Introduced Birds and the Fate of Hawaiian Rainforests

Abstract:  The Hawaiian Islands have lost nearly all their native seed dispersers, but have gained many frugivorous birds and fleshy-fruited plants through introductions. Introduced birds may not only aid invasions of exotic plants but also may be the sole dispersers of native plants. We assessed seed dispersal at the ecotone between native- and exotic-dominated forests and quantified bird diets, seed rain from defecated seeds, and plant distributions. Introduced birds were the primary dispersers of native seeds into exotic-dominated forests, which may have enabled six native understory plant species to become reestablished. Some native plant species are now as common in exotic forest understory as they are in native forest. Introduced birds also dispersed seeds of two exotic plants into native forest, but dispersal was localized or establishment minimal. Seed rain of bird-dispersed seeds was extensive in both forests, totaling 724 seeds of 9 native species and 2 exotics with over 85% of the seeds coming from native plants. Without suitable native dispersers, most common understory plants in Hawaiian rainforests now depend on introduced birds for dispersal, and these introduced species may actually facilitate perpetuation, and perhaps in some cases restoration, of native forests. We emphasize, however, that restoration of native forests by seed dispersal from introduced birds, as seen in this study, depends on the existence of native forests to provide a source of seeds and protection from the effects of ungulates. Our results further suggest that aggressive control of patches of non-native plants within otherwise native-dominated forests may be an important and effective conservation strategy.     

Effects of Forest Fragmentation on Seed Dispersal and Seedling Establishment in Ornithochorous Trees

Abstract: Habitat fragmentation increases seed dispersal limitation across the landscape and may also affect subsequent demographic stages such as seedling establishment. Thus, the development of adequate plans for forest restoration requires an understanding of mechanisms by which fragmentation hampers seed delivery to deforested areas and knowledge of how fragmentation affects the relationship between seed‐deposition patterns and seedling establishment. We evaluated the dispersal and recruitment of two bird‐dispersed, fleshy‐fruited tree species (Crataegus monogyna and Ilex aquifolium) in fragmented secondary forests of northern Spain. Forest fragmentation reduced the probability of seed deposition for both trees because of decreased availability of woody perches and fruit‐rich neighborhoods for seed dispersers, rather than because of reductions in tree cover by itself. The effects of fragmentation went beyond effects on the dispersal stage in Crataegus because seedling establishment was proportional to the quantities of bird‐dispersed seeds arriving at microsites. In contrast, postdispersal mortality in Ilex was so high that it obscured the seed‐to‐seedling transition. These results suggest that the effects of fragmentation are not necessarily consistent across stages of recruitment across species. Habitat management seeking to overcome barriers to forest recovery must include the preservation, and even the planting, of fleshy‐fruited trees in the unforested matrix as a measure to encourage frugivorous birds to enter into open and degraded areas. An integrative management strategy should also explicitly consider seed‐survival expectancies at microhabitats to preserve plant‐population dynamics and community structure in fragmented landscapes.     

Birds as Suppliers of Seed Dispersal in Temperate Ecosystems: Conservation Guidelines from Real‐World Landscapes

Abstract: Seed dispersal by animals is considered a pivotal ecosystem function that drives plant‐community dynamics in natural habitats and vegetation recovery in human‐altered landscapes. Nevertheless, there is a lack of suitable ecological knowledge to develop basic conservation and management guidelines for this ecosystem service. Essential questions, such as how well the abundance of frugivorous animals predicts seeding function in different ecosystems and how anthropogenic landscape heterogeneity conditions the role of dispersers, remain poorly answered. In three temperate ecosystems, we studied seed dispersal by frugivorous birds in landscape mosaics shaped by human disturbance. By applying a standardized design across systems, we related the frequency of occurrence of bird‐dispersed seeds throughout the landscape to the abundance of birds, the habitat features, and the abundance of fleshy fruits. Abundance of frugivorous birds in itself predicted the occurrence of dispersed seeds throughout the landscape in all ecosystems studied. Even those landscape patches impoverished due to anthropogenic disturbance received some dispersed seeds when visited intensively by birds. Nonetheless, human‐caused landscape degradation largely affected seed‐deposition patterns by decreasing cover of woody vegetation or availability of fruit resources that attracted birds and promoted seed dispersal. The relative role of woody cover and fruit availability in seed dispersal by birds differed among ecosystems. Our results suggest that to manage seed dispersal for temperate ecosystem preservation or restoration one should consider abundance of frugivorous birds as a surrogate of landscape‐scale seed dispersal and an indicator of patch quality for the dispersal function; woody cover and fruit resource availability as key landscape features that drive seedfall patterns; and birds as mobile links that connect landscape patches of different degrees of degradation and habitat quality via seed deposition.     

