Effects of contagious distributions of parasitoid eggs per host and of sampling vagaries on Nicholson’s area of discovery

Summary

Hassell andVarley described a negative relationship between the Nicholsonianarea of discovery and parasitoid density that permits coexistence of two or more parasitoid species and yields stability. They ascribed the relationship to “mutual interference” among searching parasitoid adults, leading to a “decreased efficiency”. This matter has recently been studied in more detail byHassell and coworkers. Negative relationships can occur for several reasons. In the present paper it is shown that the negative relation occurs under certain assumptions about the way parasitoid eggs are distributed among hosts. Several “contagious” (or “clustered”) distributions were studied. Those assuming variation in host susceptibility or accessibility (negative binomial and added zeros distributions) yielded negative relations. Those assuming variation in parasitoid aggressiveness and fecundity (Neyman Type A distributions) yielded variable results depending on how the distributions of parasitoids per host and of eggs per parasitoid were combined. It was found that, when analyzing actual data, sampling fluctuations can introduce strong negative relations either with or without “contagion”. In all instances, the relation betweenarea of discovery and parasitoid density is affected by host density and is, in general, not linear. The foregoing findings raise serious questions about thearea of discovery concept. An alternative approach is suggested.