Dynamics and drivers of mycorrhizal fungi after glacier retreat.

The development of terrestrial ecosystems depends greatly on plant mutualists such as mycorrhizal fungi. The global retreat of glaciers exposes nutrient-poor substrates in extreme environments and provides a unique opportunity to study early successions of mycorrhizal fungi by assessing their dynamics and drivers. We combined environmental DNA metabarcoding and measurements of local conditions to assess the succession of mycorrhizal communities during soil development in 46 glacier forelands around the globe, testing whether dynamics and drivers differ between mycorrhizal types. Mycorrhizal fungi colonized deglaciated areas very quickly (< 10 yr), with arbuscular mycorrhizal fungi tending to become more diverse through time compared to ectomycorrhizal fungi. Both alpha- and beta-diversity of arbuscular mycorrhizal fungi were significantly related to time since glacier retreat and plant communities, while microclimate and primary productivity were more important for ectomycorrhizal fungi. The richness and composition of mycorrhizal communities were also significantly explained by soil chemistry, highlighting the importance of microhabitat for community dynamics. The acceleration of ice melt and the modifications of microclimate forecasted by climate change scenarios are expected to impact the diversity of mycorrhizal partners. These changes could alter the interactions underlying biotic colonization and belowground-aboveground linkages, with multifaceted impacts on soil development and associated ecological processes.

Local climate modulates the development of soil nematode communities after glacier retreat.

The worldwide retreat of glaciers is causing a faster than ever increase in ice-free areas that are leading to the emergence of new ecosystems. Understanding the dynamics of these environments is critical to predicting the consequences of climate change on mountains and at high latitudes. Climatic differences between regions of the world could modulate the emergence of biodiversity and functionality after glacier retreat, yet global tests of this hypothesis are lacking. Nematodes are the most abundant soil animals, with keystone roles in ecosystem functioning, but the lack of global-scale studies limits our understanding of how the taxonomic and functional diversity of nematodes changes during the colonization of proglacial landscapes. We used environmental DNA metabarcoding to characterize nematode communities of 48 glacier forelands from five continents. We assessed how different facets of biodiversity change with the age of deglaciated terrains and tested the hypothesis that colonization patterns are different across forelands with different climatic conditions. Nematodes colonized ice-free areas almost immediately. Both taxonomic and functional richness quickly increased over time, but the increase in nematode diversity was modulated by climate, so that colonization started earlier in forelands with mild summer temperatures. Colder forelands initially hosted poor communities, but the colonization rate then accelerated, eventually leveling biodiversity differences between climatic regimes in the long term. Immediately after glacier retreat, communities were dominated by colonizer taxa with short generation time and r-ecological strategy but community composition shifted through time, with increased frequency of more persister taxa with K-ecological strategy. These changes mostly occurred through the addition of new traits instead of their replacement during succession. The effects of local climate on nematode colonization led to heterogeneous but predictable patterns around the world that likely affect soil communities and overall ecosystem development.

Eco-evolutionary drivers of avian migratory connectivity.

Migratory connectivity, reflecting the extent by which migrants tend to maintain their reciprocal positions in seasonal ranges, can assist in the conservation and management of mobile species, yet relevant drivers remain unclear. Taking advantage of an exceptionally large (~150,000 individuals, 83 species) and more-than-a-century-long dataset of bird ringing encounters, we investigated eco-evolutionary drivers of migratory connectivity in both short- and long-distance Afro-Palearctic migratory birds. Connectivity was strongly associated with geographical proxies of migration costs and was weakly influenced by biological traits and phylogeny, suggesting the evolutionary lability of migratory behaviour. The large intraspecific variability in avian migration strategies, through which most species geographically split into distinct migratory populations, explained why most of them were significantly connected. By unravelling key determinants of migratory connectivity, our study improves knowledge about the resilience of avian migrants to ecological perturbations, providing a critical tool to inform transboundary conservation and management strategies at the population level.

The Mitogenome Relationships and Phylogeography of Barn Swallows (Hirundo rustica).

