Conspecific Negative Density Dependence Research Articles

Conspecific negative density dependence in rainy season enhanced seedling diversity across habitats in a tropical forest.

Conspecific negative density dependence (CNDD) could be one of the most important local-scale mechanisms shaping plant species coexistence. However, the spatial and temporal changes in the strength CNDD and the implications for the plant diversity remain unknown. We used 10years of seedling data, in a seasonal tropical rainforest, to discover how CNDD influences tree seedling survival across habitats and seasons. We also evaluated the relation between CNDD and species diversity. We found the strength of CNDD in the valley habitat was significantly stronger than in ridge habitat in rainy season, but not significantly different in dry season. Corresponding to expectations of CNDD as mechanism of diversity maintenance, seedling species diversity was significantly higher in valley habitat than in ridge habitat and significantly correlated with CNDD. Additionally, conspecific and heterospecific seedling neighbour densities positively affected the survival of tree seedlings, but heterospecific adult neighbour density had a weak effect. Our study finds that CNDD varied significantly across habitats and was correlated with local seedling diversity. Our results highlight the importance of CNDD in driving species diversity at the local scale. Recognizing the spatial and temporal variation in the strength of CNDD will aid efforts to model and understand species coexistence.

Positive density dependence acting on mortality can help maintain species-rich communities.

Conspecific negative density dependence is ubiquitous and has long been recognized as an important factor favoring the coexistence of competing species at local scale. By contrast, a positive density-dependent growth rate is thought to favor species exclusion by inhibiting the growth of less competitive species. Yet, such conspecific positive density dependence often reduces extrinsic mortality (e.g. reduced predation), which favors species exclusion in the first place. Here, using a combination of analytical derivations and numerical simulations, I show that this form of positive density dependence can favor the existence of equilibrium points characterized by species coexistence. Those equilibria are not globally stable, but allow the maintenance of species-rich communities in multispecies simulations. Therefore, conspecific positive density dependence does not necessarily favor species exclusion. On the contrary, some forms of conspecific positive density dependence may even help maintain species richness in natural communities. These results should stimulate further investigations into the precise mechanisms underlying density dependence.

Trait-mediated neighbor effects on plant survival depend on life stages and stage-specific traits in a temperate forest

Among the multiple ecological processes that determine species coexistence and community dynamics, conspecific negative density dependence (CNDD) and niche partitioning are perhaps among the most important. However, the relative strength of these mechanisms may change across life stages and depend on species ecological strategies. We used a dataset that includes information on stage-specific functional traits and a long-term, spatially explicit data of plant survival including 7971 individuals and 41 species monitored for seedling and adult life stages to examine the relative role of these mechanisms influencing individual survival across ontogenetic stages in a 9-ha temperate forest plot in China. Our results showed that for seedlings, CNDD became weaker for species with larger leaf area, while for adults, the conspecific neighbors had a positive effect on survival and this effect became stronger with decreasing seed mass. Also, the results examing the effects of neighboring heterospecifics suggest a prevalence of niche partitioning at the seedling stage but a prevalence of environmental filtering or hierarchical competition for the adult stage. In particular, the strength of these processes at the adult life stage increased with higher maximum height and smaller specific leaf area. Finally, we found that the effect of traits on individual survival for seedling and adult life stages was dependent on both the prevalence of conspecifics and functional diversity of heterospecifics in the neighborhood. Overall, our results demonstrates that ecological processes tend to have a stronger diversifying effect for seedlings than for adults and these effects depend on stage-specific species traits. Considering trait variation and their interacting effects with biotic factors across life stages provides further insights into ecological mechanisms shaping plant communities and driving its dynamics.

