Infaunal Community Research Articles

Thyasirid species composition (Bivalvia: Thyasiridae) and genetic connectivity of Parathyasira equalis (A. E. Verrill & K. J. Bush, 1898) in deep basins of sub-Arctic fjords

BackgroundThyasirid bivalves are often recorded as a dominant component of macrobenthic infaunal communities in depositional environments such as fjord basins. Fjord basins comprise patchy soft-bottom habitats bounded by steep walls and sills; however, little is known how this semi-isolated nature of fjords affects benthic populations. Accordingly, data on the composition and population connectivity of thyasirids can provide valuable information on the ecology of these ecosystems.ResultsThe species composition of thyasirid bivalves has been studied in the basins of three sub-Arctic fjords (Nordland, Northern Norway). Overall, six thyasirid species were recorded: Parathyasira equalis, Parathyasira dunbari, Mendicula ferruginosa, Genaxinus eumyarius, Thyasira sarsii, and Thyasira obsoleta. The species composition remained stable within the basins during the sampling period (2013–2020) and suggested the importance of local reproduction over advection of individuals for population dynamics. Only one species, Parathyasira equalis, was common in all fjords. We have further investigated the population genetics of this species by combining two types of genetic markers: a 579 bp fragment of the cytochrome c oxidase subunit I (COI) gene and 4043 single-nucleotide polymorphisms (SNPs) generated by genotyping-by-sequencing. The latter provided a more in-depth resolution on the population genetics of this species and revealed a weak but significant differentiation of populations within fjords, further indicating limited connectivity between basins.ConclusionBased on our findings, we conclude that limited dispersal between the basin communities results in weakly connected populations and might be an important structuring factor for macrobenthic communities.

Invertebrate Responses to Large- and Small-Scale Drivers in Coastal Phragmites australis Beds in the Northern Baltic Sea

Phragmites australis is a common helophyte, covering much of the sheltered and shallow soft bottoms along the coasts of the Baltic Sea. Despite the expansion of P. australis over the past decades, there is little information on aquatic macroinvertebrates within P. australis beds. In this study, we examined the effect of large-scale (wave exposure, nutrients) and small-scale (distance from the seaward edge, live and dead stalk density, epiphyte and rhizome biomass) drivers on the density, taxa richness, diversity, and community structure of epifauna and infauna in monospecific P. australis beds around the Åland Islands and the Archipelago Sea. We found that higher wave exposure and nutrient levels generally supported higher epi- and infauna abundance and taxa richness. The effects on Shannon–Wiener diversity were less evident apart from an increase of the infauna diversity in the Archipelago Sea with increasing nutrient levels. On a local scale, the distance from the seaward edge, live and dead stalk density, and epiphyte biomass had varying effects on both epi- and infauna communities in the different regions. Rhizome biomass had no effect on either the epi- or infauna abundance, taxa richness, or diversity. Furthermore, according to existing studies, other habitats, e.g., Zostera marina meadows, Fucus vesiculosus belts, and vegetated soft-bottomed shallow bays, are generally characterized by more abundant fauna, except for the infauna, which had a higher density in P. australis beds than in vegetated soft-bottomed shallow bays. P. australis are a widespread, expanding, and understudied habitat with an important role in supporting coastal biodiversity.

Effects of environmental and climatic drivers on abyssal macrobenthic infaunal communities from the NE Pacific nodule province

Effects of environmental and climatic drivers on abyssal macrobenthic infaunal communities from the NE Pacific nodule province

Biogeoacoustic variability in muddy ocean bottom sediment

Both physical and biological benthic processes can influence seabed heterogeneity and contribute to spatiotemporal variability in geoacoustic properties. In particular, how biological processes affect both sediment acoustic properties and their variability is poorly understood. To address this deficiency, recent measurements investigated spatial variability in the upper few decimeters of sediment near the water-seabed interface within a fine-grained sediment deposit on the New England shelf. At each measurement location, acoustic probes were inserted into the sediment to collect direct in situ measurements of sediment sound speed and attenuation at near-ambient conditions, after which cores were collected from the inter-probe propagation paths for ex situ analysis of sediment physical, biological, and acoustic properties. Relationships among sediment properties, such as bulk density, porosity, grain size distribution, organic matter composition, infaunal community composition, and acoustic measurements spanning several frequency decades (10–1000 kHz) will be explored in this paper. Frequency dependence of sediment acoustic properties will also be discussed in the context of sediment acoustics models for mud based on the viscous grain shearing and extended Biot theories. [Sponsored by ONR.]

