The previous section dealt with analyses directed at phases in the life cycle, but these often ascribe the effects occurring in particular phases to factors or processes - food, predation, etc. - known to operate during those phases. An alternative has been to study the role of particular factors in the determination of abundance directly, by relating the level or presence of the factor (the amount of food, the presence of predators) either to abundance itself or to population growth rate, which is obviously the proximate determinant of abundance. This mechanistic approach has the advantage of focusing clearly on the particular factor, but in so doing it is easy to lose sight of the relative importance of that factor compared to others.
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