RESUMO

BACKGROUND: Bats are important long-distance dispersers of many tropical plants, yet, by consuming fruits, they may disperse not only the plant's seeds, but also the mycobiota within those fruits. We characterized the culture-dependent and independent fungal communities in fruits of Ficus colubrinae and feces of Ectophylla alba to determine if passage through the digestive tract of bats affected the total mycobiota. RESULTS: Using presence/absence and normalized abundance data from fruits and feces, we demonstrate that the fungal communities were significantly different, even though there was an overlap of ca. 38% of Amplicon Sequence Variants (ASVs). We show that some of the fungi from fruits were also present and grew from fecal samples. Fecal fungal communities were dominated by Agaricomycetes, followed by Dothideomycetes, Sordariomycetes, Eurotiomycetes, and Malasseziomycetes, while fruit samples were dominated by Dothideomycetes, followed by Sordariomycetes, Agaricomycetes, Eurotiomycetes, and Laboulbeniomycetes. Linear discriminant analyses (LDA) show that, for bat feces, the indicator taxa include Basidiomycota (i.e., Agaricomycetes: Polyporales and Agaricales), and the ascomycetous class Eurotiomycetes (i.e., Eurotiales, Aspergillaceae). For fruits, indicator taxa are in the Ascomycota (i.e., Dothideomycetes: Botryosphaeriales; Laboulbeniomycetes: Pyxidiophorales; and Sordariomycetes: Glomerellales). In our study, the differences in fungal species composition between the two communities (fruits vs. feces) reflected on the changes in the functional diversity. For example, the core community in bat feces is constituted by saprobes and animal commensals, while that of fruits is composed mostly of phytopathogens and arthropod-associated fungi. CONCLUSIONS: Our study provides the groundwork to continue disentangling the direct and indirect symbiotic relationships in an ecological network that has not received enough attention: fungi-plants-bats. Findings also suggest that the role of frugivores in plant-animal mutualistic networks may extend beyond seed dispersal: they may also promote the dispersal of potentially beneficial microbial symbionts while, for example, hindering those that can cause plant disease.

RESUMO

Ecosystem services of Amazonian forests are disproportionally produced by a limited set of hyperdominant tree species. Yet the spatial variation in the delivery of ecosystem services by individual hyperdominant species across their distribution ranges and corresponding environmental gradients is poorly understood. Here, we use the concept of habitat quality to unravel the effect of environmental gradients on seed production and aboveground biomass (AGB) of the Brazil nut, one of Amazonia's largest and most long-lived hyperdominants. We find that a range of climate and soil gradients create trade-offs between density and fitness of Brazil nut trees. Density responses to environmental gradients were in line with predictions under the Janzen-Connell and Herms-Mattson hypotheses, whereas tree fitness responses were in line with resource requirements of trees over their life cycle. These trade-offs resulted in divergent responses in area-based seed production and AGB. While seed production and AGB of individual trees (i.e., fitness) responded similarly to most environmental gradients, they showed opposite tendencies to tree density for almost half of the gradients. However, for gradients creating opposite fitness-density responses, area-based seed production was invariable, while trends in area-based AGB tended to mirror the response of tree density. We conclude that while the relation between environmental gradients and tree density is generally indicative of the response of AGB accumulation in a given area of forest, this is not necessarily the case for fruit production.

