Habitat Structural Complexity Research Articles

Escarpments within Mediterranean seagrass Posidonia oceanica meadows increase habitat heterogeneity and structural complexity enhancing fish diversity and biomass

Seagrass meadows provide important ecosystem services including carbon sequestration, coastal protection from erosion, and sustained biodiversity and fisheries thereby improving the wellbeing and livelihoods of coastal communities. The erosion of millenary deposits of intertwined roots and rhizomes of Posidonia results in the formation of three-dimensional structures named escarpments that constitute a biogenic reef habitat. However, the natural history of seagrass escarpments including their formation processes and their role as habitat for reef fauna and flora remains poorly understood. This research located and characterized Posidonia oceanica escarpments in Menorca (Balearic Islands) and compared structural complexity and fish assemblages among seagrass escarpments, seagrass meadows, rocky substrates and bare sand with emphasis on its role as habitat and shelter for typical rocky fish. Fish abundance and biomass were similar between seagrass escarpments and rocky substrates (P > 0.05), but significantly lower in seagrass meadows (P < 0.001). The large number of caves found along seagrass escarpments provide shelter to fish, including species only associated to rocky substrates. Seagrass meadows form a rather homogenous habitat within their canopy, but the presence of seagrass escarpments enhances habitat heterogeneity and structural complexity along with fish abundance and biomass at the seascape level. This study enhances understanding on the ecological importance of seagrass escarpments.

Structural attributes and macrofaunal assemblages associated with rose gorgonian gardens (Leptogorgia sp. nov.) in Central Chile: opening the door for conservation actions

Gorgonians (like corals) are important habitat-forming organisms that support a diversity of macrofauna. This study explored structural attributes of gorgonian gardens formed by rose gorgonians (Leptogorgia sp. nov.) and associated macrofaunal assemblages in Caleta Pichicuy (Central Chile). Hierarchical sampling was conducted at 20 m depth (maximum colony abundances) in order to assess spatial variability in abundance and colony attributes at two spatial scales (among sites and rocky walls). The abundance and composition of the associated vagile and sessile macrofauna were also examined using univariant (Taxa richness and Shannon index (H’e)) and multivariant approaches and were compared with adjacent bare rocky habitats. Our results showed a high abundance of gorgonians (ca. 28.9–36.5 colonies m−2) compared to other gorgonian gardens in the world. For structural attributes, our results showed smaller colonies with thicker holdfasts in more exposed sites, suggesting the influence of hydrodynamic forces on the colony morphology. Taxa richness and H’e of vagile fauna showed threefold and twofold, respectively, higher values in gorgonian gardens compared to bare walls, but no differences were observed for sessile fauna. In addition, PCoA and PERMANOVA evidenced a distinctive assemblages’ composition between habitats for both vagile and sessile fauna. Correlation analyzes and dbRDA showed, however, little association between structural attributes and associated faunal assemblages (R2 = 0.06, and ca. 3–9.4% of the total variation explained, respectively). Our results constitute the first assessment of structural habitat complexity and accompanying fauna in these gorgonian gardens and establish the baseline for understanding possible future changes associated to human activities.

Effects of Water Temperature and Structural Habitat Complexity on the Routine Swimming Speed and Escape Response of Post-Settlement Stage White Seabream

Coastal habitats are increasingly threatened by multiple anthropogenic-related activities, which include ocean warming and loss of structural habitat complexity. These two pressures have the potential to severely affect the structure and function of marine biodiversity. Early life stages of many fish species recruit to coastal habitats at the end of their pelagic phase, benefiting from access to food, shelter and protection. However, changes in temperature have been shown to influence ecologically relevant behaviours in post-settlement stage fish, and the loss of structural habitat complexity has been related to low recruitment and deleterious behaviours of fish in coastal habitats. Here, we evaluated the individual and interactive effects of prolonged exposure to increasing temperature and changed structural habitat complexity on routine swimming speed and escape response of post-settlement white seabream, Diplodus sargus (Linnaeus, 1758). Fish were reared under different temperatures (control 19 °C; high 22 °C) and structural habitat complexity (low and high) scenarios, in a cross-experimental design, and the routine swimming and escape responses were analyzed after 6 weeks of exposure. Change in temperature did not induce alterations at the behavioural level, but loss of structural habitat complexity increased speed and distance travelled during routine swimming, and responsiveness to a stimulus during the escape response behaviour. The interaction of the two factors did not influence performance. Determining how species are affected by changes in their environment, and the mechanisms that underlie these changes, will be critical to understanding the fish recruitment and populations’ fitness and survival.

