Structural Refuge Research Articles

Habitat complexity mediates spatiotemporal niche partitioning among native and invasive seed predators

Red imported fire ants dominate certain food webs in North America due to their omnivorous and aggressive feeding behavior. Structural habitat complexity can mediate effects of invasive predators on native insects, by either facilitating or tempering antagonistic interactions, with potential consequences at lower trophic levels. Along a gradient of habitat complexity, we evaluated how refuge affects the activity and foraging behavior of fire ants and native invertebrate seed predators using video surveillance, and then determined their consumption of pigweed seeds in seed removal assays. Fire ants dominated seed predation in our system, and structural complexity increased both their activity and seed removal, while reducing that of native seed predators. In structurally simple habitats with high sun exposure, fire ant activity was restricted to the evening when temperatures were milder, while native seed predators foraged during the heat of the day, possibly to avoid antagonistic interactions with fire ants. In relatively shaded, structurally complex habitat, the active foraging period for fire ants expanded earlier into the heat of the day, and almost totally overlapped with that of native seed predators. Our data suggest that structural refuge may facilitate antagonism and site/resource domination by fire ants.

The Specialist Marine Herbivore Elysia papillosa Grows Faster on a Less Utilized Algal Diet.

AbstractMany small specialist herbivores utilize their food resources both for nutrition and as a structural refuge or resource. Trophic linkage cannot solely be inferred from physical association of herbivores with a potential food item, because herbivores may temporarily inhabit algae or plants on which they do not feed. Elysia papillosa, a small sacoglossan sea slug, consumes and sequesters chloroplasts from the siphonaceous, chlorophytic alga Penicillus capitatus; it also maintains moderate densities on this alga. Recently, E. papillosa was also infrequently found in association with the alga Penicillus lamourouxii, which displays density similar to that of P. capitatus. After collecting E. papillosa from each of the two algal species from a shallow-water site along the west central coast of Florida, we used DNA barcoding of the rbcL gene sequences in order to determine whether the slug was consuming both algal species. The molecular data indicated that E. papillosa consumed and sequestered chloroplasts from the same algal species from which they were collected. A laboratory feeding experiment tested whether algal diet (P. capitatus or P. lamourouxii) had an impact on slug growth rate as measured by change in body size (mm). After 3 weeks E. papillosa fed P. lamourouxii achieved a mean body length that was 1.5-2 times that recorded for slugs fed P. capitatus, but maximum growth depended on the original field host. Thus, while the highest densities of E. papillosa in the field occurred on P. capitatus, slugs grew much faster on P. lamourouxii in the laboratory. The observed association of E. papillosa with P. capitatus must be related to other factors, such as foraging efficiency, algal morphology, algal biochemistry, or algal suitability as a refuge.

Colour dimorphism in labrid fishes as an adaptation to life on coral reefs.

Conspicuous coloration displayed by animals that express sexual colour dimorphism is generally explained as an adaptation to sexual selection, yet the interactions and relative effects of selective forces influencing colour dimorphism are largely unknown. Qualitatively, colour dimorphism appears more pronounced in marine fishes that live on coral reefs where traits associated with strong sexual selection are purportedly more common. Using phylogenetic comparative analysis, we show that wrasses and parrotfishes exclusive to coral reefs are the most colour dimorphic, but surprisingly, the effect of habitat is not influenced by traits associated with strong sexual selection. Rather, habitat-specific selective forces, including clear water and structural refuge, promote the evolution of pronounced colour dimorphism that manifests colours less likely to be displayed in other habitats. Our results demonstrate that environmental context ultimately determines the evolution of conspicuous coloration in colour-dimorphic labrid fishes, despite other influential selective forces.

Ecology shapes the evolutionary trade-off between predator avoidance and defence in coral reef butterflyfishes.