Bioturbating megafauna limits the expansion of Cymodocea nodosa seagrass meadows

In seagrass systems, there is a realization that biotic interactions affecting sediment stability might be as important as abiotic stressors in shaping the growth and distribution patterns of seagrass vegetation. However, few studies have looked at the interaction between seagrass and large (mega) bioturbating species, although through sediment reworking activity they may have the capacity to cause large scale impact on seagrass establishment. We assessed the effect of bioturbation by mobile megabioturbators (species lager than 5 cm) on subtidal seagrass (Cymodocea nodosa) using a manipulative field experiment where access by megabioturbators was restricted for 4 months during the growth season. There was a negative effect of megabioturbators on both the biomass and density of young seagrass ramets extending outwards from a mature seagrass meadow, but megabioturbators did not have a notable effect on seagrass growth in the meadows. Interestingly, the exclusion of megabioturbators in all tested habitats significantly modified the infaunal community composition, indicating the presence of complexity in biotic interactions and the possibility of indirect effects caused by the megabioturbators. Overall, megabioturbators have direct and indirect impact in seagrass habitats, playing a significant role in seagrass distribution patterns.

Spatial patterns of sediment contamination and their influence on benthic infaunal communities in a highly tidal and industrial estuary in Atlantic Canada

Sediment contamination can be elevated in ports, harbours, and estuaries with legacies of exploitation, negatively impacting infaunal invertebrate communities. Saint John Harbour (45.25° N, 66.05° W), New Brunswick, Canada, is an active harbour with strong tides and a long history of human activity. To examine spatial patterns of sediment contamination, samples were collected between 2011 and 2021 from subtidal sites near potential contaminant sources. Invertebrate data from the same samples were used to investigate potential effects on biological communities. Contaminant concentrations in the inner parts of the harbour were elevated compared to reference sites, but generally did not reach levels comparable to other highly contaminated harbours in the region. Effects on invertebrates were detectable, particularly at sites with higher contamination, although physical factors (depth, sediment grain size) were more important. Dynamic tidal conditions in the harbour may reduce the accumulation of contaminants in subtidal sediments and their impacts on infaunal communities.

Infaunal invertebrate community relationships to water column and sediment abiotic conditions

Infaunal invertebrates are affected by the overlying water and the sediment in which they live. Therefore, understanding how these environmental conditions impact infauna is critical for evaluating how they may respond to future changes in these conditions due to climate change. Here, we considered which abiotic variables, for example, salinity, sediment characteristics (i.e. mean grain size, sorting), and water column and sediment carbonate chemistry, influence infaunal invertebrate communities and juvenile bivalve abundance at intertidal sites. We used data from sites in two regions in New Brunswick, Canada with contrasting tidal regimes and oceanographic conditions, the Bay of Fundy and the Southern Gulf of St. Lawrence. We were particularly interested in bivalve recruitment due to the importance of bivalves in ecosystem services and predicted sensitivity to climate change impacts. Using data collected in 2020 and 2021, statistical modeling was done to determine which abiotic variables were potential drivers of multivariate community composition as well as species richness, total abundance, and juvenile bivalve abundance. We found that carbonate chemistry variables, both sediment and water, explained a large amount of variation (~ 7–44%) in infaunal invertebrate communities in the two regions in both our multivariate and univariate analyses. Sediment pH explained the most variation (16.9%) in the multivariate analyses for the Bay of Fundy sites. However, in the Southern Gulf of St. Lawrence, salinity explained the most variation (9.8%) in the multivariate community composition. In the univariate modeling, alkalinity, either water column or sediment, was included in all top models for all four dependent variables, suggesting the importance of this carbonate chemistry variable for bivalves and infaunal communities. Climate change is expected to have large impacts on carbonate chemistry conditions in the oceans, specifically pH, carbonate availability, and alkalinity. The influence of carbonate chemistry parameters on infaunal invertebrate communities in these regions shows the potential sensitivity these animals have to future oceanic conditions.

