We have a new paper on megafauna and human impacts on plants in Proceedings of the National Academy of Sciences this week. There are many New World plant species that produce large fruits – for example, avocados, cherimoya, or cacao, or some species of Pouteria and Inga. These species were originally dispersed (via consumption and subsequent excretion in another location) by megafaunal species that are now extinct – animals like ground sloths and gomphotheres that were hunted to extinction by humans after the last ice age. The large fruits and seeds of these species have traditionally been thought of as ‘ghosts of mutualisms past’ or ‘dispersal anachronisms’ – traits that reflected past adaptation, but that now prevent effective dispersal. It has been proposed that the slow decline of these plant species is an indirect consequence of human extirpation of megafauna, leading to further loss of ecosystem functioning.
The direct role of humans in this narrative has been unclear. Humans may have also modified the geographic ranges and genetic diversity of these plant species. In this study, we found that many of these plant species are not declining at all. Species that that were once consumed by megafauna, and that are now consumed by humans, have wider geographic and environmental ranges that those consumed by megafauna but not humans. This suggests that humans have actively taken the disperser role of the megafauna they earlier hunted to extinction, and have actively expanded the geographic and environmental ranges of a large number of plant species in the last ten to fifteen thousand years. Moreover, many plant species used only by megafauna are restricted to only one continent, while those also used by humans now occur on both continents.
Thus we are forced to accept a more complex role for humans in the New World – both driving the extinction of many animals, but also driving the spread of many plants. Many apparent dispersal anachronisms have actually turned out to be pre-adaptations to human use. The narrative of wild plant population decline may actually be better understood in terms of human promotion.
Moreover, the legacy of humans’ past use of plants for food has undoubtedly left a large legacy on the genetic diversity of each species. Better exploring the history and population genetics of each species may now help to improve and diversify the set of species that our species depends on for sustenance in a changing world.
The study was led by Maarten van Zonneveld at the World Vegetable Center, and I am grateful to have been able to make a few small contributions. You can read a copy at the PNAS website or download a PDF reprint.