Comparison between community structures of fishes in <Emphasis Type="Italic">Enhalus acoroides</Emphasis>- and <Emphasis Type="Italic">Thalassia hemprichii</Emphasis>-dominated seagrass beds on fringing coral reefs in the Ryukyu Islands,Japan

To clarify differences in community structures and habitat utilization patterns of fishes in Enhalus acoroides- and Thalassia hemprichii-dominated seagrass beds on fringing coral reefs, visual censuses were conducted at Iriomote and Ishigaki islands, southern Japan. The numbers of fish species and individuals were significantly higher in the E. acoroides bed than in the T. hemprichii bed, although the 15 most dominant fishes in each seagrass bed were similar. Cluster and ordination analyses based on the number of individuals of each fish species also demonstrated that fish community structures were similar in the two seagrass beds. Species and individual numbers of coral reef fishes which utilized the seagrass beds numbered less than about 15% of whole coral reef fish numbers, although they comprised about half of the seagrass bed fishes. Of the 15 most dominant species, 5 occurred only in the two seagrass beds, including seagrass feeders. Ten other species were reef species, their habitat utilization patterns not differing greatly between the two seagrass beds. Some reef species, such as Lethrinus atkinsoni and L. obsoletus, showed ontogenetic habitat shifts with growth, from the seagrass beds to the coral areas. These results indicate that community structures and habitat utilization patterns of fishes were similar between E. acoroides- and T. hemprichii-dominated seagrass beds, whereas many coral reef fishes hardly utilized the seagrass beds.     

Contrasting effects of habitat structure on the recruitment and mortality of an epibiotic macroalga

Field surveys over 2 years in contiguous beds of the seagrassesZostera capricorni andPosidonia australis showed that the green algaCodium duthieae was consistently more abundant inZ. capricorni than inP. australis. In 1 year, mature plants were also more abundant at the boundary between the seagrass beds than in either bed. Field experiments and programmes of sampling were used to investigate three potential explanations for the unusual distribution of this alga: (1) that the availability of substrata suitable for attachment of the alga differed between the two seagrass beds; (2) that mortality of matureC. duthieae differed between the seagrass beds; and/or (3) that the intensity of recruitment was different in the two seagrass beds.C. duthieae plants were exclusively epibionts of the bivalveAnadara trapezia. Detailed sampling showed that the abundance ofA. trapezia was similar in both seagrass beds and that the distribution of bivalves suitable as substrata forC. duthieae plants was not obviously related to proximity to the boundary between the beds. Two experiments investigated the survival ofC. duthieae plants in each bed. In the first, matureC. duthieae plants transplanted into theP. australis bed suffered similar rates of mortality to plants which were disturbed and moved within theZ. capricorni bed or which were left undisturbed in theZ. capricorni bed. Fewer of the host bivalves were recovered from theZ. capricorni bed, however, indicating that the mechanism of mortality differed between the beds, hosts being more frequently dislodged in theZ. capricorni bed. Removal of the leaves of the seagrasses had consistently greater effects on near-bottom current velocities in theZ. capricorni bed than in theP. australis bed and significantly increased mortality ofC. duthieae in theZ. capricorni bed. Survival of plants was greater in plots of artificial leaves ofP. australis placed in theZ. capricorni bed than in plots of the naturalZ. capricorni leaves or plots where the natural leaves were removed. Most mortality in theZ. capricorni bed was due to dislodgement of the alga and its bivalve substratum. Corresponding manipulations of leaves in theP. australis bed had consistently smaller effects on survivorship of both the alga and its host. Patterns in the recruitment of the alga most clearly reflected the distribution of adults.C. duthieae recruits were 5 times more abundant in theZ. capricorni bed and at the boundary between the two beds than in theP. australis bed. The results demonstrate how habitat structure, provided by the canopy of leaves of the two species of seagrass, can have contrasting effects on the recruitment and mortality of a macroalga. In the case ofC. duthieae, it appears that the differential pattern of recruitment is the primary determinant of the distribution of adult plants.     

