Density-dependent facilitation cascades determine epifaunal community structure in temperate Australian mangroves

Co-occurring foundation species can determine biological community structure via facilitation cascades. We examined the density dependencies of facilitation cascades, including how the density of a basal foundation species influences the density of secondary foundation species, and how the density of secondary foundation species influences community structure. The system in which we assessed density dependencies was a temperate mangrove forest in which pneumatophores trap the fucoid alga Hormosira banksii and provide substrate for the oyster, Saccostrea glomerata. The alga and oyster in turn determine benthic community structure. In the field, algal biomass was positively correlated with pneumatophore density. Oysters, by contrast, were highly over-dispersed and correlated with the presence/absence of pneumatophores. Epifaunal abundance and species richness were positively correlated with algal and oyster abundance, but their effects were independent. The positive effect of pneumatophore density on epifauna was primarily an indirect effect of trapping more algae. Pneumatophores did not directly influence invertebrate communities. Experiments revealed that, at very low pneumatophore densities, algal retention was insufficient to facilitate epifauna above that found on pneumatophores alone. At higher densities, however, increasing the density of pneumatophores increased algal retention, and the density and diversity of associated invertebrates. Shading by the mangrove canopy reduced algal biomass but did not modify the density-dependent nature of the cascade. Our results extend facilitation theory by showing that the density of both basal and secondary foundation species can be critical in triggering facilitation cascades. Our study also reveals that, where foundation species co-occur, multiple, independent cascades may arise from a single basal facilitator. These findings enhance our understanding of the role of density-dependent facilitation cascades in community assembly.     

Individual- and Assemblage-Level Effects of Anthropogenic Sedimentation on Snails in Lake Tanganyika

Abstract:  Human impacts on aquatic biodiversity are often measured at the assemblage or community level, although it has been suggested that individual-level measures are more sensitive. We evaluated the effects of anthropogenic sedimentation on endemic snails in Lake Tanganyika, East Africa, by comparing assemblage-level (i.e., species richness, evenness, and abundance) and individual-level (i.e., frequencies of predation and parasitism, fecal organic content, life history) data between sediment-disturbed and reference sites. Previous studies have indicated that sedimentation kills snails and reduces mollusc diversity in this system, but we found little evidence of changes in species richness, evenness, or snail abundance at the levels of sedimentation recorded. In contrast, individual-level data revealed a variety of differences associated with sedimentation. Frequencies of shell scarring by predatory crabs and castration by parasitic trematodes were significantly lower at disturbed sites, indicating shifts in interspecific interactions. Snails ingested large amounts of inorganic sediments at disturbed sites, suggesting a reduction in food quality. In addition, sedimentation was associated with a large downward shift in size distribution within some species and reproduction at smaller size. These strong patterns in individual-level data contrast with the lack of effects at the assemblage level. We argue that incorporating individual-level measures will often enhance the sensitivity of impact surveys and may reveal effects of disturbance on important interspecific interactions.     

Using parasites to inform ecological history: comparisons among three congeneric marine snails

Species introduced to novel regions often leave behind many parasite species. Signatures of parasite release could thus be used to resolve cryptogenic (uncertain) origins such as that of Littorina littorea, a European marine snail whose history in North America has been debated for over 100 years. Through extensive field and literature surveys, we examined species richness of parasitic trematodes infecting this snail and two co-occurring congeners, L. saxatilis and L. obtusata, both considered native throughout the North Atlantic. Of the three snails, only L. littorea possessed significantly fewer trematode species in North America, and all North American trematodes infecting the three Littorina spp. were a nested subset of Europe. Surprisingly, several of L. littorea's missing trematodes in North America infected the other Littorina congeners. Most likely, long separation of these trematodes from their former host resulted in divergence of the parasites' recognition of L. littorea. Overall, these patterns of parasitism suggest a recent invasion from Europe to North America for L. littorea and an older, natural expansion from Europe to North America for L. saxatilis and L. obtusata.     

