An invasive crab alters interaction webs in a marine community

Over the last decade, the non-native, filter-feeding crab Petrolisthes armatus invaded oyster reefs of the South Atlantic Bight at densities of thousands m⁻². Mesocosm and field experiments demonstrated that P. armatus at ∼10–75% of mean summer densities: (1) suppressed growth of small oysters, biomass of benthic microalgae, and recruitment of native mud crabs, (2) enhanced oyster, mussel, and total bivalve recruitment, macroalgal cover, and survivorship of predatory oyster drills, but (3) did not affect native taxonomic richness. Laboratory feeding assays, field tethering experiments, and population changes in field and mesocosm experiments suggest that P. armatus is a preferred prey for native mud crabs and other consumers, thus relieving predation on native species and enhancing recruitment or survival of bivalves and oyster drills. In contrast, the invasive crab can consume crustacean larvae and via this feeding may suppress recruitment of native mud crabs. Our findings should be conservative given the low densities of P. armatus seeded into experimental plots and our inability to run longer-term experiments due to controls rapidly being colonized by non-native crabs recruiting from the plankton. Invasive crabs commonly impact native communities via predation, but community impacts of this invasive crab may be as much due to its role as a preferred prey of native consumers as to its predation on native prey. Given that oysters are foundation species for shallow reefs in the South Atlantic Bight, the long-term effects of this invasion could be considerable.     

TRACING SPECIES INVASION IN CODIUM,A SIPHONOUS GREEN ALGA,USING MOLECULAR TOOLS

The siphonous green alga Codium fragile occurs in many temperate marine regions and is composed of a number of distinct subspecies. Included in this taxon is the common open coast C. fragile subsp. fragile of the northeast Pacific and the weedy C. fragile subsp. tomentosoides which has invaded temperate marine communities in the Atlantic and Pacific oceans. The center of origin of this weedy subspecies is not known, although it is thought to have dispersed from the northwest Pacific. To examine the relationship of the weedy subspecies to the indigenous northeast Pacific form, chloroplast DNA was compared. Each of these subspecies has a restriction map that is uniform throughout its geographic distribution, and the patterns are distinct from each other and from other Codium species examined. However, the two share an almost identical genome size and arrangement of genes. A population in San Francisco Bay was found to be indistinguishable from the weed C. fragile subsp. tomentosoides from the Atlantic. The potential for using molecular data in solving systematic problems in Codium has been clearly demonstrated.     

Contrasting patterns of spread in interacting invasive species: Membranipora membranacea and Codium fragile off Nova Scotia

In the Northwest Atlantic, overgrowth of the competitively dominant, native kelps by an invasive bryozoan Membranipora membranacea increases frond erosion, which has facilitated the establishment and spread of the invasive macroalga Codium fragile ssp fragile. To document the spread of both introduced species along the Atlantic coast of Nova Scotia from initial introduction points (the ‘epicentre’) southwest of Halifax, we conducted video-surveys of shallow rocky habitats along the southwestern shore of Nova Scotia (100 km linear distance, encompassing the range of M. membranacea) in 2000, and then along the entire Atlantic coast in 2007 (650 km). Membranipora membranacea was observed continuously throughout the surveyed ranges in 2000 and 2007, wherever kelps were present, suggesting natural dispersal via planktonic larvae. Codium fragile was observed along 95 km of the surveyed range in 2000 and along 445 km in 2007, with a relatively patchy distribution beyond the epicentre, suggesting a combination of natural and anthropogenic dispersal mechanisms. Rockweed-dominated (Fucus spp.) or mixed algal assemblages common outside the epicentre may alter the interaction between M. membranacea and C. fragile, since seaweeds other than kelp are not subject to defoliation by the bryozoan. Percent cover of kelp at the epicentre generally increased from 2000 to 2007, while that of C. fragile generally decreased. Codium fragile was the dominant canopy alga at 54% of sites in 2000 and at only 15% of sites in 2007. These findings indicate that, at near decadal timescales, C. fragile does not prevent re-colonization by native kelps.     

From introduced species to invader: what determines variation in the success ofCodium fragile ssp.tomentosoides (Chlorophyta) in the North Atlantic Ocean?

