Sustainability is a core concept in an ever-broadening concern for the fate of the earth and the ecological communities that occupy it. In this chapter we deal with two key aspects of ecological management - the control of pests and the management of harvests of wild populations. Each depends on an understanding of population interactions (discussed in Chapters 8-14) and each has sustainability as a primary aim.
One might imagine that the goal of pest control is total eradication but this is generally restricted to cases where a new pest has invaded a region and a rapid effort is made to completely eliminate it. Usually, the aim is to reduce the pest population to a level at which it does not pay to achieve yet more control (the economic injury level or EIL). In this way, we can see that economics and sustainability are intimately tied together. When a pest population has reached a density at which it is causing economic injury, however, it is generally too late to start controlling it. More important, then, is the economic threshold (ET): the density of the pest at which action should be taken to prevent it reaching the EIL.
We describe the tool kit of chemical pesticides and herbicides. These are a key part of the armory of pest managers but they
Figure 15.20 (a) Schematic representation of a network of marine reserves (white) and fished areas (gray). The fraction of coastline in the reserves is c, the fraction of larvae produced in the reserves is F, and the fraction of larvae produced in reserves that are exported is 1 — F. (b) The combination of values of the fraction of coastline in reserves, c, and mean width of reserve (in units of mean dispersal distance) that yield a value of 0.35 for the fraction of larvae that are retained within reserves, F, along with similar combinations for other values of F. The arrow indicates the configuration that produces the maximum fishing yield outside the reserves. (After Hastings & Botsford, 2003.)
have to be used with care because of the possibility of 'target pest resurgence' (when treatment affects the pest's natural enemies more than the pest itself) and 'secondary pest outbreaks' (when natural enemies of 'potential' pests are strongly affected, allowing potential pests to become actual pests). Pests are also adept at evolving resistance to pesticides.
An alternative to chemical pesticides is biological manipulation of the natural enemies of the pests. Biological control may involve: (i) 'introduction', with the expectation of long-term persistence, of a natural enemy from another geographic area (often the one from where the pest originated); (ii) the manipulation of natural predators already present ('conservation biological control'); (iii) the periodic release of an agent that is unable to persist through the year but provides control for one or a few pest generations ('inoculation'); or (iv) the release of large numbers of enemies, which will not persist, to kill only those pests present at the time ('inundation', sometimes called, by analogy, biological pesticides). Biological control is by no means always environmentally friendly. Examples are coming to light where even carefully chosen and apparently successful introductions of biological control agents have impacted on nontarget species, both by affecting nontarget species related to the pest and by affecting other species that interact in food webs with the nontarget species.
Integrated pest management (IPM) is a practical philosophy of pest management that is ecologically based but uses all methods of control, including chemicals, when appropriate. It relies heavily on natural mortality factors such as weather and natural enemies.
Whenever, a natural population is exploited by harvesting there is a risk of overexploitation. But harvesters also want to avoid under-exploitation, where potential consumers are deprived and those who harvest are underemployed. Thus, as with many areas of applied ecology, there are important economic and sociopolitical perspectives to consider.
The concept of the maximum sustainable yield (MSY) has been a guiding principle in harvest management. We describe the different approaches to obtain an MSY - taking a fixed quota, regulating harvest effort, harvesting a constant proportion or allowing constant escapement - and we point out the shortcomings of each. More reliable approaches to sustainable harvesting are also discussed, including dynamic pool models (which recognize that all individuals in the harvested population are not equivalent and incorporate population structure into the population models) and approaches that explicitly incorporate economic factors (dealing with economically optimum yield, OEY, rather than simply MSY). We also note that no data are available for many of the world's fisheries, especially in developing areas of the world; in these cases, simple 'dataless' management principles may be the best that ecologists can propose.
Finally, many populations, including those of pests and harvested populations, exist in a heterogeneous environment, sometimes as metapopulations. Managers need to be aware of this possibility, for instance when determining which biological control agent to use in an agricultural landscape or when designing a network of 'no-take' zones as part of a fisheries management strategy.
Communities and / / Ecosystems
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