Spatially biased dispersal of acorns by a scatter-hoarding corvid may accelerate passive restoration of oak habitat on California’s largest island

Abstract

Scatter hoarding by corvids (crows, jays, magpies, and nutcrackers) provides seed dispersal for many large-seeded plants, including oaks and pines. When hoarding seeds, corvids often choose nonrandom locations throughout the landscape, resulting in differential survival of seeds. In the context of habitat restoration, such disproportional storing of seeds in areas suitable for germination and establishment can accelerate expansion and recovery of large-seeded tree populations and their associated ecosystems. Here, we investigate the spatial preferences of island scrub jays Aphelocoma insularis during scatter hoarding of acorns (Quercus spp.) on Santa Cruz Island. We use a large behavioral data set on the birds’ behavior in combination with seedling surveys and spatial analysis to determine whether 1) island scrub jays disproportionally cache seeds in specific habitat types, and 2) whether the preferred habitat type is suitable for oak regeneration. Our results show that the jays nonrandomly cache acorns across the landscape; they use chaparral and coastal sage scrub disproportionally while avoiding open and grassy areas. The areas used most often for caching were also the areas with the highest oak seedling densities. We discuss the potential role of these findings for the recovery of Santa Cruz Island’s oak habitat since the 1980s.

Oak habitat recovery on California's largest islands: Scenarios for the role of corvid seed dispersal

“…Such facilitated movement of seeds across fragmented and degraded landscapes is considered one of the key advantages of restoring seed dispersal mutualisms for conservation management (McConkey et al., ). Our results, which suggest the pace of oak habitat recovery on Santa Cruz Island required jay scatter hoarding, also support the hypothesis that removal of >45,000 sheep starting in the 1980s restored a vital seed dispersal mutualism and enabled it to catalyse oak habitat expansion on Santa Cruz Island (Morrison et al., ; Pesendorfer et al., ; Pesendorfer, Sillett, et al., ).…”

“…First, the seed predation and dispersal submodel treats dispersal as a mostly homogenous process across spatial scales. Corvid scatter hoarding of seeds, however, is often context dependent, so that dispersal rates and distances change with seed abundance and local habitat composition (Pesendorfer et al., ; Pesendorfer et al., ; Pesendorfer, Sillett, et al., ). Agent‐based models are able to integrate such details of animal behaviour, but due to the large spatial and temporal scales covered in our study, such an approach would have posed computational challenges (Aben et al., ).…”

“…The model was constructed in MatLab 9.1 (The MathWorks Inc., Natick, MA, USA, 2016). Code, maps and data used for parameterization are publicly available (Pesendorfer, Sillett, & Morrison, ; Pesendorfer et al., ).…”

“…Individual island scrub‐jays scatter‐hoard 3,500 to 5,500 acorns a year, with dispersal distances ≤400 m from the parent plant (Pesendorfer et al., ). The birds spatially bias caching of acorns towards areas of high oak seedling recruitment and avoid open or grassy areas (Pesendorfer et al., ). The birds also transport more than 50% of acorns uphill or across ridges and canyons in ways that cannot be achieved by gravity (Pesendorfer, ).…”

“…The model code, vegetation maps and data used for parametrization are publicly accessible at figshare https://doi.org/10.6084/m9.figshare.5413987.v1 (Pesendorfer et al., ).…”