The green algaCodium fragile ssp.tomentosoides (Chlorophyta) has been introduced accidentally and successfully from Japan to many shores of the northern and southern hemispheres, including those of the Northeast and Northwest Atlantic Ocean. On most European coasts,Codium occurs regularly but at low abundances in the intertidal zone and is absent from subtidal habitats. In contrast,Codium is extremely abundant in subtidal kelp beds in the Northwest Atlantic Ocean where it often reaches nuisance proportions. This differential success cannot be accounted for by either the properties of the invader or by physico-chemical differences between invaded coasts. A theoretical comparison between two regions on opposite sides of the Atlantic Ocean, i.e. Eastern Nova Scotia, Canada, and south central Britain, illustrates how the resident benthic community may determine the difference in relative abundance ofCodium in subtidal habitats between northeast America and Europe. In this review, low floral species diversity, biological disturbance and facilitation by a previous species invasion are suggested as potential factors for the establishment, success and abundance ofCodium in the Northwest Atlantic Ocean, but these require testing in field experiments.     

The impact of simulated chronic nitrogen deposition on the biomass and N2-fixation activity of two boreal feather moss–cyanobacteria associations

Bryophytes achieve substantial biomass and play several key functional roles in boreal forests that can influence how carbon (C) and nitrogen (N) cycling respond to atmospheric deposition of reactive nitrogen (N_r). They associate with cyanobacteria that fix atmospheric N₂, and downregulation of this process may offset anthropogenic N_r inputs to boreal systems. Bryophytes also promote soil C accumulation by thermally insulating soils, and changes in their biomass influence soil C dynamics. Using a unique large-scale (0.1 ha forested plots), long-term experiment (16 years) in northern Sweden where we simulated anthropogenic N_r deposition, we measured the biomass and N₂-fixation response of two bryophyte species, the feather mosses Hylocomium splendens and Pleurozium schreberi. Our data show that the biomass declined for both species; however, N₂-fixation rates per unit mass and per unit area declined only for H. splendens. The low and high treatments resulted in a 29% and 54% reduction in total feather moss biomass, and a 58% and 97% reduction in total N₂-fixation rate per unit area, respectively. These results help to quantify the sensitivity of feather moss biomass and N₂ fixation to chronic N_r deposition, which is relevant for modelling ecosystem C and N balances in boreal ecosystems.     

Discovery and quantification of anaerobic nitrogen metabolisms among oxygenated tropical Cuban stony corals

Coral reef health depends on an intricate relationship among the coral animal, photosynthetic algae, and a complex microbial community. The holobiont can impact the nutrient balance of their hosts amid an otherwise oligotrophic environment, including by cycling physiologically important nitrogen compounds. Here we use ¹⁵N-tracer experiments to produce the first simultaneous measurements of ammonium oxidation, nitrate reduction, and nitrous oxide (N₂O) production among five iconic species of reef-building corals (Acropora palmata, Diploria labyrinthiformis, Orbicella faveolata, Porites astreoides, and Porites porites) in the highly protected Jardines de la Reina reefs of Cuba. Nitrate reduction is present in most species, but ammonium oxidation is low potentially due to photoinhibition and assimilatory competition. Coral-associated rates of N₂O production indicate a widespread potential for denitrification, especially among D. labyrinthiformis, at rates of ~1 nmol cm⁻² d⁻¹. In contrast, A. palmata displays minimal active nitrogen metabolism. Enhanced rates of nitrate reduction and N₂O production are observed coincident with dark net respiration periods. Genomes of bacterial cultures isolated from multiple coral species confirm that microorganisms with the ability to respire nitrate anaerobically to either dinitrogen gas or ammonium exist within the holobiont. This confirmation of anaerobic nitrogen metabolisms by coral-associated microorganisms sheds new light on coral and reef productivity.Subject terms: Biogeochemistry, Biogeochemistry     