Release thresholds strongly determine the range of seed dispersal by wind

The effect of the seed abscission process on the dispersal distance of seeds has never been studied explicitly and is often ignored in studies that aim to estimate the seed shadows of species. To examine the importance of the abscission process for the seed shadow we used a seed trajectory model that keeps track of the release threshold dynamics of the individual seeds on mother plant. We defined the release threshold as the critical wind speed that induces a mechanical force that is just large enough to release a seed from its mother plant. The model used real wind speed sequences and seed appearance over time on the mother plant.Several calculations were performed to investigate the effect of release thresholds dynamics on seed shadow of two herbaceous species with contrasting terminal velocity values (V_t): Centaurea jacea (V_t = 4.1 m s⁻¹) and Hypochaeris radicata (V_t = 0.49 m s⁻¹).Release thresholds were responsible for a two-fold increase of median dispersal distances in both species. Tails of the seed shadows, the fraction of seeds that travel furthest, were even more sensitive and increased with a factor 4.5 for Centaurea and 7.0 for Hypochaeris. Our work indicates that the abscission process appears to be very important and suggests that dispersal distance of plants is currently severely underestimated, which, in turn, has major consequences for our current understanding of the distribution, metapopulation dynamics and survival of plant species.     

Evaluating the efficacy of restoration plantings through DNA barcoding of frugivorous bird diets

Frugivores are critical components of restoration programs because they are seed dispersers. Thus, knowledge about bird–plant trophic relationships is essential in the evaluation of the efficacy of restoration processes. Traditionally, the diet of frugivores is characterized by microscopically identifying plant residues in droppings, which is time‐consuming, requires botanical knowledge, and cannot be used for fragments lacking detectable morphological characteristics (e.g., fragmented seeds and skins). We examined whether DNA barcoding can be used as a universal tool to rapidly characterize the diet of a frugivorous bird, Eurasian blackcap (Sylvia atricapilla). We used the DNA barcoding results to assess restoration efforts and monitor the diversity of potentially dispersed plants in a protected area in northern Italy. We collected 642 Eurasian Blackcap droppings at the restored site during the autumn migration over 3 years. Intact seeds and fragmented plant material were analyzed at 2 plastidial barcode loci (rbcL and trnH‐psbA), and the resulting plant identifications were validated by comparison with a reference molecular data set of local flora. At least 17 plant species, including 7 of the 11 newly transplanted taxa, were found. Our results demonstrate the potential for DNA barcoding to be used to monitor the effectiveness of restoration plantings and to obtain information about fruit consumption and dispersal of invasive or unexpected plant species. Such an approach provides valuable information that could be used to study local plant biodiversity and to survey its evolution over time.     

Effects of Artificial Roosts for Frugivorous Bats on Seed Dispersal in a Neotropical Forest Pasture Mosaic

Abstract:  In the Neotropics ongoing deforestation is producing open and heavily fragmented landscapes dominated by agriculture, mostly plantations and cattle pastures. After some time agriculture often becomes uneconomical and land is abandoned. Subsequent habitat regeneration may be slow because seed inputs are restricted by a lack of incentives—such as suitable roost sites—for seed dispersers to enter deforested areas. Increasing environmental awareness has fostered growing efforts to promote reforestation. Practical and cost-efficient methods for kick-starting forest regeneration are, however, lacking. We investigated whether artificial bat roosts for frugivorous bat species can attract these key seed dispersers to deforested areas, thereby increasing seed rain. We installed artificial bat roosts in a forest-pasture mosaic in the Costa Rican Atlantic lowlands and monitored bat colonization and seed dispersal. Colonization occurred within a few weeks of installation, and 10 species of bats occupied the artificial roosts. Five species of frugivorous or nectarivorous bats colonized artificial roosts permanently in both primary habitat and in deforested areas, in numbers similar to those found in natural roosts. Seed input around artificial roosts increased significantly. Sixty-nine different seed types, mostly of early-successional plant species, were transported by bats to artificial roosts in disturbed habitats. The installation of artificial bat roosts thus successfully attracted frugivorous bats and increased seed inputs into degraded sites. This method is likely to speed up early-vegetation succession, which in turn will attract additional seed dispersers, such as birds, and provide a microhabitat for seeds of mid- and late-successional plants. As well as supporting natural forest regeneration and bat conservation, this cost-efficient method can also increase environmental awareness among landowners.     

