Distance Decay Of Similarity Research Articles

Biogeography of larval trematodes in the freshwater snail, Semisulcospira libertina: a comparison of the morphological and molecular approaches

While biogeographic patterns of free-living organisms are well documented, the biogeography of parasitic fauna remains largely unclear. Due to morphological similarities, parasites are often difficult to identify without the aid of molecular genetics, further complicating the interpretation of their biogeographic patterns. We investigated trematode parasites infecting the East Asian freshwater snail Semisulcospira libertina to understand their biogeography and to evaluate how molecular approaches influence the interpretation of biogeographic patterns of the trematode fauna. We identified 46 genetically delimited species from 19 morphologically distinguishable trematodes infecting S. libertina and found that their species richness was negatively correlated to latitude. We also found that potential definitive host (fishes) richness and host body size were positively correlated with trematode species richness, suggesting that host attributes are essential factors shaping the biogeographic pattern in trematodes. These trends were observed irrespective of species identification methods, demonstrating that classical morphological identification can also effectively identify the latitudinal gradient pattern in trematodes. We further detected the distance decay of similarity in trematode communities, although this trend was only detectable in the biogeographic dataset based on molecular identification. Our study showed that morphological identification sufficiently reflects the latitudinal richness gradient while molecular identification is essential to estimate accurate local species richness and increase the resolution of the large-scale pattern of population similarities in the trematode communities.

Taxonomic distinctness and diversity patterns of a polychaete (Annelida) community on the continental shelf of the Southern Gulf of Mexico.

The spatial patterns of taxonomic diversity of annelid polychaete species from the continental shelf in the Southern Gulf of Mexico were examined in this study. We used taxonomic distinctness and its spatial variations to explore the diversity patterns and how they change between Southern Gulf of Mexico regions. In addition, using taxonomic distinctness as a dissimilarity measure and Ward's Clustering, we characterized three distinct faunal assemblages. We also investigated patterns of richness, taxonomic distinctness, and distance decay of similarity between sampling stations as a ß-diversity measure. Finally, we examined the spatial relationships between polychaete assemblages and environmental variables to test the relative importance of spatial and environmental components in annelid polychaete community structure from the Southern Gulf of Mexico. We used a combination of eigenvector-based multivariate analyses (dbMEMs) and distance-based redundancy analysis (dbRDA) to quantify the relative importance of these explanatory variables on the spatial variations of taxonomic distinctness. The significance level of spatial and environmental components to the distribution of polychaete species showed that the combined effect of spatial processes and sediment characteristics explained a higher percentage of the variance than those parameters could alone.

Distinct orchid mycorrhizal fungal communities among co-occurring Vanilla species in Costa Rica: root substrate and population-based segregation.

Despite being the second largest family of flowering plants, orchids represent community structure variation in plant-microbial associations, contributes to niche partitioning in metacommunity assemblages. Yet, mycorrhizal communities and interactions remain unknown for orchids that are highly specialized or even obligated in their associations with their mycorrhizal partners. In this study, we sought to compare orchid mycorrhizal fungal (OMF) communities of three co-occurring hemiepiphytic Vanilla species (V. hartii, V. pompona, and V. trigonocarpa) in tropical forests of Costa Rica by addressing the identity of their OMF communities across species, root types, and populations, using high-throughput sequencing. Sequencing the nuclear ribosomal internal transcribed spacer (nrITS) yielded 299 fungal Operational Taxonomic Units (OTUs) from 193 root samples. We showed distinct segregation in the putative OMF (pOMF) communities of the three coexisting Vanilla hosts. We also found that mycorrhizal communities associated with the rare V. hartii varied among populations. Furthermore, we identified Tulasnellaceae and Ceratobasidiaceae as dominant pOMF families in terrestrial roots of the three Vanilla species. In contrast, the epiphytic roots were mainly dominated by OTUs belonging to the Atractiellales and Serendipitaceae. Furthermore, the pOMF communities differed significantly across populations of the widespread V. trigonocarpa and showed patterns of distance decay in similarity. This is the first report of different pOMF communities detected in roots of wild co-occurring Vanilla species using high-throughput sequencing, which provides evidence that three coexisting Vanilla species and their root types exhibited pOMF niche partitioning, and that the rare and widespread Vanilla hosts displayed diverse mycorrhizal preferences.

