What do humans have in common with the dinosaurs that trampled through ancient forests? It turns out that both may have a surprising impact on the size of seeds in the fruits growing around them. When researchers mapped the evolution of seed sizes onto that of land animals, they observed that when land animals got bigger, so did fruit seeds—with a few outsize exceptions. A recent study in Palaeontology illustrates how, over the course of natural history, gigantic megafauna such as dinosaurs curbed the growth of seed sizes by physically altering the ecosystem, influencing forest light levels. Today that role may be filled by a much tinier species: humans.
The idea that land animals can alter their environments is “fairly straightforward and well substantiated in a variety of scales,” explains Clive G. Jones, an ecologist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., who was not involved in the new study. For instance, savanna elephants push down trees and tear at shrubs, transforming the plant landscape. But even this elephantine influence is minor in comparison to that of prehistoric creatures.
The researchers’ new model suggests dinosaurs caused a level of destruction that suppressed an evolutionary tendency for seeds to grow bigger, says study lead author Christopher E. Doughty, an earth system scientist at Northern Arizona University. Bigger seeds tend to attract bigger animals for dispersal and to sprout taller plants, Doughty explains; both factors can give plants better access to sunlight in crowded conditions. But this was generally not the case when there were “big lumbering dinosaurs knocking things down, opening up the environment” and thinning forests, Doughty says.
[Sign up for Today in Science, a free daily newsletter]
After dinosaurs went extinct, forest understories became about 20 percent darker. This change “reset the slate” for plants and other animals, Doughty says. And “during this time the canopy became more closed,” notes Brian Atkinson, a University of Kansas paleobotanist not involved in the study. This growth would have placed evolutionary pressure on seeds to get larger again, Atkinson says, which is also reflected in fossil data. Another dip in seed size occurred with the emergence of early giant mammals and persisted until they died out.
But even though we’re far from megafauna-sized, humans’ influence on forests—particularly via logging—resembles that of those long-extinct giants, Doughty says. If we continue at this rate, our effect on fruit seeds might someday rival that of dinosaurs.
Jones notes that humans influence plant life in many other ways as well. “Agriculture [is] one obvious example,” he says, along with “introducing exotic species, clearing forests to make suburbia, to make cities, and so on.”
That complexity is one reason it could be difficult for the model to predict future fruit seed sizes, Doughty says. Another important factor to consider is the rapid pace at which human technology tends to develop in realms such as farming. Although the model provides a good analytical comparison of forest density alterations by megafauna and by humans, developments such as agriculture mean “normal ecological rules don’t really apply anymore.”