森林土壤种子库研究进展

土壤种子库与物种多样性存在密切相关性,森林土壤种子库是植被天然更新的物质基础。文章阐述了森林土壤种子库的内涵、森林生态系统土壤种子库的基本特征及其主要研究方法,探讨了影响种子库的基本因素及当前极为关注的热点问题。在受损森林生态系统目标树种培养、植被群落快速恢复和生态系统科学管理等领域仍存在一些理论和实践急待解决的问题,如随着群落进展演替土壤种子库种子数量在增加而质量却在逐渐下降,面临如何解决生态系统健康稳定发育和物种多样性长期维持问题。文章认为今后应加强土壤种子库与生物多样性保护、防止外来物种入侵乡土物种利用及对全球变化的响应等领域的研究工作,以期为从事森林土壤种子库研究和退化森林植被群落恢复实践提供理论指导。     

Meta‐Analysis of the Effects of Human Disturbance on Seed Dispersal by Animals

Animal‐mediated seed dispersal is important for sustaining biological diversity in forest ecosystems, particularly in the tropics. Forest fragmentation, hunting, and selective logging modify forests in myriad ways and their effects on animal‐mediated seed dispersal have been examined in many case studies. However, the overall effects of different types of human disturbance on animal‐mediated seed dispersal are still unknown. We identified 35 articles that provided 83 comparisons of animal‐mediated seed dispersal between disturbed and undisturbed forests; all comparisons except one were conducted in tropical or subtropical ecosystems. We assessed the effects of forest fragmentation, hunting, and selective logging on seed dispersal of fleshy‐fruited tree species. We carried out a meta‐analysis to test whether forest fragmentation, hunting, and selective logging affected 3 components of animal‐mediated seed dispersal: frugivore visitation rate, number of seeds removed, and distance of seed dispersal. Forest fragmentation, hunting, and selective logging did not affect visitation rate and were marginally associated with a reduction in seed‐dispersal distance. Hunting and selective logging, but not fragmentation, were associated with a large reduction in the number of seeds removed. Fewer seeds of large‐seeded than of small‐seeded tree species were removed in hunted or selectively logged forests. A plausible explanation for the consistently negative effects of hunting and selective logging on large‐seeded plant species is that large frugivores, as the predominant seed dispersers for large‐seeded plant species, are the first animals to be extirpated from hunted or logged forests. The reduction in forest area after fragmentation appeared to have weaker effects on frugivore communities and animal‐mediated seed dispersal than hunting and selective logging. The differential effects of hunting and selective logging on large‐ and small‐seeded tree species underpinned case studies that showed disrupted plant‐frugivore interactions could trigger a homogenization of seed traits in tree communities in hunted or logged tropical forests. Meta Análisis de los Efectos de la Perturbación Humana sobre la Dispersión de Semillas por Animales     

Forest Restoration on a Closed Landfill: Rapid Addition of New Species by Bird Dispersal

Urban areas often contain sizeable pockets of degraded land, such as inactive landfills, that could be reclaimed as wildlife habitat and as connecting links to enhance remnant natural areas. In the northeastern U.S., many such lands fail to undergo natural succession to woodland, instead retaining a weedy, herbaceous cover for many years. We hypothesize that seed dispersal is a limiting factor, and that a form of secondary succession could be stimulated by introducing clusters of trees and shrubs to attract avian seed dispersers. As a direct test, we censused a 1.5-ha experimental plantation on the Fresh Kills Landfill (Staten Island, New York) one year after installation, in search of evidence that the plantation was spreading or increasing in diversity. The 17 planted species, many from coastal scrub forests native to this region, were surviving well but contributed almost no seedlings to the area, in part because only 20% of the installed trees or shrubs were reproductive. Of the 1079 woody seedlings found, 95% came from sources outside the plantation; most (71%) were from fleshy-fruited, bird-dispersed plants from nearby woodland fringes. Although the restoration planting itself had not begun to produce seedlings, it did function as a site for attracting dispersers, who enriched the young community with 20 new species. One-fourth of all new recruits were from nine additional wind-dispersed species. Locations with a high ratio of trees to shrubs had proportionately more recruits, indicating that plant size contributed to disperser attraction. The density of new recruits of each species was dependent on distance from the nearest potential seed source. Introducing native species with the capacity to attract avian dispersers may be the key to success of many restoration programs.     

