Enhanced seed dispersal of Prunus africana in fragmented and disturbed forests?

Forest destruction and disturbance can have long-term consequences for species diversity and ecosystem processes such as seed dispersal. Understanding these consequences is a crucial component of conserving vulnerable ecosystems. In the heavily fragmented and disturbed Kakamega Forest, western Kenya, we studied seed dispersal of Prunus africana (Rosaceae). In the main forest, five forest fragments, and differently disturbed sites, we quantified the overall frugivore community as an indicator for species diversity. Furthermore, we determined the frugivores on 28 fruiting P. africana trees, estimated seed dispersal, crop size and the general fruit availability of surrounding trees. During the overall frugivore census we recorded 49 frugivorous species; 36 of them were observed visiting P. africana trees and feeding on their fruits. Although overall frugivore species richness was 1.1 times lower in fragments than in main forest sites and 1.02 times higher in highly disturbed than in less disturbed sites, P. africana experienced 1.1 times higher numbers of frugivores in fragments than in main forest sites and 1.5 times higher numbers of frugivores in highly disturbed than in less disturbed sites. Correspondingly, seed dispersal was 1.5 times higher in fragments than in main forest sites and 1.5 times higher in more disturbed than less disturbed sites. Fruit availability of surrounding trees and crop size influenced the number of visitors to some degree. Thus, the number of dispersed seeds seemed to be slightly higher in fragmented and highly disturbed sites. This indicates that loss of single species does not necessarily lead to a decrease of ecosystem services. However, loss of diversity could be a problem in the long term, as a multitude of species might act as buffer against future environmental change.     

Can seed dispersal by human activity play a useful role for the conservation of European grasslands?

Objective: To review the recent research into human‐mediated dispersal (HMD) in the European rural landscape, and explore the potential positive aspect of HMD for grassland conservation, in contrast to it's common association with the spread of invasive species. Methods: A literature search was undertaken to identify HMD vectors in the rural landscape for discussion regarding dispersal potential past and present, implications for management, and the identification of future research needs. Results: Grazing animals are important propagule dispersers, but the reduced movement of livestock through the landscape has also meant a reduction in seeds dispersed in this way. Other, non‐standard human‐mediated dispersal vectors such as clothing and motor vehicles can also transport seeds of many species, and HMD vectors often transport seeds with a variety of dispersal specialisations. Recommendations: There should be a greater movement of grazing animals throughout the landscape, either within larger grazing areas or between existing grasslands. Where this is not possible, other, more directed dispersal of propagules from species‐rich communities to target sites should be considered. The potential of non‐standard HMD vectors to make a positive contribution to biodiversity should be considered, but more research into all types of HMD vectors is important if we are to fully understand their role in the dispersal of plant species in fragmented landscapes.     

Endozoochorous dispersal of aquatic plants: does seed gut passage affect plant performance?

The ingestion of seeds by vertebrates can affect the germinability and/or germination rate of seeds. It is, however, unclear if an earlier germination as a result of ingestion affects later plant performance. For sago pondweed, Potamogeton pectinatus, the effects of seed ingestion by ducks on both germinability and germination rate have been previously reported from laboratory experiments. We performed an experiment to determine the effects of seed ingestion by ducks on germination, seedling survival, plant growth and asexual multiplication. Both at the start and end of the winter, seeds were fed to three captive shovelers (Anas clypeata) and planted outdoors in water-filled containers. Plant biomass and its allocation to vegetative parts (shoot and roots), tubers, and seeds were determined in autumn. More duck-ingested seeds than control (uningested) seeds germinated in early winter, but this difference disappeared for seeds planted in late winter, when the treatments were first stratified for 3 mo. None of the variables for measuring seedling survival and plant performance varied between treatments. Under our experimental conditions (no herbivory or competition), ingestion by ducks in early winter resulted in increased performance for seeds surviving gut passage due to enhanced seed germinability, without other costs or benefits for the seedlings.     

Fruit and seed traits of native and invasive plant species in Hawai‘i: implications for seed dispersal by non-native birds

Biological Invasions - For alien invasive plant species dependent on frugivores for seed dispersal, traits that influence consumption can be important determinants of invasion and spread. However,...     

Are predatory birds effective secondary seed dispersers?

