It’s a mystery how many species of woodland herbs recolonized New England and Southern Canada following the end of the last Ice Age. Glaciers scoured away the topsoil, leaving no ungerminated seeds, and statistical models suggest that many species of woodland herbs could not have re-expanded their range as rapidly as they did. Canada wild ginger (Asarum canadensis) is an example of a woodland herb that recolonized deglaciated territory faster than models suggest is possible. Wild ginger relies on ants for dispersal. Ants carry the fleshy seeds to their nests where the fatty covering is consumed. This is known as myrmecochory. The seeds are then discarded and will eventually germinate. Wild ginger also spreads via roots. Scientists calculated how far this species could expand its range considering its dependence on ants and root growth. Ants can transport the seeds up to 35 meters in 1 season. They determined this species could only have expanded by 30 miles over 16,000 years. Instead, this species expanded its range by over 960 miles. Scientists are stumped, but I think I’ve figured out an obvious solution.
A colony of Canada wild ginger.
Range map of Canada wild ginger. It expanded its range into New England and southern Canada within the past 16,000 years from refugia south of the Ice Sheet.
Passenger pigeon range (before extinction). Note how closely the breeding range of this species corresponds with the range of Canada wild ginger. I hypothesize passenger pigeons spread wild ginger seeds in their dung, and that explains how wild ginger expanded its range so rapidly following the end of the last Ice Age.
Ants spread wild ginger seeds, but ant propagation does not explain how wild ginger expanded its range so rapidly into recently deglaciated regions.
A few explanations for the rapid recolonization of New England and southern Canada by wild ginger have been proposed. Perhaps, the seeds adhered to the hooves or fur of migrating mammals, such as caribou or bison; or a storm blew the seeds a great distance. Wild ginger refugia may have existed in nunataks–unglaciated territory, usually on elevated hills, that occurred within glaciers. However, I hypothesize an obvious solution to this mystery. It seems likely passenger pigeons (Ectopistes migratorius) fed upon wild ginger seeds and defecated the viable seeds at a much greater distance than ants carry them away. It may not be coincidence the area recolonized by wild ginger happened to closely correlate with the breeding range of this species. Armies of passenger pigeons used to forage on the forest floor in eastern North America devouring all the acorns, nuts, and seeds. I think wild ginger and other woodland herbs recolonized deglaciated North America thanks to passenger pigeons. I can’t figure out how to test this hypothesis. An experiment could be conducted to see if extant species of doves or pigeons actually will eat wild ginger seeds. The isotopic signature of wild ginger can’t be distinguished from other species passenger pigeon ate so we can’t get the answer by analyzing passenger pigeon specimens in museums. We could also see, if wild ginger seeds remain viable when they pass through a pigeon gut.
Wild ginger is not related to true ginger (Zingiber oficianale) but reportedly has a similar spicy aroma. Indians used the root as a seasoning and medicine, but it is a carcinogen, like tobacco–another cancer-causing plant Indians introduced to Western culture. Snakeroot oil is made by grinding up wild ginger roots and distilling the liquid. Modern medical scientists do not recommend its use.
Reference:
Cain, M.; H. Dumany, and A. Muir
“Seed Dispersal and the Holocene Migration of Woodland Herbs”
Ecological Monographs August 1998
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