Responses of Seed‐Dispersing Birds to Amount of Rainforest in the Landscape around Fragments

Habitat loss and fragmentation alter the composition of bird assemblages in rainforest. Because birds are major seed dispersers in rainforests, fragmentation‐induced changes to frugivorous bird assemblages are also likely to alter the ecological processes of seed dispersal and forest regeneration, but the specific nature of these changes is poorly understood. We assessed the influence of fragment size and landscape forest cover on the abundance, species composition, and functional properties of the avian seed disperser community in an extensively cleared, former rainforest landscape of subtropical Australia. Bird surveys of fixed time and area in 25 rainforest fragments (1–139 ha in size across a 1800 km² region) provided bird assemblage data which were coupled with prior knowledge of bird species’ particular roles in seed dispersal to give measurements of seven different attributes of the seed disperser assemblage. We used multimodel regression to assess how patch size and surrounding forest cover (within 200 m, 1000 m, and 5000 m radii) influenced variation in the abundance of individual bird species and of functional groups based on bird species’ responses to fragmentation and their roles in seed dispersal. Surrounding forest cover, specifically rainforest cover, generally had a greater effect on frugivorous bird assemblages than fragment size. Amount of rainforest cover within 200 m of fragments was the main factor positively associated with abundances of frugivorous birds that are both fragmentation sensitive and important seed dispersers. Our results suggest a high proportion of local rainforest cover is required for the persistence of seed‐dispersing birds and the maintenance of seed dispersal processes. Thus, even small rainforest fragments can function as important parts of habitat networks for seed‐dispersing birds, whether or not they are physically connected by vegetation. Respuestas de Aves Dispersoras de Semillas al Incremento de Selvas en el Paisaje Alrededor de Fragmentos     

Incorporating differences between genetic diversity of trees and herbaceous plants in conservation strategies

Reviews that summarize the genetic diversity of plant species in relation to their life history and ecological traits show that forest trees have more genetic diversity at population and species levels than annuals or herbaceous perennials. In addition, among-population genetic differentiation is significantly lower in trees than in most herbaceous perennials and annuals. Possible reasons for these differences between trees and herbaceous perennials and annuals have not been discussed critically. Several traits, such as high rates of outcrossing, long-distance pollen and seed dispersal, large effective population sizes (N_e), arborescent stature, low population density, longevity, overlapping generations, and occurrence in late successional communities, may make trees less sensitive to genetic bottlenecks and more resistant to habitat fragmentation or climate change. We recommend that guidelines for genetic conservation strategies be designed differently for tree species versus other types of plant species. Because most tree species fit an LH scenario (low [L] genetic differentiation and high [H] genetic diversity), tree seeds could be sourced from a few populations distributed across the species’ range. For the in situ conservation of trees, translocation is a viable option to increase N_e. In contrast, rare herbaceous understory species are frequently HL (high differentiation and low diversity) species. Under the HL scenario, seeds should be taken from many populations with high genetic diversity. In situ conservation efforts for herbaceous plants should focus on protecting habitats because the typically small populations of these species are vulnerable to the loss of genetic diversity. The robust allozyme genetic diversity databases could be used to develop conservation strategies for species lacking genetic information. As a case study of reforestation with several tree species in denuded areas on the Korean Peninsula, we recommend the selection of local genotypes as suitable sources to prevent adverse effects and to insure the successful restoration in the long term.     

