Herbivores limit the population size of big‐leaf mahogany trees in an Amazonian forest

The Janzen–Connell hypothesis proposes that specialized herbivores maintain high numbers of tree species in tropical forests by restricting adult recruitment so that host populations remain at low densities. We tested this prediction for the large timber tree species, Swietenia macrophylla, whose seeds and seedlings are preyed upon by small mammals and a host‐specific moth caterpillar Steniscadia poliophaea, respectively. At a primary forest site, experimental seed additions to gaps – canopy‐disturbed areas that enhance seedling growth into saplings – over three years revealed lower survival and seedling recruitment closer to conspecific trees and in higher basal area neighborhoods, as well as reduced subsequent seedling survival and height growth. When we included these Janzen–Connell effects in a spatially explicit individual‐based population model, the caterpillar's impact was critical to limiting Swietenia's adult tree density, with a > 10‐fold reduction estimated at 300 years. Our research demonstrates the crucial but oft‐ignored linkage between Janzen–Connell effects on offspring and population‐level consequences for a long‐lived, potentially dominant tree species.     

Strong distance-dependent effects for a spatially aggregated tropical species

A fundamental aspect of the Janzen–Connell Hypothesis (JCH) is that distance- and density-dependent mortality reduce the local dominance of species and cause regular rather than random or aggregated spatial patterns. Despite this explicit linkage between process and pattern, very few studies have explored how JCH processes translate into the spatial distributions of adult populations. In field experiments, we assessed germination, mortality and growth of conspecific and heterospecific seedlings beneath and away from Esenbeckia leiocarpa, a highly aggregated tropical tree species. We also investigated the effects of vertebrates using exclosures in the field, and the effects of pathogens in a soil sterilization shadehouse experiment. Germination of conspecifics underneath Esenbeckia was reduced by 64 % and mortality increased 28–123 % when compared to seedlings growing under other tree species; 99–100 % of Esenbeckia seedlings died under conspecifics. Heterospecifics were much less affected by Esenbeckia canopies. However, we found no evidence that either vertebrate herbivores or soil pathogens affected seed germination and seedling performance. Although many tropical tree species are aggregated, our results are the first to demonstrate strong negative distance-dependence for an aggregated species and one of the few to explore germination in the context of the JCH and show differences between conspecifics and heterospecifics, thus suggesting a broader role for Janzen–Connell processes as determinants of the distribution and abundance of tree species in tropical forests.     

Testing for Janzen–Connell Effects in a West African Montane Forest

The Janzen–Connell hypothesis proposes that density dependent seed and seedling mortality, combined with increasing seed and seedling survival away from the parent tree, together promote regular spacing of species and thus α diversity. This hypothesis has rarely been tested in tropical Africa, and rarely in montane forests anywhere. We tested this hypothesis using a combination of field experiments and observations in the most floristically diverse dry submontane forest in Nigeria. We investigated distance effects on seedling herbivory, seedling survival and seedling height growth. We found a significant decrease in herbivory with distance from conspecific adult trees for all three species of experimentally planted seedlings (Entandrophragma angolense, Deinbollia pinnata and Sterculia setigera), and also for naturally occurring seedlings of Pouteria altissima but not of Newtonia buchananii or Isolona pleurocarpa. The relative density of large seedlings/saplings of P. altissima, N. buchananii and I. pleurocarpa increased significantly at greater distance from conspecific adult trees; however, we found no significant distance effect on survival or height growth over 3 mo for all three experimentally planted species. Taken together, our results are some of the first to show that Janzen–Connell effects occur on the African continent and in particular montane tropical forest and suggest that such effects may be pantropical.     

Recruitment Distance from the Nearest Reproductive Conspecific Increases with Tree Size in Tropical Premontane Wet Forests

Studies have shown that median distances of plants from the nearest reproductive conspecific (recruitment distance) shifts outward with increasing age or size class, and that plant spatial distribution changes over time in a predictable manner. However, observations and empirical evidence for such predictable changes are limited, and underlying mechanisms explaining such patterns for a wide range of individual sizes are not fully explored. In Costa Rican premontane wet forests, I empirically tested whether recruitment distance changes in a predictable manner with increasing size for five animal‐dispersed tree species by considering all post‐germination sizes. Specifically, I tested the Janzen–Connell hypothesis and the colonization hypothesis by considering distance, density, size, herbivory, biotic infection, and light availability simultaneously. Recruitment distance increased with increasing size (16–22 m) for the four non‐pioneer species, suggesting eventual regeneration success for seeds dispersed away from reproductive conspecifics. During the 2 years of this study, I found positive distance‐dependent survivorship and light availability were important for post‐seedling survivorship, in agreement with the Janzen–Connell hypothesis and the colonization hypothesis, respectively, but only for seedlings. However, seedlings did not escape aboveground herbivory or biotic infection better when the seeds were dispersed greater distances. Results highlight the importance of seed dispersal for successful regeneration, and suggest that changes in spatial distribution over time may be predictable in the vicinity of maternal trees for some non‐pioneer tree species.     

