Comparative species richness,composition, and abundance of invertebrates in Caribbean seagrass (Thalassia testudinum) meadows (Panamá)

The results of a year-long study in which epibenthic invertebrates were collected monthly from seagrass (Thalassia testudinum) meadows along the Caribbean coast of Panamá and the Panamá Canal Zone are described in this paper. Differences in species composition and abundance among sites were primarily due to the proximity of surrounding habitats, especially coral reefs, which contain a number of species that utilize the seagrass meadows. In contrast to many previous characterizations of tropical marine habitats, important seasonal fluctuations in both species number and abundance took place at each of the sites. Data on breeding activity among several species of decapod crustaceans indicate year-round reproduction, although considerable seasonal differences occur in the percentage of ovigerous females. These interspecific differences in observed reproductive output may be explained by differences in life-cycle length, a factor not often considered in discussions of seasonal breeding patterns in tropical marine invertebrates. Overall species composition was qualitatively similar to that reported in comparable studies of tropical and subtropical seagrass meadows elsewhere, although caridean shrimp and xanthid orab species were reduced in number and total abundance were much lower than in previous studies.     

Continuous plankton records: overwintering and annual fluctuations in the abundance of zooplankton

A study of seasonal variations in the pattern of year-to-year changes, for 1948 to 1982, in the abundance of four zooplankton taxa (Pseudocalanus elongatus Acartia clausi, Calanus finmarchicus and Hyperiidea) for the North Sea suggests that an element of the variations, represented at least in part by a quasi-linear downward trend, has its origins in winter and its occurrence through the year is a function of inherent persistence. A downward linear trend is the dominant pattern in the annual fluctuations in abundance of many species of zooplankton throughout the north-east Atlantic and the North Sea. It is argued that it is reasonable to extrapolate from the limited data set included in the detailed study and to assume a winter origin for the pattern wherever it occurs. Some implications for ideas about the dynamics of the plankton ecosystem are considered.     

Vertical distribution of Calanus finmarchicus and C. helgolandicus in relation to the development of the seasonal thermocline in the Celtic Sea

The geographical distribution and annual mean abundance of Calanus finmarchicus (Gunnerus) and C. helgolandicus (Claus) in the northern North Atlantic Ocean were shown in relation to the seasonal and annual fluctuations of abundance of the species in the Celtic Sea from 1960 to 1981. These congeneric copepods, although showing allopatric distributions over most of their geographical range, have sympatric distributions in the Celtic Sea where they dominate the dry weight biomass of the plankton throughout the year. The two species respond differently to the development of the seasonal thermocline and halocline by taking up different vertical distributions in the water column. C. finmarchicus occurred in the colder, more saline water below the thermocline, while C. helgolandicus occurred in the warmer, less saline water above the thermocline. This behaviour is postulated as a mechanism by which these morphologically similar copepods more fully exploit the resources of their temporally and spatially heterogeneous environment and also minimise interspecific competition. The species have the same foraging techniques and are able to exploit the same size spectrum of particulates. The vertical depth strata in which the populations are found for most of the year in the Celtic Sea means that both species exploit the diatom bloom in early spring but, thereafter, C. helgolandicus grazes on the daily production of the autotrophs in the euphotic zone while C. finmarchicus, below the thermocline, has to rely for its food on sedimenting particulates (whole cells, detritus and faecal material). The isolating mechanisms whereby these two populations partition the habitat in the Celtic Sea are discussed.     

A Mechanistic Approach to Evaluation of Umbrella Species as Conservation Surrogates

Abstract:  Although species with large area requirements are sometimes used as umbrella species, their general utility as conservation tools is uncertain. We surveyed the species diversity of birds, butterflies, carabids, and forest-floor plants in forest sites across an area (1600 km²) in which we delineated large breeding home ranges of Northern Goshawk ( Accipiter gentilis ). We tested whether protection of the home ranges could serve as an effective umbrella to protect sympatric species of the four taxa. We also used an empirical habitat model of occupancy of home range to examine mechanisms by which the Northern Goshawk acts as an umbrella species. Among species richness, abundance, and species composition of the four taxa, only abundance and species composition of birds differed between sites located inside and outside home ranges, which was due to greater abundance of bird species that were prey of Northern Goshawks inside the home ranges. Thus, although home range indicated areas with high abundance of certain bird prey species, it was not effective as an indicator of the species diversity of all four taxa. We also did not find any difference in species richness, abundance, and species composition between sites predicted as occupied and unoccupied using the habitat model. In contrast, when we selected sites on the basis of each habitat variable in the model, habitat variables that selected sites either in agricultural or forested landscapes encompassed sites with high species richness or particular species composition. This result suggests that the low performance of the Northern Goshawk as an umbrella species is due to this species' preference for habitat in both agricultural and forested landscapes. Species that can adjust to changes in habitat conditions may not act as effective umbrella species despite having large home ranges.     