Tree mycorrhizal type mediates the strength of negative density dependence in temperate forests

Abstract Recent plant–soil feedback experiments suggest that arbuscular mycorrhizal (AM) tree species experience stronger conspecific negative density dependence (CNDD) than ectomycorrhizal (EM) tree species. Yet, how these findings inform our understanding of natural systems is limited because the roles of local soil conditions, light availability and tree species abundances in influencing CNDD for AM and EM species are not clear. Here we examined seedling and sapling survival in two temperate old‐growth forests (broad‐leaved pine and spruce‐fir forests) in Northeast China, to evaluate the effects of both conspecific and heterospecific neighbour density, as well as the soil and light environments, on the survival of AM‐ and EM‐dependent trees at early life stages. While light availability increased the survival of EM seedlings, soil organic resources increased EM sapling survival in the spruce‐fir plot. AM tree species suffered stronger CNDD than did EM tree species in both plots. In the spruce‐fir plot, soil factors and light availability mediated species CNDD but their effects differed between AM and EM species, and also between seedlings and saplings. For seedlings in both plots, we found that AM species exhibited a positive relationship between species abundance and CNDD strength, whereas this relationship was negative for EM species. Synthesis. Our results provide one of the few tests of how fungal symbioses determine species responses to intraspecific and interspecific interactions and the direct effects of local environmental conditions on seedling and sapling survival. We show that mycorrhizal type mediates the strength of CNDD and its relationship with species abundance. These results suggest that tree mycorrhizal association can determine the strength of CNDD effect on both rare and common species, and these CNDD differences are likely to influence the community composition of temperate forests.

The asymmetric relationships of the distribution of conspecific saplings and adults in forest fragments

Abstract Aims With the increase of land-use change, habitat fragmentation has become a major factor affecting plant diversity. Generally, when the adult density is high, the survival rate of conspecific saplings may decline and provide more resources and space for other species to maintain a high local species diversity. Therefore, the spatial relationship of conspecific saplings and adults (SRCSA) can regulate plant diversity. However, very few studies have assessed SRCSA within fragmented forests, and we still largely unknown the relationship between SRCSA and species diversity in fragmented landscapes. Methods We calculated the mean strength of SRCSA using the spatial distribution data of saplings and adults with a log-transformed hierarchical offset-power model on 27 islands in a land-bridge island system. The higher strength of SRCSA reflects a more negative relationship between the density of conspecific saplings and adults on an island (i.e. the looser pattern of SRCSA). We tested the relationships among island attributes (i.e. island area, the distance to the mainland—MD, and the distance to the nearest island—ND), SRCSA and species diversity. Important Findings The strength of SRCSA increased with ND. Meanwhile, the species diversity significantly increased with the strength of SRCSA, and island area and the strength of SRCSA independently explained 26% and 6% of variation of species diversity, respectively, and their interactions explained 8%. Shade-intolerant and low-abundant species showed looser patterns of SRCSA. Our study suggests that SRCSA may have the effect of excluding species with a relatively low abundance in isolated island forests, which illustrates the importance of biotic interactions in maintaining plant diversity in fragmented forests. Our results also emphasize that we should consider forest connectivity when testing the conspecific negative density dependence.

Variations of density-dependent seedling survival in a temperate forest

Seedling dynamics are driven by various biotic factors (for example, neighborhood interactions) and abiotic conditions (for example, light availability, topography, and soil nutrients). Therefore, quantifying the drivers of seedling survival is critical for understanding seedling dynamics within plant communities. However, how the importance of certain biotic and abiotic factors changes over growth stages is still relatively unknown. In the present study, the importance of density and distance dependence were examined, as well as the effects of abiotic factors, in regard to the survival of both newly germinated and established seedlings and saplings (<1 cm DBH; taller than 30 cm in height; and at least 2 years of age). A total of 1305 seedlings belonging to 21 species of a temperate forest in northeastern China were monitored. It was found there were significant conspecific negative density dependence (CNDD) effects impacting the survival of the established seedlings. In addition, the newly germinated seedlings were found to be less affected by the CNDD. The abiotic effects on seedling survival were not found to be significant. These results suggested that at the community level, the density dependence effects may be stronger during the seedling stage when compared with the abiotic factors. The distance dependence was observed to be mainly effective within a radius of approximately 5 m for both the newly germinated and established seedling communities. The directions and magnitudes of the aforementioned factors were affected by different life-history strategies. The seedlings which were wind-dispersed and the shade-tolerant species tended to suffer less from the CNDD. Therefore, based on an unusually large set of observations in a temperate forest, this study identified and analyzed the factors affecting seedling survival for different life-history strategies. The obtained results will potentially provide new perspectives for deepening the understanding of the dynamics of temperate forests.

