DO PARASITES REDUCE THE CHANCES OF TRIANGULATION IN A REAL FOOD-WEB

At least ten different static patterns have been suggested by the anal ysis of food webs. However, the existence of many of these patterns ha s been questioned in recent years. One pattern that has remained immun e from such criticism is intervality. In this paper, we develop a cont inuous measure of niche overlap, by calculating the number of non-tria ngulated quads (the simplest structure that confounds intervality) in a web. In contrast to intervality, this measure is applicable to webs of any size. We then explore the implications for niche overlap of inc luding parasite - host links in a real food web, that of the Ythan est uary, Aberdeenshire. increasing the number of parasite-host links incr eases the number of non-triangulated quads in a web. This increase is greater than that predicted by the cascade model. One explanation for the high incidence of intervality in real food webs is that species ar e ordered into a hierarchy dependent on body size. Parasites should ob viate this ordering, since they are smaller than their hosts. Splittin g the parasite species into 'trophospecies' according: to their separa te life history stages reduces the number of non-triangulated quads. T his indicates that parasites do not reduce the chances of intervality (or increase the number of non-triangulated quads) merely because of t heir smaller body sizes, but because of their complex life cycles.