Nitrogen deposition mediates the effects and importance of chance in changing biodiversity

Nitrogen deposition is changing biodiversity on Earth. We need to understand the underlying mechanisms to conserve biodiversity better. Both selection and chance are potential mechanisms, and they may operate concurrently. Then, what are the respective effects of selection and chance, what is their relative importance and how do they change with increasing nitrogen deposition rate? Here, we performed a 6‐year nitrogen addition experiment (0–28 g N/m²/year) in a typical steppe ecosystem of Inner Mongolia to investigate the community structure of plants, bacteria and ammonia‐oxidizing Archaea (AOA). We developed an experimentally based calculation method to first separate the structural variations between plots into the effects of selection (S) and chance (C), and then calculate their relative importance. Our results showed that as nitrogen addition rate increased, S for both plants and bacteria increased, but C for plants first increased and then decreased, and C for bacteria also increased; meanwhile, both S and C for AOA changed nonlinearly. As nitrogen addition rate increased, the importance of chance decreased on the whole for all these communities, but it decreased nonlinearly for plants and bacteria, with a local increase at certain intermediate rates. At all treatments, the importance of chance was <0.5 for plants, but >0.5 for AOA. These results demonstrated that nitrogen deposition changed biodiversity by mediating the effects and importance of chance, implicating different strategies should be adopted in conserving biodiversity according to nitrogen deposition rate and community properties.     

Can high tree species richness be explained by Hubbell’s null model?

We examine several features of Hubbell’s nonequilibrium, or “null”, model of tree dynamics, which holds that species-rich tropical tree communities are maintained on a local scale by a balance of extinction and immigration, and on a global scale by a balance of extinction and speciation. All species are held to be ecologically equivalent, such that species having equal initial abundances have equal probabilities of extinction or fixation. We show here that the null model is not robust to relaxation of the assumption of ecological equivalence. Recently, 32 ; J. Theor. Biol. 188: 361–367) showed that persistence times decrease when unequal colonization rates are allowed, but their results still permit very long persistence times in stands of hundreds of thousands of stems or more. We extend their work by allowing tree mortality rates to differ across species, as is seen in all natural tree communities. As a result, persistence times drop dramatically, and forest composition becomes highly deterministic, such that long-lived species drop out of the community much more slowly than short-lived species. We also note that the use of tree deaths (instead of years) as a measure of time inflates estimates of persistence times. In summary, calculated persistence times of tropical tree species, even those in very large stands, no longer reach time scales plausible for speciation.     

Neutral theory in community ecology

One of the central goals of community ecology is to understand the forces that maintain species diversity within communities. The traditional niche-assembly theory asserts that species live together in a community only when they differ from one another in resource uses. But this theory has some difficulties in explaining the diversity often observed in specie-rich communities such as tropical forests. As an alternative to the niche theory, Hubbell and other ecologists introduced a neutral model. Hubbell argues that the number of species in a community is controlled by species extinction and immigration or speciation of new species. Assuming that all individuals of all species in a trophically similar com-munity are ecologically equivalent, Hubbell's neutral theory predicts two important statistical distributions. One is the asymptotic log-series distribution for the metacommunities under point mutation speciation, and the other is the zero-sum multinomial distribution for both local communities under dispersal limitation and metacommunities under random fission speciation. Unlike the niche-assembly theory, the neutral theory takes similarity in species and individuals as a starting point for investigating species diversity. Based on the fundamental processes of birth, death, dispersal and spe-ciation, the neutral theory provided the first mechanistic explanation of species abundance distribution commonly observed in natural communities. Since the publication of the neutral theory, there has been much discussion about it, pro and con. In this paper, we summarize recent progress in the assumption, prediction and speciation mode of the neutral theory, including progress in the theory itself, tests about the assumption of the theory, prediction and speciation mode at the metacommunity level. We also suggest that the most important task in the future is to bridge the niche-assembly theory and the neutral theory, and to add species differences to the neutral theory and more stochasticity to the niche theory.     