The barn swallow (Hirundo rustica) poses a number of fascinating scientific questions, including the taxonomic status of postulated subspecies. Here, we obtained and assessed the sequence variation of 411 complete mitogenomes, mainly from the European H. r. rustica, but other subspecies as well. In almost every case, we observed subspecies-specific haplogroups, which we employed together with estimated radiation times to postulate a model for the geographical and temporal worldwide spread of the species. The female barn swallow carrying the Hirundo rustica ancestral mitogenome left Africa (or its vicinity) around 280 thousand years ago (kya), and her descendants expanded first into Eurasia and then, at least 51 kya, into the Americas, from where a relatively recent (<20 kya) back migration to Asia took place. The exception to the haplogroup subspecies specificity is represented by the sedentary Levantine H. r. transitiva that extensively shares haplogroup A with the migratory European H. r. rustica and, to a lesser extent, haplogroup B with the Egyptian H. r. savignii. Our data indicate that rustica and transitiva most likely derive from a sedentary Levantine population source that split at the end of the Younger Dryas (YD) (11.7 kya). Since then, however, transitiva received genetic inputs from and admixed with both the closely related rustica and the adjacent savignii. Demographic analyses confirm this species' strong link with climate fluctuations and human activities making it an excellent indicator for monitoring and assessing the impact of current global changes on wildlife.

Metabarcoding data reveal vertical multitaxa variation in topsoil communities during the colonization of deglaciated forelands.

Ice-free areas are expanding worldwide due to dramatic glacier shrinkage and are undergoing rapid colonization by multiple lifeforms, thus representing key environments to study ecosystem development. It has been proposed that the colonization dynamics of deglaciated terrains is different between surface and deep soils but that the heterogeneity between communities inhabiting surface and deep soils decreases through time. Nevertheless, tests of this hypothesis remain scarce, and it is unclear whether patterns are consistent among different taxonomic groups. Here, we used environmental DNA metabarcoding to test whether community diversity and composition of six groups (Eukaryota, Bacteria, Mycota, Collembola, Insecta, and Oligochaeta) differ between the surface (0-5 cm) and deeper (7.5-20 cm) soil at different stages of development and across five Alpine glaciers. Taxonomic diversity increased with time since glacier retreat and with soil evolution. The pattern was consistent across groups and soil depths. For Eukaryota and Mycota, alpha-diversity was highest at the surface. Time since glacier retreat explained more variation of community composition than depth. Beta-diversity between surface and deep layers decreased with time since glacier retreat, supporting the hypothesis that the first 20 cm of soil tends to homogenize through time. Several molecular operational taxonomic units of bacteria and fungi were significant indicators of specific depths and/or soil development stages, confirming the strong functional variation of microbial communities through time and depth. The complexity of community patterns highlights the importance of integrating information from multiple taxonomic groups to unravel community variation in response to ongoing global changes.

Topsoil organic matter build-up in glacier forelands around the world.

Since the last glacial maximum, soil formation related to ice-cover shrinkage has been one major sink of carbon accumulating as soil organic matter (SOM), a phenomenon accelerated by the ongoing global warming. In recently deglacierized forelands, processes of SOM accumulation, including those that control carbon and nitrogen sequestration rates and biogeochemical stability of newly sequestered carbon, remain poorly understood. Here, we investigate the build-up of SOM during the initial stages (up to 410 years) of topsoil development in 10 glacier forelands distributed on four continents. We test whether the net accumulation of SOM on glacier forelands (i) depends on the time since deglacierization and local climatic conditions (temperature and precipitation); (ii) is accompanied by a decrease in its stability and (iii) is mostly due to an increasing contribution of organic matter from plant origin. We measured total SOM concentration (carbon, nitrogen), its relative hydrogen/oxygen enrichment, stable isotopic (13 C, 15 N) and carbon functional groups (C-H, C=O, C=C) compositions, and its distribution in carbon pools of different thermal stability. We show that SOM content increases with time and is faster on forelands experiencing warmer climates. The build-up of SOM pools shows consistent trends across the studied soil chronosequences. During the first decades of soil development, the low amount of SOM is dominated by a thermally stable carbon pool with a small and highly thermolabile pool. The stability of SOM decreases with soil age at all sites, indicating that SOM storage is dominated by the accumulation of labile SOM during the first centuries of soil development, and suggesting plant carbon inputs to soil (SOM depleted in nitrogen, enriched in hydrogen and in aromatic carbon). Our findings highlight the potential vulnerability of SOM stocks from proglacial areas to decomposition and suggest that their durability largely depends on the relative contribution of carbon inputs from plants.