Abundance and size distribution dynamics of abyssal epibenthic megafauna in the northeast Pacific

The importance of interannual variation in deep-sea abundances is now becoming recognized. There is, however, relatively little known about what processes dominate the observed fluctuations. The abundance and size distribution of the megabenthos have been examined here using a towed camera system at a deep-sea station in the northeast Pacific (Station M) from 1989 to 2004. This 16-year study included 52 roughly seasonal transects averaging 1.2 km in length with over 35600 photographic frames analyzed. Mobile epibenthic megafauna at 4100 m depth have exhibited interannual scale changes in abundance from one to three orders of magnitude. Increases in abundance have now been significantly linked to decreases in mean body size, suggesting that accruals in abundance probably result from the recruitment of young individuals. Examinations of size-frequency histograms indicate several possible recruitment events. Shifts in size-frequency distributions were also used to make basic estimations of individual growth rates from 1 to 6 mm/month, depending on the taxon. Regional intensification in reproduction followed by recruitment within the study area could explain the majority of observed accruals in abundance. Although some adult migration is certainly probable in accounting for local variation in abundances, the slow movements of benthic life stages restrict regional migrations for most taxa. Negative competitive interactions and survivorship may explain the precipitous declines of some taxa. This and other studies have shown that abundances from protozoans to large benthic invertebrates and fishes all have undergone significant fluctuations in abundance at Station M over periods of weeks to years.     

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     

Induction of sexual reproduction and resting egg production in Brachionus plicatilis by a diet of salt-grown Nannochloris oculata

Rates of population increase in early spring and the sizes of overwintering stocks were calculated for the planktonic copepods Pseudocalanus elongatus and Acartia clausi for a set of areas covering the open waters of the north-east Atlantic Ocean and the North Sea for the period 1948 to 1979. For both species, the rates of population increase were higher in the open ocean than in the North Sea and appear to be related to temperature. The overwintering stocks in the North Sea were larger than those in the open ocean and are probably related to phytoplanton concentration. P. elongatus shows higher overwintering stocks and lower rates of population increase than A. clausi, resulting in different levels of persistence in the stocks of the two species. It is suggested that this difference in persistence is responsible for differences between the two species with respect to geographical distribution in summer and different patterns of year-to-year fluctuations in abundance.     

Quelques données sur le déterminisme ecologique de la reproduction des annélides polychètes

The geographical distribution of marine invertebrate species — Annelida Polychaeta in the present case — is not random. It permits the definition of partially overlapping biogeographical groups. These groups are disposed in a line corresponding to a north-south axis. The seasonal cycles of occurrence of pelagic larvae are considered in southern Scandinavia (on the basis of literature sources), in the western Mediterranean Sea, and in the Indian Ocean (both on the basis of own data). The criterium of reproduction is the presence of meroplanktonic larvae of benthonic species. In this way, it has been possible to distinguish stocks of boreal, temperate and sub-tropical affinity. Within a given stock, the breeding season depends on the site of observation and on ecological factors. The breeding season is retarded for 6 months in southarn Scandinavia as compared to the western Mediterranean Sea, although these species belong to the same stock. This type of comparative study allows emphasis of the role of certain ecological factors related to reproduction (temperature, abundance of phytoplankton, etc.). Breeding may be triggered by different ecological factors, even in animals belonging to one and the same geographical stock: in the Mediterranean Sea, the breeding of the so-called subtropical species seems to be controlled by temperature (monthly variations); in the Indian Ocean, by high levels of phytoplankton abundance. A comparison of temperate species with those of southern Scandinavia and the Mediterranean Sea reveals differences in breeding seasons, which are correlated with thermal regime; breeding does not occur below or above a specific temperature, but is restricted to an optimum value, which appears to be a physiological species characteristic.     