Human activities and environmental variables drive infaunal community structure and functioning in West African mangroves

In West African tropical estuaries, such as the Saloum Delta (Senegal), mangroves provide specific socio-economic functions including various natural food resources for local human populations. Mangrove-habitat loss is therefore of great concern, in terms of conservation and management, and highlights the need to study West African mangrove under human-induced pressures. Based on data from 2007 to 2008, this study aimed to establish a conditions report for two of the main Southern West Senegal mangroves areas: the Sine Saloum and the Petite Côte. We assessed the influence of environmental variables and human-induced pressures (i.e. shellfish harvesting and heavy metals (potentially toxic elements, PTEs)) on infaunal community structure and functions through trait-based approaches.Reflecting for example changing seasons and hypersaline events, the Saloum environmental variables and their temporal dynamics greatly impacted infaunal communities in the subtidal zone. In the intertidal area, closer to anthropogenic disturbances, pressures related to human activities took precedence over environmental variables in explaining infaunal community structure. Shellfish harvesting resulted in a significant decrease (p < 0.05) in total macrofaunal abundance (933.79 ± 626.34 ind m−2 in unexploited stations compared to 140.92 ± 71.99 ind m−2 in exploited stations; mean ± SE) in addition to a decline in biomass and taxonomic richness. Intertidal species composition was affected by both shellfish harvesting and PTEs. PTEs were also associated with a significant decrease in functional diversity and particularly with a change in macrofaunal living habits. The combination of an RLQ and fourth-corner analysis highlighted a significant association between feeding modes and sediment grain size in both tidal areas.This study provides a needed deeper understanding of the unintended effects of human activities on infaunal communities and their functioning, in a highly valuable system for humans. Pinpointed factors driving infaunal community structure and functions provide important insights for future research, as well as reference points for the implementation of management measures.

Monitoring Intertidal Habitats for Effects from Biosolids Applications onto an Adjacent Forestry Plantation

Stabilised organic solids derived from sewage sludge (“biosolids”) are applied to land as an alternative to disposal as landfill. This study evaluated the long-term effects of biosolids applied to forestry plantations on the adjacent intertidal habitats of Rabbit Island (New Zealand). On this island, biosolids are applied to enhance the growth of trees (Pinus radiata). Shoreline topography, macroalgal cover, sediment grain size, the concentrations of nutrients, trace metals, and faecal indicator bacteria, and benthic infaunal communities were studied in 2008, 2014, and 2019 at twelve intertidal transect sites (four “reference” and eight “application”) adjacent to forestry blocks where biosolids have been applied over a period of 24 years. The sediment composition did not differ significantly between the survey years or between the reference and application sites. Total nitrogen concentrations in the sediments increased over time at some transects, but such increases were not consistent among the application transects. No symptoms of excessive algal growth, sediment anoxia, and hydrogen sulphide odours were observed at most sites. Key infaunal taxa were similar between the reference and application transects. Overall, no long-term adverse changes to intertidal habitats attributed to biosolids application were detected between the reference and application sites. This study shows that biosolids application can co-occur without detectable adverse effects on nearby intertidal environments. In a global context of rising concern over climate change, environmental pollution, and resource scarcity, forest fertilisation with biosolids can facilitate biomass production and soil development while protecting valued coastal ecosystems.

Temporal and Spatial Trends in Benthic Infauna and Potential Drivers, in a Highly Tidal Estuary in Atlantic Canada

Infaunal invertebrate communities of coastal marine sediments are often impacted by human activities, particularly in harbours and estuaries. However, while many studies have attempted to identify the key factors affecting benthic infauna, few have done so for highly energetic tidal estuaries. Samples were collected over a decade (2011–2020) from a series of reference sites in Saint John Harbour (45.25° N, 66.05° W), a highly tidal estuary in the Bay of Fundy, Canada. These data were used to examine spatial and temporal trends in infaunal invertebrate communities and sediment properties and to determine the extent to which the biological patterns were driven by measured physical and chemical variables. There were substantial differences among sites in infaunal invertebrate abundance (median ranging from 688 to 13,700 individuals per square meter), infaunal species richness (median ranging from 8 to 22), and Shannon diversity (median ranging from 1.26 to 2.34); multivariate analysis also revealed variation in species composition among sites. Sediment contaminant concentrations also varied among sites, but differences tended to be smaller (e.g. median chromium concentrations ranging from 21.6 to 27.6 mg/kg). Sample contaminant concentrations were all below probable effect levels, and almost all below threshold effect levels (Canadian interim sediment quality guidelines), but relationships with biological data were still detectable. However, physical variables (depth, sediment characteristics) were better predictors of biological variables and community composition. These results confirm the importance of physical factors in shaping infaunal communities in soft-sediment habitats in tidally influenced coastal waters.