Antipredator defensive traits are thought to trade-off evolutionarily with traits that facilitate predator avoidance. However, complexity and scale have precluded tests of this prediction in many groups, including fishes. Using a macroevolutionary approach, we test this prediction in butterflyfishes, an iconic group of coral reef inhabitants with diverse social behaviours, foraging strategies and antipredator adaptations. We find that several antipredator traits have evolved adaptively, dependent primarily on foraging strategy. We identify a previously unrecognised axis of diversity in butterflyfishes where species with robust morphological defences have riskier foraging strategies and lack sociality, while species with reduced morphological defences feed in familiar territories, have adaptations for quick escapes and benefit from the vigilance provided by sociality. Furthermore, we find evidence for the constrained evolution of fin spines among species that graze solely on corals, highlighting the importance of corals, as both prey and structural refuge, in shaping fish morphology.

Direct and Indirect Interactions Between Lower Estuarine Mangrove and Saltmarsh Habitats and a Commercially Important Penaeid Shrimp

Vegetated habitats in estuaries may provide a structural refuge and food supply in the same place, but benefits are also derived where a productive food source and suitable habitat are adjacent to each other. Quantifying these relationships is fundamental to understanding the structural and functional characteristics of estuarine ecosystems and for informing management actions. Effective juvenile habitat (habitat that contributes greater-than-average numbers of recruits to the adult population), recruitment patterns and trophic relationships were studied for Eastern King Prawn (Penaeus plebejus) in the lower Clarence River estuary, New South Wales, between 2014 and 2016. Effective juvenile habitat was identified in both the north arm and main river channel of the estuary, and these areas also supported a higher abundance of juvenile prawns. There was minimal recruitment to the southern channels of the estuary, possibly due to reduced connectivity with the incoming tide arising from a rock wall. Trophic relationships in parts of the lower estuary were evaluated using stable isotopes, and saltmarsh grass (Sporobolus virginicus) was the dominant primary producer supporting juvenile Eastern King Prawn productivity across the area. Mangroves were of minimal importance, and seagrass cover was minimal in the area studied. The patterns observed indicate that nursery function of different areas within the lower estuary is a product of connectivity, recruitment and nutrition derived from primary productivity of vascular plants. Habitats within the lower Clarence River estuary have seen substantial degradation over decadal time scales, and the implications of our findings for targeting future habitat repair are discussed.

Contrasting complexity of adjacent habitats influences the strength of cascading predatory effects

Although cascading effects of top predators can help structure communities, their influence may vary across habitats that differentially protect prey. Therefore, to understand how and to what degree habitat complexity can affect trophic interactions in adjacent habitats, we used a combination of a broad regional-scale survey, manipulative field trials, and an outdoor mesocosm experiment to quantify predator-prey interaction strengths across four trophic levels. Within estuaries of the southeastern USA, bonnethead sharks (Sphyrna tiburo) hunt blue crabs on mudflats and adjacent oyster reefs, two habitats with vastly different aboveground structure. Using 12-h tethering trials of blue crabs we quantified habitat-dependent loss rates of 37% on reefs and 78% on mudflats. We hypothesized that the sharks' predatory effects on blue crabs would cascade down to release a lower-level mud crab predator, which subsequently would increase juvenile oyster mortality, but that the cascade strength would be habitat-dependent. We experimentally manipulated predator combinations in split-plot mesocosms containing reef and mudflat habitats, and quantified oyster mortality. Bonnetheads exerted strong consumptive and non-consumptive effects on blue crabs, which ceased eating oysters in the sharks' presence. However, mud crabs, regardless of shark and blue crab presence, continued to consume oysters, especially within the structural refuge of the reef where they kept oyster mortality high. Thus, bonnetheads indirectly boosted oyster survival, but only on the mudflat where mud crabs were less active. Our work demonstrates how structural differences in adjacent habitats can moderate trophic cascades, particularly when mesopredators exhibit differential use of structure and different sensitivities to top predators.

