Herbivores in the tundra interact with vegetation through several mechanisms, especially defoliation, trampling and nutrient addition through urine and faeces. Through these mechanisms, herbivores drive shifts in plant species composition, richness and diversity. As reindeer effects on vegetation accumulate over time, they might cascade to other trophic levels, but how and when this happens is poorly understood. Since it is methodologically demanding to measure biodiversity across spatial gradients, an alternative approach is to assess it indirectly via biodiversity indices of vascular plants. Values from the Index of Biodiversity Relevance were coupled with vegetation data from a network of 96 fenced and paired grazed plots across Fennoscandia. We analysed the role herbivory has on plant richness and diversity, and on the number of organisms that depend on the vegetation according to the index values. We also explored how herbivores affect the competitive effects of shrubs on other plants since the dominance of a vegetation type links directly to biodiversity. Vegetation richness and diversity did not present any differences between treatments, yet reindeer had an increasing effect on plant diversity when testing the interaction between grazing and herbaceous vegetation. Three out of six biodiversity indexes were higher in fenced plots indicating a higher number of interactions between plants and organisms from other trophic levels. Finally, herb abundance was negatively related to shrubs in both treatments but with a faster decline in the absence of herbivores, suggesting that herbivory increases plant diversity and decreases the diversity of other taxa by reducing shrub abundance. This study highlights the importance of maintaining herbivore populations in the Arctic to prevent the expansion of climate-driven biodiversity into the tundra. The effect of herbivores on ecological communities is not merely a product of plant diversity but can be quantitatively and qualitatively different.

While the evidence supports a role for the soil microbiome to modulate the environmental tolerance of various plant species, its role in the invasion success remains seldom assessed. Here we show results from two complementary experiments aimed at understanding the role of the soil microbiome on the performance of T. officinale (dandelion) plants. Since the relative importance of soil microbiome on plant fitness can differ between native versus introduced origins, we conducted a full cross-transplant experiment to compare the plant performance from different origins (native/introduced) in their original or exotic soils, along with manipulated soil microbiome. In addition, since the relevance of soil microbiome for plant fitness can depend on the level of environmental stress, we compared the plant performance under different soil microbiome treatments in an introduced latitudinal gradient. We found positive effects of soil microbiome on performance traits for T. officinale plants from most of the evaluated populations, being particularly relevant for plants in the introduced range and under stressful conditions.

Deterministic and stochastic factors shape ecological communities. However, a quantitative synthesis of the factors underlying the balance among different assembly processes is lacking. Here, we synthesized data from 149 datasets covering major biotic groups and ecosystem types globally. We used a null model approach based on Raup-Crick dissimilarities and Bayesian meta-regression to analyze the data. We found that communities were more under homogeneous selection than heterogeneous selection across biotic taxa and ecosystems. Environment selected species homogeneously more often at small scales while heterogeneously more often at large scales. Stochasticity also showed scale-dependence as stochastic community assembly increased with study scale. Homogeneous and heterogeneous selection were strongest at high latitudes while stochastic factors were strongest in tropics. Marine systems had the highest degree of homogeneous selection and the lowest stochasticity. We provide the first analysis of community assembly across taxa and ecosystems which should be important for a better understanding of how communities respond to environmental change.

Plant-associated microbes play a key role in mediating the relationship between plant diversity and productivity. However, previous studies have generally focused on a sole microbial guild (i.e. plant-beneficial microbes or pathogens), and on either aboveground or belowground microbes. As a result, the interplay among different microbial guilds and the overall impact of above- and belowground microbes on plant diversity-productivity relationships have rarely been investigated. Here we carried out an experiment where we applied microbial inocula collected from leaves and soils in the field onto plant leaves and soil in a greenhouse experiment with a herbaceous plant community. We showed that microbial inoculation of leaves reduced plant productivity and this negative effect was weaker at higher plant diversity, which promoted positive diversity-productivity relationships through complementarity effects. In contrast, microbial inoculation of soil alone had no impact on plant diversity-productivity relationships, but it counteracted the negative effects of leaf inoculum on plant productivity and weakened the leaf microbe-induced positive diversity-productivity relationships. We found that the abundance of arbuscular mycorrhizal fungi and Streptomyces bacteria increased when soil microbes were inoculated, and such increase was more significant at lower plant diversity, potentially explaining the effects of soil inoculation on plant productivity. These results suggest that the belowground plant beneficial microbes can counteract the effect of aboveground plant pathogens in mediating positive plant diversity-productivity relationships. Simultaneous study of plant-pathogenic and -beneficial microbes both above- and belowground is required to better understand the contributions of plant-associated microbes to biodiversity-ecosystem function relationships.

A document by Raquel Díaz Borrego. Click on the document to view its contents.

