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Rodent preference for scatter-hoarding large seeds has been widely considered to favor the evolution of large seeds. Previous studies supporting this conclusion were primarily based on observations at earlier stages of seed dispersal, or on a limited sample of successfully established seedlings. Because seed dispersal comprises multiple dispersal stages, we hypothesized that differential foraging preference on seed size by animal dispersers at different dispersal stages would ultimately result in medium-sized seeds having the highest dispersal success rates. In this study, by tracking a large number of seeds for 5 yr, we investigated the effects of seed size on seed fates from seed removal to seedling establishment of a dominant plant Pittosporopsis kerrii (Icacinaceae) dispersed by scatter-hoarding rodents in tropical forest in southwest China. We found that small seeds had a lower survival rate at the early dispersal stage where more small seeds were predated at seed stations and after removal; large seeds had a lower survival rate at the late dispersal stage, more large seeds were recovered, predated after being cached, or larder-hoarded. Medium-sized seeds experienced the highest dispersal success. Our study suggests that differential foraging preferences by scatter-hoarding rodents at different stages of seed dispersal could result in conflicting selective pressures on seed size and higher dispersal success of medium-sized seeds.

Cache pilfering is a pervasive phenomenon among seed‐hoarding rodent species; nonetheless, the influence of seed species and pilferer identity on cache pilfering risk at the community level remains underexplored. This study examined the cache pilfering risk of the scatter‐hoarding rodent Leopoldamys edwardsi and investigated the subsequent fate of the pilfered seeds of two dominant tree species, Camellia oleifera and Castanopsis henryi. The experiments were conducted in semi‐natural enclosures, which simulate natural habitat conditions while allowing for controlled observation of rodent behavior. Sympatric pilfering rodent species involved in the study also comprised Apodemus chevrieri, A. draco, Niviventer confucianus, and N. fulvescens, all commonly found in the study area. Our findings revealed that L. edwardsi showed no significant preference between the seeds of C. henryi and C. oleifera, but the pilferage rate of C. oleifera seeds was significantly higher. The cache pilfering risk posed by sympatric rodent species varied significantly, with scatter‐hoarding pilferers being primarily responsible for most cache losses. Moreover, seed species exerted a substantial impact on the cache pilfering risk imposed by these pilferers. Notably, we discovered that scatter‐hoarding pilferers selectively re‐cached pilfered seeds on the basis of seed species, a process that is anticipated to play a pivotal role in secondary seed dispersal and, consequently, plant regeneration. Our findings revealed that the cache pilfering risk posed by sympatric rodent species varied significantly, with scatter‐hoarding pilferers being primarily responsible for most cache losses. Moreover, seed species exerted a substantial impact on the cache pilfering risk imposed by these pilferers.

1. Habitat loss and fragmentation often leads to defaunation of large-bodied mammals, and their loss could trigger release from top-down control or food resource competition for small mammal seed dispersers, which in turn may affect the effectiveness of seed disperal by alrering the number of dispersed seeds or the manner in which they are dispersed. Although rodents are primary seed dispersers in habitat subjected to defaunation, changes in seed dispersal effectiveness of rodents along mammalian defaunation gradients, and empirical support for mechanisms underlying alteration of this ecological process, are unclear. 2. We assessed the direct and indirect effects of forested area and isolation on seed dispersal effectiveness of rodents on 21 study islands with varying levels of defaunation in the Thousand Island Lake, China. We used camera sampling, live traps and semi-quantitative acorn counts to assess occurrence of large-bodied mammal species, relative abundance of small rodent species and seed crop size respectively. Seed dispersal, post-dispersal seed survival, seedling emergence, and seedling survival were estimated by tracking fates of tagged acorns and by planting acorns in exclosures. 3. Forested area had positive indirect effects on seed dispersal effectiveness through defaunation and rodent competition for acorns, whereas isolation had negative direct and weaker positive indirect effects on seed dispersal effectiveness mediated by loss of large-bodied mammals and rodent competition for acorns. Loss of large-bodied mammals negatively affected seed dispersal effectiveness indirectly by virtue of its impact on rodent competition for acorns. Seed dispersal effectiveness exhibited a unimodal relationship with intensity of rodent competition for acorns, peaking at intermediate levels. 4. Synthesis. Indirect effects of island attributes mediated by defaunation of large-bodied mammals on small or isolated islands appear to drive altered competition for food among rodents and decreased seed dispersal effectiveness. Altered interactions between acorns and their rodent consumers/dispersers can substantially affect oak population demography in the Thousand Island Lake system. More broadly, our findings highlight the importance to the seed dispersal process of multiple interwoven effects between habitat fragmentation and defaunation of large-bodied mammals.