Soil and bark biodiversity forms discrete islands between vineyards that are not affected by distance or management regime.

Within geographic regions, the existing data suggest that physical habitat (bark, soil, etc.) is the strongest factor determining agroecosystem microbial community assemblage, followed by geographic location (site), and then management regime (organic, conventional, etc.). The data also suggest community similarities decay with increasing geographic distance. However, integrated hypotheses for these observations have not been developed. We formalized and tested such hypotheses by sequencing 3.8 million bacterial 16S, fungal ITS2 and non-fungal eukaryotic COI barcodes deriving from 108 samples across two habitats (soil and bark) from six vineyards sites under conventional or conservation management. We found both habitat and site significantly affected community assemblage, with habitat the stronger for bacteria only, but there was no effect of management. There was no evidence for community similarity distance-decay within sites within each habitat. While communities significantly differed between vineyard sites, there was no evidence for between site community similarity distance-decay apart from bark bacterial communities, and no correlations with soil and bark pH apart from soil bacterial communities. Thus, within habitats, vineyard sites represent discrete biodiversity islands, and while bacterial, fungal and non-fungal eukaryotic biodiversity mostly differs between sites, the distance by which they are separated does not define how different they are.

Environmental distances are more important than geographic distances for predicting earthworm gut bacterial community composition

Distance-decay of similarity is one of the most widely studied patterns in biogeography. Both geographic distance and environmental distance can cause distance-decay of similarity in ecological communities. Here, we studied distance-decay relationships of bacteria in the gut of an earthworm species, Eisenia nordenskioldi, which is widely distributed in Eurasian boreal region. Using high-throughput sequencing, we investigated bacterial community composition in different gut sections, together with the surrounding soil. We specifically quantified the effects of geographic distance and environmental distance on distance-decay pattern of both soil and earthworm gut bacteria. Environmental distance significantly affected soil bacteria, and bacteria in the foregut and midgut of earthworms, with those in the foregut changing most, whereas geographic distance only significantly affected bacteria in the foregut of earthworms. Both, environmental and geographic distance explained bacterial community composition in the gut of earthworms and in surrounding soil, with environmental distance being more important than geographic distance. The results indicate that bacteria in the foregut of earthworms are most sensitive to environmental changes than those in the other sections. Overall, the results suggest that homogenization processes during gut passage of earthworms contribute to similarity of soil microbial communities in space due to the production of casts of similar bacterial community composition.

Assembly processes of eukaryotic plankton communities in the world's largest drinking water diversion project

The largest engineered water diversion project–the Middle Route of the South to North Water Diversion Project (MRP), is of strategic importance in solving the problem of the northern water shortage in China. Eukaryotic plankton are important to the water quality stability in the MRP, but little has been reported about their dynamics and assembly processes, especially for abundant and rare communities. In this study, amplicon sequencing was used to investigate the eukaryotic plankton communities. The results revealed both abundant and rare communities exhibited similar distance–decay patterns, but abundant communities were particularly subject to environmental heterogeneity and played an important role in determining seasonal differences in eukaryotic plankton communities and alpha diversity. In the MRP, with its strong hydrodynamic exchange, abundant and rare communities were mainly affected by stochastic processes, especially homogenizing dispersal. In addition, abundant communities were subject to moderate variable selection (25 %) and rare communities were affected by a higher proportion of dispersal limitation (27 % vs. 10 %). The variation in water temperature and water velocity led to a shift from a stochastic to a deterministic process dominating the assembly of abundant communities. This study extends insights into the dynamics and assembly processes of abundant and rare eukaryotic plankton communities in the large, engineered drinking water diversion project, which is also useful for the management and regulation of the MRP.

