Effect of nutrient availability, grazer assemblage and seagrass source population on the interaction between Thalassia testudinum (turtle grass) and its algal epiphytes

Seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity and has been implicated in declines of seagrass meadows worldwide. The net effect of epiphytes on seagrass growth and morphology depends on the independent and interactive effects of a variety of factors, including nutrient availability and the intensity of grazing on epiphytes. Here I report the results of a mesocosm experiment designed to test the effects of nutrient addition and within-functional group variation (grazer species composition and the source population of seagrass) on the strength of the interactions among grazers, epiphytes, and turtle grass (Thalassia testudinum). Turtle grass ramets from two sites in the northern Gulf of Mexico were cleared of epiphytes and transplanted into common-garden mesocosms. Replicate ramets were grown in a split-split plot design with two levels of dissolved nutrients and four different grazer species combinations (Tozeuma carolinense alone, Pagurus maclaughlinae alone, both species together, and no grazers present). As expected, grazers had a significant negative effect on epiphyte biomass/leaf area and a significant positive effect on turtle grass growth in the mesocosms. The two species were more similar in their direct effects on epiphyte biomass than in their indirect effects on turtle grass growth; this may reflect differences in epiphyte community composition under different grazer treatments. The effect of nutrient addition on turtle grass growth depended critically on the intensity of grazing: in the presence of grazers, turtle grass tended to produce a greater biomass of new leaf tissue in the tanks with nutrients added than in the control tanks. However, when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants. The two source populations of turtle grass differed significantly in epiphyte biomass/leaf area accrued in the mesocosms as well as in the strength of the effect of grazers on turtle grass growth. This suggests that population differentiation in seagrass interactions with epiphytes, as well as spatial and temporal variation in resources and grazer community composition, can greatly effect the role of epiphytes in limiting seagrass productivity.     

Are macroinvertebrate communities influenced by seagrass structural complexity?

A study was undertaken within a sub-tidal Zostera marina seagrass bed (Devon, U.K.), with the aim of elucidating the relationship between seagrass structural complexity and the size and composition of the associated macroinvertebrate community. Samples of macroinvertebrates were recovered from three designated areas of shoot density. Various physical characteristics were measured for individual plants, and an a priori complexity index was determined relevant to the associated target organisms. Resulting data were analysed using linear regression and multivariate techniques. Significant relationships were found between shoot density and number of leaves/shoot, leaf length, stem length and algal epiphyte biomass. Neither the number of species nor abundance of macroinvertebrates was significantly related with the derived complexity index. Multivariate analysis indicated that macroinvertebrate communities from the three areas of shoot density were significantly different, the pattern of macroinvertebrate community composition being best explained by sea-grass biomass. Linear regression of seagrass biomass with macroinvertebrate number of species and abundance revealed significant positive relationships. Regression also indicated that there was no significant increase in complexity with increasing seagrass biomass. The results suggest that within a seagrass bed the size and composition of the associated macroinvertebrate community is not determined by the structural complexity of the plants, but by the amount of plant available. This finding indicates a simple species-area relationship, and arguably one brought about as a result of a sampling artefact. Thus, the current paradigm that structural complexity of seagrass is responsible for increased species diversity, can only be justifiably applied to comparisons between seagrass and other habitats, and not within a seagrass bed itself.     

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.     

Qualitative and quantitative aspects of the epiphytic component in a mixed seagrass meadow from Papua New Guinea

