Seagrass bed patchiness: effects on epifaunal communities in San Diego Bay, USA

Seagrass habitat structure influences epifaunal density, diversity, community composition and survival, but covariation of structural elements at multiple scales (e.g., shoot density or biomass per unit area, patch size, and patch configuration) can confound studies attempting to correlate habitat structure with ecological patterns and processes. In this study, we standardized simulated seagrass shoot density and bed area among artificial seagrass beds in San Diego Bay, California, USA to evaluate the singular effect of seagrass bed configuration (“patchiness”) on the density and diversity of seagrass epifauna. Artificial seagrass beds all were 1 m², but were composed of a single large patch (“continuous” treatment), four smaller patches (“patchy” treatment), or 16 very small patches (“very patchy” treatment). We allowed epifauna to colonize beds for 1 month, and then sampled beds monthly over the next 3 months. Effects of seagrass bed patchiness on total epifaunal density and species-specific densities were highly variable among sampling dates, and there was no general trend for the effects of fragmentation on epifaunal densities to be positive or negative. Epifaunal diversity (measured as Simpson's index of diversity) was highest in very patchy or patchy beds on two out of the three sampling dates. Very patchy beds exhibited the highest dissimilarity in community composition in the first two sampling periods (August and September), but patchy beds exhibited the highest dissimilarity in the third sampling period (October). Our results indicate that seagrass patch configuration affects patterns of epifaunal density, diversity, and community composition in the absence of covarying bed area or structural complexity, and that patchy seagrass beds may be no less valuable as a habitat than are continuous seagrass beds. The spatial pattern employed when harvesting or planting seagrass may influence epifaunal habitat use and should be a key consideration in restoration plans.     

Effects of habitat edges on American lobster abundance and survival

Habitat edges frequently possess distinct ecological conditions that affect interactions such as competition and predation. Within a species' preferred habitat, the structural complexity and resource availability of adjacent habitats may influence the effect of edges on ecological processes. In nearshore waters of New England, American lobsters (Homarus americanus) inhabit fragmented cobble reefs that often are bordered by unvegetated sediment and occasionally by seagrass. We determined whether proximity to cobble patch edges, microhabitat characteristics within cobble habitat, and the type of habitat adjacent to cobble patches (seagrass or unvegetated sediment) influence the density and survival of juvenile and adult American lobsters in Narragansett Bay, Rhode Island, USA. We surveyed naturally occurring cobble patches and artificial cobble reefs to determine how the odds of finding lobsters varied with distance from the edge and habitat type. Additionally, we tethered lobsters at different distances from the edge inside and outside of cobble patches to determine how lobster relative survival varied with edge proximity and habitat type. In cobble habitat, the odds of finding large lobsters (adolescents and adults > 40 mm carapace length (CL)) were highest near patch edges regardless of adjacent habitat type, whereas smaller lobsters (e.g. emergent juveniles 15-25 mm CL) were more abundant in patch interiors when seagrass bordered cobble patches. The odds of finding lobsters also increased with the relative amount of cobble cover within patches. In predation experiments, lobster relative survival after 6 h was lowest outside of cobble and increased toward cobble patch interiors, but after 24 h this trend disappeared or reversed. Seagrass appeared to offer greater refuge for lobsters than did unvegetated sediment. Our results suggest that proximity to patch edges influences lobster distribution and survival, and that edge effects on lobsters vary with life history phase and with the type of habitat adjacent to cobble patches.     

Effects of seagrass habitat fragmentation on juvenile blue crab survival and abundance

