CONFLICT BETWEEN LOCAL AND GLOBAL CHANGES IN PLANT DIVERSITY THROUGH GEOLOGICAL TIME

Reliable measurement of the original diversity of paleo-vegetation mus t account for many influences: the amount of time and space represente d by the sample, the number of plant parts in the sample, the stature of the paleo-vegetation in, relation to sample area, and the preservat ional quality of the sample, as well as the number of species in the s ource community and their relative abundances. Plant compression fossi l assemblages from river or delta floodplain environments generally ha ve undergone minimal time and space averaging, which allows species ri chness to be measured and compared at a variety of temporal and spatia l scales. Comparison of late Paleozoic and early Cenozoic wet floodpla in. vegetation reveals little difference in species richness or divers ity at the smallest measurable spatial scale, based on samples of a Je w square meters representing 0.1-0.5 hectares of source vegetation. Ou tcrop transects several hundred meters to several kilometers long that sample across minor variations in wet floodplain habitat also show si milar levels of diversity in the Paleozoic and Cenozoic, with the exce ption of one Cenozoic transect with very high diversity. Diversity dif ferences between Paleozoic and Cenozoic sites and transects are far le ss than would be expected based on global species richness curves for the Devonian through Pleistocene. The small differences in the diversi ty of wet floodplain vegetation probably relate to several factors, in cluding the number of trees that can coexist in a small area, difficul t edaphic conditions on wet floodplains, and perhaps the geologically early colonization and saturation of these habitats. The difficulty of making reliable measures of plant diversity in the fossil record, and the absence of change observed with relatively high quality data, sug gest that biological explanations for changes in global species richne ss through geological time are premature.