Effects of macrophyte-specific olfactory cues on fish preference patterns

Vegetated habitats provide numerous benefits to nekton, including structural refuge from predators and food sources. However, the sensory mechanisms by which fishes locate these habitats remain unclear for many species, especially when environmental conditions (such as increased turbidity) are unfavorable for visual identification of habitats. Here, a series of laboratory experiments test whether three species of adult fish (golden topminnow Fundulus chrysotus Gunther 1866, sailfin molly Poecilia latipinna Lesueur 1821, and western mosquitofish Gambusia affinis Baird and Girard 1853) use plant chemical cues to orient to one of two habitats [hydrilla Hydrilla verticillata (L.f.) Royle or water hyacinth Eichhornia crassipes (Mart.) Solms]. First, experiments in aquaria were conducted offering fish a choice of the two habitats to determine preference patterns. Next, a two-channel flume, with each side containing flow originating in one of the two habitats, was used to determine whether preferences were still exhibited when fish could only detect habitats through olfactory means. While patterns among the three fish species tested here were variable, results did indicate consistent habitat preferences despite the lack of cues other than olfactory, suggesting that these organisms are capable of discriminating habitats via chemical exudates from plants. As such, olfactory mechanisms likely provide vital information about the surrounding environment and future work should be directed at determining how anthropogenic inputs such as eutrophication and sediment runoff affect the physiology of these sensory capabilities.

Naïve prey exhibit reduced antipredator behavior and survivorship.

Prey naiveté has been hypothesized to be one of the major driving forces behind population declines following the introduction of novel predators or release of inexperienced prey into predator rich environments. In these cases, naïve prey may lack sufficient antipredator behavior and, as a result, suffer increased mortality. Despite this, some evidence suggests that many prey utilize a generalized response to predators. Here, the naiveté hypothesis is tested using a predator–prey pair sharing an evolutionary history: the red swamp crayfish (Procambarus clarkii Girard, 1852) and largemouth bass (Micropterus salmoides Lacépède, 1802). Using farm-reared, naïve crayfish and wild-caught, experienced individuals, laboratory experiments demonstrated that naïve, farmed crayfish lack behavioral responses to chemical cues from bass, both in terms of movement and use of structural refuge. In contrast, experienced crayfish responded strongly to the same cues. In a subsequent field tethering experiment, these naïve individuals suffered a three-fold increase in predation rate. Based on these results, recognition of predators may not be innate in all prey, and previous experience and learning likely play a key role in the development of antipredator behavior.

Habitat structure influences the survival and predator–prey interactions of early juvenile red king crab Paralithodes camtschaticus

Highly structured nursery habitats promote the survival of juvenile stages of many species by providing foraging opportunities and refuge from predators. Through integrated laboratory and field experiments, we demonstrate that nursery habitat structure affects survival and predator-prey interactions of red king crab Paralithodes camtschaticus. Crabs (<1 yr old (Age 0); 8 to 10 mm carapace length (CL)) preferred complex biogenic habitats formed by struc- tural invertebrates and macroalgae over structural mimics and sand in the absence of predators in laboratory experiments, yet they associated with any available structural habitat when fish preda- tors were present. Survival was higher in the presence of complex habitat for Age 0 crabs (5 to 7.5 mm CL) with Pacific cod Gadus macrocephalus predators in the laboratory and for Age 0 (4 to 8 mm CL) and Age 1 (16 to 28 mm CL) crabs with fish and invertebrate predators in the field. Crab activity and refuge response behavior varied with crab stage and habitat. Age 0 crabs were cryptic, avoiding predators by associating with habitat structure or remaining motionless in the absence of structure, and were less likely to respond to an attack. In contrast, Age 1 crabs were more likely to respond to an attacking predator and were less likely to remain motionless in the absence of structural refuge, suggesting an ontogenetic shift in behavior. Complex habitats, cryp- tic behavior, and direct defense improve juvenile red king crab survival against certain predators, including demersal fishes.