RESUMO

Patterns of seed dispersal and seed mortality influence the spatial structure of plant communities and the local coexistence of competing species. Most seeds are dispersed in proximity to the parent tree, where mortality is also expected to be the highest, because of competition with siblings or the attraction of natural enemies. Whereas distance-dependent mortality in the seed-to-seedling transition was often observed in tropical forests, few studies have attempted to estimate the shape of the survival-distance curves, which determines whether the peak of seedling establishment occurs away from the parent tree (Janzen-Connell pattern) or if the peak attenuates but remains at the parent location (Hubbell pattern). In this study, we inferred the probability density of seed dispersal and two stages of seedling establishment (new recruits, and seedlings 20 cm or taller) with distance for 24 tree species present in the 50-ha Forest Dynamics Plot of Barro Colorado Island, Panama. Using data from seed traps, seedling survey quadrats, and tree-census records spanning the 1988-2014 period, we fit hierarchical Bayesian models including parameters for tree fecundity, the shape of the dispersal kernel, and overdispersion of seed or seedling counts. We combined predictions from multiple dispersal kernels to obtain more robust inferences. We find that Hubbell patterns are the most common and Janzen-Connell patterns are very rare among those species; that distance-dependent mortality may be stronger in the seed stage, in the early recruit stage, or comparable in both; and that species with larger seeds experience less overall mortality and less distance-dependent mortality. Finally, we describe how this modeling approach could be extended at a community scale to include less abundant species.

Assuntos

RESUMO

The top-down and indirect effects of insects on plant communities depend on patterns of host use, which are often poorly documented, particularly in species-rich tropical forests. At Barro Colorado Island, Panama, we compiled the first food web quantifying trophic interactions between the majority of co-occurring woody plant species and their internally feeding insect seed predators. Our study is based on more than 200 000 fruits representing 478 plant species, associated with 369 insect species. Insect host-specificity was remarkably high: only 20% of seed predator species were associated with more than one plant species, while each tree species experienced seed predation from a median of two insect species. Phylogeny, but not plant traits, explained patterns of seed predator attack. These data suggest that seed predators are unlikely to mediate indirect interactions such as apparent competition between plant species, but are consistent with their proposed contribution to maintaining plant diversity via the Janzen-Connell mechanism.

Assuntos

RESUMO

One of the hypothesized benefits of seed dispersal is to escape density- and distance-responsive, host-specific, natural enemies near maternal plants where conspecific seed and seedling densities are high. Such high conspecific neighbor densities typically result in lower offspring growth and survival (i.e., negative density-dependent effects), yet many dispersal modes result in clumped seed distributions. New World leaf-nosed bats transport fruits to their feeding roosts and deposit seeds, thereby creating high-density seed/seedling patches beneath feeding roosts in heterospecific trees away from maternal trees, which seemingly nullifies a key benefit of seed dispersal. Such dispersal may still be adaptive if negative density-dependent effects are reduced under feeding roosts or if the benefit of being dispersed away from maternal trees outweighs negative effects of conspecific seed/seedling density below roosts. We mapped the entire post-germination population of a bat-dispersed tree species Calophyllum longifolium (Calophyllaceae) in a 50-ha plot on Barro Colorado Island, Panama in each of three successive years. We tested two hypotheses: (1) distance-dependent effects are stronger than density-dependent effects on seedling performance because seedlings far from conspecific adults are more likely to escape natural enemies even when at high densities and (2) negative density-dependent effects will be reduced far from vs. near conspecific adults. Density and distance were naturally decoupled, as expected. However, in contrast to our expectation, we found positive density effects on seedling survival and density-dependent effects did not differ with distance from conspecific adults. Both density and distance had positive effects on seedling survival when considered together, while only year had a significant effect on seedling growth. Thus, both being dispersed under bat feeding roosts and escaping the vicinity of conspecific adults were beneficial for C. longifolium seedling survival, supporting the directed dispersal and escape hypotheses, respectively. Despite resulting in high densities of conspecific seedlings, favorable habitat under bat feeding roosts and lack of negative density-dependent effects appear to provide evolutionary advantages in C. longifolium.

