The effects of dispersion on frequency-dependent predation of polymorphic prey

Does prey dispersion affect frequency-dependent predation by wild birds?

“…In contrast, Gianino and Jones (1989) found apostatic selection at a density of 2 baits m A2 when prey were randomly distributed and anti-apostatic selection when aggregated (although it must be noted that their``aggregated'' dispersion was qualitatively dierent to ours and probably less realistic, consisting of a line of rare prey types along one side of their experimental plot). We suggest that the non-signi®cant eect of dispersion in our study resulted primarily from the way in which the small experimental plots and high prey densities in¯uenced prey perception by the avian foragers.…”

“…We suggest that the appropriate analysis is, as we have performed here, to treat sites as experimental units and perform a repeated measures ANOVA. Such an analysis would certainly reduce the P-values of Gianino and Jones (1989) results; indeed, it is worth noting that our data produces signi®cant eects of dispersion on frequency-dependence if individual foraging trials are treated as independent. On a broader scale, there is a risk of pseudoreplication in many analyses of selective predation carried out in the wild (including those previously performed by us).…”

“…We must also sound a note of caution with regards to the statistical analysis of Gianino and Jones (1989). They treated each foraging trial as independent, even though data were obtained from only two experimental sites, albeit over a period of 2 years.…”

“…Since the ®rst few prey encountered by a predator in a foraging period may aect subsequent selection decisions (Cook 1965) it is conceivable that any deviation from randomness could aect the strength and direction of selection. Gianino and Jones (1989) have demonstrated, using pastry baits at a density of 2 m A2 , that changes in dispersion can reverse the direction of frequency-dependent selectivity by vertebrate foragers. Clearly, it is necessary to understand the eects of dispersion on prey selection before any generalisations can be made from experiments with random distributions to real-world situations.…”

Reversible frequency-dependent predation of a puffer, Takifugu niphobles (Pisces: Tetraodontidae), related to spatial distribution of colour-polymorphic prey

“…The reversal of frequency dependence predicts that predators can generate poly‐ and monomorphism in the same prey population, although such natural populations have not been found. Reversed frequency dependence has also been observed when the spatial distribution of prey shifts between being random and aggregated (Gianino & Jones, 1989; Church, Jowers & Allen, 1997).…”