Effects of Structural Refuge and Density on Foraging Behaviour and Mortality of Hungry Tadpoles Subject to Predation Risk

Theoretical models of prey behaviour predict that food-limited prey engage in risk-prone foraging and thereby succumb to increased mortality from predation. However, predation risk also may be influenced by factors including prey density and structural cover, such that the presumed role of prey hunger on predation risk may be obfuscated in many complex predator–prey systems. Using a tadpole (prey) – dragonfly larva (predator) system, we determined relative risk posed to hungry vs. sated prey when both density and structural cover were varied experimentally. Overall, prey response to perceived predation risk was primarily restricted to increased cover use, and hungry prey did not exhibit risk-prone foraging. Surprisingly, hungry prey showed lower activity than sated prey when exposed to predation risk, perhaps indicating increased effort in search of refuge or spatial avoidance of predator cues among sated animals. An interaction between hunger level and predation risk treatments indicated that prey state affected sensitivity to perceived risk. We also examined the lethal implications of prey hunger by allowing predators to select directly between hungry and sated prey. Although predators qualitatively favoured hungry prey when density was elevated and structural cover was sparse, the overall low observed variation in mortality risk between hunger treatments suggests that preferential selection of hungry prey was weak. This implies that hunger effects on prey mortality risk may not be readily observed in complex landscapes with additional factors influencing risk. Thus, current starvation-predation trade-off theory may need to be broadened to account for other mechanisms through which undernourished prey may cope with predation risk.

Effects of larger heterospecifics and structural refuge on the survival of a coral reef fish, Thalassoma hardwicke

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 407:197-207 (2010) - DOI: https://doi.org/10.3354/meps08569 Effects of larger heterospecifics and structural refuge on the survival of a coral reef fish, Thalassoma hardwicke Shane W. Geange* School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand *Email: shane.geange@vuw.ac.nz ABSTRACT: Structural refuge and competition can influence the survival of coral reef fishes; however, the effect of structural refuge on the outcome of competition remains unclear. In Moorea, French Polynesia, surveys were used to quantify habitat association for juvenile Thalassoma hardwicke and 7 sympatric labroid species. Having identified the 3 species most similar to focal T. hardwicke in habitat association, a field assay was used to determine if the presence of larger individuals of these species adversely affected the survival of focal individuals. Finally, a field experiment was used to test how competition with larger residents and structural refuge interact to influence survival of transplanted T. hardwicke. Surveys demonstrated that Gomphosus varius, Pseudocheilinus hexataenia, and T. quinquevittatum were most similar to T. hardwicke in habitat association. In the field assay, the presence of T. quinquevittatum had the greatest negative effect on survival of T. hardwicke. The field experiment revealed that competition with T. quinquevittatum and structural refuge both altered T. hardwicke survival, although their effects were not interactive. Survival of T. hardwicke was 2.3 times greater in treatments without T. quinquevittatum relative to those with T. quinquevittatum, and 2.8 times greater in treatments with structural refuge relative to those without structural refuge. The additive effect may have occurred via the establishment of social dominance hierarchies between transplanted T. hardwicke and resident T. quinquevittatum independently of structural refuge, suggesting that it is important to account for interactions occurring independently of resources when examining resource competition. KEY WORDS: Habitat complexity · Competition · Structural refuge · Reef fish · Labroid Full text in pdf format PreviousNextCite this article as: Geange SW (2010) Effects of larger heterospecifics and structural refuge on the survival of a coral reef fish, Thalassoma hardwicke. Mar Ecol Prog Ser 407:197-207. https://doi.org/10.3354/meps08569 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 407. Online publication date: May 20, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

You can’t run but you can hide: refuge use in frog tadpoles elicits density-dependent predation by dragonfly larvae