DIETARY INDUCTION OF OPISTHOBRANCH MORPHOLOGY: PLACIDA DENDRITICA (ALDER & HANCOCK, 1843) ON DIFFERENT GREEN ALGAL HOSTS

The relationship of phenotypic morphological and behaviouralvariation was investigated for the ascoglossan (=sacoglossan)opisthobranch Placida dendritica. Morphological attributes weredocumented for individual slugs collected from three algal hostspecies along the central Oregon coast, USA, the green algaeCodium setchellii, C. fragile, and Bryopsis corticulans. Becauseindividuals on B. corticulans generally were substantially largerthan conspecifics on Codium spp., the species appeared polymorphic.Comparably sized slugs from different host genera also differedsignificantly in morphology: individuals from Codium spp. hadmore numerous, shorter cerata on wider bodies than did conspecificsfrom B. corticulans. Principal components analysis indicatedthat despite considerable overlap of morphological characters,ceratal shape features were diagnostic of algal host. When slugscollected from C. fragile were maintained on C. fragile andB. corticulans for 4 days in the laboratory, the size and shapeof slugs fed different diets diverged, and changes were consistentwith field data. Thus, phenotype was labile and changed as aresult of modified diet. Although a variety of molluscs exhibitdietary induction of morphology, a general awareness of thephenomenon has only recently begun to emerge, despite its importantecological and taxonomic implications. (Received 5 February 1996; accepted 2 July 1996)     

Salt marsh shoreline geomorphology influences the success of restored oyster reefs and use by associated fauna

Restoration is increasingly implemented as a strategy to mitigate global declines in biogenic habitats, such as salt marshes and oyster reefs. Restoration efforts could be improved if we knew how site characteristics at landscape scales affect the ecological success of these foundation species. In this study, we determined how salt marsh shoreline geomorphologies (e.g. with variable hydrodynamic energy, fetch, erosion rates, and slopes) affect the success of restored intertidal oyster reefs, as well as how fauna utilize restored reefs and forage along marsh habitats. We constructed oyster reefs along three marsh shoreline geomorphologies in May 2012: 1) “creek” (small‐fetch, gradual‐sloped shoreline), “ramp” (large‐fetch, gradual‐sloped shoreline), and “scarp” (large‐fetch, steep‐sloped shoreline). Following recruitment, oyster spat density was greatest on ramp reefs; however, 2 years later, the highest adult oyster densities were found on creek reefs. Total nekton and blue crab catch rates in trawl nets were highest in the creek, while piscivore catch rates in gill nets were highest along the scarp shoreline. We found no difference in predation on snails in the salt marsh behind constructed reef and nonconstructed reference sites, but there were more snails consumed in the creek shoreline, which corresponded with the distribution of their major predator—blue crabs. We conclude that oyster reef construction was most successful for oysters in small‐fetch, gradual‐sloped, creek environments. However, nekton abundance did not always follow the same trends as oyster density, which could suggest constructed reefs may offer similar habitat‐related functions (prey availability and refuge) already present along existing salt marsh borders.     

Ecological niche differentiation between native and non-native shrimps in the northern Baltic Sea

Invasions of non-native species are modifying global biodiversity but the ecological mechanisms underlying invasion processes are still not well understood. A degree of niche separation of non-native and sympatric native species can possibly explain the success of novel species in their new environment. In this study, we quantified experimentally and in situ the environmental niche space of caridean shrimps (native Crangon crangon and Palaemon adspersus, non-native Palaemon elegans) inhabiting the northern Baltic Sea. Field studies showed that the non-native P. elegans had wider geographical range compared to native species although the level of habitat specialization was similar in both Palaemon species. There were clear differences in shrimp habitat occupancy with P. elegans inhabiting lower salinity areas and more eutrophicated habitats compared to the native species. Consequently, the non-native shrimp has occupied large areas of the northern Baltic Sea that were previously devoid of the native shrimps. Experiments demonstrated that the non-native shrimp had higher affinity to vegetated substrates compared to native species. The study suggests that the abilities of the non-native shrimp to thrive in more stressful habitats (lower salinity, higher eutrophication), that are sub-optimal for native shrimps, plausibly explain the invasion success of P. elegans.     