Scale‐dependent effects of conspecific negative density dependence and immigration on biodiversity maintenance

Conspecific negative density dependence (CNDD) has been highlighted as a main driver of biodiversity maintenance. However, while there is general consensus on the scale‐dependent and interacting nature of ecological processes, there is limited knowledge about the relative importance of CNDD across spatial scales and on its interaction with other processes, such as dispersal and immigration. While many studies have detected CNDD at local scales, it remains unknown whether its effects on biodiversity maintenance scale‐up to landscapes and regions. Here, we use a generic dynamic and spatially‐explicit simulation model to assess the interacting relative effects of local‐scale CNDD and immigration from a metacommunity for biodiversity maintenance across spatial scales. For this purpose, we systematically varied immigration rates, the average strength and the variation of CNDD among species. We found that CNDD only determined species richness in strongly isolated communities with little or no interspecific variation in CNDD. In closed communities, plausible interspecific variation in CNDD led to a strong reduction in species richness. In open communities, realistic levels of immigration overwhelmed the effects of CNDD on diversity maintenance. From these results, we suggest that local CNDD is unlikely to be a main driver of biodiversity in real communities, especially at larger spatial scales. This study provides a first step towards improved integration of local‐scale coexistence theory with large‐scale metacommunity theory and highlights the importance of considering the interacting and scale‐dependent nature of ecological processes.

Abiotic niche partitioning and negative density dependence across multiple life stages in a temperate forest in northeastern China

Abstract Negative density dependence (NDD) and niche partitioning have been proposed to explain the species coexistence in plant communities. However, the majority of previous studies have examined either only resource partitioning or NDD, or have focused only on a single life‐history stage. We thus lack knowledge regarding the relative importance of biotic neighbourhood interactions and abiotic conditions on survival at the various life stages of a tree. Using tree and seedling demographics data, and combined with spatially explicit abiotic and biotic data, we assessed the relative importance of abiotic and biotic variables for survival from the seedling to tree stage in a temperate forest in northeastern China. We provide strong evidence that both biotic and abiotic neighbourhood variables affect individual survival. We also observed that their relative importance varied with the life‐history stages of a tree. The strength of the effect of conspecific tree neighbours decreases along ontogeny, while the roles of the topographic and edaphic variables increase at later life stages. Heterospecific tree neighbours show consistent negative effects on individual survival at all life stages, yet had the strongest influence on survival at the seedling stage. We demonstrated that species vary widely in their probability of survival under different environmental conditions, as well as in response to differences in neighbour identity and density. Overall, the effects of conspecific neighbours varied more among species than the effects of heterospecific neighbour density. Wide variation among species in response to abiotic variables is a necessary requirement for niche differentiation and coexistence via resource partitioning processes. Our findings indicate that conspecific NDD and abiotic niche partitioning are major structuring mechanisms in our temperate forest in northeastern China. Synthesis. Both biotic and abiotic neighbourhood variables affected individual survival, but the relative importance varied with the life‐history stages and among species. Our results indicated that niche partitioning may contribute to the coexistence through differential responses to abiotic factors, as well as to conspecific negative density dependence (NDD). Finally, our results demonstrated the necessity to incorporate multiple life stages and multiple influence factors when elucidating the maintenance of species diversity in temperate forests.