A Two-Part Measure of Degree of Invasion for Cross-Community Comparisons

Abstract:  Invasibility is a critical feature of ecological communities, especially for management decisions. To date, invasibility has been measured in numerous ways. Although most researchers have used the richness (or number) of exotic species as a direct or indirect measure of community invasibility, others have used alternative measures such as the survival, density, or biomass of either a single or all exotic species. These different measures, even when obtained from the same communities, have produced inconsistent results and have made comparisons among communities difficult. Here, we propose a measure of the degree of invasion (DI) of a community as a surrogate for community invasibility. The measure is expressed as 2 independent components: exotic proportion of total species richness and exotic proportion of total species abundance (biomass or cover). By including richness and abundance, the measure reflects that the factors that control invasibility affect both of these components. Expressing exotic richness and abundance relative to the richness and abundance of all species in a community makes comparisons across communities of different sizes and resource availability possible and illustrates the importance of dominance of exotic species relative to natives, which is a primary management concern associated with exotic species.     

Effects of Forest Fragmentation on a Dung Beetle Community in French Guiana

Abstract:  Fragmentation is the most common disturbance induced by humans in tropical forests. Some insect groups are particularly suitable for studying the effects of fragmentation on animal communities because they are taxonomically and ecologically homogenous. We investigated the effects of forest fragmentation on a dung beetle species community in the forest archipelago created in 1994–1995 by the dam of Petit Saut, French Guiana. We set and baited an equal number of pitfall traps for dung beetles on three mainland sites and seven island sites. The sites ranged from 1.1 to 38 ha. In 250 trap days, we captured 50 species in 19 genera. Diversity indices were high (2.18–4.06). The lowest diversity was on the small islands and one mainland site. Species richness and abundance were positively related to fragment area but not to distance from mainland or distance to the larger island. The islands had lower species richness and population than mainland forest, but rarefied species richness was relatively invariant across sites. There was a marked change in species composition with decreasing fragment that was not caused by the presence of a common fauna of disturbed-area species on islands. Small islands differed from larger islands, which did not differ significantly from mainland sites. Partial correlation analyses suggested that species richness and abundance of dung beetle species were positively related to the number of species of nonflying mammals and the density index of howler monkeys ( Alouatta seniculus ), two parameters positively related to fragment area.     

Influence of a dominant consumer species reverses at increased diversity

Theory and experiments indicate that changes in consumer diversity affect benthic community structure and ecosystem functioning. Although the effects of consumer diversity have been tested in the laboratory and the field, little is known about effects of consumer diversity in the subtidal zone, one of the largest marine habitats. We investigated the grazing effects of sea urchins on algal abundance and benthic community structure in a natural subtidal habitat of the Galápagos Islands. Three species of urchins (Eucidaris, Lytechinus, and Tripneustes) were manipulated in inclusion cages following a replacement design with three levels of species richness (one, two, and three species) with all possible two-species urchin combinations. Identity was the main factor accounting for changes in the percentage of substrate grazed and benthic community structure. Two out of the three two-species assemblages grazed more than expected, suggesting a richness effect, but analyses revealed that this increased grazing was due to a sampling effect of the largest and commercially valued urchin species, Tripneustes. Benthic community structure in treatments with Eucidaris, Lytechinus, and Tripneustes alone was significantly different at the end of the experiment, suggesting that resource use differentiation occurred. Communities in Tripneustes enclosures were characterized by abundant crustose coralline algae and grazed substrate, while those without it contained abundant green foliose algae (Ulva sp.). An unexpected emergent property of the system was that the most species-rich urchin assemblage underyielded, grazing less than any other assemblage with Tripneustes, effectively reversing its dominant influence observed in the two-species treatments. While further experiments are needed to discern the mechanisms of underyielding, it may be related to changing interspecific interactions as richness increases from two to three species or to density-dependent Tripneustes grazing. This study highlights the general importance of evaluating consumer richness effects across the entire range of species richness considered, as the performance of the most species-rich consumer assemblage could not be predicted by manipulations of intermediate levels of consumer species richness.     

Patterns of animal diversity in different forms of tree cover in agricultural landscapes.