Spatio-Temporal Variation of the Bacterial Communities along a Salinity Gradient within a Thalassohaline Environment (Saline di Tarquinia Salterns, Italy).

The "Saline di Tarquinia" salterns have been scarcely investigated regarding their microbiological aspects. This work studied the structure and composition of their bacterial communities along the salinity gradient (from the nearby sea through different ponds). The communities showed increasing simplification of pond bacterial diversity along the gradient (particularly if compared to those of the sea). Among the 38 assigned phyla, the most represented were Proteobacteria, Actinobacteria and Bacteroidetes. Differently to other marine salterns, where at the highest salinities Bacteroidetes dominated, preponderance of Proteobacteria was observed. At the genus level the most abundant taxa were Pontimonas, Marivita, Spiribacter, Bordetella, GpVII and Lentibacter. The α-diversity analysis showed that the communities were highly uneven, and the Canonical Correspondence Analysis indicated that they were structured by various factors (sampling site, sampling year, salinity, and sampling month). Moreover, the taxa abundance variation in relation to these significant parameters were investigated by Generalized Linear Models. This work represents the first investigation of a marine saltern, carried out by a metabarcoding approach, which permitted a broad vision of the bacterial diversity, covering both a wide temporal span (two years with monthly sampling) and the entire salinity gradient (from the nearby sea up to the crystallisation ponds).

Sex-dependent carry-over effects on timing of reproduction and fecundity of a migratory bird.

Life of many organisms flows as a sequence of annual cycles. Timing of cyclical events is shaped by natural selection also via the domino effects that any life history stage has on the stages that follow. Such 'carry-over effects' have major consequences for evolutionary, ecological and demographic processes, but the causes that generate their individual-level variation, including the effect of sex, are poorly understood. We used light-level geolocators to study carry-over effects on the year-round life cycle of the long-distance migratory barn swallow (Hirundo rustica) and sex-dependent variation in their strength. Correlation analyses showed that timing of breeding influenced departure time for autumn migration in females but not in males. In addition, strong, time-mediated carry-over effects of timing of departure from the wintering areas in sub-Saharan Africa for spring migration on timing of arrival to the breeding grounds in Italy and Switzerland operated in both sexes. However, carry-over effects of spring migration phenology on breeding date and seasonal fecundity were observed among females but not among males. We used partial least squares path modelling to unveil the complex carry-over effects of phenology during the non-breeding season in combination with the ecological conditions experienced by individual swallows in the wintering area, as gauged by Normalized Difference Vegetation Index values (NDVI), on breeding performance. Phenology during the non-breeding season combined with NDVI during wintering accounted for as much as 65-70% of variation in subsequent seasonal fecundity in females, while such carry-over effects on breeding success of males were weaker. Intense, sex-specific carry-over effects can have impacted on evolutionary processes, including sexual selection, and affected phenological response to climate change, causing the large population decline observed in this species.

Diversity and Assembling Processes of Bacterial Communities in Cryoconite Holes of a Karakoram Glacier.

Cryoconite holes are small ponds that form on the surface of glaciers that contain a dark debris, the cryoconite, at the bottom and host active ecological communities. Differences in the structure of bacterial communities have been documented among Arctic and mountain glaciers, and among glaciers in different areas of the world. In this study, we investigated the structure of bacterial communities of cryoconite holes of Baltoro Glacier, a large (62 km in length and 524 km2 of surface) glacier of the Karakoram, by high-throughput sequencing of the V5-V6 hypervariable regions of the 16S rRNA gene. We found that Betaproteobacteria dominated bacterial communities, with large abundance of genera Polaromonas, probably thanks to its highly versatile metabolism, and Limnohabitans, which may have been favoured by the presence of supraglacial lakes in the area where cryoconite holes were sampled. Variation in bacterial communities among different sampling areas of the glacier could be explained by divergent selective processes driven by variation in environmental conditions, particularly pH, which was the only environmental variable that significantly affected the structure of bacterial communities. This variability may be due to both temporal and spatial patterns of variation in environmental conditions.

Wing morphology, winter ecology, and fecundity selection: evidence for sex-dependence in barn swallows (Hirundo rustica).

