First record of potential bird pollination in the holoparasitic genus Orobanche L.

Pollinators play an important role in the reproduction of zoophilous plants. A shift in pollinators has often been observed for oceanic island plants, probably because of the differences in fauna. In this study, we obtained data on pollinator shifts from insects to birds in Orobanche boninsimae (Orobanchaceae), a holoparasitic plant species endemic to the Bonin (Ogasawara) Islands, oceanic islands in the Pacific. We observed pollination and measured seed viability in O. boninsimae and its continental sister species O. coerulescens. We found that two passerine birds, the Japanese white-eye (Zosterops japonicus) and bulbul (Hypsipetes amaurotis squameiceps), visited the flowers and sucked the nectar of O. boninsimae, while only insects visited those of O. coerulescens. Viable seeds were produced under pollinator-excluded treatments in the two Orobanche species, indicating that the seeds were produced by automatic self-pollination and/or apomixis. These results suggest that O. boninsimae may be pollinated by birds and can produce seeds by automatic self-pollination/apomixis. This is the first record of visitation of the genus Orobanche by birds. Studies of pollination systems in native plants on the Bonin Islands are few compared to those on other oceanic islands, and O. boninsimae may provide a valuable example of pollinator shifts in the Bonin Islands.     

Quantitative analysis of the effects of the exotic Argentine ant on seed-dispersal mutualisms

Although it is increasingly clear that exotic invasive species affect seed-dispersal mutualisms, a synthetic examination of the effect of exotic invasive species on seed-dispersal mutualisms is lacking. Here, we review the impacts of the invasive Argentine ant (Linepithema humile) on seed dispersal. We found that sites with L. humile had 92 per cent fewer native ant seed dispersers than did sites where L. humile was absent. In addition, L. humile did not replace native seed dispersers, as rates of seed removal and seedling establishment were all lower in the presence of L. humile than in its absence. We conclude that potential shifts in plant diversity and concomitant changes in ecosystem function may be a consequence of Argentine ant invasions, as well as invasions by other ant species. Because very few studies have examined the effects of non-ant invasive species on seed-dispersal mutualisms, the prevalence of disruption of seed-dispersal mutualisms by invasive species is unclear.     

Patterns of Seed Dispersal and Dispersal Failure in a Hawaiian Dry Forest Having Only Introduced Birds

Dry forests are among the most endangered natural communities in the Hawaiian Islands. Most have been reduced to isolated trees and small forest fragments in which native tree species reproduce poorly. The replacement of native birds by introduced generalists may be contributing to dry forest decline through modification of seed dispersal patterns. To document seed dispersal by introduced birds, we conducted foraging observations on fleshy-fruited trees and measured seed rain under trees and in adjacent open areas for 1 year in a dry forest dominated by native trees. Although trees covered only 15.2 percent of the study area, 96.9 percent of the bird-dispersed seeds were deposited beneath them. The Japanese white-eye (Zosterops japonicus) was the principal dispersal agent. Among bird-dispersed seeds, those of the invasive tree Bocconia frutescens accounted for 75 percent of all seeds collected beneath trees (14.8 seeds/m²/yr) and the invasive shrub Lantana camara accounted for 17 percent. Although nearly 60 percent of the reserve's native woody species possess fleshy fruits, introduced birds rarely disperse their seeds. Native trees accounted for <8 percent of all bird-dispersed seeds and are consequently experiencing dispersal failure by falling directly under parent trees. Smaller-seeded non-native plants, in contrast, may be benefiting from dispersal by introduced birds. Current dispersal patterns suggest that these readily disseminated non-native plants may eventually replace the remaining native flora.     

Potential for ants and vertebrate predators to shape seed-dispersal dynamics of the invasive thistles <Emphasis Type="Italic">Cirsium arvense</Emphasis> and <Emphasis Type="Italic">Carduus nutans</Emphasis> in their introduced range (North America)

