1. The operation of ‘negative density-dependence' in seedling cohorts in tropical forests is empirically well-established, but only at a phenomenological level that leaves open the question of why ...seedlings conspecific with an overtopping parent tree experience higher mortality than heterospecifics. The distinction is of theoretical importance because distinct mechanisms are involved. 2. We consider the two most debated possibilities: seedling mortality resulting from classical Lotka-Volterra density-dependence and seedling mortality resulting from the action of biotic agents, as postulated for the Janzen-Connell mechanism. Our study is the first to identify the full spectrum of mortality factors affecting both conspecific and heterospecific members of seedling cohorts. 3. We took advantage of the occurrence of ‘seedling carpets', dense concentrations of seedlings that appear following fruiting events under reproductive individuals of some species. In these ‘carpets', seedlings conspecific with the overtopping parent tree predominate numerically, but heterospecific seedlings are also typically present. Here, we investigated the differential survival of conspecific versus heterospecific seedlings under focal trees of four species: Calatola microcarpa (Icacinaceae), Clarisia racemosa (Moraceae), Matisia cordata (Bombacaceae/Malvaceae) and Sorocea pileata (Moraceae). 4. We show that mortality rates of conspecific seedlings are much higher than those of heterospecific seedlings and that most conspecific mortality (64-100%) resulted from host-restricted arthropod herbivores and/or fungal pathogens, whereas the mortality of heterospecific seedlings resulted from a variety of other causes. 5. Synthesis. Conspecific seedlings died following attack by apparently host-restricted arthropods or fungi and eventually experienced 100% mortality. The results are inconsistent with classical intra- and inter-specific competition and consistent with the actions of distance-responsive and/or density-responsive ‘enemies', as postulated 40 years ago by Janzen and Connell.
We monitored a close-spaced grid of 289 seed traps in 1.44 ha for 8.4 yr in an Amazonian floodplain forest. In a tree community containing hundreds of species, a median of just three to four species ...of tree seeds falls annually into each 0.5-m² establishment site. The number of seed species reaching a given site increased linearly with time for the duration of the monitoring period, indicating a roughly random arrival of seed species in a given site-year. The number of seed species captured each year over the entire grid ranged from one-third to one-half of the total captured over the 8.4 yr of monitoring, revealing a substantial temporal component of variation in the seed rain. Seed rain at the 0.5-m² scale displayed extreme spatial variability when all potentially viable seeds were tallied, whereas the rain of dispersed seeds was scant, more nearly uniform, and better mixed. Dispersal limitation, defined as failure of seeds to reach establishment sites, is ≥99% per year for a majority of species, explaining why seed augmentation experiments are often successful. Dispersal limitation has been evoked as an explanation for distance-dependent species turnover in tropical tree communities, but that interpretation contrasts with the fact that many Amazonian tree species possess large geographical ranges that extend for hundreds or thousands of kilometers. A better understanding of the processes that bridge the gap between the scales of seedling establishment and the regulation of forest composition will require new methodologies for studying dispersal on scales larger than those yet achieved.
Phenology has long been hypothesized as an avenue for niche partitioning or interspecific facilitation, both promoting species coexistence. Tropical plant communities exhibit striking diversity in ...reproductive phenology, but many are also noted for large synchronous reproductive events. Here we study whether the phenology of seed fall in such communities is nonrandom, the temporal scales of phenological patterns, and ecological factors that drive reproductive phenology. We applied multivariate wavelet analysis to test for phenological synchrony versus compensatory dynamics (i.e., antisynchronous patterns where one species' decline is compensated by the rise of another) among species and across temporal scales. We used data from long‐term seed rain monitoring of hyperdiverse plant communities in the western Amazon. We found significant synchronous whole‐community phenology at multiple timescales, consistent with shared environmental responses or positive interactions among species. We also observed both compensatory and synchronous phenology within groups of species (confamilials) likely to share traits and seed dispersal mechanisms. Wind‐dispersed species exhibited significant synchrony at ~6‐month scales, suggesting these species might share phenological niches to match the seasonality of wind. Our results suggest that community phenology is shaped by shared environmental responses but that the diversity of tropical plant phenology may partly result from temporal niche partitioning. The scale‐specificity and time‐localized nature of community phenology patterns highlights the importance of multiple and shifting drivers of phenology.
