Documenting Natural and Human-Caused Plant Invasions Using Paleoecological Methods

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Paleoecologists and ecologists are concerned with ecological dynamics, but typically at different temporal, spatial, and taxonomic scales. A key interest for both groups is documentation of species invasions and assessment of their causes and consequences. However, the termsinvasionandinvaderhave different connotations for each group owing to a differential in scale. Ecologists tend to view invasion as the introduction and expansion of a nonindigenous (NI) species within a region, typically occurring rapidly (10’ to 102years) against a backdrop of constant climate and moderate-to-intense landscape disturbance related to human activities (e.g., Elton 1958; Mooney and Drake 1986). In contrast, paleo-ecologists perceive invasions as spatial and population expansions of species related ultimately to secular changes in climate and other environmental factors. These “natural” invasions occur over longer time spans (102to 106years) and are typically detected at broad spatial scales (10’ to 104km) (e.g., Huntley and Webb 1988). Environmental changes, usually substantial at these time scales, mediate the ecological processes underlying the invasions (MacDonald 1993; Webb 1986).