As tropical regions are converted to agriculture, conservation of biodiversity will depend not only on the maintenance of protected forest areas, but also on the scope for conservation within the agricultural matrix in which they are embedded. Tree cover typically retained in agricultural landscapes in the neotropics may provide resources and habitats for animals, but little is known about the extent to which it contributes to conservation of animal species. Here, we explore the animal diversity associated with different forms of tree cover for birds, bats, butterflies, and dung beetles in a pastoral landscape in Nicaragua. We measured species richness and abundance of these four animal taxa in riparian and secondary forest, forest fallows, live fences, and pastures with high and low tree cover. We recorded over 20,000 individuals of 189 species including 14 endangered bird species. Mean abundance and species richness of birds and bats, but not dung beetles or butterflies, were significantly different among forms of tree cover. Species richness of bats and birds was positively correlated with tree species richness. While the greatest numbers of bird species were associated with riparian and secondary forest, forest fallows, and pastures with >15% tree cover, the greatest numbers of bat species were found in live fences and riparian forest. Species assemblages of all animal taxa were different among tree cover types, so that maintaining a diversity of forms of tree cover led to conservation of more animal species in the landscape as a whole. Overall, the findings indicate that retaining tree cover within agricultural landscapes can help conserve animal diversity, but that conservation efforts need to target forms of tree cover that conserve the taxa that are of interest locally. Preventing the degradation of remaining forest fragments is a priority, but encouraging farmers to maintain tree cover in pastures and along boundaries may also make an important contribution to animal conservation.     

Spatial and temporal variation in a soft-bottom fish assemblage in St Vincent Bay,New Caledonia

Monthly trawl surveys were performed in 1989 in North Bay and South Bay of St Vincent (New Caledonia) with both a shrimp trawl and fish trawl to produce a reference standard of the natural variability of an unexploited tropical soft-bottom fish assemblage. A total of 230 species belonging to 62 families were recorded. The mean density and biomass were 0.18 fishes m^-2 and 4.31 g m^-2, respectively. The major variations were explained by spatial factors. Species richness, density and biomass were greater in South Bay (204 species, 0.26 fishes m^-2 and 5.90 g m^-2) than in North Bay (105 species, 0.10 fishes m^-2 and 2.71 g m^-2), 34% of the species being present in both areas. The North Bay assemblage was characterized by four abundant benthic species (Saurida undosquamis, Gerres ovatus, Secutor ruconius and Upeneus moluccensis) and by numerous pelagic species (Carangidae, Sphyraenidae and Scombridae). The South Bay assemblage was characterized by several Mullidae, Bothidae and Balistidae, and by some rare species usually found on coral reefs (Pomacentridae and Chaetodontidae). These differences were induced by the physical and benthic characteristics of the two bays. North Bay was an homogeneous, confined, deposit area with few benthic organisms, whereas the substrate was more heterogeneous and the benthic organisms more diversified and abundant in South Bay, which was connected to the adjacent reef lagoon. Species richness remained stable in time, except in January when a hurricane disturbed the environment. Seasonal tendencies in species composition were evidenced in North Bay, with an autumn-winter structure opposed to a spring-summer structure, and characterized by the relative importance of the major species. No seasonal tendencies were observed in the organization of the South Bay assemblage. Nevertheless, mean density and biomass were at a minimum in summer in both bays; maxima occurred in winter. Biomass was negatively correlated to both temperature and rainfall, and reflected the population variations of the main species, particularly their reproductive migrations. Thus, the soft-bottom fish assemblages were strongly organized spatially in New Caledonia, but remained relatively stable over time.     