During 1982, structural and functional aspects of the epiphytic component in a tropical mixed seagrass meadow, have been investigated for each seagrass species separately. This meadow consisted of the seagrasses Thalassia hemprichii (Ehrenb.) Aschers., Cymodocea serrulata (R.Br.) Aschers. et Magnus, C. rotundata Ehrenb. et Hempr. ex Aschers., Halodule uninervis (Forssk.) Aschers. and Syringodium isoetifolium (Aschers.) Dandy.No significant differences were observed in floristic composition, number of algal species, abundance and diversity of the epiphytic component. On an area basis, annual mean above-ground biomass (seagrass leaves and epiphytes), amounted to 82 g ADW, of which 18% could be ascribed to the epiphytic component. The contribution of the epiphytic component to the annual mean above-ground production ranged from 16% on leaves of Thalassia hemprichii to 33% on leaves of Cymodocea serrulata. Total annual mean epiphyte production was 4.6 g ADW m⁻² sediment surface day⁻¹ (19%).When including the macroalgal component of this mixed seagrass meadow, total annual mean above-ground plant biomass amounted to 93 g ADW (212 g DW) on an area basis, of which the epiphytes contributed 15.5% (28.5% DW), the macroalgal component 12% (32.5% DW) and the seagrass leaves 72.5% (39.5% DW). Aspects of the epiphytic component (e.g., floristic composition, abundance, biomass and production) in monospecific and mixed seagrass communities are discussed.     

Habitat Use by Fishes in Coral Reefs,Seagrass Beds and Mangrove Habitats in the Philippines

Understanding the interconnectivity of organisms among different habitats is a key requirement for generating effective management plans in coastal ecosystems, particularly when determining component habitat structures in marine protected areas. To elucidate the patterns of habitat use by fishes among coral, seagrass, and mangrove habitats, and between natural and transplanted mangroves, visual censuses were conducted semiannually at two sites in the Philippines during September and March 2010–2012. In total, 265 species and 15,930 individuals were recorded. Species richness and abundance of fishes were significantly higher in coral reefs (234 species, 12,306 individuals) than in seagrass (38 species, 1,198 individuals) and mangrove (47 species, 2,426 individuals) habitats. Similarity tests revealed a highly significant difference among the three habitats. Fishes exhibited two different strategies for habitat use, inhabiting either a single (85.6% of recorded species) or several habitats (14.4%). Some fish that utilized multiple habitats, such as Lutjanus monostigma and Parupeneus barberinus, showed possible ontogenetic habitat shifts from mangroves and/or seagrass habitats to coral reefs. Moreover, over 20% of commercial fish species used multiple habitats, highlighting the importance of including different habitat types within marine protected areas to achieve efficient and effective resource management. Neither species richness nor abundance of fishes significantly differed between natural and transplanted mangroves. In addition, 14 fish species were recorded in a 20-year-old transplanted mangrove area, and over 90% of these species used multiple habitats, further demonstrating the key role of transplanted mangroves as a reef fish habitat in this region.     

Consumer Depletion Alters Seagrass Resistance to an Invasive Macroalga

Few field studies have investigated how changes at one trophic level can affect the invasibility of other trophic levels. We examined the hypothesis that the spread of an introduced alga in disturbed seagrass beds with degraded canopies depends on the depletion of large consumers. We mimicked the degradation of seagrass canopies by clipping shoot density and reducing leaf length, simulating natural and anthropogenic stressors such as fish overgrazing and water quality. Caulerpa racemosa was transplanted into each plot and large consumers were excluded from half of them using cages. Potential cage artifacts were assessed by measuring irradiance, scouring by leaf movement, water flow, and sedimentation. Algal invasion of the seagrass bed differed based on the size of consumers. The alga had higher cover and size under the cages, where the seagrass was characterized by reduced shoot density and canopy height. Furthermore, canopy height had a significant effect depending on canopy density. The alteration of seagrass canopies increased the spread of C. racemosa only when large consumers were absent. Our results suggest that protecting declining habitats and/or restoring fish populations will limit the expansion of C. racemosa. Because MPAs also enhance the abundance and size of fish consuming seagrass they can indirectly promote algal invasion. The effects of MPAs on invasive species are context dependent and require balancing opposing forces, such as the conservation of seagrass canopy structure and the protection of fish grazing the seagrass.     