Plant density can increase invertebrate postdispersal seed predation in an experimental grassland community

Janzen–Connell effects are negative effects on the survival of a plant's progeny at high conspecific densities or close to its conspecifics. Although the role of Janzen–Connell effects on the maintenance of plant diversity was frequently studied, only few studies targeted Janzen–Connell effects via postdispersal seed predation in temperate grassland systems. We examined effects of conspecific density (abundance of conspecific adult plants) on postdispersal seed predation by invertebrates of three grassland species (Centaurea jacea, Geranium pratense, and Knautia arvensis) in experimental plant communities. Additionally, we examined the impact of plant species richness and different seed predator communities on total and relative seed predation (= seed predation of one plant species relative to others). We offered seeds in an exclusion experiment, where treatments allowed access for (1) arthropods and slugs, (2) arthropods only, (3) small arthropods only, and (4) slugs only. Treatments were placed in plots covering a gradient of abundance of conspecific adults at different levels of plant species richness (1, 2, 3, 4, 8 species). Two of the plant species (C. jacea and K. arvensis) experienced higher rates of seed predation and relative predation with increasing abundance of conspecific adults. For C. jacea, this effect was mitigated with increasing plant species richness. Differences in seed predator communities shifted seed predation between the plant species and changed the magnitude of seed predation of one plant species relative to the others. We exemplify density‐dependent increase in seed predation via invertebrates in grassland communities shaping both the total magnitude of species‐specific seed predation and seed predation of one species relative to others. Further differences in seed predator groups shift the magnitude of seed predation between different plant species. This highlights the importance of invertebrate seed predation to structure grasslands via density‐dependent effects and differing preferences of consumer groups.     

The Relative Importance of Fungal Infection,Conspecific Density and Environmental Heterogeneity for Seedling Survival in a Dominant Tropical Tree

A gap remains in our understanding of how host‐specific fungal pathogens impact negative density dependence (NDD). Here, we investigated survival of Cinnamomum subavenium Miq. seedlings, the dominant canopy species in a seasonal tropical evergreen forest, Thailand. It is infected by a host‐specific fungus that is easily identifiable in the field. We quantified the effects of conspecific seedling and adult density on fungal infection and seedling survival over a wide range of environmental heterogeneity in elevation, understory vegetation and presence of forest gaps. Generalized linear mixed models (GLMMs) for seedling survival revealed that fungal infection significantly reduced survival and had the strongest effect on seedling survival as compared with conspecific density and environmental heterogeneity. Adult conspecific density was not, however, significantly correlated with the probability of infection, and conspecific seedling density was positively associated with increased infection only at high elevations. In contrast to infection, we found a significant positive correlation between conspecific seedling density and the probability of seedling survival. Consequently, our results demonstrate that fungal infection can have major impacts on seedling survival, but not in a manner consistent with local NDD effects on seedlings, as assumed in the Janzen–Connell hypothesis. Our study provides an example of how quantifying the interaction between environmental heterogeneity and a host‐specific plant‐pathogen can yield unexpected insights into the dynamics of seedling populations. The combined effects of host‐specific pathogens and environmental heterogeneity on survival of dominant seedling species may ultimately provide a chance for rarer species to recruit.     

