Vertebrate herbivory impacts seedling recruitment more than niche partitioning or density-dependent mortality

In tropical forests, resource-based niches and density-dependent mortality are mutually compatible mechanisms that can act simultaneously to limit seedling populations. Differences in the strengths of these mechanisms will determine their roles in maintaining species coexistence. In the first assessment of these mechanisms in a Congo Basin forest, we quantified their relative strengths and tested the extent to which density-dependent mortality is driven by the distance-dependent behavior of seed and seedling predators predicted by the Janzen-Connell hypothesis. We conducted a large-scale seed addition experiment for five randomly selected tropical tree species, caging a subset of seed addition quadrats against vertebrate predators. We then developed models to assess the mechanisms that determine seedling emergence (three months after seed addition) and survival (two years after seed addition). As predicted, both niche differentiation and density-dependent mortality limited seedling recruitment, but predation had the strongest effects on seedling emergence and survival. Seedling species responded differently to naturally occurring environmental variation among sites, including variation in light levels and soil characteristics, supporting predictions of niche-based theories of tropical tree species coexistence. The addition of higher densities of seeds into quadrats initially led to greater seedling emergence, but survival to two years decreased with seed density. Seed and seedling predation reduced recruitment below levels maintained by density-dependent mortality, an indication that predators largely determine the population size of tree seedlings. Seedling recruitment was unrelated to the distance to or density of conspecific adult trees, suggesting that recruitment patterns are generated by generalist vertebrate herbivores rather than the specialized predators predicted by the Janzen-Connell hypothesis. If the role of seed and seedling predation in limiting seedling recruitment is a general phenomenon, then the relative abundances of tree species might largely depend on species-specific adaptations to avoid, survive, and recover from damage induced by vertebrate herbivores. Likewise, population declines of herbivorous vertebrate species (many of which are large and hunted) may trigger shifts in species composition of tropical forests.     

Seed predation by Neotropical rain forest mammals increases diversity in seedling recruitment

Seed dispersal and seedling recruitment (the transition of seeds to seedlings) set the spatiotemporal distribution of new individuals in plant communities. Many terrestrial rain forest mammals consume post-dispersal seeds and seedlings, often inflicting density-dependent mortality. In part because of density-dependent mortality, diversity often increases during seedling recruitment, making it a critical stage for species coexistence. We determined how mammalian predators, adult tree abundance, and seed mass interact to affect seedling recruitment in a western Amazonian rain forest. We used exclosures that were selectively permeable to three size classes of mammals: mice and spiny rats (weighing <1 kg), medium-sized rodents (1-12 kg), and large mammals (20-200 kg). Into each exclosure, we placed seeds of 13 tree species and one canopy liana, which varied by an order of magnitude in adult abundance and seed mass. We followed the fates of the seeds and resulting seedlings for at least 17 months. We assessed the effect of each mammalian size class on seed survival, seedling survival and growth, and the density and diversity of the seedlings that survived to the end of the experiment. Surprisingly, large mammals had no detectable effect at any stage of seedling recruitment. In contrast, small- and medium-sized mammals significantly reduced seed survival, seedling survival, and seedling density. Furthermore, predation by small mammals increased species richness on a per-stem basis. This increase in diversity resulted from their disproportionately intense predation on common species and large-seeded species. Small mammals thereby generated a rare-species advantage in seedling recruitment, the critical ingredient for frequency dependence. Predation by small (and to a lesser extent, medium-sized) mammals on seeds and seedlings significantly increases tree species diversity in tropical forests. This is the first long-term study to dissect the effects of various mammalian predators on the recruitment of a diverse set of tree species.     