Relationships between species richness, evenness, and abundance in a southwestern savanna

Species richness and evenness are components of biological diversity that may or may not be correlated with one another and with patterns of species abundance. We compared these attributes among flowering plants, grasshoppers, butterflies, lizards, summer birds, winter birds, and rodents across 48 plots in the grasslands and mesquite-oak savannas of southeastern Arizona. Species richness and evenness were uncorrelated or weakly negatively correlated for each taxonomic group, supporting the conclusion that richness alone is an incomplete measure of diversity. In each case, richness was positively correlated with one or more measures of abundance. By contrast, evenness usually was negatively correlated with the abundance variables, reflecting the fact that plots with high evenness generally were those where all species present were about equally uncommon. Therefore richness, but not evenness, usually was a positive predictor of places of conservation value, if these are defined as places where species of interest are especially abundant. Species diversity was more positively correlated with evenness than with richness among grasshoppers and flowering plants, in contrast to the other taxonomic groups, and the positive correlations between richness and abundance were comparatively weak for grasshoppers and plants as well. Both of these differences can be attributed to the fact that assemblages of plants and grasshoppers were numerically dominated by small subsets of common species (grasses and certain spur-throated grasshoppers) whose abundances differed greatly among plots in ways unrelated to species richness of the groups as a whole.     

Effects of long-term physical disturbance by commercial scallop fishing on subtidal epifaunal assemblages and habitats

 This paper examines spatial differences in the distribution of by-catch assemblages from the scallop [Pecten maximus (L.) and Aequipecten opercularis (L.)] fishing grounds in the North Irish Sea, during 1995. The sites examined have been exposed to differing known levels of fishing disturbance by scallop dredging, based on unusually high-resolution data extracted from fishermens' logbooks. Uni- and multi-variate techniques have been used on a production dataset (a value which incorporates both abundance and biomass figures), as well as abundance and biomass data individually. The original species list was reduced to higher taxonomic groupings in line with the theory that the latter is more appropriate for detecting anthropogenic change. Species diversity and richness, total number of species, and total number of individuals all decrease significantly with increasing fishing effort. Species dominance increases with effort. Total abundance, biomass and production, and the production of most of the major individual taxa investigated decrease significantly with increasing effort. Multivariate analysis reveals a significant relationship between fishing effort and by-catch assemblage structure. The taxa most responsible for the differences are the echinoids and cnidarians, but prosobranch molluscs and crustaceans also contribute to the differences. By-catch assemblage structure is more closely related to fishing effort than any other environmental parameter investigated, including depth and sediment type. We observed an approximately linear decrease in diversity with increasing fishing disturbance, and suggest this is primarily due to selective removal of sensitive species and, more importantly, habitat homogenisation. These results were interpreted in the light of ecological theories relating disturbance to community structure. The argument that invertebrate scavenger populations benefit from prolonged exposure to fishing disturbance was also examined, but no supporting evidence was found. Received: 29 January 2000 / Accepted: 31 May 2000     

Crustacean epifauna of seagrass and macroalgae in Apalachee Bay,Florida, USA

Epifaunal crustaceans on turtlegrass (Thalassia testudinum) and five dominant macroalgae (Anadyomene stellata, Digenia simplex, Halimeda incrassata, Laurencia poitei and Penicillus lamourouxii) were quantitatively sampled bimonthly over a one-year period from September 1979 to September 1980 in a subtropical seagrass meadow in Apalachee Bay, Florida (northeastern Gulf of Mexico). These plant species exhibited a wide range of morphologies, with surface area-to-biomass ratios differing by over 2.5 times. A similar suite of crustaceans occurred on all macrophytes despite differences in shape or architecture among plant species. Relative abundances of many crustaceans, however, varied among plant hosts. Similarity analysis indicated that the epifaunal associates of T. testudinum were distinct from those of the macroalgae. Species richness was generally higher on turtlegrass than on any of the macroalgae. Abundances of total crustaceans per plant biomass or per plant surface area, on the other hand, were greater on all macroalgal species compared to the seagrass. Abundances (per plant biomass or plant surface area) of 14 of the 16 numerically dominant epifaunal species differed significantly among macrophytes. Twelve of the 16 species had greater abundance on one or more macroalgae, while only two species were more abundant on T. testudinum. Almost half of the dominant species had greatest abundances on the branching red alga L. poitei. Although abundances per plant biomass and plant surface area were greater on macroalgae relative to turtlegrass, densities (individuals per meter square of bottom) of animals associated with T. testudinum were significantly greater than those associated with macroalgae, primarily because of the greater abundance of turtlegrass in the grass bed. Both surface area-to-biomass ratios and degree of branching were poorly correlated with epifaunal abundance and number of species. Neither structural feature is an adequate predictor of faunal abundance and species richness among plant species, especially when macrophytes with very different morphologies are compared.     