Density dependence is not very prevalent in a heterogeneous subtropical forest

There is a growing body of evidence demonstrating that tree survival is influenced by negative density‐dependence, but it is still controversial how the effect may vary with life‐stage, and to what extent it plays a role in regulating tree survival in heterogeneous subtropical forests. In this study, we investigated density‐dependent effects on tree survival of six tree species in a 5‐ha subtropical forest in eastern China. The roughly 45 000 individuals in the forest were fully censused in 2003 and 2008. For each of these species, we used an inhomogeneous pair‐correlation function to quantify the change in spatial distribution for different size classes, and a case‐control design to study seedling–adult associations in 2003. Autologistic regression was used to determine the influence of neighborhood factors on individual survival from 2003 to 2008. We found that seedlings of five species were repulsed by distance to nearest conspecific adults in terms of their survival, consistent with predictions of the Janzen–Connell mechanism. By contrast, only the least shade‐tolerant Schima superba had a negative relationship with individual survival and conspecific distance‐weighted basal area. This suggests that the Janzen–Connell effect is only prevalent at the early seedling stage or seed‐to‐seedling phase. The strength of clustering significantly declined at sapling–pole and pole–adult transitions for Sycopsis sinensis and at seedling–sapling transition for Cleyera pachyphylla. Correlations between individual survival and conspecific abundance for these species were consistent with trends in the strength of clustering. These results suggest that density dependence plays a limited role in individual survival and species spatial structure beyond the early seedling stage (i.e. after true leaves growing) in this forest. In addition, this study indicates that including individuals from early life‐stages and factoring out potential confounding factors such as habitat preference are important in studies that seek evidence for density dependence in forest trees.     

Leaf diseases drive the Janzen–Connell mechanism regardless of light conditions: a 3-year field study

In forests, negative density/distance-dependent seedling mortality (NDD) caused by natural enemies plays a key role in maintaining species diversity [Janzen–Connell (J–C) model]. However, the relative importance of natural enemies in mediating NDD under heterogeneous light conditions has remained unclear. We examined the relative importance of pathogens (i.e., soil pathogens, leaf diseases) on seedling performance in forest understories (FUs) and gaps (gaps) during a 3-year period (results of first year of our study have been previously reported). For the hardwood, Prunus grayana, we investigated seedling mortality, morbidity agents, growth, and root infection by arbuscular mycorrhizal fungi (AMF) beneath conspecific and heterospecific adults in FUs and gaps. Seedling mortality was higher beneath conspecific than heterospecific adults throughout 3 years at both sites, mainly due to continuous leaf disease (i.e., angular leaf spot), whereas damping-off diseases caused mortality only in the first year. Beneath each adult, seedling mortality was higher in FUs than in gaps until second year, but it did not differ between two habitat types in the third year, because leaf diseases caused severe damage even in gaps. Seedling mass was significantly lower beneath conspecific adults. AMF infection of seedlings was also lower beneath conspecific adults, while it was higher in gaps than in FUs beneath both adults. This study demonstrates that the J–C model in a hardwood tree, P. grayana is mainly driven by high NDD seedling mortality caused by airborne leaf diseases, which continuously attack seedlings in a NDD manner regardless of environmental light conditions.     

Tree seedling richness,but not neighborhood composition,influences insect herbivory in a temperate deciduous forest community

Insect herbivores can serve as important regulators of plant dynamics, but their impacts in temperate forest understories have received minimal attention at local scales. Here, we test several related hypotheses about the influence of plant neighborhood composition on insect leaf damage in southwestern Pennsylvania, USA. Using data on seedlings and adult trees sampled at 36 sites over an approximately 900 ha area, we tested for the effects of total plant density, rarefied species richness (i.e., resource concentration and dietary‐mixing hypotheses), conspecific density (i.e., Janzen–Connell hypothesis), and heterospecific density (i.e., herd‐immunity hypothesis), on the proportion of leaf tissue removed from 290 seedlings of 20 species. We also tested for the effects of generic‐ and familial‐level neighborhoods. Our results showed that the proportion of leaf tissue removed ranged from zero to just under 50% across individuals, but was generally quite low (<2%). Using linear mixed models, we found a significant negative relationship between insect damage and rarefied species richness, but no relationship with neighborhood density or composition. In addition, leaf damage had no significant effect on subsequent seedling growth or survival, likely due to the low levels of damage experienced by most individuals. Our results provide some support for the resource concentration hypothesis, but suggest a limited role for insect herbivores in driving local‐scale seedling dynamics in temperate forest understories.     

Spatial Patterns of Seed Dispersal by White-Faced Capuchins in Costa Rica: Evaluating Distant-Dependent Seed Mortality

Spatial patterns of seed dispersal are the focus of numerous theoretical examinations of endozoochory. Here, we examine the spatial pattern of seed dispersal by white-faced capuchin monkeys Cebus capucinus in Santa Rosa National Park, Costa Rica, and the neighborhood characteristics and distance variables most closely associated with seed survival and germination, and seedling survival and growth in various locations. Overall, distance to the nearest fruiting conspecific tree has the most positive, consistent effect on growth and survival variables, which supports a variation of the Janzen–Connell seed escape hypothesis.     