Modifications of the specific diversity and feeding guilds in an intertidal sediment colonized by an eelgrass meadow (Zostera marina) (Brittany,France)

A sandy beach located in a bay of north Brittany (France) is partly colonized by an eelgrass meadow. The overall macrobenthic fauna, i.e. epifauna, endofauna and fish, living in this seagrass and in non-vegetated sands was compared in terms of species, abundance and biomass in the different feeding guilds. Seagrass makes this habitat more complex by increasing both its surface by 5.45–fold and volume by 4.79–fold. Eighteen species among the 23 living in the non-vegetated sands were observed in the seagrass assemblage. Their abundance and biomass were increased by 7.33–fold and 3.54–fold, respectively. The occurrence of Zostera was associated with the presence of 124 new species and 34 new feeding guilds. In the Zostera meadow, a spatial segregation exists among the taxa from the different microhabitats explored for food-quest. The balanced distribution of biomass within the various microhabitats may suggest that an optimization in the exploitation of feeding resources goes hand in hand with a higher complexity of the trophic web. It looks as if the architecturally increasing complexity of the habitat resulting from the development of a seagrass meadow on sediment both stimulates the abundance and biomass of the indigenous macrobenthic assemblage, increases the specific and functional biodiversity and favours the specialization of feeding guilds.     

New aerial survey and hierarchical model to estimate manatee abundance

Monitoring the response of endangered and protected species to hydrological restoration is a major component of the adaptive management framework of the Comprehensive Everglades Restoration Plan. The endangered Florida manatee (Trichechus manatus latirostris) lives at the marine-freshwater interface in southwest Florida and is likely to be affected by hydrologic restoration. To provide managers with prerestoration information on distribution and abundance for postrestoration comparison, we developed and implemented a new aerial survey design and hierarchical statistical model to estimate and map abundance of manatees as a function of patch-specific habitat characteristics, indicative of manatee requirements for offshore forage (seagrass), inland fresh drinking water, and warm-water winter refuge. We estimated the number of groups of manatees from dual-observer counts and estimated the number of individuals within groups by removal sampling. Our model is unique in that we jointly analyzed group and individual counts using assumptions that allow probabilities of group detection to depend on group size. Ours is the first analysis of manatee aerial surveys to model spatial and temporal abundance of manatees in association with habitat type while accounting for imperfect detection. We conducted the study in the Ten Thousand Islands area of southwestern Florida, USA, which was expected to be affected by the Picayune Strand Restoration Project to restore hydrology altered for a failed real-estate development. We conducted 11 surveys in 2006, spanning the cold, dry season and warm, wet season. To examine short-term and seasonal changes in distribution we flew paired surveys 1–2 days apart within a given month during the year. Manatees were sparsely distributed across the landscape in small groups. Probability of detection of a group increased with group size; the magnitude of the relationship between group size and detection probability varied among surveys. Probability of detection of individual manatees within a group also differed among surveys, ranging from a low of 0.27 on 11 January to a high of 0.73 on 8 August. During winter surveys, abundance was always higher inland at Port of the Islands (POI), a manatee warm-water aggregation site, than in the other habitat types. During warm-season surveys, highest abundances were estimated in offshore habitat where manatees forage on seagrass. Manatees continued to use POI in summer, but in lower numbers than in winter, possibly to drink freshwater. Abundance in other inland systems and inshore bays was low compared to POI in winter and summer, possibly because of low availability of freshwater. During cold weather, maps of patch abundance of paired surveys showed daily changes in manatee distribution associated with rapid changes in air and water temperature as manatees sought warm water with falling temperatures and seagrass areas with increasing temperatures. Within a habitat type, some patches had higher manatee abundance suggesting differences in quality, possibly due to freshwater flow. If hydrological restoration alters the location of quality habitat, postrestoration comparisons using our methods will document how manatees adjust to new resources, providing managers with information on spatial needs for further monitoring or management. Total abundance for the entire area was similar among survey dates. Credible intervals however were large on a few surveys, and may limit our ability to statistically detect trends in total abundance. Additional modeling of abundance with time- and patch-specific covariates of salinity, water temperature, and seagrass abundance will directly link manatee abundance with physical and biological changes due to restoration and should decrease uncertainty of estimates. © 2011 The Wildlife Society.     