The potential role of prey refuges in stabilizing predator-prey interactions is of longstanding interest to ecologists, but mechanisms underlying a sigmoidal predator functional response remain to be fully elucidated. Authors have disagreed on whether the stabilizing effect of prey refuges is driven by prey- versus predator-centric mechanisms, but to date few studies have married predator and prey behavioural observations to distinguish between these possibilities. We used a dragonfly nymph-tadpole system to study the effect of a structural refuge (leaf litter) on the predator's functional response, and paired this with behavioural observations of both predator and prey. Our study confirmed that hyperbolic (type II) functional responses were characteristic of foraging predators when structural cover was low or absent, whereas the functional response was sigmoidal (type III) when prey were provided with sufficient refuge. Prey activity and refuge use were density independent across cover treatments, thereby eliminating a prey-centric mechanism as being the genesis for density-dependent predation. In contrast, the predator's pursuit length, capture success, and handling time were altered by the amount of structure implying that observed shifts in density-dependent predation likely were related to predator hunting efficiency. Our study advances current theory by revealing that despite fixed-proportion refuge use by prey, presence of a prey refuge can induce density-dependent predation through its effect on predator hunting strategy. Ultimately, responses of predator foraging decisions in response to changes in prey availability and search efficiency may be more important in producing density-dependent predation than the form of prey refuge use.

Observations on an Association Between the Dexaminid Amphipod Polycheria antarctica f. acanthopoda and Its Ascidian Host Distaplia cylindrica

Abstract The Antarctic circumpolar ascidian Distaplia cylindrica harbors individuals of the amphipod Polycheria antarctica form acanthopoda that live in domiciles excavated by the amphipod. There was no evidence this amphipod consumed its host ascidian tissues. This burrowing association with D. cylindrica may provide an important structural or chemical refuge from prospective fish predators.

Organism responses to habitat fragmentation in two shallow-water brackish environments: the Goro Lagoon (Adriatic Sea) and the Padrongiano Delta (Tyrrhenian Sea)

Habitat fragmentation from natural or human-mediated causes is a common phenomenon in terrestrial and aquatic environments. In this study, the effects of varying the size of habitat patches on the abundance of benthic invertebrates inhabiting date mussel (Musculista senhousia) patches was studied at two different transition environments, the Goro Lagoon (Adriatic Sea) and the Padrongiano Delta (Tyrrhenian Sea). Benthic fauna responded to habitat patchiness in a complex manner that varied according to habitat type, taxon and animal body size (small: 0.5–2.0 mm; large &gt;2 mm). Small invertebrates were mostly polychaetes, nemertea, amphipods and isopods. Large invertebrates were mostly large polychaetes, bivalves, gastropods and crabs. Invertebrate population size and diversity seemed to be maximized in landscapes that include both small and large patches of mussel beds ‘embedded’ in a continuous matrix. Musculista senhousia patches served as a critical refuge and foraging habitat for many species. Patchy and continuous areas may promote the persistence of organisms with different life histories, especially in environments like those studied where mussel patches represent the only structural refuge available.

Crop domestication creates a refuge from parasitism for a native moth

Summary Insect herbivores are generally more abundant in agro‐ecosystems than in natural ecosystems. One possible cause is that artificial selection of crop plants may have disrupted parasitoid–herbivore interactions. We studied a native tritrophic system to determine if crop domestication has created a refuge for the sunflower moth Homoeosoma electellum from its major parasitoid Dolichogenidea homoeosomae. Parasitoid foraging behaviour on infested wild and agricultural sunflowers was studied using event‐recording software. We used detailed observations on larval feeding location and parasitoid probing behaviour to determine if the lower parasitism on agricultural sunflowers was the result of a structural refuge or a change in parasitoid foraging behaviour. Greater than 20 times more larvae fed on seeds of agricultural sunflowers than on seeds of wild sunflowers. Seed‐feeding enabled larvae to escape parasitoids because their ovipositors were too weak to penetrate the seed. As a consequence, larvae were more likely to be parasitized on wild sunflowers than on agricultural sunflowers, and floret‐feeding larvae were more likely to be parasitized than seed‐feeding larvae. Parasitoids were more stimulated to probe for larvae on wild sunflowers than on agricultural sunflowers. However, they responded to larval feeding location rather than plant genotype. Thus lower parasitism on agricultural sunflowers was due to a structural refuge for larvae rather than a change in parasitoid foraging behaviour. Synthesis and applications. Crop domestication has created a structural refuge from parasitism. The presence of refuges on agricultural plants helps explain why some insects are difficult to regulate using biological control. Studying parasitoid activity on wild relatives of agricultural plants can identify how plant variation contributes to the creation of a refuge. Beneficial plant traits can then potentially be bred back into the crop germplasm in order to enhance the biological control of insect pests in agroecosystems.