Seagrasses form temporally dynamic, fragmented subtidal landscapes in which both large- and small-scale habitat structure may influence faunal survival and abundance. We compared the relative influences of seagrass (Zostera marina L.) habitat fragmentation (patch size and isolation) and structural complexity (shoot density) on juvenile blue crab (Callinectes sapidus Rathbun) survival and density in a Chesapeake Bay seagrass meadow. We tethered crabs to measure relative survival, suction sampled for crabs to measure density, and took seagrass cores to measure shoot density in patches spanning six orders of magnitude (ca. 0.25-30,000 m²) both before (June) and after (September) seasonally predictable decreases in seagrass structural complexity and increases in seagrass fragmentation. We also determined if juvenile blue crab density and seagrass shoot density varied between the edge and the interior of patches. In June, juvenile blue crab survival was not linearly related to seagrass patch size or to shoot density, but was significantly lower in patches separated by large expanses of unvegetated sediment (isolated patches) than in patches separated by <1 m of unvegetated sediment (connected patches). In September, crab survival was inversely correlated with seagrass shoot density. This inverse correlation was likely due to density-dependent predation by juvenile conspecifics (i.e. cannibalism); juvenile blue crab density increased with seagrass shoot density, was inversely correlated with crab survival, and was greater in September than in June. Shoot density effects on predator behavior and on conspecific density also likely caused crab survival to be lower in isolated patches than in connected patches in June. Isolated patches were either large (patch area >3000 m²) or very small (<1 m²). Large isolated patches had the lowest shoot densities, which may have allowed predators to easily find tethered crabs. Very small isolated patches had the highest shoot densities and consequently a high abundance of predators (=juvenile conspecifics). Though shoot density did not differ between the edge and the interior of patches, crabs were more abundant in the interior of patches than at the edge. These results indicate that seagrass fragmentation does not have an overriding influence on juvenile blue crab survival and density, and that crab cannibalism and seasonal changes in landscape structure may influence relationships between crab survival and seagrass habitat structure. Habitat fragmentation, structural complexity, faunal density, and time all must be incorporated into future studies on faunal survival in seagrass landscapes.     

Gradients in the number of species at reef-seagrass ecotones explained by gradients in abundance

Gradients in the composition and diversity (e.g. number of species) of faunal assemblages are common at ecotones between juxtaposed habitats. Patterns in the number of species, however, can be confounded by patterns in abundance of individuals, because more species tend to be found wherever there are more individuals. We tested whether proximity to reefs influenced patterns in the composition and diversity ('species density' = number of species per area and 'species richness' = number of species per number of individuals) of prosobranch gastropods in meadows of two seagrasses with different physiognomy: Posidonia and Amphibolis. A change in the species composition was observed from reef-seagrass edges towards the interiors of Amphibolis, but not in Posidonia meadows. Similarly, the abundance of gastropods and species density was higher at edges relative to interiors of Amphibolis meadows, but not in Posidonia meadows. However, species richness was not affected by proximity to reefs in either type of seagrass meadow. The higher number of species at the reef-Amphibolis edge was therefore a consequence of higher abundance, rather than species richness per se. These results suggest that patterns in the composition and diversity of fauna with proximity to adjacent habitats, and the underlying processes that they reflect, likely depend on the physiognomy of the habitat.     

Structural complexity and fish body size interactively affect habitat optimality

Predator–prey interactions are strongly influenced by habitat structure, particularly in coastal marine habitats such as seagrasses in which structural complexity (SC) may vary over small spatial scales. For seagrass mesopredators such as juvenile fishes, optimality models predict that fitness will be maximized at levels of SC that enhance foraging but minimize predation risk, both of which are functions of body size. We tested the hypothesis that in eelgrass (Zostera marina) habitat, optimal SC for juvenile giant kelpfish (Heterostichus rostratus), an abundant eelgrass mesopredator in southern California, changes through ontogeny. To do this, we quantified eelgrass SC effects on habitat associations, relative predation risk, and foraging efficiency for three size classes of juvenile giant kelpfish. We found that habitat selection differed with fish size: small fish selected dense eelgrass, whereas larger fish selected sparse eelgrass. Small kelpfish experienced the lowest relative predation risk in dense eelgrass but also had higher foraging efficiency in dense eelgrass, suggesting that dense eelgrass is selected by these fish because it minimizes risk and maximizes potential for growth. Surprisingly, larger kelpfish did not experience lower predation risk than small kelpfish. However, larger kelpfish experienced higher foraging efficiency in sparse eelgrass vs. dense eelgrass, suggesting that they select sparse eelgrass to maximize foraging efficiency. Our study highlights that trade-offs between predation risk and foraging can occur within a single habitat type, that studies should consider how habitat value changes through ontogeny, and that seagrass habitat value may be maximal when within-patch variability in SC is high.     

