Temporal variations in seed dispersal patterns of a bird‐dispersed tree, Swida controversa (Cornaceae), in a temperate forest
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Seed dispersal syndrome
Frugivore
Diaspore (botany)
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Seed dispersal syndrome
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Propagule
Hoarding (animal behavior)
Mutualism
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Frugivore
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Biological invasions are one of the main threats to biodiversity and ecosystem functioning in the Anthropocene. Fleshy‐fruited invasive plants establish mutualistic interactions with native seed dispersers and may affect the mutualisms between native partners, negatively by disrupting plant dispersal through competition for seed dispersers or positively by facilitating seed dispersal via the attraction of fruit‐eating animals. Moreover, the invaders' density in the neighborhood of native plants may modulate the direction and/or magnitude of such effects on natives. In mountains of central Argentina, the cessation of fruiting of the dominant native tree Lithraea molleoides overlaps with the beginning of the fruiting of invasive shrubs ( Pyracantha spp.). This partial overlap allows for testing opposite predictions regarding the effect of invasive fruits on native seed dispersal. We compared frugivory by seed disperser birds on L. molleoides during non‐overlapping and overlapping periods, at six sites that differed in Pyracantha shrub density (high, low and no‐invasion). We counted frugivory during 2 h on 15 individuals of L. molleoides at each site and period, totaling 360 h of observation. Frugivory on the native tree was similar among sites during the non‐overlapping period and remained constant in both periods at non‐invaded sites. At invaded sites, frugivory increased from non‐overlapping to overlapping period and was greater at highly invaded sites. The resource provided by Pyracantha did not disrupt seed dispersal of the dominant native tree. Rather, it facilitated frugivory by seed dispersers and this effect may be exacerbated with higher fruit availability of invasives. Our results provide a counterview to the prevailingly negative impacts of invasive species on the seed dispersal of native species since the fruiting of invasive plants facilitated the seed dispersal of native species in a continental region contrasts with the mutualism disruption commonly observed in insular environments.
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In tropical forest ecosystems, majority of plant species depend on frugivorous vertebrates for seed dispersal. Because primates constitute a large portion of frugivore biomass in neotropical and paleotropical forests, the roles of primates as seed dispersers have been examined since 1970's. The process of seed dispersal by vertebrates can be divided into three phases: (1) pre-dispersal phase in which animals select particular fruits as attractive food; (2) dispersal phase in which animals handle, transport, and deposit seeds during their foraging activities; (3) post-dispersal phase in which dispersed seeds germinate and grow to reproductive age. To understand roles of primates as seed dispersers, this paper marshals the previous achievements in order of the three phases. During pre-dispersal and dispersal phases, quantitative and qualitative effectiveness of seed dispersal are generally related to several anatomical traits and behavioral pattern of each primate taxon, and particularly, large-bodied frugivorous primates often perform effectively. However, during post-dispersal phase, high mortality and unpredictable fates of dispersed seeds dramatically hurt the effectiveness of primate seed dispersal. Large-bodied frugivorous primates are recognized as vulnerable taxa to human disturbance such as deforestation and bushmeat hunting. Recently, in the forests where such primates are locally extinct or reduced, researchers have demonstrated that the loss of their seed dispersal services drives low density of seedlings and saplings and low rates of gene flow among populations. These facts paradoxically suggest that primates contribute regeneration of plant populations even their effectiveness is lessened in post-dispersal phase. As future issues, integration between seed dispersal research and plant demographic study will develop our understanding of the primates' roles in plant population dynamics over many generations. Moreover, considering the critical situations in empty forests, there is also an urgent need to argue prioritized conservation of high-performance primates for maintaining regeneration and vegetative restoration in degraded forests.
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Frugivores are the main seed dispersers in many ecosystems, such that behaviorally driven, nonrandom patterns of seed dispersal are a common process; but patterns are poorly understood. Characterizing these patterns may be essential for understanding spatial organization of fruiting trees and drivers of seed-dispersal limitation in biodiverse forests. To address this, we studied resulting spatial associations between dispersed seeds and adult tree neighbors in a diverse rainforest in Madagascar, using a temporal and phylogenetic approach. Data show that by using fruiting trees as seed-dispersal foci, frugivores bias seed dispersal under conspecific adults and under heterospecific trees that share dispersers and fruiting time with the dispersed species. Frugivore-mediated seed dispersal also resulted in nonrandom phylogenetic associations of dispersed seeds with their nearest adult neighbors, in nine out of the 16 months of our study. However, these nonrandom phylogenetic associations fluctuated unpredictably over time, ranging from clustered to overdispersed. The spatial and phylogenetic template of seed dispersal did not translate to similar patterns of association in adult tree neighborhoods, suggesting the importance of post-dispersal processes in structuring plant communities. Results suggest that frugivore-mediated seed dispersal is important for structuring early stages of plant-plant associations, setting the template for post-dispersal processes that influence ultimate patterns of plant recruitment. Importantly, if biased patterns of dispersal are common in other systems, frugivores may promote tree coexistence in biodiverse forests by limiting the frequency and diversity of heterospecific interactions of seeds they disperse.
