Crop Size and Fruit Neighborhood Effects on Bird Visitation to Fruiting Schefflera morototoni Trees in Puerto Rico1

Intrinsic and extrinsic drivers of intraspecific variation in seed dispersal are diverse and pervasive

There is growing realization that intraspecific variation in seed dispersal can have important ecological and evolutionary consequences. However, we do not have a good understanding of the drivers or causes of intraspecific variation in dispersal, how strong an effect these drivers have, and how widespread they are across dispersal modes. As a first step to developing a better understanding, we present a broad, but not exhaustive, review of what is known about the drivers of intraspecific variation in seed dispersal, and what remains uncertain. We start by decomposing 'drivers of intraspecific variation in seed dispersal' into intrinsic drivers (i.e. variation in traits of individual plants) and extrinsic drivers (i.e. variation in ecological context). For intrinsic traits, we further decompose intraspecific variation into variation among individuals and variation of trait values within individuals. We then review our understanding of the major intrinsic and extrinsic drivers of intraspecific variation in seed dispersal, with an emphasis on variation among individuals. Crop size is the best-supported and best-understood intrinsic driver of variation across dispersal modes; overall, more seeds are dispersed as more seeds are produced, even in cases where per seed dispersal rates decline. Fruit/seed size is the second most widely studied intrinsic driver, and is also relevant to a broad range of seed dispersal modes. Remaining intrinsic drivers are poorly understood, and range from effects that are probably widespread, such as plant height, to drivers that are most likely sporadic, such as fruit or seed colour polymorphism. Primary extrinsic drivers of variation in seed dispersal include local environmental conditions and habitat structure. Finally, we present a selection of outstanding questions as a starting point to advance our understanding of individual variation in seed dispersal.

Fruiting phenology and nutrient content variation among sympatric figs and the ecological correlates

BACKGROUND

Figs are key resources for tropical frugivores and display unique fruiting patterns. While monoecious figs support both seeds and wasp rearing, dioecious plants perform the tasks separately and produce seeded figs in smaller asynchronous crops. Thus dioecious females, compared to monoecious figs, may afford to invest more efforts to maximize seediness, or increase fruit pulp, water content, and nutrient rewards to attract frugivores for better seed dispersal. Yet size variation among and within fig species in either breeding system may lead to complicated resource allocation. We assessed fruiting phenology, measured fig morphological traits, and analyzed fig nutrient contents of the monoecious Ficus caulocarpa and F. subpisocarpa and the dioecious F. ampelas and F. irisana in a sympatric tropical forest to investigate species differences and size effects on fig functional traits and their ecological correlates.

RESULTS

All four species fruited nearly year-round. Monoecious figs' inter-tree asynchronous crops had high peak mature crop sizes over much shorter fruiting periods than dioecious figs. Among trees, F. subpisocarpa and F. irisana were greater in fig-size and size variation, F. caulocarpa and F. ampelas comparatively displayed large variation in fig compositions. As fig size increased, water contents gradually increased in large-fig species, but seediness with a decreasing trend in small-fig species. Dioecious figs had lower pulp-seed ratio but tended to have higher water contents than monoecious figs, particularly within a similar size range. Dioecious figs also had higher carbohydrates, whereas monoecious figs contained higher fiber and lipid contents.

CONCLUSIONS

Our study revealed species differences in certain fig functional traits that were correlated with fig size or their breeding systems, with substantial inter-tree variation. This partially supported the predictions regarding their fruiting strategies of aiding seed dispersal by frugivores, yet suggests a fruiting plasticity of individual trees subject to environmental constraints and their biotic interactions.