Tree species traits affect which natural enemies drive the Janzen-Connell effect in a temperate forest

A prominent tree species coexistence mechanism suggests host-specific natural enemies inhibit seedling recruitment at high conspecific density (negative conspecific density dependence). Natural-enemy-mediated conspecific density dependence affects numerous tree populations, but its strength varies substantially among species. Understanding how conspecific density dependence varies with species’ traits and influences the dynamics of whole communities remains a challenge. Using a three-year manipulative community-scale experiment in a temperate forest, we show that plant-associated fungi, and to a lesser extent insect herbivores, reduce seedling recruitment and survival at high adult conspecific density. Plant-associated fungi are primarily responsible for reducing seedling recruitment near conspecific adults in ectomycorrhizal and shade-tolerant species. Insects, in contrast, primarily inhibit seedling recruitment of shade-intolerant species near conspecific adults. Our results suggest that natural enemies drive conspecific density dependence in this temperate forest and that which natural enemies are responsible depends on the mycorrhizal association and shade tolerance of tree species.

Differential soil fungus accumulation and density dependence of trees in a subtropical forest.

The mechanisms underlying interspecific variation in conspecific negative density dependence (CNDD) are poorly understood. Using a multilevel modeling approach, we combined long-term seedling demographic data from a subtropical forest plot with soil fungal community data by means of DNA sequencing to address the feedback of various guilds of soil fungi on the density dependence of trees. We show that mycorrhizal type mediates tree neighborhood interactions at the community level, and much of the interspecific variation in CNDD is explained by how tree species differ in their fungal density accumulation rates as they grow. Species with higher accumulation rates of pathogenic fungi suffered more from CNDD, whereas species with lower CNDD had higher accumulation rates of ectomycorrhizal fungi, suggesting that mutualistic and pathogenic fungi play important but opposing roles.

Fungal influence on density dependence

Forest Ecology Tree species in highly diverse tropical forests tend to exhibit conspecific negative density dependence, a phenomenon whereby individuals of the same species tend to grow at a distance from one another. This is understood to be a key driver of species coexistence. The strength of negative density dependence varies between species, but the mechanisms driving this variation are unknown. Chen et al. studied tree species in a subtropical forest in China and found an important role for soil-dwelling fungi in this variation. Elevated accumulation of pathogenic fungi leads to stronger negative density dependence, whereas elevated accumulation of mutualistic fungi leads to weaker negative density dependence. Science , this issue p. [124][1] [1]: /lookup/doi/10.1126/science.aau1361

Bias in the detection of negative density dependence in plant communities.

Regression dilution is a statistical inference bias that causes underestimation of the strength of dependency between two variables when the predictors are error-prone proxies (EPPs). EPPs are widely used in plant community studies focused on negative density-dependence (NDD) to quantify competitive interactions. Because of the nature of the bias, conspecific NDD is often overestimated in recruitment analyses, and in some cases, can be erroneously detected when absent. In contrast, for survival analyses, EPPs typically cause NDD to be underestimated, but underestimation is more severe for abundant species and for heterospecific effects, thereby generating spurious negative relationships between the strength of NDD and the abundances of con- and heterospecifics. This can explain why many studies observed rare species to suffer more severely from conspecific NDD, and heterospecific effects to be disproportionally smaller than conspecific effects. In general, such species-dependent bias is often related to traits associated with likely mechanisms of NDD, which creates false patterns and complicates the ecological interpretation of the analyses. Classic examples taken from literature and simulations demonstrate that this bias has been pervasive, which calls into question the emerging paradigm that intraspecific competition has been demonstrated by direct field measurements to be generally stronger than interspecific competition.

Shade tolerance and mycorrhizal type may influence sapling susceptibility to conspecific negative density dependence

Abstract The maintenance of tree diversity has been explained by multiple mechanisms. One of the most thoroughly studied is conspecific negative density dependence, in which specialist plant enemies reduce survivorship of seeds, seedlings or saplings located near adult conspecifics. Although there is much support that conspecific negative density dependence occurs in temperate forests, only a subset of the species investigated thus far exhibit this recruitment pattern. It remains unclear what drives differential susceptibility to conspecifics among tree species. Previous investigators have considered shade tolerance and mycorrhizal type (arbuscular mycorrhizal vs. ectomycorrhizal association) as two traits that might explain differential susceptibility to conspecific negative density dependence. Here, we test whether these two plant traits predict susceptibility of tree saplings to conspecific negative density dependence in a temperate hardwood forest using three responses: spatial point patterns of saplings, sapling growth and sapling survival. Spatial patterns of saplings indicate that shade tolerant species are less sensitive to conspecifics than shade intolerant species, but show no differences based on mycorrhizal type. Conversely, shade tolerant saplings exhibit reduced growth, but not survival, when located in areas with high conspecific density. We interpret this finding in light of the conservative functional strategies of shade tolerant species, which typically have low leaf nitrogen levels and slower growth to divert resources to tissue defence against enemies. We found an effect of mycorrhizal type interacting with adult conspecific density, where arbuscular mycorrhizal species show a greater reduction in growth than ectomycorrhizal species in areas dense with conspecifics. Synthesis. We conclude that the shade tolerance level and the mycorrhizal type of temperate forest saplings may influence how their growth and survival respond to the adult conspecific trees in their neighbourhoods.

