Caching territoriality and site preferences by a scatter‐hoarder drive the spatial pattern of seed dispersal and affect seedling emergence

Abstract

For plants with seeds dispersed by scatter‐hoarders, decision‐making by animals when caching determines the spatial pattern of seed dispersal and lays the initial template for recruitment, driving the regeneration of many plant species. However, the mechanism by which animal behaviour shapes seed distributions in spatially complex landscapes is not well understood. We investigated caching territoriality and site preferences to determine the spatial pattern of seed caching at different scales and whether scatter‐hoarding behaviour drives the spatial distribution of seedling emergence. We used radio‐tracking and automatic wildlife cameras to monitor holm oak (Quercus ilex) acorn caching by Eurasian magpies (Pica pica), who are effective scatter‐hoarders in agroforestry systems. We assessed the effect of caching territories, distance to seed source, habitat, sub‐habitat, microsites and caching material in the spatial pattern of acorn dispersal by magpies. In addition, we analysed the relationship between the density of cached acorns and of emerged seedlings in different habitats. Breeding magpies cached the acorns inside their caching territories, where they preferred tilled areas over oak plantations and mostly avoided old fields. These differences in habitat preference were maximized at relatively short to medium dispersal distances, where most acorns were cached, and decreased or disappeared at long distances. Within tree plantations, magpies preferred high plant‐productivity sites over low productivity ones. At the finest spatial scale, magpies preferred structures built by animals, such as rabbit grit mounds and latrines and ant litter mounds, to cache the acorns. In many sites, magpies selected uncommon materials such as stones and litter to cover caches. In the subsequent spring, seedling emergence was positively correlated with acorn cache density. Synthesis. Scatter‐hoarding is a hierarchical process in which caching sites are selected using different criteria at different spatial scales driven by territoriality and site preferences. Territoriality constrained dispersal distance and the habitats available for acorn caching. Magpie territoriality therefore indirectly drives oak seedling emergence and can determine oak recruitment and forest regeneration.

Drivers of oak establishment in Mediterranean old fields from 25-year-old woodland islets planted to assist natural regeneration

“…Additionally, shrubs can be planted around the islets to act as "nurses" to facilitate recruitment of later successional species (Gómez-Aparicio et al 2008;Andivia et al 2017;Cruz-Alonso et al 2019b). Conserving isolated nesting trees or placing new nesting sites for seed dispersers such as the magpie inside the old fields and near islets can increase seed arrival and consequently recruitment (Martínez-Baroja et al 2021). This study may help increase the effectiveness of woodland islet plantation for restoring Mediterranean old fields and other areas with harsh climates.…”

Linking animal behaviour and tree recruitment: Caching decisions by a scatter‐hoarder corvid determine seed fate in a Mediterranean agroforestry system

“…We created randomly distributed experimental habitat types of equal size but with contrasting soil compaction and grass cover, two features that determine cache site selection for this bird and corvids in general (Bossema, 1979;Martínez-Baroja et al, 2021). We also inserted landmarks that simulated local cues that could potentially guide cache site selection and cache recovery.…”

“…The decisions made by the animal to cache the seeds and to recover them later may have strong consequences for seed fate and seedling establishment. At the landscape scale, scatter hoarders cache seeds in a variety of habitat types that may affect seed survival and seedling establishment (Gómez, 2003; Martínez‐Baroja et al, 2021; Morán‐López et al, 2015; Pesendorfer et al, 2017; Vander Wall, 1990). At the microsite scale, seed burial or concealment under litter (a common caching behaviour in scatter hoarders; Vander Wall, 1990) may increase seedling establishment because seeds are better protected against pilfering and are less exposed to harsh environmental conditions (Briggs et al, 2009; García et al, 2002; Longland et al, 2001; Vander‐Wall, 1990; Yi et al, 2013, and references therein).…”

“…The studies addressing the role of scatter hoarders as vectors for seed dispersal in the wild have relied mostly on observational approaches in which seed dispersal is monitored from the source (either natural trees or feeder stations) to the caches (Gómez, 2003; Gómez et al, 2008; Kuprewicz, 2015; Martínez‐Baroja et al, 2021). This procedure sheds light on the habitat and microhabitat selection by the animal.…”

“…We used walnut ( Juglans regia L., a tree secularly cultivated in western Europe) caching by the Eurasian magpie ( Pica pica L., a common scatter‐hoarder corvid in agroecosystems of Eurasia, Castro et al, 2017; Martínez‐Baroja et al, 2019) as the study system. We created randomly distributed experimental habitat types of equal size but with contrasting soil compaction and grass cover, two features that determine cache site selection for this bird and corvids in general (Bossema, 1979; Martínez‐Baroja et al, 2021). We also inserted landmarks that simulated local cues that could potentially guide cache site selection and cache recovery.…”