Stable isotopes reveal intertidal fish and crabs use bivalve farms as foraging habitat in Puget Sound, Washington

Bivalves such as oysters and clams have been farmed in intertidal zones across the Puget Sound region of the Salish Sea for thousands of years. The variety of gear types used on bivalve farms creates complex vertical structure and attachment points for aquatic epiphytes and invertebrates which increases habitat structural complexity, but may alter eelgrass cover in areas where bivalve farms and eelgrass meadows overlap. Eelgrass meadows are highly productive and ecologically foundational nearshore habitats that provide valuable ecosystem services including the provision of nursery, refuge, and foraging habitat. Aquaculture has been a key feature of the environment in the Puget Sound for millennia, however, little is known about how well aquaculture practices are integrated into the system, and what services they provide to mobile species assemblages relative to unfarmed eelgrass meadows. We used stable isotope mixing models to estimate, for several species of nearshore fish and crab in two areas of North Puget Sound, Washington, the percent diet originating from either a natural bottom habitat (eelgrass meadows), farm habitat (oyster farms), or pelagic planktonic sources. Our results indicate that several species of nearshore fish such as surf perch and staghorn sculpin derive a significant proportion of their diets from farm areas, while crabs derive most of their diets from eelgrass habitat, and stickleback derive a significant proportion of their diets from planktonic sources. The results indicate that foraging habitat uses are species specific, and that several species that spatially overlap bivalve farms obtained a large percentage of their diets from adjacent bivalve farm habitat.

The effect of habitat structural complexity on gastropods in an arid mangrove wetland

The effect of habitat structural complexity on gastropods in an arid mangrove wetland

Biological Invasions in Fresh Waters: Micropterus salmoides, an American Fish Conquering the World

Biological invasions in fresh waters cause biodiversity loss and impairment of ecosystem functioning. Many freshwater invasive species are fish, including the largemouth bass Micropterus salmoides, which is considered one of the 100 worst invasive species in the world. Fast individual growth rates, high dispersal ability, ecological tolerance, and trophic plasticity are among the characteristics contributing to its success. The negative impact of M. salmoides on littoral fish communities is believed to be mitigated by habitat structural complexity resulting from aquatic vegetation and coarse woody debris, while the main limits on its spread seem to be strong water flows and high turbidity, which impairs visual predation. Together with the human overexploitation of its potential fish antagonists, habitat alteration could result in M. salmoides having seriously detrimental effects on native biodiversity. The purpose of this study is to critically review the life history and ecology of M. salmoides, its impact on ecosystems outside North America, and the effects of anthropogenic activities on its spread. This will highlight environmental factors that favor or limit its invasive success, helping to identify management measures that might mitigate its negative effects on freshwater biodiversity.

Counterintuitive prey strategies against predators with finite budgets: protection heterogeneity among sites matters more than their number.

Combining the search and pursuit aspects of predator-prey interactions into a single game, where the payoff to the Searcher (predator) is the probability of finding and capturing the Hider (prey) within a fixed number of searches was proposed by Gal and Casas (J. R. Soc. Interface 11, 20140062 (doi:10.1098/rsif.2014.0062)). Subsequent models allowed the predator to continue its search (in another 'round') if the prey was found but escaped the chase. However, it is unrealistic to allow this pattern of prey relocation to go on forever, so here we introduce a limit of the total number of searches, in all 'rounds', that the predator can carry out. We show how habitat structural complexity affects the mean time until capture: the quality of the location with the lowest capture probability matters more than the number of hiding locations. Moreover, we observed that the parameter space defined by the capture probabilities in each location and the budget of the predator can be divided into distinct domains, defining whether the prey ought to play with pure or mixed hiding strategies.