Using Parasitic Trematode Larvae to Quantify an Elusive Vertebrate Host

Abstract: Digenean trematode parasites require multiple host species to complete their life cycles, and their abundance can often be strongly correlated with the abundance of their host species. Species richness and abundance of parasites in easily sampled host species may yield an accurate estimate of the species richness and abundance of other hosts in a parasite's life cycle that are difficult to survey directly. Accordingly, we investigated whether prevalence and mean abundance of trematodes could be used to estimate the abundance of one of their host species, diamondback terrapins (Malaclemys terrapin), which are difficult to sample and are designated as near threatened (by the International Union for Conservation of Nature [IUCN Red List]) along some U.S. coasts. As an adult the trematode Pleurogonius malaclemys is specific to terrapins. Its larval stages live first inside mud snails (Ilyanassa obsoleta) and are subsequently shed into the environment where they form external metacercarial cysts on hard surfaces such as snail opercula. The life cycle of P. malaclemys is completed when terrapins ingest these cysts. At 12 sites along the coast of Georgia (U.S.A.), we determined the prevalence of internal P. malaclemys larvae in mud snails (proportion of infected snails in a population) and the prevalence and mean abundance of external trematode cysts. We examined whether these data were correlated with terrapin abundance, which we estimated with mark‐recapture methods. The abundance of external cysts and salinity explained ≥59% of the variability in terrapin abundance. We suggest that dependent linkages between the life stages of multihost parasites make them reliable predictors of host species’ abundance, including hosts with abundances that are challenging to quantify directly.     

An evaluation of the ABC-method (abundance/biomass comparison) as applied to macrozoobenthic communities living on tidal flats in the Dutch Wadden Sea

The ABC-method proposed by Warwick (1986) for detecting pollution effects on marine zoobenthic communities, was tested on distribution patterns of numbers and biomass among species in macrozoobenthos samples taken annually for 13 yr in a uniform way at 15 tidal-flat stations in the western part of the Dutch Wadden Sea. Along the margins of the tidal-flat area studied, where exposition to either drainage or water movements is extreme, the k-dominance plots for numbers were generally situated above those for biomass. In these areas values for biomass and species richness were relatively low and a few small-sized species such asCorophium volutator andHydrobia ulvae were very numerous. In the central part of the area, where environmental conditions are less severe, values for biomass and species richness were higher. Biomass was dominated by large adults ofMya arenaria, Mytilus edulis and/orArenicola marina. However, in only a restricted part of this area were plots for numbers consistently below biomass plots. In most of the central area high numbers ofH. ulvae occasionally occurred, causing plots for numbers to be situated above those for biomass during such periods. There appears to be no reason to interprete a high abundance ofH. ulvae as a sign of pollution or any other kind of disturbance or stress. It is concluded that the ABC method cannot be applied to tidal flat communities without reference to long-term and spatial series of control samples. In areas where such smallsized and sometimes numerous species asH. ulvae occur in strongly fluctuating numbers, the method appears useless for assessing the pollution status of a benthic community.     

Role of food subsidies and habitat structure in influencing benthic communities of shell mounds at sites of existing and former offshore oil platforms

Mussels (Mytilus californianus, M. galloprovincialis) and other organisms sloughed from offshore oil platforms provide a food subsidy to benthic consumers and alter underlying soft bottom habitat by creating hard substrate. The removal of overlying platforms eliminates this food subsidy, but large shell mounds remain. The distribution, abundance, and population characteristics of mobile macroinvertebrates differed among shell mounds beneath existing offshore oil platforms, shell mounds at the former sites of offshore oil platforms, and soft bottom. Predatory and omnivorous echinoderm and mollusk species were more abundant and generally larger on shell mounds under platforms than on shell mounds without platforms. Omnivorous and deposit feeding echinoderms were the most abundant macroinvertebrate taxa sampled on mound-only sites. The brown rock crab (Cancer antennarius), known to have a strong preference for hard substrate, was significantly more abundant on shell mounds, with or without platforms, than adjacent soft bottom sites. Results suggest that the effects of platform removal differed among benthic species according to trophic level, degree of mobility, and substrate preference. Although the shell mound habitat persists after removal of platform structures, species abundance and the composition of the associated benthic community is altered by removal of the platform structure.Communicated by P.W. Sammarco, Chauvin     