Characterization and differentiation of sublittoral sandbanks in the southeastern North Sea

Marine sublittoral sandbanks are essential offshore feeding grounds for larger crustaceans, fish and seabirds. In the southern North Sea, sandbanks are characterized by considerable natural sediment dynamics and are subject to chronic bottom trawling. However, except for the Dogger Bank, sandbanks in the southeastern North Sea have been only poorly investigated until now. We used an extensive, multi-annual dataset covering ongoing national monitoring programmes, environmental impact assessments, and basic research studies to analyse benthic communities on sublittoral sandbanks, evaluating their ecological value against the backdrop of similar seafloor habitats in this region. The analysis revealed complex spatial structuring of sandy seafloor habitats of the southeastern North Sea. Different infauna clusters were identified and could be specified by their composition of characteristic species. The sandbanks shared common structural features in their infauna community composition although they were not necessarily characterized by particularly high biodiversity compared to other sandy habitats. A close association of one of the main bioturbators in the southern North Sea, the sea urchin Echinocardium cordatum, with sandbanks was detected, which may promote the sediment-bound biogeochemical activity in this particular seafloor habitat. This would corroborate the status of sandbanks as sites of high ecological value calling for consideration in marine conservation.

The role of non-skeletal carbonate component in Mediterranean Coralligenous: new insight from the CRESCIBLUREEF project

In the frame of the project FISR_04543 “CRESCIBLUREEF - Grown in the blue: new technologies for knowledge and conservation of Mediterranean reefs”, we present preliminary data on the ecological and depositional implications of micritic sediments in a coralligenous bioconstruction formed along the Mediterranean shelf in front of the Marzamemi village (Sicily, Italy). The framework of the build- up is mainly built by crustose coralline algae, which in turn create the substrate for a high-diversified epi- and infaunal community. Two types of microcrystalline calcite, tentatively interpreted as allochthonous and autochthonous micrite, strictly related to fine skeletal debris, have been detected. The allochthonous micrite derive from abiotic accumulation of fine sediments in the framework cavities. The autochthonous micrite is deposited in situ through organic-mediated processes. The occurrence of this component allows hypothesizing a possible contribution of non-skeletal carbonate in the strengthening of the primary framework due to its syndepositional cementation.

Structuring forces of intertidal infaunal communities on the northern coast of British Columbia, Canada: assessing the relative importance of top-down, bottom-up, middle-out, and abiotic variables

While trophic and habitat-related abiotic variables (predation, competition, tolerance, etc.) are known to influence community structure in many ecosystems, some systems appear to be only minimally influenced by these variables. Sampling multiple tidal flat communities in northern BC, Canada, we investigated the relative importance of top-down and middle-out (mesopredators) variables, competition for resources (bottom up), and abiotic variables in structuring an infaunal community (invertebrates living in sediment). Similar to previous studies on mudflats in the Bay of Fundy (also at a north temperate latitude), we determined that these variables accounted for a minor (0%–9%) proportion of the observed variation in this infaunal community, suggesting that these variables play a small role in structuring this community. Based on the results of our study and in combination with previous experiments on infaunal recovery patterns post disturbance, we posit that the main factors influencing these infaunal communities likely operate at a scale of sites (kilometres) and(or) plot (metres or less) but not transects (10–100 m within site). Candidate forces structuring these intertidal communities that need future examination include regional species pools and the variables that affect these pools, sediment biogeochemistry, and disturbance/recovery history of a site. The similarity of our Pacific coast findings to those from the north temperate Atlantic coast suggests some similarity in the processes structuring these distinct infaunal communities.