Generalist birds promote tropical forest regeneration and increase plant diversity via rare-biased seed dispersal

Regenerated forests now compose over half of the world's tropical forest cover and are increasingly important as providers of ecosystem services, freshwater, and biodiversity conservation. Much of the value and functionality of regenerating forests depends on the plant diversity they contain. Tropical forest diversity is strongly shaped by mutualistic interactions between plants and fruit-eating animals (frugivores) that disperse seeds. Here we show how seed dispersal by birds can influence the speed and diversity of early successional forests in Puerto Rico. For two years, we monitored the monthly fruit production of bird-dispersed plants on a fragmented landscape, and measured seed dispersal activity of birds and plant establishment in experimental plots located in deforested areas. Two predominantly omnivorous bird species, the Northern Mockingbird (Mimus polyglottos) and the Gray Kingbird (Tyrannus dominicensis), proved critical for speeding up the establishment of woody plants and increasing the species richness and diversity of the seed rain in deforested areas. Seed dispersal by these generalists increased the odds for rare plant species to disperse and establish in experimental forest-regeneration plots. Results indicate that birds that mix fruit and insects in their diets and actively forage across open and forested habitats can play keystone roles in the regeneration of mutualistic plant-animal communities. Furthermore, our analyses reveal that rare-biased (antiapostatic) frugivory and seed dispersal is the mechanism responsible for increasing plant diversity in the early-regenerating community.

Strong among population variation in frugivory strength by functional diverse frugivores: a 'reciprocal translocation' experiment

Fruit-frugivore interactions are critical for the dynamics and evolution of many plant communities. The strength of the interactions between a given plant species and different frugivore guilds (e.g., seed dispersers, seed predators) often vary in space due to changes in plant extrinsic factors (e.g., frugivore abundances) and plant traits (e.g., fruit size and reward). By reciprocally translocating Pyrus bourgaeana ripe fruits representative of five Mediterranean localities during 2 consecutive years, we experimentally quantified guild-specific variations among populations in frugivory strength, while accounted for plant-intrinsic and- extrinsic factors. Though overall fruit removal did not differ among localities, there were strong guild-specific differences in fruit removal strength. Fruit removal by pulp feeders, seed dispersers, and fruit predators varied among populations up to 8.5-, 5.6-, and 4.0-folds, respectively. These strong variations seemed mediated by changes in frugivore relative abundances rather than on availability of alternative fruits. As expected, all fruit traits considered (e.g., fruit size, pulp amount) markedly varied among tree populations. However, no frugivore guild showed preference for fruits from any locality, suggesting that fruit traits did not contribute much to differences in frugivory strength among populations. Since the functional diverse frugivore guilds played contrasting roles in P. bourgaeana dynamics (e.g., seed dispersal vs. seed predation), our study highlights the importance of accounting for functional diversity in frugivore guilds when estimating spatial variations in the strenght of seed dispersal. This investigation also illustrates a neglected but widely applicable experimental approach to identify the relative importance of extrinsic factors and fruit traits in mediating fruit-frugivore interactions.

Intraspecific variation in fruit-frugivore interactions: effects of fruiting neighborhood and consequences for seed dispersal

The extent of specialization/generalization continuum in fruit-frugivore interactions at the individual level remains poorly explored. Here, we investigated the interactions between the Neotropical treelet Miconia irwinii (Melastomataceae) and its avian seed dispersers in Brazilian campo rupestre. We built an individual-based network to derive plant degree of interaction specialization regarding disperser species. Then, we explored how intraspecific variation in interaction niche breadth relates to fruit availability on individual plants in varying densities of fruiting conspecific neighbors, and how these factors affect the quantity of viable seeds dispersed. We predicted broader interaction niche breadths for individuals with larger fruit crops in denser fruiting neighborhoods. The downscaled network included nine bird species and 15 plants, which varied nearly five-fold in their degree of interaction specialization. We found positive effects of crop size on visitation and fruit removal rates, but not on degree of interaction specialization. Conversely, we found that an increase in the density of conspecific fruiting neighbors both increased visitation rate and reduced plant degree of interaction specialization. We suggest that tracking fruit-rich patches by avian frugivore species is the main driver of density-dependent intraspecific variation in plants' interaction niche breadth. Our study shed some light on the overlooked fitness consequences of intraspecific variation in interaction niches by showing that individuals along the specialization/generalization continuum may have their seed dispersed with similar effectiveness. Our study exemplifies how individual-based networks linking plants to frugivore species that differ in their seed dispersal effectiveness can advance our understanding of intraspecific variation in the outcomes of fruit-frugivore interactions.