Effects of habitat heterogeneity at multiple spatial scales on fish community assembly

Habitat variability at multiple spatial scales may affect community structure within a given habitat patch, even within seemingly homogenous landscapes. In this context, we tested the importance of habitat variables at two spatial scales (patch and landscape) in driving fish community assembly using experimental artificial reefs constructed across a gradient of seagrass cover in a coastal bay of The Bahamas. We found that species richness and benthic fish abundance increased over time, but eventually reached an asymptote. The correlation between habitat variables and community structure strengthened over time, suggesting deterministic processes were detectable in community assembly. Abundance of benthic fishes, as well as overall community structure, were predicted by both patch- and landscape-scale variables, with the cover of seagrass at the landscape-scale emerging as the most important explanatory variable. Results of this study indicate that landscape features can drive differences in community assembly even within a general habitat type (i.e., within seagrass beds). A primary implication of this finding is that human activities driving changes in seagrass cover may cause significant shifts in faunal community structure well before complete losses of seagrass habitat.     

Effects of native predators and eelgrass habitat structure on the introduced Asian mussel Musculista senhousia (Benson in Cantor) in southern California

The ability of predators to control the abundance of non-native species has been little explored in marine systems. Native predators may be used to control non-native species or may confer invasion resistance to communities if predation rates on invaders are density-dependent. We studied the response of southern California native predators to the density of Musculista senhousia (Benson in Cantor, 1842), a small, fast growing mussel that has been introduced from Japan to several coastlines worldwide. We performed field experiments to determine if M. senhousia proportional mortality is density-dependent and if eelgrass Zostera marina L. habitat structure influenced mussel density-dependent mortality. We also evaluated the effect of seagrass habitat structure on the aggregative and functional responses of the predatory gastropod Pteropurpura festiva (Hinds, 1844) to Asian mussel density. In the summer of 2002, P. festiva aggregated in plots with high mussel density and was responsible for nearly all predation on M. senhousia. However, M. senhousia proportional mortality was inversely density-dependent at all levels of eelgrass above-ground and below-ground habitat structure. Asian mussel proportional mortality also was inversely density-dependent and was not influenced by eelgrass habitat structure in the spring of 2004 when wading birds were the chief predator of mussels. In contrast to results for mussel proportional mortality, the aggregative and functional responses of P. festiva varied with seagrass habitat structure. P. festiva density increased with Asian mussel density in plots with low simulated habitat structure, but the relationship between P. festiva density and Asian mussel density was parabolic at zero, intermediate and high levels of habitat structure. In field enclosures, P. festiva exhibited a Type I (linear) functional response to Asian mussel density at low levels of eelgrass structure, and a Type II (hyperbolic) functional response to mussel density at high levels of eelgrass structure. Our results and those of others suggest that the degree to which local benthic communities in southern California are resistant to Asian mussel invasion depends on habitat structure, mussel settlement rates, and the density and diversity of predators.     

Seagrass landscapes: a terrestrial approach to the marine subtidal environment

Subtidal seagrass habitats are prime candidates for the application of principles derived from landscape ecology. Although seagrass systems are relatively simple compared to their terrestrial counterparts in terms of species diversity and structural complexity, seagrasses do display variation in spatial patterns over a variety of scales. The presence of a moving water layer and its influence on faunal dispersal may be a distinguishing feature impacting ecological processes in the subtidal zone. Studying seagrass-dominated landscapes may provide a novel approach to investigating questions regarding self-similarity of spatial patterns, and offers a new perspective for analysing habitat change in a variety of marine environments.     