Navigating the scales of diversity in subtropical and coastal fish assemblages ascertained by eDNA and visual surveys

Environmental DNA (eDNA) metabarcoding emerges as a powerful method, allowing a more exhaustive investigation of fish fauna than any other methods. Yet, the general use of eDNA as a replacement of traditional methods such as visual surveys or physical sampling remains debatable. Therefore, a prior understanding of eDNA’s spatial resolution is necessary. This study aimed to compare the assessments of fish diversity at three spatial scales by eDNA, underwater visual census (UVC), and diver-operated video (DOV) surveys across 21 reef sites in northern Taiwan. The specific objectives were to explore the regional species pool (γ-diversity), reveal spatial patterns of fish assemblages (β-diversity), and disentangle the relationships between fish assemblages and benthic composition (α-diversity). At the γ-diversity level, a total of 438 marine fish species were detected across methods. eDNA exhibits an extraordinary power to explore the regional species pool given sufficient replication, a power which is unachievable by DOV and UVC. At the β-diversity level, all the methods successfully revealed the same spatial patterns of beta diversity and the distance decay of similarity in fish assemblages. At the α-diversity level, none of the methods is capable of investigating the entire resident fish fauna, but visual surveys are more suitable for scrutinizing interactions between fish and benthos. Instead of undiscriminatingly recommending a combination of eDNA with traditional survey methods, we suggest implementing specific surveys in accordance with the ecological questions of interest.

Multi-group biodiversity distributions and drivers of metacommunity organization along a glacial–fluvial–limnic pathway on the Tibetan plateau

Extensive global glacial retreats are threatening cryosphere ecosystem functioning and the associated biota in glacier-fed water systems. Understanding multi-group biodiversity distributions and compositional variation across diverse but hydrologically linked habitats under varying glacial influences will help explain the mechanisms underlying glacial community organization and ecosystem processes. However, such data are generally lacking due to the difficulty of obtaining biodiversity information across wide taxonomic ranges. Here, we used a multi-marker environmental DNA metabarcoding approach to simultaneously investigate the spatial patterns of community compositions and assembly mechanisms of four taxonomic groups (cyanobacteria, diatoms, invertebrates, and vertebrates) along the flowpaths of a tributary of Lake Nam Co on the Tibetan Plateau—from its glacier headwaters, through its downstream river and wetlands, to its estuary. We detected 869 operational taxonomic units: 119 cyanobacterial, 395 diatom, 269 invertebrate, and 86 vertebrate. Taxonomic richnesses consistently increased from upstream to downstream, and although all groups showed community similarity distance decay patterns, the trend for vertebrates was the weakest. Cyanobacteria, diatom, and invertebrate community compositions were significantly correlated with several environmental factors, while the vertebrate community was only correlated with waterway width. Variation partitioning analysis indicated that varying extents of environmental conditions and spatial factors affected community organizations for different groups. Furthermore, stochastic processes contributed prominently to the microorganisms' community assembly (Sloan's neutral model R2 = 0.77 for cyanobacteria and 0.73 for diatoms) but were less important for macroorganisms (R2 = 0.21 for invertebrates and 0.15 for vertebrates). That trend was further substantiated by modified stochasticity ratio analyses. This study provides the first holistic picture of the diverse biotic communities residing in a series of hydrologically connected glacier-influenced habitats. Our results both uncovered the distinct mechanisms that underlie the metacommunity organizations of different glacial organisms and helped comprehensively predict the ecological impacts of the world's melting glaciers.

Microbial community diversity and function analysis of Aconitum carmichaelii Debeaux in rhizosphere soil of farmlands in Southwest China.

