbert et al., 1998) Patterns ranged from undisturbed (control) to isolated patches and included broken or unbroken corridors as intermediate treatments Species richness declined with isolation and, of particular relevance for IPM, this effect was reflected for predatory species The importance of isolation will vary from species to species according to its vagility For species that are able to move readily between habitat patches in a highly disturbed environment, survival may be possible even if the size of individual patches is such that the chances of extinction within each is high The term ‘‘metapopulation’’ was coined by Levins (1969) (see Chapter 3) to describe such an arrangement The sustainability of such systems demands that the factors that may cause extinction not apply synchronously to all patches (Hanski, 1997) for this would not allow recolonization from occupied patches (Figure 7.1) Metapopulations are acknowledged to be important in the dynamics of natural enemy (e.g Bonsall and Hassell, 2000) and pest (e.g Jervis, 1997) systems 7.4 SLOSS and IPM The considerations sketched out above developed into a controversy over the optimal design of nature reserves that became known as the SLOSS (Single Large or Several Small) debate (Dobson and Rodriguez, 2001) From a biodiversity conservation perspective, the SLOSS debate essentially revolves Figure 7.1 Schematic representation of the metapopulation model in which a species is present in only some of the potentially suitable patches of habitat between which individuals are able to move (Adapted from Pullin, A S., 2002 Conservation Biology Cambridge University Press, Cambridge.) 227 228 S D Wratten et al around whether a single large reserve with robust populations is better than having the same total land area divided into several small reserves (Figure 7.2a) The latter scenario may support fewer species and the population of each may be at a relatively high risk of extinction but it can ‘‘capture’’ a wider range of habitat types and decreases the likelihood of extinction from a catastrophic event (such as fire or epidemic) which could affect a species confined to a single large reserve The relevance of the SLOSS debate has not, to the best of our knowledge, been considered from the perspective of IPM but at least some ecological studies indicate its relevance For vertebrate pests, which require relatively large areas, theory suggests scope to effect control by reducing the size of patches of suitable habitat (‘‘reserves’’) located in farming landscapes to below the size that would support a viable population This concept is supported by empirical data (Newmark, 1987) but the minimum sizes implied by this relationship are relatively large in relation to the typical size of remnant vegetation patches found in many agricultural landscapes Reducing the size of such habitat may also be incompatible with the broader objective of maintaining biodiversity in agricultural landscapes An alternative is to site vulnerable enterprises in locations most remote from habitat associated with risk An analysis of the Figure 7.2 Examples of reserve/refuge areas of varying shape classified ‘‘better’’ and ‘‘worse’’ according to predictions of the theory of island biogeography but which may be challenged by the ‘‘SLOSS debate’’ and demands of on-farm IPM (see text for explanation) (Adapted from Diamond, J M., 1975 The island dilemma: lessons of modern biogeographic studies for the design of natural reserves In: Biological Conservation, Volume Elsevier Science, Amsterdam pp 129–146.) Conservation, biodiversity, and integrated pest management factors affecting lynx predation on sheep in the French Jura, for example, showed the importance of forested areas (Stahl et al., 2002) Achieving adequate protection from vertebrate pest damage can, however, be difficult, especially for highly mobile species such as birds, because shortterm movement over large distances is possible, demanding the clearing of suitable habitat from large areas The same challenge applies also to some arthropod pest species that may undertake long-range dispersal (e.g Helicoverpa spp [Fitt, 1989]) The SLOSS debate is less relevant to considerations of species that are undesired, such as pests as considered above, than to desired species where conservation is the goal For the natural enemies of pests, conservation is a component of IPM for which there is growing interest (Barbosa, 1998; Pickett and Bugg, 1998) An important difference, however, between conservation biology and conservation of natural enemies in IPM is that the aim of the latter is to maximize the impact of predators and parasitoids on pests within crops, rather than simply conserving populations of natural enemies in separate reserves/refuges Many species of natural enemies have small body