March 2010

Helen Esser and Yorick Liefting from Wageningen University, the Netherlands, are studying the community composition of ticks and their mammalian hosts in the Barro Colorado Nature Monument. More specifically, they want to now how mammal density and diversity impacts tick density and diversity.  In order to do so, they are sampling various islands and peninsulas surrounding BCI by placing remote cameras and collecting ticks using the drag-sampling method. Island size and isolation are among the most important determinants of species richness, and it is a well documented fact that ecosystem fragmentation causes a decline in species richness. Therefore, a fairly large range in mammal abundance and diversity is expected between the numerous islands and peninsulas of the BCNM, which vary both in size and isolation from adjacent forests. With a total area of 1600 ha, BCI is the largest of the islands in the Gatun Lake and most species of mammals which were there before the creation of the canal are still there. Some species have disappeared from the island, such as crab eating raccoons, rabbits, olingos and owl monkeys. Other species such as pumas and jaguars are able to swim on and off the island, so even though they are not permanent residents, the close proximity of BCI to the mainland allows these species to reach the island. One of the islands Helen and Yorick are working on, Frijoles Island (65.5 ha), is connected to the mainland by the Panama Canal railroad.

It appears that this rail link is being used by predatory mammals to get to the island. In one camera location, Helen and Yorick caught a pair of agoutis in an intimate moment. At this same site, they also captured photos of an ocelot and a yaguarundi! Given the size of the island, these cats are probably not permanent residents, but are using the rail link to move on and off the island. In addition to these exciting photos, they also found a species of rabbit which is no longer found on BCI.

You would think a hungry agouti would be bad news for a seed – munch munch.  However, agoutis living in ares with few food trees may also move further, and therefore disperse seeds further than those living the easy life in a grove of palm trees.  Seed dispersal is critical for tree regeneration, so this question of tree ‘density dependence’ on seed survival has important implications.

Enter STRI intern Veronica Zamora-Gutierrez.  “Vero” spent five months with our agouti project on BCI to help track seeds that were scatter-hoarded by agoutis. Her study question was how local food abundance affects seed dispersal by agoutis. Unlike previous studies, Veronica tracked seeds continuously, even after the seeds were removed from their initial caches, with the help of new technology

Vero and the team placed tagged Astrocaryum palm seeds in the home ranges of several radio-tagged agoutis that varied in how many palm trees they had. Some agoutis had home ranges full of fruits and seeds, others had almost nothing. The seed tags have small radio-transmitters that allowed us to find them when they moved.  They also were motion sensitive, so they start transmitting a signal when the seed gets excavated and moved.  These signals show up on the Automated Radio Tracking System on BCI, allowing Vero to know each day which seeds had been moved and then go out into the field to see exactly where it had been moved, and if it had been eaten or buried again in a new cache.

What followed was a logistical tour de force. The seeds were being moved all the time! Many seeds were recovered and re-cached many times in a row, producing multi-step dispersal over impressive distances (some >250m). This was much more movement than we expected, Vero and the team worked overtime for many weeks to keep track of them.

In a nice MSc thesis, Veronica showed that seeds moved most often and furthest in areas that had least fruits – so hungry animals moved seeds further. Dispersal, however, came at a high cost: every time a seed was dug up it increased the risk that the seed was eaten rather than re-cached.

Veronica obtained her MSc degree from the University of Leiden in the Netherlands last week: congratulations!

Zamora-Gutierrez, V. 2010. Effects of food availability on seed dispersal by the Central American agouti (Dasyprocta punctata). Leiden University, Leiden, The Netherlands.

The agouti team 2009

Literature Cited

1.     Gutierrez, V.Z. 2010. Effects of food availability on seed dispersal by the Central American agouti (Dasyprocta punctata). ***UNMAPPED REFMGR FIELD #30***, Leiden University, Leiden, The Netherlands.