Statistical modeling of seasonal and environmental influences on the population dynamics of an estuarine fish community

A statistical modeling study was performed on the population fluctuations of the 15 commonest fish species frequenting the tidal Scheldt estuary in Belgium. These included marine juvenile and seasonal visitors, estuarine residents and diadromous fish species that were recorded on the filter screens of a power plant cooling-water intake between September 1991 and April 2001. The species population abundance was regressed against a candidate set of 6 environmental variables and 13 instrumental variables, accounting for seasonality and long-term trends present in the data. Population abundances of the different species were, in general, best described by seasonal variables. Seasonal components contributed, on average, up to 63.8% of the variance explained by the models. Ten species were found to show a slightly negative, though significant, trend over the period of the survey. Most models also included at least one environmental variable, and 25.4% of the explained variance could be attributed to environmental fluctuations. Of all physico-chemical variables, dissolved oxygen was the most important predictor of fish abundance, suggesting that the estuary suffered from poor water quality during the survey. Temperature, salinity, freshwater flow, suspended solids and chlorophyll a concentrations were minor determinants of fish abundance.Communicated by O. Kinne, Oldendorf/Luhe     

洱海冬季水鸟群落结构与水位变化的潜在关系

水深是影响水鸟栖息地利用最重要的因素,而湿地的水位波动会导致水深发生变化。为研究水位波动对水深岸陡型高原湖泊冬季水鸟群落的影响,本研究以云南大理洱海为研究地点,在水位下降前后,即2014年和2017年的冬季(11月至次年1月)对洱海北部湖湾5个观察点各进行10次和8次水鸟统计调查,结果 2014年冬季共记录到22个物种,11 611只个体;2017年冬季共记录到30个物种,17 278只个体。对比涉禽、钻水鸭、潜水类三类功能集团的变化,发现水位下降后钻水鸭和潜水类的物种丰富度和多度都有所增加,而涉禽的物种丰富度不变,多度下降,这一结果与浅水滩地型湿地的响应变化规律不同。6种钻水鸭2017年冬季同时在洱海出现,说明可能为共同的环境因素所吸引。研究结果说明水位下降是洱海钻水鸭类物种丰富度和多度增加的必要条件。     

Quaternary geographical sibling speciation and population structuring in the Eastern Atlantic skates (suborder Rajoidea) Raja clavata and R. straeleni

The European Raja clavata and the South African R. straeleni are marine skates which exhibit highly conserved morphological and ecological traits. Owing to this, taxonomic and evolutionary relationships between the two taxa have not yet fully elucidated. Here, we have tested the hypothesis that restricted gene flow and genetic divergence between these taxa might be associated with climatic/oceanographic discontinuities by surveying genetic variation in ten geographical samples at control region (CR) and amplified fragment length polymorphism (AFLP) loci. The clustering of CR haplotypes in two reciprocally monophyletic clades consistent with taxon zoogeography and the significant AFLP F values between the European and South African populations indicated the two taxa as recently diverged peripatric sibling species. Within each species, significant spatial genetic heterogeneity among samples at both markers revealed population structuring. We argued that structured populations and isolated sibling species might represent two stages of geographical speciation.     

Disturbance of the Macroinvertebrate communities of bank-root biotope of a river in southern Nigeria

The impact of disturbance on the macroinvertebrate community was investigated in the biotope of a river catchment area. A total of 56 and 18 taxa were recorded at Stations 1 and 2, accounting for 71 and 29% individual organisms, respectively. Twelve taxa were common at both stations. Baetidae (Ephemeroptera) constituted the predominant group with the 46% density at the upstream biotope of Station 1, whereas, family Hydrobidae (Mollusca) dominated numerically the downstream Station 2. Data revealed that months of sampling at stations significantly influenced the occurrence and abundance of the major groups. Three distinct patterns of seasonal affiliation among the benthos were observed. Monthly variation in species diversity occurred. The macroinvertebrate community was affected by disturbances at the downstream bank-root biotope, limiting the occurrence of life forms to a few adaptable groups.     