Plant–animal associations in two species of seagrasses in Western Australia

Relationships between algal epiphytes and epifaunal invertebrates (amphipods, molluscs and polychaetes) occurring within meadows of the seagrasses Posidonia sinuosa and Amphibolis griffithii were compared along the south west coast of Western Australia. Although the seagrasses are very different structurally, many species of algal epiphytes and epifaunal grazers were common to both. However, meadows of Amphibolis supported a greater number of both algal epiphyte and epifaunal species. The long-lived stems of Amphibolis supported a larger biomass of algal epiphytes and grazers than did the leaves of either Posidonia or Amphibolis. The densities and biomass of epifauna were variable but on a comparison adjusted to the biomass of seagrass, both the density and biomass of the taxonomic groups were similar between seagrass species except that the density of grazing gastropods and the biomass of polychaetes were greater in Amphibolis (by 238% and 252%, respectively). Nested analyses of variance (ANOVA) indicated that variations in plant and animal biomass differed at all spatial scales (sites, meadows within sites and replicates) and the pattern was inconsistent amongst biota. However, a significant proportion of the variability occurred between replicate samples. Canonical correlation and multiple regression analyses indicated that associations between algal epiphytes and epifauna were also inconsistent and differed between seagrass species. These patterns highlight the importance of seagrass species and structural complexity in affecting both the epiphytic and grazer community. The importance of spatial scales at which seagrasses and their associated communities are sampled are equally important because of the differing levels of spatial patchiness.     

Scale-dependant control of motile epifaunal community structure along a coral reef fishing gradient

Large-scale fishing is mostly conducted using towed gears that reduce the biomass and diversity of benthic invertebrates. However, it is impossible to differentiate between the physical disturbance effect of towed gears from the effect of fish predator removal upon benthic invertebrate communities. Here we explore the impact of fish removal alone on the community structure of small motile coral reef invertebrates (epifauna) along a subsistence fishing intensity gradient in the Lau group, Fiji. We deployed settlement plates at three areas in each of six fishing grounds and examined the density and class richness of the motile epifaunal communities and the associated algal communities in relation to the structure of fish and benthic communities. Motile epifaunal density was unrelated to fishing intensity. However, at smaller inter-area scale (0.5-10 km) motile epifaunal density was negatively related to plate algal biomass, whereas at the larger inter-fishing-ground scale (4-180 km) motile epifaunal density was positively related to the rugosity (substrate complexity) of the surrounding benthos. The class richness and diversity (Margalef's d) of motile epifaunal communities were negatively related to fishing intensity, but unrelated to grazing intensity, rugosity or algal biomass at either scale. Benthic community structure varied significantly with fishing intensity; hard-coral cover was lower and turf-algal cover was higher at high fishing pressure. The variation in benthic community structure was associated with variation in fish community structure, which in turn varied with fishing intensity. Motile epifaunal community structure upon plates was linked to the structure of the surrounding benthic community, but was not directly linked to the plate algal community. We suggest the decline in richness of the motile epifauna community along the fishing gradient is attributable to either to exploiter-mediated coexistence or the reduction in ‘habitat quality’ of the surrounding benthos. At the large spatial scale substrate complexity is the key determinant of motile epifaunal density, suggesting predation by fishes plays an important structuring role at this scale. Assuming that rugosity is inversely related to predation risk then this study represents the first evidence for spatial-dependence on the top-down (predation) vs. bottom-up (algal biomass) control of community structure. We argue fisheries exploitation, in the absence of a physical disturbance can negatively influence motile epifaunal community structure at large spatial scales.     

Population Genetic Analyses of Transplanted Eelgrass (Zostera marina) Beds Reveal Reduced Genetic Diversity in Southern California

Seagrass ecosystems fulfill ecologically and economically valuable functions in coastal marine environments. Unfortunately, seagrass beds are susceptible to natural and human disturbances, and their distrubution is declining worldwide. Although intentional disturbance of seagrass beds must be mitigated pursuant to U.S. law, to date mitigation of seagrass beds has not prevented a net loss of habitat. Transplantation of vegetative material from small areas of nearby beds is the primary method of seagrass mitigation. Restoration research on seagrasses has focused primarily on establishment of the plants and secondarily on the functional equivalency of the habitats. We questioned whether transplanted seagrass beds were comparable to “natural” beds in terms of genetic diversity and structure. We sampled Zostera marina L. (eel-grass) from 12 sites in the highly urbanized area of San Diego County and from pristine sites in Baja California. Using allozyme electrophoresis, we determined that genetic diversity (percentage of polymorphic loci, allele richness, expected and observed heterozygosities, and proportion of genetically unique individuals) was significantly reduced in transplanted eelgrass beds. Eelgrass from Baja California exhibited the highest genetic diversity. Based on Wright's F statistics, most of the genetic variation was distributed within rather than among sites (F_ST= 0.139), and the degree of genetic structure was only moderate at the greatest geographical scale (San Diego—Baja). Using a spatial statistical analysis (second-order analysis), we found virtually no evidence for nonrandom distribution of alleles or genotypes at scales of 3–50 m within beds. We discuss several hypotheses for reduced genetic diversity in transplanted eelgrass beds, including transplantation protocol, small size of transplantations, and reduced or failed sexual reproduction.     