We have studied the unusual phenomenon of secondary seed dispersal of Lycium intricatum seeds on a small oceanic Atlantic island (Alegranza, Canarian Archipelago) in which a small frugivorous lizard ( Gallotia atlantica ) and two different predatory birds participate, a shrike ( Lanius excubitor ) and a kestrel ( Falco tinnunculus ). Endemic lizards that are common prey of both bird species consume Lycium fruits. Lizard remains were significantly matched with the presence of Lycium fruits in the regurgitation pellets of the two predatory birds. Seeds were found in 7.3% of the lizard droppings, 31.0% of kestrel pellets and 55.7% of shrike regurgitations. The mean number of seeds per dropping or pellet was 4.8 ±4 in lizard, 20.2 ±34.5 in shrike and 6.7 ±8.1 in kestrel. The percentage of viable seeds showed significant differences among all four treatments, decreasing in the following direction: seeds collected directly from plants (98.0%), shrikes (88.0%), lizards (72.3%), and kestrels (31.7%). Seeds from Lycium fresh fruits and shrike pellets showed significantly higher germination rates than those from lizard droppings and kestrel pellets. While lizards and shrikes are effective seed dispersers, kestrel gut treatment decreases seed viability. Seed viability is always higher than seed germination in each of the four treatments. In this island environment, Lycium seeds are under an important random influence during the seed dispersal process. Secondary seed dispersal seems to acquire a relevant dimension in small and remote insular environments or isolated continental systems where interactions among the different elements involved are intense, all of them are abundant native residents, and they have been coexisting for a long time. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 345–352.     

Can assignment tests measure dispersal?

Individual-based assignment tests are now standard tools in molecular ecology and have several applications, including the study of dispersal. The measurement of natal dispersal is vital to understanding the ecology of many species, yet the accuracy of assignment tests in situations where natal dispersal is common remains untested in the field. We studied a metapopulation of the grand skink, Oligosoma grande, a large territorial lizard from southern New Zealand. Skink populations occur on isolated, regularly spaced rock outcrops and are characterized by frequent interpopulation dispersal. We examined the accuracy of assignment tests at four replicate sites by comparing long-term mark-and-recapture records of natal dispersal with the results of assignment tests based on microsatellite DNA data. Assignment tests correctly identified the natal population of most individuals (65-100%, depending on the method of assignment), even when interpopulation dispersal was common (5-20% dispersers). They also provided similar estimates of the proportions of skinks dispersing to those estimated by the long-term mark-and-recapture data. Fully and partially Bayesian assignment methods were equally accurate but their accuracy depended on the stringency applied, the degree of genetic differentiation between populations, and the number of loci used. In addition, when assignments required high confidence, the method of assignment (fully or partially Bayesian) had a large bearing on the number of individuals that could be assigned. Because assignment tests require significantly less fieldwork than traditional mark-and-recapture approaches (in this study < 3 months vs. > 7 years), they will provide useful dispersal data in many applied and theoretical situations.     

Does the morphology of animal foraging pits influence secondary seed dispersal by ants?

Secondary seed dispersal by ants (myrmecochory) is an important process in semi‐arid environments where seeds are transported from the soil surface to an ant nest. Microsites from which ants often remove seeds are the small pits and depressions made by native and exotic animals that forage in the soil. Previous studies have demonstrated greater seed retention in the pits of native than exotic animals, but little is known about how biotic factors such as secondary seed dispersal by ants affect seed removal and therefore retention in these foraging pits. We used an experimental approach to examine how the morphology of burrowing bettong (Bettongia lesueur), greater bilby (Macrotis lagotis), short‐beaked echidna (Tachyglossus aculeatus) and European rabbit (Oryctolagus cuniculus) foraging pits and ant body size influenced ant locomotion and seed removal from pits along an aridity gradient. Ants took 3.7‐times longer to emerge from echidna pits (19.6 s) and six‐times longer to emerge from bettong pits (30.5 s) than from rabbit pits (5.2 s), resulting in lower seed removal from bettong pits than other pit types. Fewer seeds were removed from pits when cages were used to exclude large body‐sized (>2 mm) ants. Few seeds were removed from the pits or surface up to aridity values of 0.5 (humid and dry sub‐humid), but removal increased rapidly in semi‐arid and arid zones. Our study demonstrates that mammal foraging pit morphology significantly affects ant locomotion, the ability of ants to retrieve seeds, and therefore the likelihood that seeds will be retained within foraging pits.     

Do mobbing calls affect the perception of predation risk by forest birds?