Plant pathogens are important for ecosystem functioning and community assembly and respond to a variety of biotic and abiotic factors, which change along elevation gradients. Thus elevational gradients are a valuable model system for exploring how plant community, soil properties, and environmental factors influence pathogens. Yet, how these factors influence pathogens in nature remains poorly understood. We tested patterns and potential mechanisms of plant fungal pathogens along elevational gradients by combining a field survey in the Tibetan Plateau with a global meta-analysis. We found that increasing elevation was associated with a decrease in soil fungal pathogen richness but not foliar fungal disease symptoms. Elevation mainly related to soil fungal pathogen richness through abiotic factors. Whereas no evidence supported association between elevation and foliar fungal disease. The meta-analysis suggests some generality in the results of the field survey: elevation was associated with a decrease in soil fungal pathogen richness, but had no consistent relationship with foliar fungal disease or pathogens. Our study reveals distinct patterns of above- and belowground plant pathogen along elevation gradients and provides new insight into the potential mechanisms in shaping these patterns.

Dissolved organic matter and nutrients released from forest leaf litter are important cross-ecosystem resources that support freshwater ecosystems. Dissolved organic matter released from leaf litter is one of the allochthonous sources for heterotrophic organisms in freshwater communities. However, the role of macro- and micronutrients released from leaf litter in producing autochthonous organic matter in aquatic ecosystems is not necessarily well understood. In this study, therefore, we investigated how dissolved nutrients released from leaf litter affect the algal growth, biomass production, and cell elemental quotas. Specifically, we focused on the responses of the green algae to TDN, TDP, and micronutrients released from the leaf litter of 11 temperate tree species. We found that the algal growth rate increased with TDP when micronutrients were amended. In contrast, the algal biomass production increased with TDN, regardless of micronutrient amendment. Micronutrient limitation of algal growth rate was found in the leaf litter leachates from oak, Japanese elm, and Japanese hemlock. However, algal biomass production was limited by micronutrients only in the leaf litter leachate from Japanese hemlock. More importantly, leaf litter leachates from different tree species altered algal cell quotas and C:N:P ratios, which would affect secondary production. These results suggest that forest vegetation change or succession affect the quantity and quality of autochthonous organic matter and thus the mass transfer efficiency in the aquatic community by changing the stoichiometry of nutrient input from the leaf litter.

Declining enemy release predicts that invasive plants accumulated more soil natural enemies, and the increase in enemies may inhibit growth of invasive plants themselves. But most studies focus on historical time rather than short-term. We designed a fully crossed factorial experiment, we grew individuals of four congeneric pairs of invasive and native plant species in 2.5 L pots that contained live or sterilized field soil under two harvest time (first vs second). Results shows that soil microbes tended to have a slight positive effect on total biomass of the native plant species over time in short-term, while the effect of soil microbes on invasive plants as their total biomass tended to change from promotion to inhibition over time in short-term. Overall, these results suggest that regardless of the direction and strength of plant-soil feedback on invasive plant species, invasive plant species consistently may grow larger than co-occurring native plant species over time in short-term.

Disturbance and connectivity control biodiversity, ecosystem functioning and their interactions across connected aquatic and terrestrial ecosystems, that form a meta-ecosystem. In rivers, detrital organic matter (OM) is transported across terrestrial-aquatic boundaries and along the river network and decomposed on the way by diverse communities of organisms, including microorganisms and invertebrates. Drying naturally fragments most river networks and thereby modify organism dispersal and OM transfers across ecosystems. This may prevent organisms from reaching and consuming OM, generating mismatches between community composition and decomposition. However, little evidence of the effects of drying on river network-scale OM cycling exists. Here, we aim to examine the effects of fragmentation by drying on the structure of consumer communities and ecosystem functioning within interacting aquatic-terrestrial river ecosystems. We monitored leaf resource stocks, invertebrate communities and decomposition rates in the instream and riparian habitats of 20 sites in a river network naturally fragmented by drying. Although instream resource quantity and quality increased with drying severity, decomposition decreased due to changes in invertebrate communities and particularly leaf-decomposer abundance. Invertebrate-driven decomposition peaked at intermediate levels of upstream connectivity, suggesting that intermediate levels of fragmentation can promote the functioning of downstream ecosystems. We found that the variability in community composition was unrelated to variability in decomposition at sites with low connectivity and high drying severity, suggesting that such conditions can promote mismatches between community composition and decomposition. Decomposition instream was correlated to decomposition in the riparian area, revealing one of the first network-scale evidence of the links between ecosystem functions across terrestrial-aquatic boundaries. Our river network-scale study thus demonstrates the paramount effect of drying on the dynamics of resources, communities and ecosystem functioning in river networks, with crucial implications for the adaptive management of river networks and preservation of their functional integrity.