The Role of Arboreal Seed Dispersal Groups on the Seed Rain of a Lowland Tropical Forest1

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.     

Seed Dispersal by Kerama Deer (Cervus nippon keramae) on Aka Island,the Ryukyu Archipelago,Japan1

Endozoochorous seed dispersal by kerama deer (Cervus nippon keramae) was investigated at four sites on Aka Island, the Ryukyu Archipelago, Japan. Kerama deer feces contained seeds of 35 plant species. Most of the germinated seeds were small (1.3 ± 0.18 mm) and had a hard coat, comparable to the germinated seeds in several other studies of endozoochorous seed dispersal by herbivores. Both the numbers and species compositions of seeds differed among the four sites, reflecting differences in the food available to kerama deer among habitats. Although many graminoid seeds were found in the two open habitats (Sites A and B) and in the adjoining forest habitat (Site C), woody plant seeds dispersed endozoochorously by birds and mammals were dominant in the forest (Site D), away from the marsh. Although a majority of the graminoid species was growing in open habitats such as the marsh and open fields, few were growing in the forest. Therefore, the kerama deer spread many open‐habitat graminoid seeds to the open and forest habitats. If a high density of kerama deer persists for a long time and gaps in forests are created by browsing kerama deer or by other means, graminoid species may spread substantially on Aka Island. Therefore, it is possible that seed dispersal by forest deer contributes to the expansion of grasslands.     

Mammal Abundances and Seed Traits Control the Seed Dispersal and Predation Roles of Terrestrial Mammals in a Costa Rican Forest

In Neotropical forests, mammals act as seed dispersers and predators. To prevent seed predation and promote dispersal, seeds exhibit physical or chemical defenses. Collared peccaries (Pecari tajacu) cannot eat some hard seeds, but can digest chemically defended seeds. Central American agoutis (Dasyprocta punctata) gnaw through hard‐walled seeds, but cannot consume chemically defended seeds. The objectives of this study were to determine relative peccary and agouti abundances within a lowland forest in Costa Rica and to assess how these two mammals affect the survival of large seeds that have no defenses (Iriartea deltoidea, Socratea exorrhiza), physical defenses (Astrocaryum alatum, Dipteryx panamensis), or chemical defenses (Mucuna holtonii) against seed predators. Mammal abundances were determined over 3 yrs from open‐access motion‐detecting camera trap photos. Using semi‐permeable mammal exclosures and thread‐marked seeds, predation and dispersal by mammals for each seed species were quantified. Abundances of peccaries were up to six times higher than those of agoutis over 3 yrs, but neither peccary nor agouti abundances differed across years. Seeds of A. alatum were predominantly dispersed by peccaries, which did not eat A. alatum seeds, whereas non‐defended and chemically defended seeds suffered high levels of predation, mostly by peccaries. Agoutis did not eat M. holtonii seeds. Peccaries and agoutis did not differ in the distances they dispersed seeds. This study shows that seed fates are contingent upon many factors such as seed defenses, frugivore–granivore abundances, and seed‐handling capabilities. Mammal–seed interactions are complex; the outcomes of these interactions depend on the inherent characteristics of seeds and their potential dispersers.     

Mammalian seed dispersal in Cantabrian woodland pastures: Network structure and response to forest loss

We evaluated the role of wild large mammals as dispersers of fleshy-fruited woody plants in woodland pastures of the Cantabrian range (N Spain). By searching for seeds in mammal scats across four localities, we addressed how extensive seed dispersal was in relation to the fleshy-fruited plant community, and applied a network approach to identify the relative role of mammal species in the seed dispersal process. We also tested the response of mammalian dispersers to forest availability at increasing spatial scales. Five carnivores and three ungulates dispersed seeds of eight fleshy-fruited trees and shrubs. Mammalian seed dispersal did not mirror community-wide fruit availability, as abundant fruiting trees were scarce whereas thorny shrubs were over-represented among dispersed species. The dispersal network was dominated by bramble (Rubus ulmifolius/fruticosus), the remaining plants being rarer and showing more restricted disperser coteries. Fox (Vulpes vulpes), badger (Meles meles), and wild boar (Sus scrofa) dispersed mostly bramble, whereas martens (Martes sp.) dispersed mostly wild rose (Rosa sp.). Ungulates occasionally dispersed holly (Ilex aquifolium) and hawthorn (Crataegus monogyna). The empirical network reflected a skewed distribution of interactions and some functional complementarity (as judged from the low levels of connectance and nestedness), but also some degree of specialization. Mammals overused uncovered microsites for seed deposition, and increased their disperser activity in those landscape sectors devoid of forest. Combined with previous findings on avian seed dispersal, this study suggest a strong functional complementarity coming from the low overlap in the main plant types that mammals and birds disperse – thorny shrubs and trees, respectively – and the differential patterns of seed deposition, with mammals mostly dispersing into deforested areas, and birds into forest-rich landscapes.     

