One of the big discoveries of our project is that agoutis disperse Astrocaryum palm nuts in a complex step-wise manner: agoutis bury a seed, then dig it up and move it to another site over and over again. While collecting data in the field, and following seeds from one place to another over many weeks, we noticed that the movements didn’t seem to be completely random with respect to the surrounding trees. Astrocaryum palm trees are very conspicuous because they are covered in big spines, you have to keep an eye out for them when your walking off trail to avoid getting pricked. When tracking down radio-tagged seeds, it almost seemed as though the seeds ended up in areas far away from other Astrocaryum trees. If this was true, this could be a really important phenomenon, because of something known as “Janzen-Connell effects”.
Janzen-Connell effects, first identified by Drs Janzen and Connell (of course), result in seeds falling below their parent tree being attacked by enemies while those that escape their mother’s shadow are more likely to also escape their enemies (parasites and predators). For this reason, our previous result that step-wise dispersal led to long-distance dispersal was so important. By carrying these seeds away from their parent trees, agoutis are helping these seeds survive, and helping trees to reproduce.
BUT, this phenomenon comes with a big caveat. It is possible that when agoutis take a seed away from its mother tree, the seed might be buried right underneath another Astrocaryum tree, and therefore at high risk for predation or parasitism. This pattern is called contagious dispersal because if agoutis dispersed seeds in this manner, they would simply carry them from one Atrocaryum tree site to another Astrocayum tree site. If certain pests (such as insects and fungus) are more common when plants are found at high densities, seeds should survive best when not surrounded by many trees or seeds of the same species. For this reason, contagious dispersal has been hypothesized to be a major limiting factor in seed survival in some systems. (For those interested in more on the subject of distance and density dependent mortality, John Terborgh recently published a great paper in American Naturalist where he discusses this subject.)
So, if our impression that agoutis carried seeds away from adult trees is confirmed, agoutis could be dispersing seeds in a manner which would be really helpful to these trees. The question then was: how do we test this question? Luckily, we had the data at our fingertips. During our study we recorded the exact location of each seed cache, and we also recorded the coordinates of all Astrocaryum trees in the area. We were thus able to calculate the number of adult trees near (within 25m) our seed locations using a GIS and see if the seeds went from high to low density areas.
We found that the seeds indeed went from high to low density areas. But, could this have happened randomly? If a seed starts out in a high density area, no matter which way it moves, it would end up in a lower density area. To really test this question properly we created a computer model of random dispersal: we had pretend seeds moving in random directions and random distances (based on the observed dispersal kernel), and then compared the random movements to the observed movements actually made by agouti-carried seeds. What we found was that the observed seeds indeed moved to areas with fewer adult Astrocaryum trees than the random movements.
Ultimately, agoutis carried seeds to areas with 36% lower density than their original locations. This should greatly benefit the survival of Astrocaryum seeds. But the agoutis aren’t going the extra mile just for the seeds sake, as our earlier research discusses, seeds buried in areas with fewer Astrocaryum trees are less likely to be stolen by other agoutis. So whats good for the cacher is also good for the cachee.
By Ben Hirsch