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.

Microbes play a critical role in regulating the size, composition, and turnover of dissolved organic matter (DOM), which is one of the largest pools of carbon in aquatic ecosystems. Global change may alter DOM-microbe associations with implications for biogeochemical cycles, although disentangling these complex interactions remains a major challenge. Here we develop a framework called Energy-Diversity-Trait integrative Analysis (EDTiA) to examine the associations between DOM and bacteria along temperature and nutrient gradients in a manipulative field experiment on mountainsides in contrasting subarctic and subtropical climates. In both study regions, the chemical composition of DOM correlated with bacterial communities, and was primarily controlled by nutrients and to a lesser degree by temperature. At a molecular-level, DOM-bacteria associations depended strongly on the molecular traits of DOM, with negative associations indicative of decomposition as molecules are more biolabile. Using bipartite networks, we further demonstrated that negative associations were more specialized than positive associations indicative of DOM production. Nutrient enrichment promoted specialization of positive associations, but decreased specialization of negative associations particularly at warmer temperatures in subtropical climate. These global change drivers influenced specialization of negative associations most strongly via molecular traits, while both molecular traits and bacterial diversity similarly affected positive associations. Together, our framework provides a quantitative approach to understand DOM-microbe associations and wider carbon cycling across scales under global change.

Freshwater biodiversity is declining dramatically, and the current biodiversity crisis requires defining bold goals and mobilizing substantial resources to meet the challenges. While the reasons are varied, both research and conservation of freshwater biodiversity lag far behind efforts in the terrestrial and marine realms. We identify fifteen pressing global needs to support informed global freshwater biodiversity stewardship. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated action towards its sustainable management and conservation.