Pathogens are hypothesized to play an important role in the maintenance of tropical forest plant species richness. Notably, species richness may be promoted by incomplete filling of niche space due ...interactions of host populations with their pathogens. A potentially important group of pathogens are endophytic fungi, which asymptomatically colonize plants and are diverse and abundant in tropical ecosystems. Endophytes may alter competitive abilities of host individuals and improve host fitness under stress, but may also become pathogenic. Little is known of the impacts of endophytes on niche-space filling of their hosts.Here we evaluate how a widespread fungal endophyte infecting a common tropical palm influences its recruitment and survival in natural ecosystems, and whether this impact is modulated by the abiotic environment, potentially constraining host niche-space filling. Iriartea deltoidea dominates many wet lowland Neotropical forests. Diplodia mutila is a common asymptomatic endophyte in mature plants; however, it causes disease in some seedlings. We investigated the effects of light availability on D. mutila disease expression.We found I. deltoidea seedlings to preferentially occur under shady conditions. Correspondingly, we also found that high light triggers endophyte pathogenicity, while low light favors endosymbiotic development, constraining recruitment of endophyte-infested seedlings to shaded understory by reducing seedling survival in direct light. Pathogenicity of D. mutila under high light is proposed to result from light-induced production of H(2)O(2) by the fungus, triggering hypersensitivity, cell death, and tissue necrosis in the palm. This is the first study to demonstrate that endophytes respond to abiotic factors to influence plant distributions in natural ecosystems; and the first to identify light as a factor influencing where an endophyte is placed on the endosymbiont-pathogen continuum. Our findings show that pathogens can indeed constrain niche-space filling of otherwise successful tropical plant species, providing unoccupied niche space for other species.
Treefall gaps have long been a central feature of discussions about the maintenance of tree diversity in both temperate and tropical forests. Gaps expose parts of the forest floor to direct sunlight ...and create a distinctive microenvironment that can favor the recruitment into the community of so-called gap pioneers. This traditional view enjoys strong empirical support, yet has been cast into doubt by a much-cited article claiming that gaps are inherently "neutral" in their contribution to forest dynamics. We present concurrent data on seedfall and sapling recruitment into gaps vs. under a vertically structured canopy in an Amazonian floodplain forest in Peru. Our results strongly uphold the view of gaps as important generators of tree diversity. Our methods differed significantly from those employed by the neutralist group and can explain the contrasting outcomes. We found that seedfall into gaps differs both quantitatively and qualitatively from that falling under a multi-tiered canopy, being greatly enriched in wind-dispersed and autochorus species and sharply deficient in all types of zoochorous seeds. Despite a reduced input of zoochorous seeds, zoochorous species made up 79% of saplings recruiting into gaps, whereas wind-dispersed species made up only 1%. Cohorts of saplings recruiting into gaps are less diverse than those recruiting under a closed canopy (Fisher's alpha = 40 vs. 100) and compositionally distinct, containing many light-demanding species that rarely, if ever, recruit under shaded conditions. Saplings recruiting into gaps appear to represent a variable mix of shade-tolerant survivors of the initiating treefall and sun-demanding species that germinate subsequently.
1. The term ‘dispersal limitation' represents two distinct component processes: the number of seeds produced (fecundity) and the spatial pattern of the seed rain (distribution). We present a ...quantitative evaluation of these component processes of dispersal limitation for a tropical forest tree community. 2. Using a regularly spaced grid of 289 seed traps (0.5 m² each), we monitored the seed rain into 1.44 ha of upper Amazonian floodplain forest for 6 years whilst concurrently monitoring sapling recruitment in a 0.81-ha subplot centred within the seed-trapping grid. This arrangement allowed us to compare the spatial pattern of seed rain with that of sapling recruitment. 3. We endeavoured to distinguish between undispersed and dispersed seeds by applying a series of criteria to seeds collected in the traps and by removing from certain analyses all seeds that fell under reproductive conspecifics. Gross fecundity of 30 common species that contribute to the advanced regeneration was uniformly low and the rain of dispersed seeds was lower still, being <1.0 m⁻² year⁻¹ in every case. 4. The rain of dispersed seeds with respect to conspecific reproductives closely matched the recruitment of saplings, whereas gross seed rain (all seeds, including undispersed seeds) did not. 5. Synthesis.‘Dispersal limitation' in this faunally intact Amazonian forest is primarily attributable to a scant rain of dispersed seeds, i.e. fecundity limitation, whereas the distribution of dispersed seeds, being random for most species, appears adequate. Evidence from this and earlier research at the same site indicates that the per-capita success of dispersed seeds is many times higher than that of undispersed seeds. Thus, seed dispersal kernels that do not distinguish between dispersed and undispersed seeds are likely to be biologically misleading.
