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Convergence of eco-physiological function in harsh habitats due to environmental filtering is well-known, however, some evidence of eco-physiological divergence in coexisting species also exists. Experimental data to verify whether divergence of eco-physiological function in coexisting species increases with increasing environmental stress remain limited. Here, we measured bulk leaf oxygen isotope and functional traits along a climatic stress gradient in a grassland in Tibetan Plateau where drought, excess radiation, and temperature increased from east to west. Enrichment (Δ18O) in bulk leaf oxygen isotope was used as proxy for time-integrated gs. We found that the leaf water use (Δ18O and leaf area [LA]) spectrum was independent of the leaf economic spectrum, indicating that coexisting species shifted from high nitrogen use efficiency (high ratio of leaf carbon to nitrogen) and water loss (high LA) in low-stress habitats to high photosynthetic capacity (high leaf nitrogen) and low water loss (low gs) in high-stress habitats. As the stress intensity of climate increases, higher mean and maximum and lower skewness (varying from positive to negative values) of Δ18O in coexisting species confirmed that the abundance of water-saving species increased to cope with climatic stress, while the greater standard deviation and kurtosis further indicated that climatic stress drove the divergence in gs in coexisting species. This study revealed that coexisting species adopt more diversified water use strategies and the abundance of water-saving species increases to cope with climatic stress, regardless of decreases in overall species abundance.

Abstract Background Stellera chamaejasme L. is a poisonous plant widely distributes in degraded grasslands in China. The mechanism underlying its spread remains unknown. In some degraded grasslands, S. chamaejasme has gradually replaced previous dominant species, such as Leymus chinensis, Stipa krylovii, Artemisia eriopoda on typical steppes. Apart from its unpalatability by livestock, we hypothesized that the survival strategy (nutrient uptake and water use efficiency) of S. chamaejasme in degraded grasslands could be distinct from other coexisting species in the community. Recently, ecological stoichiometry has been suggested as a new approach for studying the demand for natural resources of plants in a changing world, and the leaf carbon isotopic composition (δ13C leaf) as a rapid and effective high throughput phenotyping method for water use efficiency (WUE), both of which can reveal the survival and adaptive strategies of plants. Therefore, in this study we aimed to fill the knowledge gap concerning ecological stoichiometry in the leaf, stem, and root of S. chamaejasme and its surrounding soil on grasslands with different degrees of degradation, and comparing the leaf nutrient content and δ13C of S. chamaejasme with the coexisting species (L. chinensis, S. krylovii, A. eriopoda) in the communities. Toward this goal, we conducted a field survey in which plants and soils were sampled from four different degraded grasslands on typical steppes in China. Results Our results showed that there is no significant difference of carbon content (C%) and nitrogen content (N%) in leaves of S. chamaejasme in different degraded grasslands, and all element contents and element ratios in stems did not differ significantly. Meanwhile, ecological stoichiometry of S. chamaejasme is distinct from the coexisting species, with low C%, high N% and phosphorus content (P%) in the leaf, indicating high nutrient uptake efficiency of S. chamaejasme in nutrient-poor environments like degraded grasslands. Additionally, S. chamaejasme showed significant higher WUE than other species. Conclusions Our results indicated that high nutrient uptake efficiency and high WUE of S. chamaejasme might together contribute to the spread of S. chamaejasme in degraded grasslands.

We investigate the rate of increase of protists introduced at low density into experimental communities comprising different sets of coexisting species. The experiment separates (1) the main effect of six introduced species, (2) the main effect of eleven resident communities, and (3) the introductioncommunity interaction. Introduced species differ markedly in their capacity to invade, and only one, Paramecium,invades all of the communities. Most of the introduced species invade some communities, and there is a large introductioncommunity interaction that comes about from bringing together particular combinationsof species. For example, the rate of increase of Amoeba is greater when introduced into communities containing Paramecium than when introduced into communities in which Paramecium is absent. The omnivore Blepharisma increases faster in the presence of one of its prey Tetrahymena, notwithstanding the fact that it is also a potential competitorwith this prey for bacteria. There is little indication that the species richness of resident communities (over the limited range available) affects the success of invasion. The results suggest that understanding invasions depends as much on detailed knowledge of idiosyncratic biological interactions as on general properties of community structure. [ABSTRACT FROM AUTHOR]