Effects of accumulations of sediments and drift algae on recruitment of sessile organisms associated with oyster reefs

Increases in sediment and drift algae accumulations have caused degradation of coastal lagoons worldwide. It is well known that these factors are stressors of seagrass beds, sediment fauna and coral reefs. However, little is known about the impacts on temperate hard-bottom assemblages within soft-bottom lagoons. To test if accumulations of sediment and drift algae (stress) affected recruitment of sessile oyster reef organisms, we constructed cages in Hog Island Bay, Virginia that trapped drifting macroalgae (≈ 2.7 kg WW m²) and facilitated sedimentation (≈ 7 mm per 2-3 month). The stress treatments and unstressed controls were placed in front, between, and behind reefs (position) to represent wave exposed (≈ 0.3 m, windy conditions), current exposed (≈ 0.2 m s^− 1, peak tide) and protected (≈ 0 m, 0.0 m s^− 1) habitats. The percentage cover of recruited taxa onto bricks was mapped 4 times during a 1-year period. There were strong significant effects of stress on the total assemblage, plant (but not animal) richness, total plant and animal cover, and cover of the most common taxa. Unstressed bricks had high plant richness, high animal and plant cover, and high cover of the oyster Crassostrea virginica, the alien algae Gracilaria vermiculophylla and Codium fragile, the alga Agardhiella subulata, and high to medium cover of the opportunistic algae Ulva curvata and Enteromorpha spp. In comparison, sediment-stressed bricks had low plant richness, low animal and plant cover, and low cover of C. virginica, G. vermiculophylla, C. fragile, A. subulata, U. curvata and Enteromorpha spp. Similarly, algae-stressed bricks also had low cover of animals, C. virginica, G. vermiculophylla, C. fragile, and A. subulata, but intermediate plant richness and plant cover and high cover of U. curvata and Enteromorpha spp. Although reef position caused significant multivariate results, this factor was clearly less important than the stress factor. Our study shows that accumulations of sediments and drift algae have an adverse impact on sessile temperate reef organisms, reducing richness and abundance, but favoring a few small opportunistic taxa. As the reef-generating oysters themselves performed poorly under these stressors, the long-term impact of the causes of these stressors, eutrophication and urbanization, is likely to be diminished reefs with cascading adverse effects on sessile reef organisms.     

Production of N2 through Anaerobic Ammonium Oxidation Coupled to Nitrate Reduction in Marine Sediments

In the global nitrogen cycle, bacterial denitrification is recognized as the only quantitatively important process that converts fixed nitrogen to atmospheric nitrogen gas, N₂, thereby influencing many aspects of ecosystem function and global biogeochemistry. However, we have found that a process novel to the marine nitrogen cycle, anaerobic oxidation of ammonium coupled to nitrate reduction, contributes substantially to N₂ production in marine sediments. Incubations with ¹⁵N-labeled nitrate or ammonium demonstrated that during this process, N₂ is formed through one-to-one pairing of nitrogen from nitrate and ammonium, which clearly separates the process from denitrification. Nitrite, which accumulated transiently, was likely the oxidant for ammonium, and the process is thus similar to the anammox process known from wastewater bioreactors. Anaerobic ammonium oxidation accounted for 24 and 67% of the total N₂ production at two typical continental shelf sites, whereas it was detectable but insignificant relative to denitrification in a eutrophic coastal bay. However, rates of anaerobic ammonium oxidation were higher in the coastal sediment than at the deepest site and the variability in the relative contribution to N₂ production between sites was related to large differences in rates of denitrification. Thus, the relative importance of anaerobic ammonium oxidation and denitrification in N₂ production appears to be regulated by the availability of their reduced substrates. By shunting nitrogen directly from ammonium to N₂, anaerobic ammonium oxidation promotes the removal of fixed nitrogen in the oceans. The process can explain ammonium deficiencies in anoxic waters and sediments, and it may contribute significantly to oceanic nitrogen budgets.     

