Guest post & graphics by Nitin Sekar, Princeton University
All around the planet, the world’s largest animal species are becoming very rare. Whether we speak of giant tortoises, large wild cows like gaur, rhinoceroses, hippos, tigers, or whales, local extinction or extreme reductions in range size and population are the norm for species that are too big or too slow-reproducing to withstand human conflict or the human hunter. Most people would probably agree that it is a shame to lose these species. But what about functionally? Are the largest animals critical to any ecological processes? When there are declines in honey bee populations, human agriculture suffers; if nitrogen fixing plants are absent from a forest, other plant species may be affected. When the largest animal species go missing, do any other species miss them? Aside from a few cases, we don’t know! This is an active area of research.
We decided to explore the role of Asia’s largest land mammal—the Asian elephant (Elephas maximus)—in seed dispersal, which helps maintain the diversity of tropical forests. Elephants are thought to be amazing seed dispersers. They can eat lots of fruit but digest only about 40% of what they ingest, leaving a lot of intact seeds. They can move many kilometers in a day, taking seeds to new, varied environments; this allows a parent plant to “put their eggs in different baskets,” making it less likely that a drought or abundance of seed predators in one area would kill all their offspring. Finally, elephants deposit seeds with a large dollop of fertilizer thought to be advantageous to the germinating seeds. Surely, some tree species would miss dispersal by elephants if Asian elephants went extinct?
To gauge whether Asian elephants are as important for seed dispersal as expected, we examined the dispersal ecology of the plant species most likely to be dependent on elephants: Dillenia indica. Known in Hindi and Bengali as the chalta tree, this tropical evergreen tree endemic to Asia seemed to be made for elephant dispersal. Its fruits are super hard, and it seemed as though only the largest animal species would be able to crunch down on them. Plus, scientists and villagers alike reported that elephants loved chowing down on chalta fruits. Of course, the chalta is an unusual fruit—most fruits are not so inaccessible to other frugivores (fruit-eating animals), and so can have multiple dispersers. We figured that if even the chalta turned out to not need elephants for dispersal, then Asian elephants weren’t likely to be critical for seed dispersal. In a sense, the chalta was to serve as a proof of concept, the concept being that a plant could be solely or primarily reliant on elephants for dispersal.
So we set out to measure how important elephants were compared to other frugivores in the dispersal of chalta. We first focused on fruit removal—in order for an animal to be important for seed dispersal, it has to remove seeds from the fruit and discard them away from the tree. So who removes the most chalta fruit? We spent over 100 hours staring at chalta fruit in the canopy (before they fell), and used camera traps over more than 10 months over two years to see what animals removed chalta fruit. The majority—about 56.6% of all fruit, and 63.3% of fruit taken from camera traps—were eaten by elephants! This was twice more than the next most-common frugivore, bovids (gaur, domestic cattle, and domestic buffalo). Chalta fruit were just too hard for most animals to eat. We saw many giant squirrels and monkeys struggling to bite through the fruit’s tough exterior, but usually failing. Some of the camera sequences even showed a cow trying to eat a fruit, giving up because it was too hard, and disappearing—then, later that night an elephant would make off with the same fruit!
Initially, then, one would expect that, without elephants, the chalta would be in trouble when it came to dispersal. However, we discovered the chalta had a back-up plan. If elephants don’t pick up the hard chalta fruit, they… transformed! It would grow brown and soft, and then suddenly all sorts of animals—macaques, small squirrels, even rats—could easily access the chalta’s seeds. They may not disperse the seeds as far, but at least the chalta’s seeds wouldn’t all be stuck in the hard fruit, where they may rot. We collected seeds from both the hard and softened fruits and showed that they both germinate, demonstrating that the soft fruit were not yet rotten.
So what does the chalta tell us about the importance of elephants for seed dispersal? Well, on the one hand, it suggests that our intuition was right—elephants must be pretty great dispersers if evolution would push a species to depend so heavily on just one animal species for seed dispersal! On the other hand, though, this research indicates why losing big species may not immediately seem to have knock-on effects on biodiversity. Since elephants are already big and rare, the chalta have a back-up plan ready if elephants don’t show up – they rely on smaller animals. So if elephants never show up (e.g, because they are locally extinct), the chalta may suffer, but not disappear quickly. If this is how other species that rely on large partners (like hippos or whales) also respond to their disappearance, it might make it difficult to spot the effects of even the largest creatures when they go missing – at least, for a short time. But eventually, sooner or later, such species will have to evolve other solutions or themselves vanish.
Sekar, N., & Sukumar, R. (2013). Waiting for Gajah: an elephant mutualist’s contingency plan for an endangered megafaunal disperser Journal of Ecology, 101 (6), 1379-1388 DOI: 10.1111/1365-2745.12157