Response of fauna in seagrass to habitat edges,patch attributes and hydrodynamics

This study has investigated the taxon‐specific responses of fauna to patch edges, and how these relate to patch attributes (patch size, seagrass biomass and water depth), and hydrodynamics in the seagrass habitat. Faunal abundances were sampled at the edge, 2 m in from the edge, and in the middle of 10 seagrass patches of variable size in Port Phillip Bay, Australia. Five of nine taxa showed edge effects. There were higher abundances at the edge compared with the middle for porcellid harpacticoids, and an increase in abundance from the edge to the middle of the patches for tanaids and isopods. For caprellid and gammarid amphipods, the edge effect varied across patches. Changes in current within the patch and patch size were related to the variability in the edge effect pattern of caprellids. None of the measured environmental variables (seagrass biomass, current and water depth) or patch size had a role in the variable edge effect pattern of gammarid amphipods. At the patch level, the distribution of six of nine taxa in this study, namely isopods, polychaetes, ‘other harpacticoids’, porcellid harpacticoids, cumaceans and gammarid amphipods, was related to differences in average water depth, average seagrass biomass and patch size. Our study indicates that the faunal response to edges cannot be generalized across seagrass habitat, and the implications of habitat area loss will vary depending on the taxon under consideration.     

Temporal variation in nest predation risk along habitat edges between grassland and secondary forest in Central Europe

Habitat fragmentation alters many ecological processes, including trophic cascades. For example, increased predation pressure along habitat edges has often been observed in fragmented landscapes. Here, we studied how nest predation risk varies along the transition zone between grassland and mixed forest in Central Europe. Using artificial nests, we tested the two mechanisms that are expected to underlie higher predation rates along edges: (1) the matrix effect model that supposes predator penetration from a habitat type with higher predator density to one with lower predator density and (2) the ecotonal effect model that assumes specific predator preferences for habitat edges. Although our results do not fully support either of these scenarios, our data show high temporal instability in nest predation along forest–grassland edges. Predation was higher in habitat interiors compared to edges during the first year, whereas the opposite pattern was observed during the subsequent year. In addition, dramatic between-year differences in the species composition of nest predators were observed. Therefore, we hypothesise that the effect of edges on nest predation is difficult to predict in landscapes with high predator diversity. In addition, our data indicate that a high abundance of wild boar considerably increases the risk of predation for ground-nesting birds.     

Edge effects on fauna in fragmented seagrass meadows

Abstract Fragmented habitats are a common occurrence in many marine systems, but remain poorly studied in comparison to their terrestrial counterparts. Here, I show that crustaceans inhabiting fragmented Zostera seagrass meadows show a dramatic response (change in abundance) to patch edges, with 11 out of 12 tests showing greatest abundance at the boundary between sand and seagrass. These patterns occurred on a scale of 0.25–1 m around the patch edge. Changes in seagrass biomass are unlikely to explain this pattern, as seagrass biomass increased smoothly at the patch edge, and did not decline towards patch interiors. In contrast to crustaceans, only a few polychaete taxa responded to the patch edge (9 of 25 tests), and bivalves generally did not show a response (1 of 5 tests). These latter groups are predominantly infaunal, and their lack of response may be partly due to the presence of substantial quantities of seagrass root and rhizome material in the sand habitat, which was defined visually based on the lack of above‐ground seagrass components only.     