Low‐cost management interventions and their impact on multilevel trade‐offs in agricultural grasslands

Abstract Cost‐effective strategies to increase biodiversity are a fundamental requirement to reconcile conservation and food production in agricultural landscapes. Key for the implementation of such strategies is an accurate quantification of both their benefits and potential associated trade‐offs. We therefore assessed, in a commercially managed grassland, biodiversity responses to two low‐cost management interventions and their mediating effects on ecosystem services. In a 6‐year experiment, we showed that a one‐time seed bank activation treatment had strong initial impacts on biodiversity, increasing plant richness in year 1 by 61%. Long‐term effects, which were also driven by the second management intervention, the propagation of the keystone species yellow rattle, were weaker but nonetheless substantial. These positive biodiversity responses improved ecosystem multifunctionality through additive positive effects of richness, evenness and phylogenetic distinctiveness on nectar production and structural habitat complexity. In contrast, hay biomass production was negatively affected by both management interventions, resulting in a multilevel trade‐off between biomass production competing with biodiversity conservation and the provisioning of other ecosystem services. Synthesis and applications. In this study, we demonstrate that the maximisation of either biodiversity or biomass production requires largely different land management practices. The evaluation of this trade‐off, however, is strongly dependent on its social, economic and ecological context and requires clearly defined land management priorities for both food production and biodiversity conservation.

Quantifying the impacts of an invasive weed on habitat quality and prey availability for tiger snakes (Notechis scutatus) in urban wetlands

Invasive plants are a threat to natural ecosystems worldwide, with urban wetlands being some of the most susceptible and highly modified environments of all. The tiger snake (Notechis scutatus) is a top predator that persists in urban wetlands of south-western Australia, many of which have been degraded by introduced kikuyu grass (Cenchrus clandestinus). To evaluate the potential impact of kikuyu grass on habitat quality for tiger snakes we quantified the structural features of habitats within wetlands degraded by kikuyu grass and compared them to wetlands with native vegetation. We also examined tiger snake prey availability, assessed predation risk for juvenile snakes using clay models, and measured the thermal quality of the vegetation. Proliferation of kikuyu grass has reduced habitat structural heterogeneity by reducing available bare ground and increasing vegetation density. This homogenisation of habitat structure had little effect on the predation risk for juveniles or the thermal properties of tiger snake shelter sites; however, one key prey species, the motorbike frog, had significantly lower abundance in the most impacted habitat. Habitat types with more structural complexity also offered tiger snakes more stable thermal regimes and lower predation risk. These findings indicate that the current extent of kikuyu grass invasion offers overall similar habitat quality for tiger snakes to native vegetation and may contribute to their persistence in urban wetlands; however, both tiger snakes and their anuran prey may benefit from increased habitat structural complexity.

Remote sensing of Antarctic polychaete reefs (Serpula narconensis): reproducible workflows for quantifying benthic structural complexity with action cameras, remotely operated vehicles and structure‐from‐motion photogrammetry

