Geographic variation in salt marsh structure and function

We examined geographic variation in the structure and function of salt marsh communities along the Atlantic and Gulf coasts of the United States. Focusing on the arthropod community in the dominant salt marsh plant Spartina alterniflora, we tested two hypotheses: first, that marsh community structure varies geographically, and second, that two aspects of marsh function (response to eutrophication and addition of dead plant material) also vary geographically. We worked at eleven sites on the Gulf Coast and eleven sites on the Atlantic Coast, dividing each coast up into two geographic areas. Abiotic conditions (tidal range, soil organic content, and water content, but not soil salinity), plant variables (Spartina nitrogen content, height, cover of dead plant material, but not live Spartina percent cover or light interception), and arthropod variables (proportional abundances of predators, sucking herbivores, stem-boring herbivores, parasitoids, and detritivores, but not total arthropod numbers) varied among the four geographic regions. Latitude and mean tidal range explained much of this geographic variation. Nutrient enrichment increased all arthropod functional groups in the community, consistent with previous experimental results, and had similar effects in all geographic regions, contrary to our hypothesis, suggesting widespread consistency in this aspect of ecosystem function. The addition of dead plant material had surprisingly little effect on the arthropod community. Our results caution against the uncritical extrapolation of work done in one geographic region to another, but indicate that some aspects of marsh function may operate in similar ways in different geographic regions, despite spatial variation in community structure.     

Relationships between Sediment Microbial Communities and Pollutants in Two California Salt Marshes

Salt marshes are important ecosystems whose plant and microbial communities can alter terrestrially derived pollutants prior to coastal water discharge. However, knowledge regarding relationships between anthropogenic pollutant levels and salt marsh microbial communities is limited, and salt marshes on the West Coast of the United States are rarely examined. In this study, we investigated the relationships between microbial community composition and 24 pollutants (20 metals and 4 organics) in two California salt marshes. Multivariate ordination techniques were used to assess how bacterial community composition, as determined by terminal restriction fragment length polymorphism and phospholipid fatty acid analyses, was related to pollution. Sea urchin embryo toxicity measurements and plant tissue metabolite profiles were considered two other biometrics of pollution. Spatial effects were strongly manifested across marshes and across channel elevations within marshes. Utilizing partial canonical correspondence analysis, an ordination technique new to microbial ecology, we found that several metals were strongly associated with microbial community composition after accounting for spatial effects. The major patterns in plant metabolite profiles were consistent with patterns across microbial community profiles, but sea urchin embryo assays, which are commonly used to evaluate ecological toxicity, had no identifiable relationships with pollution. Whereas salt marshes are generally dynamic and complex habitats, microbial communities in these marshes appear to be relatively sensitive indicators of toxic pollutants.     

The natural regeneration of salt marsh on formerly reclaimed land

Question: Does the vegetation of restored salt marshes increasingly resemble natural reference communities over time? Location: The Essex estuaries, southeast England. Methods: Abandoned reclamations, where coastal defences had been breached in storm events, and current salt marsh recreation schemes were surveyed giving a chronosequence of salt marsh regeneration from 2 to 107 years. The presence, abundance and height of plant species were recorded and comparisons were made with adjacent reference salt marsh communities at equivalent elevations. Results: Of the 18 paired sites surveyed, 13 regenerated marshes had fewer species than their adjacent reference marsh, three had an equal number and two had more. The plant communities of only two de‐embankment sites matched that of the reference community. 0–50 year old sites and 51–100 year old sites had fewer species per quadrat than the 101+ year sites and the reference salt marshes. There was a weak relationship between differences in species richness for regenerated and reference marshes and the time since sites were first re‐exposed to tidal inundation. Cover values for the invasive and recently evolved Spartina anglica were greater within regenerated than reference marshes. Conclusions: Salt marsh plants will colonise formerly reclaimed land relatively quickly on resumption of tidal flooding. However, even after 100 years regenerated salt marshes differ in species richness, composition and structure from reference communities.     