Using community‐level metrics to monitor the effects of marine protected areas on biodiversity

Marine protected areas (MPAs) are used to protect species, communities, and their associated habitats, among other goals. Measuring MPA efficacy can be challenging, however, particularly when considering responses at the community level. We gathered 36 abundance and 14 biomass data sets on fish assemblages and used meta‐analysis to evaluate the ability of 22 distinct community diversity metrics to detect differences in community structure between MPAs and nearby control sites. We also considered the effects of 6 covariates—MPA size and age, MPA size and age interaction, latitude, total species richness, and level of protection—on each metric. Some common metrics, such as species richness and Shannon diversity, did not differ consistently between MPA and control sites, whereas other metrics, such as total abundance and biomass, were consistently different across studies. Metric responses derived from the biomass data sets were more consistent than those based on the abundance data sets, suggesting that community‐level biomass differs more predictably than abundance between MPA and control sites. Covariate analyses indicated that level of protection, latitude, MPA size, and the interaction between MPA size and age affect metric performance. These results highlight a handful of metrics, several of which are little known, that could be used to meet the increasing demand for community‐level indicators of MPA effectiveness.     

Predation,recruitment and the dynamics of communities of coral-reef fishes

I investigated the ability of predators to influence the patterns of species richness and abundance of non-piscivorous fishes on small, artificial reefs replenished by natural recruitment. Periodic removal of predators effectively reduced the species richness and abundance of predators on removal reefs. The difference between the number of predators on control and removal reefs was greatest immediately following the removal of predators and attenuated between removals. During periods of recruitment, species richness and total abundance of recently-recruited, non-piscivorous fishes were generally greater on predator-removal reefs than on control reefs. Species richness and total abundance of resident non-piscivorous fishes were not affected by the removal of predators in the first year of the experiment. Both abundance and species richness of residents, however, were greater on the removal reefs during the second year of the experiment. The difference in the responses of the two age classes to the removal of predators suggests that predators may affect community patterns of older age classes through time-lagged effects on the survivorship of younger age classes. At the end of the experiment, species richness was positively related to abundance for recruits and residents. The effects of removing piscivorous fishes on the abundance of non-piscivorous fishes were similar for species considered separately. A greater number of species of recruit and resident fishes were more abundant on reefs from which predators had been removed. These data suggest that predators can play an important role in structuring communities of fishes on coral reefs.     

Impact of Native Plants on Bird and Butterfly Biodiversity in Suburban Landscapes

Abstract:  Managed landscapes in which non-native ornamental plants are favored over native vegetation now dominate the United States, particularly east of the Mississippi River. We measured how landscaping with native plants affects the avian and lepidopteran communities on 6 pairs of suburban properties in southeastern Pennsylvania. One property in each pair was landscaped entirely with native plants and the other exhibited a more conventional suburban mixture of plants—a native canopy with non-native groundcover and shrubs. Vegetation sampling confirmed that total plant cover and plant diversity did not differ between treatments, but non-native plant cover was greater on the conventional sites and native plant cover was greater on the native sites. Several avian (abundance, species richness, biomass, and breeding-bird abundance) and larval lepidopteran (abundance and species richness) community parameters were measured from June 2006 to August 2006. Native properties supported significantly more caterpillars and caterpillar species and significantly greater bird abundance, diversity, species richness, biomass, and breeding pairs of native species. Of particular importance is that bird species of regional conservation concern were 8 times more abundant and significantly more diverse on native properties. In our study area, native landscaping positively influenced the avian and lepidopteran carrying capacity of suburbia and provided a mechanism for reducing biodiversity losses in human-dominated landscapes.     