The Interplay of Habitat and Seed Type on Scatterhoarding Behavior in a Fragmented Afromontane Forest Landscape

Scatterhoarding by rodents, whereby seeds are collected and stored for later consumption, can result in seed dispersal. Seeds may be covered in litter on the forest floor (cached) or buried. This is particularly so in the Neotropics for large, nutritious seeds, and where primary dispersers are rare or missing. In African forests, contemporary anthropogenic pressures such as hunting, forest degradation, and fragmentation are contributing toward major declines in large frugivores, yet the potential for scatterhoarding to mitigate this loss is largely unknown. In this study, we used thread‐marked seed to explore the balance between seed predation and dispersal by rodents in Afromontane forest. We studied two tree species in three habitats: (1) continuous forest; (2) continuous forest edge, and (3) small, degraded riparian forest patches. We found that seed removal rates were high and almost the same in all three habitats for both tree species, but that the predation/dispersal balance differed among habitats. In continuous forest, more seeds of each species were scatterhoarded than depredated, and rates of scatterhoarding differed between the two species. In all habitats, burying seeds up to 2 cm belowground was more common than caching. Distances seeds were moved was approximately five times greater in continuous forest than in forest edge or riparian patches. We found strong evidence to suggest that the African pouched rat, Cricetomys sp. nov was responsible for the scatterhoarding.     

Postdispersal Seed Fate of Tropical Montane Trees in an Agricultural Landscape, Southern Costa Rica

Variation in postdispersal seed fate is an important factor driving patterns of forest regeneration. Because most previous studies have not tracked final seed fate and have commonly equated seed removal with predation without considering the possibility of secondary dispersal, little is known about individual seed mortality factors in successional and degraded habitats. This study tracked the postdispersal fate of large-seeded tropical montane trees in abandoned pasture, young secondary forests, and small forest fragments during two consecutive years in an agricultural landscape in southern Costa Rica. The incidence of secondary dispersal by animals, scatterhoarding in particular, and the effects of seed burial on germination were measured. Overall, seeds survived through germination more often in secondary forests with high levels of mortality occurring in abandoned pastures and forest fragments. The majority of seed mortality was caused by rodent predation in forest fragments, insects and fungal pathogens in secondary forests, and a combination of desiccation, insects, and fungal pathogens in pastures. Seeds were frequently secondarily dispersed in larger forest fragments, whereas they were only rarely moved in pastures and secondary forests. Burial tended to improve germination in pastures and was important for an often scatterhoarded species, Otoba novogranatensis, in all habitats. The results of this study suggest that: (1) seed mortality factors differ in response to the type of habitat degradation; (2) large-seeded species have high potential for survival when dispersed to young secondary forests; and (3) seed removal is not a reliable proxy for seed predation, particularly in less degraded forest fragments.     

Inter‐group variability in seed dispersal by white‐handed gibbons in mosaic forest

Seed dispersers, like white‐handed gibbons (Hylobates lar), can display wide inter‐group variability in response to distribution and abundance of resources in their habitat. In different home ranges, they can modify their movement patterns along with the shape and scale of seed shadow produced. However, the effect of inter‐group variability on the destination of dispersed seeds is still poorly explained. In this study, we evaluate how seed dispersal patterns of this arboreal territorial frugivore varies between two neighboring groups, one inhabiting high quality evergreen forest and one inhabiting low quality mosaic forest. We predicted a difference in seed dispersal distance between the two groups (longer in the poor quality forest). We hypothesized that this difference would be explained by differences in home range size, daily path length, and ranging tortuosity. After 6 months of data collection, the evergreen group had a smaller home range (12.4 ha) than the mosaic group (20.9 ha), significantly longer daily path lengths (1507 m vs. 1114 m respectively) and greater tortuosity (39.1 vs. 16.1 respectively). Using gut passage times and displacement rates, we estimated the median seed dispersal distance as 163 m for the evergreen group (high quality forest) and of 116 m for the mosaic group (low quality forest). This contradiction with our initial prediction can be explained in term of social context, resource distribution, and habitat quality. Our results indicate that gibbons are dispersers of seeds between habitats and that dispersal distances provided by gibbons are influenced by a range of factors, including habitat and social context.     