Effect of time on the natural regeneration of salt marsh

Abstract. The effect of time on natural regeneration of two salt marshes was studied in relation to plant and edaphic factors. The study was carried out in two naturally restoring salt marshes, differing in restoration time, in Txingudi (Bay of Biscay). After 20 yr, the younger salt marsh had the same plant species richness and high species similarity as a 35 yr old salt marsh (17 and 16, respectively, similarity index = 0.9), but both sites had lower species richness and similarity than a nearby natural salt marsh (36 plant species and similarity indices of 0.45 with the 35 yr old marsh and 0.46 with the 20 yr old marsh). Plant species present in the two recovering salt marshes followed a similar distribution pattern in relation to organic matter, conductivity and moisture content although this zonation differed from the natural salt marsh. The range of edaphic factors measured was also similar, but differed from those in the natural salt marsh. The process of plant species recolonization and spatial distribution might be delayed by a low probability of species arrival and by the time need for the restoration of hydrologic and edaphic factors. This study supports the necessity of long‐term monitoring in measuring coastal salt marsh restoration.     

Using generalized dissimilarity modelling to analyse and predict patterns of beta diversity in regional biodiversity assessment

Generalized dissimilarity modelling (GDM) is a statistical technique for analysing and predicting spatial patterns of turnover in community composition (beta diversity) across large regions. The approach is an extension of matrix regression, designed specifically to accommodate two types of nonlinearity commonly encountered in large-scaled ecological data sets: (1) the curvilinear relationship between increasing ecological distance, and observed compositional dissimilarity, between sites; and (2) the variation in the rate of compositional turnover at different positions along environmental gradients. GDM can be further adapted to accommodate special types of biological and environmental data including, for example, information on phylogenetic relationships between species and information on barriers to dispersal between geographical locations. The approach can be applied to a wide range of assessment activities including visualization of spatial patterns in community composition, constrained environmental classification, distributional modelling of species or community types, survey gap analysis, conservation assessment, and climate-change impact assessment.     

Biogeography of salt marsh plant zonation on the Pacific coast of South America

Aim

The aim of this study was to investigate the biogeography of plant zonation in salt marshes on the Pacific coast of South America; to examine whether salt marsh plant zonation varies with latitude; and to explore the relative importance of climatic, tidal, edaphic and disturbance factors in explaining large‐scale variation in salt marsh plant community structure.

Location

A 2,000‐km latitudinal gradient on the Pacific coast in Chile, with a climate shift from hyper‐arid at low to hyper‐humid at high latitudes.

Methods

Plant zonation was quantified in field surveys of ten marshes. Climate, tidal regimes, edaphic factors and disturbances (tsunami and rainfall floods) were determined. We used multivariate analyses to explore their relative importance in explaining large‐scale variation in salt marsh plant community structure.

Results

Across latitude, marshes were dominated by distinct plant communities in different climate regions, especially at the extreme dry and wet latitudes. Intertidal plant species zonation was present in hyper‐arid and semi‐arid climates, but not in arid, humid and hyper‐humid climates. Latitudinal variation in low‐marsh plant communities (regularly flooded at high tide) was largely a function of precipitation, while at high marshes (never flooded at high tide) latitudinal variation was explained with precipitation, temperature, tidal cycles, soil salinity and disturbances.

Main conclusions

Salt marshes on the Pacific coast of South America belong to Dry Coast and Temperate biogeographic types. Salt marsh plant zonation varies across latitude, and is explained by climatic, tidal, edaphic and disturbance factors. These patterns appear to be mechanistically explained by extrapolating experimentally generated community assembly models and have implications for predicting responses to climate change.     

Responses of Salt Marsh Ecosystems to Mosquito Control Management Practices along the Atlantic Coast (U.S.A.)

