The ecological mechanisms hypothesized to structure species-rich communities range from strict local determinism to neutral ecological drift. We assessed the degree of ecological determinism in tropical plant community structure by analyses of published demographic data; a broad range of spatial, historical, and environmental variables; and the distributions of 33 herbaceous species (plot size = 0.02 ha) and 61 woody species (plot size = 0.09 ha) among 350 plots in a 16-km² forest landscape (Barro Colorado Island, Panama). We found a strong degree of cross-landscape dominance by a subset of species whose identities were predictable from sapling survivorship rates under shade. Using canonical ordination we found that spatial and environmental-historical factors were of comparable importance for controlling within-landscape variability in species composition. Past land use had a strong. impact on species composition despite ceasing 100-200 years ago. Furthermore, edaphic-hydrological factors, tree fall gaps, and an edge effect all had unique impacts on species composition. Hence, ecological determinism was evident in terms of both cross-landscape dominance and within-landscape variability in species composition. However, at the latter scale, the large portion of the explained variance in species composition among plots uniquely attributed to spatial location pointed to an equally important role for neutral processes. [ABSTRACT FROM AUTHOR]