Understanding how microbial communities affect plant growth is crucial for sustainable productivity and ecological health. However, in contrast with the crop system, there is limited information on the microbial community associated with the medicinal plant. We observed that altitude was the most influential factor on the soil microbial community structures of Aconitum carmichaelii Debeaux. For community composition, bacterial reads were assigned to 48 phyla, with Proteobacteria, Acidobacteriota, and Actinobacteriota being the dominant phyla. The fungal reads were assigned to seven phyla, and Ascomycota was the predominant phylum detected in most groups. The four dominant phyla were categorized as keystone taxa in the co-occurrence networks, suggesting that they may be involved in soil disease suppression and nutrient mobility. Bacterial co-occurrence networks had fewer edges, lower average degree, and lower density at YL1, HQ1, HQ2, BC, and DL than fungal networks, creating less intricate rhizosphere network patterns. Furthermore, the bacterial and fungal communities showed strong distance decay of similarity across the sampling range. Overall, this study improves our understanding of regulating rhizosphere microbial communities in soil systems and also provides potential production strategies for planting A. carmichaelii.

Ecology of testate amoebae in waterbodies of the Central Highlands and South-Central Coast provinces of Vietnam with the description of new species Difflugia quangtrani sp. nov.

This study provides new data on ecology of testate amoebae in the waterbodies of the Central Highlands and South-Central Coast regions of Vietnam. A total of 108 species and subspecies including six new records for Vietnam were identified. One new species Difflugia quangtrani sp. nov. was described. Difflugia, Arcella, and Centropyxis were the most species-rich genera, and Centropyxis aculeata was the most common species. The results of NMDS and ANOSIM analyses for the first time showed significant differences in composition of tropical freshwater testate amoeba communities within both various types of waterbodies (reservoir, river, and wetland) and biotopes (plankton and benthos). Average species richness per sample in different types of waterbodies was similar, while that of plankton was statistically higher than in benthos. The distance decay of similarity in tropical freshwater testate amoeba community of both plankton and benthos was observed for the first time indicating the importance of geographical distance in testacean species composition changes between samples. Redundancy analyses followed by the forward selection have determined elevation, pH, and dissolved oxygen as significant factors that affect tropical freshwater testacean community. More large-scale climatic and small-scale hydrological and hydrochemical variables should be included into further studies to find out most important factors determining structure of freshwater testacean assemblages.

Generalists and Specialists Determine the Trend and Rate of Soil Fungal Distance Decay of Similarity in a 20-ha Subtropical Forest

Fungi are an important component of microbial communities that serve a variety of important roles in nutrient cycling and are essential for plant nutrient uptake in forest soils. Distance decay of similarity (DDS) is one of the few ubiquitous phenomena in community ecology. However, the contribution of specialist and generalist fungal species in shaping DDS remains poorly investigated. Through removing operational taxonomic units (OTU) with low or high frequencies, we rigorously quantified the impact of specialists or generalists on the change in slope, initial similarity, and halving distance of DDS of undefined saprotroph, plant mutualist, and plant putative pathogen communities in a 20-ha subtropical evergreen forest plot in Yunnan Province, Southwest China. We hypothesized that (1) the soil fungal co-occurrence networks are different between the three fungal guilds; (2) specialists and generalists contribute to the spatial turnover and nestedness of beta diversity, respectively; and (3) the removal of specialists or generalists will have opposite effects on the change of slope, initial similarity, and halving distance of DDS. Co-occurrence network analysis showed that the undefined saprotroph network had a much more complicated structure than mutualist and pathogen networks. Ascomycota and Basidiomycota were the two most abundant phyla in soil fungal communities. We found that partly in line with our expectations, the change in initial similarity increased and decreased when removing specialists and generalists, respectively, but there was always one exception guild of out of the three communities for the change in slope and halving distance. We identified that such change was mainly due to the change in turnover and nestedness of beta diversity. Furthermore, the results show that species turnover rather than species nestedness drove fungal beta diversity across functional guilds for both specialists and generalists.