Open marsh water management (OMWM) of salt marshes modifies grid‐ditched marshes by creating permanent ponds and radial ditches in the high marsh that reduce mosquito production and enhance fish predation on mosquitoes. It is preferable to using pesticides to control salt marsh mosquito production and is commonly presented as a restoration or habitat enhancement tool for grid‐ditched salt marshes. Monitoring of nekton, vegetation, groundwater level, soil salinity, and bird communities before and after OMWM at 11 (six treatment and five reference sites) Atlantic Coast (U.S.A.) salt marshes revealed high variability within and among differing OMWM techniques (ditch‐plugging, reengineering of sill ditches, and the creation of ponds and radial ditches). At three marshes, the dominant nekton shifted from fish (primarily Fundulidae species) to shrimp (Palaemonidae species) after manipulations and shrimp density increased at other treatment sites. Vegetation changed at only two sites, one with construction equipment impacts (not desired) and one with a decrease in woody vegetation along existing ditches (desired). One marsh had lower groundwater level and soil salinity, and bird use, although variable, was often unrelated to OMWM manipulations. The potential effects of OMWM manipulations on non‐target salt marsh resources need to be carefully considered by resource planners when managing marshes for mosquito control.     

Geography,environment, and spatial turnover of species in China's grasslands

Environment and spatial processes are key factors in shaping species composition in a community. These two factors make competing predictions concerning the decay of species composition similarity with environmental divergence and geographic distance. Unfortunately, these can be difficult to test independently because changes in environment are commonly well correlated with geographic distance. However, an opportunity is provided by exploiting marked regional differences in the spatial structure of the environment. In this study, we test the predictions of environment filtering and dispersal in explaining species turnover using > 300 study sites spanning ?4000 km, across three major grasslands in China in which the environment is spatially structured to different degrees. We find that species composition similarity decayed with environmental divergence in the same way in all three regions, and even across biogeographic regions between which dispersal barriers are evident; in contrast, the decay of species composition similarity with geographic distance depended largely on the spatial structure of the environment. We conclude that, at the scale of study, environmental filtering rather than spatial processes best explains patterns of species turnover in China's grasslands.     

Assessment of coastal salt marsh plants on the Arabian Gulf region

Salt marshes form along coastlines and are very interesting ecosystems due to their function and services. In the future, salt marsh plants might provide food and medicine as crops irrigated via seawater in hyper-arid regions. In the Arabian Gulf, little is known about salt marsh vegetation. Therefore, a targeted search on scientific literature was performed to provide a comprehensive assessment. Hence, current knowledge of the extent and status of salt marsh in the Arabian Gulf region was reviewed, based on literature-based analysis. Then, historic trends of salt marsh publications were carefully inspected. This study provides a list of salt marsh families and their genera and species, with a total of 51 family 179 genera 316 species in the Arabian Gulf. The largest family was Chenopodiaceae followed by Poaceae, Asteraceae. Moreover, this study identified some of the gaps that could help future directions for scientific research, and help making decisions of conservation, management policies and procedures.     

Biogeographical patterns of rocky intertidal communities along the Pacific coast of North America

Aim Our aim in this paper is to present the first broad‐scale quantification of species abundance for rocky intertidal communities along the Pacific coast of North America. Here we examine the community‐level marine biogeographical patterns in the context of formerly described biogeographical regions, and we evaluate the combined effects of geographical distance and environmental conditions on patterns of species similarity across this region. Location Pacific coast of North America. Methods Data on the percentage cover of benthic marine organisms were collected at 67 rocky intertidal sites from south‐eastern Alaska, USA, to central Baja California Sur, Mexico. Cluster analysis and non‐metric multidimensional scaling were used to evaluate the spatial patterns of species similarity among sites relative to those of previously defined biogeographical regions. Matrices of similarity in species composition among all sites were computed and analysed with respect to geographical distance and long‐term mean sea surface temperature (SST) as a measure of environmental conditions. Results We found a high degree of spatial structure in the similarity of intertidal communities along the coast. Cluster analysis identified 13 major community structure ‘groups’. Although breaks between clusters of sites generally occurred at major biogeographical boundaries, some of the larger biogeographical regions contained several clusters of sites that did not group according to spatial position or identifiable coastal features. Additionally, there were several outliers – sites that grouped alone or with sites outside their region – for which localized features may play an important role in driving community structure. Patterns of species similarity at the large scale were highly correlated with geographical distance among sites and with SST. Importantly, we found community similarity to be highly correlated with long‐term mean SST while controlling for the effects of geographical distance. Main conclusions These findings reveal a high degree of spatial structure in the similarity of rocky intertidal communities of the north‐east Pacific, and are generally consistent with those of previously described biogeographical regions, with some notable differences. Breaks in similarity among clusters are generally coincident with known biogeographical and oceanographic discontinuities. The strong correlations between species similarity and both geographical position and SST suggest that both geography and oceanographic conditions have a large influence on patterns of intertidal community structure along the Pacific coast of North America.     