Effects of an Exotic Plant Invasion on Native Understory Plants in a Tropical Dry Forest

Abstract: The dry forests of southern India, which are endangered tropical ecosystems and among the world's most important tiger (Panthera tigris) habitats, are extensively invaded by exotic plants. Yet, experimental studies exploring the impacts of these invasions on native plants in these forests are scarce. Consequently, little is known about associated implications for the long‐term conservation of tigers and other biodiversity in these habitats. I studied the impacts of the exotic plant Lantana camara on understory vegetation in a dry‐forest tiger habitat in southern India. I compared the richness, composition, and abundance of tree seedlings, herbs, and shrubs and the abundance of grass among plots in which Lantana was cleared or left standing. These plots were distributed across two blocks—livestock free and livestock grazed. Removal of Lantana had an immediate positive effect on herb–shrub richness in the livestock‐free block, but had no effect on that of tree seedlings in either livestock block. Tree‐seedling and herb–shrub composition differed significantly between Lantana treatment and livestock block, and Lantana removal significantly decreased survival of tree seedlings. Nevertheless, the absence of trees, in any stage between seedling and adult, indicates that Lantana may stall tree regeneration. Lantana removal decreased the abundance of all understory strata, probably because forage plants beneath Lantana are less accessible to herbivores, and plants in Lantana‐free open plots experienced greater herbivory. Reduced access to forage in invaded habitats could negatively affect ungulate populations and ultimately compromise the ability of these forests to sustain prey‐dependent large carnivores. Additional research focused on understanding and mitigating threats posed by exotic plants may be crucial to the long‐term protection of these forests as viable tiger habitats.     

Effects of Long-Term Ungulate Exclusion and Recent Alien Species Control on the Preservation and Restoration of a Hawaiian Tropical Dry Forest

Abstract: Although the destruction of tropical rain forests receives much attention, tropical dry forests are in general far more threatened and endangered. Eliminating grazing ungulates is often considered a key first step toward protecting these ecosystems, but few studies have investigated the long-term effects of this technique. We examined the effects of ungulate exclusion from a 2.3-ha native dry-forest preserve on the island of Hawaii by comparing its present flora to the flora of an adjacent area subjected to continuous grazing since the preserve was fenced over 40 years ago. Relative to this adjacent area, the fenced preserve contained a more diverse flora with substantially greater coverage of native overstory and understory species. Until recently, however, regeneration of native canopy trees within the preserve appears to have been thwarted by a dominant herbaceous cover of alien fountain grass (   Pennisetum setaceum ) and predation by alien rodent species. Our results indicate that although ungulate exclusion may be a necessary and critical first step, it is not sufficient to adequately preserve and maintain Hawaii's remaining tropical dry forest remnants. Our recent efforts to control the dominant alien species within the fenced preserve suggest that this practice may facilitate both the regeneration of native species and the colonization and potential invasion of new alien plants. Comparisons of seedlings of the dominant native canopy tree Diospyros sandwicensis growing in sites both dominated by and free of fountain grass suggested that fountain grass inhibits Diospyros seedling growth and photosynthesis but may increase survival if seedlings are protected from ungulates.     

Are functional traits good predictors of demographic rates? Evidence from five neotropical forests

A central goal of comparative plant ecology is to understand how functional traits vary among species and to what extent this variation has adaptive value. Here we evaluate relationships between four functional traits (seed volume, specific leaf area, wood density, and adult stature) and two demographic attributes (diameter growth and tree mortality) for large trees of 240 tree species from five Neotropical forests. We evaluate how these key functional traits are related to survival and growth and whether similar relationships between traits and demography hold across different tropical forests. There was a tendency for a trade-off between growth and survival across rain forest tree species. Wood density, seed volume, and adult stature were significant predictors of growth and/or mortality. Both growth and mortality rates declined with an increase in wood density. This is consistent with greater construction costs and greater resistance to stem damage for denser wood. Growth and mortality rates also declined as seed volume increased. This is consistent with an adaptive syndrome in which species tolerant of low resource availability (in this case shade-tolerant species) have large seeds to establish successfully and low inherent growth and mortality rates. Growth increased and mortality decreased with an increase in adult stature, because taller species have a greater access to light and longer life spans. Specific leaf area was, surprisingly, only modestly informative for the performance of large trees and had ambiguous relationships with growth and survival. Single traits accounted for 9-55% of the interspecific variation in growth and mortality rates at individual sites. Significant correlations with demographic rates tended to be similar across forests and for phylogenetically independent contrasts as well as for cross-species analyses that treated each species as an independent observation. In combination, the morphological traits explained 41% of the variation in growth rate and 54% of the variation in mortality rate, with wood density being the best predictor of growth and mortality. Relationships between functional traits and demographic rates were statistically similar across a wide range of Neotropical forests. The consistency of these results strongly suggests that tropical rain forest species face similar trade-offs in different sites and converge on similar sets of solutions.     