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 scatter‐hoarding 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. 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. 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. 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. Our field results support laboratory findings that caching rates and distances by scatter‐hoarding 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.

1. Mast seeding in animal-dispersed plants has previously been accounted for by two main hypotheses: the predator satiation hypothesis (that it increases seed survival and establishment before dispersal) and the predator dispersal hypothesis (that it increases seed dispersal or dispersal distance). However, neither hypothesis has been rigorously tested with simultaneous data on seed production, seed predation and seed dispersal by vertebrate seed predators. 2. We studied oil tea Camellia oleifera (Theaceae) seed production for eight years (2002-2009) in a subtropical forest in south-west China, and investigated how annual seed and rodent abundance determined per capita seed availability for rodent seed predators and seed dispersers and how seed and rodent abundance were related to seed dispersal and seed survival via scatter-hoarding. We predicted the patterns of seed dispersal and survival to test the two hypotheses about mast seeding. Edward's long-tailed rat Leopoldamys edwardsi acted as the principal seed disperser of oil tea seeds because of scatter-hoarding, while other sympatric rodent species acted only as seed predators. 3. We first provided a reasonable method to estimate per capita seed availability based on annual seed abundance and annual metabolic rodent abundance (corrected for metabolic-scaling body mass of each rodent species). We found that annual seed abundance, annual metabolic rodent abundance and per capita seed availability all had some significant effects on different estimators of seed fates (including dispersal distances) across each stage from seedfall to seedling establishment. Both annual seed abundance and per capita seed availability were positively correlated with pre-dispersal seed survival, but negatively correlated with scatter-hoarding (and recaching), seed survival after dispersal and dispersal distances. However, annual metabolic rodent abundance had a positive effect on scatter-hoarding, but had a negative effect on the time to cache recovery. 4. Synthesis. Since greater seed production was associated with improvement in pre-dispersal survival of oil tea seeds but a reduction in dispersal (including secondary dispersal and dispersal distance), our long-term study indicates that, compared with the predator dispersal hypothesis, the predator satiation hypothesis provides a better mechanism predicting seed dispersal and seed survival in animal-dispersed plants by integrating seed abundance and animal abundance.

1. It is well known that direct effects of seed predators or dispersers can have strong effects on seedling establishment. However, we have limited knowledge about the indirect species interactions between seeds of different species that are mediated by shared seed predators and/or dispersers and their consequences for plant demography and diversity. Because scatter-hoarding rodents as seed dispersers may leave some hoarded seeds uneaten, scatter hoarding may serve to increase seed survival and dispersal. Consequently, the presence of heterospecific seeds could alter whether the indirect interactions mediated by scatter-hoarding rodents have a net positive effect, creating apparent mutualism between seed species, or a net negative effect, creating apparent competition between seed species. 2. We present a testable framework to measure short-term indirect effects between co-occurring plant species mediated by seed scatter-hoarding rodents. We tested this framework in a subtropical forest in south-west China using a replacement design and tracked the fate of individually tagged seeds in experimental patches. We manipulated the benefits to rodents by using low-tannin dormant chestnuts as palatable food and high-tannin non-dormant acorns as unpalatable food. 3. We found that seed palatability changed the amount of scatter hoarding that occurred when seeds co-occurred either among or within patches. Consistent with our predictions, scatter-hoarding rodents created apparent mutualism through increasing seed removal and seed caching, and enhancing survival, of both plant species in mixed patches compared with monospecific patches. However, if we ignore scatter hoarding and treat all seed harvest as seed prédation (and not dispersal), then apparent competition between palatable chestnuts and unpalatable acorns was also observed. 4. This study is the first to demonstrate that foraging decisions by scatter-hoarding animals to scatter hoard seeds for later consumption (or loss) or consume them can influence indirect effects among co-occurring seeds, and rodent-mediated indirect effects vary depending on whether the harvested seeds are hoarded or eaten.