Neutral theory in community ecology

One of the central goals of community ecology is to understand the forces that maintain species diversity within communities. The traditional niche-assembly theory asserts that species live together in a community only when they differ from one another in resource uses. But this theory has some difficulties in explaining the diversity often observed in specie-rich communities such as tropical forests. As an alternative to the niche theory, Hubbell and other ecologists introduced a neutral model. Hubbell argues that the number of species in a community is controlled by species extinction and immigration or speciation of new species. Assuming that all individuals of all species in a trophically similar community are ecologically equivalent, Hubbell’s neutral theory predicts two important statistical distributions. One is the asymptotic log-series distribution for the metacommunities under point mutation speciation, and the other is the zero-sum multinomial distribution for both local communities under dispersal limitation and metacommunities under random fission speciation. Unlike the niche-assembly theory, the neutral theory takes similarity in species and individuals as a starting point for investigating species diversity. Based on the fundamental processes of birth, death, dispersal and speciation, the neutral theory provided the first mechanistic explanation of species abundance distribution commonly observed in natural communities. Since the publication of the neutral theory, there has been much discussion about it, pro and con. In this paper, we summarize recent progress in the assumption, prediction and speciation mode of the neutral theory, including progress in the theory itself, tests about the assumption of the theory, prediction and speciation mode at the metacommunity level. We also suggest that the most important task in the future is to bridge the niche-assembly theory and the neutral theory, and to add species differences to the neutral theory and more stochasticity to the niche theory. __________ Translated from Journal of Plant Ecology, 2006, 30(5): 868–877 [译自:植物生态学报]     

亚高山草甸植物群落物种多样性与群落C、N、P生态化学计量的关系

通过测定祁连山东部亚高山草甸256个群落样方的群落物种数和地上植物的C、N、P元素含量, 探究了该地区高产草地和低产草地植物群落物种多样性与植物群落C、N、P生态化学计量特征的关系。结果表明: 高产样地植物群落C、N、P含量的平均值分别为53.05%、1.99%、0.22%; 而低产样地植物群落C、N、P含量的平均值分别为52.51%、2.28%、0.19%。高产和低产样地植物群落C、N、P元素含量均呈现显著差异。高产样地上植物群落的物种数与N含量和N:P呈显著正相关关系, 与C:N呈显著负相关关系, 但与C、P含量和C:P无明显相关关系; 低产样地上植物群落的物种数与N、P含量呈极显著负相关关系, 与C:N和C:P呈显著正相关关系, 但与C含量和N:P无明显相关关系。说明高产草地和低产草地植物群落物种多样性与养分元素化学计量特征显示出一定的分异性, 高产样地上植物群落的物种数主要受N含量的限制, 与N含量呈正相关关系; 低产样地上植物群落的物种数则受N、P含量共同限制, 与N、P含量呈负相关关系。     

Mechanisms determining the fate of dispersed bacterial communities in new environments

Recent work has shown that dispersal has an important role in shaping microbial communities. However, little is known about how dispersed bacteria cope with new environmental conditions and how they compete with local resident communities. To test this, we implemented two full-factorial transplant experiments with bacterial communities originating from two sources (freshwater or saline water), which were incubated, separately or in mixes, under both environmental conditions. Thus, we were able to separately test for the effects of the new environment with and without interactions with local communities. We determined community composition using 454-pyrosequencing of bacterial 16S rRNA to specifically target the active fraction of the communities, and measured several functional parameters. In absence of a local resident community, the net functional response was mainly affected by the environmental conditions, suggesting successful functional adaptation to the new environmental conditions. Community composition was influenced both by the source and the incubation environment, suggesting simultaneous effects of species sorting and functional plasticity. In presence of a local resident community, functional parameters were higher compared with those expected from proportional mixes of the unmixed communities in three out of four cases. This was accompanied by an increase in the relative abundance of generalists, suggesting that competitive interactions among local and immigrant taxa could explain the observed ‘functional overachievement''. In summary, our results suggest that environmental filtering, functional plasticity and competition are all important mechanisms influencing the fate of dispersed communities.     

The efficiency of purifying selection in Mammals vs. Drosophila for metabolic genes

The nearly neutral theory of molecular evolution states that the efficiency of natural selection depends on the effective population size. By using a wide range of multispecies data on nucleotide polymorphism, we have tried to ascertain whether there are any differences in the level of selective constraints of metabolic process genes between Mammals and Drosophila species. The results are consistent with a higher selective constraint in Drosophila than in Mammals, according to the expected under the nearly neutral model: purifying selection seems to be more efficient in species with a larger effective population size.