Scatterhoarding and tree regeneration

Seed-eating animals are well known as predators of seeds, but they also function as seed dispersers. This dissertation deals with the interaction between nut-bearing trees and scatterhoarding rodents, animals that 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.

Patrick Jansen used novel video surveillance and thread-marking techniques to follow the fate of individual seeds throughout the dispersal process, from seed shedding until either death or establishment of a seedling. Within these seed fate experiments, he varied seed size and seed abundance to determine how these plants 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 Myroprocta exilis, Carapa procera, Meliaceae.

Patrick Jansen graduated with distinction February 7, 2003.

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