Predators, Parasitoids and Population Stability

Abstract

One of the lessons from the analysis of populations in Chapter 7 was that natural enemies may sometimes cause density-dependent mortality of their prey and may be a major regulatory factor. This potential is one of the reasons why the role of such natural antagonists has been intensively investigated in the past 20 years or so. The other main reason is that there has been increasing interest in the biological control of pest and disease organisms, stimulated by the interrelated problems of increased pesticide costs, reduced rate of production of new agro-chemicals, increased pest and disease resistance to agro-chemicals, dangers of ‘pest trading’ and pest resurgence and concern about environmental disruption and pollution. The details of these applied aspects of ecology are beyond the scope of this book but are dealt with in Wratten and Watt (1984); however, Figures 9.1 and 9.2 and Table 9.1 illustrate some of the background to current concern about the farm ecosystem. However, in some countries, such as the UK, biocontrol attempts in the field are a rarity and lag far behind the theory. Elsewhere, when ecologists have initiated biocontrol programmes, selection of the most appropriate enemy has often not been based on clear ecological criteria, follow-up of the consequences of release has been poor and success rate has not been good (Table 9.2).