Spatial Variation In Community Structure Research Articles

Identifying patterns in the multitrophic community and food-web structure of a low-turbidity temperate estuarine bay

Food web dynamics outline the ecosystem processes that regulate community structure. Challenges in the approaches used to capture topological descriptions of food webs arise due to the difficulties in collecting extensive empirical data with temporal and spatial variations in community structure and predator–prey interactions. Here, we use a Kohonen self-organizing map algorithm (as a measure of community pattern) and stable isotope-mixing models (as a measure of trophic interaction) to identify food web patterns across a low-turbidity water channel of a temperate estuarine-coastal continuum. We find a spatial difference in the patterns of community compositions between the estuarine and deep-bay channels and a seasonal difference in the plankton pattern but less in the macrobenthos and nekton communities. Dietary mixing models of co-occurring dominant taxa reveal site-specific but unchanging food web topologies and the prominent role of phytoplankton in the trophic base of pelagic and prevalent-detrital benthic pathways. Our approach provides realistic frameworks for linking key nodes from producers to predators in trophic networks.

Spatial Variation in Community Structure and the Underlying Environmental Variation

Spatial Variation in Community Structure and the Underlying Environmental Variation

Data from monogenean and endohelminth communities in twospot livebearer Pseudoxiphophorus bimaculatus (Teleostei: Poeciliidae) populations in a neotropical river.

The data presented in this article are related to the research article entitled “Competition from sea to mountain: interactions and aggregation in low diversity monogenean and endohelminth communities in twospot livebearer Pseudoxiphophorus bimaculatus (Teleostei: Poeciliidae) populations in a neotropical river.” accepted for publication in Ecology and Evolution. The data describes the communities of helminth parasites in 11 populations of a small poeciliid freshwater fish Pseudoxiphophorus bimaculatus (Heckel, 1848) sampled along the La Antigua river basin in Veracruz, Mexico. We examined 19 P bimaculatus from one locality, 21 from another locality, and 20 from each of the other nine locations sampled in June 2016. A total of 220 individual fish were examined, and in this paper we provide the data for 18 helminth parasite taxa recorded from them. The material in this Data paper comprised the raw data on the abundance, i.e. the number of helminth individuals of each of 18 taxa found in each one individual of P. bimaculatus from each of 11 localities. The data set is contained in a single text-table including one matrix containing each of the 220 host P. bimaculatus examined from 11 localities (lines). Measures for each host P. bimaculatus include total length, standard length, maximum deep and sex, documented for everyone fish examined, plus data of the number of individual helminth of each taxa collected by each examined fish are placed in the columns. These data might be used to examine spatial distribution of helminth parasite taxa. These data might be reused to examine the spatial variation in community structure of helminth parasites of freshwater fish. This kind of data could be used to provide an assessment of human environmental impacts, or for public awareness of conservation objectives.

Coastal geomorphology and oceanographic features shape subtidal benthic communities in management areas of central Chile

Abstract The spatial variation in community structure among 30 areas for the management and exploitation of benthic resources (AMEBRs) in central Chile was assessed from surveys conducted during the same spring–summer season. One hundred taxa identified in the subtidal surveys were grouped into 10 functional groups. The influence and relative importance of external factors over benthic associations were assessed through nonmetric multidimensional scaling (NMDS) and path analysis. Three main categories of external factors were considered: ‘terrain traits’ (e.g. coastline exposure), ‘surface‐ocean conditions’ (e.g. sea surface temperature, SST), and ‘fishery‐related variables’ (e.g. cumulative landings). The NMDS revealed a strong association between benthic community structure and both terrain and surface‐ocean conditions (42% of variance explained by permutational multivariate analysis of variance), but not with fishery‐related variables. Assemblages typical of rocky substrates were associated with larger seasonal fluctuations in SST and surface chlorophyll a. The presence of soft‐bottom functional groups was associated with higher levels of surface fluorescence and larger seasonal fluctuations in a satellite‐derived proxy for river‐plume influence (nLw645). A path analysis model for hard‐bottom communities reached a 49% goodness of fit (GOF), with the presence of epifaunal filter‐feeders, grazers, predators, and scavengers being significantly correlated with leathery and corticated foliose algae, which in turn depend on the fraction of rocky substrate in the AMEBR; the fraction of rocky substrate is positively correlated with coastline exposure. The model for soft‐bottom communities reached a 52% GOF with the presence of infaunal filter‐feeders and epifaunal detritivores depending directly on SST and surface chlorophyll a. The fishery‐related variables considered in the model had no apparent effect on the structure of either type of subtidal community. These results provide a hierarchy of physical–biological associations determining the functional composition of subtidal benthic communities in management areas along central Chile. This knowledge may improve future assessments of ecosystem response to changes in environmental conditions and management strategies.