Variation in wing morphology results from the combination of diverse selection pressures. Wing feather morphology within species varies with sex and ontogenetic effects, and also with ecological factors. Yet, the direction of causation for the wing morphology-ecology association remains to be elucidated. Under the 'ecology-dependence' hypothesis, wing morphology covaries with ecological conditions, because the latter affect feather molt. Alternatively, the 'habitat choice' hypothesis posits that individuals with different wing morphology choose different habitats because of the habitat-dependent advantages of a specific wing morphology. We tested these competing hypotheses in the migratory, aerially insectivorous barn swallow (Hirundo rustica). We quantified wing morphology (isometric size, pointedness, and convexity) on the same individuals during consecutive breeding seasons (i.e., before and after molt in sub-Saharan wintering areas) and located wintering areas using light-level geolocators. Wing pointedness of females but not males during 1 year negatively correlated with vegetation vigor (gauged by the Normalized Difference Vegetation Index; NDVI) in the African area where individuals spent the next winter. Partial least-squares path modelling showed that the association between wing morphology and NDVI was sex-dependent. Conversely, NDVI during wintering did not predict wing morphology in the next breeding season. Because wing morphology can have carry-over effects on subsequent performance, we investigated selection on wing traits and found strong positive fecundity selection on wing size of females. Our results suggest that female barn swallows choose their wintering habitat depending on their wing morphology. In addition, directional fecundity selection operates on females, suggesting sex-dependence of current selection on the flight apparatus.

Clock gene polymorphism, migratory behaviour and geographic distribution: a comparative study of trans-Saharan migratory birds.

Migratory behaviour is controlled by endogenous circannual rhythms that are synchronized by external cues, such as photoperiod. Investigations on the genetic basis of circannual rhythmicity in vertebrates have highlighted that variation at candidate 'circadian clock' genes may play a major role in regulating photoperiodic responses and timing of life cycle events, such as reproduction and migration. In this comparative study of 23 trans-Saharan migratory bird species, we investigated the relationships between species-level genetic variation at two candidate genes, Clock and Adcyap1, and species' traits related to migration and geographic distribution, including timing of spring migration across the Mediterranean Sea, migration distance and breeding latitude. Consistently with previous evidence showing latitudinal clines in 'circadian clock' genotype frequencies, Clock allele size increased with breeding latitude across species. However, early- and late-migrating species had similar Clock allele size. Species migrating over longer distances, showing delayed spring migration and smaller phenotypic variance in spring migration timing, had significantly reduced Clock (but not Adcyap1) gene diversity. Phylogenetic confirmatory path analysis suggested that migration date and distance were the most important variables directly affecting Clock gene diversity. Hence, our study supports the hypothesis that Clock allele size increases poleward as a consequence of adaptation to the photoperiodic regime of the breeding areas. Moreover, we show that long-distance migration is associated with lower Clock diversity, coherently with strong stabilizing selection acting on timing of life cycle events in long-distance migratory species, likely resulting from the time constraints imposed by late spring migration.

The timing of life history events in the presence of soft disturbances.

We study a model for the evolutionarily stable strategy (ESS) used by biological populations for choosing the time of life-history events, such as arrival from migration and breeding. In our model we account for both intra-species competition (early individuals have a competitive advantage) and a disturbance which strikes at a random time, killing a fraction 1-p of the population. Disturbances include spells of bad weather, such as freezing or heavily raining days. It has been shown by Iwasa and Levin (1995) that when the disturbance is so strong that it kills any individual present when it strikes (hard disturbance, p=0), then the ESS is a mixed strategy (individuals choose their arrival date in an interval of possible dates, according to a certain probability distribution). In this case, individuals wait for a certain time and afterwards start arriving (or breeding) every day. In this paper we explore a biologically more realistic situation whereby the disturbance kills only a fraction of the individuals (soft disturbance, p>0). We also remove some technical assumptions which Iwasa and Levin made on the distribution of the disturbance. We prove that the ESS is still a mixed choice of times, however with respect to the case of hard disturbance, a new phenomenon arises: whenever the disturbance is soft, if the competition is sufficiently strong, the waiting time disappears and a fraction of the population arrives at the earliest day possible, while the rest will arrive throughout the whole period during which the disturbance may occur. This means that under strong competition, the payoff of early arrival balances the increased risk of being killed by the disturbance. We study the behaviour of the ESS and of the average fitness of the population, depending on the parameters involved. We also investigate how the population may be affected by climate change: namely the occurrence of more extreme weather events, which may kill a larger fraction of the population, and time shifts of the distribution of the disturbance. We show how the ESS and the average fitness change under the new climate and discuss which is the impact of the new climate on a population that still follows the old strategy. In particular, we show that, at least under some conditions, extreme weather events imply a temporary decrease of the average fitness of the population due to an increased mortality. In addition, if the population adapts to the new climate, the population may have a larger fitness.