Changes in the macrobenthic infaunal community of the Southern California continental margin over five decades in relation to oceanographic factors

Climate change has altered the physiochemical conditions of the coastal ocean but effects on infaunal communities have not been well assessed. Here, we used multivariate ordination to examine temporal patterns in benthic community composition from 4 southern California continental shelf monitoring programs that range in duration from 30 to 50 yr. Temporal changes were compared to variations in temperature, oxygen, and acidification using single-taxon random forest models. Species richness increased over time, coupled with a decline in overall abundance. Continental shelf macrobenthic communities from the 2010s comprised a broader array of feeding guilds and life history strategies than in the 1970s. Changing water temperature was associated with northward shifts in geographic distribution and increases in species abundance, while acidification was associated with southward shifts and declines in abundance of other species. Acidification was also associated with changes in depth distribution of benthic fauna, with shelled molluscs declining in abundance at depths most associated with increasing exposure to acidification. This broad-scale community-level analysis establishes causal hypotheses that set the stage for more targeted studies investigating shifts in abundance or distribution for taxa that appear to be responding to climate change-related disturbances.

Infaunal community structure, diversity, and function in Pacific-Arctic shelf sediments: a comparison of meiofaunal- and macrofaunal-sized nematodes

Meiofauna, and nematodes in particular, perform essential roles in benthic ecosystems and serve as bioindicators of disturbance and environmental change. In the Pacific Arctic, a region experiencing rapid environmental change, this component of the fauna has received little attention, and the role of meiofauna in ecosystem processes is poorly understood. We collected multi-core samples at 10 stations in the northern Bering and southern Chukchi Seas in June 2018 and characterized the sedimentary environment. We also assessed meiofauna community structure and abundance at higher taxonomic levels and evaluated genus-level nematode composition in meiofaunal (63-500 µm) and macrofaunal (&gt;500 µm) size fractions. Nematodes were classified by feeding type and life-history strategies. Total meiofauna abundance and biomass varied greatly, with 1449-12875 ind. 10 cm-2 and 373-2325 µg dry weight 10 cm-2 for the upper 5 cm of sediment. Estimated production of meiofaunal-sized nematodes was 5-28 g C m-2 yr-1. Four distinct communities of meiofaunal-sized nematodes were identified in different sub-regions reflecting food availability and substrate type. The meiofaunal- and macrofaunal-sized nematodes represented 2 distinct communities. The unique taxonomic composition and large standing stock of the macrofaunal-sized nematodes (22 ± 15% of total nematode biomass) suggest they are critical components of the infauna and merit further research to assess their role in critical ecosystem functions. This study provides the first genus-level characterization of nematodes and some of the first measurements of meiofauna standing stock in the region, contributing important data for assessing ecosystem function in a rapidly changing Arctic.

Benthic infauna associated with a shallow-water hydrothermal system of Punta Mita (Mexico)

Abstract The shallow-water hydrothermal system of Punta Mita in Banderas Bay is located on the fault called Fisura de las Coronas off Punta Pantoque beach. In this area, three sites with hydrothermal vents were studied at a depth of 9 m. This study aimed to characterize the structure of the benthic infauna communities that coexist in this hydrothermal system; therefore, physicochemical parameters were measured and the organisms found in the sediment samples were identified up to the class taxonomic level. The highest temperatures (89°C) were recorded within the hydrothermal influence area, which was reflected in an inverse relationship with pH, conductivity and salinity. Sediment temperature profiles increased at greater depth. A total of 371 individuals were found and these were grouped into eight classes: Malacostraca, Maxillopoda, Gastropoda, Bivalvia, Scaphopoda, Polychaeta, Leptocardii and Stenolaemata. The Malacostraca class was the most abundant with 240 individuals, while the Scaphopoda was the lowest with 3. The organic matter contents in the sediments were higher in the areas adjacent to the hydrothermal activity. The high temperature of the hydrothermal discharges structures the benthic community since it was the factor that differed most significantly in the study sites, causing lower abundances in the area of hydrothermal influence compared with the more distant areas. Despite the above, it is concluded that the benthic community of the area with hydrothermal activity is part of the community adjacent to this influence, only in lower numbers.