Temporal variation of macrobenthic community zonation over nearly 60 years and the effects of latitude and depth in the southern Yellow Sea and East China Sea

To examine the temporal variation of macrobenthic community zonation over nearly 60 years and the effects of latitude and depth in the southern Yellow Sea and East China Sea, a total of 1386 box-corer samples from four large spatial scale studies during 1958–1959, 2000–2004, 2011–2013 and 2014–2016 period, respectively, were compiled. A total of 26, 14, 13 and 18 communities were identified, respectively during the four periods with the same analytical method. The Yellow Sea Cold Water Mass (YSCWM) community, restricted to the 34°N latitude in the south and 50 m isobaths in the west, varied little in its spatial pattern over nearly 60 years, while other communities did not. The representative species in the YSCWM community changed from the polychaetes to the brittle stars during 1958–2016. However, in other large spatial scale communities except the YSCWM community, the representative species changed from the echinoderms, nemerteans or crustaceans to the polychaetes. For the revisited locations across studies, significant temporal and spatial variations in community structure at both species and family levels were detected. Macrobenthic species with high consecutive contributions to the community similarity were significantly influenced by latitude, depth, temperature and salinity, among which latitude and depth were the first two most significant environmental variables. Species diversity increased from 32 to 37°N during 1958–1959, but decreased during 2014–2016. It seems that the latitude of 32°N is an ecological barrier for macrobenthic community and diversity, but its effects weakened from 1958 to 2016. Species diversity roughly showed a first increasing then decreasing trend with the increase of the water depth in the southern Yellow Sea and East China Sea.

Soil pH and C/N ratio determines spatial variations in soil microbial communities and enzymatic activities of the agricultural ecosystems in Northeast China: Jilin Province case

Soil microbes and their functions play important roles in the sustainable development of agricultural ecosystems but are vulnerable to climate change and human activity. We measured the contents of microbial phospholipid fatty acids (PLFAs) and enzymatic activities in various soil types (dark-brown soil, albic soils, black soil, chestnut soil, and chernozem) in three climatic zones (humid zone, subhumid zone, and semi-arid zone) of the Songnen Plain in northeastern China. The objectives were to investigate the spatial variations in community structure of soil microbes and to evaluate the functional potential of the microbes in croplands in northeastern China. The C-acquiring activities were highest in the chernozem soil of the semi-arid zone, but N-acquiring and P-acquiring activities and peroxidase were highest in the albic soil of the humid and subhumid zone. Phosphorus deficiencies were indicated by the reduced ratios of N- relative to P-acquiring activity and C-relative to P-acquiring activity in the croplands compared with the global average. Different groups of PLFA contents were highest in the dark-brown soil of the humid zone and the chestnut soil of the semi-arid zone. The community compositions of the microbes varied amongst the soil types and climatic zones, with the ratios of fungi/bacteria and Gram-positive/Gram-negative bacteria highest in chernozemic soil of the semi-arid zone. The variations in soil enzyme activities and microbial community structure were more dependent on climate conditions than soil type. Soil pH, soil organic C content, and the C/N ratio were the most important factors affecting the spatial patterns of the soil enzyme activities and microbial-community compositions across soil types and climatic zones. Mean annual precipitation and clay content strongly influenced enzymatic activities and microbial-community compositions, respectively, when each variable was used as the only factor. Their effects, however, were negligible when combined with other factors. Balanced chemical fertilizer with scientific N-P-K ratio should be applied instead of only N fertilizers. The other combination of different conservational agricultural practices, such as application of organic fertilizer, not tilling, crop rotation, and intercropping, would be more beneficial for restoring the community compositions and functions of soil microbes and achieving sustainable agricultural development.