Explanation of beta diversity in European alpine grasslands changes with scale

AbstractThe importance of environmental difference among sites and dispersal limitations of species to the explanation of diversity differs among biological systems and geographical regions. We hypothesized that climate and then dispersal limitation will predominantly explain the similarity of alpine vegetation at increasing distances between pairs of regions at subcontinental extent. We computed the similarity of all pairs of 23 European mountain regions below 50° N after dividing the species lists of each region by calcareous or siliceous substrates. Distance decay in similarity was better fitted by a cubic polynomial than a negative exponential function, and the fit was better on calcareous than on siliceous substrate. Commonality analysis revealed that the proportion of explanation of beta diversity by climatic difference had unimodal patterns on a gradient of increasing distance between regions, while explanation by dispersal limitation had consistently rising patterns on both substrates. On siliceous substrate, dispersal limitation explained more of the variation in beta diversity only at longer distances, but it was predominant at all distances on calcareous substrate. The steeper response to distance at <1600 km and >2600 km may indicate dispersal limitation at different temporal scales, and the uptick in the response to distance at the longest distances may reflect how isolated some regions have been before and since the last glacial maximum.

Small‐scale spatiotemporal variability and distance–decay relationships in intertidal micro‐ and meiobenthic assemblages

AbstractCompositional similarity generally decreases with increasing geographic distance between sites (distance decay of similarity, DDS). Two non‐exclusive mechanisms have been proposed to explain this pattern: increasing differences in environmental conditions and pure spatial effects of dispersal limitation. On regional and global scales, environmental selection is considered the major factor for microbial assemblages, whereas pure spatial effects are generally more profound for larger organisms with limited dispersal ability. We investigated the DDS relationships at a small spatial scale (decimeters ‐ tens of meters) for three groups of benthic organisms differing in body size: diatoms and flagellates (DF), ciliates (CL), and harpacticoids (HA). All groups demonstrated definite DDS relationships. The pure spatial effects were strongest for the smallest‐size protists (DF), relatively weaker for CL and negligible for HA. In contrast, environmental factors (i.e., the differences in sediment properties) were more important for the HA and CL assemblages than for DF assemblages. In addition, we revealed a considerable temporal component of DDS relationships. First, similarity decreased with the time interval between sampling events (“temporal DDS”). Second, the average spatial similarity itself increased with time. These effects were significant for DF and CL only but were weak or undetectable for HA. Thus, our results are opposite to those commonly observed at larger (regional or global) scales. We suppose that the DDS relationships obtained at different spatial scales, despite formal similarity, reflect different phenomena driven by different mechanisms. At the geographical scale, species distribution patterns are mainly driven by long‐distance dispersal processes operated at the population level and dependent on their size‐related traits, which, taken in common, offer the advantage of small‐sized groups. The local‐scale distribution is primarily determined by individual motility and within‐habitat environmental heterogeneity. Small protists are relatively less motile, finely perceived and, therefore, have higher rates of spatial decay.

Epiphytic fungal communities vary by substrate type and at submetre spatial scales.

Fungal species have numerous important environmental functions. Where these functions occur will depend on how fungi are spatially distributed, but the spatial structures of fungal communities are largely unknown, especially in understudied hyperdiverse tropical tree canopy systems. Here we explore fungal communities in a Costa Rican tropical rainforest canopy, with a focus on local-scale spatial structure and substrate specificity of fungi. Samples of ~1cm3 were collected from 135 points along five adjacent tree branches, with intersample distances from 1 to 800cm, and dissected into four substrates: outer host tree bark, inner bark, dead bryophytes and living bryophytes. We sequenced the ITS2 region to characterize total fungal communities. Fungal community composition and diversity varied among substrate types, even when multiple substrates were in direct contact. Fungi were most diverse in living bryophytes, with 39% of all operational taxonomic units (OTUs) found exclusively in this substrate, and the least diverse in inner bark. Fungal communities had significant positive spatial autocorrelation and distance decay of similarity only at distances less than 1m. Similarity among samples declined by half in less than 10cm, and even at these short distances, similarities were low with few OTUs shared among samples. These results indicate that community turnover is high and occurs at very small spatial scales, with any two locations sharing very few fungi in common. High heterogeneity of fungal communities in space and among substrates may have implications for the distributions, population dynamics and diversity of other tree canopy organisms, including epiphytic plants.

Characterizing the spatial signal of environmental DNA in river systems using a community ecology approach.