Assuntos

RESUMO

Conspecific negative density dependence (CNDD) is thought to promote plant species diversity. Theoretical studies showing the importance of CNDD often assumed that all species are equally susceptible to CNDD; however, recent empirical studies have shown species can differ greatly in their susceptibility to CNDD. Using a theoretical model, we show that interspecific variation in CNDD can dramatically alter its impact on diversity. First, if the most common species are the least regulated by CNDD, then the stabilising benefit of CNDD is reduced. Second, when seed dispersal is limited, seedlings that are susceptible to CNDD are at a competitive disadvantage. When parameterised with estimates of CNDD from a tropical tree community in Panama, our model suggests that the competitive inequalities caused by interspecific variation in CNDD may undermine many species' ability to persist. Thus, our model suggests that variable CNDD may make communities less stable, rather than more stable.

Assuntos

RESUMO

Negative population feedbacks mediated by natural enemies can promote species coexistence at the community scale through disproportionate mortality of numerically dominant (common) tree species. Simultaneously, associations with arbuscular mycorrhizal fungi (AMF) can result in positive effects on tree populations. Coupling data on seedling foliar damage from herbivores and pathogens and DNA sequencing of soil AMF diversity, we assessed the effects of these factors on tree seedling mortality at local (1 m2 ) and community (16 ha plot) scales in a tropical rainforest in Puerto Rico. At the local scale, AMF diversity in soil counteracted negative effects from foliar damage on seedling mortality. At the community scale, mortality of seedlings of common tree species increased with foliar damage while rare tree species benefited from soil AMF diversity. Together, the effects of foliar damage and soil AMF diversity on seedling mortality might foster tree species coexistence in this forest.

Assuntos

RESUMO

Although one of the most widely studied hypotheses for high tree diversity in the tropics, the Janzen-Connell hypothesis (JC), and the community compensatory trend upon which it is based, have conflicting support from prior studies. Some of this variation could arise from temporal variation in seedling survival of common and rare species. Using 10 years of data from La Selva Biological Station in Costa Rica, we analyzed annual seedling survival and found that negative density-dependence (negative DD) was significantly stronger for rare species than for common species in 2 years and was significantly stronger for common species than for rare species in 4 years. This temporal variation in survival was correlated with climatic variables: in warmer and wetter years, common species had higher negative DD than rare species. The relationship between climate and variation in JC effects on seedling survival of common and rare species could have important consequences for the maintenance of tree species diversity in Central America, which is predicted to experience warmer and wetter years as global change proceeds.

Assuntos

RESUMO

The coexistence of numerous tree species in tropical forests is commonly explained by negative dependence of recruitment on the conspecific seed and tree density due to specialist natural enemies that attack seeds and seedlings ('Janzen-Connell' effects). Less known is whether guilds of shared seed predators can induce a negative dependence of recruitment on the density of different species of the same plant functional group. We studied 54 plots in tropical forest on Barro Colorado Island, Panama, with contrasting mature tree densities of three coexisting large seeded tree species with shared seed predators. Levels of seed predation were far better explained by incorporating seed densities of all three focal species than by conspecific seed density alone. Both positive and negative density dependencies were observed for different species combinations. Thus, indirect interactions via shared seed predators can either promote or reduce the coexistence of different plant functional groups in tropical forest.

Assuntos

RESUMO

Seed ingestion by frugivorous vertebrates commonly benefits plants by moving seeds to locations with fewer predators and pathogens than under the parent. For plants with high local population densities, however, movement from the parent plant is unlikely to result in 'escape' from predators and pathogens. Changes to seed condition caused by gut passage may also provide benefits, yet are rarely evaluated as an alternative. Here, we use a common bird-dispersed chilli pepper (Capsicum chacoense) to conduct the first experimental comparison of escape-related benefits to condition-related benefits of animal-mediated seed dispersal. Within chilli populations, seeds dispersed far from parent plants gained no advantage from escape alone, but seed consumption by birds increased seed survival by 370% - regardless of dispersal distance - due to removal during gut passage of fungal pathogens and chemical attractants to granivores. These results call into question the pre-eminence of escape as the primary advantage of dispersal within populations and document two overlooked mechanisms by which frugivores can benefit fruiting plants.