Arthropod assemblages in a focal tree species (Eucalyptus microcarpa) depends on the species mix in restoration plantings

The composition of arthropods in trees has long been a topic of interest, with many studies exploring ways in which arthropod communities differ among tree species or entire forests. Few studies, however, have examined arthropods from trees in restoration plantings, and little is known about how different tree plantings might lead to different biodiversity outcomes. The aim of our study was to determine if a focal tree species hosted a different arthropod fauna depending on its context. We examined arthropod assemblages from the foliage of Eucalyptus microcarpa (Grey Box) trees present in two kinds of plantings: (i) simple plantings with only Eucalyptus trees, and (ii) mixed plantings with both Eucalyptus and Acacia trees. We examined the composition of the assemblages, and looked for associations between ant and psyllid (Hemiptera) diversity across each kind of planting. We found more species of psyllids, beetles and ants in E. microcarpa trees when they were grown in simple plantings compared with the mixed species plantings. We also found that psyllid richness and abundance was positively correlated with ant richness and abundance across all plantings. Our study shows that a more diverse context (i.e. a mixed planting) is not necessarily associated with a more diverse fauna within a focal tree species, and that simple Eucalyptus-only plantings may encourage higher numbers of insect herbivores. The apparent association between psyllids and ants suggests a complex interaction that may drive patterns in tree arthropod assemblages.     

Response of seagrass epiphyte loading to field manipulations of fertilization, gastropod grazing and leaf turnover rates

This study evaluates the bottom-up and top-down controls on epiphyte loads under low nutrient additions. Nutrients and gastropod grazers were manipulated in a field experiment conducted within a Thalassia testudinum meadow in Florida Bay, FL, USA. The effect of seagrass leaf turnover rate on epiphyte loading was also evaluated using novel seagrass short-shoot mimics that “grow,” allowing for the manipulation of leaf turnover rates. During the summer growing season and over the course of one seagrass leaf turnover period, low-level water column nutrient additions increased total epiphyte load, epiphyte chlorophyll a, and epiphyte autotrophic index. T. testudinum leaf nutrients (N and P) and leaf productivity also increased. Epiphyte loading and T. testudinum shoot biomass and productivity did not respond to a 60% mean increase in gastropod abundance. Manipulations of seagrass leaf turnover rates at minimum wintertime and maximum summertime rates resulted in a 20% difference in epiphyte loading. Despite elevated grazer abundances and increased leaf turnover rates, epiphyte loads increased with nutrient addition. These results emphasize the sensitivity of T. testudinum and associated epiphytes to low-level nutrient addition in a nutrient-limited environment such as Florida Bay.     

Wintering bird communities in newly-formed wetland in the Yangtze River estuary

We compared wintering bird communities and their habitats among three shoals at Jiuduansha, a newly-formed wetland in the Yangtze River estuary. The highest species richness and diversity were recorded in Shangsha, which is the highest shoal, and the highest abundance and lowest species diversity were recorded in Xiasha, which is the lowest shoal. Shangsha had the largest abundance of perching birds whereas Xiasha was the most abundant in waterbirds. Bird assemblages showed different associations with the different habitat types—perching birds were favored by reed (Phragmites australis) communities, shallow water foragers and dabbling ducks preferred sea-bulrush (Scirpus mariqueter) communities, and moist-soil foragers and gulls showed a preference for bare intertidal zones. All bird assemblages, however, avoided the smooth cordgrass (Spartina alterniflora) communities, which are dominated by an alien invasive plant. The composition of avian communities was related to habitat types at the three shoals. Our results suggest that the newly-formed tidelands can provide suitable habitats for waterbirds and that the lower tidelands can attract more waterfowl than the higher tidelands. Because the shoal with low species diversity could have exclusive bird species, conservation efforts should not concentrate only on the area with high species diversity. The estuarine wetlands should be considered as a whole when conservation strategies are designed. The alien invasive plant should, moreover, be effectively controlled, to provide suitable habitats for birds.     