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Most tropical plants produce fleshy fruits that are dispersed primarily by vertebrate frugivores. Behavioral disparities among vertebrate seed dispersers could influence patterns of seed distribution and thus forest structure. This study investigated the relative importance of arboreal seed dispersers and seed predators on the initial stage of forest organization—seed deposition. We asked the following questions: (1) To what degree do arboreal seed dispersers influence the species richness and abundance of the seed rain? and (2) Based on the plant species and strata of the forest for which they provide dispersal services, do arboreal seed dispersers represent similar or distinct functional groups? To answer these questions, seed rain was sampled for 12 months in the Dja Reserve, Cameroon. Seed traps representing five percent of the crown area were erected below the canopies of 90 trees belonging to nine focal tree species: 3 dispersed by monkeys, 3 dispersed by large frugivorous birds, and 3 wind-dispersed species. Seeds disseminated by arboreal seed dispersers accounted for ca 12 percent of the seeds and 68 percent of the seed species identified in seed traps. Monkeys dispersed more than twice the number of seed species than large frugivorous birds, but birds dispersed more individual seeds. We identified two distinct functional dispersal groups, one composed of large frugivorous birds and one composed of monkeys, drop dispersers, and seed predators. These groups dispersed plants found in different canopy strata and exhibited low overlap in the seed species they disseminated. We conclude it is unlikely that seed dispersal services provided by monkeys could be compensated for by frugivorous birds in the event of their extirpation from Afrotropical forests.
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Abstract: Seed dispersal effectiveness (SDE) is related to the role of seed dispersal agents in realizing the reproductive potential of plants through seed dispersal and subsequent plant recruitment. The SDE of a given seed disperser may vary spatially and temporally, with important implications for our understanding of the mutualistic relationships involving plants and frugivores. Here we observed 22 frugivorous bird species visiting an individual tree ( Cabralea canjerana ) over a 9-y period in a fragment of Atlantic forest in south-eastern Brazil to document the temporal variation in SDE. The quantitative (that takes into account the frequency of feeding visits to the focal plant and the number of fruits removed per visit) and qualitative (that considers the probability of seed deposition on a suitable site for plant recruitment) components of SDE varied in two and one order of magnitude both among bird species and among years within bird species, respectively. As a result, the SDE of bird species fluctuates over the years, especially for a migratory species that is the main seed disperser of C. canjerana (Chivi vireo Vireo chivi ) whose quantitative component (i.e. the product of visit and fruit intake rates) varied from 3.0 to 7.1. This study illustrates the dynamic nature of the seed-dispersal environment to which a plant is subjected to over the course of its life, which has important consequences for the plant's individual fitness.
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Previous studies have estimated distributions of seed dispersal distance by a variety of methods. But apart from direct observation, these methods cannot clarify the absolute numbers of seeds dispersed by each disperser. Without the ability to calculate absolute spreading volumes, each dispersers' contribution cannot be quantitatively evaluated. To resolve this problem, we estimated and compared quantities and distances of seed dispersal by mammalian seed dispersers. We focused on the fruits of Cerasus leveilleana and five species of frugivorous mammals that forage on it, central Japan. We tracked wild individuals of each species and gut retention times of seeds in captive individuals. By observing wild C. leveilleana trees for 2 years, we calculated fruit production, visitation frequency, and numbers of fruits removed per visit by each species. The mammal species differed greatly in body size and foraging behavior, and thus in the seed dispersal patterns that they generated: not only in dispersal distance, especially long distance, but also in the number of seeds removed. In particular, total seed removal and long-distance dispersal tended to be greater when fruit production was greater, and they were strongly affected by occasional visitation by bears. Therefore, understanding the relative functional importance of seed dispersers requires direct comparison of their effectiveness at seed dispersal. Large differences in the seed disperser fauna between years due to interannual variation in fruit production suggest that observations of many trees with different amounts of fruit production over multiple years are needed to clarify how fruiting trees and seed dispersers interact.
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Seed dispersal syndrome
Diaspore (botany)
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