Assuntos

RESUMO

Many models have been proposed to explain the possible role of pests in the coexistence of a high diversity of plant species in tropical forests. Prominent among them is the Janzen-Connell model. This model suggests that specialized herbivores and pathogens limit tree recruitment as a function of their density or proximity to conspecifics. A large number of studies have tested the predictions of this model with respect to patterns of recruitment and mortality at different life stages, yet only a few have directly linked those density or distance-dependent effects to pest attack. If pest-attack is an important factor in density or distance-dependent mortality, there should be spatial heterogeneity in pest pressure. I studied the spatial distribution of leaf damage in saplings of six common Inga species (Fabaceae: Mimosoideae) in the 50ha forest dynamic plot of Barro Colorado Island, Panama. The percent leaf damage of Inga saplings was not heterogeneous in space, and the density of conspecific, congener or confamilial neighbors was uncorrelated with the observed damage levels in focal plants. One of the focal species did suffer density-dependent mortality, suggesting that spatial variation in plant performance in these species is not directly driven by leaf damaging agents. While multiple studies suggest that density-dependent effects on performance are common in tropical plant communities, our understanding of the mechanisms that drive those effects is still incomplete and the underlying assumption that these patterns result from differential herbivore attack deserves more scrutiny.

Se han propuesto muchos modelos para explicar la coexistencia de una alta diversidad de especies de árboles en bosques tropicales. Prominente, entre estos modelos es el de Janzen-Connell, que sugiere que los herbívoros especialistas limitan la colonización de árboles en función de la densidad o proximidad de con-específicos. Si este efecto es en realidad el resultado de ataque por herbívoros, debiera haber heterogeneidad espacial en la herbivoría. Aquí se evalúa esta hipótesis estudiando la distribución espacial de la herbivoría en juveniles de seis especies comunes de Inga (Fabaceae: Mimosoideae) en la parcela de 50ha de la Isla de Barro Colorado, en Panamá. Análisis de autocorrelación espacial no mostraron heterogeneidad en la herbivoría de estas especies, y la densidad local de con-específicos, congéneres o confamiliares no se correlacionó con la herbivoría de las plantas estudiadas. Solo una de las especies de estudio sufrió mortalidad densidad-dependiente en 20 años de censos. Aunque muchos estudios han demostrado que los efectos densidad-dependientes en la mortalidad de las plantas son comunes en bosques tropicales, nuestro entendimiento de los mecanismos que causan esos efectos es aún limitado, y la suposición de que estos resultan de heterogeneidad espacial en el ataque de herbívoros merece más escrutinio.

Assuntos

RESUMO

Seed production, seed dispersal and recruitment are critical processes in population dynamics, because they are almost never completely successful. We recorded the recruitment dynamics for the population of Dipteryx oleifera in a tropical rainforest in eastern Nicaragua (12°05’ N., 83°55’ W.) from March 2002 to August 2006. Seeds and seedlings had highly clumped distributions, while sapling distributions appeared to be random. Seedling survival increased away from the nearest conspecifc adult tree, where seedling density is lower. Since relative growth rates of seedlings are not correlated with the distance to the nearest conspecific adult, seedling survival appears to be independent of seedling growth. Seedling density is inversely correlated with seedling insect herbivory damage. Seedling survival correlated negatively with the number of saplings per sub-plot (10x10m), suggesting that insect herbivore may also cue in on saplings rather than only on adult D. oleifera trees in order to locate seedlings. Seedling establishment is significantly clumped with respect to the nearest adult tree. Larger clumps of seedlings seems more ephemeral than isolated smaller clumps located away from the nearest D. oleifera tree. These results support current empirical evidence presented earlier for the Janzen-Connell hypothesis for Dipteryx oleifera at seed and seedling stages and, the Recruitment Limitation hypothesis at the sapling stage, because sapling individuals might have recruited after random light-gap formation. Rev. Biol. Trop. 57 (1-2): 321-338. Epub 2009 June 30.

