Quasi-Parallel Shock Reformation Seen by Magnetospheric Multiscale and Ion-Kinetic Simulations.

Shock waves in collisionless plasmas are among the most efficient particle accelerators in space. Shock reformation is a process important to plasma heating and acceleration, but direct observations of reformation at quasi-parallel shocks have been lacking. Here, we investigate Earth's quasi-parallel bow shock with observations by the four Magnetospheric Multiscale spacecraft. The multi-spacecraft observations provide evidence of short large-amplitude magnetic structures (SLAMS) causing reformation of the quasi-parallel shock. We perform an ion-kinetic Vlasiator simulation of the bow shock and show that SLAMS reforming the bow shock recreates the multi-spacecraft measurements. This provides a method for identifying shock reformation in the future.

Joint species distributions reveal the combined effects of host plants, abiotic factors and species competition as drivers of species abundances in fruit flies.

The relative importance of ecological factors and species interactions for shaping species distributions is still debated. The realised niches of eight sympatric tephritid fruit flies were inferred from field abundance data using joint species distribution modelling and network inference, on the whole community and separately on three host plant groups. These estimates were then confronted the fundamental niches of seven fly species estimated through laboratory-measured fitnesses on host plants. Species abundances depended on host plants, followed by climatic factors, with a dose of competition between species sharing host plants. The relative importance of these factors mildly changed among the three host plant groups. Despite overlapping fundamental niches, specialists and generalists had almost distinct realised niches, with possible competitive exclusion of generalists by specialists on Cucurbitaceae. They had different assembly rules: Specialists were mainly influenced by their adaptation to host plants, while generalist abundances varied regardless of their fundamental host use.

Assessing metacommunity processes through signatures in spatiotemporal turnover of community composition.

Although metacommunity ecology has been a major field of research in the last decades, with both conceptual and empirical outputs, the analysis of the temporal dynamics of metacommunities has only emerged recently and consists mostly of repeated static analyses. Here we propose a novel analytical framework to assess metacommunity processes using path analyses of spatial and temporal diversity turnovers. We detail the principles and practical aspects of this framework and apply it to simulated datasets to illustrate its ability to decipher the respective contributions of entangled drivers of metacommunity dynamics. We then apply it to four empirical datasets. Empirical results support the view that metacommunity dynamics may be generally shaped by multiple ecological processes acting in concert, with environmental filtering being variable across both space and time. These results reinforce our call to go beyond static analyses of metacommunities that are blind to the temporal part of environmental variability.

Modeling competition, niche, and coexistence between an invasive and a native species in a two-species metapopulation.

Modeling the dynamics of competition and coexistence between species is crucial to predict long-term impacts of invasive species on their native congeners. However, natural environments are often fragmented and variable in time and space. In such contexts, regional coexistence depends on complex interactions between competition, niche differentiation and stochastic colonization-extinction dynamics. Quantifying all these processes at landscape scale has always been a challenge for ecologists. We propose a new statistical framework to evaluate metapopulation parameters (colonization and extinction) in a two-species system and how they respond to environmental variables and interspecific competition. It requires spatial surveys repeated in time, but does not assume demographic equilibrium. We apply this model to a long-term survey of two snails inhabiting a network of freshwater habitats in the West Indies. We find evidence of reciprocal competition affecting colonization or extinction rates, modulated by species-specific sensitivity to environmental variables. Simulations using model estimates allow us to predict species dynamics and explore the role of various coexistence mechanisms described by metacommunity theory in our system. The two species are predicted to stably coexist, because niche partitioning, source-sink dynamics and interspecific differences in extinction-colonization parameters all contribute to reduce the negative impacts of competition. However, none of these mechanisms is individually essential. Regional coexistence is primarily facilitated by transient co-occurrence of the two species within habitat patches, a possibility generally not considered in theoretical metacommunity models. Our framework is general and could be extended to guilds of several competing species.