Distance- and density-dependent leaf dynamics of seedlings of a tropical rainforest tree

Parental distance and plant density dependence of seedling leaf turnover and survival was examined to investigate predictions of the Janzen–Connell hypothesis. The focal study species, Shorea macroptera is a canopy tree species in a lowland rain forest in peninsular Malaysia. We found that the peak of the distribution of plants shifted from 3–6 m to 6–9 m during the course of the change from seedling to sapling stage. The leaf demography of the seedlings was influenced by their distance from the adult tree and also by the seedling density. Although significant density- and distance dependence in leaf production was not detected, seedling leaf loss decreased with distance from the parent tree and with seedling density. Similarly, leaf damage was not found to be distance- or density-dependent, but net leaf gain of seedlings increased with distance from the parent tree. Although no significant distance- or density-dependence was evident in terms of leaf damage, significant distance dependence of the net leaf gain was found. Thus, we concluded that positive distance dependence in the leaf turnover of seedlings may gradually contribute to a shift in the distribution pattern of the progeny through reductions in growth and survivorship.     

Janzen–Connell effects partially supported in reef-building corals: adult presence interacts with settler density to limit establishment

In many plant and sessile marine invertebrate (SMI) taxa, population and community dynamics are heavily influenced by processes occurring during the dispersal and establishment phases. The Janzen–Connell (J–C) hypothesis predicts increased survival of early life stages with decreasing conspecific density and increased distance from conspecific adults. Evidence of J–C effects in maintaining diversity is common in plant communities, but its importance in SMI communities remains unclear. Under controlled aquarium conditions, we examined the effect of density-dependence and adult conspecific water treatments (absent/present) on propagule settlement success and settler post-settlement survival, along with associated spatial patterns, for six broadcast-spawning, reef-building coral species from three families. We also tested if settlement success was linked to increasing propagule species diversity for three coral species from two families. We found that the probability of settlement was density independent and not influenced by adult present water treatments. Yet, adult present water treatments and settler density did have a synergistic negative effect on the probability of short-term settler survival for all species examined. Settlers also showed greater spatial aggregation as their numbers increased, but were less aggregated in adult present water treatments compared to those in adult absent water treatments. We further show evidence of significant species interactions among propagules, as settlement in single-species trials was four-fold higher compared to mixed-species trials. Our findings from controlled experimental arenas indicate that the early establishment of corals was predominantly limited by density-dependent settler–adult interactions among conspecifics and propagule–propagule interactions among heterospecifics. Thus, the proximity to established conspecific adults, settler density and species diversity of propagules are relevant drivers of local coral community diversity and structure. Based on these outcomes, we suggest that the J–C hypothesis, with demonstrated importance for plants, is partially upheld for reef corals.     

Intraspecific and phylogenetic density-dependent seedling recruitment in a subtropical evergreen forest

Recent evidence suggests that plant performance can be influenced by the phylogenetic diversity of neighboring plants. However, no study to date has examined the effect of such phylogenetic density dependence on the transition from seed to seedling. Using 6 years of data on seedling recruitment and seed rain of 13 species from 130 stations (one 0.5 m² seed trap and three adjacent 1 m² seedling plots) in a subtropical evergreen forest, we asked: (1) Does negative density dependence act across seed to seedling stages? (2) Is there evidence for phylogenetic density dependence during the seed to seedling transition? (3) Does the strength of density dependence vary among years? Generalized linear mixed-effects models were used to model seed to seedling transition as a function of conspecific seed and seedling densities, heterospecific seed and seedling densities, and mean phylogenetic distance of heterospecific seeds and seedling. Conspecific seed density had a significant negative effect on seedling transition rates for 12 of 13 focal species. In contrast, conspecific seedling density had a positive effect for 7 species, suggesting species-specific habitat preferences. Few species were significantly affected by the density or phylogenetic relatedness of heterospecific seeds and seedlings. Only conspecific seed density effects varied among years for most focal species. Overall, our results reveal that conspecific seed and seedling densities play a more important role than the density or relatedness of heterospecific seeds and seedlings during the seed to seedling stage, suggesting that species-specific seed predators, along with habitat preferences, may contribute to diversity maintenance in this forest.     

