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 gametes, they also need to develop a means to disperse their offspring. The apple, in this case, really does need to fall far from the tree.
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 have to deal with inbreeding. It also diminishes the species’ ability to survive environmental disruptions.
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 in one blow!
So, plants need to disperse seeds, but how do they do it? Many of the plant species we see today follow the same “animals, water, or wind” approach that they use for fertilization (think bees pollinating flowers or pine pollen clouds sweeping through the forest).
For plants which have evolved to take advantage of 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 some later time (and preferably at a location that isn’t right near where they found the fruit in the first place).
Like all organisms, the evolution of plants and their fruit is driven by environmental pressures and many of those pressures comes 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 imposed by bird preferences.
That’s all fantastic, 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?
In some cases, it’s logical to assume that the plant disappears as well, but in others the seed-dispersal mechanisms developed by those plants have a degree of robustness that allows the plant(s) to persist long after the “preferred seed-disperser(s)” no longer comes calling.
Those adaptations, however, still have hallmarks of their dependence on the original seed dispersers; leading the plant/fruit to appear out-of-place under current 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 fruits at great energetic expense and those fruits would often just fall to the ground where most them were left to rot. Few animals seemed interested in eating them and their morphology didn’t line-up with the feeding behaviors of the nearby birds and small mammals 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 weren’t really suited to any of the surrounding biological or physical environmental conditions.
Working with Paul Martin (a paleontologist), Janzen hypothesized that these fruits originally developed in response to seed-dispersal by large animals like giant ground sloths and gomphotheres (gigantic, elephant-like mammals) that have since gone extinct.
Following the loss of these megafauna seed-disperser(s), the plants managed to persist because smaller animals like deer (in the case of Spondia mombin) had picked-up just enough of the seed dispersal slack.
Though Janzen’s theory was formulated in the tropics, it has been extended to other biomes in an effort to explain why some plants in other parts of the world have been observed to produce ecologically ‘out-of-place’ fruits.
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 these fruit typically disperse 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 spurred the development 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.
But you don’t need to travel to Texas or the rainforest to see an example of this phenomenon first-hand. More 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 (fruits not protected by spines are out of reach of even the largest local browsing herbivores) have also been advanced as examples of plants suffering from this lost-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.
References and Further Reading
- Book: Ghosts of Evolution (every purchase helps support Corner of the Cabinet)
- Duan, Q., Goodale, E., & Quan, R. C. (2014). Bird fruit preferences match the frequency of fruit colours in tropical Asia. Scientific reports, 4, 5627. (Link)
- Guimarães Jr, P. R., Galetti, M., & Jordano, P. (2008). Seed dispersal anachronisms: rethinking the fruits extinct megafauna ate. PloS one, 3(3), e1745. (Link)
- Janzen, D. H., & Martin, P. S. (1982). Neotropical anachronisms: the fruits the gomphotheres ate. Science, 215(4528), 19-27. (PDF)
- Jansen, Patrick, et al. 2012. Thieving rodents as substitute dispersers of megafaunal seeds. PNAS 109: 12610-12615. (PDF)
- Rodríguez-Flórez, C. D., Rodríguez-Flórez, E. L., & Rodríguez, C. A. (2009). Revision of Pleistocenic Gomphotheriidae Fauna in Colombia and case report in the Department of Valle Del Cauca. Boletín Científico. Centro de Museos. Museo de Historia Natural, 13(2), 78-85. (Link [Spanish])