Ecology of the fruit-colour polymorphism in Rubus spectabilis

Although some studies have focused on the colour polymorphisms of flowers and fruits, little is known of their ecological and evolutionary significance. We investigated the potential contribution of several factors to the maintenance of fruit-colour polymorphism in Rubus spectabilis, a common shrub in the temperate rainforests of southeast Alaska. Fruits occur in two colours (red and orange), whose frequencies vary geographically. The two colour morphs have similar size, weight, seed load and nutrient composition. Colour preferences of avian frugivores, in the aviary and in the field, varied among individuals, but the majority favoured red fruits. Seed predators (mostly rodents) did not discriminate between seeds from different morphs. The effect of seed passage through the digestive tract of frugivores (birds and bears) on germination was similar for both morphs, although there were significant differences among frugivores. The type of soil on which the seeds are deposited influenced their germination behaviour, suggesting that some soils could favour one morph over the other. Such differences may contribute to the regional differences in frequencies of the two morphs. This study emphasizes the need to investigate fruit and seed characteristics that correlate with fruit colour; the colour preferences of consumers is only one of several selection pressures that determine the frequency distribution of fruit colours. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fruit colour polymorphism in Acacia ligulata: seed and seedling performance, clinal patterns, and chemical variation

Fruit colour polymorphisms are widespread in nature, but their ecological and evolutionary dynamics remain poorly understood. Here we examine Acacia ligulata, a shrub of the Australian arid zone which exhibits a red/orange/yellow aril colour polymorphism. We asked whether the polymorphism had a genetic basis; whether selection acted differentially on morphs during the seed and seedling stages; whether geographic variation in morph frequencies was correlated with environmental factors; and whether morphs differed in physical or chemical characteristics that might influence selection on them. When grown to maturity in a common greenhouse environment, maternal families of seeds showed phenotypic patterns consistent with biparental genetic control of the polymorphism. In contrast to other fruit-colour polymorphic species, progeny of A. ligulata morphs did not vary in rates of seedling emergence or survival in a common garden. Sampling along a 580 km transect revealed clinal variation in morph frequencies. Frequencies of the yellow morph decreased, and frequencies of the red morph increased, across a gradient of decreasing temperature and increasing rainfall. Morphs did not differ in seed mass, aril mass, or in profiles of fatty acids and flavonoids in either arils or seeds. However, morphs showed consistent differences in carotenoid profiles' and elemental content of arils, suggesting that selection by avian and insect seed dispersers, seed predators and herbivores should be investigated. These patterns indicate that both abiotic and biotic factors may contribute to selection on the A. ligulata polymorphism. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of type of avian frugivory on the fitness ofPistacia terebinthus L.

Summary The fruits ofPistacia terebinthus, a circum-Mediterranean tree/shrub, are consumed by an array of bird species that differ in feeding methods and in relative frequencies of visits to plants. In this study I document interindividual variation in the proportion of fruits consumed by three types of frugivores: legitimate dispersers, pulp-consumers and seed predators. The results show that the relative frequencies of each kind of frugivore notably influence the final reproductive output (absolute number of viable seeds dispersed) and in fact prevail over the effects of pre-dispersal factors acting on plant fitness. Those relative frequencies are not associated with any of the plant traits related to fitness, such as fruit crop size and the number of viable seeds produced, suggesting that the type of avian frugivory exerts a negligible, if not null, selective pressure on such plant attributes. Plant specialization to attract the most effective seed dispersers seems to be precluded, given the small scale at which the high variation in seed dispersal success takes place.     

Signal convergence in fruits: a result of selection by frugivores?

The Dispersal Syndrome hypothesis remains contentious, stating that apparently nonrandom associations of fruit characteristics result from selection by seed dispersers. We examine a key assumption under this hypothesis, i.e. that fruit traits can be used as reliable signals by frugivores. We first test this assumption by looking at whether fruit colour allows birds and primates to distinguish between fruits commonly dispersed by birds or primates. Second, we test whether the colours of fruits dispersed by primates are more contrasting to primates than the colours of bird‐dispersed fruits, expected if fruit colour is an adaptation to facilitate the detection by seed dispersers. Third, we test whether fruit colour has converged in unrelated plant species dispersed by similar frugivores. We use vision models based on peak sensitivities of birds’ and primates’ cone cells. We base our analyses on the visual systems of two types of birds (violet and ultraviolet based) and three types of primates (trichromatic primates from the Old and the New Worlds, and a dichromatic New World monkey). Using a Discriminant Function Analysis, we find that all frugivore groups can reliably discriminate between bird‐ and primate‐dispersed fruits. Fruit colour can be a reliable signal to different seed dispersers. However, the colours of primate‐dispersed fruits are less contrasting to primates than those of bird‐dispersed fruits. Fruit colour convergence in unrelated plants is independent of phylogeny and can be better explained by disperser type, which supports the hypothesis that frugivores are important in fruit evolution. We discuss adaptive and nonadaptive hypotheses that can potentially explain the pattern we found.     

