Imagine you’re a tree. You’re rooted to the ground and can’t move. You have a crown full of seeds ready to usher in the next generation of your species, but all you can do is drop them straight down, where they are doomed by the shade of your own crown, not to mention the predators and parasites attracted to this bounty of seeds. Meanwhile, all these animals are scurrying and flying around you, flaunting their mobility. You’ve got a serious case of movement envy. You need to move your seeds as far away as possible to give them a chance to survive and germinate. You need mobility assistance. Maybe the animals can help.
Animals do help. Many trees have co-evolved relationships with animals and offer them some reward for moving their seeds around – a bit of nutritious fruit-bribe surrounding the seeds. But some animals are not helpful, eating the fruit without moving the seed away from the mother tree, or even consuming the seed along with the fruit. Trees don’t appreciate these cheaters, but evolution’s rule book allows it. The trees can fight back by co-evolving with certain species of animals that are the most beneficial to them, and putting up defenses against the others. When evaluating these relationships biologists consider the ‘disperser effectiveness’ as the value of an animal species as a disperser for a given tree species.
Palm trees have been bribing animals and fighting cheaters for millennia, evolving a variety of sweet fruit lures surrounding hard nuts. The trees “want” a cooperative animal to eat the fruit and move the seed away from the mother tree. Our earlier work using cameras to monitor animals feeding under fruiting Astrocaryum palms trees found Agoutis, Spiny Rats, Red-tailed Squirrels, and Collard Peccaries to make up 99% of the visitors. The next question was to evaluate the disperser effectiveness for this group. Lieneke Bakker took this challenge on as her MS thesis project at Wageningen University.
Lieneke used small radio-transmitters to follow the movement and fate of Astrocaryum seeds, and used camera traps to determine which species of animal was doing the moving. She considered a number of factors to evaluate the effectiveness of each species as a disperser including: how often they moved seeds, how far they moved them, rather or not they buried them and, if so, how long the seeds survived.
Agoutis moved by far the most seeds in Lieneke’s experiments, followed by squirrels and then rats. Peccaries did not move any seeds, probably because Lieneke cleaned the fruit off the seeds for her experiments, and peccaries only eat the fruit part, being unable to crack the hard seed. All three rodents moved seeds about the same distance (average 9-15m), but they treated them much different, with agoutis and rats burying most of their seeds in underground caches, and squirrels taking about half of them up into the trees to eat or store them. Finally, the rat-buried seeds were typically dug back up after just 1-2 days, while the squirrel or agouti buried seeds remained underground for three weeks, or more.
Lieneke defended her Masters Thesis this month, concluding that agoutis are by far the most effective initial disperser of Astrocaryum seeds, moving more of them, burying more of them, and leaving them in their underground caches longer than the other rodent species. Add to this Veronica’s earlier results showing that once a seed is dug up from an agouti burrow it might get re-buried into another underground cache (i.e. secondary dispersal), and Agoutis come out as the main mobility assistant for Astrocaryum trees.
Congratulations to Lieneke on an outstanding Masters thesis.