Scatter-hoarding corvids as seed dispersers for oaks and pines: A review of a widely distributed mutualism and its utility to habitat restoration

Pesendorfer, Mario B.; Sillett, T. Scott; Koenig, Walter D.; Morrison, Scott

doi:10.1650/condor-15-125.1

Cited by 117 publications

(116 citation statements)

References 190 publications

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“…Jays are known to be highly mobile scatter‐hoarders that gather several thousand acorns in a single fruiting season and often carry them over long distances (Pesendorfer et al . ). Hence, they could rapidly enlarge the suite of trees they forage on as more individuals reach maturity through the expansion process.…”

Section: Discussionmentioning

confidence: 97%

Temporal change and determinants of maternal reproductive success in an expanding oak forest stand

2016

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Summary1. Global change is generating widespread local-scale expansions of tree populations. During demographic expansions, even small differences in individual reproductive success can generate large differences in the genetic composition of the resulting population. Colonizing tree populations almost invariably show highly skewed distributions of seed production, but the evolution of this reproductive skew during plant recruitment remains understudied. 2. We examine how recruit mortality modifies initial patterns of acorn dispersal and maternal reproductive success in a recently established, expanding Pedunculate oak (Quercus robur) forest stand and test different predictors of successful recruit establishment. 3. We mapped and genotyped a cohort of seedlings (n = 809) emerging within and around the stand (n = 254 adult trees), identified their habitats and monitored their fate until an age of 3 years. Their mother trees were inferred by Bayesian parentage analysis, and the number of assigned descendants was used as a proxy for tree maternal fertility. We examined temporal trends in tree fertility and patterns of dispersal, and tested effects of seed number-quality trade-offs, dispersal failure, dispersal distance and habitat of propagule arrival on the probability of seedling establishment. 4. We observed marked inequality in maternal reproductive success at the time of seedling emergence. A positive relationship between tree fertility and the proportion of non-dispersed seedlings pointed to disperser satiation. An overall seedling mortality of 69% generated considerable reshuffling in the fertility ranking of individual trees. Non-dispersal more than doubled seedling mortality. The establishment success of actually dispersed seedlings was independent of mother fertility and dispersal distance but strongly related to the habitat of arrival. Recruit survival was markedly higher in pine plantations or non-forested areas than in broadleaved forest. 5. Synthesis. A few highly fertile trees dominated reproduction in the stand. Differential recruitment success tended to reduce their initial advantage, however, suggesting that reproductive inequality might rapidly decline as more trees start to reproduce along the expansion process. Our results also suggest that field estimates of size or seed production are unlikely to provide reliable estimates of lifetime reproductive success in trees.

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Section: Discussionmentioning

confidence: 97%

Temporal change and determinants of maternal reproductive success in an expanding oak forest stand

2016

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“…If pre‐dispersal predation by insects drives interspecific variation in acorn harvesting, these findings would thus suggest a pattern opposite to the one found: in years of low overall acorn crops, acorn harvesting rates by birds that avoid infested acorns should be lower, as the proportion of infested acorns is higher (Pesendorfer & Koenig ; Pesendorfer et al . ).…”

Section: Discussionmentioning

confidence: 97%

“…), seed dispersal quality by California scrub‐jays is greater because they scatter‐hoard seeds in the ground (Pesendorfer et al . ). In contrast, acorn woodpeckers are cooperative breeders that store acorns in specialized ‘granary’ trees (MacRoberts ) and only disperse acorns when inadvertently dropping them during transport, thus providing comparatively low‐quality seed dispersal.…”

Section: Introductionmentioning

confidence: 97%

Competing for seed dispersal: evidence for the role of avian seed hoarders in mediating apparent predation among oaks