Resumen Estudiamos la dinámica de regeneración de la población de Dipteryx oleifera en un bosque húmedo tropical del este de Nicaragua. Semillas y plántulas se encuentran altamente agregadas, pero la distribución de vástagos podría ser al azar. La supervivencia de plántulas aumenta con la distancia al congéner más cercano, donde la densidad de plántulas es más baja. Como las tasas de crecimiento relativo de plántulas no se correlacionan con la distancia al congénere más cercano, la supervivencia de las plantas pareciera no estar determinada por el crecimiento de plántulas. La densidad de plántulas está inversamente correlacionada con los niveles de daño de herbivoría insectívora. La supervivencia de plántulas se correlaciona negativamente con el número de vástagos, lo cuál sugiere que los insectos herbívoros podrían estar localizando vástagos y árboles y de allí, las plántulas. La supervivencia de plántulas presentó una distribución significativamente agrupada lejos de los congéneres más cercanos. Los parches de plántulas más grandes fueron más efímeros que los parches pequeños y aislados ubicados lejos de los árboles adultos. Los resultados coinciden con la hipótesis de Janzen-Connell sobre estados de semilla y plántula. Además, ofrecen evidencia en favor de la hipótesis de Limitación de Reclutamiento para los vástagos, ya que los vástagos podrían estar estableciéndose en claros de luz pequeños que se forman al azar.

Assuntos

RESUMO

Hunting may have drastic effects on some populations of frugivores and seed dispersal agents, which in turn may affect patterns of forest regeneration and plant diversity. However, when a species disappears, it is possible that the population of other species increases due to competition release, compensating or not, their ecological roles. The main aim of this study was to measure density compensation in primate communities, their possible effects on seedling and sapling diversity, and compensation of seed removal rates in two canopy trees (Apeiba aspera and Hymenaea oblongifolia). The study site was Amacayacu National Park (Colombian Amazon), where we carried out a census of mammal species using line transect methods in two forests with different levels of hunting pressure. Vegetation plots were used to quantify plant diversity for seedlings and saplings (4 m² and 25 m² respectively), and fruit traps were set up to estimate seed removal of A. aspera and H. oblongifolia. Large primates were less frequently encountered near human settlements, while small primates exhibited the opposite pattern, suggesting a density compensation effect. The diversity of regenerating plants was higher in the forest where large primates occur. Seed removal was higher in forests with large primates for A. aspera, but not for H. oblongifolia. Overall, the results support the hypothesis of density compensation on the primate community; however, there is no strong evidence of ecological compensation in terms of seed dispersal and regeneration in the species studied. Finally, the presence of large seed dispersers (e.g. Lagothrix lagothricha) was associated with higher plant diversity. Rev. Biol. Trop. 56 (3): 1531-1542. Epub 2008 September 30.

La cacería ha tenido fuertes efectos en algunas poblaciones de frugívoros y dispersores de semillas, lo cual en consecuencia puede afectar los patrones de diversidad de plantas y regeneración del bosque. Sin embargo, es posible que extinciones locales promuevan el incremento de otras especies, que pueden o no compensar roles ecológicos. El principal objetivo de este estudio fue examinar la posibilidad de compensación por densidad en las comunidades de primates y sus posibles efectos sobre la diversidad de plántulas y juveniles, así como también la remoción de semillas de dos árboles (Apeiba aspera e Hymenaea oblongifolia). El trabajo fue realizado en el Parque Nacional Natural Amacayacu (Amazonas - Colombia). Hicimos censos de mamíferos usando transectos lineales en dos bosques con diferentes presiones de cacería. Además, levantamos parcelas de vegetación para plántulas y juveniles (4 m² y 25 m² respectivamente), y ubicamos trampas de frutos bajo al menos seis árboles de cada especie, en cada bosque, para estimar la remoción de semillas. Los encuentros con primates grandes fueron menos frecuentes en bosques cerca de las comunidades indígenas, mientras que para los primates pequeños el patrón fue contrario, sugiriendo un efecto de compensación por densidad. Por otra parte, la diversidad de plántulas y juveniles fue más alta en bosques donde la densidad de primates grandes es mayor. La remoción de semillas fue mayor en bosques con mayor densidad de primates grandes para A. aspera, pero para H. oblongifolia las diferencias entre bosques no fueron significativas. En general, los resultados de este estudio apoyan la hipótesis de compensación por densidad en la comunidad de primates, sin embargo, no hay evidencia fuerte de compensación ecológica en términos de dispersión de semillas y regeneración. Finalmente, la presencia de dispersores grandes (ej. Lagothrix lagothricha) estuvo asociada con una mayor diversidad de plantas.