Abundance of macrofauna in dense seagrass is due to habitat preference,not predation

Summary Two main hypotheses compete to explain why prey abundance decreases when seagrass density is reduced. One proposes that predators are more successful amongst seagrass of lower density; the other invokes habitat choice by prey. We reduced the density of seagrass in the presence, and in the absence, of predators in a field experiment to discriminate between these hypotheses. When seagrass was manipulated abundances of all six prey species decreased simultaneously both in the presence and in the absence of predators. We conclude that correlations of prey abundance and shoot density within a seagrass bed are proximately due to habitat preference of dense seagrass by prey. We report another experiment which supports this conclusion and shows that habitat preference is exercised at the earliest opportunity. However, the habitat preferences may have been selected by predation pressure.     

Microhabitat use by a post-settlement stage estuarine fish: evidence from relative abundance and predation among habitats

Seagrass beds provide food and shelter for many fish species. However, the manner in which fishes use seagrass bed habitats often varies with life stage. Juvenile fishes can be especially dependent on seagrass beds because seagrass and associated habitats (drift macroalgae) may provide an effective tradeoff between shelter from predation and availability of prey. This study addressed aspects of habitat use by post-settlement pinfish, Lagodon rhomboides (Linneaus), an abundant and trophically important species in seagrass beds in the western North Atlantic and Gulf of Mexico. Abundance of post-settlement fish in seagrass beds was positively related to volume of drift macroalgae, but not to percent cover of seagrass, indicating a possible shelter advantage of the spatially complex algae. Tethering experiments indicated higher rates of predation in seagrass without drift macroalgae than in seagrass with drift macroalgae. Aquarium experiments showed lower predation with higher habitat complexity, but differences were only significant for the most extreme cases (unvegetated bottom, highest macrophyte cover). Levels of dissolved oxygen did not differ between vegetated and unvegetated habitats, indicating no physiological advantage for any habitat. Seagrass beds with drift macroalgae provide the most advantageous tradeoff between foraging and protection from predation for post-settlement L. rhomboides. The complex three-dimensional shelter of drift macroalgae provides an effective shelter that is embedded in the foraging habitat provided by seagrass. Drift macroalgae in seagrass beds is a beneficial habitat for post-settlement L. rhomboides by reducing the risk of predation, and by providing post-settlement habitat within the mosaic (seagrass beds) of adult habitat, thus reducing risks associated with ontogenetic habitat shifts.     

Long-term changes in amphipod population dynamics in a temperate estuary following ecosystem restoration

The Mondego estuary (Portugal) has suffered severe ecological stress over the last two decades, as manifested in the replacement of seagrasses by opportunistic macroalgae, degradation of water quality and increased turbidity. A restoration plan was implemented in 1998, which aimed to reverse the eutrophication effects, and especially to restore the original natural seagrass (Zostera noltii) community. This article explores the long-term changes in Ampithoe valida and Melita palmata (Amphipoda) populations in response to eutrophication (with consequent seagrass loss and macroalgal proliferation) and to the subsequent restoration plan (with progressive seagrass recovery and macroalgal biomass decline). Until the early 1990s, high densities of A. valida and M. palmata were recorded in the Mondego estuary, especially during the occurrence of the macroalgal bloom and during all the periods in which green macroalgae were available. After the implementation of the restoration plan, species abundance, biomass and production levels decreased considerably due to the progressive decline of green macroalgae. This implied the virtual disappearance of the amphipod population, mainly A. valida. Distinct behaviours displayed by the two species could be related to different food strategies and habitat preferences. Ampithoe valida showed feeding preferences for ephemeral softer, filamentous or bladed algae (e.g. Ulva sp.) due to its high caloric content, using the Z. noltii bed only as a habitat for protection against predators or shelter from wave action. On the other hand, M. palmata did not suffer a strong decline in its population density, biomass and production, which may indicate that this species is probably not a primary consumer of green macroalgae and may readily shift to alternative ecological niches. Handling editor: P. Viaroli     

Dynamic changes in seagrass assemblages under eutrophication and implications for recovery

