Consider the avocado: a large fruit, delicious, full of nutrition that is easily accessible with the merest pressure of the thumb against the ripe skin. A hard, round seed at the center poses little in the way of an obstacle to your enjoyment. Add a little cumin, lime juice, salt, and a finely diced tomato, and the compact, creamy flesh becomes a delicious, satisfying dip. It tastes so good, it should probably be bad for you, but it's packed with healthy fats, potassium and B vitamins.
Close your eyes. Can you taste it? That is the scrumptious, irresistible flavor of megafaunal dispersal syndrome. Pretty good, huh?
Plants, it turns out, must make trade-offs when it comes to reproduction (as with everything else), which means that a number of different strategies have evolved to make sure that enough seeds are dispersed to the appropriate locations and are able to make more plants, given the constraints of limiting resources and natural selection. One such trade-off is between seed size and seed dispersal; simply put, gravity tends to ensure that big things don't go very far, as a general rule. One way around this, however, is thought to be the evolution of megafaunal dispersal syndrome, wherein plants get big animals to move the big seeds around for them (Guimarães et al., 2008). That delicious avocado (that I now cannot stop thinking about) is thought to be an example of one such fruit.
The idea of megafaunal dispersal syndrome was first suggested by Dan Janzen, a tropical ecologist. He noticed that there were a number of large elephant-ear tree fruits in the deciduous forests of Costa Rica that just rotted on the forest floor-- unless cattle had been introduced, in which case the cows went about gobbling up as many of the fruits as they could find. Janzen called such fruits "neotropical anachronisms," hypothesizing that large, fleshy-fruited seeds had co-evolved with megafaunal dispersers that have-- in the Americas, at least-- been extinct for over twelve thousand years (Janzen & Martin 1982). In other words, as author Connie Barlow puts it, these incongruous fruits are "ghosts of evolution," living out of evolutionary context with their co-evolved dispersers (Barlow 2000). Many large-fruited trees have have other life history traits that are handy around large herbivores. Some, like honey locust, have large thorns or other defenses, presumably to make sure that the herbivores don't do more harm than good. "Come closer," say the trees, beckoning with their tasty seed pods. "But not too close." Others, like osage orange, compensate for overzealous herbivores with clonal or apomictic reproductive strategies.
How are plants faring in the absence of their extinct dispersers? In the case of the avocado and other cultivated fruits, there's obviously little to worry about. Humans, it turns out, make pretty good megafaunal dispersers, too. For breadfruit, for example, humans have been an effective disperser, selecting particular varieties and transporting them across the islands of Oceania for many millennia (Zerega, et al. 2006). But what about the osage orange, the honey locust, the pawpaw, or the elephant ear tree, which Janzen observed in Costa Rica? Did they once taste as delicious to the ground sloths, horses, and gomphotheres (basically cousins of elephants) that once roamed the Americas as the avocado does to me? In the absence of modern dispersers, scientists have speculated that large-fruited plants may be struggling, living in greatly restricted ranges compared to where they could grow if the mastodons and other megafauna were still eating-- and dispersing-- their seeds today. Osage orange's name comes from the fact that its range fell within the territory of the Osage people, who purportedly controlled its trade for the use of bows. Following European colonization of North America, the tree's range expanded rapidly to new regions, suggesting it had previously been dispersal-limited (Barlow, 2000).
What are the consequences of being an ecological anachronism? Are such species on the road to extinction, barring intervention? Did "thieving rodents" or other herbivores take up the slack (Jansen et al., 2012)? Megafaunal-dispersed plants are a high-profile example of the consequences of the Pleistocene extinctions, but so far there have been surprisingly few studies to test what have been, thus far, very compelling just-so stories. One of the projects I'm developing for my postdoc is to assess whether plants with extinct dispersers have more restricted ranges than we'd expect given their climatic tolerances, and whether such plants are at greater risk of climate change if they have trouble keeping up with their habitats in the future without a little help. As a paleoecologist interested in plant-herbivore interactions, it's hard to touch the thorns of a honey locust and not think of the mastodon in the room, so to speak. If we can help keep the "fruits the gomphotheres ate" alive, maybe it will help us to remember the gomphotheres, too.
Barlow, Connie. 2000. The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and other Ecological Anachronisms. Basic Books, New York, NY.
Guimarães, Paulo R., et al. 2008. Seed Dispersal Anachronisms: Rethinking the Fruits Extinct Megafauna Ate. PLoS ONE 3(3): e1745.
Janzen, Daniel & Paul S. Martin. 1982. Neotropical anachronisms: The fruits the gomphotheres ate. Science 215: 19-27
Jansen, Patrick, et al. 2012. Thieving rodents as substitute dispersers of megafaunal seeds. PNAS 109: 12610-12615
Zerega, N.J.C., D. Ragone, and T.J. Motley. 2006. Genetic diversity and origins of domesticated breadfruit. In Darwin’s Harvest: New Approaches to the Origins, Evolution, and Conservation of Crops, ed. T.J. Motley, N.J.C. Zerega, and H.B. Cross. Columbia University Press, New York.