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How human disturbance of tropical rainforest can influence avian fruit removal
Abstract
Fruit consumption by birds is an important ecological interaction that contributes to seed dispersal in tropical rainforests. In this field experiment, we asked whether moderate human disturbance alters patterns of avian frugivory: we measured fruit removal by birds in the lower montane rainforest of Tobago, West Indies, using artificial infructescences made with natural fruits from two common woody plants of the forest understory (Psychotria spp., Rubiaceae). Displays were mounted simultaneously in three forest habitats chosen to represent a gradient of increasing habitat disturbance (primary, intermediate and disturbed), caused by subsistence land use adjacent to a protected forest reserve. We measured the numbers of fruits removed and the effect of fruit position on the likelihood of removal, along with the abundances of all fruits and fruit-eating birds at the study sites. Fruit removal was highly variable and there was not a significant difference in removal rate among forest habitats; however, the trend was for higher rates of removal from displays in primary forest. Canopy cover, natural fruit availability, and frugivore abundance were not good predictors of fruit removal. Birds preferred more accessible fruits (those proximal to the perch) in all habitats, but in disturbed forest, there was a tendency for distal fruits to be chosen more frequently than in the other forest types. One possible explanation for this pattern is that birds in disturbed forests were larger than those in other habitats, and hence were better able to reach the distal fruits. Coupled with differences in bird community composition among the forest types, this suggests that different suites of birds were removing fruit in primary versus disturbed forest. As frugivore species have different effectiveness as seed dispersers, the among-habitat differences in fruit removal patterns that we observed could have important implications for plant species experiencing disturbance; these possible implications include altered amounts of seed deposition and seedling recruitment in Tobago's tropical rainforest.
... Indeed, other multiple-year studies evaluating the assemblage of small mammals in other gallery forests within our protected area did not document it, and it has only been rarely documented elsewhere. However, it is essential to note that C. lanatus plasticity in vertical space use can significantly affect the species' survival in different deforestation scenarios and have relevant implications for forest dynamics (e.g., Havmøller et al. 2021;Lefevre and Rodd 2009). Thus, more studies on the natural history and ecology of this rare and elusive Neotropical marsupial are warranted. ...
The brown-eared woolly opossum (Caluromys lanatus) is an elusive frugivorous Neotropical canopy specialist marsupial considered primarily arboreal, but here we report the first records of individuals from this species captured on the ground in the Cerrado. We hypothesize two main reasons to explain this behavior: first related to canopy openness, which would force the animals to the ground to keep moving across the landscape; and the second related with search for food on the ground during periods of food scarcity due to increased canopy gaps. Our findings can have important implications for the species conservation and understanding forest ecological dynamics.
... Thus, patchworks of small-scale diverse agroforests, home gardens, and secondary forests can increase the local tree diversity, structural heterogeneity, and foods available for small-island bird communities in forest landscapes relative to secondary forests alone Lefevre et al. 2012;. While secondary forests themselves become less structurally diverse as the canopy closes, light loving nitrogen-fixing trees die off Helmer et al. 2018), and initial increases in tree (and flower, fruit, and seed) diversity begin to decline in the decades following scrub regrowth Lefevre & Rodd 2009;Oberleitner et al. 2021). ...
Knowledge of species habitat requirements, movements, and life cycles is important for understanding their ecology and functions in ecosystems, and is critical for enabling their effective conservation and monitoring, particularly in small-island states where conservation resources are limited. Island biogeography and ecological release theories make broadly supported predictions about the evolution of generalist traits among small-island birds in the Caribbean, and this research examines the influence of small-island conditions on the life cycle adaptations, diets, and distribution of birds and their responses to human land-use such as agriculture to inform conservation research and management. Using mist-netting, I found that 10 common terrestrial bird species on Grenada (3 endemic to the East Caribbean) exhibited similar patterns of low breeding period seasonality, followed by synchronized peaks in moult intensity and uncommonly high rates of moult-breeding overlap during peak rainfall and high arthropod abundance, despite the wide range in size, morphology, and phylogeny of these species. I studied patterns of seasonal diet composition and diet-overlap among species using DNA meta-barcoding of feces, which suggested that moult-breeding overlap may result from restricted time periods over which arthropod availability is sufficient for breeding and moult to occur. Diet analyses also indicated that agricultural habitats provide important year-round food sources. Lastly, repeated point-count surveys indicated increased bird diversity and abundance at high elevations and in closed-canopy habitats during the dry season, while many birds moved to low elevations and open-canopy habitats in the wet season. Much of Grenada consists of a diverse mosaic of closed-canopy secondary forest patches, interspersed among more open agroforests and small-scale cropping and grazed areas. These agroecosystems appear to provide important habitats and foods to birds throughout the year, especially more-open canopy agroforests in the wet season, while intact canopy forests provide important dry season habitat, which may be particularly important for species in lowland coastal fringe areas experiencing more intensive agricultural, urban, and resort development. This research demonstrates life history adaptations of birds on small islands, the importance of maintaining habitat diversity at local and landscape scales for species conservation, and the contribution of diverse small-scale agroecosystems to supporting small-island bird communities.
... These kinds of disturbances may hinder floral reproductive success and thus alter floral demographics and distribution (Aizen & Feinsinger, 1994;Sodhi et al., 2009). At the same time, disturbance can disrupt the biotic interactions which are essential for the maintenance of the ecosystem's integrity (Lefevre & Rodd, 2009). In the case of this study, it seems that similar with the report of Matson et al. (1997); agricultural expansions have provided anthropogenic alterations to the disturbed forest. ...
Despite the notable plant biodiversity of the Philippines, the country still exhibits increasing anthropogenic activities in many forests of the archipelago that pose a threat to many floral species of the country. Hence, to test the hypothesis that there are differences in plant distributions and diversity between disturbed and undisturbed tropical forest patches, a rapid floristic assessment in Malinao, Aklan was conducted. A total of 106 species were sampled in both sites, with 68 present in the disturbed site, 53 present in the undisturbed site, and 12 present in both. Rarefied species accumulation curves and calculations of Shannon-Wiener Index (H) showed higher species richness and evenness in the undisturbed forest. Analysis of the similarities in species composition using the nonmetric dimensional scaling combined with the statistical test PERMANOVA showed clear significant difference between the two forest categories. In addition, one species (Artocarpus blancoi (Elmer) Merr.) that is classified as “Vulnerable” according to the IUCN was noted. Thus, standard calls for the establishment of priority areas for forest conservation in the country are recommended.
... Fruit and flower abundance might be higher in moderately disturbed habitats because of improved light conditions, and fruits in open habitats may be especially sugary and attract more birds than fruits in the more shaded forest (Lumpkin and Boyle, 2009). Irrespective of the mechanism, even low levels of disturbance in tropical forests can significantly change community composition and affect species interactions (Lefevre and Rodd, 2009;Lefevre et al., 2012). The potential effects of habitat disturbances on birds that engage in mixed flocks in Cotapata NP is still unknown, and could represent an important predictor of forest health and ability to harbor functional biodiversity. ...