Plathelminth abundance in North Sea salt marshes: environmental instability causes high diversity

Although supralittoral salt marshes are habitats of high environmental instability, the meiofauna is rich in species and abundance is high. The community structure of free-living Plathelminthes (Turbellaria) in these salt marshes is described. On an average, 104 individuals are found below an area of 10 cm². The average species density in ungrazed salt marshes is 11.3 below 10 cm² and 45.2 below 100 cm², indicating strong small-scale heterogenity. The faunal similarity between sediment and the corresponding above-ground vegetation is higher than between adjacent sample sites. Species prefer distinct ranges of salinity. In the lower part of the supralittoral salt marshes, the annual fluctuations of salinity are strongest and highly unpredictable. This region is richest in plathelminth species and abundance; diversity is highest, and the faunal composition of parallel samples is quite similar. In the upper part of the supralittoral salt marshes, the annual variability of salinity is lower, plathelminths are poor in species diversity and abundance. Parallel samples often have no species in common. Thus, those salt marsh regions with the most unstable environment are inhabited by the most diverse species assemblage. Compared to other littoral zones of the North Sea, however, plathelminth diversity in salt marshes is low. The observed plathelminth diversity pattern can apparently be explained by the dynamic equilibrium model (Huston, 1979).     

Vegetation and seed bank dynamics in a tidal freshwater marsh

Questions: What are the feedbacks among the seed bank, parent vegetation, and landscape structure that control plant species turnover in space and time in a tidal freshwater marsh? How can these feedbacks be used to better understand marsh community dynamics and to establish restoration practices that seek to restore vegetation diversity of this important and widely distributed ecosystem? Location: Potomac River, Virginia, United States (15 km south of Washington, DC). Methods: We sampled the seed bank and standing vegetation in a tidal freshwater marsh and explored similarities between seed bank and vegetation composition through space and time. We then investigated marsh surface elevation, distance to nearest tidal channels, and life history of component species as potential explanations for the observed vegetation patterns. Results: The composition of individual plots changed considerably from year to year; however, the composition at broader spatial (whole marsh) and temporal (4 years) scales was relatively stable. Species composition of the seed bank was dissimilar to both the previous and current year's standing vegetation, and similarity to standing vegetation was particularly low in plots dominated by annual species. Landscape structure and life history characteristics of individual species best explained the spatiotemporal variability in marsh vegetation. Conclusions: Restoration designs should be landscape‐dependent and explicitly incorporate spatially structured elements such as elevation gradients to maximize community diversity in reconstructed tidal freshwater marshes. Optimal designs include areas of high seed input, areas of high species turnover, as well as other areas of greater stability.     

Similarity between seed banks and above‐ground vegetation along a salinity gradient

Abstract. Zonation of above‐ground vegetation often occurs in salt marshes along salinity and moisture gradients. The above‐ground vegetation and seed bank in four physiognomically different vegetation zones in a salt marsh were compared to determine their level of similarity using percent similarity as a distance measure. 10‐m transects were established along a salinity gradient through four different vegetation zones; a Salicornia zone, a Salicornia‐Atriplex zone, an Atriplex zone and an Atriplex‐Hordeum zone. A UPGMA cluster analysis demonstrated that the above‐ground vegetation was not usually highly correlated with the seed bank composition of zonal communities. Since seeds of these annual salt marsh species occurred in all zones, the levels of salt stress may be the main factor determining which species were found in the above‐ground vegetation.     

