Stephen T. Jackson, University of Wyoming
Background/Question/Methods Climate-driven range expansion and contraction patterns respectively depend on local processes of colonization and extirpation. Paleoecology has necessarily focused more on the broad-scale patterns of change and less on the underlying processes. However, a number of detailed paleoecological studies offer revealing glimpses of biological processes underlying the biogeographic dynamics. These studies typically combine high spatial density of temporally and taxonomically detailed, spatially precise records with independent records of paleoclimate. Although they include a variety of paleoecological archives (lakes, peatlands, rodent middens, tree-rings) and span diverse parts of the globe, most are concentrated in the past few thousand years, and nearly all focus on range expansion rather than contraction.
Results/Conclusions Detailed paleoecological studies indicate that biology does matter in biogeographic range expansion. Propagule dispersal appears to have been limiting in some range expansions, although long-distance dispersal events (10s of km) are documented in several cases. Interactions among life-history traits, disturbances, and climate variability have regulated rates and patterns of expansion of woody species in both semiarid and temperate regions. These interactions have introduced transient to persistent legacy effects in biogeographic ranges. Positive feedbacks (via incumbency, propagule flux density, and population coalescence) are also implicated in several past range expansions, as are dampening effects of incumbent competitors. Genetic processes (e.g., drift, selection) are not well documented paleoecologically, but integration of modern genetic and ecological studies with paleoecological studies (including in some cases aDNA) can support assessment of genetic changes and processes.
Less is known, paleoecologically, about detailed processes underlying range contraction, owing to a combination of factors (including less scientific attention, fewer late Holocene contractions at broad scales, low taxonomic precision). However, a number of opportunities exist at local to regional scales for the late Holocene (tree-line shifts and other elevational contractions, regional fragmentations), and additional opportunities are available for the early Holocene and (especially) late-glacial periods. Studies to date indicate roles of fire and transient climate events in driving widespread mortality. Targeted studies would provide a basis for examination of processes that resist or delay extirpation (e.g., incumbency, ability to recolonize after widespread mortality, ability to shift habitats within a region) as well as positive feedbacks that can accelerate or ensure eventual extinction (e.g., fragmentation, loss of genetic diversity, Allee effects, reduced propagule flux).