Spatial patterns of an endemic Mediterranean palm recolonizing old fields

Throughout Europe, increased levels of land abandonment lead to (re)colonization of old lands by forests and shrublands. Very little is known about the spatial pattern of plants recolonizing such old fields. We mapped in two 21-22-ha plots, located in the Doñana National Park (Spain), all adult individuals of the endozoochorous dwarf palm Chamaerops humilisL. and determined their sex and sizes. We used techniques of spatial point pattern analysis (SPPA) to precisely quantify the spatial structure of these C. humilis populations. The objective was to identify potential processes generating the patterns and their likely consequences on palm reproductive success. We used (1) Thomas point process models to describe the clustering of the populations, (2) random labeling to test the sexual spatial segregation, and (3) mark correlation functions to assess spatial structure in plant sizes. Plants in both plots showed two critical scales of clustering, with small clusters of a radius of 2.8-4 m nested within large clusters with 38-44 m radius. Additional to the clustered individuals, 11% and 27% of all C. humilis individuals belonged to a random pattern that was independently superimposed to the clustered pattern. The complex spatial pattern of C. humilis could be explained by the effect of different seed-dispersers and predators' behavior and their relative abundances. Plant sexes had no spatial segregation. Plant sizes showed a spatial aggregation inside the clusters, with a decreasing correlation with distance. Clustering of C. humilis is strongly reliant on its seed dispersers and stressful environmental conditions. However, it seems that the spatial patterns and dispersal strategies of the dwarf palm make it a successful plant for new habitat colonization. Our results provide new information on the colonization ability of C. humilis and can help to develop management strategies to recover plant populations.

The impact of anthropogenic food supply on fruit consumption by dusky-legged guan (Penelope obscura Temminck, 1815): potential effects on seed dispersal in an Atlantic forest area

Frugivorous birds are important seed dispersers and influence the recruitment of many plant species in the rainforest. The efficiency of this dispersal generally depends on environment quality, bird species, richness and diversity of resources, and low levels of anthropogenic disturbance. In this study, we compared the sighting number of dusky-legged guans (Penelope obscura) by km and their movement in two areas of Serra do Japi, one around the administrative base (Base) where birds received anthropogenic food and a pristine area (DAE) with no anthropogenic resource. We also compared the richness of native seeds in feces of birds living in these two areas. Although the abundance of P. obscura was higher in the Base, these individuals moved less, dispersed 80% fewer species of plants and consumed 30% fewer seeds than individuals from DAE. The rarefaction indicated a low richness in the frugivorous diet of birds from the Base when compared to the populations from DAE. We conclude that human food supply can interfere in the behavior of these birds and in the richness of native seeds dispersed.

Mantled howler monkey spatial foraging decisions reflect spatial and temporal knowledge of resource distributions

An animal's ability to find and relocate food items is directly related to its survival and reproductive success. This study evaluates how mantled howler monkeys make spatial foraging decisions in the wild. Specifically, discrete choice models and agent-based simulations are used to test whether mantled howler monkeys on Barro Colorado Island, Panama, integrate spatial information in order to maximize new leaf flush and fruit gain while minimizing distance traveled. Several heuristic models of decision making are also tested as possible alternative strategies (movement to core home range areas instead of individual trees, travel along a sensory gradient, movement along arboreal pathway networks without a predetermined destination, straight-line travel in a randomly chosen direction, and random walks). Results indicate that although leaves are the single most abundant item in the mantled howler monkey diet, long-distance travel bouts target the areas with the highest concentrations of mature fruits. Observed travel patterns yielded larger estimated quantities of fruit in shorter distances traveled than all alternative foraging strategies. Thus, this study both provides novel information regarding how primates select travel paths and suggests that a highly folivorous primate integrates knowledge of spatiotemporal resource distributions in highly efficient foraging strategies.