The Potential for Created Oyster Shell Reefs as a Sustainable Shoreline Protection Strategy in Louisiana

Coastal protection remains a global priority. Protection and maintenance of shoreline integrity is often a goal of many coastal protection programs. Typically, shorelines are protected by armoring them with hard, non‐native, and nonsustainable materials such as limestone. This study investigated the potential shoreline protection role of created, three‐dimensional Eastern oyster (Crassostrea virginica) shell reefs fringing eroding marsh shorelines in Louisiana. Experimental reefs (25 × 1.0 × 0.7 m; intertidal) were created in June 2002 at both high and low wave energy shorelines. Six 25‐m study sites (three cultched and three control noncultched) were established at each shoreline in June 2002, for a total of 12 sites. Shoreline retreat was reduced in cultched low‐energy shorelines as compared to the control low‐energy shorelines (analysis of variance; p < 0.001) but was not significantly different between cultched and noncultched sites in high‐energy environments. Spat set increased from 0.5 ± 0.1 spat/shell in July 2002 to a peak of 9.5 ± 0.4 spat/shell in October 2002. On average, oyster spat grew at a rate of 0.05 mm/day through the duration of the study. Recruitment and growth rates of oyster spat suggested potential reef sustainability over time. Small fringing reefs may be a useful tool in protecting shorelines in low‐energy environments. However, their usefulness may be limited in high‐energy environments.     

<Emphasis Type="Italic">Melilotus officinalis</Emphasis> (yellow sweetclover) causes large changes in community and ecosystem processes in both the presence and absence of a cover crop

Non-native species are hypothesized to decrease native species establishment and cover crops are hypothesized to decrease non-native species abundance. Although many studies have compared invaded to non-invaded habitats, relatively few studies have experimentally added non-native species to directly examine their effects. In a greenhouse mesocosm experiment, we tested the effects of non-native forbs (Melilotus officinalis, Verbascum thapsus, and Lespedeza cuneata), a proposed C₃ grass cover crop (Pascopyrum smithii), and a commonly seeded non-native C₃ grass (Bromus inermis) on the establishment of target native C₄ prairie grass species. All treatments contained the same seed density of target C₄ species and were begun on bare soil collected from the field. The legume M. officinalis strongly decreased the abundance of all other species, species diversity, and light and soil moisture levels. Surprisingly, M. officinalis took up relatively large amounts of labeled nitrogen (¹⁵N) from the soil early in its development, but M. officinalis fixed nitrogen, thus increasing nitrogen in biomass nearly fivefold by the end of the study. We found few effects of either C₃ grass species on non-native forbs or C₄ target species, but seeded P. smithii did increase species diversity. Non-native plants therefore impeded native C₄ grass establishment through long-lasting effects of target species seedbank depletion (death of most target seedlings) and altered nutrient availability. The effects of M. officinalis were not reduced by the presence of a cover crop.     

GROWTH OF THE GREEN ALGA CODIUM FRAGILE IN A CONNECTICUT ESTUARY1

An in situ comparison of environmental and physiological factors was undertaken in 1971–72 (15 months) in the Niantic River estuary to elucidate some of the important aspects, of the growth and development of the seaweed Codium fragile. In general, Codium in the estuary has a growing season of from 6 to 9 months. Growth increments during this period are relatively constant. Temperature and salinity are the main limiting factors to growth, although low summer concentrations of inorganic nitrogen may also be involved. Reproduction by means of swarmers occurs only in late summer or autumn. Codium appears highly adapted to the role of a colonizing species. It not only possesses the capability to occupy the harshest of environments, but also a system of reproductive alternatives which facilitates rapid colonization. The availability of substrate for attachment is the chief factor limiting its spread in this estuarine system.     

Temperature,Herbivory and Epibiont Acquisition as Factors Controlling the Distribution and Ecological Role of an Invasive Seaweed

The invasive canopy alga, Codium fragile ssp. tomentosoides, first observed at the Isles of Shoals in 1983, has become the dominant canopy species to 8 m throughout the islands. Codium populations are replacing themselves at most sites in what appears to be a new, climax, canopy species. However, Codium densities have declined in protected Gosport Harbor areas where it first became established. Codium has only slowly expanded its presence in adjacent nearshore subtidal habitats. Recent studies suggest a combination of factors that may be influencing the relative success of populations between habitats. The herbivorous sea slug, Placida dendritica, may be reducing populations in protected areas in spite of predators such as the green crab, Carcinus maenas, while surge may inhibit herbivore buildup in exposed habitats. Temperature instability due to localized, wind-driven upwelling may be slowing the buildup of subtidal Codium populations in nearshore sites. The combination of Codium dominance and the acquisition of increasing epibiont diversity are producing a new, potentially more complex community state than the previous kelp-dominated climax typical of the Gulf of Maine.     