Biodiversity of an Antarctic rocky subtidal community and its relationship with glacier meltdown processes

Glacier meltdown is a major environmental response to climate change in the West Antarctic Peninsula. Yet, the consequences of this process for local biodiversity are still not well understood. Here, we analyse the diversity and structure of a species-rich marine subtidal macrobenthic community (consumers and primary producers) across two abiotic environmental gradients defined by the distance from a glacier (several km) and depth (between 5 and 20 m depth) in Fildes Bay, King George Island. The analysis of spatially extensive records of seawater turbidity, high-frequency temperature and salinity data, and suction dredge samples of macrobenthic organisms revealed non-linear and functional group-dependent associations between biodiversity, glacier influence, and depth. Turbidity peaked in shallow waters and in the nearby of the glacier. Temperature and salinity, on the other hand, slightly decreased in the proximity of the glacier relative to reference sites. According to the spatial pattern in turbidity, species richness of consumers was lowest in shallow waters and near to the glacier. Also, Shannon's diversity of consumers significantly decreased in the nearby of glacier across depths. Moreover, the spatial variation in community structure of consumers and primary producers depended on both glacier distance and depth. These results suggest that glacier melting can have significant effects on diversity and community structure. Therefore, the accelerated glacier meltdown may have major consequences for the biodiversity in this ecosystem.

The effect of environmental gradient on biodiversity and similarity of invertebrate communities in eelgrass (Zostera marina) beds

AbstractEnvironmental filtering and dispersal limitation are important processes within the metacommunity concept. Non‐random species turnover occurs in places where environmental filtering plays the key role in determining local community structure, whereas dispersal limitation causes nested patterns of species assemblages organized by non‐random colonization processes. However, factors that modify the relative importance of these processes remain unclear for many ecosystems. We tested whether salinity gradient affect the relative importance of environmental filtering and dispersal limitation for structuring epifaunal and infaunal communities in three lagoons in Hokkaido, Japan, that have different salinity gradients. Specifically, we compared patterns of species diversity and similarity of eelgrass‐associated invertebrate assemblages across space. Beta diversity (i.e., species turnover among different sites in each lagoon) was highest in Akkeshi, the lagoon with the salinity gradients. Variation partitioning of similarity components showed that spatial variation in the community assemblage pattern was mostly explained by environmental filtering in Akkeshi, but that it was explained more by species dispersal patterns and the difference in eelgrass biomass and shoot density in Notoro and Saroma, the lagoons without clear salinity gradient. Redundancy analysis showed that spatial variation in community structure was related to salinity and eelgrass biomass in Akkeshi, and to eelgrass aboveground biomass in Notoro and Saroma. Our findings highlight the effects of environmental heterogeneity on beta diversity and community structure and indicate that environmental gradients can be a key factor causing a shift in the relative importance of different metacommunity processes and the role of the foundation species in provisioning habitat.