Biogeographical patterns of freshwater micro‐ and macroorganisms: a comparison between phytoplankton,zooplankton and fish in the eastern Mediterranean

Aim We investigated the biogeographical patterns of phytoplankton, zooplankton and fish in freshwater ecosystems. We tested whether spatial distance or environmental heterogeneity act as potential factors controlling community composition. Location Northern and central Greece, eastern Mediterranean. Method Data on 310 phytoplankton, 72 zooplankton and 37 fish species were collected from seven freshwater systems. Species occurrence data were used to generate similarity matrices describing community composition. We performed Mantel tests to compare spatial patterns in community composition of phytoplankton, zooplankton and fish. Next, we examined the correlation between geographical distance and the degree of similarity in community composition. The analysis was repeated for different taxonomic, trophic and size‐based groups of the microorganisms studied. We assessed different environmental variables (topographic and limnological) as predictors of community composition. Results Phytoplankton community composition showed a strong positive correlation with environmental heterogeneity but was not correlated with the geographical distance between systems. Zooplankton community composition was unrelated to geographical distance and was only weakly correlated with environmental variables. In contrast, fish community similarity decayed significantly with distance. We found no relationship along all pairwise comparisons of the compositional matrices of the three groups. The pairwise comparisons of the different taxonomic, trophic and size‐based groups of the microorganism communities studied were in accordance with the results for the entire microorganism community. Main conclusions Our results support the proposition that the biogeography of microorganisms does not demonstrate a distance–decay pattern and further suggest that, in reality, the drivers of distribution depend on the specific community examined. In contrast, the biogeography of macroorganisms was affected by geographical distance. These differences reflect the dispersal abilities of the different organisms. The microorganisms exhibit passive dispersal through the air, with local environmental conditions structuring their community composition. On the other hand, for macroorganisms such as fish, the terrestrial environment could pose barriers to their dispersal; with fish structuring distinctive communities over greater distances. Overall, we suggest that the biogeography of freshwater phytoplankton and zooplankton reflects contemporary environmental conditions, while the biogeographical patterns for fish inhabiting the same systems are related to factors affecting their dispersal ability.     

Composition and abundance of resident marsh-surface nekton: comparison between tidal freshwater and salt marshes in Virginia,USA

Previous research on intertidal nekton communities has identifiedimportant determinants of community structure and distribution; however, fewstudies have compared nekton utilization of disparate marsh habitats. Inthis study, abundance and distribution patterns of resident nekton werecompared between tidal freshwater marsh and salt marsh surfaces varying inflooding depth and duration. Nekton were collected in pit traps installedalong elevational transects at four marshes in coastal Virginia (twofreshwater, two saline) from April through November 1992–1993. Thedominant fish collected at all sites was the mummichog Fundulusheteroclitus. The daggerblade grass shrimp Palaemonetes pugio was thedominant nekton species collected at salt marsh sites, and was seasonallyabundant on tidal freshwater marshes. A positive correlation betweenflooding depth and nekton abundance was observed on salt marshes; anopposite pattern was observed on tidal freshwater marshes. Tidal floodingregime influences the abundance of resident nekton, however, the effect maybe confounded by other environmental variables, including variation insurface topography and seasonal presence or absence of submerged aquaticvegetation (SAV) in adjacent subtidal areas. In mid-Atlantic tidalfreshwater wetlands, SAV provides a predation refuge and forage site forearly life stages of marsh-dependent nekton, and several species utilizethis environment extensively. Salt marshes in this region generally lackdense SAV in adjacent subtidal creeks. Consequently, between-sitedifferences in species and size-specific marsh surface utilization byresident nekton were observed. Larvae and juveniles represented 79%and 59% of total fish collected at tidal freshwater and salt marshsites, respectively. The resident nekton communities of tidal freshwater andsalt marsh surfaces are characterized by a few ubiquitous species with broadenvironmental tolerances. This revised version was published online in July 2006 with corrections to the Cover Date.     

