DETECTING SMALL-SCALE SPATIAL PATTERNS IN LOTIC PREDATOR-PREY RELATIONSHIPS - STATISTICAL-METHODS AND A CASE-STUDY

We mapped the exact locations of two predaceous stream insects, a caddis larva (Rhyacophila obliterata) and a stonefly nymph (Isoperla grammatica), in 1-m(2) sampling windows in relation to their major prey, larval black flies (mostly Simulium sublacustre), which were quantified within the same areas by extensive systematic sampling. In mid-June, when black flies were abundant and highly clustered, R. obliterata was also aggregated on a scale of 5-10 cm. After the emergence of black flies in the beginning of July, the point pattern of the rhyacophilid was random. Isoperla grammatica was abundant only on the earlier sampling occasion, at which time it was randomly distributed. We also analysed spatial association between the two predators and found a weak indication of small-scale inhibition between them. In the range of current speeds of 40-60 cm/s, microhabitat overlap between the caddisfly and black fly was intensive, leading to high consumption of this semi-sessile prey. The second-order methods used proved powerful in detecting small-scale spatial patterns and could be more widely used by ecologists in place of classical single- statistic indices whenever detailed mapping of individuals is feasible.