Diversity,distribution and host-species associations of epiphytic orchids in Nepal

Associations between epiphytes and their hosts are among the main factors affecting the biodiversity and distribution of epiphytes. While several previous studies explored the association between epiphyte diversity and host characteristics, very little is known about the generality of such associations at larger spatial scales. We aim to explore the associations between diversity and distribution of epiphytic orchids and host characteristics in different localities in Nepal. Epiphytic orchids and their hosts were recorded along the transects in total of 23,539 host individuals. To describe the diversity of orchids in the different localities, a rarefaction function was used. Univariate and multivariate analyses were carried out to explore the associations with locality, host characteristics, and their interactions with locality. In total, we recorded 141 species of orchids growing on 192 host species. The five localities significantly differed in orchid diversity and abundance. The number of orchid individuals per host species significantly increased with increasing number of host individuals. Species richness, abundances, occupancy of orchid species on host species and composition of orchids varied across the localities. Species richness and abundance was significantly higher on hosts in the higher strata and differed between families of host species. Abundance was higher on evergreen hosts. Composition of orchid communities are also associated with host characteristics, such as habit (shrub/tree/climber), bark texture, nature (deciduous/evergreen) and the plant families of host species. This study revealed a high diversity of epiphytic orchids in the localities studied and strong associations between the orchids and their hosts. Future studies looking at the relationships between epiphyte communities and host characteristics need to identify relationships at a wider scale in order to determine whether they are really general rather than site-specific.     

Local Competition and Metapopulation Processes Drive Long-Term Seagrass-Epiphyte Population Dynamics

It is well known that ecological processes such as population regulation and natural enemy interactions potentially occur over a range of spatial scales, and there is a substantial body of literature developing theoretical understanding of the interplay between these processes. However, there are comparatively few studies quantifying the long-term effects of spatial scaling in natural ecosystems. A key challenge is that trophic complexity in real-world biological communities quickly obscures the signal from a focal process. Seagrass meadows provide an excellent opportunity in this respect: in many instances, seagrasses effectively form extensive natural monocultures, in which hypotheses about endogenous dynamics can be formulated and tested. We present amongst the longest unbroken, spatially explict time series of seagrass abundance published to date. Data include annual measures of shoot density, total above-ground abundance, and associated epiphyte cover from five Zostera marina meadows distributed around the Isles of Scilly, UK, from 1996 to 2011. We explore empirical patterns at the local and metapopulation scale using standard time series analysis and develop a simple population dynamic model, testing the hypothesis that both local and metapopulation scale feedback processes are important. We find little evidence of an interaction between scales in seagrass dynamics but that both scales contribute approximately equally to observed local epiphyte abundance. By quantifying the long-term dynamics of seagrass-epiphyte interactions we show how measures of density and extent are both important in establishing baseline information relevant to predicting responses to environmental change and developing management plans. We hope that this study complements existing mechanistic studies of physiology, genetics and productivity in seagrass, whilst highlighting the potential of seagrass as a model ecosystem. More generally, this study provides a rare opportunity to test some of the predictions of ecological theory in a natural ecosystem of global conservation and economic value.     

The effects of seagrass (Zostera japonica) canopy structure on associated fauna: a study using artificial seagrass units and sampling of natural beds