The effects of seafloor habitat complexity on survival of juvenile fishes: Species-specific interactions with structural refuge

Marine fishes are often associated with structurally complex microhabitats that are believed to provide a refuge from predation. However, the effects of habitat complexity on predator foraging success can be strongly modified by predator and prey behaviors. We conducted a series of laboratory experiments to evaluate the effects of sea floor habitat complexity on juvenile fish survivorship using multiple predator (striped searobin and summer flounder) and prey (winter flounder, scup, and black sea bass) species to identify potentially important species–habitat interactions. Three habitats of varying complexity (bare sand, shell, and sponge) common to coastal marine environments were simulated in large aquaria (2.4 m diameter, 2400 L volume). Prey survivorship increased significantly with greater habitat complexity for each species combination tested. However, examination of multiple prey and predator species across habitats revealed important effects of predator × habitat and prey × habitat interactions on prey survival, which appeared to be related to species-specific predator and prey behavior in complex habitats. Significant species × habitat interactions imply that the impact of reduced seafloor habitat complexity may be more severe for some species than others. Our results indicate that the general effects of seafloor habitat complexity on juvenile fish survivorship may be broadly applicable, but that the interaction of particular habitats with search tactics of predators as well as habitat affinities and avoidance responses of prey can produce differences among species that contribute to variable mortality.

Density, abundance and survival of the blue crab in seagrass and unstructured salt marsh nurseries of Chesapeake Bay

Structured benthic habitats such as salt marshes, seagrass beds and oyster reefs are recognized as critical nurseries for fish, crustaceans and mollusks in coastal and estuarine systems. Yet most estuaries and coastal habitats have extensive, relatively unstructured shallow-water habitats such as subtidal mud and sand flats, which are generally viewed as inconsequential nursery grounds. We tested this paradigm with the blue crab, Callinectes sapidus Rathbun, in shallow and deep benthic habitats of the York River, Chesapeake Bay. Juvenile blue crabs (<100 mm carapace width) were sampled quantitatively in deep channel muds (DCM, >2 m depth), in shallow unstructured subtidal mud flats (SMF) and sand flats (SSF) adjoining salt marshes, and in beds of submerged aquatic vegetation (SAV—eelgrass, Zostera marina, and widgeongrass, Ruppia maritime) in three river zones (Upriver, Downriver, Mouth) across 60 km of the river axis. Survival of juveniles 25–55 mm carapace width was examined experimentally in all shallow habitats. SAV habitats were examined only at the Mouth zone; SAV did not occur in the Downriver and Upriver zones. Juvenile blue crab density was nearly an order of magnitude lower in SMF and SSF than in SAV habitats; density was lowest in DCM. Density in Upriver SMF and SSF habitats was 4- to 10-fold higher than that in Mouth and Downriver SMF and SSF, and DCM. Consequently, the two areas harboring the greatest fractions of York River juveniles were shallow: Mouth SAV (∼50%) and Upriver SMF and SSF (∼40%). Upriver expanses of SMF and SSF adjoining extensive salt marshes near the turbidity maximum harbored an approximately equal abundance of juvenile crabs as the downriver SAV beds, despite the density difference. Survival of tethered juveniles was significantly higher in Upriver SMF and SSF habitats than in Mouth SAV, SMF and SSF habitats, despite the lack of structural refuge in SMF and SSF; crabs in Upriver SMF and SSF survived four times as long as crabs in SAV, Mouth SMF and Mouth SSF. We conclude that shallow subtidal mud and sand flats near upriver salt marshes and in marsh coves are vital nursery grounds for the blue crab, and thus warrant conservation and restoration efforts at the level provided to SAV. The production potential of the blue crab and other estuarine species that utilize salt marshes has likely been severely reduced due not only to direct salt marsh destruction, but also due to indirect degradation of shallow subtidal mud and sand flats fringing salt marshes.