Who is in the neighborhood? Conspecific and heterospecific responses to perceived density for breeding habitat selection

Theoretical models of habitat selection often incorporate negative density dependence. Despite strong negative density‐dependent effects on habitat selection, more recent studies indicate that animals settle near members of their own (conspecific) and other species (heterospecific) when selecting habitat with social cues. Social cue use for habitat selection is particularly common among songbirds, but few studies have investigated if songbirds use social cues to assess conspecific or heterospecific density (as opposed to just presence/absence) when making settlement decisions. We conducted a playback experiment to evaluate if yellow warblers (Setophaga petechia) and willow flycatchers (Empidonax traillii), two potential competitors for breeding habitat, use social cues to assess density (conspecific for warblers and heterospecific for flycatchers) when selecting breeding locations at two spatial scales. We simulated yellow warbler density to be high or low at multiple treatment plots (3.14 ha) with song playback and then evaluated settlement decisions by comparing yellow warbler and willow flycatcher abundances across plots (broad‐scale habitat selection) and individual space use within plots (fine‐scale territory establishment). Yellow warbler density treatments did not affect habitat selection by yellow warblers at the broad scale, but caused individuals to cluster territories at high‐density treatments. Willow flycatchers were most abundant at high‐density treatment plots, but yellow warbler density treatments did not affect territory locations. The results indicate that perceived density affects the habitat selection process for both conspecifics and heterospecifics.     

Neighbour-regulated mortality: the influence of positive and negative density dependence on tree populations in species-rich tropical forests

Density‐dependent mortality has long been posited as a possible mechanism for the regulation of tropical forest tree density. Despite numerous experimental and phenomenological investigations, the extent to which such mechanisms operate in tropical forests remains unresolved because the demographical signature of density dependence has rarely been found in extensive investigations of established trees. This study used an individual‐based demographical approach to investigate the role of conspecific and heterospecific neighbourhood crowding on tree mortality in a Panamanian and a Malayan tropical forest. More than 80% of the species investigated at each site were found to exhibit density‐dependent mortality. Furthermore, most of these species showed patterns of mortality consistent with the Janzen–Connell hypothesis and the rarely explored hypothesis of species herd protection. This study presents some of the first evidence of species herd protection operating in tree communities.     

Biochemical recognition in seeds: Germination of Rumex obtusifolius is promoted by leaves of facilitative adult conspecifics

Seeds express various germination behaviors in response to competitor plants. However, germination behaviors in response to facilitator plants are not yet well understood. Rumex obtusifolius seedlings usually appear on the ground near adult conspecific plants, and their survival rate under the canopy of adult conspecifics is higher than that outside the canopy, indicating that adult R. obtusifolius plants facilitate their seedling establishments. We hypothesized that emergence of R. obtusifolius seedlings is promoted by cues from adult conspecifics, but emergence of heterospecific seedlings is not. To test this, we investigated emergence responses of seedlings of R. obtusifolius and three other species that grow with R. obtusifolius in the presence of R. obtusifolius leaf phytochemicals. Emergence of R. obtusifolius seedlings was promoted by the presence of R. obtusifolius leaves. In contrast, emergence of other species seedlings was not promoted by R. obtusifolius leaves. We conclude that germination of R. obtusifolius seeds is facilitated in the presence of conspecifics, via water-soluble chemical exposure, and that recognizing these chemicals has adaptive value.     

Sexual differentiation and seasonal variation in response to conspecific and heterospecific acoustic signals

Interspecific territoriality is frequently reported between closely related species; however, few studies have demonstrated interspecific territoriality between distantly related species living in sympatry. We conducted playback experiments to investigate territorial behaviour in male and female White‐bellied Wrens (Uropsila leucogastra) in response to simulated conspecific and heterospecific intruders during the breeding and non‐breeding seasons. We explored whether heterospecific songs of the Happy Wren (Pheugopedius felix), a distantly related species and ecological competitor, elicited antagonistic responses from focal White‐bellied Wrens, and whether such responses differed between the sexes. We also examined whether male and female responses to conspecific and heterospecific rivals varied with season. We found that male White‐bellied Wrens always responded to conspecific song, and responded significantly more to heterospecific song compared to a control stimulus (Tropical Parula, Setophaga pitiayumi). In contrast, although female White‐bellied Wrens responded strongly to conspecific song, their response to heterospecific song did not differ significantly from the control stimulus. The proportion of males that responded to heterospecific songs and the proportion of females that responded to conspecific songs varied seasonally, showing significantly lower responses during the breeding season. The intense responses of male White‐bellied Wrens to playback of heterospecific songs suggest that they recognise ecological competitors based on their vocal signals. Furthermore, the decrease in agonistic interactions during the breeding season is in line with the hypothesis that aggressive behaviour may be detrimental to reproductive and parental activity, and the hypothesis that heterospecific animals pose less of a threat during the breeding season.     