We report the results of two field experiments assessing whether exposure to mobbing calls, which usually indicates the presence of a predator, elicits changes in risk perception as measured by the willingness of forest birds to enter or cross gaps in forest cover. In the first experiment, we tested the prediction that wintering black-capped chickadees,Poecile atricapilla , will travel less in open areas to seek food from a feeder during a simulated mobbing event than they will under control conditions. The number of visits to feeders decreased at increasing distances from forest edges (0-10 m) when mobbing calls were played. Mobbing calls increased the overall rate of visits to feeders, but lowered the rate of visits to feeders located at 10 m from forest edges. By contrast, when a stuffed merlin, Falco columbarius, was placed near the feeder, chickadees rarely visited the feeder, regardless of the distance to forest edge. In the second experiment, we tested the prediction that breeding male red-eyed vireos, Vireo olivaceus, would be less attracted to the song of a nearby red-eyed vireo after a playback of a mobbing event than after a ‘neutral’ sound stimulus, especially if they had to cross an open area. Vireos were as willing to enter or cross gaps in forest cover following exposure to mobbing calls, although they responded more frequently to conspecific song under forest cover than across gaps. Our results suggest that forest birds under cover assess the risk of a mobbing situation largely on the basis of the distance to the cause of mobbing. It follows that a trade-off between risk and reward may not occur with mobbing birds, unless mobbing occurs far outside forest cover and/or at very close range (<5 m) to a predator. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Are tortoises important seed dispersers in Amazonian forests?

According to most studies on seed dispersal in tropical forests, mammals and birds are considered the main dispersal agents and the role played by other animal groups remains poorly explored. We investigate qualitative and quantitative components of the role played by the tortoise Chelonoidis denticulata in seed dispersal in southeastern Amazon, and the influence of seasonal variation in tortoise movement patterns on resulting seed shadows. Seed shadows produced by this tortoise were estimated by combining information on seed passage times through their digestive tract, which varied from 3 to 17 days, with a robust dataset on movements obtained from 18 adult C. denticulata monitored with radio transmitters and spoon-and-line tracking devices. A total of 4,206 seeds were found in 94 collected feces, belonging to 50 seed morphotypes of, at least, 25 plant genera. Very low rates of damage to the external structure of the ingested seeds were observed. Additionally, results of germination trials suggested that passage of seeds through C. denticulata’s digestive tract does not seem to negatively affect seed germination. The estimated seed shadows are likely to contribute significantly to the dispersal of seeds away from parent plants. During the dry season seeds were dispersed, on average, 174.1 m away from the location of fruit ingestion; during the rainy season, this mean dispersal distance increased to 276.7 m. Our results suggest that C. denticulata plays an important role in seed dispersal in Amazonian forests and highlight the influence of seasonal changes in movements on the resulting seed shadows.     

Does invasion by an alien plant species affect the soil seed bank?

Questions: How does invasion affect old‐field seed bank species richness, composition and density? How consistent are these effects across sites? Does the soil seed bank match vegetation structure in old‐fields? Location: Menorca, Balearic Islands, Spain, western Mediterranean basin. Methods: We monitored seed germination in soils from old‐fields that were both uninvaded and invaded (legacy effect) by the annual geophyte Oxalis pes‐caprae. We also added O. pes‐caprae bulbs to uninvaded soils to test O. pes‐caprae interference with seedling emergence (competitive effect). We compared species composition in the seed bank with that of the vegetation. Results: Species richness in the seed bank and in the vegetation was not significantly different between invaded and uninvaded areas. Uninvaded areas did not have larger seed banks than invaded areas. More seedlings, especially of geophytes, emerged when O. pes‐caprae bulbs were added to the soil. Species similarity between invaded and uninvaded areas was higher in the seed bank (74%) than in the vegetation (49%). Differences in species composition were as important as differences among sites. The degree of species similarity between the seed bank and the vegetation was very low (17%). Conclusions: Despite invasion by O. pes‐caprae not affecting species richness, the variation in the seed bank species composition in invaded and uninvaded areas, and the differences between the seed bank and the mature vegetation, highlights that even if the invader could be eradicated the vegetation could not be restored back to the exact composition as found in uninvaded areas.     

When will plant morphology affect the shape of a seed dispersal "kernel"?

