Applied issues with predators and predation: editor’s introduction
*Correspondence: Prof. S. J. Ormerod: Tel.: 01222 875871, Fax: 01222 874305, E-mail: email@example.com
- 1 The effects of predation are among the most pervasive in ecology. As parasitoids, parasites, grazers or top carnivores, predators have large influences on the distribution, density, dynamics and evolved traits of other organisms. Effects scale-up to influence community attributes such as species coexistence and ecosystems processes such as production or trophic cascades.
- 2 Increasingly, however, some of the largest predation issues fall clearly within the scope of applied ecology. They include instances where, due to their ecological attributes and trophic position: (i) predators are valuable to nature conservation, as biocontrol agents, as natural enemies, or as grazers used in rangeland or ecosystem management; (ii) natural or introduced predators are viewed negatively due to effects on conservation, agriculture, forestry, hunting or disease transmission; (iii) predators are affected by human activities such as resource exploitation, or from exposure to factors such as biomagnified pollutants and disturbance; (iv) predators are controversial because different groups view them as either desirable or undesirable.
- 3 In all these cases, ecologists have a pivotal rôle in facilitating appropriate management. For valued predators, this involves developing sufficient ecological understanding to optimize habitat, increase prey abundance or to reinforce, establish or reintroduce desirable species. For predators considered undesirable, management can involve direct control. In other cases, predation and its consequences can be mitigated by deterrent, exclusion, supplementary feeding, habitat management to favour prey, predator swamping, or by compensating losses financially. These latter strategies are often used where predators are themselves considered too valuable to remove or control.
- 4 This collection of seven papers illustrates many of these themes by examining contrasting aspects of the applied ecology of Eurasian lynx; by further probing the interaction between predatory birds and red grouse; by exploring the effects of weather on biocontrol; and by illustrating effects on plant species where grazing or seed predation play a dominant rôle.
- 5 A key lesson from these and other recent papers in the Journal of Applied Ecology is that the successful management of predators depends invariably on understanding adequately the exact ecological context in which predator–prey interactions take place and in which problems arise. With predator-related issues growing rather than diminishing, ecologists will need sufficient resources to maintain current research if they are to provide the understanding required to offer and evaluate sound management.
Effects stemming from predation – where one organism consumes another alive or shortly after killing it – are among the most pervasive in the whole of ecology. Whether they be parasitoids, parasites, grazers or top carnivores, predators have some of the strongest of all influences on the distribution, density and dynamics of other organisms. By their actions, predators not only affect their own quality and fitness, but ultimately shape anti-predatory mechanisms among their prey, such as coloration, chemical defence, morphology and anti-predatory behaviour. At the ecosystem level, predation has major effects on species coexistence, food-web character, trophic cascading and eventually major ecosystem attributes such as production. With all of these features such strong forces in ecosystems, it is unsurprising that predators have been a long-standing and traditional focus of basic ecological research.
More and more, however, the rôle of predation and the status of predators have become central themes in applied ecology: predators figure with pronounced regularity in all volumes of the Journal of Applied Ecology. Indeed, management problems and applied issues now provide some of the key developmental themes in the whole of predation research (Table 1). First, are those instances where predators are viewed as intrinsically valuable, for example to nature conservation, in biocontrol, as natural enemies or in ecosystem management. Secondly, and in contrast, are instances where natural or introduced predators are viewed as problem species due to their impacts on organisms important to conservation, agriculture, forestry or hunting. Thirdly are instances where predators are negatively affected by human activities, for example because their trophic position places them either directly or indirectly at risk from the exploitation of key resources such as fisheries, or from the effects of factors such as pollution and disturbance. Some, as a consequence, figure as valuable biological indicators. Lastly, and probably most controversially, are instances where a given species is perceived by different sectors as being either desirable or undesirable, for example where species with high conservation importance might also be responsible for impacts on valued resources.
