The links among aboveground and belowground biotic communities are believed to be multiple, reciprocal, and specific, but the patterns of these interactions are not well-established in space and lime. To better understand aboveground-belowground interactions, terrestrial plant and soil protozoan (testate amoeba) communities were surveyed along a proglacial chronosequence in Kenai Fjords, Alaska. Unlike many comparisons among functionally different groups, the diversity patterns and the communities' response to site variables shown in this study are remarkably similar for both plants and testate amoebae. Tight correlations between these groups suggest that, contrary to traditional successional theory, biotic interactions may influence ecosystem development more than changes in abiotic site conditions. The apparent importance of interactions between aboveground and belowground biota in this boreal-maritime ecosystem highlights the controlling role of soil organisms in ecosystem development and supports the correlation of diversity patterns for different groups across extended spatial and temporal gradients.