Life history trade-offs in tropical trees and lianas

It has been hypothesized that tropical trees partition forest light environments through a life history trade-off between juvenile growth and survival; however, the generality of this trade-off across life stages and functional groups has been questioned. We quantified trade-offs between growth and survival for trees and lianas on Barro Colorado Island (BCI), Panama using first-year seedlings of 22 liana and 31 tree species and saplings (10 mm < dbh < 39 mm) of 30 tree species. Lianas showed trade-offs similar to those of trees, with both groups exhibiting broadly overlapping ranges in survival and relative growth rates as seedlings. Life history strategies at the seedling stage were highly correlated with those at the sapling stage among tree species, with all species showing an increase in survival with size. Only one of 30 tree species demonstrated a statistically significant ontogenetic shift, having a relatively lower survival rate at the sapling stage than expected. Our results indicate that similar life history trade-offs apply across two functional groups (lianas and trees), and that life history strategies are largely conserved across seedling and sapling life-stages for most tropical tree species.     

Wood Density as a Conservation Tool: Quantification of Disturbance and Identification of Conservation‐Priority Areas in Tropical Forests

Abstract: Inventories of tree species are often conducted to guide conservation efforts in tropical forests. Such surveys are time consuming, demanding of expertise, and expensive to perform and interpret. Approaches to make survey efforts simpler or more effective would be valuable. In particular, it would be good to be able to easily identify areas of old‐growth forest. The average density of the wood of a tree species is closely linked to its successional status. We used tree inventory data from eastern Borneo to determine whether wood density can be used to quantify forest disturbance and conservation importance. The average density of wood in a plot was significantly and negatively related to disturbance levels, with plots with higher wood densities occurring almost exclusively in old‐growth forests. Average wood density was unimodally related to the diversity of tree species, indicating that the average wood density in a plot might be a better indicator of old‐growth forest than species diversity. In addition, Borneo endemics had significantly heavier wood than species that are common throughout the Malesian region, and they were more common in plots with higher average wood density. We concluded that wood density at the plot level could be a powerful tool for identifying areas of conservation priority in the tropical rain forests of Southeast Asia.     

Plant-Mammal Interactions in Tropical Bolivian Forests with Different Hunting Pressures

Abstract: We studied plant-animal interactions and vegetation structure in two geographically close tropical Bolivian forests subjected to different hunting intensities. We hypothesized that reduction of mammals openface> 1 kg in an "intensively hunted forest," compared with an "occasionally hunted forest," should correlate with decreased seed predation and seedling trampling, increased seedling survival and density, and decreased tree-species diversity at the seedling stage in relation to the adult stage. The occasionally hunted forest held 1.7 times as many mammalian species as the intensively hunted forest. As predicted, predation of Astrocaryum murumuru seeds was 34.2% lower in the intensively hunted forest. Similarly, trampling of model seedlings was 5.4 times lower and seedling survival was 1.15 times greater in the intensively hunted forest than that in the occasionally hunted forest. But the intensively hunted forest displayed lower seedling densities and a higher ratio of seedling diversity to tree diversity than did the occasionally hunted forest. Reduction of peccaries from the intensively hunted forest may explain much of the between-site differences in seed predation, trampling, and seedling survival. Lack of consistent differences in seedling density and diversity could mean that reduced granivory and trampling may be counteracted by reduced seed dispersal. The reduction of mammalian populations may produce a complex mosaic of forest patches with distinct degrees of structural change, depending on the intensity of defaunation.     