size and short individual longevity and some are flightless These factors demand that viable populations are maintained close to crops if they are to effect control of its pests The limited powers of dispersal of many natural enemies may also require a departure from the idealized, circular shaped reserve with minimal edge to area ratio Indeed, linear features with a high edge:area ratio (Figure 7.2b) may be demanded also by the practicalities of such features being accommodated on farms Ways in which this may be accomplished are further explored below in relation to two further aspects of conservation biology theory; the declining population and small population paradigms 7.5 Declining population and small population paradigms The likelihood of local (and ultimately global) extinctions of populations can be predicted by examining the factors causing their declines and assessing the extent to which their size is likely to contribute to their loss (Caughley, 1994; Caughley and Gunn, 1996) The ‘‘small population paradigm’’ highlights how stochastic factors affect the persistence of populations, whereas the ‘‘declining population paradigm’’ focuses on how deterministic factors influence their trajectories Caughley (1994) argues strongly that the small population paradigm has little to contribute to conservation biology in practice outside of a general conceptual underpinning, and that the effective diagnosis of declines and their treatment 229 230 S D Wratten et al through management ‘‘cures’’ (i.e the declining population paradigm) was the major mechanism by which conservation biologists could contribute to the conservation of species The declining population paradigm requires that we identify deterministic causes of declines (negative growth) in populations to identify systemic pressures on the persistence of populations The identification of key threats is critical to the effective management of the declining populations and relies on clear diagnoses of all factors contributing to the declines through experimental approaches and predictive modeling (Caughley and Gunn, 1996; Brook et al., 2000) In an agricultural system, for example, densities of natural enemy species may decline in response to factors such as use of monocultures, intensive tillage practices, use of fire, large field sizes, efficient weed control, and impoverished non-crop vegetation Some factors will cause obvious direct mortality of natural enemies while others may act in less immediate fashion by, for example, decreasing availability of key resources such as nectar, pollen, alternative prey or hosts, as well as habitats with moderated microclimates Assessments of declines in systems reveal a willingness to accept ‘‘intuitive, common sense’’ explanations for possible declines, even in the absence of data supporting them (Caughley and Gunn, 1996) For instance, management options for conserving the declining populations of the large blue butterfly, Maculinea arion, in the UK focused on apparent anthropogenic impacts at coarse scales (e.g grazing, collecting) rather than the complex obligate relationship the lycaenid butterfly had with ants (Elmes and Thomas, 1992) Inappropriate management responses contributed to the ongoing decline of populations and the ultimate extinction of the species in the UK Further case studies (outlined in Caughley and Gunn, 1996) highlight the importance of clear, experimental approaches to identifying factors that influence the trajectory of declining populations so that appropriate management actions can be taken This call for an increased emphasis on experimental approaches in conservation biology is made by numerous proponents (Thomas, 1996; Debinski and Holt, 2000) and has recently been echoed for conservation biological control (Gurr et al., 2003a) In the case of impacts of natural enemies of crop pests, there is good evidence that use of conservation biological control approaches as simple as providing a strip of nectar-producing flowers in the field margin can improve local densities (Landis et al., 2000) There are, however, examples of habitat manipulation attempts failing to bring about any increase in pest suppression or reduction in crop damage (Gurr et al., 2000) These illustrate the applicability to IPM of the need for experimental studies to identify the causes Conservation, biodiversity, and integrated pest management of declining populations, as discussed for conservation biology in the above paragraph In the case of parasitoids of Pseudaletia unipuncta in the USA, for example, studies showed that percent parasitism was significantly higher in ‘‘complex’’ landscapes containing woodlots than in a simple landscape but was unaffected by proximity to hedgerows at the field scale within each type of landscape (Marino and Landis, 1996) Work on a large