The population dynamics of invasive plants are influenced by positive and negative associations formed with members of the fauna present in the introduced range. For example, mutualistic associations formed with pollinators or seed dispersers may facilitate invasion, but reduced fitness from attack by native herbivores can also suppress it. Since population expansion depends on effective seed dispersal, interactions with seed dispersers and predators in a plant species introduced range may be of particular importance. We explored the relative contributions of potential seed dispersers (ants) and vertebrate predators (rodents and birds) to seed removal of two diplochorous (i.e., wind- and ant-dispersed), invasive thistles, Cirsium arvense and Carduus nutans, in Colorado, USA. We also conducted behavior trials to explore the potential of different ant species to disperse seeds, and we quantified which potential ant dispersers were prevalent at our study locations. Both ants and vertebrate predators removed significant amounts of C. arvense and C. nutans seed, with the relative proportion of seed removed by each guild varying by location. The behavior trials revealed clear seed preferences among three ant species as well as differences in the foragers’ abilities to move seeds. In addition, two ant species that acted as potential dispersal agents were dominant at the study locations. Since local conditions in part determined whether dispersers or predators removed more seed, it is possible that some thistle populations benefit from a net dispersal effect, while others suffer proportionally more predation. Additionally, because the effectiveness of potential ant dispersers is taxon-specific, changes in ant community composition could affect the seed-dispersal dynamics of these thistles. Until now, most studies describing dispersal dynamics in C. arvense and C. nutans have focused on primary dispersal by wind or pre-dispersal seed predation by insects. Our findings suggest that animal-mediated dispersal and post-dispersal seed predation deserve further consideration.     

Impacts of introduced species on the biota of an oceanic archipelago: the relative importance of competitive and trophic interactions

Introduced species negatively impact native species through competitive and trophic interactions, particularly on oceanic islands that have never been connected to any continental landmass. However, there are few studies on the relative importance of competitive interactions (resource competition with introduced species) and trophic interactions (predation or herbivory by introduced species) with respect to the negative impacts on native organisms on oceanic islands. A literature review on introduced and native species of the oceanic Ogasawara (Bonin) Islands in the western Pacific Ocean indicated that many native species (e.g., bees, beetles, damselflies, butterflies, land snails, birds, and plants) have been negatively impacted by introduced predators and herbivores (e.g., lizards, rats, flatworms, and goats). Several native plants and bees have been negatively affected by introduced competitors. However, the native species that have competed with introduced species have also suffered from either intense herbivory or predation by other introduced species. Thus, introduced predators and herbivores have had greater impacts on native species than introduced competitors in the Ogasawara Islands.     

Reconstructing past ecological networks: the reconfiguration of seed-dispersal interactions after megafaunal extinction

The late Quaternary megafaunal extinction impacted ecological communities worldwide, and affected key ecological processes such as seed dispersal. The traits of several species of large-seeded plants are thought to have evolved in response to interactions with extinct megafauna, but how these extinctions affected the organization of interactions in seed-dispersal systems is poorly understood. Here, we combined ecological and paleontological data and network analyses to investigate how the structure of a species-rich seed-dispersal network could have changed from the Pleistocene to the present and examine the possible consequences of such changes. Our results indicate that the seed-dispersal network was organized into modules across the different time periods but has been reconfigured in different ways over time. The episode of megafaunal extinction and the arrival of humans changed how seed dispersers were distributed among network modules. However, the recent introduction of livestock into the seed-dispersal system partially restored the original network organization by strengthening the modular configuration. Moreover, after megafaunal extinctions, introduced species and some smaller native mammals became key components for the structure of the seed-dispersal network. We hypothesize that such changes in network structure affected both animal and plant assemblages, potentially contributing to the shaping of modern ecological communities. The ongoing extinction of key large vertebrates will lead to a variety of context-dependent rearranged ecological networks, most certainly affecting ecological and evolutionary processes.     

Introduced galliforms as seed predators and dispersers in Hawaiian forests

In altered communities, novel species’ interactions may critically impact ecosystem functioning. One key ecosystem process, seed dispersal, often requires mutualistic interactions between frugivores and fruiting plants, and functional traits, such as seed width, may affect interaction outcomes. Forests of the Hawaiian Islands have experienced high species turnover, and introduced galliforms, the largest of the extant avian frugivores, consume fruit from both native and non-native plants. We investigated the roles of two galliform species as seed dispersers and seed predators in Hawaiian forests. Using captive Kalij Pheasants (Lophura leucomelanos) and Erckel’s Francolins (Pternistis erckelii), we measured the probability of seed survival during gut passage and seed germination following gut passage. We also examined which seeds are being dispersed in forests on the islands of O’ahu and Hawai’i. We found that galliforms are major seed predators for both native and non-native plants, with less than 5% of seeds surviving gut passage for all plants tested and in both bird species. Gut passage by Kalij Pheasants significantly reduced the probability of seeds germinating, especially for the native plants. Further, larger-seeded plants were both less likely to survive gut passage and to germinate. In the wild, galliforms dispersed native and non-native seeds at similar rates. Overall, our results suggest the introduced galliforms are a double-edged sword in conservation efforts; they may help reduce the spread of non-native plants, but they also destroy the seeds of some native plants. Broadly, we show mutualism breakdown may occur following high species turnover, and that functional traits can be useful for predicting outcomes from novel species’ interactions.

