Analyzing maps of dispersal around a single focus

The spatial pattern of organisms may be used to characterize their dispersa l, quantify spread or estimate the point of introduction of an alien specie s. Their distribution may be represented by maps of individuals, or by coun ts or by presence/absence at known positions within a sampled area. The pro blems and relative merits of these different forms of data for spatial infe rence are discussed. Three datasets concerning dispersal from a single focus are analyzed: count s of aphids, Rhopalosiphum padi and Sitobion avenae, on barley plants, Hord eum vulgare, grown in experimental trays; mapped locations of couch grass, Elymus repens, tillers within plots of a field experiment; locations of sig htings of the lupin aphid, Macrosiphum albifrons, as it invaded Great Brita in between 1981 and 1984. A method for generating maps from counts is proposed to overcome problems c aused by recording imprecision. Several statistics are used to quantify dispersal and spatial pattern in th e experimental data and together provide a clear picture of the spatial pat tern observed; they enabled several effects of the experimental treatments to be identified. The value of the statistics are compared. Estimates of the source of the lupin aphid invasion are obtained using the backtracking methods of Ferry (1995b) and do not contradict previous sugges tions.