施肥模式对砂姜黑土区土壤种子库影响的研究

通过对砂浆黑土区小麦-大豆轮作制地不同施肥模式的杂草土壤种子库（0～15 cm）进行调查,分析种子库的大小、组成、多样性等特征,探讨施肥模式与群落演替速率之间的关系。结果表明,施肥模式显著影响着土壤种子库的大小、组成、多样性、群落演替速率。土壤样品中共检出杂草种子18种,隶属于11科,且各处理优势种组成差异明显。种子库密度按NPK〉NP〉NK〉CK〉PK顺序依次递减且差异极显著（P〈0.01）。各处理物种组成不同,种数按CK〉NPK〉PK=NK〉NP顺序依次递减,群落均匀度指数按NP〉CK〉NK〉PK〉NPK顺序依次递减且差异显著（P〈0.05）,物种多样性指数按CK〉NP〉NK〉PK〉NPK顺序依次递减,群落优势度指数按NPK〉PK=NK〉CK〉NP顺序依次递减。我们初步认为适应与竞争机制影响土壤种子库的物种组成、密度的差异;施肥模式间土壤种子库的优势种变化尤为突出,引起均匀度的改变,导致施肥模式下土壤种子库多样性的差异。Whittaker指数按CK〈NPK〈PK=NK〈NP顺序依次增大,表明土壤种子库的群落演替速率依次加快,说明长期平衡施肥模式（NPK）对土壤种子库群落的影响明显小于PK、NP、NK施肥处理,群落结构较PK、NP、NK施肥处理更稳定。气候、取样时间、除草剂等因素导致年际间、种植季间的杂草土壤种子库特征差异明显。本研究认为长期平衡施肥既有利于作物的优质高产,也有利于农田土壤种子库群落的稳定,是建立合理、高效、稳定的农业生态系统以及取得良好的经济效益和生态效益的重要施肥方式。     

Effects of Growth Form and Functional Traits on Response of Woody Plants to Clearing and Fragmentation of Subtropical Rainforest

The conservation implications of large‐scale rainforest clearing and fragmentation on the persistence of functional and taxonomic diversity remain poorly understood. If traits represent adaptive strategies of plant species to particular circumstances, the expectation is that the effect of forest clearing and fragmentation will be affected by species functional traits, particularly those related to dispersal. We used species occurrence data for woody plants in 46 rainforest patches across 75,000 ha largely cleared of forest by the early 1900s to determine the combined effects of area reduction, fragmentation, and patch size on the taxonomic structure and functional diversity of subtropical rainforest. We compiled species trait values for leaf area, seed dry mass, wood density, and maximum height and calculated species niche breadths. Taxonomic structure, trait values (means, ranges), and the functional diversity of assemblages of climbing and free‐standing plants in remnant patches were quantified. Larger rainforest patches had higher species richness. Species in smaller patches were taxonomically less related than species in larger patches. Free‐standing plants had a high percentage of frugivore dispersed seeds; climbers had a high proportion of small wind‐dispersed seeds. Connections between the patchy spatial distribution of free‐standing species, larger seed sizes, and dispersal syndrome were weak. Assemblages of free‐standing plants in patches showed more taxonomic and spatial structuring than climbing plants. Smaller isolated patches retained relatively high functional diversity and similar taxonomic structure to larger tracts of forest despite lower species richness. The response of woody plants to clearing and fragmentation of subtropical rainforest differed between climbers and slow‐growing mature‐phase forest trees but not between climbers and pioneer trees. Quantifying taxonomic structure and functional diversity provides an improved basis for conservation planning and management by elucidating the effects of forest‐area reduction and fragmentation. Efectos de la Forma de Crecimiento y Atributos Funcionales en la Respuesta de Plantas Leñosas al Desmonte y Fragmentación de Bosque Lluvioso Subtropical     

Comparison of three chemical tests to assess seed viability: the seed dispersal system of the Macaronesian endemic plant <Emphasis Type="Italic">Rubia fruticosa</Emphasis> (Rubiaceae) as an example

Summary. Three chemical viability tests were evaluated in the seed dispersal system of Rubia fruticosa, in which three main groups of dispersers participate: reptiles, birds and mammals. Tetrazolium chloride (TTC) and indigo carmine (IC) indicated a lower viability of seeds from droppings of introduced rabbits (Oryctolagus cuniculus) than of those from control plants and the native dispersers, lizards and gulls. In the rabbit seed treatment, significant differences were observed between results obtained with TTC and IC tests. Interpretation of these data, due to the presence of doubtful embryo staining, was more difficult using the IC test. Furthermore, some seeds that were clearly dead had been underestimated. In contrast with results obtained from the two staining methods, the EC test did not confirm that viability of control seeds and those seeds consumed by native dispersers were clearly higher than in seeds ingested by O. cuniculus. Further, compared to the other two tests, the EC method requires more careful handling of the embryo during the extraction process to avoid errors in viability estimation, since this method measures concentration of electrolytes that are released through cellular membranes. Thus, TTC was the most reliable test to assess seed viability in the seed dispersal system of R. fruticosa, and these results agree with those obtained in previous germination experiments made on the same set of seeds given the same treatments.     