Vertebrate Fruit Removal and Ant Seed Dispersal in the Neotropical Ginger Renealmia alpinia (Zingiberaceae)*

Plants frequently display fruit characteristics that support multiple seed‐dispersal syndromes. These ambiguous characteristics may reflect the fact that seed dispersal is usually a complex process involving multiple dispersers. This is the case for the Neotropical ginger Renealmia alpinia (Zingiberaceae). It was originally suggested that the aromatic fruits of R. alpinia located at the base of the plant are adapted for terrestrial mammal seed dispersal. However, the dark‐purple coloration of the fruits and bright orange aril surrounding the seeds suggest that birds may play a role in R. alpinia seed dispersal. At La Selva Biological Station, Costa Rica, we used camera traps to record vertebrate visits to infructescences of R. alpinia. Most visitors were toucans and aracaris (Ramphastidae). However fruits were also removed by terrestrial mammals (coatis and armadillos). In addition to vertebrate fruit removal, some of the fruits dehisce and the seeds that fall on the ground are dispersed by ants. Fruitfall traps showed that 77 percent of fruits are removed by vertebrates. However, 15 percent of fruits fall to the base of parent plants to be potentially dispersed by ants. Experiments using a laboratory ant colony showed that ants are effective seed dispersers of R. alpinia. Ant seed manipulation increased germination success and reduced time to germination. In conclusion, primary seed dispersal in the Neotropical ginger R. alpinia is mostly performed by birds, additionally ants are effective dispersers at short distances. Seed dispersal in R. alpinia is a complex process involving a diverse array of dispersal agents.     

Importance of animal and plant traits for fruit removal and seedling recruitment in a tropical forest

The traits of animals and plants influence their interaction networks, but the significance of species' traits for the resulting ecosystem functions is poorly understood. A crucial ecosystem function in the tropics is seed dispersal by animals. While the importance of species' traits for structuring plant–frugivore networks is supported by a number of studies, no study has so far identified the functional traits determining the subsequent processes of fruit removal and seedling recruitment. Here, we conducted a comprehensive field study on fruit removal by frugivorous birds and seedling recruitment along an elevational gradient in the Colombian Andes. We measured morphological traits of birds (body mass, bill width, Kipp's index) and plants (plant height, crop mass, fruit width and seed mass) which we expected to be related to fruit removal and seedling recruitment. We tested 1) which bird and plant traits influence fruit removal, and 2) whether network metrics at plant species level, functional identities of frugivores (community‐based mean trait values) and/or plant traits were the main determinants of seedling recruitment. We found that large‐bodied bird species contributed more to fruit removal than small‐bodied bird species and that small‐sized fruits were more frequently removed than large‐sized fruits. Small plant species and plants with heavy seeds recruited more seedlings than did large plants and plants with light seeds. Network metrics and functional identities of seed dispersers were unrelated to seedling recruitment. Our findings have two important implications. First, large birds are functionally more important than small birds in tropical seed‐removal networks. Second, the detected tradeoff between fruit size and seed mass in subsequent recruitment processes suggests that the adaptability of forest plant communities to a loss of large frugivores is limited by life‐history constraints. Hence, the protection of large‐bodied frugivores is of primary importance for the maintenance of diverse tropical plant communities.     

