During the Pleistocene, North America was populated by a diverse array of megafaunal browsers and grazers. By the onset of the Holocene, thirty-three genera of these megaherbivores and their predators were extinct, as part of a global, time-transgressive extinction wave that was taxonomically restricted to mammals and highly size-selective, disproportionately targeting animals >50kg. While the cause of the late-Quaternary extinctions have been the subject of extensive research and debate, the ecological consequences of that event have received little attention. The influence of ice-age megaherbivores on forest community composition, structure and function has been largely unknown; previous attempts to correlate vegetation change with megafaunal declines have relied largely on cross-comparisons between fossil pollen records and megafaunal remains, which have been hampered by dating uncertainties and the poor spatiotemporal and taxonomic resolution of fossil data. New paleorecords using Sporormiella, a coprophilous fungus that produces spores preserved in lake sediments, allow for the direct interpretation of vegetation change in the context of local megaherbivore population declines.
We present several records from the Great Lakes region that suggest that keystone megaherbivores may have 1) altered ecosystem structure and function through the release of palatable hardwoods from herbivory pressure, and 2) contributed to widespread shifts in fire activity due to the build-up of landscape fuel loads. The megafaunal extinctions were coeval with the emergence of so-called "no-analogue" plant communities, vegetation assemblages that were compositionally unlike any found in North America today. In the Midwest, these communities were characterized by higher-than-modern abundances of boreal conifers (e.g. spruce and larch) coexisting with temperate deciduous taxa (e.g. ash, hop-hornbeam), as well as anomalously low abundances of other boreal types (pine, birch, and alder). Previous hypotheses for the no-analogue communities have focused on dissimilar climates during the Pleistocene-Holocene transition. We sampled several lakes from within the peak no-analogue region, and found that vegetation dissimilarity and charcoal influx increased immediately following the Sporormiella decline (~13,500 BP); vegetation change may thus have been a consequence, rather than a cause of local megafaunal decline, though the influence of climate on both the extinctions and no-analogue vegetation cannot be ruled out. Modern North American ecosystems are currently depauperate of megaherbivores; these findings suggest that the further extinction or (re)introduction of herbivores could have unexpected consequences to temperate deciduous forests, particularly in the context of changing climates.