Disentangling the drivers of reduced long-distance seed dispersal by birds in an experimentally fragmented landscape

Seed dispersal is a crucial component of plant population dynamics. Human landscape modifications, such as habitat destruction and fragmentation, can alter the abundance of fruiting plants and animal dispersers, foraging rates, vector movement, and the composition of the disperser community, all of which can singly or in concert affect seed dispersal. Here, we quantify and tease apart the effects of landscape configuration, namely, fragmentation of primary forest and the composition of the surrounding forest matrix, on individual components of seed dispersal of Heliconia acuminata, an Amazonian understory herb. First we identified the effects of landscape configuration on the abundance of fruiting plants and six bird disperser species. Although highly variable in space and time, densities of fruiting plants were similar in continuous forest and fragments. However, the two largest-bodied avian dispersers were less common or absent in small fragments. Second, we determined whether fragmentation affected foraging rates. Fruit removal rates were similar and very high across the landscape, suggesting that Heliconia fruits are a key resource for small frugivores in this landscape. Third, we used radiotelemetry and statistical models to quantify how landscape configuration influences vector movement patterns. Bird dispersers flew farther and faster, and perched longer in primary relative to secondary forests. One species also altered its movement direction in response to habitat boundaries between primary and secondary forests. Finally, we parameterized a simulation model linking data on fruit density and disperser abundance and behavior with empirical estimates of seed retention times to generate seed dispersal patterns in two hypothetical landscapes. Despite clear changes in bird movement in response to landscape configuration, our simulations demonstrate that these differences had negligible effects on dispersal distances. However, small fragments had reduced densities of Turdus albicollis, the largest-bodied disperser and the only one to both regurgitate and defecate seeds. This change in Turdus abundance acted together with lower numbers of fruiting plants in small fragments to decrease the probability of long-distance dispersal events from small patches. These findings emphasize the importance of foraging style for seed dispersal and highlight the primacy of habitat size relative to spatial configuration in preserving biotic interactions.     

A method for building spatial model of annual weed seed dispersal from experimental data and its application to simulating Bromus sterilis population dispersal

Spatial model of annual weed seed dispersal, in this article, was theoretically derived. According to the requirements of building the spatial model, we designed and done an indoor experiment of weed seed dispersal by wind. In the experiment, the seeds of Bromus sterilis were released at 100 cm height under different wind velocity conditions. Based on the experimental data, the spatial models of seed dispersal of the weed species were built, which were divided into three types according to the coefficient β < 0, β = 0, β > 0. The results showed that dispersal of annual weed seed in any direction obeyed an approximate Gaussian distribution; under the experimental conditions, spatial distribution type of weed seed dispersal changed with variation of wind velocity. Well-known Howard et al.'s model (Howard et al., 1991) of Bromus sterilis seed dispersal is an especial example of the model built in this article. The result of model analysis indicated that the distribution type described by Howard's model was similar to that of seed dispersal of the weed species at the height of 100 cm under the condition of lower wind velocity (about 2.18 m/s). Using CA simulation analysis we found that mean control agent applying to a cell with weed should have a decrease with an increase of wind velocity to prevent weed with the initial configuration from spreading, which implicated less herbicide needs spraying in every cell with weed on average when wind velocity increases.     

