Contagious seed dispersal beneath heterospecific fruiting trees and its consequences

The timing of frugivore-mediated seed dispersal effectiveness

The seed dispersal effectiveness framework allows assessing mutualistic services from frugivorous animals in terms of quantity and quality. Quantity accounts for the number of seeds dispersed and quality for the probability of recruitment of dispersed seeds. Research on this topic has largely focused on the spatial patterns of seed deposition because seed fates often vary between microhabitats due to differences in biotic and abiotic factors. However, the temporal dimension has remained completely overlooked despite these factors-and even local disperser assemblages-can change dramatically during long fruiting periods. Here, we test timing effects on seed dispersal effectiveness, using as study case a keystone shrub species dispersed by frugivorous birds and with a fruiting period of 9 months. We evaluated quantity and quality in different microhabitats of a Mediterranean forest and different periods of the fruiting phenophase. We identified the bird species responsible for seed deposition through DNA barcoding and evaluated the probability of seedling recruitment through a series of field experiments on sequential demographic processes. We found that timing matters: The disperser assemblage was temporally structured, seed viability decreased markedly during the plant's fruiting phenophase, and germination was lower for viable seeds dispersed in the fruiting peak. We show how small contributions to seed deposition by transient migratory species can result in a relevant effectiveness if they disperse seeds in a high-quality period for seedling recruitment. This study expands our understanding of seed dispersal effectiveness, highlighting the importance of timing and infrequent interactions for population and community dynamics.

Frugivores bias seed-adult tree associations through nonrandom seed dispersal: a phylogenetic approach

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.

Interspecific associations in seed arrival and seedling recruitment in a Neotropical forest

Contagious seed dispersal refers to the tendency for some sites to receive many dispersed seeds while other sites receive few dispersed seeds. Contagious dispersal can lead to interspecific associations in seed arrival, and this in turn might lead to interspecific associations in seedling recruitment. We evaluate the extent of spatially contagious seed arrival, the frequency of positive interspecific associations in seed arrival, and their consequences for seedling recruitment at the community level in a tropical moist forest. We quantified seed arrival to 200 passive seed traps for 28 yr of weekly censuses and seedling recruitment to 600 1-m² quadrats for 21 yr of annual censuses on Barro Colorado Island, Panama. We assessed whether spatially contagious seed dispersal was more important among zoochorous species than among anemochorous species, increased in importance with similarity in fruiting times, and led to interspecific associations in seed arrival and seedling recruitment. We controlled adult seed source associations statistically to evaluate predicted relationships. We found that spatially contagious seed arrival was widespread among zoochorous species, but also occurred among anemochorous species when the strong, consistent trade winds were present. Significant interspecific associations in seed arrival were more likely for pairs of species with zoochorous seeds and similar fruiting times and persisted through seedling recruitment. Thus, interspecifically contagious seed dispersal affects local species composition and alters the mixture of interspecific interactions through the seed, germination, and early seedling stages in this forest. Future investigations should consider the implications of interspecific association at the regeneration stages documented here for later life stages and species coexistence.

Closing the gaps for animal seed dispersal: Separating the effects of habitat loss on dispersal distances and seed aggregation

Habitat loss can alter animal movements and disrupt animal seed dispersal mutualisms; however, its effects on spatial patterns of seed dispersal are not well understood. To explore the effects of habitat loss on seed dispersal distances and seed dispersion (aggregation), we created a spatially explicit, individual‐based model of an animal dispersing seeds (SEADS—Spatially Explicit Animal Dispersal of Seeds) in a theoretical landscape of 0%–90% habitat loss based on three animal traits: movement distance, gut retention time, and time between movements. Our model design had three objectives: to determine the effects of (1) animal traits and (2) habitat loss on seed dispersal distances and dispersion and (3) determine how animal traits could mitigate the negative effects of habitat loss on these variables. SEADS results revealed a complex interaction involving all animal traits and habitat loss on dispersal distances and dispersion, driven by a novel underlying mechanism of fragment entrapment. Unexpectedly, intermediate habitat loss could increase dispersal distances and dispersion relative to low and high habitat loss for some combinations of animal traits. At intermediate habitat loss, movement between patches was common, and increased dispersal distances and dispersion compared to continuous habitats because animals did not stop in spaces between fragments. However, movement between patches was reduced at higher habitat loss as animals became trapped in fragments, often near the parent plant, and dispersed seeds in aggregated patterns. As movement distance increased, low time between movements and high gut retention time combinations permitted more movement to adjacent patches than other combinations of animal traits. Because habitat loss affects movement in a nonlinear fashion under some conditions, future empirical tests would benefit from comparisons across landscapes with more than two levels of fragmentation.

Fruit availability, frugivore satiation and seed removal in 2 primate-dispersed tree species

During a mast-fruiting event we investigated spatial variability in fruit availability, consumption, and seed removal at two sympatric tree species, Manilkara bidentata and M. huberi (Sapotaceae) at Nouragues Natural Reserve, French Guiana. We addressed the question of how Manilkara density and fruits at the community level might be major causes of variability in feeding assemblages between tree species. We thus explored how the frugivore assemblages differed between forest patches with contrasting relative Manilkara density and fruiting context. During the daytime, Alouatta seniculus was more often observed in M. huberi crowns at Petit Plateau (PP) with the greatest density of Manilkara spp. and the lowest fruit diversity and availability, whereas Cebus apella and Saguinus midas were more often observed in M. bidentata crowns at both Grand Plateau (GP), with a lowest density of M. bidentata and overall greater fruit supply, and PP. Overall, nearly 53% and 15% of the M. bidentata seed crop at GP and PP, respectively, and about 47% of the M. huberi seed crop were removed, otherwise either spit out or defecated beneath trees, or dropped in fruits. Small-bodied primates concentrated fallen seeds beneath parent trees while large-bodied primate species removed and dispersed more seeds away from parents. However, among the latter, satiated A. seniculus wasted seeds under conspecific trees at PP. Variations in feeding assemblages, seed removal rates and fates possibly reflected interactions with extra-generic fruit species at the community level, according to feeding choice, habitat preferences and ranging patterns of primate species.