Neighborhood and community interactions determine the spatial pattern of tropical tree seedling survival

Factors affecting survival and recruitment of 3531 individually mapped seedlings of Myristicaceae were examined over three years in a highly diverse neotropical rain forest, at spatial scales of 1-9 m and 25 ha. We found convincing evidence of a community compensatory trend (CCT) in seedling survival (i.e., more abundant species had higher seedling mortality at the 25-ha scale), which suggests that density-dependent mortality may contribute to the spatial dynamics of seedling recruitment. Unlike previous studies, we demonstrate that the CCT was not caused by differences in microhabitat preferences or life history strategy among the study species. In local neighborhood analyses, the spatial autocorrelation of seedling survival was important at small spatial scales (1-5 m) but decayed rapidly with increasing distance. Relative seedling height had the greatest effect on seedling survival. Conspecific seedling density had a more negative effect on survival than heterospecific seedling density and was stronger and extended farther in rare species than in common species. Taken together, the CCT and neighborhood analyses suggest that seedling mortality is coupled more strongly to the landscape-scale abundance of conspecific large trees in common species and the local density of conspecific seedlings in rare species. We conclude that negative density dependence could promote species coexistence in this rain forest community but that the scale dependence of interactions differs between rare and common species.     

Indirect Effects of Pandemic Deer Overabundance Inferred from Caterpillar‐Host Relations

Externally feeding phytophagous insect larvae (i.e., caterpillars, here, larval Lepidoptera and sawflies, Hymenoptera: Symphyta) are important canopy herbivores and prey resources in temperate deciduous forests. However, composition of forest trees has changed dramatically in the eastern United States since 1900. In particular, browsing by high densities of white‐tailed deer (Odocoileus virginianus) has resulted in forests dominated by browse‐tolerant species, such as black cherry (Prunus serotina), and greatly reduced relative abundance of other tree species, notably pin cherry (Prunus pensylvanica) and birches (Betula spp.). To quantify effects of these changes on caterpillars, we sampled caterpillars from 960 branch tips of the 8 tree species that comprise 95% of trees in Allegheny hardwood forests: red maple (Acer rubrum), striped maple (Acer pensylvanicum), sugar maple (Acer saccharum), sweet birch (Betula lenta), yellow birch (Betula allegheniensis), American beech (Fagus grandifolia), black cherry, and pin cherry. We collected 547 caterpillar specimens that belonged to 66 Lepidoptera and 10 Hymenoptera species. Caterpillar density, species richness, and community composition differed significantly among tree species sampled. Pin cherry, nearly eliminated at high deer density, had the highest density and diversity of caterpillars. Pin cherry shared a common caterpillar community with black cherry, which was distinct from those of other tree hosts. As high deer density continues to replace diverse forests of cherries, maples, birches, and beech with monodominant stands of black cherry, up to 66% of caterpillar species may be eliminated. Hence, deer‐induced changes in forest vegetation are likely to ricochet back up forest food webs and therefore negatively affect species that depend on caterpillars and moths for food and pollination. Efectos Indirectos de la Sobreabundancia de Venados Pandémicos Inferida de Relaciones Orugas‐Huéspedes     

Long-term CO2 enrichment of a forest ecosystem: implications for forest regeneration and succession.

The composition and successional status of a forest affect carbon storage and net ecosystem productivity, yet it remains unclear whether elevated atmospheric carbon dioxide (CO2) will impact rates and trajectories of forest succession. We examined how CO2 enrichment (+200 microL CO2/L air differential) affects forest succession through growth and survivorship of tree seedlings, as part of the Duke Forest free-air CO2 enrichment (FACE) experiment in North Carolina, USA. We planted 2352 seedlings of 14 species in the low light forest understory and determined effects of elevated CO2 on individual plant growth, survival, and total sample biomass accumulation, an integrator of plant growth and survivorship over time, for six years. We used a hierarchical Bayes framework to accommodate the uncertainty associated with the availability of light and the variability in growth among individual plants. We found that most species did not exhibit strong responses to CO2. Ulmus alata (+21%), Quercus alba (+9.5%), and nitrogen-fixing Robinia pseudoacacia (+230%) exhibited greater mean annual relative growth rates under elevated CO2 than under ambient conditions. The effects of CO2 were small relative to variability within populations; however, some species grew better under low light conditions when exposed to elevated CO2 than they did under ambient conditions. These species include shade-intolerant Liriodendron tulipifera and Liquidambar styraciflua, intermediate-tolerant Quercus velutina, and shade-tolerant Acer barbatum, A. rubrum, Prunus serotina, Ulmus alata, and Cercis canadensis. Contrary to our expectation, shade-intolerant trees did not survive better with CO2 enrichment, and population-scale responses to CO2 were influenced by survival probabilities in low light. CO2 enrichment did not increase rates of sample biomass accumulation for most species, but it did stimulate biomass growth of shade-tolerant taxa, particularly Acer barbatum and Ulmus alata. Our data suggest a small CO2 fertilization effect on tree productivity, and the possibility of reduced carbon accumulation rates relative to today's forests due to changes in species composition.     