Most models of dispersal assume that plants are point sources. In reality, the scale in height over which seed sources are distributed is often of the same order as the scale in distance over which most individual seeds are dispersed. But is this sufficient to affect the fundamental shapes of dispersal frequency distributions? Most published conclusions about the effects of canopy structure on dispersal are subjective. A model is developed to explore the consequences of plant canopies for the shapes of whole-plant seed dispersal "kernels". The canopies were described by simple geometric shapes, while an empirical probability density function (PDF) was used for dispersal from a point source. It was found that the resulting whole-plant PDF for dispersal distance was almost invariably peaked, whereas the PDF for the density of seed rain (as would be measured by pitfall traps) could either be peaked or monotonic according to the canopy shape, position of seeds in the canopy, and mean dispersal distance. The shapes of kernels from whole plants (distributed seed sources) can be very different from those derived from a point source under certain circumstances.     

How much is a lot? Seed dispersal by white-faced capuchins and implications for disperser-based studies of seed dispersal systems

The quantity of seeds dispersed is considered one of several means to determine the dispersal effectiveness of an animal. However, there is little consistency in the manner in which quantities are measured or presented. Here, we quantify seed dispersal by white-faced capuchin monkeys (Cebus capucinus) in Santa Rosa National Park, Costa Rica by measuring: degree of frugivory, number of plant species consumed, the number of seeds consumed per unit time, the number of seeds dispersed intact per unit time, and the number of seeds dispersed intact per unit space. Forty-nine percent of C. capucinus diet is composed of the fruit of 39 species, 4 of which constitute 82% of the frugivory. Seventy-four percent of consumed fruits contain seeds that pass intact through the capuchin digestive system. Capuchins pass a mean of 15.7 seeds of a mean of 1.3 species per defecation, and defecate 25.4 times per 12-h day. These numbers are compared with extant data for C. capucinus, and possible reasons for discrepancies among results between studies are discussed. We propose a standardization of quantitative measures of seed dispersal so that quantifications of seed dispersal can be compared within species, and eventually across species.     

Could poor seed dispersal contribute to predation by introduced rodents in a Hawaiian dry forest?

Dry forests are among the most diverse, yet threatened, communities in Hawai’i. Dry forests throughout the archipelago suffer from a lack of natural regeneration of trees. Two factors that may limit tree recruitment include poor seed dispersal and seed predation by rodents. Poor or limited dispersal of fleshy-fruited species results in seeds and fruits falling directly under parents. Dispersed and non-dispersed seeds may differ in their vulnerability to predation. We tested effects of seed location (under/away from parent trees) and pulp (presence/absence) on predation of four native species that suffer from limited dispersal and one readily-dispersed alien species in Kanaio Natural Area Reserve, Maui. Three natives (Diospyros sandwicensis, Pleomele auwahiensis, Santalum ellipticum), had significantly more seeds removed under parent trees than in exposed sites away from trees. For the one alien (Bocconia frutescens) and two native trees (D. sandwicensis, P. auwahiensis) that were evaluated, significantly more intact fruits were removed than were cleaned seeds. Presence of teeth marks and gnawed seed husk fragments indicate introduced rodents are destroying many of the seeds they remove. These results suggest that seed predation is disproportionately concentrated among poorly-dispersed seeds and may contribute to recruitment failure.     

Fruiting phenology as a “triggering attribute” of invasion process: Do invasive species take advantage of seed dispersal service provided by native birds?

Mechanisms underlying biological invasion of highly disturbed ecosystems are well known, yet mechanisms responsible for biological invasion of undisturbed or weakly disturbed ecosystems are less understood. The triggering attribute (TA) approach, proposed as a mechanism that explains plant invasion success in undisturbed or weakly disturbed systems, considers that the spread of alien species depends on specific vegetative or regenerative traits in invasive species, discontinuously distributed in comparison to the resident community. In mountain Chaco woodland, fruiting phenology of ornithocorous invasive plants has been proposed as a TA, because it would allow invasive species to benefit from seed dispersal service, which is unused by native plants during a specific period of the year (winter). Under the seed dispersal ecology framework, we evaluated if fruiting phenology (fructification largely uncoupled with native species) of the fleshy-fruited invasive Pyracantha angustifolia affects bird fruit consumption, and allows the invasive to take advantage of the unused seed dispersal service during winter. If uncoupled fructification phenology represents a TA, seed disperser, seed predator, and pulp consumer diversity, abundance, and fruit consumption on P. angustifolia (which fructifies in winter), will be higher than on its exotic congeneric P. coccinea during summer, when fructification overlaps with native Celtis ehrenbergiana and many other native species. We found that: (1) disperser bird abundance and fruit consumption did not differ between P. angustifolia and P. coccinea; (2) the most diverse frugivorous assemblage was observed on C. ehrenbergiana, yet it had the lowest proportion of seed dispersers and the highest fruit consumption by seed predators and, (3) we also observed higher proportion of seed predators on P. angustifolia (uncoupled fructification scenario) than on P. coccinea (coupled fructification scenario). Our results suggest that invasive uncoupled fructification phenology does not represent a true TA which facilitates plant invasion processes in undisturbed or weakly disturbed ecosystem.     