In this special profile of seven papers, and through a brief review of some of our recent papers, the Journal of Applied Ecology draws attention to applied issues related to predator–prey interactions, and to the management responses examined or proposed by ecologists.
The right predators in the right places: predators viewed positively
Some of the most positive of all views of true predators arise in conservation biology, where their naturally scarce, often threatened and frequently charismatic status gives them particular value. Research on such species published in this Journal often aims to assess factors influencing distribution or dynamics, to understand habitat requirements (Suarez et al. 2000), to manage habitats optimally for predators (Palomares 2001), to augment the production of prey (Ausden, Sutherland & James 2001), and sometimes to re-introduce species back into areas from which they have been lost (Schadt et al. 2002). Large-scale work typical of the type we publish often shows how predators important to conservation can sometimes be accommodated – at least with sensitive management – in landscapes where productive land-use continues (Suarez et al. 2000; Palomares 2001). In other instances, there is no alternative but to safeguard important predators through adequately designated and adequately sized nature reserves. At least where predators are vertebrates, their large size, mobility and extensive home-range will ensure that they figure prominently in debates about protected areas (Schadt et al. 2002), and in landscape issues such as connectivity or habitat fragmentation (Clevanger, Chruscz & Gunson 2001). At contrasting scales, work published by applied ecologists illustrates how molecular approaches can have important bearing on applied ecological issues affecting predators; for example, in resolving the genetic attributes of wild cats at risk of outbreeding with feral domestic cats (Daniels et al. 2001).
BIOCONTROL TO RANGE MANAGEMENT
While predation beyond natural limits can have detrimental effects on valued resources (see below), predators and their interactions with prey can also be managed positively to substantial benefit in environmental systems. For example, in the sphere of pest control, where chemical interventions are increasingly problematic due to unwanted non-target effects, the use of predators as agents of biological control can offer management solutions that are widely considered to be both environmentally sound and sustainable. Many are still developmental (see References in Norris, Memmott & Lovell 2002), but there are already direct examples of predatory invertebrates from a range of feeding guilds released or reinforced specifically to control unwanted organisms. Several recent instances published in the Journal of Applied Ecology involve empirical studies or modelling exercises where either the targets of control (i.e. the ‘pest’) or the predatory control agent were introduced species either of animals (e.g. Kean & Barlow 2001; Sheppard, Smyth & Swirepik 2001) or plants (Heard & Winterton 2000; Rees & Hill 2001). As is so often the case with papers in this Journal, the optimization of management has been a clear goal, for example with respect to the timing or circumstances of release most likely to promote the establishment of control agents (Shea & Possingham 2000; Norris, Memmott & Lovell 2002). In other instances, the aim of biological control is the reinforcement of predation from natural invertebrate enemies. Investigations published recently have aimed to understand ways that prey communities might be managed to increase generalist predator numbers in agricultural lands (Bilde, Axelsen & Toft 2000; Harwood, Sunderland & Symondson 2001). Alternatively, investigations have aimed to augment the capacity of predators for feeding on key pests such as aphids which not only have top-down effects on plants, but can also have undesirable bottom-up effects on valued predators such as game birds (Beck & Toft 2000; Borg & Toft 2000). As with other issues in predation, the dynamics of natural enemies or biocontrol agents interact with other ecological factors such as habitat structure, nutrient status, weather or disturbance regimes that can modify both the responses of predators and their intended prey (Frampton et al. 2000; Heard & Winterton 2000; Lee, Menalled & Landis 2001; Rees & Hill 2001). Increasingly, also, the importance of explicit spatial patterns in predator activity and density has emerged as a potentially important feature in managing invertebrate predators as natural enemies (Thomas et al. 2001). Such patterns underlie both the frequency and intensity of encounters that predators have with prey taxa, but also reveal the importance of refuge habitats where natural enemies can avoid adverse impacts such as from pesticides.