Tropical forest restoration: tree islands as recruitment foci in degraded lands of Honduras.

Tropical forest recovery in pastures is slowed by a number of biotic and abiotic factors, including a lack of adequate seed dispersal and harsh microclimatic extremes. Accordingly, methods to accelerate forest recovery must address multiple impediments. Here, we evaluated the ability of "tree islands" to serve as "recruitment foci" in a two-year study at three sites in northern Honduras. Islands of three sizes (64, 16, and 4 m2) and at two distances to secondary forest (20 and 50 m) were created by planting 2 m tall vegetative stakes of two native species: Gliricidia sepium (Fabaceae) and Bursera simaruba (Burseraceae), each in monoculture. Open-pasture "islands" of equal sizes served as controls. Tree islands reduced temperature and light (PAR) extremes as compared to open pasture, creating a microenvironment more favorable to seedling establishment. Seed-dispersing birds (quantified at one site only) showed an overwhelming preference for islands; 160 visits were recorded to islands compared with one visit to open pasture. Additionally, frugivores visited large islands more often, and for longer time periods, than small islands, thereby increasing the likelihood of a dispersal event there. In total, 144 140 seeds belonging to 186 species were collected in islands; more than 80% were grasses. Tree islands increased zoochorous tree seed rain; seed density and species richness were greater in tree islands than in open pasture, and large islands had greater seed density than smaller islands (Gliricidia only), suggesting that they are more effective for restoration. Distance to forest did not affect seed rain. A total of 543 seedlings and 41 species established in islands; > 85% were zoochorous. Seedling density did not differ among treatments (mean 0.2 seedlings/m2 for islands vs. 0.1 seedlings/m2 for pasture), although an increasing trend in tree islands over the course of two years suggests that seedling recruitment is accelerated there. Lastly, similar seedling densities were censused in the 1 m perimeter surrounding islands, suggesting that islands can expand outward into pasture. Planting vegetative stakes to create tree islands in pastures accelerates forest recovery by overcoming a number of impediments, and presents a simple, broadly applicable alternative for facilitating forest regeneration in abandoned pastures.     

Specificity between Neotropical tree seedlings and their fungal mutualists leads to plant-soil feedback

A growing body of evidence obtained largely from temperate grassland studies suggests that feedbacks occurring between plants and their associated soil biota are important to plant community assemblage. However, few studies have examined the importance of soil organisms in driving plant-soil feedbacks in forested systems. In a tropical forest in central Panama, we examined whether interactions between tree seedlings and their associated arbuscular mycorrhizal fungi (AMF) lead to plant-soil feedback. Specifically, do tropical seedlings modify their own AMF communities in a manner that either favors or inhibits the next cohort of conspecific seedlings (i.e., positive or negative feedback, respectively)? Seedlings of two shade-tolerant tree species (Eugenia nesiotica, Virola surinamensis) and two pioneer tree species (Luehea seemannii, Apeiba aspera) were grown in pots containing identical AMF communities composed of equal amounts of inoculum of six co-occurring AMF species. The different AMF-host combinations were all exposed to two light levels. Under low light (2% PAR), only two of the six AMF species sporulated, and we found that host identity did not influence composition of AMF spore communities. However, relative abundances of three of the four AMF species that produced spores were influenced by host identity when grown under high light (20% PAR). Furthermore, spores of one of the AMF species, Glomus geosporum, were common in soils of Luehea and Eugenia but absent in soils of Apeiba and Virola. We then conducted a reciprocal experiment to test whether AMF communities previously modified by Luehea and Apeiba differentially affected the growth of conspecific and heterospecific seedlings. Luehea seedling growth did not differ between soils containing AMF communities modified by Luehea and Apeiba. However, Apeiba seedlings were significantly larger when grown with Apeiba-modified AMF communities, as compared to Apeiba seedlings grown with Luehea-modifed AMF communities. Our experiments suggest that interactions between tropical trees and their associated AMF are species-specific and that these interactions may shape both tree and AMF communities through plant-soil feedback.     