Over the last 20 years, loss of seagrass beds, often related with increased eutrophication, became a common problem worldwide. In the Mondego estuary (Portugal), eutrophication has triggered serious biological changes, which led to an overall increase in primary production and to a progressive replacement of seagrass Zostera noltii beds by coarser sediments and opportunistic macroalgae.The effects of this eutrophication on benthic assemblages were studied along a spatial gradient in the Mondego estuary from 1993 to 1995. Over these short temporal and small spatial scales, distinct changes in the structure of the macrobenthic communities were observed. One of the main structural modifications was the decrease in species diversity along the eutrophication gradient and over time, with a marked impoverishment of the most disturbed inner area. Other changes included an increase in detritivores and a decline in herbivores together with a significant increase in small deposit-feeding polychaetes.In the long term, sustained eutrophication of this estuary is expected to lead to complete replacement of seagrass habitat by unvegetated coarser sediments, occasionally covered by green macroalgal blooms and dominated by opportunistic invertebrate taxa. Recovery from this situation may not only require reduction in nutrient loadings to the estuary, but also active seagrass restoration programmes to reverse the positive feedback processes thought to be presently taking place.     

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.     

Modification of animal habitat by large plants: mechanisms by which seagrasses influence clam growth

Summary Field experiments withMercenaria mercenaria in a relatively high-energy environment demonstrated that clams on unvegetated sand flats failed to grow during autumn while those within seagrass beds grew substantially. Clam growth rates at the seagrass margin that first receives the faster-flowing, flood-tidal currents were about 25% less than at the opposite edge. In a second experiment, pruning, which reduced average blade length by 50–75%, was shown to enhance near-bottom current velocities and to reduce shell growth ofMercenaria during summer by about 50%. As in the first experiment, clams in the unvegetated sand flats exhibited no net growth. Clam mortality, caused mostly by predatory crabs and whelks, was much higher on sand flats than in seagrass beds and intermediate in clipped seagrass. Although consistent with some previous reports, these growth results are still surprising given that they contradict the generalization that suspension feeders grow faster under more rapid current regimes.Three types of indirect interactions might explain the observed effect of seagrass on growth of buried clams: (1) altering food supply; (2) changing the intensity of biological disturbance on feeding clams; and/or (3) affecting the physical stability of the sediments. Previous research on this question has focused almost exclusively on processes that alter food supply rates. In this study, food concentrations, as indicated by suspended chla, were 30% higher inside than outside one seagrass bed, whereas chla concentrations in two other beds were not different from those on adjacent sand flats. This result is sufficient to show that more intense food depletion was not induced by the reduction in flow velocities under the seagrass canopy. Nevertheless, the possible small difference in food concentrations between vegetated and unvegetated bottom seems insufficient to explain the absence of growth of sand-flat clams, especially given the virtual lack of food limitation among suspension feeders in this system. Two data sets demonstrated that the effects of biological disturbance agents cannot be ignored. An outdoor laboratory experiment showed that even in the absence of physical contact between predator and prey the presence of a whelk reduces the amount of time spent feeding byMercenaria. This result suggests that sand flats, where predation rates are higher, may be sites of lower clam growth than seagrass beds because of greater consumer interference with clam feeding. Furthermore, clam siphons are proportionately larger inside seagrass than on sand flats, implying that siphon nipping may not be as intense inside seagrass. This process, too, would reduce net growth of sand-flat clams. Finally, no explicit test was conducted of the hypothesis that enhanced sediment transport in the absence of flow baffling and root binding by seagrass inhibits net growth of clams on high-energy sand flats. Nevertheless, this is a reasonable explanation for the pattern of enhanced growth of seagrass clams, and could serve to explain the otherwise unexplained pattern of lower clam growth at the edge of the seagrass bed that experiences the faster flood-tidal current velocities. Each broad process, changing fluid dynamics, altering consumer access, and varying sediment stability, represents a mechanism whereby habitat structure, provided by the dominant plant, has an important indirect influence on the functional value of the habitat for resident animals.     