In addition to the utility of predators in biological control, the use of grazing organisms as predators of living plants represents one of the most important aspects of the management of habitats such as agroecosystems, wetlands, rangelands or other grasslands. The Journal of Applied Ecology has published a special profile on ‘Grasslands, grazing and biodiversity’ (Watkinson & Ormerod 2001) and parallel themes have continued to attract research attention. These include continued attempts to assess the contrasting ecology and rôle of different types of grazing organisms in management (Palmer & Hester 2000; Menard et al. 2002), continued activity to predict how different plant species respond to grazing (Díaz, Noy-Meir & Cabido 2001; Vesk & Westoby 2001) and attempts to resolve the consequences for extinction, richness, production, biomass, important species, undesirable species or habitat condition in grazed lands (Humphrey & Patterson 2000; Bullock et al. 2001; Hunt, 2001; Jansen & Roberston 2001; Oba, Vetass & Stenseth 2001; Noy-Meir & Briske 2002). These studies reveal how conclusions often depend on the ecological context in which grazing takes place, for example with respect to natural disturbances such as fires or climate (Roques, O’Connor & Watkinson 2001).
The wrong predators in the wrong places?
EXOTIC SPECIES AND UNDESIRABLE PREDATION BY NATIVE SPECIES
Almost in direct contrast to the re-introduction, reinforcement or conservation of desirable predators are instances in which predators are sometimes perceived as having adverse effects on organisms considered valuable to nature conservation (Schneider 2001), hunting (Kenward et al. 2001) or other economic activity such as agriculture and forestry (Greentree et al. 2000; Nystrand & Granström 2000; Allen & Sparkes 2001; Harmer 2001; Stahl et al. 2001; Tourenq et al. 2001). Some predators are also considered undesirable because of possible rôles in the transmission of disease (Baker et al. 2001; G.C. Smith et al. 2001). The problems are particularly acute where exotic predators that have been introduced outside their normal range have dramatic and sometimes runaway effects on prey species (Rushton et al. 2000). In all these examples, a wide range of organisms from vertebrates to invertebrates is involved (Van den Berg & Soehardi 2000). Not surprisingly given their occasional non-desirability, the control of predators, or the mitigation of predation and its consequences, figure frequently in the work we publish. A whole hierarchy of strategies have been suggested or investigated and depend often on the exact circumstances in which problems arise. They include financial compensation to cover losses to predators (Stahl et al. 2001), managed exclusion of predators from small (Harmer 2001; Jackson 2001) or large areas (Allen & Sparkes 2001), deterrents such as guard dogs (Stahl et al. 2002), supplementary feeding to divert predatory pressure (Patterson & Fuchs 2001; Redpath et al. 2001), prey management to swamp predators at local scales (Kenward et al. 2001), prey reduction to reduce predator numbers (Thirgood et al. 2002), or habitat management to favour prey over predators (Kenward et al. 2001). Under the most extreme circumstances, particularly where predators are exotic, control measures include direct culling (Guillemette & Brousseau 2001), poisoning (Greentree et al. 2000; Allen & Sparkes 2001; Baker et al. 2001; G.C. Smith et al. 2001), or more stealthy and so far largely theoretical strategies such as purposefully released disease (Barlow 2000; Courchamp & Cornell 2000). With the impacts of introduced species likely to grow in future despite the best efforts of legislation (Manchester & Bullock 2000), addressing such issues will require large investment and sustained research effort. In addition, control measures are being increasingly scrutinized by those involved with animal welfare (Barlow 2000), while the sheer scale of investigations required to evaluate the benefits of predator control presents considerable challenge (Greentree et al. 2000). In all these instances, as recognized elsewhere in this short review, the effects of predation invariably interact with other natural or anthropogenic attributes of ecosystems such as habitat fragmentation (Rushton et al. 2000; Schneider 2001) and changes in land use (Whitfield et al. 2001). Predator and prey dynamics also present difficult problems in their own right, for example due to processes that might reflect density dependence or interactions between different geographical scales (Bosch et al. 2000; Frederiksen, Lebreton & Bregnballe 2001). Applied ecologists will therefore need substantial skill, ingenuity and resources if they are to successfully evaluate predator management problems that are often of great complexity (Norbury 2001). Probably in many instances, the best approaches will be management solutions that address key issues simultaneously, evaluating predator management alongside other major options such as landscape management either to favour prey (Rushton et al. 2000; Kenward et al. 2001; Schneider 2001) or predators (Palomares 2001).
