PS 18-168 - Linking animal behaviour and tree recruitment: caching decisions by a scatter hoarder corvid determine seed fate in an agroforestry system

Background/Question/Methods

Seed dispersal by scatter-hoarder corvids is of paramount relevance for the establishment of important tree species from the Holarctic region such as the walnut (Juglans regia). However, the factors that drive animal decisions to cache seeds in specific locations and the consequences of these decisions on seed fate are poorly understood. We experimentally created four habitat types that differed in their characteristics in a Mediterranean agricultural landscape where the Eurasian magpie (Pica pica) is a common scatter-hoarder: soft bare soil (SBS); compacted bare soil (CBS); compacted soil with a dense herbaceous cover (CHC); and soft lineal bare soil (SLBS). We also included visual landmarks (stones, sticks, and bunches of dry plants) that magpies might use to locate the caches. Walnut dispersal from feeders to the habitat type plots was monitored with radio-tracking. A sowing experiment simulating natural caches was conducted to test the effect of caching characteristics on seed germination and seedling emergence. Seed mass was controlled for the dispersal and sowing experiments.

Results/Conclusions

Magpies selected SBS, SLBS, and to a lesser extent the CHC habitats, but avoided CBS to cache nuts. Seed mass did not affect dispersal distance; however, it affected caching characteristics, as larger seeds were cached more often under litter and in the CHC, whereas lighter seeds were more often buried in the SBS. Seed mass did not affect either germination or emergence, but burial under soil or litter was critical, with virtually no emergence from nuts left on the surface. There was no evidence of any effect of the visual landmarks used in this experiment. The results support that the consequences of seed caching for seedling establishment are driven by a fine decision-making process of the magpie, which seems to ponder characteristics of the habitat and the microhabitat together with the seed mass to determine where and how to cache each nut. The consequence of such behaviour is that magpies reinforced the quantitative and qualitative components of seed dispersal effectiveness for walnuts in the studied agroforestry system by caching nuts in habitats and microhabitats that benefit seed survival and seedling emergence.