AbstractQuantifying the structural complexity provided by biogenic habitat structures is important in ecology, conservation and management, and yet remains a challenging task, particularly in deep sea and polar environments, that current photogrammetry tools can alleviate. In this study, we demonstrate how small remotely operated vehicles and compact underwater GoPro® action cameras can be easily integrated into coastal Antarctic surveys to quantify structural complexity of under‐ice benthos via underwater photogrammetry. Forty‐four pairs of 1 m2 quadrats at 1 cm resolution, each comprising an orthomosaic and three‐dimensional reconstructions, were analyzed to describe relationships between benthic cover and structural complexity metrics. The study case provided insights into a unique biogenic habitat, highlighting the role of integrating structural complexity metrics in Antarctic benthic surveys. Although no clear relationships between structural complexity and biodiversity were found, high cover of live reef‐building polychaetes was associated with higher levels of structural complexity, particularly fractal dimension (D). Further, broken biogenic structures, product of disturbance events retain habitat structural complexity known to be associated with larvae settlement and biogenic reef growth. This suggests that D can be used as a metric for detecting subtle changes in biogenic structural complexity. We build from available open‐source code, a reproducible scientific workflow that is expected to facilitate the acquisition and analysis of structural complexity metrics. The workflow presented aims to encourage and accelerate the use of photogrammetry tools for benthic studies aiming to quantify biogenic structural complexity across depths and latitudes.

Genetic diversity and population structure of Capitulum mitella (Linnaeus, 1767) in Fujian (China) revealed by mtDNA COI sequences.

Background: Capitulum mitella is a widely distributed and ecologically important stalked barnacle that settles extensively on rocky shores. This species contributes to the structural complexity of intertidal habitats and plays a critical role in the marine ecosystem. This study aimed to reveal the genetic diversity and population structure of C. mitella by analyzing the mitochondrial cytochrome oxidase I (COI) gene. Methods: A 683bp fragment of the COI gene was sequenced from 390 individuals sampled from six localities in Fujian, China. Results: A total of 84 distinct haplotypes were identified through the analysis of 82 polymorphic sites, resulting in an average haplotype diversity (h) of 0.660 and nucleotide diversity (π) of 0.00182. Analysis of molecular variance (AMOVA) and pairwise F ST statistics showed no significant population structure. Neutrality tests and mismatch distributions provided evidence of recent population expansion for the species. Conclusions: We suggest that the species' high dispersal ability, and ocean currents coupled with limited physical barriers in the region, contribute to its current phylogeographic structure. These findings enhance our comprehension of the genetic diversity and population structure of C. mitella, providing valuable insights for future conservation efforts.

Hierarchical drivers of cryptic biodiversity on coral reefs

AbstractDeclines in habitat structural complexity have marked ecological outcomes, as currently observed in many of the world's ecosystems. Coral reefs have provided a model for such changes in marine ecosystems; still our understanding has been centered on corals and fishes at broad spatial scales when metazoan diversity on coral reefs is dominated by small cryptic taxa (herein: “cryptofauna”). Given the paucity of studies and high taxonomic complexity of the cryptofauna, both of which limit a priori hypotheses, we asked whether hierarchical structuring theory provides a compelling framework to impose order and quantify patterns. In general terms, we explored whether cryptic communities are sufficiently described by broad seascape parameters or limited by a set of processes operating at their distinctly nested microhabitat scale. To address this theory and gaps in knowledge for the cryptofauna, we characterized community structure in coral rubble, an eroded coral condition where biodiversity proliferates. Rubble was sampled along a depth and exposure gradient at Heron Island on the Great Barrier Reef, Australia, to parameterize environmental and morphological indicators of sessile taxa and motile cryptofauna communities. We used a hierarchical study framework from microhabitat to seascape scales, which were evaluated using nonstructured multivariate analyses and Bayesian structural equation modeling. While the nonstructured analyses showed the effects of seascape on the cryptobenthos and its community, this approach overlooked the finer hierarchical patterns in rubble ecology revealed only in the structured model. Seascape parameters (exposure and depth) influenced microhabitat complexity (i.e., rubble branchiness), which determined the cover of sessile organisms on rubble pieces, which shaped the motile cryptofauna community. Rubble is likely to be increasingly prevalent on coral reefs in the Anthropocene and is typically associated with low seascape‐level complexity and reduced macrofaunal richness. Parallel with hierarchical structuring theory, we showed a similar response operating at the microhabitat scale whereby low rubble complexity (i.e., branchiness) reduced cryptobenthic structure, diversity and size spectra. In a future ocean, we expect there may be an initial increase in biodiversity and trophodynamic processes derived from branching rubble, but a delay in ecosystem‐scale outcomes if coral, and thus rubble, generation and complexity is not sustained.