Lionfish alter benthic invertebrate assemblages in patch habitats of a subtropical estuary

Invasive lionfish (primarily Pterois volitans) have spread throughout the Caribbean region, Gulf of Mexico, and the Southeast US coast, and resulting impacts on reef fish populations have been well documented. We examined whether lionfish can likewise affect benthic invertebrate communities, using an in situ caging experiment, in the Loxahatchee Estuary, Florida, USA. We found that lionfish caused significant declines in the three most abundant benthic invertebrate species, driving an overall shift in assemblage composition. For example, grass shrimp (Palaemonidae) abundance was reduced by nearly 90 % in the presence of lionfish. Species richness of benthic organisms was significantly higher when lionfish were present, suggesting potentially complex emergent effects of lionfish predation on benthic assemblages. Despite the fact that this experiment was conducted in just a single location using relatively small experimental units, we show altered benthic invertebrate communities could well be an additional outcome of the lionfish invasion.     

Relating bird host distribution and spatial heterogeneity in trematode infections in an intertidal snail—from small to large scale

Shorebird abundance and spatial distribution of larval trematodes in the New Zealand mudsnail, Zeacumantus subcarinatus, were investigated in soft-sediment intertidal bays within Otago Harbour, South Island, New Zealand. In a small-scale study, recruitment of trematodes to caged sentinel snails and the prevalence of infection in free-living snails were examined across a grid of fifteen 50×25 m plots arranged in a representative area of an intertidal bay, in relation to within-plot shorebird abundance (definitive hosts) and tidal height. In a large-scale study, natural spatial variation of larval trematodes in Z. subcarinatus was examined across 12 bays in relation to local abundance of shorebirds. Our results revealed that trematode prevalence in snails was positively correlated with bird abundance across bays (R ²=0.503, P=0.006). In contrast, despite a difference in bird abundance between tidal heights, there was no evidence that trematode prevalence reflected the spatial distribution of birds in the small-scale study, suggesting that factors related to differences in submersion time may override the differential input of trematode eggs from birds.     

Associations of Forest Cover,Fragment Area,and Connectivity with Neotropical Understory Bird Species Richness and Abundance

Theoretical and empirical studies demonstrate that the total amount of forest and the size and connectivity of fragments have nonlinear effects on species survival. We tested how habitat amount and configuration affect understory bird species richness and abundance. We used mist nets (almost 34,000 net hours) to sample birds in 53 Atlantic Forest fragments in southeastern Brazil. Fragments were distributed among 3 10,800‐ha landscapes. The remaining forest in these landscapes was below (10% forest cover), similar to (30%), and above (50%) the theoretical fragmentation threshold (approximately 30%) below which the effects of fragmentation should be intensified. Species‐richness estimates were significantly higher (F= 3715, p = 0.00) where 50% of the forest remained, which suggests a species occurrence threshold of 30–50% forest, which is higher than usually occurs (<30%). Relations between forest cover and species richness differed depending on species sensitivity to forest conversion and fragmentation. For less sensitive species, species richness decreased as forest cover increased, whereas for highly sensitive species the opposite occurred. For sensitive species, species richness and the amount of forest cover were positively related, particularly when forest cover was 30–50%. Fragment size and connectivity were related to species richness and abundance in all landscapes, not just below the 30% threshold. Where 10% of the forest remained, fragment size was more related to species richness and abundance than connectivity. However, the relation between connectivity and species richness and abundance was stronger where 30% of the landscape was forested. Where 50% of the landscape was forested, fragment size and connectivity were both related to species richness and abundance. Our results demonstrated a rapid loss of species at relatively high levels of forest cover (30–50%). Highly sensitive species were 3‐4 times more common above the 30–50% threshold than below it; however, our results do not support a unique fragmentation threshold. Asociaciones de la Cobertura Forestal, Superficie del Fragmento y Conectividad con la Riqueza y Abundancia de Aves Neotropicales de Sotobosque