... The results of our study indicate that land-use negatively affected frugivore abundance. Our findings are congruent with other studies that found a decline of frugivore abundance with increasing anthropogenic land-use intensity (Azhar et al., 2013;Cordeiro and Howe, 2003;Lefevre and Rodd, 2009). The intensification of anthropogenic landuse involves a destruction of natural habitats, which not only entails a loss of food resources, but also a loss of a variety of habitat characteristics that are indispensable for the survival of frugivore species, e.g. ...
Invasive plant species threaten ecosystems worldwide. Management of invasive plants considerably benefits from an understanding of the processes that drive invasive spread, like seed dispersal. In the subtropics, many invasive species rely on vertebrate frugivores for dispersal. In turn, frugivore distribution across the landscape depends on habitat and fruit availability. Therefore, the loss of habitat owing to land-use change, and low fruit availability may reduce the local frugivore abundance and thereby impact invasive fruit removal and spread. However, the interplay among these factors remains poorly understood. In the intensively used agricultural landscape in KwaZulu-Natal, eastern South Africa, we investigated the effects of land-use and native and invasive fruit availability on frugivore abundance and analyzed how potential changes translate into fruit removal rates of the invasive fleshy-fruiting shrub Solanum mauritianum Scopoli (Solanaceae). Increasing land-use intensity reduced frugivore abundance, which in turn reduced fruit removal rates of the invasive species. Increasing native fruit availability did not influence frugivore abundance, but reduced fruit removal rates of S. mauritianum, indicating a fruit preference of frugivores for the simultaneously fruiting native species. In conclusion, fruit removal of the species is highest in locations with low land-use and low native fruit availability. Thus, these areas should be priority areas for invasive species management. Additionally, plantings of native fruiting species may provide alternative preferred food resources, reduce invasive fruit removal and ultimately improve invasive species management.
... The habitats are described fully in Lefevre and Rodd (2009) and are characterized as follows: (1) control plots were located in Primary forest in the reserve's interior, where the canopy was dominated by large, buttressed trees, with an understory of shrubs and small trees. The only known human disturbance was due to walking trails and possibly some wildlife poaching. ...
Forests worldwide are experiencing rapid environmental change due to human activity. We aimed to increase understanding of anthropogenic impacts on community composition and species interactions. In a natural experiment, we asked whether subsistence human land use has altered the community composition of a Neotropical rain forest on the island of Tobago, in the West Indies. We surveyed fruiting plants and birds in three adjacent habitat types that varied in level of disturbance, and used multivariate analyses to determine whether changes in the plant community were associated with differences in avifauna composition. The three forest habitats had similar plant and bird diversities, yet markedly different species compositions and abundances. Primary forest had the most diverse plant community, while disturbed habitats had a more homogeneous plant composition. Primary and disturbed forest had distinct community compositions, with canopy cover and the relative abundance of plant types explaining 83 percent of the variation in bird species assemblages. Seemingly moderate human disturbance has led to substantial changes in the plant and bird assemblages of Tobago's rain forest, outside of a protected reserve. Our study highlights the direct links between human disturbance and the structure of rain forests, underscoring the impact of even moderate activity on community composition.
... Thus, the widely accepted notion that fig trees are keystone resources for tropical frugivores (Kissling et al. 2007) appears to be context-dependent and could for instance depend on the composition of the local frugivore community. These findings underline that changes in frugivore communities may differently affect plants presenting different types of fruits (Lefevre andRodd 2009, Moran et al. 2009) and may pose new selective pressures on fleshy-fruited plants in disturbed habitats, for instance due to better seed-dispersal services for small-fruited plant species in the absence of large seed dispersers. ...
Forest fragmentation and local disturbance are prevailing threats to tropical forest ecosystems and affect frugivore communities and animal seed dispersal in different ways. However, very little is known about the effects of anthropogenic forest edges and of local disturbance on the structure and robustness of plant–frugivore networks. We carried out focal tree observations to record the frugivore species feeding on eight canopy tree species in the forest interior and at forest–farmland edges in a little and a highly disturbed part of a Kenyan rain forest. For each frugivore species, we recorded its body mass and its forest dependence. We examined how forest edge and local disturbance affected the abundance, the richness and the composition of the frugivore community and tested whether forest edge and local disturbance affected plant frugivore networks. Abundance and species richness of frugivores were higher at edges than in the forest interior. Forest visitors and small-bodied frugivores increased, while forest specialists decreased in abundance at forest edges. The changes in frugivore community composition resulted in plant–frugivore networks that were more connected, more nested and more robust against species extinctions at forest–farmland edges than in the forest interior. Network specialization was lower at forest edges than in the forest interior because at the edges plant specialization on frugivores was very low in small-fruited species. In contrast, small-fruited plants were more specialized than large-fruited plants in the forest interior. Our findings suggest that forest-visiting birds may stabilize seed-dispersal services for small-fruited plant species at rain forest margins, while seed-dispersal services for large-fruited plant species may be disrupted at forest edges due to the decrease of large-bodied frugviores. To assess the ultimate consequences of bird movements from farmland to forest edges for ecosystem functioning, future studies are required to investigate the seed-dispersal qualities provided by forest-visiting bird species in the tropics.
... Even in some wind dispersed tropical tree species dispersal may be insufficient to achieve effective colonisation of hospitable habitat within an ecologically relevant time scale (Kettle et al., 2011b). In many developing countries fragmentation occurs in unison with the defaunation of forest and loss of many frugivore communities which is leading to further reduction in seed dispersal (Anzures-Dadda et al., 2011;Galetti et al., 2006;Lefevre and Rodd, 2009). Recent modelling studies of forest successional dynamics also indicate that secondary forests in highly fragmented landscapes with small isolated patches are likely to lead to arrested early successional stage forests of low diversity due to lack of seed dispersal from intact forest . ...
... In human disturbed landscapes, a concentration of mobile organisms within suitable forest fragments has been reported (Lovejoy et al. 1986, Bowman et al. 2002), resulting from limited habitat availability, but also from a certain habitat attractiveness including for instance shelter, nesting sites and food resources for many species. However, our results showed that crop size and local fruit availability had no significant effect on either bird visitation rates or on total fruit removal which also has been reported in previous studies (Kirika et al. 2008b, Lefevre and Rodd 2009). As suggested by Thompson and Willson (1978), it is not higher fruit availability, but rather higher perceptibility of fruit resources that attracts more fruit-consuming birds in disturbed sites, like forest edges or gaps. ...
Human disturbance threatens and modifies forest ecosystems worldwide. Previous studies have investigated the effects of human impact on local bird communities in disturbed forests, but we still lack information on how bird species richness and ecological processes respond to different forest modifications present at a landscape scale. In a heterogeneous South African landscape, we chose six types of indigenous scarp forest, differing in the intensity of human disturbance: continuous natural forests and natural forest fragments in nature reserves, forest fragments in eucalyptus plantations, fragments in the agricultural matrix, forest gardens and secondary forests in game reserves. In 36 study sites, we investigated the bird community using point counts and observed the seed removal of birds at the native tree species Celtis africana. Species richness did not differ among the forest types, but abundance varied significantly with most birds observed in fragments in the agricultural matrix, forest gardens, and secondary forests. The higher bird abundance in these forests was mainly due to forest generalists, shrubland and open country species whereas forest specialists were rarely present. Changes in species composition were also confirmed by multivariate analysis which clearly separated bird communities by forest type. Frugivore abundance in C. africana was highest in natural forest fragments, fragments in the agricultural matrix, forest gardens and secondary forests. The same trend was found for the estimated total number of fruits removed per C. africana tree, though the differences among forest types were not significant. Consequently, modified forests seem to maintain important ecological functions as they provide food sources for generalist species which may, due to their mobility, enhance natural plant regeneration. However, we could show that protected forest habitats are important refugees for specialist species sensitive to human disturbance.