Seed trait-mediated selection by rodents affects mutualistic interactions and seedling recruitment of co-occurring tree species

As mutualists, seed dispersers may significantly affect mutualistic interactions and seedling recruitment of sympatric plants that share similar seed dispersers, but studies are rare. Here, we compared seed dispersal fitness in two co-occurring plant species (Armeniaca sibirica and Amygdalus davidiana) that inhabit warm temperate deciduous forest in northern China. We tested the hypothesis that seed trait-mediated selection by rodents may influence mutualistic interactions with rodents and then seedling establishment of co-occurring plant species. A. davidiana seeds are larger and harder (thick endocarps) than A. sibirica seeds, but they have similar levels of nutrients (crude fat, crude protein), caloric value and tannin. More A. sibirica seedlings are found in the field. Semi-natural enclosure tests indicated that the two seed species were both harvested by the same six rodent species, but that A. sibirica had mutualistic interactions (scatter hoarding) with four rodent species (Apodemus peninsulae, A. agrarius, Sciurotamias davidianus, Tamias sibiricus), and A. davidiana with only one (S. davidianus). Tagged seed dispersal experiments in the field indicated that more A. sibirica seeds were scatter-hoarded by rodents, and more A. sibirica seeds survived to the next spring and became seedlings. A. sibirica seeds derive more benefit from seed dispersal by rodents than A. davidiana seeds, particularly in years with limited seed dispersers, which well explained the higher seedling recruitment of A. sibirica compared with that of A. davidiana under natural conditions. Our results suggest that seed dispersers may play a significant role in seedling recruitment and indirect competition between co-occurring plant species.

Fruit resource tracking by hornbill species at multiple scales in a tropical forest in India

The fruit-tracking hypothesis predicts a positive association between frugivores and fruit abundance over space and time. We documented hornbill diets and examined the relationship between fruit abundance and abundance of three hornbill species (Buceros bicornis, Rhyticeros undulatus and Aceros nipalensis) in the Eastern Himalaya from 2009–2012. The study was carried out at three scales: at the largest scale of the study area (15 km2), at the intermediate scale – eight 3-ha patches within the study area and at the smallest scale of individual fruiting trees. Ninety-one per cent of the 64 foraging sightings of the great hornbill were on figs while more than 50% of the foraging sightings of the wreathed (83) and rufous-necked hornbills (87) were on non-fig fruits. At the largest scale, wreathed hornbill abundance and ripe fruit abundance peaked in the non-breeding season. At the intermediate scale, wreathed hornbill abundance was positively associated with non-fig fruit availability while rufous-necked hornbill abundance was negatively associated with non-fig fruit availability. At the smallest scale, great and rufous-necked hornbill abundances were correlated with fig and non-fig fruit crop sizes, respectively. The three hornbill species track fruit availability at different scales based on diet, which has implications for their role in seed dispersal.

Interactions between frugivorous birds and plants in savanna and forest formations of the Cerrado

We recorded interactions between frugivorous birds and plants in the Cerrado and we assessed the role and importance of birds as potential seed dispersers. We analyzed the distribution of recorded feeding events, bird-plant interactions, and bird species composition between savanna and forest formations and between the dry and rainy seasons. Samplings were carried out from August 2009 to October 2010 and from November 2011 to August 2012 by means of line transects and focal observations. We recorded 348 feeding events and 187 interactions involving 44 plant species and 60 bird species. Most of the feeding events were observed in the forests and during the dry season (χ2 = 39.529; gl = 1; p < 0.001). However, no significant difference was found in the number of interactions (χ2 = 15.975; gl = 1; p = 0.06) between the two vegetation formations and between seasons. The bird species composition differed between savanna and forest formations (ANOSIM, R = 0.238; p < 0.001) and between the dry and rainy seasons (ANOSIM, R = 0.223; p < 0.001). Most of the potential seed dispersers were generalist birds that preferentially occupy forests during the dry season. Records of feeding events in forest formations increased in the dry season, indicating that birds use these sites as foraging areas during this period. We suggest that the preservation of forests in predominantly savanna-like ecosystems such as the Cerrado is extremely important for frugivorous birds and for frugivore-plant interactions.