Variation in structural and physiological leaf traits of eight species in karst desertification area of China

Leaf traits have long been recognized as influential factors in the acquisition and processing of resources by plants. However, there is less knowledge of between-species variations in seasonal changes in leaf traits and trait interrelationships. Therefore, we examined variations in leaf area (LA), dry biomass (DM), specific leaf area (SLA), and leaf gas-exchange parameters in one non-native and seven native tree species under field environmental conditions, in a karst area in China subjected to desertification. Measurements were taken three times during the growing season. The results show that the seven native trees had higher LA, DM, and water-use efficiency (WUE) than the non-native Cinnamomum camphora. In contrast, all the native tree species except Ligustrum lucidum had lower photosynthetic rates (P _N) than the non-native species. In all species, the relationship between LA and DM was less variable than the relationship between SLA and LA. However, leaves of the non-native C. camphora and native species Sterculia lanceolata, Cleidiocarpon cavalerei and Cyclobalanopsis glauca were highly sensitive to seasonal conditions, leaves of Sapindus mukorossi and Ligustrum lucidum were less sensitive to seasonal changes, and leaves of Syzygium cumini and Cephalomappa sinensis were insensitive. An understanding of leaf traits will aid the selection of suitable species for land restoration.     

Native vegetation structure,landscape features and climate shape non-native plant richness and cover in New Zealand native shrublands

Aim

Studies investigating the determinants of plant invasions rarely examine multiple factors and often only focus on the role played by native plant species richness. By contrast, we explored how vegetation structure, landscape features and climate shape non-native plant invasions across New Zealand in mānuka and kānuka shrublands.

Location

New Zealand.

Method

We based our analysis on 247 permanent 20 × 20-m plots distributed across New Zealand surveyed between 2009 and 2014. We calculated native plant species richness and cumulative cover at ground, understorey and canopy tiers. We examined non-native species richness and mean species ground cover in relation to vegetation structure (native richness and cumulative cover), landscape features (proportion of adjacent anthropogenic land cover, distance to nearest road or river) and climate. We used generalized additive models (GAM) to assess which variables had greatest importance in determining non-native richness and mean ground cover and whether these variables had a similar effect on native species in the ground tier.

Results

A positive relationship between native and non-native plant species richness was not due to their similar responses to the variables examined in this study. Higher native canopy richness resulted in lower non-native richness and mean ground cover, whereas higher native ground richness was associated with higher native canopy richness. Non-native richness and mean ground cover increased with the proportion of adjacent anthropogenic land cover, whereas for native richness and mean ground cover, this relationship was negative. Non-native richness increased in drier areas, while native richness was more influenced by temperature.

Main Conclusions

Adjacent anthropogenic land cover seems to not only facilitate non-native species arrival by being a source of propagules but also aids their establishment as a result of fragmentation. Our results highlight the importance of examining both cover and richness in different vegetation tiers to better understand non-native plant invasions.     

Positive Effects of Non-Native Grasses on the Growth of a Native Annual in a Southern California Ecosystem

Fire disturbance is considered a major factor in the promotion of non-native plant species. Non-native grasses are adapted to fire and can alter environmental conditions and reduce resource availability in native coastal sage scrub and chaparral communities of southern California. In these communities persistence of non-native grasses following fire can inhibit establishment and growth of woody species. This may allow certain native herbaceous species to colonize and persist beneath gaps in the canopy. A field manipulative experiment with control, litter, and bare ground treatments was used to examine the impact of non-native grasses on growth and establishment of a native herbaceous species, Cryptantha muricata. C. muricata seedling survival, growth, and reproduction were greatest in the control treatment where non-native grasses were present. C. muricata plants growing in the presence of non-native grasses produced more than twice the number of flowers and more than twice the reproductive biomass of plants growing in the treatments where non-native grasses were removed. Total biomass and number of fruits were also greater in the plants growing in the presence of non-native grasses. Total biomass and reproductive biomass was also greater in late germinants than early germinants growing in the presence of non-native grasses. This study suggests a potential positive effect of non-native grasses on the performance of a particular native annual in a southern California ecosystem.