No abstract

Intermittent rivers and ephemeral streams (IRES) may represent over half the global stream network, but their contribution to respiration and carbon dioxide (CO2) emissions is largely undetermined. In particular, little is known about the variability and drivers of respiration in IRES sediments upon rewetting, which could result in large pulses of CO2. We present a global study examining sediments from 200 dry IRES reaches spanning multiple biomes. Results from standardized assays show that mean respiration increased 32‐fold to 66‐fold upon sediment rewetting. Structural equation modeling indicates that this response was driven by sediment texture and organic matter quantity and quality, which, in turn, were influenced by climate, land use, and riparian plant cover. Our estimates suggest that respiration pulses resulting from rewetting of IRES sediments could contribute significantly to annual CO2 emissions from the global stream network, with a single respiration pulse potentially increasing emission by 0.2–0.7%. As the spatial and temporal extent of IRES increases globally, our results highlight the importance of recognizing the influence of wetting‐drying cycles on respiration and CO2 emissions in stream networks.

Summary 201. Niche segregation on fine-scale soil-moisture gradients is found in many plant 21 communities, but the contribution of this to community structure has yet to be analysed 22 in a fully spatial manner. We introduce a univariate and multivariate analytical 23 approach that, taking spatial autocorrelation into account, decomposes the spatial 24 structure of species composition into components that correlate with linear and non-25 linear soil-moisture gradients plus a residual. 26 2. The analysis is applied to two contrasting plant communities in Western and Central-27Northern Spain, a seasonally dry dehesa meadow that is rich in both annual and 28 perennial herbs and an alpine meadow community. In each we: (i) determined whether 29 the community was structured on a soil moisture gradient, (ii) tested whether there was 30 spatial segregation into hydrological niches, (iii) Characterised the hydrological niches 31 of individual species, and (iv) controlling for spatial autocorrelation, determined how 32 much of the spatial structure in the community was due to variation in hydrology. In the 33 dehesa community we also compared annuals and perennials with respect to questions 34 1-4. 35 3. We found that both plant communities were primarily structured along hydrological 36 gradients and that spatial segregation into hydrological niches occurred among 37 perennial species, though not among the annuals in the dehesa community. Dehesa 38 annuals were spatially aggregated in the driest niches. Hydrological variation shaped 39 the responses of 60% of the annual and about 70% of the perennial species in both the 40 dehesa meadow and the alpine community. Most responses were either monotonic or 41 hump-shaped. Finally, spatially structured hydrological variation proved to be the main 42 3 driver of spatially structured species composition in all three cases. 43 4. Synthesis Consistent univariate and multivariate results showed that linearly (gradient 44 of slope) and topographically (at a fine scale) structured variation in hydrology is the 45 main driver of spatially structured species composition in both test communities. Our 46 results support the hypothesis that spatial niche segregation on soil-moisture gradients 47is an important mechanism of coexistence for perennials in both test communities, 48though not for the species-rich sub-community of annuals in the dehesa meadow. 49 50

Questions Does the absence of equalizing mechanisms after cessation of grazing unleash strong above‐ground competitors to create large patches in the community? Do these competitive intraspecific aggregations displace and exclude other species, thereby reducing species diversity? Location Atlantic grasslands, Aralar Natural Park, Basque Country, Northern Iberian Peninsula. Methods Large herbivores were experimentally excluded from three sites (50 m × 50 m exclusion fences) during 9 yr in a productive semi‐natural grassland system with a long history of grazing, using adjacent grazed plots as experimental controls. Sampling was carried out by systematically placing 100 quadrats (0.5 m × 0.5 m) in each of the six plots. Floristic composition and abundance, as well as eight hydrological and chemical soil properties, were measured in each quadrat. The spatial structures created by competitive species were analysed using RDA in conjunction with Moran's eigenvector maps, and soil variables were simultaneously included in the analyses, thus disentangling the structures likely created by niche effects. Competitive exclusion was further determined using linear regressions between species richness and abundance of competitive species. Results Grazing exclusion unleashed competitive species such as Festuca microphylla and Agrostis capillaris, which became dominant in the exclusion plots and created large spatial patches. Furthermore, a negative linear relationship, consistent across exclusion plots, was observed between species richness and abundance of competitive species, indicating that strong above‐ground competitors outcompeted other species when herbivores were excluded. However, the outcome of grazing exclusion across sites depended to some extent on local environmental conditions (niche effects). Conclusions This work confirms that the powerful equalizing mechanism of disturbance by herbivores is crucial for species co‐existence in productive grasslands. However, important differences observed in environmental effects across sites suggest that, even in highly productive grasslands, plant traits and local environmental characteristics (niche effects) do matter for species co‐existence.

The degree of trophic specialization determines the ability of predators to cope with changing foraging conditions, but in predators that prey on hundreds of species it is challenging to assess, especially when prey identity varies among predator individuals and across space and time. Here, we test the hypothesis that a bat species foraging on flying insects like moths will show ample flexibility in trophic niche, and this irrespective of phylogenetic relationships among moths, so as to cope with a high diversity of prey types that vary across seasons. We predict that individual bats will show functional dietary differences consistent with energetic requirements and hunting skills. We used DNA metabarcoding to determine the diet of 126 Mediterranean horseshoe bats (Rhinolophus euryale) from two different sites during three seasons. Simultaneously, we measured moth availability and characterized the traits of 290 moth taxa. Next, we explored the relationship between phylogeny and traits of all consumed and available moth taxa. Finally, we assessed the relationship between individual traits of bats and traits related to prey profitability, for which we used the RLQ and fourth‐corner statistical techniques. Seasonality was the main factor explaining the functional dietary variation in adult bats, with moths consumed irrespective of their phylogenetic relationships. While adults consumed moths with a broad range in wing loading, body mass and echolocation detection ability, juveniles consumed slower, smaller and lighter moths, which suggests that young individuals may undergo some fitness gain and/or psychomotor learning process during which they would acquire more effective foraging skills. Our approach revealed a degree of functional flexibility in the trophic niche previously unknown for an insectivorous bat. Rhinolophus euryale consumed a wide variety of moth taxa differing in profitability throughout seasons and between ontogenetic stages. We showed the validity of trait‐based approaches to gain new insights in the trophic specialization of predators consuming hundreds of species of prey.