Abundance and dominance become less predictable as species richness decreases

Aim To test the hypothesis that communities with higher diversity have more predictable properties by examining patterns of community structure along a species richness gradient. Location Trinidad and Tobago (11 degrees 00 N, 61 degrees 00 W), on the South American continental shelf, opposite the Orinoco River delta, north-east Venezuela. Methods We used quantile regressions to investigate how three total abundance, absolute and relative dominance measures - numerical abundance, biomass and energy use, respectively - change across a species richness gradient. We investigated which allocation rule best mimics community assembly in this species richness gradient by examining the abundance of the dominant species and comparing it with predictions of niche apportionment models. Results All measures of total abundance increase on average across the gradient, but the upper limit remains constant. On average, absolute dominance is constant, but the distance between the upper and lower limits decreases along the gradient. Relative dominance decreases with species richness. Observed dominance patterns are best described by Tokeshi's random fraction model. Main conclusions Our results show that both total abundance and absolute dominance become increasingly variable as biodiversity decreases. Consequently, our study suggests that ecosystem properties are less predictable when biodiversity is lower.