Environmental DNA (eDNA) is gaining a growing popularity among scientists but its applicability to biodiversity research and management remains limited in river systems by the lack of knowledge about the spatial extent of the downstream transport of eDNA. Here, we assessed the ability of eDNA inventories to retrieve spatial patterns of fish assemblages along two large and species-rich Neotropical rivers. We first examined overall community variation with distance through the distance decay of similarity and compared this pattern to capture-based samples. We then considered previous knowledge on individual species distributions, and compared it to the eDNA inventories for a set of 53species. eDNA collected from 28sites in the Maroni and 25sites in the Oyapock rivers permitted to retrieve a decline of species similarity with increasing distance between sites. The distance decay of similarity derived from eDNA was similar and even more pronounced than that obtained with capture-based methods (gill-nets). In addition, the species upstream-downstream distribution range derived from eDNA matched to the known distribution of most species. Our results demonstrate that environmental DNA does not represent an integrative measure of biodiversity across the whole upstream river basin but provides a relevant picture of local fish assemblages. Importantly, the spatial signal gathered from eDNA was therefore comparable to that gathered with local capture-based methods, which describes fish fauna over a few hundred metres.

Dispersal limitations and long-term persistence drive differentiation from haplotypes to communities within a tropical sky-island: Evidence from community metabarcoding.

Neutral theory proposes that dispersal stochasticity is one of the main drivers of local diversity. Haplotypes-level genetic variation can now be efficiently sampled from across whole communities, thus making it possible to test neutral predictions from the genetic to species-level diversity, and higher. However, empirical data is still limited, with the few studies to date coming from temperate latitudes. Here, we focus on a tropical mountain within the Transmexican Volcanic Belt to evaluate spatially fine-scale patterns of arthropod community assembly to understand the role of dispersal limitation and landscape features as drivers of diversity. We sampled whole-communities of arthropods for eight orders at a spatial scale ranging from 50m to 19km, using whole community metabarcoding. We explored multiple hierarchical levels, from individual haplotypes to lineages at 0.5, 1.5, 3, 5, and 7.5% similarity thresholds, to evaluate patterns of richness, turnover, and distance decay of similarity with isolation-by-distance and isolation-by-resistance (costs to dispersal given by landscape features) approaches. Our results showed that distance and altitude influence distance decay of similarity at all hierarchical levels. This holds for arthropod groups of contrasting dispersal abilities, but with different strength depending on the spatial scale. Our results support a model where local-scale differentiation mediated by dispersal constraints, combined with long-term persistence of lineages, is an important driver of diversity within tropical sky islands.

A fine-scale spatial analysis of fungal communities on tropical tree bark unveils the epiphytic rhizosphere in orchids.

Approximately 10% of vascular plants are epiphytes and, even though this has long been ignored in past research, are able to interact with a variety of fungi, including mycorrhizal taxa. However, the structure of fungal communities on bark, as well as their relationship with epiphytic plants, is largely unknown. To fill this gap, we conducted environmental metabarcoding of the ITS-2 region to understand the spatial structure of fungal communities of the bark of tropical trees, with a focus on epiphytic orchid mycorrhizal fungi, and tested the influence of root proximity. For all guilds, including orchid mycorrhizal fungi, fungal communities were more similar when spatially close on bark (i.e. they displayed positive spatial autocorrelation). They also showed distance decay of similarity with respect to epiphytic roots, meaning that their composition on bark increasingly differed, compared to roots, with distance from roots. We first showed that all of the investigated fungal guilds exhibited spatial structure at very small scales. This spatial structure was influenced by the roots of epiphytic plants, suggesting the existence of an epiphytic rhizosphere. Finally, we showed that orchid mycorrhizal fungi were aggregated around them, possibly as a result of reciprocal influence between the mycorrhizal partners.