Predators affected by human activity
While predator–prey interactions are sometimes used to great advantage in ecosystem management, in other instances the trophic position of predators means that they are at risk of adverse effects from human activities. Such effects can arise directly, where predators are sometimes at lethal risk if their feeding activities bring them into close contact with operations such as fisheries (Tuck et al. 2001). Probably more often the effects are indirect, for example because predators require the same resources that are depleted by human exploitation (Piersma et al. 2001; Stillman et al. 2001). The trophic position of predators also means they are vulnerable to the accumulation and biomagnification of pollutants with consequences that are increasingly well understood, for example with respect to the effects of xenobiotic and persistent substances on birds (Sibly, Newton & Walker 2000; Bustnes et al. 2001). Such vulnerability can include direct lethal effects, but also sublethal effects on attributes such as immune function which in turn might give rise to increased risk of disease or, in one case, increased parasite burden (Sagerup et al. 2000). This latter case was one of the few recently published in the Journal to focus on parasites, other than as biological control agents (Fenton et al. 2000), perhaps representing an under-subscribed area of work in applied ecology. As with parasite–host interactions, predator–prey interactions are also vulnerable to indirect disruption by pollutants. A recent example from the Journal of Applied Ecology showed how attacks by natural enemies on a leaf beetle were mediated at locations affected by air pollutants (Zvereva & Kozlov 2000).
Although management in these cases often seeks to reduce adverse ecological impacts on predators, the effects in their own right can be important in revealing or indicating management problems. Throughout applied ecology, the use of organisms as indicators is widespread (Hill et al. 2000; Davis et al. 2001). Predators can sometimes have an important function in this regard since their extensive ranges, long life span and trophic position integrate aspects of ecosystem quality over large spatio-temporal scales. For example, the biomagnification of contaminants ranging from radiocaesium to xenobiotic pesticides (Hessen et al. 2000) can show the transfer of contaminants through food webs and so reveal exposure to consumers. Such indicator functions are still evolving into novel areas, for example where the magnitude of predatory activity can illustrate disturbance (Gill, Norris & Sutherland 2001).
Predators at the centre of controversy: the right predators in the wrong places?
In a great many instances, issues connected with predators and predation are controversial. Predators can appear in guises that are revered by virtue of their conservation value, but in other cases are treated with suspicion or even vilified by virtue of their unwanted effects. Some of the greatest controversies are those instances where different user sectors view predators in opposing ways as beneficial or detrimental. Probably the clearest current examples are those where predators with intrinsic conservation importance, or species at the centre of animal welfare attention, might also be responsible for adverse effects on game and quarry species (Redpath et al. 2001), other economically valuable activities (Stahl et al. 2001), or on disease transmission (G.C. Smith et al. 2001). With some particularly large predators such as wolves, bears and cats now at the focus of conservation action, and other species such as birds of prey recovering after a recent history of persecution or the effects of pesticides, these controversial issues are being thrown into sharp public profile. Above all other cases, these are the instances where the knowledge, skill and independence of ecologists place them both at the heart of the search for sound management and at the centre of public attention.
