Jansen, P.A. (2003) Scatterhoarding and tree regeneration. Ecology of nut dispersal in a
Neotropical rainforest. PhD thesis, Wageningen University, Wageningen, the Netherlands. ISBN 90-5808-777-8; x + 166 pp.
Seed-eating animals are reputed predators of seeds, but they may also function as seed dispersers. This dissertation deals with the interaction of nut-bearing trees and scatterhoarding animals, which store important amounts of seeds as food reserves in spatially scattered soil surface caches. It studies how large cavi-like rodents - in particular the Red acouchy - disperse and predate upon the seeds of the canopy tree Carapa procera (Meliaceae) at the Nouragues Biological Station, an undisturbed tropical rainforest site in French Guiana, South America. Video surveillance and thread-marking techniques were used to follow the fate of seeds throughout the dispersal process, from shedding until either death or establishment of a seedling. Thus, seed production was linked with dispersal effectiveness and establishment success. Within these seed fate experiments, seed size and seed abundance were varied to study how these plant traits affect scatterhoarding and to test hypotheses on the evolution of large-seediness and mast seeding.
Scatterhoarding proved to be an effective dispersal mode. Seedlings did establish from cached seeds, even though the majority of seeds were eventually dug up and consumed. Large seeds were more likely to be successfully dispersed than small seeds, which opposes the paradigm that the need for dispersal causes selection against large seeds. Large seeds, however, were favoured only up to a certain point beyond which seeds apparently became increasingly difficult for the animals to manipulate. This resulted in an optimum seed size for dispersal by scatterhoarding animals. An explanation is given for the contrasting results obtained in published experiments on size-dependent seed predation.
Establishment was far more likely in years of abundant fruiting than in lean years, and the selectivity of rodents regarding the size of scatterhoarded seeds was also greater in rich years. Scatterhoarder responses to seed size and abundance alone can explain why many nut-bearing plant species have mast seeding, the alternation of years with abundant crops and years with few or no seeds.
Regeneration of C. procera in natural forest came exclusively from seeds cached by scatterhoarding rodents: seed predating insects and mammals destroyed all non-dispersed seeds. Exceptions were seeds shed by parent trees along or within treefall gaps. These high light environments permitted seedling establishment even from heavily infested seeds. Therefore, regeneration need not be at immediate risk in managed forests where scatterhoarding rodents are scarcer, but where light availability tends to be greater.
Key-words: scatterhoarding rodents, natural regeneration, seed dispersal, seed predation, seed size, mast seeding, natural selection, Myoprocta exilis, Carapa procera, Meliaceae
1. General introduction
2. Logging, seed dispersal by vertebrates, and natural regeneration of tropical timber trees
with PA. Zuidema
pp. 35-59 in R.A. Fimbel, J.G. Robinson and A. Grajal (eds). The cutting edge. Conserving wildlife in logged tropical forests. Columbia University Press, New York (2001)
rodents and tree regeneration
with P.M. Forget
pp. 275-288 in F. Bongers, P. Charles-Dominique, P. M. Forget & M. Thery (eds). Nouragues. Dynamics and plant-animal interactions in a Neotropical rainforest. Kluwer Academic Publishers, Dordrecht (2001)
change perishable seeds into long-term food supplies
with F. Bongers and H.H. T. Prins
role of seed size in dispersal by a scatterhoarding rodent
with M. Bartholomeus, J.A. Elzinga, F. Bongers, J. den Ouden and S.E. van Wieren
Pp. 209-225 in D.J. Levey, W.R. Silva & M. Galetti (eds). Seed dispersal and frugivory: Ecology, evolution and conservation. CAB International, Wallingford (2002)
Seed mass and mast seeding
enhance dispersal by scatterhoarding rodents
with F. Bongers and L Hemerik
Stabilising selection on seed mass
by a seed-dispersing rodent
with L. Hemerik, F. Bongers, F.J. Sterck, S.E. van Wieren and H.H. T. Prins
Predator escape, gap colonisation
and the recruitment pattern of three
rodent-dispersed rainforest tree species
with F. Bongers and P.J. van derMeer
Summary / Samenvatting