Migratory connectivity and effects of winter temperatures on migratory behaviour of the European robin Erithacus rubecula: a continent-wide analysis.

Many partially migratory species show phenotypically divergent populations in terms of migratory behaviour, with climate hypothesized to be a major driver of such variability through its differential effects on sedentary and migratory individuals. Based on long-term (1947-2011) bird ringing data, we analysed phenotypic differentiation of migratory behaviour among populations of the European robin Erithacus rubecula across Europe. We showed that clusters of populations sharing breeding and wintering ranges varied from partial (British Isles and Western Europe, NW cluster) to completely migratory (Scandinavia and north-eastern Europe, NE cluster). Distance migrated by birds of the NE (but not of the NW) cluster decreased through time because of a north-eastwards shift in the wintering grounds. Moreover, when winter temperatures in the breeding areas were cold, individuals from the NE cluster also migrated longer distances, while those of the NW cluster moved over shorter distances. Climatic conditions may therefore affect migratory behaviour of robins, although large geographical variation in response to climate seems to exist.

Light-level geolocators reveal covariation between winter plumage molt and phenology in a trans-Saharan migratory bird.

Contingent individual performance can depend on the environment experienced at previous life-stages. Migratory birds are especially susceptible to such carry-over effects as they periodically travel between breeding ranges and 'wintering' areas where they may experience broadly different ecological conditions. However, the study of carry-over effects is hampered by the difficulty of tracking vagile organisms throughout their annual life-cycle. Using information from light-level geolocators on the barn swallow (Hirundo rustica), we tested if feather growth bar width (GBW), a proxy of feather growth rate which depends on individual condition, and wing isometric size and shape predict the phenology of subsequent migration. GBW did not predict duration of wintering but negatively predicted the duration of spring migration and arrival date to the breeding sites, suggesting that migration phenology is not constrained by molt, and individuals in prime condition achieve both faster molt and earlier arrival. Wing morphology did not predict migration duration, as expected if wing shape were optimized for foraging, rather than migration performance, in this aerially foraging, insectivorous bird. Thus, we showed for the first time that migration phenology in a long-distance migratory bird covaries with body condition during wintering, as reflected by the growth rate of feathers.

Nestling rearing is antioxidant demanding in female barn swallows (Hirundo rustica).

Reproduction is a demanding activity, since organisms must produce and, in some cases, protect and provision their progeny. Hence, a central tenet of life-history theory predicts that parents have to trade parental care against body maintenance. One physiological cost thought to be particularly important as a modulator of such trade-offs is oxidative stress. However, evidence in favour of the hypothesis of an oxidative cost of reproduction is contradictory. In this study, we manipulated the brood size of wild barn swallows Hirundo rustica soon after hatching of their nestlings to test whether an increase in nestling rearing effort translates into an increased oxidative damage and a decreased antioxidant protection at the end of the nestling rearing period. We found that, while plasma oxidative damage was unaffected by brood size enlargement, females rearing enlarged broods showed a decrease in plasma non-enzymatic antioxidants during the nestling rearing period. This was not the case among females rearing reduced broods and among males assigned to either treatment. Moreover, individuals with higher plasma oxidative damage soon after the brood size manipulation had lower plasma non-enzymatic antioxidants at the end of the nestling rearing period, suggesting that non-enzymatic antioxidants were depleted to buffer the negative effects of high oxidative damage. Our findings point to antioxidant depletion as a potential mechanism mediating the cost of reproduction among female birds.

The surface of small glaciers as radioactive hotspots: Concentration of radioisotopes during predicted intensive melting in the Alps.

