[Assessing the relative strength of the effects of food resources and predators on a population: a review of methods].

Understanding mechanisms that underlie species' distribution and abundance is one of the key problems in population ecology. Inorder to tackle this problem, it is important to assess the relative strength of the effects of food and predator (consumer) on a focal population. In this study we have analysed advantages and disadvantages of the basic methods that are used to quantify the relative strength of the two types of effects. These methods can be divided into two groups. In the first group we put the search for examples that are consistent with a proposed hypothesis, assessment of correlations of abundance on adjacent trophic levels and biomanipulations. Common for these methods is that they assume the existence of only one type of effects--either bottom-up or top-down. Methods of the second group assume simultaneous presence of both types of effects and are aimed at quantifying their relative strength. In this group we put factorial design experiments and population-dynamics approach (analysis of population growth, death and birth rates). It is shown that due to the constraints of each of the methods of the second group none of them can be considered universal. However, their joint application can be a promising approach to the assessment of the mechanisms that drive population abundance variability both in experimental and field studies.