The outcome of plant-scatterhoarder interactions changes with masting and environmental conditions

Background/Question/Methods

Interactions between plants and scatterhoarding granivores are a prominent example of conditional mutualism: they involve both costs (seed consumption) and benefits (caching seeds in favorable microsites) and the balance of these determines the net outcome. We used a recently developed theoretical model (R. Zwolak and E. E. Crone. 2012. Quantifying the outcome of plant-granivore interactions. Oikos 121:20-27) and empirical data from a beech forest in western Poland to evaluate how masting influences the outcome of beech-rodent interactions. The model uses data on seed germination from surface and from animal caches (i.e. relative benefits of caching) to estimate the lowest proportion of seeds that would have to be cached and never recovered for granivores to be beneficial (hereafter “p_C value”). The data were collected in 2009 (mast year; low rodent abundance) and 2010 (non-mast year; high rodent abundance) and included beech seed production, the magnitude of seed removal, and seedling germination from seeds that were buried and sown on surface (crossed with rodent exclusion).

Results/Conclusions

The germination experiments indicated that rodents significantly influenced seedling recruitment in both mast and non-mast year. Thus, beech recruitment appears seed limited even in mast years (although rodents removed a smaller proportion of seeds in the mast year).

Benefits from seed caching varied between the years, probably due to changes in environmental conditions.  The p_C value equaled 0.151 in the mast year, indicating that rodents would have to cache and never recover over 15% of all handled seeds. This can be considered an upper boundary of realistic values, thus the role of granivores in the mast year was equivocal. In the non-mast year, the he p_C value was only 0,031. On the other hand, cache pilferage (estimated as a ratio of germination from buried seeds with and without rodents) was over 5 times higher in the non-mast year (mast year: 12%, non-mast year: 69%). However, if the environmental conditions from year 2010 (high benefits of seed burial) were matched with a mast year, seed removal by rodents could have a positive influence on beech recruitment.