September 2010


Go where the food is – this is the basic idea behind a fancy ecological theory called the ‘Ideal Free Distribution’. This works great for a simple animal-food system, such as an agouti eating fruits.  Areas with more fruits should have more agoutis. Not complicated…till you add predators to the mix.

First, if predators are actively hunting prey they would be expected to go to areas with high prey density.  But this might not always be so obvious, since prey hide from predators, how does a hungry ocelot find a good hunting ground?  One possibility is that the predators cue in on the same thing the prey do, ocelots may be hanging out around fruiting trees to nab the agoutis they think will show up for their breakfast.  Second complication: if ocelots are hanging around the areas with the most fruit trees than agoutis might actually realize this and then avoid these areas and scrounge around the lower risk neighborhoods.  Are ocelots and agoutis playing a cat and mouse game?

These are fun questions to ask around the BCI lounge, but the complications make them a scary thesis topic that most students have avoided.  If agoutis are more common near fruit resources, and ocelots are more common around fruit resources, how do you know if the ocelot is cueing on the agoutis or the fruits?

Enter Wageningen masters student Joris Buis and his ingenious idea. In the agouti system it is relatively easy (ok, not easy, but straight forward) to count the number of fresh fruits in a tree, something agoutis and ocelots probably quantify more with their nose than their eyes.  However, once all the fresh fruit is gone there is still a storehouse of agouti food nearby, hidden in underground caches by scatterhoarding agoutis.  The agoutis know where this food is, because they put it there, but the ocelots don’t.  Thus, by taking into account the stratification of both the aboveground and belowground agouti-food over the different seasons, Joris was able to statistically tease apart rather ocelots cue in on agoutis or ripe fruiting trees.

Executing this plan required data on local ocelot and agouti abundance at specific locations, across all seasons, along with fruit data. Joris mined camera data we collected earlier as part of our year-round camera trap survey, as well as subsequent camera runs by other students as well.  All told, Joris analyzed data collected over two years from over 1000 sites on BCI and a total of almost 13,000 camera-nights recording 8671 agoutis walking by and 297 ocelots.  For each of these sites, Joris quantified the number of fruiting trees nearby from the three main agouti-food tree species (Attalea butyracea, Astrocaryum standleyanum, and Dipteryx panamensis).  Finally, Joris stratified the year by fruiting season, categorizing each tree species as having visible (ripe) fruit, invisible (buried) fruit, or no fruit.

With this natural experiment having been recorded by cameras over the last two years, Joris was in an excellent position to test the predictions of the Ideal Free Distribution in a predator-prey system. Joris found that both agoutis and ocelots were more common in areas with fresh fruit and rare in areas with few fruits.  However, in the key test, not only agoutis, but also ocelots, were more abundant in areas with hidden fruits.  Since the ocelots don’t know where these fruits are, it strongly suggests that they are cuing on the agoutis themselves, and not just hanging around fruit trees.

This is one of the few field-tests of the Ideal Free Distribution theory in a natural predator-prey situation, and Joris received strong praise when he presented his results in his Masters Thesis Defense at Wageningen University last month.  At the end of his presentation his committee had one recommendation for him “Go to where the beer is, young man.”  Congratulations Joris on an outstanding masters thesis!

Different rodent species seem to possess different techniques to handle the seed that they retrieve. For an example, we have observed that a red squirrel prefers to stick to its arboreal nature by dragging the seed towards the canopy away from the ground, a spiny rat prefers the deep dark world below the ground and an agouti prefers to bury the seed just a few centimeters below the surface. Even within agoutis, there are differences in the sites they choose to cache their seeds. We have seeds cached in the stream beds, in the gaps and in the understory of saplings, trees, lianas and vines. In order to know if there are differences in cache sites of agoutis, red squirrels and spiny rats in terms of environmental conditions such as light intensity, soil characteristics and leaf litter depth, Lieneke Bakker, a Master’s student recently completed measurements of microsite characteristics of cache sites. The seeds and fruits of black palm (Astrocaryum standleyanum) are not only preferred by agoutis, spiny rats, red squirrels, peccaries, howler monkeys, spider monkeys, but are also predated upon by insects such as Bruchid beetles and Scotylid beetles. While the fruit is in its early developmental stage, female Bruchid beetle oviposits eggs on the flesh and the newly hatched larvae emerge out of the seed through a perfectly circular hole in the seeds.

                           

We are in the midst of the wet season. Almost every afternoon, dark clouds arrive rolling towards the island and announce rainfall through its thunder and lightning. Rain seeps through tree trunks and between gaps in the palm fronds, the streams swell up with rain water and the soil remains moist under the shadow of leaf litters. We suspect that many transmitters are washed off the magnets after heavy rainfall. But surprisingly, very few transmitters are washed off the magnets. On one hand, the rain makes the surface slippery to walk on; on the other hand it serves as a perfect platform for animal species to leave their footprints. While tracking seeds, we encounter beautiful tracks of ocelots, agoutis, peccaries and deer that decorate the muddy trails. In addition to radio tracking seeds, we are also doing mammal monitoring by using motion sensitive cameras to study the diversity and abundance of terrestrial bird and mammal communities and we recently started the seed excavation project to record the diversity and abundance of palm seeds and seedlings.