The importance of seagrass canopy to associated fauna was assessed by comparing the species richness, abundance and diversity of the epi- and infaunal macroinvertebrate assemblages in a seagrass (Zostera japonica Ascherson and Graebner) bed and the adjacent unvegetated area in Hong Kong. Seagrass cover had significant effects on the composition and abundance of the associated fauna and the amount of detritus accumulated on the sediment surface. Detritus abundance was significantly higher in the seagrass bed, and was positively correlated with both the above- and belowground biomass of Z. japonica. Both the abundance and species richness of the epi- and infauna were significantly positively correlated with the belowground biomass of the seagrass and detritus standing crop. Macrofaunal species richness was higher (118) in the seagrass bed than the adjacent unvegetated areas (70), with a higher degree of similarity between the infauna than the epifauna of the two habitats. While all species recorded from the unvegetated areas were found in the seagrass bed, 48 species occurred only in the seagrass-covered areas. Species richness of epifauna was significantly higher in the seagrass bed, but there was no difference between infaunal species of the two habitats. On the contrary, faunal (epi- and infauna) abundance was significantly higher in seagrass areas. The seagrass bed also supported species of small tellinid bivalves previously not recorded from Hong Kong. Artificial seagrass units (ASUs, 0.2 m(2)) with four combinations of leaf density and leaf length and a control (bare sand) were placed at short distances from natural patches of Z. japonica. The composition, abundance and biomass of the epibenthos associated with the ASUs and the control were recorded after 3 months in the field. While species richness did not differ among the treatments, total abundance of epibenthos was significantly higher in the high density-long leaves (HL) treatment than in the control. Results of a discriminant analysis using log-transformed abundance data suggest that the gastropod Clithon oualaniensis, the mussel Musculista senhousia and the crab Thalamita sp. were important species distinguishing the assemblages in the various treatments. All the three species were significantly more abundant in the HL treatment than in the low density-short leaves (LS) treatment and the control. By contrast, there was no significant difference in the biomass of the epifauna, but discriminant analysis again separated the five treatments based on the composition of the biomass, with the same three species identified as the most important discriminating species. The species richness and abundance of the epifauna associated with the ASUs were similar to the adjacent unvegetated areas, but significantly lower than in the Zostera patches. The physical canopy structure of Z. japonica beds increased the abundance of the epibenthos, potentially through provision of canopy and indirectly through trapping of detritus.     

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.     

Overview of the Distribution,Habitat Association and Impact of Exotic Ants on Native Ant Communities in New Caledonia

Ants are among the most ubiquitous and harmful invaders worldwide, but there are few regional studies of their relationships with habitat and native ant communities. New Caledonia has a unique and diverse ant fauna that is threatened by exotic ants, but broad-scale patterns of exotic and native ant community composition in relation to habitat remain poorly documented. We conducted a systematic baiting survey of 56 sites representing the main New Caledonian habitat types: rainforest on ultramafic soils (15 sites), rainforest on volcano-sedimentary soils (13), maquis shrubland (15), Melaleuca-dominated savannas (11) and Acacia spirorbis thickets (2). We collected a total of 49 species, 13 of which were exotic. Only five sites were free of exotic species, and these were all rainforest. The five most abundant exotic species differed in their habitat association, with Pheidole megacephala associated with rainforests, Brachymyrmex cf. obscurior with savanna, and Wasmannia auropunctata and Nylanderia vaga present in most habitats. Anoplolepis gracilipes occurred primarily in maquis-shrubland, which contrasts with its rainforest affinity elsewhere. Multivariate analysis of overall ant species composition showed strong differentiation of sites according to the distribution of exotic species, and these patterns were maintained at the genus and functional group levels. Native ant composition differed at invaded versus uninvaded rainforest sites, in the absence of differences in habitat variables. Generalised Myrmicinae and Forest Opportunists were particularly affected by invasion. There was a strong negative relationship between the abundance of W. auropunctata and native ant abundance and richness. This emphasizes that, in addition to dominating many ant communities numerically, some exotic species, and in particular W. auropunctata, have a marked impact on native ant communities.     

Community structure of amphipods on shallow <Emphasis Type="Italic">Posidonia oceanica</Emphasis> meadows off Tunisian coasts

The structure, diversity and spatial distribution of the amphipod fauna associated with Posidonia oceanica meadows were studied along the Tunisian coasts in 2007. Samples were collected in nine different meadows at 2 m depth. A total of 44 species belonging to 12 families were collected. The most common species in terms of abundance were Ampithoe helleri, Hyale camptonyx and Ericthonius punctatus. The highest values of abundance and species richness and the lowest values of diversity and equitability were found in meadows with high epiphyte biomass. Multivariate analyses of data indicated that epiphyte biomass and geographical position were major determinants of the distribution and composition of amphipod assemblages along Tunisian coasts. The presence of two lessepsian amphipod species in one of the southern Posidonia oceanica meadows modified the structure of assemblage.     