Reedbed structure and habitat preference of reed passerines during the post-breeding period

Post-breeding habitats of nine passerine species were studied in the permanently flooded reed beds of ponds near the village Pusté úlany (SW Slovakia) in 2002. Structural features of reed beds and the abundance of all birds studied were sampled at two sites and eight study plots. The reed bed structure and abundance of four passerines differed considerably between the individual study plots. The variations in vegetation structure were also caused by winter reed burning in the SE part of study area. Stems in burnt reed beds were smaller and thinner than unburnt. Abundance of the reed warbler and to a lesser extent sedge warbler was higher in tall reed beds, while the bearded tit preferred thick reed stems. Position of reed beds along the shore — open water gradient was an important factor in terms of abundance at the study plots in the case of the reed bunting. This species showed a strong preference for the reed bed edge with open water. Horizontal distribution of other species seems to be random, however, the birds with the exception of Savi’s warbler preferred the upper stratum of reed bed in vertical distribution. Our field data corroborate with those found in experimental studies concerning with ecomorphological characteristics of birds. Some inconsistencies, however, could also be caused by other factors (e.g. foraging preferences). Comparison of breeding and post-breeding habitat requirements of the studied bird species confirmed a more or less random distribution of the majority of species in the post-breeding period. Some variation was found in the cases of only the reed warbler and bearded tit.     

Habitat structure mediates top–down effects of spiders and ants on herbivores

Differences in structural complexity of habitats have been suggested to modify the extent of top–down forces in terrestrial food webs. In order to test this hypothesis, we manipulated densities of generalist invertebrate predators and the complexity of habitat structure in a two-factorial design. We conducted two field experiments in order to study predation effects of ants and spiders and, in particular, of the wasp spider Argiope bruennichi on herbivorous arthropods such as grasshoppers, plant- and leafhoppers in a grassland. Predator densities were manipulated by removal in habitats of higher and lower structural diversity, and the effects on herbivore densities were assessed by suction sampling. Habitat structure was changed by cutting the vegetation to half its height and removing leaf litter.We found a significant negative effect of this assemblage of generalist predators on plant- and leafhoppers, which were 1.6 times more abundant in predator removal plots. This effect was stronger in low-structured (cut) than in uncut vegetation. Densities of the most abundant planthopper Ribautodelphax pungens (Delphacidae) were 2.2 times higher in predator removal plots. Furthermore, adult plant- and leafhoppers responded more strongly than juveniles and epigeic species more strongly than hypergeic species. The presence of predators had a positive effect on plant- and leafhopper species diversity. In a second field experiment, we tested the exclusive impact of Argiope bruennichi on its prey, and found that its effect was also significant, although weaker than the effect of the predator assemblage. This effect was stronger in grass-dominated vegetation compared to structurally more complex mixed vegetation of grasses and herbs. We conclude that habitat structure and in particular vegetation height and architectural complexity strongly modify the strength of top–down forces and indirectly affect the diversity of herbivorous arthropods.     

Seagrass complexity hierarchies: Influence on fish groups around the coast of Jersey (English Channel)

Recent research has identified a need for seagrass habitat management plans to be based on landscape-level approaches as they offer a more appropriate scale for large mobile fauna than smaller scales. Also, conservation decisions are more likely to be a choice between different seagrass beds rather than parts of individual beds. The present study examined the spatial utilisation of subtidal seagrass beds by fish around the coast of Jersey, English Channel (49°N 02°W) with the aim of identifying influential scales of complexity. A hierarchical-scale of landscape configuration and habitat characteristics was measured for eight seagrass beds using aerial photographic analysis (e.g. core area, contiguity and other landscape metrics), digital echo-sounder data (e.g. depth, canopy height) and diver surveys (e.g. epiphyte load). The contributions of these variables as predictors of functional fish groups were explored using multiple linear regression models. Results indicated that more fragmented seagrass beds supported lower numbers of fish species than more homogenous seagrass landscapes (squared semi-partial correlation coefficient, sr² = − 0.3). Densities of juveniles of larger fish species showed a negative relationship with increased fragmentation of the seagrass (sr² = − 0.34). At smaller scales of structural complexity the densities of cryptic fish were related positively to canopy height (sr² = 0.46). At night, fewer patterns could be explained by the independent variables in the model, which was attributed to the greater movement of fish between the seagrass and adjacent habitats to forage, and a breakdown in the association with seagrass habitat as a refuge from predation.     