... Weak dispersal is suggested to be a potentially critical factor in explaining the distribution of palms in TMCF (Svenning et al., 2009), especially through the alteration of populations of seed dispersers (Wright and Duber, 2001;Jorge and Howe, 2009). The populations of birds and mammals that disperse Ceroxylon (Mejía Londoño, 1999;Krabbe, 2000) have thus very likely been altered by fragmentation as these types of animals have been shown especially sensitive to fragmentation (Lefevre and Rodd, 2009;Chapman et al., 2010). This would explain the genetic bottleneck found, as commonly documented in neotropical forests (see Lowe et al. (2005) for a review). ...
To conserve tropical forests, it is crucial to characterise the disturbance threshold beyond which populations of tropical trees are no longer resilient. This approach is still not widely employed, especially with respect to the effects of moderate disturbances. Compensation effects, such as positive interactions among plants, are addressed even more rarely. We attempt to identify the extents to which the distribution of the keystone palm tree Ceroxylon echinulatum is regulated by various regimes of deforestation in a threatened tropical montane cloud forest in the North-West Andes of Ecuador. The demographic structure of this palm tree was examined in three habitats: old-growth forest, forest disturbed by selective logging, and deforested pasture. Patterns were related to stand structure, microclimate, and soil composition. Seedling desiccation owing to severe aboveground water stress led to the absence of juvenile palms in pastures, and thus was predictive of a near extinction of the species in this habitat. However, shade provided by dominant bunchgrass in pastures considerably reduced above- and belowground water stress by diminishing light intensity. Selective logging resulted in a higher density of individuals in disturbed forests than in old-growth forests, but was associated with a spoiled spatial structure. Therefore, the protection of residual old-growth forests is a prerequisite for the conservation of C. echinulatum, although secondary forests might act as provisional refuges that promote its resilience. The reduction of water stress by nurse grasses in pastures represents a promising approach to promote the resilience of tropical tree species and their associated communities after deforestation.
... Hence, here we have a case where introduced mammals (mainly ship rats and possums) have had an adverse effect on seed dispersal both directly through seed predation, and indirectly through competition with and predation upon seed dispersers. Similarly, Lefevre & Rodd [43] report that human-disturbed tropical rainforest held a substantially different suite of avian seed-dispersing species than nearby unaltered sites, and that this affected the types of fruits consumed. Cordeiro & Howe [44] investigated 80-year old forest fragments in Tanzania. ...
At a time when plant species are experiencing increasing challenges from climate change, land-use change, harvesting and invasive species, dispersal has become a very important aspect of plant conservation. Seed dispersal by animals is particularly important because some animals disperse seeds to suitable sites in a directed fashion. Our review has two aims: (i) to highlight the various ways plant dispersal by animals can be affected by current anthropogenic change and (ii) to show the important role of plant and (particularly) animal physiology in shaping seed-dispersal interactions. We argue that large-bodied seed dispersers may be particularly important for plant conservation because seed dispersal of large-seeded plants is often more specialized and because large-bodied animals are targeted by human exploitation and have smaller population sizes. We further argue that more specialized seed-dispersal systems on island ecosystems might be particularly at risk from climate change both owing to small population sizes involved but also owing to the likely thermal specialization, particularly on tropical islands. More generally, the inherent vulnerability of seed-dispersal mutualisms to disruption driven by environmental change (as well as their ubiquity) demands that we continue to improve our understanding of their conservation physiology.
... Negative effects have been recorded for 70 individual species of birds of which 24 are threatened species (Table 1, Appendix 1). The seven studies that found no effect on birds included; a paper on the Eurasian Dotterel (Charadrius morinellus) in Scotland (Watson, 1988), one paper in England on the Black Grouse (Tetrao tetrix) (Baines and Richardson, 2007), two papers in New Zealand, one on Yellow-eyed Penguins (Megadyptes antipodes) (Ratz and Thompson, 1999) and another on the Stitchbird (Notiomystis cincta) (Lindsay et al., 2008), one paper in the United States on Passerines (Riffell et al., 1996), one paper from Brazil assessing Wood Storks (Mycteria americana) (Bouton et al., 2005) and one paper in the West Indies examining Frugivores (Lefevre and Rodd, 2009) (Appendix 1). Of these studies, three examined bird abundance; four examined behavioural changes; four examined nesting success and two examined evasion responses. ...
Nature based recreation such as wildlife viewing, hiking, running, cycling, canoeing, horse riding and dog walking can have negative environmental effects. A review of the recreation ecology literature published in English language academic journals identified 69 papers from 1978 to 2010 that examined the effect of these activities on birds. Sixty-one of the papers (88%) found negative impacts, including changes in bird physiology (all 11 papers), immediate behaviour (37 out of 41 papers), as well as changes in abundance (28 out of 33 papers) and reproductive success (28 out of 33 papers). Previous studies are concentrated in a few countries (United States, England, Argentina and New Zealand), mostly in cool temperate or temperate climatic zones, often in shoreline or wetland habitats, and mostly on insectivore, carnivore and crustaceovore/molluscivore foraging guilds. There is limited research in some regions with both high bird diversity and nature based recreation such as mainland Australia, Central America, Asia, and Africa, and for popular activities such as mountain bike riding and horse riding. It is clear, however, that non-motorised nature based recreation has negative impacts on a diversity of birds from a range of habitats in different climatic zones and regions of the world.
Frugivores and the plants depending on frugivore to disperse their seeds form a mutualism in the process of evolution, but habitat fragmentation can disturb the process and break the mutualism. Habitat fragmentation often reduces the diversity of frugivores, but this situation does not happen in all ecosystems. Frugivores have different adaptability to the habitat fragmentation, being stronger for the forest generalists and food generalists. Habitat fragmentation has negative effects on most of the plants depending on frugivore to disperse their seeds, but some plants are not, and even, benefited. The seed dispersal effectiveness of the frugviores in fragmental habitat is the combined effect of seed dispersal amount, dispersal distance, seed germination, and establishment of seedling population. The changes in the foraging behavior and food choice of frugivores affect the seed dispersal amount, whereas the heterogeneity of habitat patch and the changes in food resource availability determine the seed dispersal distance. The successful establishment of seedling population depends on effective disperser. Three issues to be further studied were proposed: (1) how habitat fragmentation affects the seed dispersal and the relationship between animals and plants, especially how the spatial distribution of heterogeneous landscapes affects the seed dispersal effectiveness of frugivores, (2) how to create a new mutualism between plants and frugivores in habitat fragmentation, and (3) how the plant invasion induced by habitat fragmentation affects the seed dispersal of native plants.
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.
