Research

Professional fields of interest

Plant-animal interactions; seed dispersal; seed predation; scatter-hoarding rodents; forest dynamics; species coexistence; hunting impacts

 

Current research project:

Tropical tree seed dispersal in a multi-trophic context.

An automated radio telemetry study at Barro Colorado Island , Panama .

funded by NWO-WOTRO

 

Ongoing research

Hunting and recruitment limitation

Mammals dispersing and predating seeds play key roles in current explanations of high tree species coexistence in tropical forests. They are thought to depress recruitment success of species when those are common, at the advantage of rare species. Is this mechanism of “compensatory mortality” really happening? And what happens if mammal populations shrink in response to hunting?

We are comparing seed fate and seedling recruitment in two arboreal palm species – Astrocaryum standleyanum and Attalea butyracea – among areas with high and low palm abundance and with and without hunting.

With Joe Wright (STRI), Helene Muller-Landau ( University of Minnesota )

Students: Carol Garzon-Lopez, Pieter van Eijk, Maaike Renard, Heleen Sharrott, Kelly Elschot, Laura Vonk, Carolina Puerta-Piñero ( Granada ), and Dumas Gálvez ( Panama ).

 

Tree distributions from aerial photos

Large-seeded trees with major fruit crops tend to have large exposed crowns. We are taking advantage of this for mapping a few tree species across all 16 km of Barro Colorado Island from high-resolution aerial photos, taken from a small plane. 

We relate differences in local abundance of several tree species to abiotic factors. We compare areas of low and high abundance of two palm species to study recruitment limitation and other possible mechanisms facilitating species coexistence.

With Stephanie Bohlman ( Princeton University ), Joe Wright (STRI), Helene Muller-Landau ( University of Minnesota ).

PhD student: Carol Garzon-Lopez (RUG)

 

Radio-tracking seeds and agoutis

Many large-seeded tree species rely on rodents scatter-hoarding seeds as food supplies for seed dispersal and seedling recruitment. How does this work? We are trying to find out at Barro Colorado Island , Panama , by simultaneously following the movements and fates of seeds and agoutis using an automated radio telemetry system.

 

We use a new, custom-made seed transmitter, attached to seeds as a threadmark, which only transmits when the seed is moved. This approach allows us to follow seed removal, handling and dispersal by agoutis, no matter how far seeds travel and how often they are dug up and re-dispersed. 

Our aim is to determine how the performance of agoutis as dispersers is influenced by animal community composition. We seek to understand the causes and consequences of stepwise dispersal (the repeated recovery and re-caching of stored seeds), specifically the role of cache robbery by competing agoutis.

 

We hypothesize that agoutis will do a better job of dispersal if more food competitors are around, and that there will also be more cache robbery. ARTS allows us to link seed handling to individual radio-collared agoutis.

Also, we want to find out how stored seeds escape recovery and consumption by agoutis. We hypothesize that predators, by eating agoutis, release stored seeds from consumption by agoutis, and indirectly enhance seedling recruitment.

 

This would imply that seed dispersal by agoutis is more effective in intact communities – with more agoutis and predators – than in impoverished ones.

 

With Roland Kays ( New York State Museum ) and Martin Wikelski ( Princeton University )

 

Spatio-temporal variation in agouti abundance

The abundance of agoutis on Barro Colorado Island varies widely in space and time. What causes this variation? Is this a response to the abundance of the three major food resources; Dipteryx panamensis, Astrocaryum standleyanum, and Attalea butyraceae fruits? Or are population abundances governed more by population levels of ocelots, the major natural enemies of agoutis on BCI?

We are combining three time series to determine the relative importance of food and predation for the temporal variation. And we use island-wide maps of the three food species to explain the spatial distribution of agoutis.

With Jackie Giacalone Willis ( Montclair University ) and Roland Kays ( New York State Museum )

 

Seed size, the reserve effect, and the flexibility of emerging seedlings

A possible advantage of having large seeds is that seedlings can leave a greater proportion of the seed reserves uncommitted and can use these reserves to anticipate or overcome calamities. We experimentally tested for this “reserve effect” in two species – European oak (Quercus robur) from the Netherlands , and Crabwood (Carapa procera) from French Guiana – using the natural variation in seed mass. 

We also tested whether seedlings emerging from large seeds were more flexible in their allocation of resources to root or shoot growth than those emerging from small seeds.

With Horacio Paz (Universidad Nacional Autónoma de México), Frans Bongers and Jan den Ouden ( Wageningen University ).

MSc student: Martijn Bartholomeus

 

Spatially contagious gap formation

Tree falls and the canopy gaps they cause are important in the dynamics of tropical forests, because tree recruitment is strongly light-limited. We are testing the idea that light gaps are spatially contagious, i.e. propagate themselves, reasoning that trees adjacent to gaps are more likely to become unstable and be blown over. in Nouragues, French Guiana , we have followed the fate of 6400 trees during five years, and analyzed how tree disturbances depend on the proximity of existing gaps.

With Peter van der Meer (Alterra) and Frans Bongers ( Wageningen University ).