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 cadd is larva (Rhyacophila obliterata) and a stonefly nymph (Isoperla gramm atica), in 1-m(2) sampling windows in relation to their major prey, la rval black flies (mostly Simulium sublacustre), which were quantified within the same areas by extensive systematic sampling. In mid-June, w hen black flies were abundant and highly clustered, R. obliterata was also aggregated on a scale of 5-10 cm. After the emergence of black fl ies in the beginning of July, the point pattern of the rhyacophilid wa s random. Isoperla grammatica was abundant only on the earlier samplin g occasion, at which time it was randomly distributed. We also analyse d spatial association between the two predators and found a weak indic ation of small-scale inhibition between them. In the range of current speeds of 40-60 cm/s, microhabitat overlap between the caddisfly and b lack fly was intensive, leading to high consumption of this semi-sessi le prey. The second-order methods used proved powerful in detecting sm all-scale spatial patterns and could be more widely used by ecologists in place of classical single- statistic indices whenever detailed map ping of individuals is feasible.