Incorporating dispersal distance into the disperser effectiveness framework: frugivorous birds provide complementary dispersal to plants in a patchy environment

Fleshy-fruited plants are usually dispersed by an array of frugivores, differing in the effectiveness of the dispersal service they provide to the plant. Body size differences among frugivores are hypothesized to affect seed dispersal distances and consequently their effectiveness as dispersers. We tested this hypothesis by comparing the effectiveness of two passerine birds, grackles ( Onychognathus tristramii ) and bulbuls ( Pycnonotus xanthopygos ), dispersing the desert shrub Ochradenus baccatus . Laboratory experiments, quantifying gut retention time and the effect on germination, were combined with field observations quantifying bird movements and fruit consumption rates. An empirically parameterized mechanistic model showed that the two dispersers switch roles as a function of spatial-scale: while most seeds within the local habitat were dispersed by bulbuls, the larger grackles were exclusively responsible for between-patches, long-distance dispersal. We suggest that distance-related differences are common and important to plant fitness, and thus should explicitly be considered in studies of disperser effectiveness.     

Impact of habitat structure and fruit abundance on avian seed dispersal and fruit predation

So far, it is poorly understood how differential responses of avian seed dispersers and fruit predators to changes in habitat structure and fruit abundance along land-use gradients may translate into consequences for the seed dispersal of associated plants. We selected a gradient of habitat modification (forest, semi-natural, and rural habitat) characterized by decreasing tree cover and a high variation in local fruit availability. Along this gradient we quantified fruit removal by avian seed dispersers and fruit predators from 18 Sorbus aucuparia trees. We analyzed the relative importance of tree cover and fruit abundance in explaining species richness, abundance and fruit removal rates of both guilds from S. aucuparia trees. Species richness and abundance of seed dispersers decreased with decreasing tree cover, whereas fruit removal by seed dispersers decreased with decreasing fruit abundance independent of tree cover. Both variables had no effect on species richness, abundance and fruit removal by fruit predators. Consequently, seed dispersers dominated relative fruit removal in fruit-rich sites but the dispersal/predation ratio shifted in favor of predation in fruit-poor habitat patches. Our study demonstrates that variation in local habitat structure and fruit abundance can cause guild-specific responses. Such responses may result in a shift in fruit removal regimes and might affect the dispersal ability of dependent fruiting plants. Future studies should aim at possible consequences for plant recruitment and guild-specific responses of frugivores to disturbance gradients on the level of entire plant–frugivore associations.     

Fruit color polymorphism in a bird-dispersed shrub (Rhagodia parabolica) in Australia

Summary We examined the selective basis of fruit color, a trait assumed to affect the attractiveness of fruits to avian dispersal agents, inRhagodia parabolica in Victoria, Australia. The fleshy fruits ofR. parabolica are highly polymorphic in color; individual plants may bear red, white, or yellow fruits, or some combination of these. Red color is produced by betacyanins, yellow by betaxanthins, and white by some other flavonoid compounds. Red is the most common color, white less common, and yellow is relatively rare; red and white is the most common color combination. Fruits of these color-morphs do not differ in maximum size, weight, pulp-seed ratio, water content, or content of major nutrients (sugars, N, and lipids). The most common avian consumer,Zosterops lateralis, foraged at random on the color-morphs, although all fruit-eaters, collectively, slightly favored white fruits. Weekly fruit removal rates were equivalent for all four morphs. Seeds from color-morphs differed in seed germination behavior: seeds from red fruits germinated faster than seeds from yellow fruits, and seeds from white fruits showed the greatest response to passage throughZ. lateralis guts. Lack of strong foraging preferences by birds and the different germination behaviors may contribute to the maintenance of the color polymorphism.     

Correlated evolution of fig size and color supports the dispersal syndromes hypothesis

The influence of seed dispersers on the evolution of fruit traits remains controversial, largely because most studies have failed to account for phylogeny and or have focused on conservative taxonomic levels. Under the hypothesis that fruit traits have evolved in response to different sets of selective pressures by disparate types of seed dispersers (the dispersal syndromes hypothesis), we test for two dispersal syndromes, defined as groups of fruit traits that appear together more often than expected by chance. (1) Bird syndrome fruits are brightly colored and small, because birds have acute color vision, and commonly swallow fruits whole. (2) Mammal syndrome fruits are dull-colored and larger on average than bird syndrome fruits, because mammals do not rely heavily on visual cues for finding fruits, and can eat fruits piecemeal. If, instead, phylogenetic inertia determines the co-occurrence of fruit size and color, we will observe that specific combinations of size and color evolved in a small number of ancestral species. We performed a comparative analysis of fruit traits for 64 species of Ficus (Moraceae), based on a phylogeny we constructed using nuclear ribosomal DNA. Using a concentrated changes test and assuming fruit color is an independent variable, we found that small-sized fruits evolve on branches with red and purple figs, as predicted by the dispersal syndromes hypothesis. When using diameter as the independent variable, results vary with the combination of algorithms used, which is discussed in detail. A likelihood ratio test confirms the pattern found with the concentrated changes test using color as the independent variable. These results support the dispersal syndromes hypothesis.     