Planktonic diatom community dynamics in a tropical flood-pulse lake: the Tonle Sap (Cambodia)

It has been proposed that, within flood-pulse systems, seasonally dynamic conditions promote extremely high levels of biodiversity. In this paper, we aim to understand how variation in environmental conditions and habitat availability, across space and time, influences the structure and composition of plankton diatom communities within the Tonle-Sap Lake (Cambodia) during two flood-pulse cycles. We hypothesize that (i) communities vary considerably across space and time, (ii) environmental filtering accounts for a substantial portion of the variation in community composition, and (iii) shifts in meta-community structure occur seasonally, due to variation in both environmental conditions and habitat availability. A principal component analysis and a permutational multivariate analysis of variance were used to characterize spatio-temporal variations in environmental conditions, community structure and composition. Determinants of community variations across space were identified by redundancy analyses and variation partitioning, while seasonal changes in meta-community structure were assessed by investigating temporal changes in β-diversity. Our results indicate that (i) spatial variations in community structure and composition are largely influenced by the seasonal flood-pulse, (ii) environmental filtering is the most likely process driving the compositional changes, and (iii) changes in diatom life-form community provide insights into the hydrological functioning of the Tonle Sap. We conclude that the survey of diatom communities may constitute a relevant sensor of hydrological change in the system and could be used to derive regionally specific hypotheses about how global climate change is impacting the lake’s functioning.

Evaluating the contribution of dispersal to community structure in Neotropical passerine birds

For two centuries evolutionary biologists have sought to explain elevated biodiversity in the Neotropics. Although different process are known to be important, it is still not uncommon for researchers to emphasize a single mechanism. Recently, arguments have highlighted the importance of dispersal shaping community structure and evolution across the region. We examine this hypothesis by visualizing spatial variation in community structure for the majority of South American passerines (Aves) across the northern half of South America. By sampling over a contiguous area we show how community structure varies widely across Amazonia and surrounding regions. Our results support a combination of processes including: the inability of species to disperse across geographic barriers, Andean uplift, and variation in habitat type. Although dispersal is a factor, our results emphasize a lack of dispersal, driven primarily by features of the landscape, coupled with historical changes in climate to be important drivers of Neotropical diversity.

Identifying hierarchical spatial patterns within paleocommunities: An example from the Upper Pennsylvanian Ames Limestone of the Appalachian Basin

Identifying ecological mechanisms that produce hierarchically arrayed spatial variation in community structure can be difficult in the fossil record due to conflation of spatial and temporal patterns. However, this difficulty can be mediated by minimizing the temporal duration of deposition within the unit examined. In this study, the fauna of the Upper Pennsylvanian Ames Limestone (Conemaugh Group) was analyzed to explore whether Ames paleocommunities exhibited hierarchical structure in a spatial dimension. This widespread carbonate unit was deposited during the maximum flooding interval of a glacio-eustatically influenced fifth order sea level cycle, and preserved taxa are contemporaneous within only a few thousand years. Paleocommunity structure and variability was assessed at multiple spatial scales using samples collected from seven outcrops of the Ames Limestone throughout southeastern Ohio which form a northeast to southwest trending transect parallel to the paleoshoreline. Abundance data were collected using quadrat sampling for brachiopods, bivalves, gastropods, bryozoa, corals, crinoids, echinoids, trilobites and foraminifera. Paleocommunity structure was analyzed via cluster, ordination, guild, and abundance analyses at multiple spatial scales (within a single locality, among localities and within the total study area) to provide insight on geographic partitioning of paleocommunity variation. Multiple levels of paleocommunity organization were recovered within the Ames fauna. All levels exhibited spatial partitioning, but the inferred proximate controls shifted from abiotic environmental controls at higher hierarchical levels to biotic controls at the lowest level. At the highest level, differentiation into a northern and southern regional paleocommunity was controlled primarily by substrate consistency and habitat heterogeneity related to variation in fluvial input within the basin. Local paleocommunity differentiation reflects biotic responses to topographic and environmental conditions that were geographically distributed within the region; whereas within outcrop variation was due largely to biotic feedback mechanisms.