A special profile: applied issues with predators and predation
Together, this group of seven papers illustrates and expands nearly all of these foregoing themes. First, the group includes two papers examining contrasting aspects of the applied ecology of Eurasian lynx Lynx lynx, a felid that has been receiving much attention recently in Europe as its numbers have increased, at least locally (Palomares 2001; Stahl et al. 2001). Schadt et al. (2002) provide a clear example in which the growing field of presence–absence modelling and spatially explicit ecology is brought to bear on a clear management problem – of identifying those landscapes in Germany most suitable for lynx reintroduction. In exemplifying a high-profile use of this modelling tool, and providing insight into the importance of scale, fragmentation and connectivity for large carnivores, this contribution represents a landmark study. Stahl et al. (2002), by contrast, turn their attention to resolving aspects of lynx predation on livestock as a central management problem in lynx conservation. They show how lynx attacks on sheep are related not to sheep density, but to habitat features that expose sheep to predation, and to a small number of individual lynx that specialize on livestock. Both these features allow clear management prescriptions that are expanded in the contribution from this group of authors who continue to offer world-class leadership in predator ecology. In a similar vein with respect to predators at the centre of debate, Thirgood et al. (2002) further probe the interactions between hen harriers Circus cyaneus, peregrine falcon Falco peregrinus and red grouse Lagopus lagopus. Having examined previously whether the supplementary feeding of harriers reduced predation on grouse (Redpath et al. 2001), here these leading workers turn their attention to assessing whether habitat management might attenuate predation effects, but find little evidence for any direct benefit. Instead these authors suggest, with considerable scope for controversy, that management should aim to reduce the abundance of raptor prey on grouse moors, thus reducing harrier abundance (see also A.A. Smith et al. 2001; Tharme et al. 2001). On these contributions, as with any other, the Journal of Applied Ecology is always eager to receive the further views or evidence of informed commentators as forum articles, reviews or standard papers.
Switching to predators in biocontrol, Norris, Memmott & Lovell (2002) explore the effects of weather on the gorse thrips Sericothrips staphylinus, crucially demonstrating how the probability of establishment is affected not only by release size, but also by rainfall pattern. Again, this work leads to clear management recommendations and illustrates the value of linking studies in controlled environments with high quality field data.
Finally in this special profile are three papers with botanical focus in which grazing or seed predation are central in studies using an innovative variety of field and experimental methods (Donlan, Tershy & Croll 2002; Meeson, Robertson & Jansen 2002; Noy-Meir & Briske 2002). In keeping with many previous papers, including those reviewed above, all three illustrate marked effects by grazers as predators of plant tissues. Respectively, they involve plant communities on arid islands, seedling survival of gum trees Eucalyptus camaldulensis in riparian floodplains, and population and fitness in wild wheat Triticum dicoccoides in Mediterranean grasslands. Despite these studies arising in contrasting environments, they illustrate one consistent result: that the activities of grazers play a dominant rôle in top-down effects on target plant species or communities. Each further reveals, however, that responses are mediated by wider ecological conditions such as climatic events, flood pattern, or by the exact nature of the grazer community and the timing and duration over which plant predation occurs. In this respect, they support the overall conclusion from this themed group of papers, and from all others reviewed here: that predators and predation are as much a major feature of applied ecology as of basic ecology. Their exact outcomes and successful management require ecologists to understand not only the direct predator–prey interactions involved, but also the context in which these interactions occur.
Perhaps most of all, these papers illustrate that any dichotomy between applied and fundamental ecology in understanding the effects of predators is almost entirely artificial. On the one hand, applied ecologists draw on basic ecological concepts in understanding the rôle of predation in management issues: food webs, predator–prey interaction, functional response, prey switching, prey defence. In many respects, the management of predation depends on profound basic understanding of predators and the systems they occupy. On the other, applied issues with predators offer some of the largest current challenges to basic ecology: conserving endangered predators in the wake of global change, controlling invasive or damaging predators, managing pests through biocontrol rather than chemical intervention, harnessing natural enemies. For these reasons, the Journal of Applied Ecology, through this special profile, draws attention to applied issues in predation whose sound management requires the attention of an adequately resourced ecological community.
My thanks are due to the other editors of the Journal of Applied Ecology for their support in editing the papers contributing to this special profile, and as always to Dr Gill Kerby and Penny Baker for their unceasing help in bringing these and all the other Journal papers to publication.