Weak interactions between avian and insect frugivores: the case of Pistacia terebinthus L. (Anacardiaceae)

Vertebrate frugivores often feed on fruits upon or within which insects also feed, yet little information exists on the potential magnitude of interactions between these consumers. The Mediterranean shrub Pistacia terebinthus, the birds that consume its fruits, and the wasps that feed upon its seeds are examined in this study. P. terebinthus produces a highly variable fraction of final-sized red fruits that never become mature (green-colored). Red fruits can be immature, parthenocarpic, aborted, or attacked by wasps, and their pulp is much less nutritious than that of mature fruits. A total of 20 bird species consumed the fruits in the study area. Legitimate dispersers accounted for 39% of the total fruit removal, while pulp eaters and seed predators accounted for the remainder. Birds strongly preferred the mature fruits (only 4% of the fruits consumed were red). The incidence of wasps in the seeds ranged from 0 to 42% of the crop in 1989 and from 0 to 24% in 1990. The influence of avian and insect frugivore guilds on each other appears to be quite low because of the narrow overlap in resource utilization by birds and wasps, and an overall low intensity of wasp seed predation. From an evolutionary perspective, the possible ability of wasps to preclude fruit maturation appears not to be attributable to the present interaction with avian frugivores.     

An invasive tree alters the structure of seed dispersal networks between birds and plants in French Polynesia

Aim We studied how the abundance of the highly invasive fruit‐bearing tree Miconia calvescens DC. influences seed dispersal networks and the foraging patterns of three avian frugivores. Location Tahiti and Moorea, French Polynesia. Methods Our study was conducted at six sites which vary in the abundance of M. calvescens. We used dietary data from three frugivores (two introduced, one endemic) to determine whether patterns of fruit consumption are related to invasive tree abundance. We constructed seed dispersal networks for each island to evaluate how patterns of interaction between frugivores and plants shift at highly invaded sites. Results Two frugivores increased consumption of M. calvescens fruit at highly invaded sites and decreased consumption of other dietary items. The endemic fruit dove, Ptilinopus purpuratus, consumed more native fruit than either of the two introduced frugivores (the red‐vented bulbul, Pycnonotus cafer, and the silvereye, Zosterops lateralis), and introduced frugivores showed a low potential to act as dispersers of native plants. Network patterns on the highly invaded island of Tahiti were dominated by introduced plants and birds, which were responsible for the majority of plant–frugivore interactions. Main conclusions Shifts in the diet of introduced birds, coupled with reduced populations of endemic frugivores, caused differences in properties of the seed dispersal network on the island of Tahiti compared to the less invaded island of Moorea. These results demonstrate that the presence of invasive fruit‐bearing plants and introduced frugivores can alter seed dispersal networks, and that the patterns of alteration depend both on the frugivore community and on the relative abundance of available fruit.     

Selection on fruit traits is mediated by the interplay between frugivorous birds,fruit flies,parasitoid wasps and seed‐dispersing ants

Every organism on Earth must cope with a multitude of species interactions both directly and indirectly throughout its life cycle. However, how selection from multiple species occupying different trophic levels affects diffuse mutualisms has received little attention. As a result, how a given species amalgamates the combined effects of selection from multiple mutualists and antagonists to enhance its own fitness remains little understood. We investigated how multispecies interactions (frugivorous birds, ants, fruit flies and parasitoid wasps) generate selection on fruit traits in a seed dispersal mutualism. We used structural equation models to assess whether seed dispersers (frugivorous birds and ants) exerted phenotypic selection on fruit and seed traits in the spiny hackberry (Celtis ehrenbergiana), a fleshy‐fruited tree, and how these selection regimes were influenced by fruit fly infestation and wasp parasitoidism levels. Birds exerted negative correlational selection on the combination of fruit crop size and mean seed weight, favouring either large crops with small seeds or small crops with large seeds. Parasitoids selected plants with higher fruit fly infestation levels, and fruit flies exerted positive directional selection on fruit size, which was positively correlated with seed weight. Therefore, higher parasitoidism indirectly correlated with higher plant fitness through increased bird fruit removal. In addition, ants exerted negative directional selection on mean seed weight. Our results show that strong selection on phenotypic traits may still arise in perceived diffuse species interactions. Overall, we emphasize the need to consider diverse direct and indirect partners to achieve a better understanding of the mechanisms driving phenotypic trait evolution in multispecies interactions.     