Environment, dispersal and patterns of species similarity

Aim The aim of this paper is to evaluate the combined effects of geographical distance and environmental distance on patterns of species similarity (similarity in species composition between sites), and to identify factors affecting the rate of decay in species similarity with each type of distance. Location Israel. Methods Data on species composition of land snails and land birds were recorded in 27 sites of 1 × 1 km scattered across a rainfall gradient in Israel. Matrices of similarity in species composition between all pairs of sites were computed and analysed with respect to corresponding matrices of geographical distance and rainfall distance (defined as the difference in mean annual rainfall between sites, and used as a measure of environmental distance). Mantel tests were applied to determine the correlation between species similarity and each type of distance. Factors affecting the decay in species similarity were investigated by comparing different subsets of the data using randomization tests. Results Both rainfall distance and geographical distance had negative effects on species similarity. The effect of rainfall distance was statistically significant even after controlling for differences in geographical distance, and vice versa. The per‐unit effect of rainfall distance on species similarity decreased with increasing geographical distance, indicating that the two types of distances interacted in determining the similarity in species composition. Snails showed a higher rate of decay in species similarity with geographical distance than birds, and large snails showed a higher rate of decay than small snails, which are better passive dispersers. The per‐unit effects of both rainfall distance and geographical distance on species similarity were higher in the desert region than in the Mediterranean region. Analyses focusing on a grain size of 10 × 10 m showed a lower similarity in species composition and a lower rate of decay in species similarity with rainfall distance than analyses carried out at a grain size of 1 × 1 km. Main conclusions Patterns of similarity in species composition are influenced by the combined effects of environmental variation, the position of the area along environmental gradients, the dispersal properties of the component species, and the scale (both spatial extent and grain size) at which the patterns are examined.     

Using Functional Trajectories to Track Constructed Salt Marsh Development in the Great Bay Estuary, Maine/New Hampshire, U.S.A.

A growing number of studies have assessed the functional equivalency of restored and natural salt marshes. Several of these have explored the use of functional trajectories to track the increase in restored marsh function over time; however, these studies have disagreed as to the usefulness of such models in long‐term predictions of restored marsh development. We compared indicators of four marsh functions (primary production, soil organic matter accumulation, sediment trapping, and maintenance of plant communities) in 6 restored and 11 reference (matched to restored marshes using principal components analysis) salt marshes in the Great Bay Estuary. The restored marshes were all constructed and planted on imported substrate and ranged in age from 1 to 14 years. We used marsh age in a space‐for‐time substitution to track constructed salt marsh development and explore the use of trajectories. A high degree of variability was observed among natural salt marsh sites, displaying the importance of carefully chosen reference sites. As expected, mean values for constructed site (n = 6) and reference site (n = 11) functions were significantly different. Using constructed marsh age as the independent variable and functional indicator values as dependent variables, nonlinear regression analyses produced several ecologically meaningful trajectories (r ²> 0.9), demonstrating that the use of different‐aged marshes can be a viable approach to developing functional trajectories. The trajectories illustrated that although indicators of some functions (primary production, sediment deposition, and plant species richness) may reach natural site values relatively quickly (<10 years), others (soil organic matter content) will take longer.     

