Stochastic dilution effects weaken deterministic effects of niche-based processes in species rich forests
The mechanism in maintaining biodiversity is one of the hot spots in ecological research. There has been big controversy. The traditional niche theory comes from regions with less rich species and stresses on the effect of deterministic processes such as species interrelations and the environment heterogeneity in maintaining biodiversity. The theory cannot explain the distribution patterns of bio-diversity in regions with rich species and was challenged by a new neutral theory that stressed on the effect of stochastic process. The new theory assumed that there are no significant relations between species. However, this seems contradictory to the observations that in temperate forest and tropical rain forest, the growth and survival of plant were affected by species interactions.
To understand this contradictory, Dr WANG Xugao and coworkers proposed a stochastic dilution hyposesis: Along with the increase in species richness in a forest community, the interactions of target species and its neighbor species is diluted, resulting in that the proportion of species pair showing significant interactions in temperate forest with less specie richness is higher than that in tropical forest with higher species richness. To test the hypothesis, data from 5 sample plots with area>5 hectares were collected. The tree species numbers recorded on the plots rare 36 (Wabikon, temperate), 52 (CBS, temperate), 110 (Fushan, subtropical), 159 (Gutianshan, subtropical) and 304 (BCI, tropical). Multivariate pattern analysis was used to estimate the phylogenic and functional similarity. The species were divided into promoting species (increase species diversity), inhibiting species (decrease species diversity) and neutral species (no significant effect) to see the proportion of these 3 types and its variation along the specie richness gradient. The results confirmed the proportion of the neutral tree species should increase with the increase of the number of species.
The work indicates that the neutral hypothesis is reasonable in tropical forest, but not for temperate forest. The hypothesis enriched and developed the theory for the maintaining mechanism of forest biodiversity. The work was supported by Outstanding Young Scientist Program, CAS and NNSFC. The results were published in Ecology (97(2), 347-360).
Abstract. Recent theory predicts that stochastic dilution effects may result in species-rich communities with statistically independent species spatial distributions, even if the underlying ecological processes structuring the community are driven by deterministic niche differences. Stochastic dilution is a consequence of the stochastic geometry of biodiversity where the identitiesof the nearest neighbors of individuals of a given species are largely unpredictable. Under such circumstances, the outcome of deterministic species interactions may vary greatly among individuals of a given species. Consequently, nonrandom patterns in the biotic neighborhoods of species, which might be expected from coexistence or community assembly theory (e.g., individuals of a given species are neighbored by phylogenetically similar species), are weakened or do not emerge, resulting in statistical independence of species spatial distributions. We used data on phylogenetic and functional similarity of tree species in five large forest dynamics plots located across a gradient of species richness to test predictions of the stochastic dilution hypothesis. To quantify the biotic neighborhood of a focal species we used the mean phylogenetic (or functional) dissimilarity of the individuals of the focal species to all species within a local neighborhood. We then compared the biotic neighborhood of species to predictions from stochastic null models to test if a focal species was surrounded by more or less similar species than expected by chance. The proportions of focal species that showed spatial independence with respect to their biotic neighborhoods increased with total species richness. Locally dominant, high-abundance species were more likely to be surrounded by species that were statistically more similar or more dissimilar than expected by chance. Our results suggest that stochasticity may play a stronger role in shaping the spatial structure of species rich tropical forest communities than it does in species poorer forests. These findings represent an important step towards understanding the factors that govern the spatial configuration of local biotic communities. The stochastic dilution effect is a simple geometric mechanism that can explain why species’ spatial distributions in species-rich communities approximate independence from their biotic neighborhood, even if deterministic niche processes are in effect.
Key words: coexistence theory; forest dynamics plot; functional dissimilarity; habitat filtering; individual species–area relationship; null model; pattern reconstruction; phylogenetic dissimilarity; point pattern analysis; species interaction; stochastic dilution hypothesis.
Publication Name: Xugao Wang,1,11 Thorsten Wiegand,2,3 Nathan J. B. Kraft,4 Nathan G. Swenson,4 Stuart J. Davies,5 Zhanqing Hao,1 Robert Howe,6 Yiching Lin,7 Keping Ma,8 Xiangcheng Mi,8 Sheng-Hsin Su,9 I-Fang Sun,10 and Amy Wolf6.
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