Tall canopy trees produce many more seeds than do understory treelets, yet, on average, both classes of trees achieve the same lifetime fitness. Using concurrent data on seedfall (8 years) and ...sapling recruitment (12 years) from a long-established tree plot at the Cocha Cashu Biological Station in Peru, we show that a 40-m canopy tree must produce roughly 13 times the mass of seeds to generate a sapling as a 5-m understory treelet. Mature tree height accounted for 41% of the variance in
seed mass
per sapling recruit in a simple univariate regression, whereas a multivariate model that included both intrinsic (seed mass, tree height, and dispersal mode) and extrinsic factors (sapling mortality as a surrogate for microsite quality) explained only 31% of the variance in
number of seeds
per sapling recruit. The multivariate model accounted for less variance because tall trees produce heavier seeds, on average, than treelets. We used "intact" (mostly dispersed) seeds to parameterize the response variable so as to reduce, if not eliminate, any contribution of conspecific crowding to the difference in reproductive efficiency between canopy trees and treelets. Accordingly, a test for negative density dependence failed to expose a relationship between density of reproductive trees in the population and reproductive efficiency (seed mass per recruit). We conclude that understory treelets, some of which produce only a dozen seeds a year, gain their per-seed advantage by failing to attract enemies à la Janzen-Connell, either in ecological or evolutionary time.
The conservation of tropical forest carbon stocks offers the opportunity to curb climate change by reducing greenhouse gas emissions from deforestation and simultaneously conserve biodiversity. ...However, there has been considerable debate about the extent to which carbon stock conservation will provide benefits to biodiversity in part because whether forests that contain high carbon density in their aboveground biomass also contain high animal diversity is unknown. Here, we empirically examined medium to large bodied ground-dwelling mammal and bird (hereafter "wildlife") diversity and carbon stock levels within the tropics using camera trap and vegetation data from a pantropical network of sites. Specifically, we tested whether tropical forests that stored more carbon contained higher wildlife species richness, taxonomic diversity, and trait diversity. We found that carbon stocks were not a significant predictor for any of these three measures of diversity, which suggests that benefits for wildlife diversity will not be maximized unless wildlife diversity is explicitly taken into account; prioritizing carbon stocks alone will not necessarily meet biodiversity conservation goals. We recommend conservation planning that considers both objectives because there is the potential for more wildlife diversity and carbon stock conservation to be achieved for the same total budget if both objectives are pursued in tandem rather than independently. Tropical forests with low elevation variability and low tree density supported significantly higher wildlife diversity. These tropical forest characteristics may provide more affordable proxies of wildlife diversity for future multi-objective conservation planning when fine scale data on wildlife are lacking.
Carnivores have long been used as model organisms to examine mechanisms that allow coexistence among ecologically similar species. Interactions between carnivores, including competition and ...predation, comprise important processes regulating local community structure and diversity. We use data from an intensive camera-trapping monitoring program across eight Neotropical forest sites to describe the patterns of spatiotemporal organization of a guild of five sympatric cat species: jaguar (Panthera onca), puma (Puma concolor), ocelot (Leopardus pardalis), jaguarundi (Herpailurus yagouaroundi) and margay (Leopardus wiedii). For the three largest cat species, we developed multi-stage occupancy models accounting for habitat characteristics (landscape complexity and prey availability) and models accounting for species interactions (occupancy estimates of potential competitor cat species). Patterns of habitat-use were best explained by prey availability, rather than habitat structure or species interactions, with no evidence of negative associations of jaguar on puma and ocelot occupancy or puma on ocelot occupancy. We further explore temporal activity patterns and overlap of all five felid species. We observed a moderate temporal overlap between jaguar, puma and ocelot, with differences in their activity peaks, whereas higher temporal partitioning was observed between jaguarundi and both ocelot and margay. Lastly, we conducted temporal overlap analysis and calculated species activity levels across study sites to explore if shifts in daily activity within species can be explained by varying levels of local competition pressure. Activity patterns of ocelots, jaguarundis and margays were similarly bimodal across sites, but pumas exhibited irregular activity patterns, most likely as a response to jaguar activity. Activity levels were similar among sites and observed differences were unrelated to competition or intraguild killing risk. Our study reveals apparent spatial and temporal partitioning for most of the species pairs analyzed, with prey abundance being more important than species interactions in governing the local occurrence and spatial distribution of Neotropical forest felids.
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