Negative density dependence of seed dispersal and seedling recruitment in a neotropical palm

Negative density dependence (NDD) of recruitment is pervasive in tropical tree species. We tested the hypotheses that seed dispersal is NDD, due to intraspecific competition for dispersers, and that this contributes to NDD of recruitment. We compared dispersal in the palm Attalea butyracea across a wide range of population density on Barro Colorado Island in Panama and assessed its consequences for seed distributions. We found that frugivore visitation, seed removal and dispersal distance all declined with population density of A. butyracea, demonstrating NDD of seed dispersal due to competition for dispersers. Furthermore, as population density increased, the distances of seeds from the nearest adult decreased, conspecific seed crowding increased and seedling recruitment success decreased, all patterns expected under poorer dispersal. Unexpectedly, however, our analyses showed that NDD of dispersal did not contribute substantially to these changes in the quality of the seed distribution; patterns with population density were dominated by effects due solely to increasing adult and seed density.

Fruit removal rate depends on neighborhood fruit density, frugivore abundance, and spatial context

Fleshy-fruited plants depend fundamentally on interactions with frugivores for effective seed dispersal. Recent models of frugivory within spatially explicit networks make two general predictions regarding these interactions: rate of fruit removal increases (i.e., is facilitated) as densities of conspecific neighborhood fruits increase, and fruit removal rate varies positively with frugivore abundance. We conducted a field experiment that constitutes the first empirical and simultaneous test of these two primary predictions. We manipulated neighborhood abundances of arrowwood (Viburnum recognitum and Viburnum dentatum) fruits in southern New England's maritime shrub community and monitored removal rates by autumn-migrating birds. Focal arrowwood plants in neighborhoods with high conspecific fruit density sustained moderately decreased fruit removal rates (i.e., competition) relative to those in low-density neighborhoods, a result that agrees with most field research to date but contrasts with theoretical expectation. We suggest the spatial contexts that favor competition (i.e., high-abundance neighborhoods and highly aggregated landscapes) are considerably more common than the relatively uniform, low-aggregation fruiting landscapes that promote facilitation. Patterns of arrowwood removal by avian frugivores generally varied positively with, and apparently in response to, seasonal changes in migratory frugivore abundance. However, we suggest that dense stands of arrowwood concentrated frugivore activity at the neighborhood scale, thus counteracting geographic patterns of frugivore abundance. Our results underscore the importance of considering spatial context (e.g., fruit distribution and aggregation, frugivory hubs) in plant-avian frugivore interactions.

An ant's-eye view of an ant-plant protection mutualism

Ant protection of extrafloral nectar (EFN)-secreting plants is a common form of mutualism found in most habitats around the world. However, very few studies have considered these mutualisms from the ant, rather than the plant, perspective. In particular, a whole-colony perspective that takes into account the spatial structure and nest arrangement of the ant colonies that visit these plants has been lacking, obscuring when and how colony-level foraging decisions might affect tending rates on individual plants. Here, we experimentally demonstrate that recruitment of Crematogaster opuntiae (Buren) ant workers to the EFN-secreting cactus Ferocactus wislizeni (Englem) is not independent between plants up to 5 m apart. Colony territories of C. opuntiae are large, covering areas of up to 5,000 m(2), and workers visit between five and 34 EFN-secreting barrel cacti within the territories. These ants are highly polydomous, with up to 20 nest entrances dispersed throughout the territory and interconnected by trail networks. Our study demonstrates that worker recruitment is not independent within large polydomous ant colonies, highlighting the importance of considering colonies rather than individual workers as the relevant study unit within ant/plant protection mutualisms.

