Individual variation in response to intraspecific competition: problems with inference from growth variation measures


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1. The intensity of intraspecific competition may influence the growth of differently sized individuals to different extents. Measures of among-individual variation in growth and body size are commonly applied to assess such effects. The measure chosen is then supposed to control for the effect of mean (also influenced by competition) on variance [e.g. coefficient of variation (CV)].

2. Although there is an appealing simplicity to this approach, interpretations of such data have some apparently unrecognised underlying complexities. Here we combine empirical data from salmonid fishes and simulations to illustrate this.

3. Simulations show that even if all individuals are equally affected by competition, CV of body mass will not be constant. In fact, the CV of body mass may either decrease or increase with increasing competition.

4. Using empirical data from growth experiments, where differences among replicates in mean growth was controlled by temperature rather than competitive intensity, we show that two salmonid fish species differ with respect to how SD growth scales with mean growth. Because this slope represents a possible null-hypothesis, and will rarely be known, observed relations between competitive intensity and SD or CV growth are unlikely to yield information regarding size-specific effects of competition.

5. Based on the empirical data, simulation results with respect to body mass variation showed that the relation between mean and CV final mass varied from positive to negative across species and scenarios.

6. Insights regarding size-specific competitive effects appear unlikely to be obtained from measures of variation in either growth or body size. If attempts to do so are made, a positive correlation between SD body size and measures of competitive intensity (e.g. population density) does suggest that smaller individuals are more influenced by increased competition. However, this will be an extremely conservative test, because the appropriate null-hypothesis is a negative correlation of unknown strength.

7. Future attempts to quantify relationships between body size and impacts of competitive intensity should employ more traditional analyses of variance on individual data (i.e. interactions between body size and competition on growth rates).