BIOTURBATORS AS ECOSYSTEM ENGINEERS: ASSESSING CURRENT MODELS

ABSTRACT Bioturbating organisms can dramatically alter the physical, chemical, and hydrological properties of the sediment and promote or hinder microbial growth. They are a classic example of “ecosystem engineers” as they alter the availability of resources to other species. Multiple evolutionary hypotheses evoke bioturbation as a possible driver for historical ecological change. To test these hypotheses, researchers need reliable and reproducible methods for estimating the impact of bioturbation in ancient environments. Early efforts to record and compare this impact through geologic time focused on the degree of bioturbation (e.g., bioturbation indices), the depth of bioturbation (e.g., bioturbation depth), or the structure of the infaunal community (e.g., tiering, ecospace utilization). Models which combine several parameters (e.g., functional groups, tier, motility, sediment interaction style) have been proposed and applied across the geological timescale in recent years. Here, we review all models that characterize the impact of bioturbators on the sedimentary environment (i.e., ‘ecosystem engineering'), in both modern and fossil sediments, and propose several questions. What are the assumptions of each approach? Are the current models appropriate for the metrics they wish to measure? Are they robust and reproducible? Our review highlights the nature of the sedimentary environment as an important parameter when characterizing ecosystem engineering intensity and outlines considerations for a best-practice model to measure the impact of bioturbation in geological datasets.

Community and population impacts of an introduced meiofauna (Nippoleucon hinumensis) upon an intertidal infaunal community and its closest endemic analog (Cumella vulgaris)

Community and population impacts of an introduced meiofauna (Nippoleucon hinumensis) upon an intertidal infaunal community and its closest endemic analog (Cumella vulgaris)

Required sampling intensity for community analyses of intertidal infauna to detect a mechanical disturbance.

In coastal ecosystems, infaunal (animals living within the sediment) invertebrates are used to study and monitor disturbances. However, it is an open question as to the minimal required sampling intensity to detect that a disturbance has influenced such communities. As such, we implemented a manipulative experiment using an infaunal community with a known response (community composition and population abundances) to a mechanical disturbance (sediment scour), to determine the minimum sampling intensity required to detect differences in the infaunal community. Statistically significant differences (α = 0.05) between the infaunal community of the disturbed and reference replicates were observed in case studies consisting of 99 (4 samples per m2) to 5 (0.2 samples per m2) samples per treatment. Below 5 samples, the known statistical and biological difference was undetectable. However, at least 10 samples per treatment (0.4 samples per m2) were required for the observed infaunal community to be within 93% similarity of our most accurate assessments of the infaunal community. These findings suggest that studies attempting to identify disturbances may require a minimal sampling intensity equivalent to 0.2 samples per m2, while studies attempting to determine how the infaunal community varies with disturbances may require 0.4 samples per m2. These potential minimal required sampling intensities will be of use in the theoretical exploration of disturbances, as well as in applied conservation, restoration, and monitoring projects.

Impact of and recovery from seabed trawling in soft-bottom benthic communities under natural disturbance of summer hypoxia: A case study in subtropical Hong Kong

Bottom trawling has widespread impact on the structure and composition of benthic communities. In this study, soft-bottom macrofauna in a heavily trawled area and an adjacent marine park that has been closed to bottom trawling for more than ten years were sampled, allowing to investigate the impact of and recovery from trawling. Both areas were equally affected by seasonal summer hypoxia. Polychaetes were the most abundant at trawled (78-86%) and non-trawled (85-94%) sites in both summer and winter. The non-trawled sites inside the marine park area had a lower level of species composition and abundance as compared to the trawled sites in summer, whereas in winter the benthic communities at these trawled and non-trawled sites appeared much similar with the majority of the identified taxa being dominated by new recruiting juveniles in both sampling areas. Results of permutational multivariate analysis of variance (PERMANOVA) on transformed infaunal abundance and biomass data revealed significant differences (p&amp;lt;0.05) between trawled and non-trawled treatments and seasons, as well as their interactions. Pair-wise a posteriori PERMANOVA further indicated significant differences for the abundance and biomass data in summer but insignificant differences in winter. PERMANOVA of the biological traits of infauna weighted with transformed biomass also confirmed significant difference for seasons only. Such findings imply that the effect of season was more important than that of trawling especially on the biological trait structure of the benthic communities. In addition, bottom trawling may contribute to a higher level of dissolved oxygen when the area suffered from hypoxia, resulting a higher biodiversity in the trawled area in summer. By comparing data taken prior to closure of the marine park from trawling, the recovery of infaunal community has been slow over the 12 years as a trawl-free area. Apart from natural disturbance of summer hypoxia, such recovery could also be complicated with the presence of artificial reefs deployed within the marine park, which may affect the hydrodynamics of the benthic environment and modify the community structure and function.