spatial scale by Roland and Taylor (1997) has shown that the response of four species of parasitoid of the forest-tent caterpillar Malacosoma disstria to habitat fragmentation differs markedly in relation to parasitoid body size The species studied ranged in weight, an expression of their relative sizes, from 34 mg for Carcelia malacosomae, 41 mg for Patelloa pachypyga, 58 mg for Arachnidomyia aldrichi, and 68 mg for Leschenaultia exul The larger three species are considered most important in control of the host insect and it was for these species that areas with relatively large blocks of contiguous forests (212 to 850 meters square) were required for optimal performance Studies of this type offer clear scope to identify systems in which landscapelevel, as opposed to field-level, manipulation of the habitat is required to enhance IPM and, therefore, the extent to which a natural enemy taxon or crop systems is amenable to manipulation The risk of populations becoming extinct through a range of stochastic and deterministic processes increases immensely if they are small (Caughley and Gunn, 1996; Burgman and Lindenmayer, 1998; New, 2000) The range of mechanisms that increase the chance of extinction for small populations include random catastrophes, variation in environmental characteristics, and demographic accidents, all of which conspire to make small size one of the attributes that most threatens endangered populations Agricultural systems tend to have higher levels of disturbance than natural systems This takes the form of tillage; applications of insecticides, fungicides and herbicides; and harvest operations In the case of annual crops the latter can result in complete clearance of all vegetation from the field, though even mowing or fruit picking in hay and perennial fruit systems, respectively, may disrupt populations of natural enemies (e.g Hossain et al., 2002) 7.6 Population viability analysis (PVA) The importance of the factors explored above is highlighted in the development of approaches to population viability analysis (PVA) which predicts risks of extinction for populations using the gamut of factors to which populations are exposed (Brook, 2000; Brook et al., 2000) Although many options for PVA are now available, the process involves the 231 232 S D Wratten et al development of a deterministic model for describing the population biology of the species of interest using ecologically meaningful data (e.g age structure, predation, competition, density dependence) followed by the introduction of the effects of uncertainty, for demographic and environmental parameters (Burgman and Lindenmayer, 1998) The main use of PVA is to identify priorities for management and guide future research (Boyce, 1992) Though PVA has been applied to a variety of natural systems, its utility in agricultural IPM is yet to be explored It has been applied most often to mammals and plants, though Baguette et al (2000) have employed PVA to explore butterfly ecology Their study showed that even within a relatively discrete taxon (‘‘butterflies’’) there may be significant differences between species and that recommendations made on the basis of two of the species would not be expected to lead to the persistence of the third This has considerable, though yet to be explored, consequences for natural enemies Data from studies of re-introductions and translocations carried out for conservation purposes suggest that establishment is most successful when the numbers of animals released is large but the relationship is not strong (Fischer and Lindenmayer, 2000; Figure 7.3) In the case of biological control, it is generally accepted that releases of large numbers of individuals increases the probability of establishment (Gurr et al., 2000) with larger introductions avoiding Allee effects (Dobson and Rodriguez, 2001) resulting from dispersal and reduced mate finding (Hopper and Roush, 1993) Some experimental work has been done on optimal release numbers for Sericothrips staphylinus, Figure 7.3 Effect of release size on success (white) and failure (black) of reintroduction attempts (From: Pullin, A S., 2002 Conservation Biology Cambridge University Press, Cambridge.) ... Codling Moth Areawide Control of Fruit Flies and Other Insect Pests In Joint Proceedings of the International Conference on Area-Wide Control of Insect Pests, 28 May? ?2 June, 1998 and the Fifth International... J., Kean, J and Keller, M (20 03a) Providing plant foods for insect natural enemies in farming systems: balancing practicalities and theory In F L Wackers, p C J van Rijn and J Bruin (eds.), Plant-Derived... Genetics and Evolution San Diego: Academic Press pp 69–91 24 1 24 2 S D Wratten et al Hill, D., Andrews, J., Sotherton, N and Hawkins, J (1995) Farmlands In W J Sutherland and D A Hill (eds.), Managing