The effects of contemporary selection and dispersal limitation on the community assembly of acidophilic microalgae

Extremophilic microalgae are primary producers in acidic habitats, such as volcanic sites and acid mine drainages, and play a central role in biogeochemical cycles. Yet, basic knowledge about their species composition and community assembly is lacking. Here, we begin to fill this knowledge gap by performing the first large‐scale survey of microalgal diversity in acidic geothermal sites across the West Pacific Island Chain. We collected 72 environmental samples in 12 geothermal sites, measured temperature and pH, and performed rbcL amplicon‐based 454 pyrosequencing. Using these data, we estimated the diversity of microalgal species, and then examined the relative contribution of contemporary selection (i.e., local environmental variables) and dispersal limitation on the assembly of these communities. A species delimitation analysis uncovered seven major microalgae (four red, two green, and one diatom) and higher species diversity than previously appreciated. A distance‐based redundancy analysis with variation partitioning revealed that dispersal limitation has a greater influence on the community assembly of microalgae than contemporary selection. Consistent with this finding, community similarity among the sampled sites decayed more quickly over geographical distance than differences in environmental factors. Our work paves the way for future studies to understand the ecology and biogeography of microalgae in extreme habitats.     

Patterns of phylogenetic beta diversity in China's grasslands in relation to geographical and environmental distance

Patterns and determinants of beta (β-) diversity can be used to explore the underlying mechanisms regulating community assembly. Despite being the most commonly used measure of β-diversity, species turnover does not consider the evolutionary differences among species, treating all species equally. Incorporating information on phylogenetic non-independence or relatedness among species in the calculation of β-diversity may substantially advance our understanding of the ecological and evolutionary mechanisms structuring communities. Here, we investigate the relative influence of geographical distance and differences in environmental conditions (environmental distance) on the phylogenetic β-diversity between grassland communities expanding 4000 km across the Tibetan Plateau, the Inner Mongolia Plateau and the Xinjiang Autonomous Region in China. Both observed and standardized effect size of phylogenetic β-diversity were significantly correlated with geographical and environmental distance across all regions. However, the effect of geographical distance on the standardized effect size of phylogenetic β-diversity disappeared when environmental distance was controlled. We also found that within different regions, the effect of environmental distance on both observed and standardized effect size of phylogenetic β-diversity was more significant than geographical distance. Among environmental variables, climate played a more important role in shaping observed phylogenetic β-diversity across and within regions, and standardized effect size of phylogenetic β-diversity across regions. Soil properties played a more important role in shaping standardized effect size of phylogenetic β-diversity within regions. The phylogenetic β-diversity of species from dicot and monocot clades exhibited similar patterns along environmental and geographical distance. The results suggest that at the study scale, phylogeny of grassland communities in China is predominantly structured by environmental filtering, and the dominant environmental factors may be scale-dependent.     

A Pleistocene disturbance event describes modern diversity patterns in tidal marsh birds

There is growing evidence to support that paleo‐timescale events are important determinants in the present‐day distribution of organisms. We explored the relationship between community composition of tidal marsh birds in the northeastern United States and potential drivers of biodiversity patterns across timescales to explore the relevance of historical contingency in this ecosystem. These potential predictors represent some of the major known influences on biodiversity in tidal marshes, including 1) a recent, intense hurricane event driving a large‐scale perturbation of this ecosystem (4 yr), 2) gradual modification of marshes through installation of human infrastructure (～ 150 yr), and 3) marsh formation and development after the Last Glacial Maximum (LGM, ～ 20 000 yr). We surveyed > 1300 locations in tidal marshes from 2011–2014 using passive point count methods to measure bird community composition at these points. We found that rarefied richness, total individuals (N), and total biomass were best explained by a quadratic relationship with marsh age peaking at 40° latitude, the location of the Last Glacial Maximum of the Laurentide ice sheet. We hypothesize that formation of marsh millennia earlier in the southern part of our survey area allowed for earlier evolution of specialization to tidal marsh by bird species than those occupying much younger, northern marshes, which could have then driven differential rates of colonization in the north (by habitat generalists) and competitive exclusion in the south (by habitat specialists). We tested this theory using a novel functional diversity metric (community habitat specialization index, or CHSI) and find that community specialization decreased linearly with marsh age, supporting our hypothesis. Our findings highlight the importance for consideration of historical contingency in biodiversity research and further exploration of mechanisms operating across geological timescales.