Are large frugivorous birds better seed dispersers than medium‐ and small‐sized ones? Effect of body mass on seed dispersal effectiveness

Frugivorous birds vary in seed dispersal effectiveness (SDE) depending on their body mass. It has been suggested that large birds are more effective dispersers than small ones because they consume a large number of fruits, disperse seeds of distinct sizes, and transport seeds over long distances. Yet, few studies have evaluated the impact of body mass on SDE of birds. In this study, we compiled one database for the quantity (i.e., frequency of visits to plants and number of seeds removed per visit) and quality (i.e., germination of seeds after gut passage and gut retention time of seeds) of seed dispersal by frugivorous birds to evaluate the impact of body mass on SDE. In addition, we compiled data on plant characteristics such as life‐form, fruit type, number of seeds per fruit, and size of seed to evaluate their influence on the quantity and quality of seed dispersal. Data were analyzed with linear mixed effects models and quantile regressions to evaluate the relationship between body mass of birds and quantity, quality, and SDE, in addition to the influence of plant characteristics on SDE. The body mass of birds was negatively related to the frequency of visits to plants. Furthermore, it was positively related to the number of seeds removed per visit, although negatively related to seed size. The life‐form of plants was the only factor explaining the germination of seeds after gut passage. Yet, the body mass of birds was positively related to the gut retention time of seeds. Small and medium birds have a relatively higher SDE than large birds. These results differ from the assertion that large birds are more effective dispersers of plants. Small and medium birds are also effective dispersers of plants that should be preserved and protected from the impact of human activities.     

Can nest predation and predator type explain variation in dispersal of adult birds during the breeding season ?

Many types of predators depredate bird nests and thus potentiallyinfluence the spatial distribution of their prey. We used asimulation model of a double-brooded songbird's nesting seasonto test three predictions about the selective advantage ofdispersing different distances after nest predation by predatorswith varying home range sizes. Our results supported the predictions that (1) dispersing birds had higher success than nondispersingbirds after predation of the first nest, (2) dispersing beyondthe home range of the nest predator increased the success ofthe second nest, and (3) birds whose first nests were depredatedearly in the nesting cycle did better by dispersing fartherthan birds whose nests were depredated later in the nestingcycle. Our results provide evidence that predation and predatorcharacteristics may cause variation in adult dispersal distancesduring the breeding season. However, we did not find an advantagefor long-distance dispersal when predators with small- or medium-sizedhome ranges were responsible for the predation event. The criticaldecisions of dispersal and dispersal distance made by adultbirds are complex, but our model demonstrates that predationevents can create a selective advantage to disperse.     

Are red imported fire ants facilitators of native seed dispersal?

Invasive ants threaten native communities, in part, through their potential to disrupt mutualisms, yet invasive species may also facilitate native species. The red imported fire ant (Solenopsis invicta) is one of the most conspicuous invasive ants in North America and its high densities, combined with its potential to displace native ants, have led to concerns that it may disrupt ant-plant seed dispersal mutualisms. We examined the potential of fire ants to disperse seeds in the longleaf pine ecosystem by comparing the removal of elaiosome-bearing seeds by fire ants versus native ants. A total of 14 ant species were observed removing seeds, with fire ants responsible for more than half of all removals. While fire ants were the dominant seed remover in this system, they did not remove significantly more seeds than would be expected based on their population density (46% of ground-dwelling ants). Moreover, red imported fire ants were similar to native ants with respect to distance of seed movement and frequency of moving seeds back to the nest. Areas of higher fire ant densities were found to have greater rates of seed removal by ants without a subsequent drop in seed dispersal by native ants, suggesting that fire ant-invaded areas may experience overall higher levels of seed dispersal. Thus, fire ants may actually facilitate dispersal of elaiosome-bearing plant species in the longleaf pine ecosystem.