Spatiotemporal variation in drivers of parasitoid metacommunity structure in continuous forest landscapes

AbstractAlthough landscape spatial structure is known to influence spatial patterns of biodiversity, its effect on insect communities at higher trophic levels such as parasitoids remains poorly understood. This is particularly true in continuously distributed forests in which it can be difficult to identify clear boundaries among habitat patches. Using the metacommunity framework, we evaluate the relative importance of forest landscape structure, non‐environmental spatial structure, and host outbreak status to spatial and within‐season temporal variation in parasitoid communities. We used variation partitioning and metacommunity structure analyses to identify (1) the drivers of the metacommunity structure of parasitoids associated with the spruce budworm (Choristoneura fumiferana), and (2) how their relative influence varies through a season. We used a multi‐scale perspective to summarize landscape heterogeneity in regions of increasing size around the community sampling locations. Spruce budworm larvae and pupae were sampled during three periods during the summer 2014 in 18 locations within continuous forest landscapes in Quebec, Canada. Thirty‐two parasitoid wasp and fly species were recorded, 16 of which were found at more than one location. We found that the mechanisms shaping metacommunity structure changed over the course of a single season and that community structure varied among sites. At early and late periods in the season, we found that non‐environmental structure, forest structure, and likely inter‐specific competition were the main mechanisms influencing spatial variation in community structure. These results suggest a competition–dispersal trade‐off. In contrast, at the middle period of the season, environmental filtering by forest structure and stochastic events were found to influence community structure. This period corresponds to the transition between early and late parasitoid communities. Our findings on the role of environmental filtering and forest structure support the idea that forest manipulations have the potential to influence parasitoid populations and hence spruce budworm outbreak dynamics as hypothesized by the “enemies hypothesis.” Moreover, our study highlights the value of considering a multi‐scale approach and temporal variability of species interactions when characterizing the multiple processes shaping spatial metacommunity structure, particularly in continuous environments.

The role of dispersal mode and habitat specialization for metacommunity structure of shallow beach invertebrates.

Metacommunity ecology recognizes the interplay between local and regional patterns in contributing to spatial variation in community structure. In aquatic systems, the relative importance of such patterns depends mainly on the potential connectivity of the specific system. Thus, connectivity is expected to increase in relation to the degree of water movement, and to depend on the specific traits of the study organism. We examined the role of environmental and spatial factors in structuring benthic communities from a highly connected shallow beach network using a metacommunity approach. Both factors contributed to a varying degree to the structure of the local communities suggesting that environmental filters and dispersal-related mechanisms played key roles in determining abundance patterns. We categorized benthic taxa according to their dispersal mode (passive vs. active) and habitat specialization (generalist vs. specialist) to understand the relative importance of environment and dispersal related processes for shallow beach metacommunities. Passive dispersers were predicted by a combination of environmental and spatial factors, whereas active dispersers were not spatially structured and responded only to local environmental factors. Generalists were predicted primarily by spatial factors, while specialists were only predicted by local environmental factors. The results suggest that the role of the spatial component in metacommunity organization is greater in open coastal waters, such as shallow beaches, compared to less-connected environmentally controlled aquatic systems. Our results also reveal a strong environmental role in structuring the benthic metacommunity of shallow beaches. Specifically, we highlight the sensitivity of shallow beach macrofauna to environmental factors related to eutrophication proxies.

Ecological and genetic variation in reef-building corals on four Society Islands

We quantified benthic community structure on shallow (10 m isobath) reefs separated by 3–130 km on four islands in the south Pacific, and evaluated the roles of disturbances vs. coral recruitment as causes of spatial heterogeneity. Reefs were surveyed in 2013 on Moorea, Tahiti, Tetiaroa, and Maiao, with community structure sampled at two sites on each island using photoquadrats. The effects of coral recruitment on population structure were evaluated through genetic analyses of Pocillopora on three of the islands. Benthic community structure with functional group resolution differed among islands and generally among sites, but coral community structure (generic resolution) differed among islands, but generally not among sites. Genetic analyses of Pocillopora using the open reading frame of host mtDNA revealed varying relative abundances of Pocillopora meandrina/Pocillopora eydouxi, Pocillopora verrucosa, Pocillopora effuses, and two unnamed haplotypes on each island. These results suggest that corals on each island represent unique samplings of genetically discrete larval assemblages rather than random samplings of a single larval assemblage. Together, our findings emphasize the extent to which coral community structure varies over a scale of <200 km, and suggests that recruitment from spatially discrete pools of coral larvae plays an important role in creating spatial variation in community structure, even where reefs are connected by prevailing currents.