Leaf dynamics and standing stocks of intertidal Zostera noltii Hornem. and Cymodocea nodosa (Ucria) Ascherson on the Banc d'Arguin (Mauritania)

Leaf dynamics and standing stocks of intertidal seagrasses were studied in the Baie d'Aouatif (Parc National du Banc d'Arguin, Mauritania) in April and September 1988. Standing stocks of Zostera noltii Hornem. suggest a unimodal seasonal curve similar to what is found for populations at higher latitudes. Also, leaf growth rates (0.03 cm² cm^–2 day^–1 on average) were similar to those found at higher latitudes in these months. Variation in leaf loss over tidal depth, time and different locations in the Baie d'Aouatif was larger and more often significant than variation in leaf growth. In general, Z. noltii beds in the Baie d'Aouatif had comparable leaf growth rates and standing stocks. In both months losses were almost always higher than or equal to growth.Variation in leaf loss over time was much higher in the plots that were situated high in the intertidal than in lower plots. This is explained by differences in susceptibility to sloughing, which is presumably higher in periods with low tide around noon for shallow depths.In an experiment using artificial shading nets, in situ leaf growth was affected negatively from 94% shading onwards. This shading was observed to reduce the light intensity reaching the seagrass bed to a level below the reported range of light compensation points for Z. noltii. Cymodocea nodosa (Ucria) Ascherson on average had higher leaf area and relative growth rates than Z. noltii and much lower loss rates, resulting in a positive net increase in September. Standing stocks were also higher than for Z. noltii. A mixed seagrass bed containing the above two species and Halodule wrightii Ascherson had the highest observed total biomass: 335 g m^–2 ash-free dry weight.     

Gracilaria-Mytilus interaction on a commercial algal farm in Chile

Mytilus chilensis is an invertebrate that competes for space with the alga Gracilaria chilensis in farmed areas in Chile and, for this reason, is considered a contaminant organism. Mussel beds are considered to play a role in the regeneration of nitrogen and, as a consequence, they could be an important source of ammonium for the algae. In this study, we manipulated the mussel cover in experimental plots, creating replicated areas with 0%, 30% and 60% coverage. In half of the plots the mussels were killed with a gas torch so their effect would be mechanical, without nutrient regeneration. After 15 days, each plot was planted with 12 G. chilensis bundles (100 g each) per square meter. Ammonium concentrations increased significantly in the waters around the mussel bed in contrast to areas without mussels or dead mussels. Mussel cover had a significant negative effect on the length of the G. chilensis bundles planted in the experimental plots. However, no significant differences were detected between experimental quadrats with live mussels and those with dead mussels during a 5 month period. These results indicate that the mechanical effect of the mussel can, to some extent, be responsible for the decline in G. chilensis abundance in farms where mussel beds have been established.     

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.     

Non‐consumptive effects of avian predators on fish behavior and cascading indirect interactions in seagrasses

Top–down impacts of avian predators are often overlooked in marine environments despite evidence from other systems that birds significantly impact animal distribution and behavior; instead, birds are typically recognized for the impacts of their nutrient rich guano. This is especially true in shallow seagrass meadows where restoration methods utilize bird perches or stakes to attract birds as a passive fertilizer delivery system that promotes the regrowth of damaged seagrasses. However, this method also increases the local density of avian piscivores that may have multiple unexplored non‐consumptive effects on fish behavior and indirect impacts to seagrass communities. We utilized laboratory and field experiments to investigate whether visual cues of avian predators impacted the behavior of the dominant demersal fish in seagrass habitats, the pinfish Lagodon rhomboides, and promoted cascading interactions on seagrass‐associated fauna and epiphytes. In laboratory mesocosms, pinfish displayed species specific responses to models of avian predators, with herons inducing the greatest avoidance behaviors. Avoidance patterns were confirmed in field seagrass meadows where heron models significantly reduced the number of fish caught in traps. In a long term field experiment, we investigated whether avian predators caused indirect non‐consumptive effects on seagrass communities by monitoring fish abundances, invertebrate epiphyte grazers, and the seagrass epiphytes in response to heron models, bird exclusions, and bird stakes. On average, more fish were recovered under bird exclusions and fewer fish under heron models. However, we found no evidence of cascading effects on invertebrate grazers or epiphytes. Bird stake treatments only displayed a simple nutrient effect where higher bird abundances resulted in higher epiphyte biomass. Our results indicate that although birds and their visual cues can affect fish and epiphyte abundance through non‐consumptive effects and nutrient enrichment, these impacts do not propagate beyond one trophic level, most likely because of dampening by omnivory and larger scale processes.     