We review resilience to natural and anthropogenic disturbance of palm populations and communities in tropical America. Response
of palms to disturbance depends on their morphological traits, their reproductive strategies and the impacts of these traits
and strategies on phenology and gene flow. Human impact induces changes in genetic structure, increasing endogamy and genetic
drift in fragmented populations. Forest fragmentation and harvest of palm organs are well documented whereas effects of intermediate
disturbance like selective logging, hunting or fire remain poorly known. We recommend emphasis on long-term experiments and
on the use of mechanistic approaches in future research to facilitate integration of available data into a theoretical ecological
framework.
KeywordsArecaceae–Deforestation–Gene flow–Harvest–Tropical America–Pollination–Resilience–Seed dispersal
Begins with a general description of fruits and frugivores at La Selva and compares them with other tropical forests. Then addresses the following questions: What factors influence fruit choice? What is the role of fruit handling in successful dispersal? How do spatial and temporal variation in fruit abundance affect frugivore populations? Given information on fruit choice, fruit handling, and spatial/temporal variation in bird and fruit abundance, what can one deduce about the coevolutionary patterns and constraints in fruit-frugivore interaction? -from Authors
The morphology of fruit-eating birds has a strong and complex influence on their selection of fruits. Through a series of aviary experiments, we examined the fruit-selection behavior of frugivorous birds. Among the species we tested, there were two fairly distinct groups, separated on the basis of their morphology and foraging behavior. One group, which includes the Cotingidae, Pipridae, Trogonidae, and Tyrannidae, have fairly short, broad wings, a wide gape with a short, flat bill and comparatively little leg musculature. The second group (e.g., Rhamphastidae, Emberizinae, Thraupinae) are characterized by longer, deeper bills and stronger legs. These differences in morphology are correlated with how the birds feed and thus, their selection of fruits. Fruit placement, for example, is differentially important to the two groups. Birds of the first group will readily fly for fruits that are difficult to reach from a sturdy perch whereas birds of the second group rarely do so, preferring to reach. Flying for fruits requires a greater energy expenditure than reaching; birds of the first group display a correspondingly higher selectivity of fruits than members of the second group whose low-cost reaching apparently allows them to include lower quality items in their diets.
In 6 species of fall-fruiting temperate plants, asked whether the percentage of the fruit that was removed was related to the size of the fruit crop. -from Authors
The influence of distance between fruiting plants on choice patterns of four species of tanagers and two species of manakins was experimentally tested in a lowland rain forest in Costa Rica. Paired clusters of two fruit species (Miconia affinis and Urera caracasana) were presented on each of two perches separated by selected distances ranging from 0.3 to 6.4. When the two perches were closest together, all the birds ate the preferred berries, M. affinis, on both perches before eating the less preferred U. caracasana. At the extreme perch separation distance, all but one of the birds ate both species of fruits on the first perch before flying to the second perch. The distance at which each bird switched between the two feeding sequences was correlated with the strength of their preference for one species of fruit over another. Manakins switched at greater separation distances than did tanagers. Since manakins take most of their fruit on the wing, the difference in switch distance is likely related to their flight pattern, but no consistent correlation with simple morphological characteristics (wing-loading or aspect ratio) was found. The results suggest that, in the field, less preferred items may often be taken when more preferred fruits are spaced even moderately far apart. The sequence of feeding and, by implication, diet breadth appears to be a consequence of the abundance and spacing of available foods; hence, degree of generalization is a dynamic feature of a bird's foraging pattern. This result supports the prediction that travelling time is an important determinant of prey and patch choice.
Plants potentially compete for seed dispersal. Selection may favour temporally segregated fruiting phenologies to minimize this competition and also to maintain resident populations of dispersal agents. Alternatively, selection may favour temporally aggregated fruiting phenologies when the effectiveness of seed dispersal agents varies seasonally or when large, synchronous fruit displays enhance dispersal. These evolutionary scenarios assume that plants share seed dispersal agents. This assumption and temporal overlap in fruiting phenologies were evaluated for the Miconia and Psychotria of central Panama. These two genera accounted for 18 and 27%, respectively, of 1096 fleshy fruits found in regurgitation or faecal samples taken from 2054 birds of 103 species netted in the forest under- storey. Two species of manakins accounted for 62% (123/200) of all Miconia fruit taken. Three species of manakins and three species of migratory thrushes accounted for 97% (282/292) of all Psychotria fruits taken. There is a high potential for intrageneric competition for seed dispersal for both plant genera. Null model analyses showed that the fruiting phenologies of Miconia (14 species) are segreg- ated in time, while fruiting of Psychotria (21 species) is highly aggregated. The Miconia were found in up to 24% of the diet samples for the two manakin species, suggesting that Miconia may be a critical resource for both species. The Psychotria fruited when the diversity of understorey fruits was greatest, suggesting a high potential for both intra- and extrageneric competition. The abundance and nomad- ism of the six bird species that consumed most Psychotria fruit peaked when the Psychotria fruited, supporting the enhancement hypothesis.
Understory fruit and fruit-eating birds were censused monthly for a year in gaps, intact forest, and second-growth sites of a lowland Costa Rican rain forest. Both fruits and birds displayed significant seasonal variation. Peak fruit abundance corresponded with peak fruit-eating bird abundance. Fruits were most abundant in the mid-to-late rainy season (August-January). Crop sizes were larger on second-growth plants than on either gap or intact forest plants. Also, fruit was much more common in second growth than in gaps and more common in gaps than in intact forest. Fruit-eating birds followed the same general patterns of spatial and temporal variation. They were significantly most abundant in second growth, significantly least abundant in intact forest, and most common from October to January. A large increase in the frugivore population in October was due to an influx of temperate and altitudinal migrants. In addition, populations of some resident frugivore species increased concurrently, suggesting altitudinal migration in some of these species as well. The two most common understory frugivores molted during the period of fruit high and bred during the period of fruit low. I suggest that insect abundance may be more important than fruit abundance in determining breeding cycles of these birds. Given the extent of spatial and temporal variation in fruit and frugivore abundance, and the apparent tracking of fruit resources by birds, large areas of diverse habitats are probably necessary for the long-term maintenance of frugivore populations.
In tropical regions worldwide, large tracts of native rain forest have been replaced by agricultural systems. Appropriate management of these systems may contribute to biodiversity conservation, but we have limited knowledge of the effect of different management strategies on species persistence. We examined the capacity of tropical country ide in southern Costa Rica to provide resources for avian frugivores. Bird assemblages visiting fruiting Miconia trees were recorded in five landscape contexts: sites near (<2 km from) a large (227-ha) rain forest remnant, split between high and low agricultural intensity; sites far (5-8 km) from the remnant, similarly split; and the remnant itself.. Across all landscape contexts, 73 native bird species were observed taking fruit from Miconia. The composition of frugivore assemblages differed significantly among agricultural landscape contexts. Species richness was highest in near, low-intensity sites (21.5 +/- 1.79, mean +/- 1 SE) and lowest in far, high-intensity sites (14.1 +/- 0.89). The visitation rates of frugivores did not differ among landscape contexts for all species combined, but there were significant differences in the frequency of visits for 10 individual species. Large, socially dominant frugivores were common visitors to trees in far, high-intensity agricultural sites, whereas the visitation rates of many smaller, subordinate frugivores (e.g., small tanagers) declined with distance from the large rain forest remnant or in high-intensity sites. Also, some forest-dependent species (e.g., White-ruffed Manakin, Corapipo altera) were infrequently recorded at fruiting trees in high-intensity and far sites. However, trees in far sites provided resources to species that use both forest and agricultural habitats. Isolated fruiting trees in our study area appear to be an important resource for many avian frugivores. Use of these trees by a wide range of species is enhanced by their proximity to relatively large areas of native rain forest. Low-intensity agriculture that incorporates fruiting trees as part of live fences, windbreaks, or shade and allows for the preservation of rain forest remnants, would be beneficial to biodiversity conservation in the tropics and may facilitate the regeneration of bird-dispersed rain forest plants.