Glaciers are considered secondary sources of pollutants, including radioisotopes such as Cesium or Plutonium, with heightened concentrations compared to other ecosystems. The predicted melting of glaciers poses a substantial risk of releasing stored radioisotopes, yet understanding the glacier-specific factors influencing their concentration remains limited. This study investigates the relationship between glacier altitude, surface area, organic matter content in dark supraglacial sediment (cryoconite), precipitation, and activity concentrations of natural (210Pb) and anthropogenic radionuclides (137Cs and 241Am) across 19 Alpine glaciers. Results indicate that radioisotope concentrations depend on organic matter content in the cryoconite, highlighting the role of biotic-abiotic interactions in pollutant accumulation on glaciers. Moreover, 210Pb activity concentration decreases with glacier altitude, likely due to atmospheric variations in 222Rn. Water precipitation events, such as during peaks in 137Cs deposition and after the Chernobyl Nuclear Power Plant disaster, do not impact current activity concentrations. Importantly, radioisotope concentrations in cryoconite are higher on smaller glaciers. This directly supports the hypothesis that the cryoconite retains a significant share of radioisotopes stored in the ice during intensive melting. Since many small glaciers in the Alps are predicted to disappear within the next 50 years, we anticipate release of radioisotopes to mountain ecosystems might be higher than previously forecasted.

Microfibers in the Diet of a Highly Aerial Bird, the Common Swift Apus apus.

Microplastic pollution is a pervasive global issue affecting various ecosystems. Despite the escalating production and well-documented contamination in both aquatic and terrestrial environments, the research focused on airborne microplastics and their interaction with terrestrial birds remains limited. In this study, we collected fecal sacs from Common swifts (Apus apus) to investigate their diet and to evaluate the potential ingestion of microplastics by both adults and nestlings. The diet was mainly composed of Hymenoptera and Coleoptera and did not differ among sexes and age classes. The 33% of nestlings' and 52% of adults' fecal sacs contained anthropogenic items, the totality of which was in the shape form of fibers. The 19.4% of the anthropogenic items were chemically characterized as microplastics, either polyethylene terephthalate (PET; two microfibers) or cellophane (four microfibers). Airborne anthropogenic items, including microplastic, might be passively ingested during the Common swift aerial feeding. In addition, our findings suggest that these ingested microparticles have the potential to be transferred to the offspring through food. While further research is essential to elucidate the pathways of microplastic ingestion, our results reinforce the evidence of the transfer of anthropogenic items from the atmosphere to the biota.

The importance of species addition 'versus' replacement varies over succession in plant communities after glacier retreat.

The mechanisms underlying plant succession remain highly debated. Due to the local scope of most studies, we lack a global quantification of the relative importance of species addition 'versus' replacement. We assessed the role of these processes in the variation (ß-diversity) of plant communities colonizing the forelands of 46 retreating glaciers worldwide, using both environmental DNA and traditional surveys. Our findings indicate that addition and replacement concur in determining community changes in deglaciated sites, but their relative importance varied over time. Taxa addition dominated immediately after glacier retreat, as expected in harsh environments, while replacement became more important for late-successional communities. These changes were aligned with total ß-diversity changes, which were more pronounced between early-successional communities than between late-successional communities (>50 yr since glacier retreat). Despite the complexity of community assembly during plant succession, the observed global pattern suggests a generalized shift from the dominance of facilitation and/or stochastic processes in early-successional communities to a predominance of competition later on.

Unravelling the bacterial diversity in the atmosphere.

The study of airborne biological particles ('bioaerosols') has gained interest in recent years, due to an increasing amount of evidence suggesting that this fraction of airborne particulate matter may play a critical role in the negative effects of aerosols on biological systems. Pioneer investigations demonstrated that bacteria do exist in the atmosphere and can be metabolically active, although studies have not proved whether they actually form ecological communities or are merely assemblages of organisms passively transported from different sources. For a long time, cultivation-based methods have been the gold standard to describe and quantify airborne microorganisms. However, the use of culture-independent techniques and, more recently, of the next-generation sequencing-based methods, has improved the ability of the scientific community to investigate bioaerosols in detail and to address further research questions, such as the temporal and spatial variability of airborne bacterial assemblages, the environmental factors affecting this variability and the potential sources of atmospheric bacteria. This paper provides a systematic review of the state-of-the-art methodologies used in the study of airborne bacteria to achieve each of the aforementioned research objectives, as well as the main results obtained so far. Critical evaluations of the current state of the knowledge and suggestions for further researches are provided.