SeagrassPosidoniaescarpments support high diversity and biomass of rocky reef fishes

AbstractAlthough seagrass meadows form a relatively homogenous habitat, escarpments, which form three‐dimensional structures and originate from the erosion of seagrass peat, can provide important habitat for reef fishes. Here, we compare fish assemblages and habitat structural complexity among seagrassPosidonia australisescarpments and canopies, as well as limestone reef habitats, to understand the role of seagrass escarpments as reef fish habitat in Shark Bay, Western Australia. The total number of fish species, fish biomass, and top predator biomass were significantly higher in seagrass escarpments and reef habitats than in seagrass canopies due to lower habitat structural complexity and thus becoming suitable habitats for predators and prey in the latter. Both seagrass escarpment and reef habitats host similar assemblages of top predators and carnivorous fishes, such asEpinephelus coioides,Microcanthus strigatus, andChoerodon schoenleinii, that were absent in seagrass canopies. Seagrass escarpments provide an alternative habitat for reef fishes comparable to rocky reefs, which are limited in Shark Bay. Caves and ledges within the escarpments support 13.4 Mg of fish and 3.6 Mg of top predator species of commercial interest within the Shark Bay World Heritage Site. Additional research is needed to further understand the ecological importance of seagrass escarpments in enhancing fish biomass and biodiversity, as reproduction grounds or refuge from predators, and to investigate the role of meadow edges in ecosystem function.

Geological age and environments shape reef habitat structure

AbstractAimHabitat complexity plays an important role in the structure and function of ecosystems worldwide. On coral reefs, habitat complexity influences ecosystem services such as harvestable fish biomass and attenuation of wave energy. Here, we test how three descriptors of surface complexity—rugosity, fractal dimension, and height range—trend with the geological age of reefs (0.2–5.1 million years old), depth (1–25 m), wave exposure (1–306 kW/m), coral cover (0–80%), and three habitat types (aggregated reef, rock and boulder, and pavement).LocationWe surveyed across 234 sites and 4 degrees of latitude in the eight main Hawaiian Islands.Time PeriodApril 2019 – July 2019.Major Taxa StudiedReef building corals.MethodsWe estimate three surface descriptors (rugosity, fractal dimension and height range) using structure‐from‐motion photogrammetry. We evaluate hypothesized relationships between these descriptors and geological reef age, depth, wave exposure, coral cover and reef habitat type using generalized linear models that account for survey design.ResultsThe rugosity of reef habitats decreased with geological reef age; fractal dimension (and coral cover) decreased with wave exposure; and height range decreased with depth. Variations in these patterns were explained by the different habitat types and the way they are formed over time. Nonetheless, the three surface descriptors were geometrically constrained across all habitat types, and so habitats occupied distinctly different regions of habitat complexity space.Main ConclusionsThis study showed how broad environmental characteristics influence the structural complexity of habitats, and therefore geodiversity, which is an important first step toward understanding the communities supported by these habitats and their ecosystem services.

Intertidal habitat complexity influences the density of the non-native crab Hemigrapsus sanguineus.

Habitat structural complexity can provide protection from predators, potentially affecting population density of native and non-native prey. The invasive Asian shore crab, Hemigrapsus sanguineus, occurs in variable densities in the rocky intertidal zone of eastern North America and northern Europe, often in densities greater than in its native range. The present study examined the influence of habitat complexity on the density of H. sanguineus. Artificial shelters of concrete pavers with stones arranged in increasing complexity were deployed in the intertidal zone along a rocky shore in southeastern Massachusetts, USA, for 21 consecutive weekly intervals in 2020. Crabs consistently reached the highest densities in the most complex shelters despite their lower internal surface area. In addition, crabs exhibited shelter selectivity based on body size, with large crabs occupying artificial shelters in greater numbers than adjacent natural substrate. In a subsequent lab study, crab activity over 1 h was observed in the presence of the same artificial shelters, under simulated tidal conditions. Shelter complexity had little influence on the number of crabs under the pavers although crabs were more active when submerged in water than exposed to air. These results show that crab density increases as habitat complexity increases, and complexity may serve as a predictor of H. sanguineus density but not short-term behavior.

