Conspecific Density Dependence Research Articles

Seedling recruitment patterns in a 20 ha subtropical forest plot: hints for niche-based processes and negative density dependence

Seedling recruitment can be influenced by seed dispersal, conspecific density dependence, and environmental factors. These forces are variant in space. In this study, seedling recruitment was investigated by the inverse modeling method. The inverse modeling framework here was made up of two components: a conspecific effect and a declining function. Power functions (P) and constant conspecific (C) effects were tried. Two types of declining functions were tried: isotropic (I) and anisotropic (A). Thus, the combination of conspecific effect and declining function generated four candidate models: PI, PA, CI, CA. These four models were used to study the seedling recruitment of 13 species in a 20 ha forest plot in subtropical China. It was found that PI, PA, CI, CA are the best models for two, three, five, and three species, respectively. Negative exponents in P were found in three species, which may indicate negative density-dependent mortality. Among those species that supported an anisotropic component, all moderately shade-tolerant and shade-tolerant species except Calophyllum membranaceum had higher possibilities of successful recruitment if their altitudes were relatively low, consistent with their ecological niches. The shade intolerant species, Castanopsis fissa produces seeds weighing 6–250 times more than other species. Yet, its seedling recruitment was more successful at higher altitudes, which again was consistent with its ecological niche. Our research indicated that it is necessary to take anisotropic forces into account when investigating seed dispersal and seedling recruitment in regions with complex topography, and that the niche-based processes and density-dependent mortality at least play some part in constructing the seedling distribution pattern.

Conspecific density dependence in seedlings varies with species shade tolerance in a wet tropical forest

Density-dependent seedling mortality could increase with a species relative abundance, thereby promoting species coexistence. Differences among species in light-dependent mortality also could enhance coexistence via resource partitioning. These compatible ideas rarely have been considered simultaneously. We developed models of mortality as functions of irradiance and local conspecific density (LCD) for seedlings of 53 tropical woody species. Species varied in mortality responses to these factors, but mortality consistently increased with shading and LCD. Across species, density-dependent mortality on a per-neighbour basis was inversely related to species community abundance, but higher LCD in more common species resulted in a weak relationship between species abundance and density-dependent mortality scaled to species maximum LCD. Species mortality responses to shading and maximum LCD were strongly and positively correlated. Our results suggest that species differences in density-dependent mortality are more strongly related to physiologically based life-history traits than biotic feedbacks related to community abundance.

A theoretical model linking interspecific variation in density dependence to species abundances

Understanding the factors that govern the commonness and rarity of individual species is a central challenge in community ecology. Empirical studies have often found that abundance is related to traits associated with competitive ability and suitability to the local environment and, more recently, also to negative conspecific density dependence. Here, we construct a theoretical framework to show how a species’ abundance is, in general, expected to be dependent on its per-capita growth rate when rare and the rate at which its growth rate declines with increasing abundance (strength of stabilization). We argue that per-capita growth rate when rare can be interpreted as competitive ability and that strength of stabilization largely reflects negative conspecific inhibition. We then analyze a simple spatially implicit model in which each species is defined by three parameters that affect its juvenile survival: its generalized competitive effect on others, its generalized response to competition, and an additional negative effect on conspecifics. This model facilitates the stable coexistence of an arbitrarily large number of species and qualitatively reproduces empirical relationships between abundance, competitive ability, and negative conspecific density dependence. Our results provide theoretical support for the combined roles of competitive ability and negative density dependence in the determination of species abundances in real ecosystems, and suggest new avenues of research for understanding abundance in models and in real communities.

Density dependence is prevalent in a heterogeneous subtropical forest

Although negative conspecific density dependence among neighbours is widely studied, the general prevalence of the effects is still poorly understood due to a lack of studies from zonal forests other than the tropics. In addition, the detection of density dependence may be confounded by the influence of habitat heterogeneity. Here we examined the spatial distributions of 47 common tree species (diameter at breast height≥1 cm) using the pair-correlation function g(r) in a fully mapped 24-ha subtropical forest in China. We first investigated whether habitat heterogeneity influenced tree distributions, and then examined the conspecific tree patterns and density dependence after removing the effects of habitat heterogeneity. We found that the forest plot exhibited strong large-scale heterogeneity in the distribution of both large adult trees of different growth forms and individual species. After the habitat heterogeneity was accounted for, 39 of the 47 species (83.0%) were found to exhibit density dependence predominantly at close distances among neighbors. Our findings highlight density dependence as a prevalent mechanism for regulating the population spatial structure of most tree species in the species-rich subtropical forest studied here. Furthermore, the occurrence of density dependence is closely associated with species abundance and the strength of conspecific aggregation at local scales. Abundant species with high strength of conspecific aggregation tend to show density dependence.

Seedling recruitment in a hurricane‐driven tropical forest: light limitation, density‐dependence and the spatial distribution of parent trees

Summary We used inverse modelling to parameterize spatially‐explicit seedling recruitment functions for nine canopy tree species in the Luquillo Forest Dynamics Plot (LFDP), Puerto Rico. We modelled the observed spatial variation in seedling recruitment following Hurricane Georges as a function of the potential number of seedlings at a given location (based on local source trees and the potential contribution of parents from outside of the mapped area) and of light levels and density‐dependent mortality during establishment. We adopted the model comparison paradigm and compared the performance of increasingly complex models against a null model that assumes uniform seedling distribution across the plot. Our data supported a model in which parents must reach a threshold size before any seedling production will occur. Once parents attain that size, the relationship between tree diameter and the number of seedlings produced is fairly flat for the majority of species. These results contradict previous analyses that simply assumed a linear relationship between biomass and seedling production and a uniform size threshold for seedling production across species. The majority of species tested supported models that included at least one of a bath term (contribution from non‐local trees), conspecific density dependence and light availability after the hurricane. Density dependence shifted the mode of the effective dispersal kernel away from potential parent trees and significantly reduced the number of seedlings established near parent trees. Recruitment from non‐local sources accounted for 6–81% of observed seedling recruitment depending upon the tree species. Light availability appeared to divide species into three groups that showed more successful seedling establishment at low (< 5% of full sun), intermediate or high light levels (> 30% full sun). Differences between individual species in the importance of local vs. bath recruitment and the intensity of density dependence suggest the existence of distinct recruitment syndromes that go beyond the traditional focus of tropical tree life histories. Understanding these syndromes will provide valuable insights into the spatial distribution of tropical tree species and the maintenance of tropical forest diversity.