Hierarchical mechanisms of spatially contagious seed dispersal in complex seed-disperser networks

Intra- and interspecific spatially contagious seed dispersal has far-reaching implications for plant recruitment, distribution, and community assemblage. However, logistical and analytical limitations have curtailed our understanding concerning the mechanisms and resulting spatial patterns of contagious seed dispersal in most systems and, especially, in complex seed-disperser networks. We investigated mechanisms of seed aggregation using techniques of spatial point pattern analysis and extensive data sets on mutispecific endozoochorous seed rain generated by five frugivorous mammals in three Mediterranean shrublands over two seasons. Our novel analytical approach revealed three hierarchical and complementary mechanisms of seed aggregation acting at different levels (fecal samples, seeds, pairs of seed species) and spatial scales. First, the three local guilds of frugivores tended to deliver their feces highly aggregated at small and intermediate spatial scales, and the overall pattern of fecal delivery could be described well by a nested double-cluster Thomas process. Second, once the strong observed fecal aggregation was accounted for, the distribution of mammal feces containing seeds was clustered within the pattern of all feces (i.e., with and without seeds), and the density of fecal samples containing seeds was higher than expected around other feces containing seeds in two out of the three studied seed-disperser networks. Finally, at a finer level, mark correlation analyses revealed that for some plant species pairs, the number of dispersed seeds was positively associated either at small or large spatial scales. Despite the relatively invariant patterning of nested double-clustering, some attributes of endozoochorous seed rain (e.g., intensity, scales of aggregation) were variable among study sites due to changes in the ecological context in which seeds and their dispersers interact. Our investigation disentangles for the first time the hierarchy of synergic mechanisms of spatially contagious seed dispersal at a range of spatial scales in complex seed-disperser networks, thus providing a robust and widely applicable framework for future studies.

Early recruitment dynamics in tropical restoration

Unassisted secondary succession in abandoned tropical pastures often results in species-poor forests of pioneer trees that persist for decades. We characterize recruitment rates of woody vegetation in planting treatments during the first 60 months of experimental restoration on thin, eroded soils at Los Tuxtlas, southern Mexico. We test the hypothesis that recruitment of later-successional trees is greater in fenced plots planted with native trees than in fenced controls that simulate natural succession, and further that recruitment of such species would be greater in plots planted with animal-dispersed trees than in those planted with wind-dispersed trees. Results indicated much greater recruitment of later-successional animal-dispersed trees in planted plots as compared with controls. Three censuses per year recorded 960 recruited individuals of 44 species of trees and shrubs from 20-60 months after cattle exclusion. Ninety-six percent of recruits were not of planted species. Repeated-measures analyses of variance indicated that recruited communities included more species of pioneers than of later-successional trees and shrubs, with more individuals and species dispersed by animals than by wind. Recruitment of pioneers did not differ between control and planted plots. Later-successional recruits dispersed by animals accumulated > 10 times faster in planted than control plots, with apparent acceleration after planted Cecropia obtusifolia and Ficus yoponensis first produced fleshy fruits 48 months after cattle exclusion. Sparse later-successional wind-dispersed recruits did not differ by treatment. Our preliminary results over the first five years after cattle exclusion indicate that planted stands clearly accelerate succession through accumulation of later-successional trees and shrubs dispersed by animals.

Reliable cues and signals of fruit quality are contingent on the habitat in black elder (Sambucus nigra)

Communication mediates interactions between organisms and can be based on signals or cues. Signals are selected for their signaling function, whereas cues evolve for reasons other than signaling. To be evolutionarily stable, communication needs to be reliable on average, but the mechanisms that enforce reliability are hotly debated in light of strong environmental influence on signals and cues. While fruit quality in black elder (Sambucus nigra) is unrelated to fruit color, it is indicated by alternative pedicel phenotypes. Information on fruit quality has thus been transferred from the fruit to the developmentally associated pedicels, which are environmentally determined cues. Within each phenotype, color variation indicates fruit quality. Communication by black elder is thus reliable, but the proximate mechanisms enforcing reliability are habitat specific. High irradiance increases both the contrasts of the visual cue and fruit quality in the anthocyanin-based red pedicel phenotype, while shaded plants of the chlorophyll-based green phenotype apparently use signals by forgoing photosynthesis. This is because lower chlorophyll content in green pedicels creates contrasting pedicels, and higher contrasts indicate higher sugar content in the fruits of green pedicels. Because anthocyanins are light-induced, plants use cues when exposed to high irradiance, whereas they apparently use costly signals in the shade by reducing chlorophyll content in the pedicels. In behavioral field and laboratory experiments we document that avian seed dispersers select among pedicel phenotypes that indicate different fruit quality. Plants can thus increase their reproductive success by sending highly informative cues. Our results indicate how reliable information transfer can be maintained both in cues and signals in spite of substantial environmental influence on visual traits.