In a Costa Rican tropical wet forest community, of species with adequate sample sizes, 40% of bird species (17 of 42) and 30% of fruiting plant species (10 of 33) were found significantly more often in gaps than in intact forest. Only 5% of the bird species (2 of 42) and no plant species were found significantly more often in intact forest sites. Frugivorous and nectarivorous bird species were especially common in gaps. Birds of second-growth and forest edge were proportionately more abundant in gaps than were birds of forest interior. Canopy birds were not prevalent in gaps. Plants in gaps tended to produce more fruit over a longer period than conspecifics fruiting under a closed canopy. Large gaps had higher densities of fruiting plants and fruit-eating birds than small gaps. Gaps thus may be important sources of fruit during periods of fruit scarcity. Given the high density of fruit-eating birds in gaps, seed deposition patterns generated by these birds was probably nonrandom, and seeds may be dispersed into or around the periphery of gaps more often than into closed understory sites. Thus, fruiting shrub density affects the distribution of fruit-eating birds, which then influences the distribution of fruiting plants and birds in the next generation. -from Author
There is a voluminous literature on pollination and dispersal, very
little of which deals with the consequences of reproductive failure and
its most extreme consequence: extinction. The risk of plant extinctions
can be assessed by considering the probability of dispersal or
pollinator failure, reproductive dependence on the mutualism and
demographic dependence on seeds. Traits for ranking species rapidly
according to these three criteria are indicated. Analysis of case
studies suggests that plants often compensate for high risk in one of
the three categories by low risk in another. For example,
self-incompatible plants with rare specialist pollinators often
propagate vegetatively. Some systems, including elements of the Cape
flora and lowland tropical rain forest, lack compensatory traits and the
risk of plant extinction from failed mutualism is high. 'What escapes
the eye, however, is a much more insidious kind of extincnction: the
extinction of ecological interactions' Janzen (1974).
Dispersal-viewed as the departure of a diaspore (eg. seed or fruit) from the parent plant-is examined in terms of mechanisms, advantages (eg. in connection with escape from parental influence, and colonization), and patterns of production (eg. geographical aspects, seasonality, and intraspecific variation).-P.J.Jarvis
The suitability of several methods for estimating light conditions in the understorey of tropical forests,
and of different sampling schedules was evaluated. Light conditions at 16 understorey sites in a Panamanian lowland
forest were continuously measured for 9 mo with quantum sensors and photodiodes. Light conditions at the sites were
also assessed indirectly with hemispherical fisheye photographs, plant canopy analysis, 38-mm photographs, 24-mm
photographs and a spherical densiometer. Estimates from all indirect methods, except the spherical densiometer, were
highly correlated with the direct measurements. Short-term direct light measurements for a day or a week also
correlated with long-term light conditions. The indirect measures differed by up to c. 70% from the direct measures relative
to single site measurements. Hence, the indirect methods are inadequate where single site light conditions have to be
assessed accurately. However, because light conditions encountered in the understorey varied up to 13-fold, the indirect
methods were found to be well suited to rank understorey light conditions among a large number of sites. The results
from frequent and infrequent sampling schedules differed only slightly, suggesting that taking indirect measures at the
beginning and the end of a study offers a reasonable compromise between accuracy and sampling effort.
The results of several studies suggest that forest fragmentation affects the mating patterns and reproductive success of tropical tree species by reducing pollinator activity, pollen deposition, and outcrossing levels. The flowering synchrony of trees has also been proposed as an additional factor in controlling fruit set and regulating levels of outcrossing, particularly in disturbed habitats. We examined the effects of forest fragmentation and flowering phenology on the reproductive success and genetic structure of the progeny produced by the tropical tree Pachira quinata. We conducted our study in the dry forest of Costa Rica and compared trees in two density and environmental conditions: ( 1 ) isolated trees separated by 500 m from other adult conspecifics and located in disturbed sites and ( 2 ) trees from continuous populations of groups of 20 or more reproductive individuals per hectare surrounded by undisturbed mature forest. Our study was conducted in the Guanacaste Conservation Area, Costa Rica, and surrounding areas. To evaluate flowering phenology, trees were classified as having synchronous or asynchronous flowering. The phenological stage of individuals was classified according to the proximity of the peak flowering date of each tree with respect to the mean peak flowering of the rest of the population. Six percent of the flowers produced a fruit in trees from continuous populations, whereas in isolated trees only 3% of the flowers did so. Fruit set was not affected by the flowering phenology of trees but was influenced mainly by factors associated with forest fragmentation. Seed production per fruit was not affected by forest fragmentation or flowering phenology. Overall, total fruit production per tree was not affected by forest fragmentation, because isolated trees tended to produce more flowers than trees from continuous populations. Genetic analysis revealed that the progeny of trees from continuous populations experienced lower levels of relatedness, a tendency for higher levels of outcrossing, and/or more sires than isolated trees. Our results suggest that forest fragmentation can have an effect on the mating patterns of P. quinata, reducing the number of outcross sires represented in the progeny of isolated trees.
Resumen: Los resultados de varios estudios sugieren que la fragmentación de bosques afecta los patrones de apareamiento y de éxito reproductivo de especies de árboles tropicales al reducir la actividad de polinizadores, la deposición de polen y los niveles de exogamia. La sincronía de floración de árboles también ha sido propuesta como un factor adicional en el control de la producción de frutos y la regulación de niveles de exogamia, especialmente en hábitats perturbados. Examinamos los efectos de la fragmentación de bosques y la fenología floral sobre el éxito reproductivo y la estructura genética de la progenie producida por el árbol tropical Pachira quinata. Realizamos nuestro estudio en el bosque seco de Costa Rica y comparamos árboles en dos densidades y condiciones ambientales: ( 1 ) árboles aislados separados 500 m de otros adultos conespecíficos y localizados en sitios perturbados y ( 2 ) árboles en poblaciones continuas con grupos de 20 o más individuos reproductivos por hectárea rodeados por bosque maduro no perturbado. Nuestro estudio se llevó a cabo en el Área de Conservación de Guanacaste, Costa Rica y áreas circundantes. Para evaluar la fenología de floración, los árboles fueron clasificados en floración sincrónica o asincrónica. El estado fenológico de los individuos se clasificó de acuerdo con la proximidad al pico promedio de floración del resto de la población. Seis por ciento de las flores produjeron fruto en árboles en poblaciones continuas, mientras que en árboles aislados sólo el 3 % de las flores desarrollaron frutos. La producción de frutos no fue afectada por la fenología de floración de los árboles pero fue fluenciada principalmente por factores asociados con la fragmentación del bosque. La producción de semillas por fruto no fue afectada por la fragmentación del bosque ni por la fenología de floración. En general, la fragmentación del bosque no afectó la producción total de frutos por árbol, porque los árboles aislados tendieron a producir más flores que los árboles en las poblaciones continuas. Un análisis genético reveló que la progenie de árboles en poblaciones continuas experimentó menores niveles de afinidad, una tendencia a mayores niveles de exogamia y/o más padres que los árboles aislados. Nuestros resultados sugieren que la fragmentación de bosques puede tener un efecto sobre los patrones de apareamiento de P. quinata, reduciendo el número de padres exógamos representado en la progenie de árboles aislados.