Genetic diversity and population structure of Capitulum mitella (Linnaeus, 1767) in Fujian (China) revealed by mtDNA COI sequences

Background: Capitulum mitella is a widely distributed and ecologically important stalked barnacle that settles extensively on rocky shores. This species contributes to the structural complexity of intertidal habitats and plays a critical role in the marine ecosystem. This study aimed to reveal the genetic diversity and population structure of C. mitella by analyzing the mitochondrial cytochrome oxidase I (COI) gene. Methods: A 683bp fragment of the COI gene was sequenced from 390 individuals sampled from six localities in Fujian, China. Results: A total of 84 distinct haplotypes were identified through the analysis of 82 polymorphic sites, resulting in an average haplotype diversity (h) of 0.660 and nucleotide diversity (π) of 0.00182. Analysis of molecular variance (AMOVA) and pairwise FST statistics showed no significant population structure. Neutrality tests and mismatch distributions provided evidence of recent population expansion for the species. Conclusions: We suggest that the species' high dispersal ability, and ocean currents coupled with limited physical barriers in the region, contribute to its current phylogeographic structure. These findings enhance our comprehension of the genetic diversity and population structure of C. mitella, providing valuable insights for future conservation efforts.

Decline in cold-water coral growth promotes molluscan diversity: A paleontological perspective from a cold-water coral mound in the western Mediterranean Sea

Framework-forming scleractinian cold-water corals (CWCs) act as ecosystem engineers, building and supporting biodiversity hotspots in the deep sea worldwide. While spatial patterns and drivers of species distributions have been evaluated on modern CWC reefs, little is known about how reef diversity is affected by habitat variability over geologic time – the scale at which CWC reefs initiate, thrive, and decline. Using three CWC reef sediment cores as species diversity archives, we investigated temporal trends of molluscan diversity over the last ~13 kyr from a CWC mound in the Alboran Sea (western Mediterranean Sea) to evaluate (a) how spatial patterns of CWC-associated diversity are recorded in reef sediments, (b) the potential of CWC reefs as biodiversity hotspots when coral growth is flourishing and when it is not, and (c) which palaeoceanographic conditions or habitat characteristics may be driving biodiversity. Our results reveal that at the ecosystem scale ecological differences between CWC habitats are more pronounced than ecological signatures of molluscan assemblages associated with intervals of CWC framework (flourishing growth) or non-framework (negligible CWC growth). However, within habitats, significant differences emerge between these assemblages with lower molluscan diversity associated with flourishing CWC growth. Significant negative correlations between molluscan diversity and palaeoceanographic conditions conducive for CWC growth (high food availability, strong hydrodynamics, optimal bottom-water temperatures and salinities, and high aggradation rates indicative of flourishing CWC growth also imply that CWC growth and relevant environmental conditions contribute to reduced molluscan diversity. Additionally, high coral volume, used here as a proxy for habitat structural complexity, is positively correlated with molluscan diversity just as high habitat complexity is in living CWC reefs. Altogether, these patterns detected over geologic time resemble those observed spatially across living CWC reefs today – where competition with resources, particularly food, prevents high reef biodiversity in the immediate vicinity of dense living CWC colonies. Overall, our study demonstrates that (1) ecological paradigms of living CWCs are preserved in their sedimentary record, (2) flourishing CWC growth and conditions promoting CWC growth drive habitat-scale diversity patterns, and (3) a geological approach can be applied to study long-term diversity dynamics in CWC ecosystems.

Managing aquatic habitat structure for resilient trophic interactions.

