Life Science

Some Plants Are Still Making Fleshy Fruit to Attract Extinct Megafauna

Plants generally lack the ability to move to new locations and that presents some challenges when it comes to reproduction. Not only do they need to find ways of exchanging their genes, but they also need to disperse offspring away from the parents.

Plant parents growing in close contact with their children is undesirable. It creates a situation in which the parent and child plants have to compete for space, water, and nutrients; not to mention increasing the potential for inbreeding.

Proper gene exchange and offspring dispersal also help a species buffer against undesirable environmental conditions. Imagine if each plant species just clumped up in only one area. All it would take is a single forest fire or contagious blight and the whole species could go extinct at once.

The Avocado fruit, with its oversized seed, likely developed in response to foraging by megafauna that are no longer around
The Avocado fruit, with its oversized seed, likely developed in response to foraging by megafauna that are no longer around. (Image: Kjokkenutstyr CC BY-SA 4.0)

Many of the plant species we see today follow the same “animals, water, or wind” approach to meet this need for desirable breeding and dispersal. Think bees pollinating flowers or pine pollen clouds sweeping through the forest.

For plants that have evolved to utilize the animal-dispersal niche, fleshy fruits are an outstanding solution. They’re sugary, typically bright and conspicuous, and they’re aromatic. These delicious seed vessels are eaten by animals that, in turn, carry the seeds in their gut until they pass them in their feces at a later time.

Like all organisms, the evolution of plants (and their fruit) is driven by environmental pressures. Many of those pressures come from the nearby animals. Berries, for instance, are believed to have developed their small sizes and generally black or reddish colors in response to the evolutionary pressures of bird preferences.

But what happens when the animal(s) that have a shaped a particular plant’s (or set of plants) seed dispersal mechanism go extinct? What happens when the birds that have driven the evolution of the size/shape/colors/taste of various berries, for instance, are gone?

Spain during the Miocene
The planet looked very different when mammalian megafauna were common, due in-part to their seed dispersal services. This is Spain during the Miocene. (Illustration: Oscar Sanisidro / Institut Català de Paleontologia Miquel Crusafont)

In some cases, the plant simply disappears as well, but in others, the seed-dispersal mechanisms developed by those plants are robust enough that the plant(s) persists long after its preferred seed-disperser is gone. Those adaptations, however, still have hallmarks of their dependence on the original seed dispersers; leading the plant/fruit to appear out-of-place under modern conditions.

This phenomenon, known as ecological anachronism, was first outlined by Dan Janzen and Paul Martin following their investigation of some rotting fruit in Costa Rica.

Janzen (an award-winning ecologist) noticed that the fruits of trees like Spondias mombin and Cassia grandis were poorly suited to the available seed-dispersal mechanisms available in their modern-day habitats. The plants were producing large fleshy fruit at great energetic expense. That fruit would often just fall to the ground where most were left to rot. Few animals seemed interested in eating them and their morphology didn’t line-up with the feeding behaviors of nearby fauna or with any abiotic process (e.g. water dispersal).

Janzen documented around 40 other such cases in which a tropical plant species seemed to be producing fruit that wasn’t really suited to any of the surrounding environmental conditions.

Working with paleontologist Paul Martin, Janzen hypothesized that these fruits had evolved alongside large extinct seed dispersing mammals like giant ground sloths and gomphotheres (gigantic, elephant-like mammals). Following the loss of these megafauna, the plants managed to persist because smaller animals like deer (as in the case of Spondia mombin) had picked up just enough of the seed dispersal slack.

Ground sloths and an American mastodon can be seen in this illustration by George Teichmann)
Ground sloths and an American mastodon can be seen in this illustration by George Teichmann)

Though Janzen’s theory was forged in the tropics, it has since been extended to other biomes in an effort to explain why some plants in other parts of the world produce ecologically out-of-place fruit.

Take the Osage orange, for instance. Now confined primarily to eastern Texas, this plant produces a large fleshy fruit that isn’t really eaten by any of the local wildlife. The only way this fruit typically disperses seeds is by fallen fruits managing to roll away from the base of the tree.

Evolution just wouldn’t select for such a poor seed dispersal mechanism, unless the fruit were some sort of anachronism. It’s probable that some large, extinct animal(s) that once existed alongside the Osage orange’s drove the evolution of these large fleshy fruits and the plant just hasn’t had adequate time to catch up to the change.

Trees are often very long-lived and assuming that the large animal in question only died out around 4,000-10,000 years ago, there hasn’t been a lot of time to produce the number of generations necessary for this plant to respond to the new evolutionary pressures. Were it not for humans, who transplanted the tree species in various parts of the country due to their utility as living fences, the plant might very well have gone the way of the megafauna they once depended on.

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But you don’t need to travel to Texas or the rainforest to see an example of this phenomenon first-hand. Commonly-encountered fruits like the avocado (seeds too large for most local animals to swallow), certain types of squash (few animals eat them in their natural range), and European holly have also been advanced as examples of plants suffering from this post-megafauna syndrome.

These ghosts of evolution are all around us and it’s important to keep in mind that components of the environment aren’t always on the same page.

The natural world is in a constant state of flux and if something seems out of place, it may very well be because of what isn’t there more than what is.

This post was updated from an earlier version published in April 2017.

References and Further Reading

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