Fragmentation drives tropical forest fragments to early successional states: A modelling study for Brazilian Atlantic forests

Land use leads to massive habitat destruction and fragmentation in tropical forests. Despite its global dimensions the effects of fragmentation on ecosystem dynamics are not well understood due to the complexity of the problem. We present a simulation analysis performed by the individual-based model FORMIND. The model was applied to the Brazilian Atlantic Forest, one of the world's biodiversity hot spots, at the Plateau of São Paulo. This study investigates the long-term effects of fragmentation processes on structure and dynamics of different sized remnant tropical forest fragments (1-100 ha) at community and plant functional type (PFT) level. We disentangle the interplay of single effects of different key fragmentation processes (edge mortality, increased mortality of large trees, local seed loss and external seed rain) using simulation experiments in a full factorial design.Our analysis reveals that particularly small forest fragments below 25 ha suffer substantial structural changes, biomass and biodiversity loss in the long term. At community level biomass is reduced up to 60%. Two thirds of the mid- and late-successional species groups, especially shade-tolerant (late successional climax) species groups are prone of extinction in small fragments. The shade-tolerant species groups were most strongly affected; its tree number was reduced more than 60% mainly by increased edge mortality. This process proved to be the most powerful of those investigated, explaining alone more than 80% of the changes observed for this group. External seed rain was able to compensate approximately 30% of the observed fragmentation effects for shade-tolerant species.Our results suggest that tropical forest fragments will suffer strong structural changes in the long term, leading to tree species impoverishment. They may reach a new equilibrium with a substantially reduced subset of the initial species pool, and are driven towards an earlier successional state. The natural regeneration potential of a landscape scattered with forest fragments appears to be limited, as external seed rain is not able to fully compensate for the observed fragmentation-induced changes. Our findings suggest basic recommendations for the management of fragmented tropical forest landscapes.     

Ecological consequences of forest elephant declines for Afrotropical forests

Poaching is rapidly extirpating African forest elephants (Loxodonta cyclotis) from most of their historical range, leaving vast areas of elephant‐free tropical forest. Elephants are ecological engineers that create and maintain forest habitat; thus, their loss will have large consequences for the composition and structure of Afrotropical forests. Through a comprehensive literature review, we evaluated the roles of forest elephants in seed dispersal, nutrient recycling, and herbivory and physical damage to predict the cascading ecological effects of their population declines. Loss of seed dispersal by elephants will favor tree species dispersed abiotically and by smaller dispersal agents, and tree species composition will depend on the downstream effects of changes in elephant nutrient cycling and browsing. Loss of trampling and herbivory of seedlings and saplings will result in high tree density with release from browsing pressures. Diminished seed dispersal by elephants and high stem density are likely to reduce the recruitment of large trees and thus increase homogeneity of forest structure and decrease carbon stocks. The loss of ecological services by forest elephants likely means Central African forests will be more like Neotropical forests, from which megafauna were extirpated thousands of years ago. Without intervention, as much as 96% of Central African forests will have modified species composition and structure as elephants are compressed into remaining protected areas. Stopping elephant poaching is an urgent first step to mitigating these effects, but long‐term conservation will require land‐use planning that incorporates elephant habitat into forested landscapes that are being rapidly transformed by industrial agriculture and logging.     