Pesendorfer

Koenig

2016

Functional Ecology

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Summary In communities of large‐seeded tree species, generalist seed hoarders often link the temporally variable dynamics of various species in ways that can result in indirect ecological interactions. A special case of such interactions is ‘apparent predation’ – when variation in seed production of one tree species changes the outcome of the interaction between seed hoarders and another tree species from seed dispersal mutualism to predation. We investigated how two species of avian hoarders responded to differences in acorn availability among individual valley oaks (Quercus lobata) and across a landscape dominated by blue oaks (Q. douglasii) to determine whether seed dispersal and predation dynamics result in apparent predation between these oak species. California scrub‐jays (Aphelocoma californica), high‐quality seed dispersers with large spatial scales of resource perception, showed strong numerical and functional responses to individually large Q. lobata acorn crops when landscape‐level acorn crops were low, but the numerical response was not observed when Q. douglasii produced a good acorn crop the subsequent year. In contrast, acorn woodpeckers (Melanerpes formicivorus), which are significant acorn predators, exhibited a consistent functional response to individual Q. lobata acorn crops independent of landscape‐wide availability. Consequently, Q. lobata experienced increased predation and a near absence of high‐quality seed dispersal when Q. douglasii acorns were abundant. Our results suggest that apparent predation among oak species can arise from contrasting responses of generalist seed predators and dispersers to acorn availability on different spatial scales. Thus, the spatial scale of resource perception, which can differ greatly among generalist seed hoarders, may provide a proximate mechanism underlying indirect, temporally variable ecological affects such as apparent predation among sympatric species of large‐seeded trees. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12770/suppinfo is available for this article.

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“…A majority of evidence for context‐dependent seed dispersal in scatter‐hoarding birds, however, comes from experimental studies or anecdotes because it is difficult to quantify in the field (Pesendorfer et al . ). Here, we use behavioural field observations of a scatter‐hoarding bird to investigate the context dependence of seed caching across two temporally variable factors: seed availability and social dynamics.…”

Section: Introductionmentioning

confidence: 97%

“…Because scatter‐hoarders continue dispersing seeds even when satiated, large crops both meet dispersers’ immediate food requirements and lead scatter‐hoarders to cache more seeds than necessary to cover energetic costs (Pesendorfer et al . ). This would result in a larger number of unrecovered seed caches at greater distances from the source plant following high‐productivity years (Vander Wall & Beck ).…”

Section: Introductionmentioning

confidence: 97%

Context‐dependent seed dispersal by a scatter‐hoarding corvid

Pesendorfer

Sillett

Morrison

et al. 2016

Journal of Animal Ecology

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Summary1. Corvids (crows, jays, magpies and nutcrackers) are important dispersers of large-seeded plants. Studies on captive or supplemented birds suggest that they flexibly adjust their scatterhoarding behaviour to the context of social dynamics and relative seed availability. Because many corvid-dispersed trees show high annual variation in seed production, context-dependent foraging can have strong effects on natural corvid scatter-hoarding behaviour. 2. We investigated how seed availability and social dynamics affected scatter-hoarding in the island scrub jays (Aphelocoma insularis). We quantified rates of scatter-hoarding behaviour and territorial defence of 26 colour-marked birds over a three-year period with variable acorn crops. 3. We tested whether caching parameters were correlated with variation in annual seed production of oaks as predicted by the predator dispersal hypothesis, which states that caching rates and distances should vary with seed abundance in ways that benefit tree fitness. We also tested whether antagonistic interactions with conspecifics would affect scatter-hoarding adversely, as found in experimental studies. 4. Caching behaviour varied with acorn availability. Caching distances correlated positively with annual acorn crop size, increasing by as much as 40% between years. Caching rates declined over time in years with small acorn crops, but increased when crops were large. Acorn foraging and caching rates were also negatively correlated with rates of territorial aggression. Overall foraging rates, however, were not associated with aggression, suggesting that reduced dispersal rates were not simply due to time constraints. 5. Our field results support laboratory findings that caching rates and distances by scatterhoarding corvids are context-dependent. Furthermore, our results are consistent with predictions of the predator dispersal hypothesis and suggest that large seed crops and social interactions among scatter-hoarders affect dispersal benefits for oaks and other masting tree species.

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Scatter-hoarding corvids as seed dispersers for oaks and pines: A review of a widely distributed mutualism and its utility to habitat restoration

Cited by 117 publications

References 190 publications

Temporal change and determinants of maternal reproductive success in an expanding oak forest stand

Temporal change and determinants of maternal reproductive success in an expanding oak forest stand

Competing for seed dispersal: evidence for the role of avian seed hoarders in mediating apparent predation among oaks

Context‐dependent seed dispersal by a scatter‐hoarding corvid

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