Dispersal ability determines the role of environmental, spatial and temporal drivers of metacommunity structure.

Recently, community ecologists are focusing on the relative importance of local environmental factors and proxies to dispersal limitation to explain spatial variation in community structure. Albeit less explored, temporal processes may also be important in explaining species composition variation in metacommunities occupying dynamic systems. We aimed to evaluate the relative role of environmental, spatial and temporal variables on the metacommunity structure of different organism groups in the Upper Paraná River floodplain (Brazil). We used data on macrophytes, fish, benthic macroinvertebrates, zooplankton, periphyton, and phytoplankton collected in up to 36 habitats during a total of eight sampling campaigns over two years. According to variation partitioning results, the importance of predictors varied among biological groups. Spatial predictors were particularly important for organisms with comparatively lower dispersal ability, such as aquatic macrophytes and fish. On the other hand, environmental predictors were particularly important for organisms with high dispersal ability, such as microalgae, indicating the importance of species sorting processes in shaping the community structure of these organisms. The importance of watercourse distances increased when spatial variables were the main predictors of metacommunity structure. The contribution of temporal predictors was low. Our results emphasize the strength of a trait-based analysis and of better defining spatial variables. More importantly, they supported the view that “all-or- nothing” interpretations on the mechanisms structuring metacommunities are rather the exception than the rule.

Trait‐mediated environmental filtering drives assembly at biogeographic transition zones

Abiotic filtering is a major driver of gradients in the structure and functioning of ecosystems from the tropics to the poles. It is thus likely that environmental filtering is an important assembly process at the transition of biogeographical zones where many species occur at their range limits. Shifts in species abundances and association patterns along environmental gradients can be indicative of environmental filtering, which is predicted to be stronger in areas of high abiotic stress and to promote increased similarity of ecological characteristics among co-occurring species. Here we test these hypotheses for scleractinian corals along a broad latitudinal gradient in high-latitude eastern Australia, where corals occur at the margins of their ranges and environmental tolerances. We quantify variation in taxonomic, zoogeographic, and functional patterns combined with null model approaches and demonstrate systematic spatial variation in community structure and significant covariance of species abundance distributions and functional characteristics along the latitudinal gradient. We describe a strong biogeographic transition zone, consistent with patterns expected under abiotic filtering, whereby species are sorted along the latitudinal gradient according to their tolerances for marginal reef conditions. High-latitude coastal reefs are typified by widely distributed, generalist, stress-tolerant coral species with massive and horizontally spreading morphologies and by diminishing influence of tropical taxa at higher latitudes and closer to the mainland. Higher degree of ecological similarity among co-occurring species than expected by chance supports the environmental filtering hypothesis. Among individual traits, the structural traits corallite size and colony morphology were filtered most strongly, suggesting that characteristics linked to energy acquisition and physical stability may be particularly important for coral survival in high-latitude environments. These findings highlight interspecific differences and species interactions with the environment as key drivers of community organization in biogeographic transition zones and support the hypothesis that environmental filters play a stronger role than biotic interactions in structuring ecological communities in areas of high abiotic stress.