     

Generalist birds govern the seed dispersal of a parasitic plant with strong recruitment constraints

Mistletoes constitute instructive study cases with which to address the role of generalist consumers in the study of plant–animal interactions. Their ranges of safe sites for recruitment are among the most restricted of any plant; therefore, frugivores specializing in mistletoe have been considered almost indispensable for the seed dispersal of these parasitic plants. However, the absence of such specialists in numerous regions inhabited by many mistletoe species raises the question of whether unspecialized vectors may successfully disperse mistletoe seeds to narrowly defined safe sites. Using the European mistletoe Viscum album subsp. austriacum as a study case, we recorded a broad range of 11 bird species that disperse mistletoe seeds. For these species, we studied the mistletoe-visitation rate and feeding behavior to estimate the quantity component of dispersal effectiveness, and the post-foraging microhabitat use, seed handling, and recruitment probabilities of different microhabitats as a measure of the quality component of effectiveness. Both endozoochory and ectozoochory are valid dispersal mechanisms, as the seeds do not need to be ingested to germinate, increasing seed-dispersal versatility. Thrushes were the most effective dispersers, although they were rather inefficient, whereas small birds (both frugivores and non-frugivores) offered low-quantity but high-quality services for depositing seeds directly upon safe sites. As birds behave similarly on parasitized and non-parasitized hosts, and vectors have broad home ranges, reinfection within patches and the colonization of new patches are ensured by an ample assemblage of generalist birds. Thus, a parasitic plant requiring precision in seed dispersal can rely on unspecialized dispersers.     

Dispersal of fig seeds in the Cook Islands: introduced frugivores are no substitutes for natives

Across the Pacific, island vegetation is altering in response to changes in seed disperser assemblages brought about by extinctions and introductions of birds and other animals. On the Cook Islands in the South Pacific, the Pacific Banyan (Ficus prolixa, Moraceae) is undergoing little if any recruitment, possibly linked to a lack of dispersal agents. On Rarotonga, where F. prolixa is found in semi-urban and agricultural environments, there is no recent recruitment in contrast to the situation on Atiu where the tree is common in native forest. We examined the quality and quantity of seed dispersal offered to F. prolixa by the available frugivores on these islands, comparing the effectiveness of extant native and introduced species. The native Cook Islands fauna, particularly birds and bats, appear to be the most effective seed dispersers of F. prolixa, both in terms of quantity and quality. Whilst these are relatively numerous on Atiu, they rarely visit F. prolixa on Rarotonga. The native Chocolate hermit crab Coenobita cavipes is a previously unreported additional native seed disperser, conferring low quantity, but high quality dispersal. Introduced birds and mammals are the most numerous F. prolixa frugivores on Rarotonga and in non-forest environments on Atiu, but they act mainly as seed predators. Consequently, the losses and rarity of remaining native frugivores have not been compensated for by introduced species on Rarotonga which may be contributing to the absence of recruitment there.     

Non-redundancy in seed dispersal and germination by native and introduced frugivorous birds: implications of invasive bird impact on native plant communities

