Nature

Wild About Ecological Anachronism

Mammoths and other Pleistocene megafauna went extinct thousands of years ago, but if you look closely, you can see their ghosts. 

Within an ecosystem, many plant and animal species form symbiotic relationships where each partner provides a benefit to the other. A bee visits a flower and receives nectar and pollen in exchange for pollination. A squirrel buries an acorn for the winter, dispersing the oak tree’s seeds and giving itself a tasty snack. These relationships are everywhere in the natural world and are known as mutualism

What happens when your partner species goes extinct?

Many of the symbiotic relationships between wild plants and animals are related to either pollination or seed dispersal. For plant species who rely on now-extinct animals to spread their seeds, extinction of their animal partner might not cause their own death, but it does make life more difficult. Which brings us back to the mammoths. 

Have you ever thought about avocados? Avocados have a tough, leathery skin, rich high-fat flesh, and a single large seed. This combination of traits means that avocados are unlikely to be dispersed by birds or even by herbivores such as deer, who would not be able to swallow the large avocado pits. Animals who would have been likely to eat whole avocados? Ice-age megafauna, and specifically, giant ground sloths. The development of features such as large seeds and tough skin that were selected for an animal partner that has now been lost is known as an ecological anachronism. 

Avocados hang from the branches of a tree.
Avocados are the perfect snack for giant ground sloths. B.navez, CC BY-SA 3.0, via Wikimedia Commons

Avocados are only one example of a plant species that has lost its seed dispersal partner. The Osage-orange (which is not actually an orange) produces a hard, baseball-sized fruit that today is only found in the Red River valley. While scientists are still searching for definitive evidence, it’s likely this tree was spread by mammoths or ice-age horses. Without those partners, the Osage-orange is constrained to a much smaller geographic area. 

In other cases, plants lose their pollinators. Species that have co-evolved to have a mutual relationship with a very specific animal partner, are left with difficult options: go extinct themselves, or adapt to a method of self-pollination. Many species of orchids are exquisitely evolved to be pollinated by a single insect. When that insect goes extinct, such as with the bee orchid (Orchis apifera), self-pollination is the only option for survival. 

A bee orchid is shaped and colored to attract a specific, now extinct, species of bee.
After losing their pollinators, bee orchids are forced to self-pollinate. Björn S., CC BY-SA 3.0, via Wikimedia Commons

 Not all plant species evolve mutualistic relationships for seed dispersal or pollination. Others rely on natural forces. Lodgepole pines need the heat of a fire to open their cones and release their seeds. Those fires need to happen regularly and also to not burn at such high temperatures that the cones are destroyed. Cottonwood trees grow on river floodplains, but their seedlings require nutrients from regular flooding to grow. Even without an animal partner, these mechanisms are also increasingly likely to be disrupted as fires are suppressed and flooding is managed. 

For many of these species, humans are the factor that determines the success of a plant species whose pollination or seed dispersal has been disrupted. Avocados flourish as an agricultural crop, while Osage-oranges, with their unpalatable fruit, do not. Growing avocados is relatively easy compared to replacing animal pollinators with human technologies. 35% of our food crops are pollinated by insects and insects are in trouble

A bright green bee pollinates a pink flower.
Insects do the heavy lifting of pollination. Bob Peterson, CC BY-SA 2.0, via Wikimedia Commons

Avocados and Osage-oranges hold the memories of mammoths and giant ground sloths. They still produce fruit and other defenses (such as thorns) that are adapted to animals that have been extinct for thousands of years. Because the pace of evolution moves slowly, these plant species have not yet adapted to a new reality. 

This is why insects are so important, not only because they pollinate our food, but they maintain biodiversity and plants cannot adapt quickly enough to their sudden extinction. Save the bees and we’ll save grapes and nutmeg and kiwis and mangos, which is much better than being left with only their memories.

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