Humans are very much a part of all ecosystems. Our activities sometimes motivate us to drive towards extinction the species we identify as pests, to kill individuals of species we harvest for food or fiber while ensuring the persistence of their populations, and to prevent the extinction of species we believe to be endangered. The desired outcomes are very different for pest controllers, harvest managers and conservation ecologists, but all need management strategies based on the theory of population dynamics. Because much of the tool kit developed to manage endangered species is based on the dynamics of individual populations, we dealt with species conservation in Chapter 7 at the end of the first section of the book, which considered the ecology of individual organisms and single species populations. Pest controllers and harvest managers, on the other hand, mostly have to deal explicitly with multispecies interactions, and their work must be informed by the theory concerning population interactions covered in the book's second section (Chapters 8-14). Pest control and harvest management are the topics of the present chapter.

The importance of pest control and harvest management has grown exponentially as the human population has increased (see Section 7.1) and each touches on a different aspect of 'sustainability'. To call an activity 'sustainable' means that it can be continued or repeated for the foreseeable future. Concern has arisen, therefore, precisely because so much human activity is clearly unsustainable. We cannot continue to use the same pesticides if increasing numbers of pests become resistant to them. We cannot (if we wish to have fish to eat in future) continue to remove fish from the sea faster than the remaining fish can replace their lost companions.

Sustainability has thus become one of the core concepts -perhaps the core concept - in an ever-broadening concern for the fate of the earth and the ecological communities that occupy it. In defining sustainability we used the words 'foreseeable future'. We did so because, when an activity is described as sustainable, it is on the basis of what is known at the time. But many factors remain unknown or unpredictable. Things may take a turn for the worse (as when adverse oceanographic conditions damage a fishery already threatened by overexploitation) or some unforeseen additional problem may be discovered (resistance may appear to some previously potent pesticide). On the other hand, technological advances may allow an activity to be sustained that previously seemed unsustainable (new types of pesticide may be discovered that are more finely targeted on the pest itself rather than innocent bystander species). However, there is a real danger that we observe the many technological and scientific advances that have been made in the past and act on the faith that there will always be a technological 'fix' to solve our present problems, too. Unsustainable practices cannot be accepted simply from faith that future advances will make them sustainable after all.

The recognition of the importance of sustainability as a unifying idea in applied ecology has grown gradually, but there is something to be said for the claim that sustainability really came of age in 1991. This was when the Ecological Society of America published 'The sustainable biosphere initiative: an ecological research agenda', a 'call-to-arms for all ecologists' with a list of 16 co-authors (Lubchenco et al., 1991). And in the same year, the World Conservation Union (IUCN), the United Nations Environment Programme and the World Wide Fund for Nature jointly published Caring for the Earth. A Strategy for Sustainable Living (IUCN/UNEP/WWF, 1991). The detailed contents of these documents are less important than their existence. They indicate a growing preoccupation with sustainability, shared by scientists, pressure groups and governments, and recognition that much of what we do is not sustainable. More recently, the emphasis has shifted from a purely ecological perspective to one that incorporates the social and economic conditions influencing sustainability

'sustainability' -an aim of both pest controllers and harvest managers

(Milner-Gulland & Mace, 1998) - this is sometimes referred to as the 'triple bottomline' of sustainability.

In this chapter we deal in turn with the application of population theory to the management of pests (Section 15.2) and harvests (Section 15.3). We have seen previously how the details of spatial structuring of populations can affect their dynamics (see Chapters 6 and 14). With this in mind, Section 15.4 presents examples of the application of a metapopulation perspective to pest control and harvest management.

We discussed in Chapter 7 how predicted global climate change is expected to affect species' distribution patterns. Such conclusions were based on the mapping of species' fundamental niches onto new global patterns of temperature and rainfall. We will not dwell on this phenomenon in the current chapter, but it should be noted that global change will also impact on population parameters, such as birth and death rates and the timing of breeding (e.g. Walther et al., 2002; Corn, 2003), with implications for the population dynamics of pest and harvested (and endangered) species.

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