Review of the effects of within-patch scale structural complexity on seagrass fishes

Studies on the effects of within-patch scale structure of seagrass habitats on predator–prey fish interactions and abundance/habitat use patterns were reviewed. Most laboratory experiments have employed chase-and-attack predators, usually resulting in lower foraging efficiency in (denser) seagrass. However, a few laboratory procedures employed alternative foraging tactics, resulting in no differences in prey mortality rates. Field studies did not always result in lower prey mortality rates in seagrass habitats. Accordingly, it is premature to conclude that seagrass presence is almost always negatively related to predator foraging efficiency or that increasing seagrass abundance is usually associated with a decrease in predator efficiency. Because several categories of predator and prey fishes occur in seagrass habitats, further studies are needed with all of these predator–prey combinations, in order to fully clarify predator–prey fish interactions in association with seagrass structure. Seagrass fishes have been shown to respond to alterations in seagrass structure in various ways: seagrass height and/or density reduction or clearance resulted in decreased abundance of some species but increases or no change in others. Some explanations have been proposed, not all mutually exclusive, for these phenomena. Although within-patch scale processes have been well studied, room exists for improvement. For example, predator–prey fish interactions in relation to varying within-patch scale complexity is not yet fully understand. The relationships of patch size, edge effects and within-patch scale complexity also still remain unclear. Further studies, which add to the clarification of within-patch scale process, will in turn improve our understanding of larger spatial scale processes.     

Changes in habitat complexity negatively affect diverse gastropod assemblages in coralline algal turf

The physical structure of a habitat generally has a strong influence on the diversity and abundance of associated organisms. I investigated the role of coralline algal turf structure in determining spatial variation of gastropod assemblages at different tidal heights of a rocky shore near Sydney, Australia. The structural characteristics of algal turf tested were frond density (or structural complexity) and frond length (the vertical scale over which structural complexity was measured). This definition of structural complexity assumes that complexity of the habitat increases with increasing frond density. While frond length was unrelated to gastropod community structure, I found significant correlations between density of fronds and multivariate and univariate measures of gastropod assemblages, indicating the importance of structural complexity. In contrast to previous studies, here there were negative relationships between the density of fronds and the richness and abundance of gastropods. Artificial habitat mimics were used to manipulate the density of fronds to test the hypothesis that increasing algal structural complexity decreases the richness and abundance of gastropods. As predicted, there were significantly more species of gastropods in loosely packed than in tightly packed turf at both low- and mid-shore levels. Despite large differences between gastropod assemblages at different tidal heights, the direction and magnitude of these negative effects were similar at low- and mid-shore levels and, therefore, relatively independent of local environmental conditions. These novel results extend our previous understanding of the ecological effects of habitat structure because they demonstrate possible limitations of commonly used definitions of structural complexity, as well as distinct upper thresholds in the relationship between structural complexity and faunal species richness.     

Responses of mobile epifauna to small-scale seagrass patchiness: is fragmentation important?

Habitat fragmentation is a process involving splitting of continuous habitats into smaller, and more isolated habitat patches. To assess the effects of small-scale habitat patchiness and isolation without the confounding effect of habitat loss on benthic macrofauna, two field experiments were conducted in the Archipelago Sea, SW Finland. Using artificial seagrass units (ASUs) we contrasted continuous patches (“C”) with fragmented patches (“F”) of the same combined area as the continuous patches. The fragmentation treatment involved two isolation distances (0.5 and 3.0 m) between the ASUs (“F 0.5”) and (“F 3.0”). This design was repeated in two consecutive experiments where the patch area was 0.25 and 0.0625 m², respectively. Mobile epifauna were allowed to colonize patches for 12 days in both experiments. In both experiments, the total epifaunal density was significantly higher in the “F 0.5” treatment than in the “C” treatment, and the three dominant taxa showed positive or neutral responses to the habitat configuration. No fragmentation effect on the number of species was detected in either of the experiments, but fragmentation had a negative effect on the epifaunal diversity (Shannon’s H′) in the experiment with the largest patch area. Epifaunal diversity was significantly lower in “F 3.0” treatment than in “C” or “F 0.5” treatments in the first experiment, indicating stronger effect of isolation instead of fragmentation per se. Edge effects were indirectly tested by comparing epifaunal densities with patch edge:area ratios. The results suggest that edge effects may have a more important role than patch size for the total densities of epifaunal taxa, and that small, isolated patches have equal or higher habitat value compared to larger fragments.     