Structural habitat (the three-dimensional arrangement of physical matter, abiotic and biotic, at a location) is a foundational element for the resilience and maintenance of biodiversity, yet anthropogenic development is driving the global simplification of aquatic environments. Resource managers regularly seek to conserve aquatic food webs by increasing structural habitat complexity with expected benefits to fisheries; however, the global effectiveness of such actions is unclear. Our synthesis and theoretical analyses found that the response of a consumer-resource interaction (predatory sportfish and forage fish prey) to the addition of prey refuge habitat differed among systems with low and high rates of biomass transfer from resource to consumer (i.e., biomass potential); stabilization was not the rule. Greater prey refuge habitat availability tended to stabilize systems characterized by high biomass potential while simultaneously increasing consumer densities. In contrast, increasing prey refuge habitat availability in systems characterized by low biomass potential tended to mute energy transfer and moved consumer densities toward local extinction. Importantly, biomass potential and prey refuge can have antagonistic effects on stability and relative consumer densities, and it is therefore important to consider the local conditions of a system when using habitat manipulation as a management measure. Further development of our context-dependent perspective to whole food webs, and across different environments, may help to guide structural habitat management to better restore and protect aquatic ecosystems.

Seasonal effects of edge and habitat complexity on eelgrass epifaunal assemblages

Habitat degradation and fragmentation reduce habitat structural complexity (e.g. amount of physical features) and increase habitat edges. While many studies have focused on the effects of habitat edges or complexity on biodiversity, relatively few have disentangled them or investigated their effects over time. We investigated how proximity to the edge of eelgrassZosterasubg.Zostera marinaLinnaeus, 1753 habitat, shoot density and their interactions across seasons can influence the diversity pattern of epifaunal assemblages in meadows situated in a Mediterranean lagoon (France). We used a combination of field sampling andin situmanipulations with artificial seagrass units (ASUs) mimicking low and high shoot densities. During autumn and spring, we found that shoot density, Z. marina biomass and leaf area index (LAI) were higher inside the meadows than at the edge, while epiphyte load was the highest in spring at the edges. Epifaunal abundance and diversity were higher at the edge than inside the meadow for both natural shoots—regardless of the epiphyte load—and ASUs in spring. In autumn, epifaunal abundance varied positively with ASU density, regardless of the position in the meadow. Our results also showed that edges and habitat complexity affect the epifaunal structure differently across seasons. Therefore, we suggest that recruitment of macrofauna is the main mechanism explaining a positive edge effect during spring. This work highlights the need to consider seasonal dynamics in the assessment of habitat fragmentation and degradation.

Competing with each other: Fish isotopic niche in two resource availability contexts

IntroductionCompetition embodies species struggle for space and food and is, therefore, a critical evolutionary mechanism influencing species spatiotemporal patterns and persistence. One of the main drivers of competition is resource availability. In marine ecosystems, resource availability is determined, among other things, by habitat structural complexity, as it increases biodiversity and species abundance. In this context, our study aims at understanding how the differences in substrate complexity affect potential trophic competition between demersal fishes in Northeast Brazil.MethodsWe selected two zones contrasted in terms of substrates, one dominated by sand (zone A) and the other by complex substrates such as reefs and calcareous algae (zone B). We used Stable Isotope Analysis (SIA) to quantify intraspecific and interspecific interactions of demersal fishes in both zones. We compared the competitive interactions between zones using social network analysis (SNA), a suitable method to quantitatively study a set of interactions.ResultsIn the sand-dominated zone, demersal fish showed greater interspecific competition and occupied a larger isotopic niche suggesting that a resource-limited context led to diet diversification in the community. Some species expanded their niche through diet plasticity, while others showed higher intraspecific competition than in Zone B to cope with the greater interspecific competition.DiscussionCombining SNA and SIA provided a new method to investigate competition. This study potentially gives a perspective on the future ecological response of the demersal fishes at the community and species level. With habitat degradation and climate change, the complex substrates in zone B will probably disappear gradually, transforming this zone into an ecosystem akin to zone A. This would be detrimental to species more vulnerable to interspecific competition, particularly those targeted by fisheries.