Interactive effects of habitat selection, food supply and predation on recruitment of an estuarine fish

 Seagrass meadows are often important habitats for newly recruited juvenile fishes. Although substantial effort has gone into documenting patterns of association of fishes with attributes of seagrass beds, experimental investigations of why fish use seagrass habitats are rare. We performed two short-term manipulative field experiments to test (1) the effects of food supply on growth and densities of fish, and (2) effects of predation on the density and size distribution of fish recruits, and how this varies among habitat types. Experiments were conducted in Galveston Bay, Texas, and we focused on the common estuarine fish, pinfish Lagodon rhomboides. In the first experiment, replicate artifical seagrass and sand plots were either supplemented with food or left as controls. Recruitment of pinfish was significantly greater to seagrass than sand habitats; however, we detected no effect of food supplementation on the abundance of recruits in either habitat. Pinfish recruits in artifical seagrass grew at a significantly faster rate than those in sand habitats, and fish supplemented with food exhibited a greater growth rate than controls in both sand and artifical grass habitats. In our second experiment, we provided artificial seagrass and sand habitats with and without predator access. Predator access was manipulated with cages, and two-sided cages served as controls. Recruitment was significantly greater to the cage versus cage-control treatment, and this effect did not vary between habitats. In addition, the standard length of pinfish recruits was significantly larger in the predator access than in the predator exclusion treatment, suggesting size-selective predation on smaller settlers or density-dependent growth. Our results indicate that the impact of predation on pinfish recruits is equivalent in both sand and vegetated habitats, and thus differential predation does not explain the higher recruitment of pinfish to vegetated than to nonvegetated habitats. Since predators may disproportionately affect smaller fish, and a limited food resource appears to be more effectively utilized by fish in vegetated than in unvegetated habitats, we hypothesize that pinfish recruits may select vegetated habitats because high growth rates allow them to achieve a size that is relatively safe from predation more quickly. Received: 10 October 1996 / Accepted: 5 April 1997     

The influence of predator presence on utilization of artificial seagrass habitats by juvenile walleye pollock,Theragra chalcogramma

Synopsis Behavioral preference for a structured habitat (artificial seagrass) by juvenile walleye pollock,Theragra chalcogramma, was tested in controlled laboratory experiments. We monitored position of fish in 2000 1 tanks with and without artificial seagrass present in one half of the tank. In addition, we exposed walleye pollock to a predator model, assessing their response when a grass plot was available or unavailable as a potential refuge. In the absence of predators, the fish avoided the artificial seagrass, displaying a preference for the open water side of the experimental tanks. In the presence of a predator model, however, juvenile walleye pollock readily entered the artificial seagrass plots. In addition, they often remained in the grass canopy in proximity to the predator instead of moving out of the grass to avoid the predator (when no grass was present they consistently moved to the opposite side of the tank from the predator). The behavioral choices exhibited in this study suggest that juvenile walleye pollock modify habitat selection in response to perceived predation risk, and recognize the structure provided by artificial seagrass as a potential refuge.     