Incorporating the disease triangle framework for testing the effect of soil‐borne pathogens on tree species diversity

Abstract The enemy‐induced Janzen–Connell (JC) effect, a classic model invoking conspecific negative density dependence (CNDD) and distance dependence, is a primary biodiversity maintenance hypothesis. Yet, conflicting evidence for the JC effect leads to disagreement about its role in maintaining forest diversity. We focus this review on soil‐borne pathogens, which are the primary agent inducing the JC effect in many forest ecosystems. Although the test of the pathogen‐induced JC effect in ecology critically rests on the seedling mortality caused by soil pathogens, what has not been explicitly explored in the early literature but has increasingly received attention is the long‐recognized fact that the environment can alter virulence of pathogens and host susceptibility (thus pathogen–host interactions), as predicted by the classic disease triangle framework enlightened by pathology research in agricultural systems. Here, following the disease triangle framework we review evidence on how the pathogen‐induced JC effect may be contingent on context (e.g. environmental conditions, pathogen inoculum load and genetic divergence in host and pathogen populations). The reviewed evidence reveals and clarifies the conditions where pathogens may or may not cause disease to hosts, thus contributing to reconciling the inconsistent results about the pathogen‐induced JC effect in the literature. The context dependence of the disease triangle predicts that the pathogen‐induced JC effect would change under global change. Gaining insights from evidence that the pathogen‐induced JC effect is context‐dependent, we suggest that future tests on the JC hypothesis be conducted under the framework of disease triangle, and we stress the necessity by controlling the effect of context factors on plant–pathogen interactions when testing for the JC effect. We conclude the review by proposing three lines of future research for testing the importance of the JC effect in maintaining global forest tree species diversity, with a particular emphasis on testing the effect of global warming on the strength of pathogen–host interactions for better predicting changes of forest biodiversity under climate change. A plain language summary is available for this article.

Physical, but not chemical, antiherbivore defense expression is related to the clustered spatial distribution of tropical trees in an Amazonian forest.

The conspecific negative density dependence hypothesis states that mortality of young trees (seedlings and saplings) is higher near conspecific adults due to mechanisms such as allelopathy, intraspecific competition, and pest facilitation, explaining why in the tropics, most of plant species tend to be rare and live dispersed. However, there are some tree species that defy this expectation and grow in large clusters of conspecific juveniles and adults. We hypothesize that conspecifics living in clusters show higher and/or more variable defensive profiles than conspecifics with dispersed distributions.We evaluated our hypothesis by assessing the expression of physical leaf traits (thickness, and the resistance to punch, tear and shear) and leaf chemical defenses for six clustered and six non‐clustered tree species in Yasuní National Park, Ecuadorian Amazon. We ask ourselves whether (a) clustered species have leaves with higher physical resistance to damage and more chemical defenses variability than non‐clustered species; (b) saplings of clustered species may show higher physical resistance to damage and higher variation on chemical leaf defenses than their conspecific adults, and (c) saplings of non‐clustered species show lower resistance to physical damage and lower variation in chemical defenses compared to conspecific adults.Overall, our study did not support any of our hypotheses. Remarkably, we found that soluble metabolites were significantly species‐specific.Our study suggests that plants physical but not chemical leaf antiherbivore defenses may be a crucial strategy for explaining survivorship of clustered species. Trees in Yasuní may also fall along a suite of tolerance/escape/defense strategies based on limitations of each species physiological constraints for survival and establishment. We conclude that other mechanisms, such as those related to indirect defenses, soil nutrient exploitation efficiency, volatile organic compounds, delayed leaf‐greening, and seed dispersal mechanisms, shall be evaluated to understand conspecific coexistence in this forest.