Seed dispersal by vertebrate animals is important for the establishment of many fleshy-fruited plant species. Different frugivorous species can provide different seed dispersal services according to their specific dietary preferences as well as behaviour and body traits (e.g. body size and beak size of birds). Our aim was to study redundancies and complementarities in seed dispersal and germination between the two main native seed disperser birds and the introduced silver pheasant Lophura nycthemera in the temperate Patagonian forests. For this, we collected fresh droppings from the studied species and analyzed seed content. We conducted germination trials for four plant species common in bird droppings; two native species (Aristotelia chilensis and Rhaphithamnus spinosus) and two invasive non-native species (Rubus ulmifolius and Rosa rubiginosa). Both native frugivorous birds and the silver pheasant dispersed fruits of non- native fleshy-fruited plants, but their roles were non-redundant in terms of species dispersed and effect on seed germination. The silver pheasant dispersed a proportionally high number of non-native seeds, while native birds dispersed a high number of native seeds. In addition, the effect of gut treatment in seed germination differed between seed dispersers. Native birds promoted the germination for the two native plant species studied, while the silver pheasant promoted the germination of one non-native plant. This suggests that seed dispersal by the silver pheasant may contribute to the spread of some invasive fleshy-fruited plants in the ecosystems that otherwise would not be dispersed by any other bird. The understanding of redundancies and complementarities on seed dispersal and germination between native and introduced birds will allow improving the management of fleshy-fruited non-native plants.     

Integration of exotic seeds into an Azorean seed dispersal network

Seed dispersal plays a central role in plant ecology. Among animals, birds are particularly important seed dispersers, often incorporating exotic plants into their diets and facilitating their integration in the communities. Network theory offers a highly informative framework to study the structural and functional attributes of complex interactions networks. We used information from bird fecal samples to build a quantitative seed dispersal network for the last fragment of native laurel forest in the island of São Miguel—Azores with three specific objectives: (1) to assess the integration of exotic seeds into seed dispersal; (2) to evaluate the impact of exotic plants in network structure; (3) to test the potential of an exotic species to reduce the seed dispersal of a co-occurring native, via competition for seed dispersers. The seed dispersal network was based on the analysis of 1,121 droppings and described 74 unique interactions between 41 plant species and 7 bird species. Exotic seeds deeply infiltrated into the seed dispersal network forming the majority (59 %) of seeds in the droppings and including those of three globally invasive plants. Overall, birds depended equally on native and exotic fruits despite the lower abundance of the latter in the study area. In an experiment, birds did not show a preference for fruits of the exotic Leycesteria formosa over the native Vaccinium cylindraceum consuming them equally. However, the presence of the exotic plant negatively affected the number of native seeds dispersed, by diverting some of the consumers of the native fruits. Taken altogether the results reveal an alarming invasion level of seed dispersal systems in one of the last remnant native forests of the Azores.     

The conservation physiology of seed dispersal

At a time when plant species are experiencing increasing challenges from climate change, land-use change, harvesting and invasive species, dispersal has become a very important aspect of plant conservation. Seed dispersal by animals is particularly important because some animals disperse seeds to suitable sites in a directed fashion. Our review has two aims: (i) to highlight the various ways plant dispersal by animals can be affected by current anthropogenic change and (ii) to show the important role of plant and (particularly) animal physiology in shaping seed-dispersal interactions. We argue that large-bodied seed dispersers may be particularly important for plant conservation because seed dispersal of large-seeded plants is often more specialized and because large-bodied animals are targeted by human exploitation and have smaller population sizes. We further argue that more specialized seed-dispersal systems on island ecosystems might be particularly at risk from climate change both owing to small population sizes involved but also owing to the likely thermal specialization, particularly on tropical islands. More generally, the inherent vulnerability of seed-dispersal mutualisms to disruption driven by environmental change (as well as their ubiquity) demands that we continue to improve our understanding of their conservation physiology.     

Bird attributes, plant characteristics, and seed dispersal of Pera glabrata (Schott, 1858), (Euphorbiaceae) in a disturbed cerrado area

Several plant characteristics, such as fruit production, nutrient reward, secondary compounds, and fruit color display, affect fruit choice by birds. On the other hand, several bird attributes affect their efficiency as dispersers. Here we investigate the ornithochoric seed dispersal of Pera glabrata Schott (Euphorbiaceae) in a cerrado fragment in southeastern Brazil. A set of bird attributes, such as frequency of visits, number of diaspores eaten, time spent foraging, methods of taking and handling the diaspores and agonistic interactions were analyzed in order to infer about the potential of each species to act as a seed disperser. Birds were the unique seed dispersers of these oil-rich diaspores. We observed 414 bird visits during 60 hours of focal observations in five trees from December 1999 to January 2000. Twenty bird species from seven families ate the diaspores of P. glabrata, but only 14 species were considered potential seed dispersers because they swallowed the diaspores, increasing the probabilities for the seeds to be defecated and/or regurgitated away from the parent trees. The main potential seed dispersers were: Turdus leucomelas (Muscicapidae), Dacnis cayana (Emberizidae), Colaptes melanochloros (Picidae) and Elaenia spp. (Tyrannidae). We did not find any significant seasonal change in the number of visits on the fruiting trees throughout the day. We also did not find any relation between the number of visits per tree and fruit production. The most effective seed dispersers of P. glabrata were generalist birds, which have a high visiting rate, high fruit consumption rate, and spend short periods on the plants. The large number of species recorded as potential seed dispersers of P. glabrata, being most of them very abundant even in Brazilian disturbed areas, may guarantee seed dispersal of this plant in small fragments and regenerating areas.     