Development of a diverse epiphyte community in response to phosphorus fertilization

The role of terrestrial soil nutrient supply in determining the composition and productivity of epiphyte communities has been little investigated. In a montane Hawaiian rainforest, we documented dramatic increases in the abundance and species richness of canopy epiphytes in a forest that had been fertilized annually with phosphorus (P) for 15 years; there was no response in forest that had been fertilized with nitrogen (N) or other nutrients. The response of N-fixing lichens to P fertilization was particularly strong, although mosses and non-N-fixing lichens also increased in abundance and diversity. We show that enhancement of canopy P availability is the most likely factor driving the bloom in epiphytes. These results provide strong evidence that terrestrial soil fertility may structure epiphyte communities, and in particular that the abundance of N-fixing lichens – a functionally important epiphyte group – may be particularly sensitive to ecosystem P availability.     

Exotic tree monocultures play a limited role in the conservation of Atlantic Forest epiphytes

The Brazilian Atlantic Forest suffered a severe geographic contraction along the last five centuries that reduced drastically most vascular epiphyte populations. Among the range of man-made matrixes, tree monocultures have the potential to contribute positively to the maintenance of the regional epiphyte diversity. Here, we test the similarity in abundance, richness, and species composition between vascular epiphytic communities established in managed monocultures of exotic and native species with natural communities occurring in neighboring native Araucaria Forest patches. In the São Francisco de Paula National Forest (Rio Grande do Sul state, Brazil), we recorded 62 epiphyte species from 300 phorophytes occurring in 12, one-hectare plots of Araucaria Forest and managed plantations of Pinus, Eucalyptus and Araucaria. Species richness, rarefied richness and abundance were significantly higher in Araucaria Forest in comparison to the exotic stands. Species composition was also substantially differentiated as Araucaria Forest patches harbored a greater number of zoochorous species than those of the exotic stands. Additionally, plantations of Araucaria angustifolia, a native species, sustained more individuals and more species than the exotic plantations. Neither tree height nor DBH explained epiphyte richness; however, both phorophyte diversity and stand age together accounted for 92% of the among-site variation in epiphytic species richness. We conclude that substrate heterogeneity in combination with time available for colonization contribute significantly to beta-diversity of epiphytes in Araucaria forests. However, demographic experimental studies are necessary in order to disentangle the role of substrate quality from metapopulation processes, such as dispersal limitation, at both temporal and spatial scales.     

Microeukaryotic Communities Associated With the Seagrass Zostera marina Are Spatially Structured

Epibiotic microorganisms link seagrass productivity to higher trophic levels, but little is known about the processes structuring these communities, and which taxa consistently associate with seagrass. We investigated epibiotic microeukaryotes on seagrass (Zostera marina) leaves, substrates, and planktonic microeukaryotes in ten meadows in the Northeast Pacific. Seagrass epibiotic communities are distinct from planktonic and substrate communities. We found sixteen core microeukaryotes, including dinoflagellates, diatoms, and saprotrophic stramenopiles. Some likely use seagrass leaves as a substrate, others for grazing, or they may be saprotrophic organisms involved in seagrass decomposition or parasites; their relatives have been previously reported from marine sediments and in association with other hosts such as seaweeds. Core microeukaryotes were spatially structured, and none were ubiquitous across meadows. Seagrass epibiota were more spatially structured than planktonic communities, mostly due to spatial distance and changes in abiotic conditions across space. Seawater communities were relatively more similar in composition across sites and more influenced by the environmental component, but more variable over time. Core and transient taxa were both mostly structured by spatial distance and the abiotic environment, with little effect of host attributes, further indicating that those core taxa would not show a strong specific association with Z. marina.