Increased mortality with increasing conspecific density is expected to be driven by species-specific predators and pathogens in the vicinity of conspecifics. This negative density dependence (NDD) may to contribute to high biodiversity in tropical forests by preventing any one species from becoming too common. For NDD to contribute to coexistence it must increase the overall mortality rate of common species relative to rare species (i.e., a community compensatory trend, CCT). Previous research suggests that NDD is stronger in low-light environments where pathogen mortality is generally higher and weaker where drought stress, and thus abiotic habitat filtering, is higher. This is the first study to analyze NDD for seedlings in critically endangered tropical dry forest, to examine the interaction between NDD and understory irradiance, and to compare NDD between wet and dry forests. Using generalized linear mixed models, we analyzed seedling mortality as a function of adult con- and heterospecific density, irradiance, and overall abundance for 3490 seedlings of 19 species in two 4-ha Forest Dynamics Plots located on Hawaii Island. We predicted that density-dependent seedling mortality would be lower in dry than wet forest, decrease in high light, and increase mortality for common species.
Results/Conclusions
Overall, density dependence was quite variable among forests and interacted with resource availability and species' abundance. Contrary to predictions, adult conspecifics had nearly10-fold stronger effects on seedling mortality in dry than wet forest, and these effects were positive rather than negative. Consistent with our expectations, irradiance had a positive interaction with adult conspecifics (though only significantly for wet forest), with the highest survival at high conspecific density and in high irradiance. We found a whole-plot-level CCT with lower survival for species with high adult abundance (stems/ha). However, this trend was contradicted by higher survival for species with high basal area. Indeed, less-common species were negatively affected by adult conspecific density while common species were positively affected. Our results suggest that for common species density-dependent mortality was less important than other mechanisms such as facilitation and/or habitat filtering. However, for less-common species NDD may strongly regulate recruitment, especially in stressful environments (e.g., low water and low-irradiance) potentially contributing to species rarity. In combination with environmental heterogeneity, density dependence appears to supply abundant opportunities for species coexistence.