Influence of acorn woodpecker social behaviour on transport of coast live oak (Quercus agrifolia) acorns in a southern California oak savanna

Abstract

Summary1. Many plant species depend upon animals for seed dispersal, yet animals disperse seeds in pursuit of their own social and behavioural agendas. Animal social behaviour affects where and how they forage, so it must also shape patterns of seed dispersal. 2. At Sedgwick Reserve, California, USA, we established a study population of Quercus agrifolia to determine patterns of acorn foraging by the acorn woodpecker (Melanerpes formicivorous). This cooperative breeder lives in social groups that defend territories surrounding arboreal seed caches (granaries), foraging communally within these territories. 3. We genotyped pericarp tissue of 568 acorns, as well as 285 adult Q. agrifolia trees, including all adults within 150 m of 16 focal granaries. After quantifying genotyping error, we identified a genetically reliable subset of 524 acorns. We assigned a source tree to each acorn and estimated the number of seed sources per granary and seed source sharing among granaries. 4. We found one to eight distinct seed-source genotypes per granary, and an effective source diversity ranging from 1.0 to 6.6 seed sources. Of all transport events, 96.5% involve source trees within 150 m of the granaries. For one granary, all sampled acorns were transported from five trees located more than 1.3 km away, with all source trees within 90 m of each other. No measure of seed-source diversity was associated with density of potential seed sources, and the pattern of acorn movement fits three traditional dispersal curves poorly. 5. Woodpecker groups rarely collected acorns from overlapping sets of maternal sources. Some pairs of neighbouring granaries shared maternal sources, and we identify those that were probably maintained by the same woodpecker group. 6. Synthesis. Territoriality of woodpecker groups restricts both the spatial area of foraging and the sharing of seed sources. This foraging behaviour limits distances and directions of acorn transport from oaks located within woodpecker territories. Dispersal agents with this type of social structure will create a high degree of local genetic structure. Extreme behavioural variations may result in anomalous long-distance dispersal events that increase genetic connectivity, but are likely to do so in an episodic and erratic fashion.

Gene flow and natural selection shape spatial patterns of genes in tree populations: implications for evolutionary processes and applications

“…Woodpeckers can fly kilometers on any given day, but our analyses showed that the effective number of maternal trees found in any single granary (each containing the combined collection from a single woodpecker family's efforts) is 2–3 local trees (Scofield et al. ; Scofield et al. ), as would be predicted by optimal foraging theory (Thompson et al.…”

Integrating frugivory and animal movement: a review of the evidence and implications for scaling seed dispersal

“…For instance, parentage analysis has shown that seeds dispersed beneath a conspecific canopy need not be from that specific mother tree (Godoy & Jordano, 2001) and that the adult plant nearest to the seedling is rarely the parent (Hardesty et al, 2006;Sezen, Chazdon & Holsinger, 2009). Although some of the reviewed studies went beyond reporting distance metrics and acknowledged the fact that seed movement is anisotropic and non-random (Santamaría et al, 2007;Scofield, Sork & Smouse, 2010), few evaluated the factors that underlie spatial variance in seed dispersal (Russo et al, 2006). Although some of the reviewed studies went beyond reporting distance metrics and acknowledged the fact that seed movement is anisotropic and non-random (Santamaría et al, 2007;Scofield, Sork & Smouse, 2010), few evaluated the factors that underlie spatial variance in seed dispersal (Russo et al, 2006).…”

Impacts of Mating Behavior on Plant–Animal Seed Dispersal Mutualisms

“…Unlike umbrellabirds, acorn woodpeckers live in highly territorial social groups and tend to forage in the trees proximal to their storage sites, with each territorial group gathering acorns from non-overlapping territories and hence from different trees (Grivet et al, 2005;Scofield et al, 2010Scofield et al, , 2011). However, because lek-breeding among frugivorous bird species is essentially absent from the temperate zone, we must limit our comparisons to species that exhibit other mating systems.…”