A Study of Fruit–Frugivore Interactions in Two Species of Durian (Durio, Bombacaceae) in Sabah, Malaysia

We determined the fruit predators and seed dispersers of two species of durian ( Durio , Bombacaceae), D. graveolens and D. zibethinus , which have contrasting aril color and timing of dehiscence by observing fruiting trees in Deramakot forest reserve. In our observations in the wild, both species were extensively predated by orangutans before fruits matured. Durio graveolens was dispersed by black hornbills and D. zibethinus was dispersed by long-tailed macaques.     

黄檗的更新特点及食果实鸟类对其种子的传播

在帽儿山实验林场和哈尔滨实验林场 ,黄檗母树下没有幼苗分布 ,不能进行天然更新 ,需要靠食果实鸟类等将果实和种子传播至远离母树的其他林下。捕食黄檗果实的食果实鸟类有 9种。其中 6种是食果肉鸟类 ,吞入果实后 ,消化果肉 ,而种子完整地随粪便排出而得以传播。其余 3种是食种子鸟类 ,没有传播作用。果实在鸟类消化道内的滞留时间达 2 0～ 30min ,具有很长的潜在传播距离。将鸟类消化后的种子与完整果实和人为去果肉种子进行萌发对比实验 ,消化后种子的累计萌发率与其余二者之间均没有显著性差异 ,说明食果实鸟类的消化 (道 )过程对种子萌发没有明显影响 ,同时证明果肉对种子萌发率没有显著影响 ,果肉中不含萌发抑制物质。黄檗提供多种鸟类以食物 ,而鸟类也同时以多种肉质果植物为食物。因此食果实鸟类和肉质果植物 (包括黄檗 )之间形成了松散的互利共生关系     

Mistletoe fruit‐colour polymorphism and differential success in a habitat mosaic

Abstract Polymorphisms in fruit colour are common in nature, but mechanistic explanations for the factor(s) responsible for their maintenance are for the most part lacking. Past studies have focused on frugivore colour preferences and fruit removal rates, but until recently there has been no evidence that these factors are responsible for the maintenance of the polymorphisms. For other types of genetic polymorphisms, habitat heterogeneity has been shown to play a role in their maintenance. Here we test the habitat heterogeneity hypothesis for a polymorphic New Zealand mistletoe. We show that red‐fruited and orange‐fruited morphs of the mistletoe Alepis flavida (Hook. F) Tiegh. (Loranthaceae) differ in their growth, mortality and flowering on forest edges and in forest interior. Red‐fruited morphs, which are preferred by dispersers, grew, survived and flowered as well as orange‐fruited morphs on edges, whereas orange‐fruited morphs had much greater growth, survival, and flowering than red‐fruited morphs in the forest interior. This is the first evidence that habitat‐specific differences in growth and survival may contribute to maintaining fruit‐colour polymorphisms.     

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.     

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.     

Fruit Bats (Chiroptera: Pteropodidae) as Seed Dispersers and Pollinators in a Lowland Malaysian Rain Forest1

The aims of this study were to (1) characterize the food resources exploited by fruit bats (Pteropodidae) within an old‐growth Malaysian dipterocarp forest, (2) test the viability of the seeds they disperse, and (3) provide an estimate of the proportion of trees that are to some degree dependent upon bats for seed dispersal and/or pollination. Fruit species exploited by bats could be distinguished from those eaten by birds largely on the basis of color (as perceived by human beings). Bat‐dispersed fruits were typically inconspicuous shades of green–yellow or dull red–brown, whereas fruits eaten by birds were generally bright orange to red. Dietary overlap between bats and nonflying mammals was relatively high. In contrast to primates and squirrels, which were major seed predators for several of the plant species under investigation, fruit bats had no negative impact on seed viability. A botanical survey in 1 ha of old‐growth forest revealed that 13.7 percent of trees (?15 cm girth at breast height) were at least partially dependent upon fruit bats for pollination and/or seed dispersal.     

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.