Boating and Navigation Activities Influence the Recruitment of Fish in a Baltic Sea Archipelago Area

We studied the effects of boating and navigation activities on the recruitment of coastal fish in the Stockholm archipelago in the NW Baltic proper. The impacts were quantified by sampling metamorphosed young-of-the-year (Y-O-Y) fish in inlets adjacent to i) routes for medium-sized passenger ferries; ii) berths (small marinas) with small boats; and iii) references. Species with high preference for vegetation were negatively influenced by boating and navigation activities and species with low preference positively influenced. Pike (Esox lucius) Y-O-Y were significantly more abundant in reference areas, while bleak (Alburnus alburnus) were more abundant in dredged marinas. No statistically significant patterns were identified for perch (Perca fluviatilis) although there was a trend of low abundance along ferry routes. Many species of nearshore fishes are dependent on submerged vegetation as spawning and larval substrate, structural refuge and feeding habitat. Our results suggest that the negative effects from boating and navigation activities on the coverage and height of vegetation, especially on species of Chara and Potamogeton spp., may contribute to changes in the Y-O-Y fish community.

Antipredator behaviour of 0+ year Perca fluviatilis: effect of vegetation density and turbidity

In this study, the combined influence of vegetation density and water turbidity on habitat utilization of a prey fish, 0+ year perch Perca fluviatilis, under predation risk (pike, Esox lucius) was investigated. The vegetated habitat was overall preferred over the open habitat in the presence of a predator. The level of turbidity, and to a lesser extent vegetation density, however, influenced the response of 0+ year perch. The use of the vegetated habitat was lower in very turbid than in clear and turbid conditions, suggesting reduced antipredator behaviour in very turbid water. The effect of vegetation density on antipredator behaviour was only present in clear water, where the use of a structural refuge decreased with increasing vegetation density. No such effect was observed in turbid and very turbid water. The results showed that the structuring role of vegetation or habitat complexity may diminish with increased turbidity. The observed masking effect of turbidity suggests that predator‐prey interactions in vegetated habitats are more complex than what has generally been thought.

Scale-dependent effects of zebra mussels on benthic invertebrates in a large eutrophic lake

We studied mechanisms by which zebra mussels may elevate densities of aquatic invertebrates at local and lake-wide spatial scales in a productive lake. Locally, zebra mussels provide structural refuge from predation and deposit organic material. An experiment demonstrated that the density of benthic invertebrates increased locally as a function of zebra mussel density in Oneida Lake, New York. On a lake-wide spatial scale, light penetration increased dramatically in response to zebra mussel invasion. Consequently, the area of lake bottom likely to support benthic primary production also increased. The maximum depth of light penetration sufficient to support benthic algal photosynthesis increased from 7.5 m in pre-invasion years (1975–1991) to 10.5 m in post-invasion years, and macrophytes extended to a greater depth in post-invasion years. Algae and macrophytes may provide food or structural refuge to invertebrates. An intensive survey of one shallow bay showed that benthic invertebrate densities were higher in 1995 than in 2 pre-invasion years (1916 and 1967), even on substrates not colonized by zebra mussels. Continuous data (1967–1997) showed that invertebrates on soft substrate in Oneida Lake have not significantly increased above peak historical levels. However, the abundance of 2 grazing taxa (Amphipoda and Gastropoda) has increased in shallow relative to deep areas, suggesting that grazing invertebrate populations have responded to modification of light penetration by zebra mussels. Phosphorus levels in the lake have declined in the past 30 y, possibly keeping the increase of benthic invertebrate populations below historical levels.