Interspecific variation in conspecific negative density dependence can make species less likely to coexist.

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

The strength of density‐dependent mortality is contingent on climate and seedling size

AbstractQuestionsDensity‐dependent processes may promote species diversity in plant communities. Here, we tested whether seedling survival was density‐dependent and varied by seedling size, species and climatic factors.LocationTropical rain forest, Xishuangbanna, southwest China.MethodsGeneralized linear mixed‐effects models were used to examine seedling survival (232 tree species) across 9 years of seedling census data from a 20‐ha tropical forest dynamics plot. Our predictor variables were conspecific and heterospecific neighbour density, size of the seedling and annual variation in climatic factors.ResultsWe found significant negative effects of conspecific tree density, but positive effects of heterospecific seedling density on the survival of tree seedlings in this plot. In general, conspecific negative density dependence (CNDD) was observed most frequently for large size classes of seedlings (≥20‐cm high), while heterospecific positive density dependence (HPDD) was similar at all size classes. CNDD for large seedlings was stronger during warm years, and HPDD for large seedlings was stronger during dry years.ConclusionsOur study suggests that the strength of density dependence varied through time, and this strength was influenced by water availability and temperature. Our results highlight the potential for changes in species composition and species co‐existence that could result from increasing temperature‐strengthening CNDD effects and decreasing precipitation strengthening HPDD effects.

Large trees are surrounded by more heterospecific neighboring trees in Korean pine broad-leaved natural forests

Negative conspecific density dependence is one of the principal mechanisms affecting plant performance and community spatial patterns. Although many studies identified the prevalence of density dependent effects in various vegetation types by analyzing conspecific spatial dispersal patterns (spatial patterning) of forest trees, interactions between individuals and heterospecific neighboring trees caused by density-dependent effects are often neglected. The effects of negative density dependence lead us to expect that neighbourhood species segregation would increase with increasing tree size and that larger trees would be surrounded by more heterospecific neighbours than would smaller trees. We studied four mapped 1-Ha plots on Changbaishan Mountain in North-eastern China and used marked point pattern analysis to explore whether trees of different sizes exhibited differences in neighbourhood species segregation; we also determined whether larger trees were more likely to have heterospecific neighbours than smaller trees were. Our results show that bigger trees generally have higher species mingling levels. Neighborhood species segregation ranged from lower than expected levels to random or nearly random patterns at small scales as tree size classes increased under heterogeneous Poisson null model tests. This study provides some evidence in support of negative density dependent effects in temperate forests.

Response to Comment on "Plant diversity increases with the strength of negative density dependence at the global scale".

Chisholm and Fung claim that our method of estimating conspecific negative density dependence (CNDD) in recruitment is systematically biased, and present an alternative method that shows no latitudinal pattern in CNDD. We demonstrate that their approach produces strongly biased estimates of CNDD, explaining why they do not detect a latitudinal pattern. We also address their methodological concerns using an alternative distance-weighted approach, which supports our original findings of a latitudinal gradient in CNDD and a latitudinal shift in the relationship between CNDD and species abundance.

Response to Comment on "Plant diversity increases with the strength of negative density dependence at the global scale".

Hülsmann and Hartig suggest that ecological mechanisms other than specialized natural enemies or intraspecific competition contribute to our estimates of conspecific negative density dependence (CNDD). To address their concern, we show that our results are not the result of a methodological artifact and present a null-model analysis that demonstrates that our original findings-(i) stronger CNDD at tropical relative to temperate latitudes and (ii) a latitudinal shift in the relationship between CNDD and species abundance-persist even after controlling for other processes that might influence spatial relationships between adults and recruits.