Seed removal patterns of pioneer trees in an agricultural landscape

The seeds of most tropical plants are dispersed by animals, many of which also act as seed predators. Shifts in animal community composition, such as those driven by the clearing of native vegetation, are therefore likely to drive changes in plant recruitment. We used manipulative experiments excluding ants, small rodents, and birds to quantify the relative impacts of these granivores on animal-dispersed pioneer trees (ADPT) in fragments of savanna vegetation and adjacent soy plantations in Brazil’s Cerrado. We found that ants were the main consumers of ADPT seeds, that the rates of seed removal varied with seed size, and that removal rates were higher in savanna fragments than in soy plantations. However, we also found significant interactions between habitat type, seed species, and the type of seed predator being excluded. Our results underscore how challenging it can be to predict the influence of human disturbances on the interactions between plant and animal communities. Because ants, rodents, and birds are Cerrado’s the main seed dispersers and granivores, seedling recruitment in Cerrado landscape mosaics will depend on how these distinct but related processes are each influenced by species-specific patterns of seed size and seed abundance.     

Passive restoration following ungulate removal in a highly disturbed tropical wet forest devoid of native seed dispersers

Overabundant ungulate populations can alter forests. Concurrently, global declines of seed dispersers may threaten native forest structure and function. On an island largely devoid of native vertebrate seed dispersers, we monitored forest succession for 7 years following ungulate exclusion from a 5‐ha area and adjacent plots with ungulates still present. We observed succession from open scrub to forest and understory cover by non‐native plants declined. Two trees, native Hibiscus tiliaceus and non‐native Leucaena leucocephala, accounted for most forest regeneration, with the latter dominant. Neither species is dependent on animal dispersers nor was there strong evidence that plants dependent on dispersers migrated into the 5‐ha study area. Passive restoration following ungulate removal may facilitate restoration, but did not show promise for fully restoring native forest on Guam. Restoration of native forest plants in bird depopulated areas will likely require active outplanting of native seedlings, control of factors resulting in bird loss, and reintroduction of seed dispersers.     

Contrasting patterns of seed dispersal between alien mammals and native lizards in a declining plant species

The introduction of carnivorous mammals has led many native island species to extinction. In the Balearic Islands, the introduction of carnivorous mammals in the main islands has contributed to the extinction of the endemic lizard Podarcis lilfordi, which is an important seed disperser of many plant species. One of the introduced mammals, Martes martes (pine marten), is also known to disperse seeds and may replace the native lizard in this role in islands. However, little is known on the patterns of seed dispersal by these two different species and their possible implications for plant regeneration ability and population structure. We have compared the quality of seed deposition provided by both to the vulnerable Mediterranean shrub Cneorum tricoccon, by studying the pattern of seed distribution among sites generated by both dispersers as well as the suitability of those sites for seedling emergence and establishment. The study was carried out in two types of habitats (coastal shrublands and pine forests) and two islands (Mallorca and Dragonera). Lizards and mammals showed contrasting patterns of seed deposition and, where lizards were absent, mammals played their role as seed dispersers in pine forests but not in coastal shrublands. The lack of seed dispersal in the coastal shrubland seriously limits recruitment, by concentrating seeds under conspecifics and hindering colonisation opportunities, marking a long-term trend towards decline. The introduced predator replaces the native seed disperser in its service where it has gone extinct, although with important differences in the dispersal service provided.     

