a1 Field Museum of Natural History, Chicago, Illinois, USA
We sampled riparian frogs along 18 streams at eight localities in Borneo. At four of these sites we sampled during more than one year. Altogether 49 species were included in our study and total sample size was 13,249. We measured overlap in species occurrences and arrays of abundances within and among localities. Variation over the time span of our study was minor within communities. Overlaps between streams at a locality were generally higher than overlaps of pairs of streams from different localities. Environmental variation, particularly in stream width and gradient, had a clear effect on both intra-and inter-locality overlaps. Although rainfall varied between localities and within localities over time, that variation did not seem to affect overlaps among or within communities. Environmental factors did not account for all differences in overlaps between communities. Instead, regional processes, perhaps the timing of barriers or speciation events, appear to have been responsible for geographic restrictions of several species, leading to variation in overlap values.
(Accepted April 27 1993)