Do edge effects increase the susceptibility of rainforest fragments to structural damage resulting from a severe tropical cyclone?

Abstract If changes in the structural characteristics of rainforest at edges are caused by wind, then physical damage from a tropical cyclone might be greatest at edges or in small fragments that have a high proportion of edge. We tested whether this was true of a fragmented rainforest landscape impacted by a category 4 severe tropical cyclone in March 2006. Six structural variables (canopy cover, canopy height, cover of ground vegetation, leaf litter, stem density and counts of woody debris) were surveyed at 18 rainforest sites (six small linear remnants, and both edges and interiors of six large remnants) on the Atherton Tableland in north‐eastern Queensland, Australia. Data collected 7 and 12 months after the passage of Cyclone Larry were compared with an identical survey conducted 4 years prior to the cyclone. The cyclone had large effects across many components of forest structure. However, sites within 30 m of forest edges in small and large remnants were not impacted more than the interiors of large remnants. It is likely that the high wind intensity from severe tropical cyclones overrides the modest wind protection provided by surrounding forest. The cyclone's effects were highly patchy at local scales (0.5–1.0 km), leading to an increase in among‐site variation in forest structure and the disappearance of significant spatial autocorrelation among large remnant edge‐interior site pairs which had existed prior to the cyclone. The main effect of Cyclone Larry at these study sites was to increase the spatial heterogeneity of forest structure at local scales.     

Habitat structure mediates biodiversity effects on ecosystem properties

Much of what we know about the role of biodiversity in mediating ecosystem processes and function stems from manipulative experiments, which have largely been performed in isolated, homogeneous environments that do not incorporate habitat structure or allow natural community dynamics to develop. Here, we use a range of habitat configurations in a model marine benthic system to investigate the effects of species composition, resource heterogeneity and patch connectivity on ecosystem properties at both the patch (bioturbation intensity) and multi-patch (nutrient concentration) scale. We show that allowing fauna to move and preferentially select patches alters local species composition and density distributions, which has negative effects on ecosystem processes (bioturbation intensity) at the patch scale, but overall positive effects on ecosystem functioning (nutrient concentration) at the multi-patch scale. Our findings provide important evidence that community dynamics alter in response to localized resource heterogeneity and that these small-scale variations in habitat structure influence species contributions to ecosystem properties at larger scales. We conclude that habitat complexity forms an important buffer against disturbance and that contemporary estimates of the level of biodiversity required for maintaining future multi-functional systems may need to be revised.     

Depth-related changes in the arctic epibenthic megafaunal assemblages of Kangerdlugssuaq, East Greenland

The effects of depth-related differences in environmental factors (pressure, hydrography, habitat and disturbance) on the benthic megafauna at the mouth of Kangerdlugssuaq Fjord in Arctic Greenland were investigated using a towed camera platform and autonomous underwater vehicle photographs. Large reductions in faunal density (60,132 to 1881 individuals ha^-1) and increases in diversity (H'=0.93-2.54), through increases in richness (ES₍₂₂₀₎=7.6-18.8) and reductions in dominance (Berger-Parker index=0.77-0.38) were found between 270 and 720 m water depth. Assemblages were separated into distinct shallow, intermediate and deep faunas by multivariate community analysis. In the shallower sites there were high levels of iceberg disturbance, directly reducing diversity as well as creating a complex, patchy environment inhabited by high densities of mobile suspension-feeding epifauna. In the deeper areas there was small-scale disturbance from the deposition of drop stones, but at a relatively low frequency and magnitude, which allowed increased species diversity. Proportions of deposit-feeding epifaunal and infaunal taxa rose with depth, as indicated by an increase with depth in faunal traces. Decreases in faunal density probably resulted from decreased food supply with depth.