Dispersal and recruitment limitation in native versus exotic tree species: life‐history strategies and Janzen‐Connell effects

Life‐history traits of invasive exotic plants are typically considered to be exceptional vis‐à‐vis native species. In particular, hyper‐fecundity and long range dispersal are regarded as invasive traits, but direct comparisons with native species are needed to identify the life‐history stages behind invasiveness. Until recently, this task was particularly problematic in forests as tree fecundity and dispersal were difficult to characterize in closed stands. We used inverse modelling to parameterize fecundity, seed dispersal and seedling dispersion functions for two exotic and eight native tree species in closed‐canopy forests in Connecticut, USA. Interannual variation in seed production was dramatic for all species, with complete seed crop failures in at least one year for six native species. However, the average per capita seed production of the exotic Ailanthus altissima was extraordinary: > 40 times higher than the next highest species. Seed production of the shade tolerant exotic Acer platanoides was average, but much higher than the native shade tolerant species, and the density of its established seedlings (≥ 3 years) was higher than any other species. Overall, the data supported a model in which adults of native and exotic species must reach a minimum size before seed production occurred. Once reached, the relationship between tree diameter and seed production was fairly flat for seven species, including both exotics. Seed dispersal was highly localized and usually showed a steep decline with increasing distance from parent trees: only Ailanthus altissima and Fraxinus americana had mean dispersal distances > 10 m. Janzen‐Connell patterns were clearly evident for both native and exotic species, as the mode and mean dispersion distance of seedlings were further from potential parent trees than seeds. The comparable intensity of Janzen‐Connell effects between native and exotic species suggests that the enemy escape hypothesis alone cannot explain the invasiveness of these exotics. Our study confirms the general importance of colonization processes in invasions, yet demonstrates how invasiveness can occur via divergent colonization strategies. Dispersal limitation of Acer platanoides and recruitment limitation of Ailanthus altissima will likely constitute some limit on their invasiveness in closed‐canopy forests.     

Invertebrate Seed Predators are not all the Same: Seed Predation by Bruchine and Scolytine Beetles Affects Palm Recruitment in Different Ways

Because most invertebrate seed predators are host‐specific, they are usually expected to produce Janzen–Connell patterns. This expectation was fulfilled for Astrocaryum but not for Allagoptera, depending on the effects of bruchine and scolytine predators on the seeds of these palms. Thus, the mere existence of invertebrate predation is not sufficient for generating Janzen–Connell; what matters is seed mortality, which varies between predators and between plant species for the same predator.     

Enemies maintain hyperdiverse tropical forests

Understanding tropical forest tree diversity has been a major challenge to ecologists. In the absence of compensatory mechanisms, two powerful forces, drift and competition, are expected to erode diversity quickly, especially in communities containing scores or hundreds of rare species. Here, I review evidence bearing on four compensatory mechanisms that have been subsumed under the terms "density dependence" or "negative density dependence": (1) intra- and (2) interspecific competition and the action of (3) density-responsive and (4) distance-responsive biotic agents, as postulated by Janzen and Connell. To achieve ontological integration, I examine evidence based on studies employing seeds, seedlings, and saplings. Available evidence points overwhelmingly to the action of both host-generalist and host-restricted biotic agents as causing most seed and seedling mortality, implying that species diversity is maintained via top-down forcing. The overall effect of most host-generalist seed predators and herbivores is to even out the distribution of surviving propagules. Spatially restricted recruitment appears to result mainly, if not exclusively, from the actions of host-restricted agents, principally microarthropods and fungi, that attack hosts in a distance-dependent fashion as Janzen and Connell proposed. Near total failure of propagules close to reproductive conspecifics ensures that successful reproduction occurs through a scant rain of dispersed seeds. Densities of dispersed seeds and seedlings arising from them are so low as to generally preclude the operation of density dependence, at least during early ontogenetic stages. I conclude that Janzen and Connell were essentially correct and that diversity maintenance results from top-down forcing acting in a spatially nonuniform fashion.