1. Seed dispersal by birds is central to the passive restoration of many tree communities. Reintroduction of extinct seed dispersers can therefore restore degraded forests and woodlands. To test this, we constructed a spatially explicit simulation model, parameterized with field data, to consider the effect of different seed dispersal scenarios on the extent of oak populations. We applied the model to two islands in California's Channel Islands National Park (USA), one of which has lost a key seed disperser. 2. We used an ensemble modelling approach to simulate island scrub oak (Quercus pacifica) demography. The model was developed and trained to recreate known population changes over a 20-year period on 250-km² Santa Cruz Island, and incorporated acorn dispersal by island scrub-jays (Aphelocoma insularis), deer mice (Peromyscus maniculatus) and gravity, as well as seed predation. We applied the trained model to 215-km² Santa Rosa Island to examine how reintroducing island scrub-jays would affect the rate and pattern of oak population expansion. Oak habitat on Santa Rosa Island has been greatly reduced from its historical extent due to past grazing by introduced ungulates, the last of which were removed by 2011. 3. Our simulation model predicts that a seed dispersal scenario including island scrubjays would increase the extent of the island scrub oak population on Santa Rosa Island by 281% over 100 years, and by 544% over 200 years. Scenarios without jays would result in little expansion. Simulated long-distance seed dispersal by jays also facilitates establishment of discontinuous patches of oaks, and increases their elevational distribution. 4. Synthesis and applications. Scenario planning provides powerful decision support for conservation managers. We used ensemble modelling of plant demographic and seed dispersal processes to investigate whether the reintroduction of seed dispersers could provide cost-effective means of achieving broader ecosystem restoration goals on California's second-largest island. The simulation model, extensively parameterized with field data, suggests that re-establishing the mutualism with seed-hoarding jays would accelerate the expansion of island scrub oak, which could benefit myriad species of conservation concern.

Oak regeneration and the expansion of forested sites in Eurasia rely on acorn dispersal by animals, especially the Eurasian jay (Garrulus glandarius). However, in open agroforestry systems where jays are absent, such as old fields far from acorn sources, oak recruitment still occurs. We hypothesize that the Eurasian magpie (Pica pica), an abundant corvid in this system, substitutes the jay in its seed dispersal function. By ringing 169 magpies, video recording >7500 acorn removal events with trail cameras, and radio‐tagging 337 acorns, we quantified that (1) magpies cached 41–56% of the annual acorn production of Quercus ilex trees in single caches on the ground; (2) breeding pairs, and especially males, were the main acorn dispersers; (3) each breeding magpie cached 169–1372 acorns in 6 weeks; and (4) the effectiveness of dispersal (percentage of cached acorns resulting in seedlings) was 0.6–2.4%, which (5) yielded a high density of emerged seedlings (56–439 seedlings/ha). We evidence that magpie could be a key species in the regeneration of oak agroforestry mosaics because they massively and effectively dispersed acorns. However, in our particular study site, effectiveness was low probably due to herbivory and summer drought stress (i.e., a context limitation rather than an intrinsic limitation of the disperser). As the distributions of magpies and oaks overlap widely in Eurasia, effective acorn dispersal by magpies could have a significant role in large‐scale oak forest recovery in strongly fragmented landscapes.

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 lay summary is available for this article. Lay Summary

Persistent soil seed banks are characteristic of Arctostaphylos (Ericaceae) species in the Mediterranean‐climate California Floristic Province. While most species are obligate seeders, regeneration of stands of all Arctostaphylos species ultimately depends on post‐fire seedling recruitment. Arctostaphylos seed banks are created, in large part, by scatter‐hoarding rodents. Variation in fruit morphology, therefore, is expected to impact the Arctostaphylos–rodent interaction. Seeds produce sufficient rewards (nutritious mature embryo) to entice rodents to disperse and ultimately bury seeds in the soil. Hard seed coats increase the time required to extract the embryo, encouraging rodents to choose storage over immediate predation, and nutlets are frequently empty. We assessed the variation of fruit nutlet fusion and seed viability among 38 Arctostaphylos taxa. Factors such as latitude, elevation, life history, ploidy, and phylogenetic position were also analyzed. Generalized mixed‐effects models were used to determine the factors contributing to variation in fruit nutlet fusion and seed viability. Our results indicate that fruit volume and shape are the most important variables affecting nutlet fusion and seed viability. Additionally, other potential influences only show a weak correlation and are not predicted to significantly impact nutlet fusion or seed viability. These findings provide insights into evolved strategies used by plants to increase reproductive success via scatter‐hoarding rodents. Our study benefits the conservation and restoration of Arctostaphylos stands by emphasizing the importance of animal‐mediated dispersal and providing estimates of seed viability for different species. With the anticipated effects of climate change, such as departures from historic fire regimes, the preservation of the relationship between plants and animal foragers is crucial for the continued survival of Arctostaphylos and California's evergreen chaparral. We assessed the variation of fruit nutlet fusion and seed viability among 38 Arctostaphylos taxa. Our results indicate that fruit volume and shape are the most important variables affecting nutlet fusion and seed viability. These findings provide insight into evolved strategies used by plants to increase reproductive success via scatter‐hoarding rodents.