Integrating frugivory and animal movement: a review of the evidence and implications for scaling seed dispersal

General principles about the consequences of seed dispersal by animals for the structure and dynamics of plant populations and communities remain elusive. This is in part because seed deposition patterns emerge from interactions between frugivore behaviour and the distribution of food resources, both of which can vary over space and time. Here we advocate a frugivore-centred, process-based, synthetic approach to seed dispersal research that integrates seed dispersal ecology and animal movement across multiple spatio-temporal scales. To guide this synthesis, we survey existing literature using paradigms from seed dispersal and animal movement. Specifically, studies are discussed with respect to five criteria: selection of focal organisms (animal or plant); measurement of animal movement; characterization of seed shadow; animal, plant and environmental factors included in the study; and scales of the study. Most studies focused on either frugivores or plants and characterized seed shadows directly by combining gut retention time with animal movement data or indirectly by conducting maternity analysis of seeds. Although organismal traits and environmental factors were often measured, they were seldom used to characterize seed shadows. Multi-scale analyses were rare, with seed shadows mostly characterized at fine spatial scales, over single fruiting seasons, and for individual dispersers. Novel animal- and seed-tracking technologies, remote environmental monitoring tools, and advances in analytical methods can enable effective implementation of a hierarchical mechanistic approach to the study of seed dispersal. This kind of mechanistic approach will provide novel insights regarding the complex interplay between the factors that modulate animal behaviour and subsequently influence seed dispersal patterns across spatial and temporal scales.

Resource abundance and frugivory in two manakin species (Aves: Pipridae) inhabiting a reforested area in Colombia

Coexistence of closely related species is a central issue in community ecology, and those species are expected to present niche differentiation, particularly when resources become scarce (García & Arroyo 2005). Hence, resource abundance and diet differences might be playing a key role in determining species coexistence (Chapman & Rosenberg 1991, Stevenson et al. 2000). Trophic niche differentiation in frugivorous birds could be driven by fruit selection, morphology, behaviour (Moermond & Denslow 1983, 1985) and the spatiotemporal resource dynamics (Blake & Loiselle 1991, Saracco et al. 2005, Solórzano et al. 2000).

From whichOcotea diospyrifoliatrees doesAlouatta caraya(Primates, Atelidae) eat fruits?

Ecological and morphological traits of fruiting trees, such as spatial distribution (Leveyet al. 1984), crop size (Howe & vande Kerckhove 1981, Korineet al. 2000, Leighton 1993, Murray 1987), local availability of fruit (Sargent 1990), seed size or pulp/seed mass ratio (Julliot 1996a, Leighton 1993, McConkeyet al. 2002, Russo 2003, Stevensonet al. 2005) and nutritional value and concentration of secondary compounds (Leighton 1993, Milton 1980, Sallabanks 1993), influence fruit preference in animals. In general, frugivores maximize energy intake by consuming ripe fruit with smaller seeds, higher pulp/seed ratio, bigger crop size, more nutritional fruits or fruits with fewer secondary compounds (Howe & vande Kerckhove 1981, Julliot 1996a, Korineet al. 2000, Leighton 1993, Milton 1980, Murray 1987, Russo 2003, Stevensonet al. 2005).

Fruits of Melastomataceae: phenology in Andean forest and role as a food resource for birds

The fruits of Melastomataceae are consumed by many Neotropical frugivorous birds. Several studies have reported segregated fruiting seasons of melastomes, but this pattern is not widespread. The segregated fruiting phenologies of congeneric sympatric species may be an evolutionary response to reduce competition for seed dispersers. Alternatively, aggregated fruiting phenologies may be favoured if local fruit abundance attracts more frugivores, thus enhancing seed dispersal. We monitored melastome fruiting in transects over a 2-y period at a cloud-forest site in the Colombian Andes. Fruiting periods of nine melastome species were aggregated and fruiting peaks coincided with rainy seasons. In a separate 6-mo study, observations at focal plants revealed that 47 of 61 bird species fed on 10 species of melastome, representing 37.4% of feeding events observed. Melastomes were consumed by birds in a higher proportion than expected from their availability and peak melastome fruit abundance coincided with the breeding season of the frugivore community, when melastomes constituted 54% of feeding records. Melastomes interact with a large number of bird species throughout their annual cycles, and seem to constitute pivotal elements that sustain the frugivore community in montane forests.