The natural history, population dynamics, and community ecology of five small mammals from tropical deciduous and semideciduous (arroyo) forests were studied in western Mexico. I evaluated the influence of habitat heterogeneity and productivity on species diversity, population dynamics, and patterns of resource utilization. I expected higher biomass and species diversity and denser populations in the more complex and productive arroyo forest than in the deciduous forest. Species diversity was higher in the arroyo forest but total biomass, population density and fluctuations were very similar in both forests, despite strong differences in habitat heterogeneity and productivity. Reproduction of all species was associated to seasonality in food availability. Populations of all the species had qualitatively similar temporal patterns of reproduction and population fluctuations because they used similar food resources. Peaks in reproduction and population densities coincided with peaks in food production, suggesting that food availability is a limiting factor. Species differed in variables affecting resource utilization such as body mass, diet, and habitat selection. Results indicate that food resource partitioning and macro and microhabitat preferences permit coexistence. This study suggests that habitat heterogeneity and productivity have a profound influence in population and community ecology of small mammals in the Neotropics.