When do herbivores affect plant invasion? Evidence for the natural enemies and biotic resistance hypotheses

Two venerable hypotheses, widely cited as explanations for either the success or failure of introduced species in recipient communities, are the natural enemies hypothesis and the biotic resistance hypothesis. The natural enemies hypothesis posits that introduced organisms spread rapidly because they are liberated from their co‐evolved predators, pathogens and herbivores. The biotic resistance hypothesis asserts that introduced species often fail to invade communities because strong biotic interactions with native species hinder their establishment and spread. We reviewed the evidence for both of these hypotheses as they relate to the importance of non‐domesticated herbivores in affecting the success or failure of plant invasion.
To evaluate the natural enemies hypothesis, one must determine how commonly native herbivores have population‐level impacts on native plants. If native herbivores seldom limit native plant abundance, then there is little reason to think that introduced plants benefit from escape from these enemies. Studies of native herbivore‐native plant interactions reveal that plant life‐history greatly mediates the strength with which specialist herbivores suppress plant abundance. Relatively short‐lived plants that rely on current seed production for regeneration are most vulnerable to herbivory that reduces seed production. As such, these plants may gain the greatest advantage from escaping their specialist enemies in recipient communities. In contrast, native plants that are long lived or that possess long‐lived seedbanks may not be kept “in check” by native herbivores. For these species, escape from native enemies may have little to do with their success as exotics; they are abundant both where they are native and introduced.
Evidence for native herbivores providing biotic resistance to invasion by exotics is conflicting. Our review reveals that: 1) introduced plants can attract a diverse assemblage of native herbivores and that 2) native herbivores can reduce introduced plant growth, seed set and survival. However, the generality of these impacts is unclear, and evidence that herbivory actually limits or reduces introduced plant spread is scarce. The degree to which native herbivores provide biotic resistance to either exotic plant establishment or spread may be greatly determined by their functional and numerical responses to exotic plants, which we know little about. Generalist herbivores, through their direct effects on seed dispersal and their indirect effects in altering the outcome of native–non‐native plant competitive interactions, may have more of a facilitative than negative effect on exotic plant abundance.     

Allozyme diversity and the evolution ofSymplocos (Symplocaceae) on the Bonin (Ogasawara) Islands

To study the origin and speciation of plants in oceanic islands, electrophoretic analyses have been done on three endemic species ofSymplocos in the Bonin Islands as well as on three other species;S. kuroki, S. nakaharae andS. tanakae which are considered to be closely related to the Bonin endemics. There occur three species:S. kawakamii, S. pergracilis andS. boninensis in Bonin. The genusSymplocos is one which is considered to be diversified in the Bonin Islands. Seven enzyme systems presumed to be encoded by 18 loci were examined. The genetic diversity was low in the island species, as reported in some oceanic island plants of Hawaii and the Bonin Islands. The three endemics share high genetic identities and they clustered together in the tree drawn by the UPGMA method, suggesting that they are a monophyletic group, that is, they result from a single introduction.     

The geographic distribution of seed-dispersal mutualisms in North America

It is often difficult to determine the relative importance of different sorts of species interactions in shaping community structure or how communities function because we lack basic information on patterns of occurrence of biotic interactions. Here we determine the geographic distribution of seed-dispersal mutualisms across North America, and then test the hypotheses that those patterns are correlated with mean annual precipitation and latitude. We analyze the floras of 197 sites across North America to identify which species of native seed plants are dispersed by animals in one of three types of plant-animal mutualisms: frugivory, scatter hoarding, and ants. We identified 1655 plant species that are involved in these seed-dispersal mutualisms. 16.5 ± 6.5% of all seed plants across North America are dispersed by animals; 10.0 ± 4.2% by frugivores, 3.7 ± 1.7% by scatter hoarders, and 3.9 ± 2.2% by ants. Secondary dispersal by a different mode and vector (e.g., wind dispersal followed by scatter hoarding, or ballistic dispersal followed by myrmecochory) occurred in 16.8% of all plant mutualist records. Although each mode of dispersal showed a different pattern, the prevalence of seed-dispersal mutualisms increased with mean annual precipitation. The prevalence of seed dispersal by frugivory and scatter-hoarding decreased with increasing latitude. The prevalence of seed-dispersal interactions varies dramatically across North America. The center of greatest diversity for all three types of seed-dispersal mutualisms is the eastern United States, roughly coincident with the eastern deciduous forest. Knowing the distribution of species interactions improves our understanding of how the structure and functioning of communities varies across environmental gradients.     

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.