Studies of zoochorous seed dispersal systems often consider crop size, yet seldom consider the kinds and amounts of fruits surrounding parent plants (the fruit neighborhood) when attempting to explain among-plant variation in fruit removal. We studied avian frugivory at 24 Schefflera morototoni trees from February to May 1998 in central Puerto Rico. The number of fruits removed by avian seed dispersers per visit was similar among focal trees (typically 2-4). In contrast, visitation rate was highly variable (range: 0-71 visits per 4 h). We used multiple regression analyses to evaluate the relative roles of crop size (focal tree ripe fruit abundance) and fruit neighborhood variables (measured within 30 m of focal trees) in affecting visitation to focal trees by avian frugivores. Visitation rate was positively related to crop size (although this variable was only significant in one of four regression models considered) and negatively related to the presence or abundance of conspecific fruits, suggesting that trees competed intraspecifically for dispersers. Relationships between visitation and heterospecific fruits were mixed - some kinds of fruits appeared to enhance visitation to focal trees, while others seemed to reduce visitation. In most regression models, neighborhood variables had larger effects on visitation than focal tree fruit crop size. Our results highlight the important effects of local fruiting environments on the ability of individual plants to attract seed dispersers.
We describe fruiting characteristics for 12 species in a community of strangler figs (Moraceae: Urostigma) studied in Panama.
We quantify diurnal and nocturnal removal rates and proportions of fruits removed, and relate them to the activities of the
main dispersers of the figs: bats and birds. These results combined with previous studies show that there are clear differences
between fig species with fruit that ripen red and those with fruit that remain green(ish). In the red-fruited species, the
fruit are small, ripen asynchronously over relatively long periods, produce little scent, and are mainly taken during the
day by birds. In contrast, in the green(ish)-fruited species, the fruits are larger, span a range of sizes, ripen relatively
synchronously, produce very distinctive aromas, and are mainly taken at night by bats. This dichotomy in fruiting characteristics
suggests coadaptive links between groups of dispersers and different species within the genus Ficus. All fig species produce a range of fruit crop sizes (10–155 fuits/m2 canopy area) of which a high proportion were removed by seed dispersers (>80%). Removal rates (fruit removed per day) were
positively correlated with crop size, suggesting that trees with large crop size attract more frugivores. Removal rates of
green-fruited figs were significantly lower and persistence and abortion of ripe fruit were significant higher around full
moon, apparently due to the reduced activity of bats. We further estimate the number of bats that are sustained by a tree
fruit crop and account for the observed fruit removal. We then discuss the evidence for coadaptation between different groups
of figs and their seed dispersers, Finally, we consider the conservation implications for figs as keystone resources in tropical
forests.
Although biologists have directed much attention to estimating the extent and causes of species losses, the consequences for ecosystem functioning have been little studied.
This book examines the impact of biodiversity on ecosystem processes in tropical forests - one of the most species-rich and at the same time most endangered ecosystems on earth. It covers the relationships between biodiversity and primary production, secondary production, biogeochemical cycles, soil processes, plant life forms, responses to disturbance, and resistance to invasion. The analyses focus on the key ecological interfaces where the loss of keystone species is most likely to influence the rate and stability of ecosystem processes.
This is the second edition of a multi-author book first published in 1992. It deals with all aspects of plant regeneration by seeds, including reproductive allocation, seed dispersal and predation, longevity, dormancy and germination. All chapters have been updated, and four new chapters added on seed size, seedling establishment, the role of gaps, and regeneration from seed after fire.
We tested the effect of crop size on fruit removal from an early successional shrub, Viburnum prunifolium (Caprifoliaceae), by comparing the "survival" distributions of fruit from plants with large crops to those with small crops. We found that crop size does not affect rate of fruit removal. Our findings differ from previous researchers who demonstrated correlations between large crop size and higher removal rates from Sambucus pubens and from Viburnum dentatum shrubs. Differences between our findings and those of other researchers may be due either to differences in statistical analyses or type of avian dispersers (solitary versus flocking species). These data do not support the fruit crop size hypothesis.
Manipulated northern arrowwood fruit density in C New York, then monitored fruit removal by birds during 14 wk in the fall of 1985. Each plot had either high or low fruit density in a central 1 m diameter "crop' area, and either high or zero "neighborhood' fruit density in the rest of the 100 m2 plot. Fruits in high density neighborhood plots were removed significantly faster than fruits in plots with zero density neighborhoods. A significantly lower proportion of marked fruit was removed from plots with both low crop size and zero density neighborhoods. Some 43% of all removal of marked fruit by birds took place during a 2-wk period early in the fruiting season. Birds feeding in flocks fed in high density neighborhood plots almost exclusively, presumably causing the greater removal from these plots. Fruit abscission accelerated after week 9, and by week 14 no fruits were left on plants. White-tailed deer Odocoileus virginianus consumed 400 of the 750 marked infructescences during the 14 wk, but destroyed rather than dispersed seeds. -from Author
Using manakins (Pipridae) and tanagers and saltators (Thraupinae and Cardinalinae), 3 types of paired choice tests were presented to birds: a) 'fruit type' trials involving fruits of different species, size and ripeness; b) 'fruit accessibility' trials and c) 'behavioural titrations' in which berry type was balanced against accessibility in series of trials. The responses of tanagers and manakins suggest that their choices may be those expected from a benefit/cost viewpoint. Results of 'behavioural titrations' showed that preference based on fruit type alone can be overridden by small changes in accessibility. The amount of change necessary to cause a switch in preference depended on the pair of fruits being presented, suggesting that accessibility is a cost weighed against potential reward of the fruits. -from Authors
Tropical birds offer unique opportunities to test ecological and evolutionary theory because their life history traits are so diverse and different from temperate zone models upon which most empirical studies are based. We review recent studies on the behavioral ecology of tropical birds, studies that explore new advances in this field. Life histories and their evolution remain the focus of research on tropical birds. Clutch size manipulations in two species showed that food limitation does not explain small clutch size. In antbirds, enlarged clutches decreased post-fledging survival whereas in thrushes, enlarged broods were costly due to high nest predation. Small clutches may be favored via different ultimate selective forces and shared underlying tradeoffs between the immune, metabolic, and endocrine systems in the body may account for the commonly observed ‘slow pace of life’ in tropical birds. The physiological tradeoff between testosterone and immunocom- petence may explain the evolution of low testosterone levels in tropical passerines where adult survival is paramount. In contrast to life history theory, few studies have explored temperate-tropical differences in terri- toriality, mating systems, and song function. The idea that low breeding synchrony in tropical birds is associated with low levels of extra-pair fertilizations was supported by several new paternity studies conducted on tropical passerines. Seasonally breeding tropical birds have higher testosterone levels than tropical birds with prolonged breeding seasons, although it is unclear if this pattern is driven by mating systems per se or selection from pathogens. Recent work on relations between pair members in permanently paired tropical passerines focuses on the question of mate defense versus territorial defense and the extent of cooperation versus selfish interests in inter-sexual relations.