An ecological paradox: More woodland predators and less artificial nest predation in landscapes colonized by noisy miners

Many passerine bird populations, particularly those that have open‐cup nests, are in decline in agricultural landscapes. Current theory suggests that an increase in habitat generalist predators in response to landscape change is partially responsible for these declines. However, empirical tests have failed to reach a consensus on how and through what mechanisms landscape change affects nest predation. We tested one hypothesis, the Additive Predation Model, with an artificial nest experiment in fragmented landscapes in southern Queensland, Australia. We employed structural equation modelling of the influence of the relative density of woodland and habitat generalist predators and landscape features at the nest, site, patch and landscape scales on the probability of nest predation. We found little support for the Additive Predation Model, with no significant influence of the density of woodland predators on the probability of nest predation, although landscape features at different spatial scales were important. Within woodlands fragmented by agriculture in eastern Australia, the presence of noisy miner colonies appears to influence ecological processes important for nest predation such that the Additive Predation Model does not hold. In the absence of colonies of the aggressive native bird, the noisy miner, the influence of woodland predators on the risk of artificial nest predation was low compared with that of habitat generalist predators. Outside noisy miner colonies, we found significant edge effects with greater predation rates for artificial nests within woodland patches located closer to the agricultural matrix. Furthermore, the density of habitat generalist predators increased with the extent of irrigated land‐use, suggesting that in the absence of noisy miner colonies, nest predation increases with land‐use intensity at the landscape scale.     

Relative influence of habitat complexity and proximity to patch edges on seagrass epifaunal communities

Habitat structure at many scales influences faunal communities. Although habitat structure at different scales often covaries, studies rarely examine the relative effects of structure at multiple scales on faunal density and diversity. In shallow‐water seagrass systems, epifaunal density at local scales generally increases with increased habitat structural complexity (e.g. shoot density per unit area). In turn, structural complexity often varies with other aspects of habitat structure at patch scales, such as proximity to patch edges, which itself modifies ecological processes that structure epifaunal communities. We conducted surveys and a manipulative experiment in the eelgrass Zostera marina beds of San Diego Bay, California, USA, to determine (1) whether eelgrass structural complexity, epifaunal density and diversity, and fish (predator) density and diversity vary with proximity to patch edges, and (2) the relative influences of structural complexity, proximity to patch edges and predator presence on epifaunal distribution. Seagrass structural complexity generally increased from patch edges to patch interiors at all sites and in all sampling periods. However, patterns of epifaunal density, diversity, and biomass varied among sites and sampling periods, with density and biomass increasing from patch edges to interiors at some sites and decreasing at others. In the manipulative experiment, we allowed epifauna to colonize sparse or dense artificial seagrass habitat at both the edge and interior of a seagrass patch, and enclosed a subset of experimental units in predator exclusion cages. Overall, proximity to patch edges had a larger influence on epifaunal density and community structure than did structural complexity or predation, with the exception of some common taxa which responded more strongly to either complexity or predator exclusion. Our results emphasize the importance of addressing and evaluating habitat structure at multiple scales to better understand the distribution and interactions of organisms in a particular environment.     

Resource limitation and fish predation: their importance to mobile epifauna associated with JapaneseSargassum

The possibility that resource limits constrain the growth of mobile epifaunal populations associated withSargassum patens plants was investigated by placing plants and associated animals into field microcosms which excluded fish predators, and then comparing faunal abundance and size-structure changes in different microcosm treatments with field populations. Four different micrososm treatments were set up: two treatments containing defaunated plants inoculated with caprellid amphipods, and two control treatments with natural faunas. The estimated secondary production of faunas enclosed in all microcosm treatments rapidly settled on a constant value (5 mg/day) which was similar to that determined in experiments conducted in Western Australia using the same microcosms but for faunas associated with a seagrass rather than a macroalga. These results support the hypothesis that the secondary production of epifaunal communities associated with macrophytes is constrained by quantifiable food resource ceilings. Predation by the most common fish species in the area, the wrasseHalichoeres tenuispinis, did not appear to alter macrofaunal production in theS. patens bed; however, it did greatly affect the faunal size-structure by eliminating most of the larger animals. The majority of epifaunal animals 2.0 mm sieve-size were consumed byH. tenuispinis, while negligible numbers of 0.5-mm sieve-size animals were captured. We postulate that food resource ceilings and predatory size-selectivity are widespread phenomena, affecting epifaunal populations at a variety of locations. Predation is predicted to generally increase rather than decrease faunal abundance because the consumption of each large invertebrate by a predator frees sufficient resources to feed several smaller individuals.