Animal–sediment relationships in an Atlantic Canada marine protected area: Richness, composition and abundance in relation to sediment food indicators

Despite growing concern for biodiversity loss, many coastal areas remain unexplored and their species composition and ecology virtually unknown. This study addressed these issues for intertidal communities of one of the few Atlantic Canada marine protected areas (MPAs). A nested sampling design was used to assess patterns for infauna and indicators of sediment food supply from three main areas of the MPA and from three sites randomly selected within each area. Samples were collected using a 7 cm diameter cylinder (38.5 cm2) inserted 5 cm into the sediment. The data were used to document composition, richness and abundance and to assess their potential relationship with sediment food indicators (organic carbon, total carbon, total N, C : N ratio, and chlorophyll a in the sediment). Twenty-eight taxa, primarily deposit-feeding spionid and capitellid polychaetes as well as venerid clams, were the numerically dominant taxa in most areas and sites. Composition and abundance and the concentration of most sediment food indicators differed significantly among areas. Quadratic regression analyses identified several significant relationships (R2≥0.54) between food indicators, primarily organic carbon, carbon and nitrogen contents and several faunistic variables. In general, increases in sediment food supply were correlated with an increase and a subsequent decrease in richness and overall abundance, as well as the abundance of polychaetes and deposit feeders. Food indicators clearly contributed to, but were not the only factors driving spatial variation in community structure. Further studies addressing the physical and biological factors structuring sedimentary communities in MPAs and other coastal areas are encouraged.

Spatial variation in community structure of a subtropical evergreen broad-leaved forest: Implications for sampling design

With the full survey data for a 24-ha subtropical evergreen broad-leaved forest dynamics plot, we evaluated spatial variation in forest structure characteristics (basal area and aboveground biomass), and calculated the minimal sample size and total sampling area necessary to estimate the forest structure characteristics within 20% (±10%) of the observed values with 95% probability for particular quadrat sizes by using a computer program that is designed to simulate the sampling process by allowing different sized quadrats to be randomly located within the sampling region. We found that (1) based on the 600 20 m×20 m subplots, basal area and aboveground biomass displayed a high degree of variation, with respective coefficients of variation of 27% and 31%; (2) based on the computer simulation analysis, the variability of basal area and aboveground biomass decreased with increasing quadrat size. The number of quadrats required to achieve the specified degree of precision dropped sharply with the increase of quadrat size. However, the total sampling area increased with increasing quadrat size, suggesting that using several small quadrats across the sampling area is more efficient than using fewer larger quadrats. Results of this study are valuable for evaluating the reliability of previous research and may assist researchers in designing effective sampling strategies for future field surveys, particularly in subtropical evergreen broad-leaved forests in China.

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.

Intra-Annual Variation in Fish Communities and Habitat Associations in a Chihuahua Desert Reach of the Rio Grande/Rio Bravo Del Norte

Anthropogenic alterations to lotic systems are often implicated in global declines among stream-dwelling fishes. A primary step toward mediating fish declines is improving our understanding of species—environment relationships; unfortunately, such information is limited. The goal of this study was to assess relationships between fish communities and environmental variables in a relatively intact portion of the Rio Grande/Rio Bravo del Norte in the southwestern United States and northern Mexico. We quantified the relationship between riverine habitats and local fish communities at 7 sites in the Big Bend reach of the Rio Grande/Rio Bravo del Norte during 2006. Monthly collections yielded 10,565 individuals representing 20 species, including 6 species listed as threatened or otherwise at risk. Species richness varied among sites (range 13–19) as did diversity (1 - D: 0.49–0.72) and density (16–45 fish · 100 m-2). Fish community composition indicated spatial and temporal variations, but habitat characteristics indicated more variation among sampling sites than among months. Spatial variation in community structure correlated with site-specific habitat characteristics, and most threatened or at risk species were associated with run or riffle geomorphic units containing higher current velocities and gravel to cobble substrates. Temporal variation in community structure correlated with fall monsoonal flooding, during which composition of dominant species shifted from the habitat generalist red shiner (Cyprinella lutrensis) to the imperiled, regionally endemic Tamaulipas shiner (Notropis braytoni). Results from this study suggest that large flood pulses and maintenance of habitat heterogeneity are necessary for the persistence of both declining and intact local fish communities in the Rio Grande/Rio Bravo del Norte.