Rain forest fragmentation and the proliferation of successional trees

The effects of habitat fragmentation on diverse tropical tree communities are poorly understood. Over a 20-year period we monitored the density of 52 tree species in nine predominantly successional genera (Annona, Bellucia, Cecropia, Croton, Goupia, Jacaranda, Miconia, Pourouma, Vismia) in fragmented and continuous Amazonian forests. We also evaluated the relative importance of soil, topographic, forest dynamic, and landscape variables in explaining the abundance and species composition of successional trees. Data were collected within 66 permanent 1-ha plots within a large (approximately 1000 km2) experimental landscape, with forest fragments ranging from 1 to 100 ha in area. Prior to forest fragmentation, successional trees were uncommon, typically comprising 2-3% of all trees (> or =10 cm diameter at breast height [1.3 m above the ground surface]) in each plot. Following fragmentation, the density and basal area of successional trees increased rapidly. By 13-17 years after fragmentation, successional trees had tripled in abundance in fragment and edge plots and constituted more than a quarter of all trees in some plots. Fragment age had strong, positive effects on the density and basal area of successional trees, with no indication of a plateau in these variables, suggesting that successional species could become even more abundant in fragments over time. Nonetheless, the 52 species differed greatly in their responses to fragmentation and forest edges. Some disturbance-favoring pioneers (e.g., Cecropia sciadophylla, Vismia guianensis, V. amazonica, V. bemerguii, Miconia cf. crassinervia) increased by >1000% in density on edge plots, whereas over a third (19 of 52) of all species remained constant or declined in numbers. Species responses to fragmentation were effectively predicted by their median growth rate in nearby intact forest, suggesting that faster-growing species have a strong advantage in forest fragments. An ordination analysis revealed three main gradients in successional-species composition across our study area. Species gradients were most strongly influenced by the standlevel rate of tree mortality on each plot and by the number of nearby forest edges. Species-composition also varied significantly among different cattle ranches, which differed in their surrounding matrices and disturbance histories. These same variables were also the best predictors of total successional-tree abundance and species richness. Successional-tree assemblages in fragment interior plots (>150 m from edge), which are subjected to fragment area effects but not edge effects, did not differ significantly from those in intact forest, indicating that area effects per se had little influence on successional trees. Soils and topography also had little discernable effect on these species. Collectively, our results indicate that successional-tree species proliferate rapidly in fragmented Amazonian forests, largely as a result of chronically elevated tree mortality near forest edges and possibly an increased seed rain from successional plants growing in nearby degraded habitats. The proliferation of fast-growing successional trees and correlated decline of old-growth trees will have important effects on species composition, forest dynamics, carbon storage, and nutrient cycling in fragmented forests.     

Herbivores on a dominant understory shrub increase local plant diversity in rain forest communities

Indirect effects of trophic interactions on biodiversity can be large and common, even in complex communities. Previous experiments with dominant understory Piper shrubs in a Costa Rican rain forest revealed that increases in herbivore densities on these shrubs caused widespread seedling mortality as a result of herbivores moving from Piper to seedlings of many different plant genera. We tested components of the Janzen-Connell hypothesis by conducting focused studies on the effects of specialist and generalist Piper herbivores on local seedling diversity. Whereas specialist herbivores are predicted to increase mortality to neighboring seedlings that are closely related to the source plant, true generalists moving from source plants may cause density-dependent mortality of many species, and possibly increase richness if new species replace abundant species that have been thinned by herbivores. Therefore, we hypothesized that seedling richness would be greater in understory control plots created in patches of Piper that had normal densities of generalist herbivores compared to plots from which we removed generalist herbivores manually from all Piper shrubs. After 15 months, generalist-herbivore-removal plots had > 40% fewer seedlings, > 40% fewer species, and 40% greater seedling evenness, on average, than control plots with generalist herbivores intact. Using a complementary approach in unmanipulated plots in four forests, we used path analysis to test for a positive association between seedling diversity and herbivore damage on Piper species. In unmanipulated plots, for both generalist and specialist herbivores, our data were significant fits to the causal model that Piper herbivores decrease evenness and increase plant species richness, corroborating the experimental results. Because herbivores changed how individuals were apportioned among the species and families present (lower evenness), one interpretation of these associations between herbivores on Piper shrubs and local seedling richness is that high seedling mortality in dominant families allowed the colonization or survival of less common species. If interspecific or apparent competition allowed for a relative increase in species richness, then the Janzen-Connell hypothesis may extend its predictions to generalist seedling predators. We speculate that apparent competition may explain some of the deviations from neutral model predictions, especially at small scales.     