Annual fires reduce local species richness but do not homogenize the composition of savanna woody species

Savanna woody species evolved with fire. However, the effect of changes in the fire regimes, which tend to increase in the next decades, is not yet fully understood. Here, we tested the effects of increased fire frequency on species richness and the composition of woody plant communities. Specifically, we tested if a high frequency fire management reduces local species richness of communities and homogenizes species composition. We sampled woody plants in 40 sites, distributed in 20 pairs, in Emas National Park (Brazil). Each pair included sites with high fire frequency (firebreaks that are burned annually since 1994) and with natural fire frequency (occurring every 3–5 years). We used a paired t-test to assess effects of the two fire frequency treatments on species richness and permutation tests to compare distance decay in similarity composition and relative abundances. We found that sites with high fire frequency had lower species richness compared to their paired sites subjected to natural fire frequency. However, similarities in species composition among sites with high fire frequency were only slightly higher than among sites subjected to natural fire frequency. The rate of distance decay in similarity was similar on the two treatments. At local scales, the high frequency of fire reduced species richness, an impact that in the long run may be irreversible. However, at broader spatial scales, effects were small as two treatments included similar sets of species. Prescribed fires have the potential to modify the natural dynamics of the woody communities in the savannas we studied. In our study, fire appears to act as a hierarchical filter, selecting species at broader spatial scales and nesting communities at local scales.

Assessing the equilibrium between assemblage composition and climate: A directional distance-decay approach.

The variation of assemblage composition in space is characterised by the decrease in assemblage similarity with spatial distance. Climatic constraint and dispersal limitation are major drivers of distance-decay of similarity. Distance-decay of similarity is usually conceptualised and modelled as an isotropic pattern, that is, assuming that similarity decays with the same rate in all directions. Because climatic gradients are markedly anisotropic, that is, they have different strength in different directions, if species distributions were in equilibrium with climate, the decay of assemblage similarity should be anisotropic in the same direction as the climatic gradient, that is, faster turnover in the direction that maximises the climatic gradient. Thus, deviations from equilibrium between assemblage composition and climatic conditions would result in differences in anisotropy between distance-decay of similarity and climatic gradients. We assessed anisotropy in distance-decay patterns in marine plankton assemblages, terrestrial vertebrates and European beetles, using two procedures: (a) measuring the correlation between the residuals of a distance-decay model and the angle in which pairs of sites are separated and (b) computing two separate distance-decay models for each dataset, one using only pairwise cases that are separated on North-South direction and another one using pairwise cases separated on East-West direction. We also analysed whether the degree of anisotropy in distance-decay is related to dispersal ability (proportion of wingless species and body size) and ecological niche characteristics (main habitat and trophic position) by assessing these relationships among beetle taxonomic groups (n=21). Anisotropy varied markedly across realms and biological groups. Despite climatic gradients being steeper in North-South direction than in East-West direction in all datasets, North-South distance-decays tended to be steeper than East-West distance-decays in plankton and most vertebrate assemblages, but flatter in European amphibians and most beetle groups. Anisotropy also markedly varied across beetle groups depending on their dispersal ability, as the proportion of wingless species explained 60% of the variance in the difference between North-South and East-West distance-decay slopes. Our results suggest that the degree of equilibrium decreases from marine to terrestrial realms, and is markedly different between vertebrates and beetles. This has profound implications on the expected ability of different groups to track their suitable climates, and thus on the impact of climate change on biodiversity.

Exploring the changing geographical pattern of international scientific collaborations through the prism of cities.

Science is becoming increasingly international in terms of breaking down walls in its pursuit of high impact. Despite geographical location and distance still being major barriers for scientific collaboration, little is known about whether high-impact collaborations are similarly constrained by geography compared to collaborations of average impact. To address this question, we analyze Web of Science (WoS) data on international collaboration between global leader cities in science production. We report an increasing intensity of international city-city collaboration and find that average distance of collaboration of the strongest connections has slightly increased, but distance decay has remained stable over the last three decades. However, high-impact collaborations span large distances by following similar distance decay. This finding suggests that a larger geographical reach of research collaboration should be aimed for to support high-impact science. The creation of the European Research Area (ERA) represents an effective action that has deepened intracontinental research collaborations and the position of the European Union (EU) in global science. Yet, our results provide new evidence that global scientific leaders are not sufficiently collaborative in carrying out their big science projects.