Functional nonredundancy of elephants in a disturbed tropical forest

Conservation efforts are often motivated by the threat of global extinction. Yet if conservationists had more information suggesting that extirpation of individual species could lead to undesirable ecological effects, they might more frequently attempt to protect or restore such species across their ranges even if they were not globally endangered. Scientists have seldom measured or quantitatively predicted the functional consequences of species loss, even for large, extinction‐prone species that theory suggests should be functionally unique. We measured the contribution of Asian elephants (Elephas maximus) to the dispersal of 3 large‐fruited species in a disturbed tropical moist forest and predicted the extent to which alternative dispersers could compensate for elephants in their absence. We created an empirical probability model with data on frugivory and seed dispersal from Buxa Tiger Reserve, India. These data were used to estimate the proportion of seeds consumed by elephants and other frugivores that survive handling and density‐dependent processes (Janzen‐Connell effects and conspecific intradung competition) and germinate. Without compensation, the number of seeds dispersed and surviving density‐dependent effects decreased 26% (Artocarpus chaplasha), 42% (Careya arborea), and 72% (Dillenia indica) when elephants were absent from the ecosystem. Compensatory fruit removal by other animals substantially ameliorated these losses. For instance, reductions in successful dispersal of D. indica were as low as 23% when gaur (Bos gaurus) persisted, but median dispersal distance still declined from 30% (C. arborea) to 90% (A. chaplasha) without elephants. Our results support the theory that the largest animal species in an ecosystem have nonredundant ecological functionality and that their extirpation is likely to lead to the deterioration of ecosystem processes such as seed dispersal. This effect is likely accentuated by the overall defaunation of many tropical systems.     

Experimental manipulation of seed shadows of an Afrotropical tree determines drivers of recruitment

The loss of animals in tropical forests may alter seed dispersal patterns and reduce seedling recruitment of tree species, but direct experimental evidence is scarce. We manipulated dispersal patterns of Manilkara mabokeensis, a monkey-dispersed tree, to assess the extent to which spatial distributions of seeds drive seedling recruitment. Based on the natural seed shadow, we created seed distributions with seeds deposited under the canopy ("no dispersal"), with declining density from the tree ("natural dispersal"), and at uniform densities ("good dispersal"). These distributions mimicked dispersal patterns that could occur with the extirpation of monkeys, low levels of hunting, and high rates of seed dispersal. We monitored seedling emergence and survival for 18 months and recorded the number of leaves and damage to leaves. "Good dispersal" increased seedling survival by 26%, and "no dispersal" decreased survival by 78%, relative to "natural dispersal." Using a mixed-effects survival model, we decoupled the distance and density components of the seed shadow: seedling survival depended on the seed density, but not on the distance from the tree. Although community seedling diversity tended to decrease with longer dispersal distances, we found no conclusive evidence that patterns of seed dispersal influence the diversity of the seedling community. Local seed dispersal does affect seedling recruitment and survival, with better dispersal resulting in higher seedling recruitment; hence the loss of dispersal services that comes with the reduction or extirpation of seed dispersers will decrease regeneration of some tree species.     

Incorporating animal behavior into seed dispersal models: implications for seed shadows

Seed dispersal fundamentally influences plant population and community dynamics but is difficult to quantify directly. Consequently, models are frequently used to describe the seed shadow (the seed deposition pattern of a plant population). For vertebrate-dispersed plants, animal behavior is known to influence seed shadows but is poorly integrated in seed dispersal models. Here, we illustrate a modeling approach that incorporates animal behavior and develop a stochastic, spatially explicit simulation model that predicts the seed shadow for a primate-dispersed tree species (Virola calophylla, Myristicaceae) at the forest stand scale. The model was parameterized from field-collected data on fruit production and seed dispersal, behaviors and movement patterns of the key disperser, the spider monkey (Ateles paniscus), densities of dispersed and non-dispersed seeds, and direct estimates of seed dispersal distances. Our model demonstrated that the spatial scale of dispersal for this V. calophylla population was large, as spider monkeys routinely dispersed seeds >100 m, a commonly used threshold for long-distance dispersal. The simulated seed shadow was heterogeneous, with high spatial variance in seed density resulting largely from behaviors and movement patterns of spider monkeys that aggregated seeds (dispersal at their sleeping sites) and that scattered seeds (dispersal during diurnal foraging and resting). The single-distribution dispersal kernels frequently used to model dispersal substantially underestimated this variance and poorly fit the simulated seed-dispersal curve, primarily because of its multimodality, and a mixture distribution always fit the simulated dispersal curve better. Both seed shadow heterogeneity and dispersal curve multimodality arose directly from these different dispersal processes generated by spider monkeys. Compared to models that did not account for disperser behavior, our modeling approach improved prediction of the seed shadow of this V. calophylla population. An important function of seed dispersal models is to use the seed shadows they predict to estimate components of plant demography, particularly seedling population dynamics and distributions. Our model demonstrated that improved seed shadow prediction for animal-dispersed plants can be accomplished by incorporating spatially explicit information on disperser behavior and movements, using scales large enough to capture routine long-distance dispersal, and using dispersal kernels, such as mixture distributions, that account for spatially aggregated dispersal.