Environmental determinism and neutrality in vegetation at millennial time scales

Questions At millennial scales, is the historic evolution of tropical plant communities the result of 'random walk'? Are presence and abundance of taxa independent of environmental variability? Location Lake Peten-Itza, Guatemala, Central American lowlands. Methods We use an 86000-yr-long pollen record to study the relative contribution of neutral mechanisms and environmental determinism in configuring vegetation in the Central American lowlands. By using variance ratios, we evaluated the role of neutral drift at community and functional group levels. Additionally, regression analyses were applied to explain persistence and abundance of individual taxa as a function of regional moisture availability and fire frequency. Results Environmental fluctuations drove vegetation dynamics in the Central American lowlands during the late Pleistocene. When the data set was stratified according to moisture availability and fire frequency, variance ratios further supported our findings. Regression analyses revealed that whereas species' presence was independent of both moisture availability and fire frequency, their abundance was closely associated with both. Conclusions Neutrality, probably expressed in the face of stressors, through factors such as the existence of microrefugia, survival of relictual individuals and phenotypic and phenologic plasticity, was probably crucial in preventing biodiversity losses. Climate, however, was the main driver of vegetation dynamics in the Yucatan Peninsula during the late Pleistocene.