RANDOMNESS AND AGGREGATION - ANALYSIS OF DISPERSION IN AN EPIPHYTIC CHIRONOMID COMMUNITY

1. Patterns of dispersion in a chironomid community on the submersed m acrophyte Myriophyllum spicatum were analysed. 2. Random dispersion co mmonly occurred throughout the year, with an average of 40% of all spe cies being observed with random spatial patterns. The frequency of occ asions with random dispersion varied among chironomid species, ranging from 3.3% in Rheotanytarsus curtistylus to 56% in Thienemanniella maj uscula. 3. Estimates of the negative binomial parameter k show that 26 % of all cases demonstrate strong aggregation (0 < k < 1.0) while near ly half (47%) have quasi-random dispersion. Interspecific variation in k was not significant statistically when all the species were conside red together, although a pairwise comparison between two abundant spec ies Tvetenia calvescens and Rheotanytarsus curtistylus demonstrated a marginally significant difference. When different instars were compare d, the percentage frequency of strong aggregation (0 < k < 1.0) declin ed from first instars (49%) to later instars (II-38%, III-24% and IV-2 7%). 4. Variance/mean and m-m regressions (lit is Lloyd's mean crowdin g statistic and m is the sample mean) both fitted the data well, but t here was little indication of significant interspecific variation in p arameter values, particularly the slope of regression. 5. Dispersion p atterns were examined along with the analysis of spatial overlap in th is community. Forty-two per cent of species-pairs with reduced spatial overlap (spatially 'segregated' pairs) contained one or both species with random dispersion, while the corresponding value for spatially un segregated pairs was 57%. This suggests that spatial segregation is no t necessarily caused by strong, independent aggregation of both specie s. Comparing spatially segregated vs. unsegregated pairs, the former t end to have one species with a stronger tendency of aggregation than s pecies of the latter. 6. Patterns of dispersion observed were consider ed in the Light of 'random patch formation'. Random patch formation em phasizes the stochasticity of patch-forming processes as well as the s tochastically dynamic nature of resultant patches. Unlike terrestrial drosophilid assemblages, where strong aggregation is a predominant pat tern, this chironomid community demonstrates widely varying degrees of dispersion with high occurrence of randomness, reflecting the stochas ticity of dispersal and recolonization processes. It is suggested that , in terms of species coexistence, more emphasis should be placed on s tochasticity rather than on aggregation in this type of community.