Frugivory and seed dispersal by vertebrates in the Oriental (Indomalayan) Region

Current knowledge of frugivory and seed dispersal by vertebrates in the Oriental Region is summarized. Some degree of frugivory has been reported for many fish and reptile species, almost half the genera of non-marine mammals and more than 40% of bird genera in the region. Highly frugivorous species, for which fruit dominates the diet for at least part of the year, occur in at least two families of reptiles, 12 families of mammals and 17 families of birds. Predation on seeds in fleshy fruits is much less widespread taxonomically: the major seed predators are colobine monkeys and rodents among the mammals, and parrots, some pigeons, and finches among the birds. Most seeds in the Oriental Region, except near its northern margins, are dispersed by vertebrate families which are endemic to the region or to the Old World. Small fruits and large, soft fruits with many small seeds are consumed by a wide range of potential seed dispersal agents, including species which thrive in small forest fragments and degraded landscapes. Larger, bigger-seeded fruits are consumed by progressively fewer dispersers, and the largest depend on a few species of mammals and birds which are highly vulnerable to hunting, fragmentation and habitat loss.     

Response of ants to human‐altered habitats with reference to seed dispersal of the myrmecochore Corydalis giraldii Fedde (Papaveraceae)

Anthropogenic habitat disturbance has potential consequences for ant communities. However, there is limited information on the effects of ant responses on associated ecological processes such as seed dispersal. We investigated the effect of disturbance on the abundance, richness, and composition of ant communities and the resulting seed‐dispersal services for a herbaceous myrmecochore, Corydalis giraldii (Papaveraceae), in an undisturbed habitat (forest understory), moderately disturbed habitat (abandoned arable field), and highly disturbed habitat (road verge) on Qinling Mountains, China. In total, we recorded 13 ant species, and five out of these were observed to transport seeds. The community composition of dispersers was significantly different amongst habitats. The richness of the dispersers did not differ among the habitats, but their total abundance varied significantly across habitats and was 21% lower in the road verge than in the abandoned arable fields. The major seed‐dispersing ant species in both the forest understory and the abandoned arable field were large‐bodied (Myrmica sp. and Formica fusca, respectively), whereas the major seed‐dispersing ants found in the road verge were the small‐bodied Lasius alienus. This difference resulted in lower seed removal rates and dispersal distances in the road verge than in the other two habitats. The different dispersal patterns were attributed primarily to differences in dispersing ant abundance and identity, most likely in response to habitats with different degree of anthropogenic disturbance. The possible influence of disturbance on the ecological specialization of ant‐seed dispersal interaction is also discussed.     

Within Flood Season Variation in Fruit Consumption and Seed Dispersal by Two Characin Fishes of the Amazon

Many Amazon River fishes consume fruits and seeds from floodplain forests during the annual flood season, potentially serving as important seed dispersers and predators. Using a participatory approach, this study investigated how within-season variation in flood level relates to fruit consumption and seed dispersal by two important frugivorous fish, Colossoma macropomum and Piaractus brachypomus , in two Lower Amazon River fishing communities in Brazil. Diets of both fish species were comprised of 78–98 percent fruits, largely dominated by a few species. Diets included fruits of 27 woody angiosperms and four herbaceous species from 26 families, indicating the importance of forest and Montrichardia arborescens habitat during peak flood. A correspondence between peak fruit species richness and peak flood level was observed in one of two communities, which may reflect higher forest diversity and/or differences in selection of fishing habitat. Both fishes are seed dispersers and predators, the relative role of which did not vary by flood level, seed size, or fish size, but may vary with seed hardness. Interspecific differences in diet volume and intact seeds suggest P. brachypomus are more effective seed dispersers than C. macropomum . Overall, the spatial and temporal variation in fruit species composition and richness demonstrate plasticity in fruit consumption in relation to flood level and locally available fruits. While such diets are adaptive to the dynamic changes of Amazon floodplain habitats, the high consumption of forest fruits and seeds from mid- and late-successional species suggests that floodplain forest degradation could disrupt seed dispersal and threaten local and regional fisheries.