This study lays the foundation for determining the importance of fruit as a resource for vertebrates and to provide recommendations for management of key species and habitats through the documentation of fruit production and availability, variation in use by wildlife and how these factors vary by plant species, habitat and season. Half the species produced fruit in the fall and winter and these fruits were primarily consumed by overwintering wildlife. Study suggests that winter fruit production and consumption deserves further attention from forest managers in temperate forests as current management practices often reduce fruit availability of key species.
Seed dispersal by animals has sometimes been considered one of the factors helping to explain angiosperm diversification. Three predictions related to this hypothesis are examined: 1) biotic seed dispersal is a feature unique to angiosperms, or this group exhibits is proportionally more often than gymnosperms; 2) extant groups exhibiting this feature tend to be taxonomically more diverse than sister groups lacking it; and 3) the timing of the appearance in the fossil record differs for biotically and abiotically dispersed angiosperm taxa. Analyses have been confined to endozoochory, the most genuine and widespread of the various biotic seed-dispersal methods, and conducted on virtually the entire set of extant gymnosperm and angiosperm families. None of the tests supports a role for animal dispersal per se in angiosperm diversification. -Author
Positive and negative sources of selection on fruit-eating by birds are discussed. It is hypothesized that fruit is rarely favored as the complete diet for small nestling birds because it results in slower growth rates than those allowed by animal food. Nest predation pressure is greater in totally frugivorous birds and counteracts the advantages of the abundant and easily accessible food supply afforded by fruit. The evolutionary result of total frugivory is discussed, relating population size, rarity of nest sites, and clutch size to the rarity of totally frugivorous species. Frugivory in adult birds is discussed in relation to habitat, social behavior, and interspecific competition. It is hypothesized that total frugivory in adult birds is opposed by selection from occasional periods of low fruit abundance, when the ability to obtain animal food reaches limiting conditions. An evolutionary and ecological dichotomy between humid tropical-zone and temperate-zone birds is discussed. This dichotomy is due ...
We investigated effects of fruit colour (red, black or white), habitat (anthropogenic edges and forest interior) and fragment size on the removal of artificial fruits in semideciduous forests in south-east Brazil. Eight forest fragments ranging from 251 to 36,000 ha were used. We used artificial fruits, which were placed on shrubs between 1 and 2 m in height and checked after 48 and 96 h for peck marks in the fruits. All three variables affected the probability of consumption of our fruit models. Red and black fruits were statistically more pecked than the white fruits. The probability of fruit consumption was lower in the interior than at the edge and less in small than in large fragments. However, the decrease fruit consumption in small compared with large fragments was more accentuated for red and black fruits than for white fruits. Our results show that habitat reduction and edges affect the chances of a fruit being eaten by birds, which may ultimately affect plant fitness in forest fragments.
In 3 species of Costa Rican forest understory plants, the likelihood of bird visitation increased with increasing crop size. Among plants visited by birds, the absolute number of fruits removed increased with increasing crop size, whereas the proportion, or relative number removed, actually decreased. Thus, effects of crop size on the different components of the dispersal process (likelihood of visitation by birds, fruit removal by those that do visit, and postremoval behavior of dispersers) often conflict, such that no clear relationship between crop size and overall dispersal success is evident. Results suggest selection for synchronous ripening of large seasonal fruit crops in these plants. Models of the relationship between crop size and reproductive success in trees are largely inappropriate for understory plants: 1) because of their small stature and small fruit crops, understory herbs and shrubs are often not detected by frugivores; and 2) the fruit crops they produce are rarely large enough to encourage sedentary behavior in dispersers. The best estimator of fitness differs for plants with different life history strategies. In most plants, fitness is best approximated by the absolute number of fruits removed. In highly iteroparous species, the relative number removed may be a more suitable estimator, but only if increased present reproductive effort results in decreased adult survival or future reproduction. -from Author
Theoretical and empirical research on frugivory and seed dispersal has been influenced by concepts derived from the study of pollination. In particular, explicit and implicit analogies between seed dispersal and pollen dispersal have led to the expectation, under certain conditions, of the evolution of obligate, species-specific relationships between fruiting plants and the animals that disperse their seeds. The two systems differ in important respects, however. Plants benefit by directing pollen dispersers to a definite, recognizable "target," a conspecific flower, and they can provide incentives at flowers which serve to attract potential pollinators. In effect, there is "payment upon delivery" of the pollen. In contrast, for seeds the target (an appropriate site for germination and establishment) is seldom readily discernible, and dispersal beneath a conspecific plant may actually be undesirable. Another important difference is that frugivores are "paid in advance." Because of these differences and others, the outcomes of coevolution of fruiting plants and frugivores are expected to be different than those of flowering plants and flower visitors. There are therefore problems with drawing analogies between the two systems and using terminology derived from studies of pollination to design and interpret studies of seed dispersal
In July in northern Wisconsin, the red elderberry (Sambucus pubens (Michx.) synchronously ripens large panicles of bright red, small, watery, sweet drupes. During 1982 and 1983, I monitored bushes aggregated along an old logging track or isolated in tree-fall clearings to investigate the effects of isolation, crop size, and fruit quality on fruit removal by birds. All three factors significantly affected rates of fruit removal, although fruit removal rates were primarily a function of crop size. More fruits per bush per day were removed from large crops of both aggregated and isolated plants. Fruits were not removed disproportionately faster from plants with large than with small crops in either habitat. These results do not support the hypothesis that differential probability of dispersal success is an important selective factor in the evolution of delayed first reproduction or supraannual fruiting periodicities. However, removal rates from aggregated plants were lower than from isolated bushes in tree-fall clearings, probably an effect of local competition for frugivores. Sugar content of the pulp (but not fruit size) also significantly affected rates of fruit removal.
The fruits of Lindera benzoin (L.) Blume are dispersed by several species of birds, mainly thrushes (Turdidae) and mimids (Mimidae). In a study in Trelease Woods, Illinios, removal rates (as percentages) did not keep pace with fruit availability and exhibited a low point in midseason. Relative removal rates in tree-fall gaps initially exceeded those in forest interior, though total fruit abundance there was less. Isolated bushes had slightly higher relative removal rates initially than clumped bushes, again despite a lower availability. Percentage removal rates on bushes with small displays often exceeded those on large displays, although the absolute numbers removed from large displays were greater. Most of these trends held up when factors were considered in combination.
Habitat loss and fragmentation remain the greatest threats to the world's biodiversity1. The local extinction of plant species from habitat fragments is common2, although the reasons for this are not fully understood. Fragmentation is known to influence both birth- and death-related processes3, but the disruption of plant reproduction, especially pollination and seed production, is thought to be particularly important4, 5. The effects of fragmentation on post-pollination processes such as seed dispersal and germination have rarely been explored experimentally6. Here I show that seeds planted in forest fragments are less likely to germinate than those in continuous forest. This finding can have negative demographic consequences because it reduces the emergence of seedlings.
