Correlations with density have not been altogether absent from the approaches we have considered so far, and indeed, density dependence played a central role in our discussions of the determinants of abundance (birth, death and movement) in earlier chapters. Some studies, however, have focused much more fixedly on density dependences in their own right. In particular, many such studies have been designed to seek evidence for both direct and delayed density dependence (see Section 10.2.2). It is a problem, for example, that conventional life table analyses may fail to detect delayed density dependence simply because they are not designed to do so (Turchin, 1990). An analysis of population time series for 14 species of forest insects detected direct density dependence clearly in only five, but it revealed delayed density dependence in seven of the remaining nine (Turchin, 1990). It may be that a similar proportion of populations classified, from their life tables, as lacking density dependence are actually subject to the delayed density dependence of a natural enemy.
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