Distinct mycorrhizal communities on new and established hosts in a transitional tropical plant community

The extent to which interspecific plants share mycorrhizal fungal communities depends on the specificity of the symbiosis. For tropical forest tree seedlings, colonization by mycorrhizal fungi associated with established vegetation could have important consequences for survival and growth. I used a novel molecular technique to assess the potential for sharing of mycorrhizas in forest and pasture in southern Costa Rica, by identifying arbuscular mycorrhizal (AM) fungi in roots of the forest canopy tree species Terminalia amazonia, pasture grasses Urochloa ruziziensis and U. decumbens, and seedlings of T. amazonia planted into experimental reforestation plots. I tested the hypotheses that experimental seedlings were colonized either by the AM fungal community of the forest T. amazonia (suggesting host specificity) or of Urochloa (suggesting absence of specificity/importance of local environment). After two years, pasture-grown T. amazonia seedlings were colonized by neither community, but rather by a species of Glomus that was rarely observed on the other plants. These results suggest that conspecific seedlings planted into existing vegetation generate a distinct mycorrhizal community that may influence competitive interactions and the relative costs and benefits of the AM fungal symbiosis at early stages in the life cycle of tropical trees.     

Widespread density-dependent seedling mortality promotes species coexistence in a highly diverse Amazonian rain forest

Negative density-dependent mortality can promote species coexistence through a spacing mechanism that prevents species from becoming too locally abundant. Negative density-dependent seedling mortality can be caused by interactions among seedlings or between seedlings and neighboring adults if the density of neighbors affects the strength of competition or facilitates the attack of natural enemies. We investigated the effects of seedling and adult neighborhoods on the survival of newly recruited seedlings for multiple cohorts of known age from 163 species in Yasuni National Park, Ecuador, an ever-wet, hyper-diverse lowland Amazonian rain forest. At local scales, we found a strong negative impact on first-year survival of conspecific seedling densities and adult abundance in multiple neighborhood sizes and a beneficial effect of a local tree neighborhood that is distantly related to the focal seedling. Once seedlings have survived their first year, they also benefit from a more phylogenetically dispersed seedling neighborhood. Across species, we did not find evidence that rare species have an advantage relative to more common species, or a community compensatory trend. These results suggest that the local biotic neighborhood is a strong influence on early seedling survival for species that range widely in their abundance and life history. These patterns in seedling survival demonstrate the role of density-dependent seedling dynamics in promoting and maintaining diversity in understory seedling assemblages. The assemblage-wide impacts of species abundance distributions may multiply with repeated cycles of recruitment and density-dependent seedling mortality and impact forest diversity or the abundance of individual species over longer time scales.     

Bat- and Bird-Generated Seed Rains at Isolated Trees in Pastures in a Tropical Rainforest

Abstract: Bats are abundant and effective seed dispersers inside the forest, but what happens when a forest is fragmented and transformed into pasture? The landscape at Los Tuxtlas, Mexico, originally rainforest, is greatly fragmented and covered with pastures. We analyzed the seed rains produced by frugivorous bats and birds under isolated trees in pastures in the fragmented landscape and the contribution of this process to vegetational recovery. We surveyed bats and obtained fecal samples under isolated trees in pastures. We also collected seed rain below the canopy of 10 isolated Ficus trees, separating nocturnally dispersed seeds from diurnally dispersed seeds. We caught 652 bats of 20 species; 83% of captures were frugivores. The most abundant species were Sturnira lilium (48%), Artibeus jamaicensis (18%), Carollia perspicillata (12%), and Dermanura tolteca (11%). Fecal samples contained seeds of 19 species in several families: Piperaceae (50%), Moraceae (25%), Solanaceae (12%), Cecropiaceae (10%), and others (3%). Sturnira lilium was the most important disperser bat in pastures. Seed rain was dominated by zoochorous species (89%). We found seed diversity between day and night seed captures to be comparable, but we found a significant interaction of disperser type (  bird or bat) with season. Seven plant species accounted for 79% of the seed rain: Piper auritum (23%), Ficus (  hemiepiphytic-strangler tree) spp. (17%), Cecropia obtusifolia (10%), P. amalago (10%), Ficus (  free-standing tree) spp. (8%), P. yzabalanum (6%), and Solanum rudepanum (5%). Bats and birds are important seed dispersers in pastures because they disperse seeds of pioneer and primary species (trees, shrubs, herbs, and epiphytes), connect forest fragments, and maintain plant diversity. Consequently, they might contribute to the recovery of woody vegetation in disturbed areas in tropical humid forests.