Assuntos

RESUMO

Evidence concerning mechanisms hypothesized to explain species coexistence in hyper-diverse communities is reviewed for tropical forest plants. Three hypotheses receive strong support. Niche differences are evident from non-random spatial distributions along micro-topographic gradients and from a survivorship-growth tradeoff during regeneration. Host-specific pests reduce recruitment near reproductive adults (the Janzen-Connell effect), and, negative density dependence occurs over larger spatial scales among the more abundant species and may regulate their populations. A fourth hypothesis, that suppressed understory plants rarely come into competition with one another, has not been considered before and has profound implications for species coexistence. These hypotheses are mutually compatible. Infrequent competition among suppressed understory plants, niche differences, and Janzen-Connell effects may facilitate the coexistence of the many rare plant species found in tropical forests while negative density dependence regulates the few most successful and abundant species.

RESUMO

We present an analysis of the long-term survival of two cohorts of seedlings of the tropical canopy tree Ocotea whitei (Lauraceae) on a 1-ha plot of mature, lowland moist forest on Barro Colorado Island, Panamá. In 1980, we counted an even-aged cohort of seedlings that germinated in 1979, then measured and tagged survivors in 1981. We also measured and tagged a second, smaller cohort of seedlings that germinated in 1981. We followed the subsequent survival of all seedlings through 1985. Seedling mortality was phenotypically, temporally, and spatially non-random. Important correlates of non-random mortality included: (1) seedling size and age, (2) an El Niño drought, and (3) biotic neighborhood. Larger and older seedlings survived better than smaller and younger seedlings, respectively, and the El Niño-related drought of 1982-1983 was associated with elevated mortality rates. Seedling density, which was strongly correlated with the proximity to the nearest conspecific adult, increased mortality. The observed mortality patterns suggest that processes consistent with the Janzen-Connell hypothesis operate during the recruitment phase of O. whitei population dynamics. However, the processes causing the observed density- and distance-dependent mortality may vary with factors such as total seed number, seedling size, and climatic variation, making it difficult to determine whether time-integrated seedling-to-adult spacing mechanisms other than self-thinning operate on a given plant population. After 6 years in the hectare studied, survivors remained densest and most numerous underneath the adult trees. We conclude that only long-term demographic data, collected at a variety of scales on a variety of species, will ultimately answer the question: do Janzen-Connell effects contribute substantially to structuring tropical forests?

RESUMO

We compared the spatial distribution of stem cankers on the canopy tree Ocotea whitei (Lauraceae) in a 20-ha plot on Barro Colorado Island, Panama, with spatial and temporal patterns of mortality in this host over the previous decade. The cankers occur both on adult and juvenile individuals, aothough juveniles are much more likely the adults to show symptoms. Disease incidence is host-density dependent, and both the presence of the disease and host mortality are more likely close to than far from a conspecific adult, which resulted in a net spatial shift of the juvenile population away from conspecific adults through time. Disease incidence is lower than expected among juveniles of O. whitei growing near to adults of the non-susceptible canopy tree Beilschmiedia pendula. The coincidence of spatial patterns of canker incidence and host mortality suggest a role for the disease in regulating host spatial distribution, in agreement with predictions of the Janzen-Connell hypothesis.