Scatter-hoarding of acorns by island scrub-jays

This dissertation examines the interaction between large-seeded trees and seed dispersal by scatter-hoarding corvids (crows, jays, magpies & nutcrackers), and focuses on the acorn production of oaks (Quercus spp.) on Santa Cruz Island (SCI) in Channel Island National Park (CINP), CA, and its effect on scatter-hoarding behavior of endemic island scrub-jays (Aphelocoma insularis). Chapter 1 presents a literature review on the ecological consequences of scatter-hoarding of seeds by corvids. Many large-seeded trees rely on seed dispersal by corvids that scatter-hoard seeds in small caches throughout the landscape. The birds are often the dominant seed dispersers in their community, providing high seed dispersal effectiveness over distances rarely matched by other dispersers. Drawing on a large body of literature, I show that this mutualism is common throughout the Northern Hemisphere and that corvid dispersal enables trees to shift ranges quickly in response to changing environments. Chapter 2 is an analysis of the temporal and spatial patterns of acorn production in oak chaparral on SCI. Acorn production of island scrub oaks on SCI varied strongly from 2009 to 2012, both between years and among locations, but with little temporal synchrony or spatial autocorrelation between trees. Across CINP, crown diameter and acorns showed greater variation within than between islands. However, trees and acorns were significantly smaller on colder Santa Rosa Island, hinting at abiotic drivers of variation, rather than selection by dispersers. Chapter 3 describes the scatter-hoarding behavior of island scrub-jays and investigates whether the resulting seed dispersal varies as a function of social context and acorn availability. Seed dispersal rates and distances by scatter-hoarding island scrub-jays were positively affected by acorn abundance. Antagonistic interactions with other jays, however, reduced hoarding rates, suggesting that the jays\u27 seed dispersal effectiveness is generally context-dependent. Scatter-hoarding by island scrub-jays was also an important contributor to the recovery of oak chaparral on Santa Cruz Island after 150 years of ranching activity