In spite of a considerable literature on fruit-eating, the general evolutionary implications of fruit as a source of food for birds have been neglected. A preliminary attempt is made to explore the evolutionary and ecological consequences of fruit-eating, considered as a mutual interaction between parent plant and dispersal agent.
The relationship considered is that obtaining between fleshy fruits and the “legitimate” fruit-eating birds which digest the fleshy part of the fruit and void the seed intact. Evolutionary aspects of seed-eating are also briefly discussed.
The “strategies” adopted by fruits for dispersal by birds result in the production of abundant food supplies which are easy of access and exploitable by many species of birds. By contrast, the predation of birds on insects leads to a heterogeneous, sparse and cryptic food supply, to exploit which many different hunting techniques are necessary. Two important evolutionary developments in birds are attributed to these differences in food supply: there tend to be more species in families of insectivorous than of frugivorous birds, and lek behaviour in tropical forest has evolved in predominantly frugivorous birds.
The seasonal succession of fruits in temperate latitudes is discussed, and contrasted with the situation in the tropics, using examples from Europe and Trinidad. In general, the succession of ripe fruits in Europe seems to be adapted to the seasonal shifts of the bird populations, and the more nutritious fruits tend to have a more southerly distribution and to ripen later than the more succulent fruits. In the tropics the distinction between nutritious and succulent fruits seems to be largely one of habitat.
The constant succession of ripe fruits throughout the year in the tropics probably depends on competition for dispersal by frugivorous birds, which thus ensure the maintenance of their own food supply. This may be regarded as a symbiosis at the level of the ecosystem.
In a montane tropical forest in southwestern Colombia, we investigated how anthropogenic edges may alter bird-mediated seed dispersal from edge to forest interior as a function of edge age and presence of treefall gaps. We estimated fruit abundance and mist-netted birds at four distances from edge to forest interior (0-10, 30-40, 60-70, and 190-200 m) in three young (40 yr) edges. Fruit-sampling plots (50-m2 plots) at each of the four distances were classified into gap and intact forest. Fruit abundance and frugivore capture rates varied from edge to forest interior, but such changes depended on edge age. At new edges, the total number of fruits was higher at the forest edge than at the forest interior, whereas bird captures showed the opposite trend. At old edges, the total number of fruits and bird capture rates did not vary among the four distances. In a first group of 12 plant and four bird species, the distribution of individuals in fruit (7 species) and captures (3 species) from edge to forest interior differed between old and new edges. In a second group of 18 plant and five bird species, which included those that were not amenable for a comparison between old and new edges and those that were not influenced by edge age, the distribution of individuals in fruit (12 species) and captures (3 species) was not uniform from forest edge to forest interior. Lastly, 124 plant and 19 bird species with
In order to evaluate the ‘fruit crop size’ hypothesis, we analyzed the effect of fruit availability on the number of visits by birds and on the proportion of removed fruits, and on how much of the variation in the proportion of removed fruits could be explained by spatio-temporal variability. Fieldwork was done on the coast of the state of Veracruz, Mexico, using the small tree Bursera fagaroides (Burseraceae) and its frugivorous feeding assemblage, on four hills during four years. The number of fruits produced, the number of visits by birds, and the proportion of removed fruits varied between years with more than half an order of magnitude. The number of available fruits was associated to both the number of visits by birds and the proportion of fruits removed from each plant individual, and supports the fruit crop size hypothesis. However, the hypothesis varies in strength depending on total fruit production by the population per year and per hill. The spatio-temporal variation of the system was the effect of variation in the intensity of fructification between years and hills; the inclusion of spatio-temporal variability helped to explain most of the variation found in our results. The latter approach may explain the explain the conflicting results found by other authors in studies done in only one year or at one site, where variation is greatly reduced. Considering the number of published reports, a meta-analysis seems to be in order to determine the existence of a general effect between fruit production, the number of visits by birds and the proportion of fruits removed. If this is possible, we hypothezise that the association between fruit production and the number of visits by birds should be of low intensity.
In a series of field experiments using Costa Rican rain forest plants, we examined the effect of accessibility on fruit removal rates. We compared the effects of fruit placement in terminal and axillary infructescences on diurnal and nocturnal removal rates, visitation rates, and incidence of fruit damage. We used three different species of berries (Phytolacca rivinoides, Psychotria brachiata, and Psychotria pitteri) and worked in three different habitats (fallow fields, treefall gaps, and forest understory) and in two different seasons (July–September, a season of fruit abundance and December–January, a season of fruit scarcity.)We found that in oldfields especially, diurnal removal rates by birds were significantly greater from axillary than from terminal infructescences. Nocturnal removal from axillary infructescences-presumably by rodents-is also occassionally significant. From these data, from observations on climbing ability and fruit use in captive rodents, and from reports in the literature, we suggest that rodents are significant sources of fruit and seed loss in tropical shrubs. We hypothesize that placement of the infructescence on the plant affects fruit removal by both seed-dispersing birds and by the less agile, often seed-destroying rodents. The balance between the two rates is an important component of a plant's dispersal success.Diurnal fruit removal rates were higher during the season of fruit scarcity than during the season of fruit abundance and higher in old fields than in forest gaps or understory. Fruit damage rates-probably due to orthopterans-were slightly greater in gaps and understory than in old fields.
Disperser effectiveness is the contribution a disperser makes to the future reproduction of a plant. Although it is a key notion in studies of seed dispersal by animals, we know little about what determines the effectiveness of a disperser. The role of the present paper is to review the available information and construct a hierarchical framework for viewing the components of disperser effectiveness.Effectiveness has both quantitative and qualitative components. The quantity of seed dispersal depends on (A) the number of visits made to the plant by a disperser and (B) the number of seeds dispersed per visit. The quality of seed dispersal depends on (A) the quality of treatment given a seed in the mouth and in the gut and (B) the quality of seed deposition as determined by the probability that a deposited seed will survive and become an adult. In this paper I review the ways disperser behavior, morphology and physiology can influence these major components of disperser effectiveness, and when data permit present preliminary analyses of relationships among components.
Pseudoreplication is defined as the use of inferential statistics to test for treatment effects with data from experiments where either treatments are not replicated (though samples may be) or replicates are not statistically independent. In ANOVA terminology, it is the testing for treatment effects with an error term inappropriate to the hypothesis being considered. Scrutiny of 176 experimental studies published between 1960 and the present revealed that pseudoreplication occurred in 27% of them, or 48% of all such studies that applied inferential statistics. The incidence of pseudoreplication is especially high in studies of marine benthos and small mammals. The critical features of controlled experimentation are reviewed. Nondemonic intrusion is defined as the impingement of chance events on an experiment in progress. As a safeguard against both it and preexisting gradients, interspersion of treatments is argued to be an obligatory feature of good design. Especially in small experiments, adequate interspersion can sometimes be assured only by dispensing with strict randomization procedures. Comprehension of this conflict between interspersion and randomization is aided by distinguishing pre-layout (or conventional) and layout-specific alpha (probability of type I error). Suggestions are offered to statisticians and editors of ecological journals as to how ecologists' understanding of experimental design and statistics might be improved.