Short-term patch dynamics of macroinvertebrate colonization on decaying reed detritus in a Mediterranean lagoon (Lake Alimini Grande,Apulia, SE Italy)

In aquatic ecosystems, the ecology of macrophytal detritus aggregations has been investigated over a range of temporal scales. Yet, no attempts have been made to minimize the temporal resolution of the analysis, thus neglecting the occurrence of short-term colonization and decay dynamics for these ephemeral patches of high biological activity. The present study was carried out at two sampling sites in Lake Alimini Grande, a brackish lagoon in South-East Italy. The abundance of macroinvertebrate taxa colonizing prepared packs of reed leaf detritus and the ash-free dry mass of the packs were monitored daily for 40 days. Spectral analysis was used to investigate the fine-scale temporal dynamics of invertebrates colonization and leaf pack mass variations. The macrobenthos found on leaf packs was dominated at both sites by few taxa (i.e., the vagile Lekanesphaera monodi, Microdeutopus gryllotalpa, Neanthes caudata and the sessile Balanus sp.), whose general colonization patterns were described by unimodal curves. At a daily temporal resolution Balanus sp. was characterized by random short-term density variations, while vagile taxa showed non-random, highly site- and species-specific fluctuations generally characterized by an oscillation period comprised between 2 and 3 days. In addition, leaf packs were characterized by non-random, short-term mass oscillations, coherent and in phase with L. monodi abundance fluctuations. Our results highlight the occurrence of short-term, non-random density variations of invertebrate colonizers on leaf detritus patches. In addition, they emphasize a previously unexplored, fine-scale linkage between abundance patterns of invertebrate consumers and short-term mass variations of decaying detritus.     

Changes in Arthropod Assemblages along a Wide Gradient of Disturbance in Gabon

Abstract: Searching for indicator taxa representative of diverse assemblages, such as arthropods, is an important objective of many conservation studies. We evaluated the impacts of a wide gradient of disturbance in Gabon on a range of arthropod assemblages representing different feeding guilds. We examined 4 × 10⁵ arthropod individuals from which 21 focal taxa were separated into 1534 morphospecies. Replication included the understory of 3 sites in each of 4 different stages of forest succession and land use (i.e., habitats) after logging (old and young forests, savanna, and gardens). We used 3 complementary sampling methods to survey sites throughout the year. Overall differences in arthropod abundance and diversity were greatest between forest and open habitats, and cleared forest invaded by savanna had the lowest abundance and diversity. The magnitude of faunal differences was much smaller between old and young forests. When considered at this local scale, anthropogenic modification of habitats did not result in a monotonous decline of diversity because many herbivore pests and their associated predators and parasitoids were abundant and diverse in gardens, where plant productivity was kept artificially high year‐round through watering and crop rotation. We used a variety of response variables to measure the strength of correlations across survey locations among focal taxa. These could be ranked as follows in terms of decreasing number of significant correlations: species turnover > abundance > observed species richness > estimated species richness > percentage of site‐specific species. The number of significant correlations was generally low and apparently unrelated to taxonomy or guild structure. Our results emphasize the value of reporting species turnover in conservation studies, as opposed to simply measuring species richness, and that the search for indicator taxa is elusive in the tropics. One promising alternative might be to consider “predictor sets” of a small number of taxa representative of different functional groups, as identified in our study.     

Spring and autumn spatial distribution of zooplankton carbon requirement across the Mediterranean Sea

Zooplankton represents a key contributor to the ocean biological pump through its consumption of sinking and suspended carbon. A specific and highly sensitive method to evaluate zooplankton carbon requirement from the sinking flux is through the estimation of the activity of the electron transport system. The present study was carried out from samplings in 2006, and it was focused on the spatial 200–0?m zooplankton carbon demand across 24 sampling stations, along the Mediterranean Sea, from the island of Crete to the Strait of Gibraltar. Its potential day/night variability was evaluated. The zooplankton composition, abundance and biomass were investigated. The carbon demand per unit zooplankton biomass indicates geographical and diel differences among the sampling stations. A higher mean carbon demand was seen for the western Mediterranean with respect to the eastern Mediterranean, which can be justified through the observed ratio of gelatinous:crustacean taxa and the water temperatures recorded. Higher carbon demand was measured in samples collected during the dark hours. The relation to the presence and abundance of actively migrating euphausiids and copepods was discussed. A comparison with data from another of our study carried out in the same study area but in another seasonal period was done.     

Competitive exclusion within the predator community influences the distribution of a threatened prey species

While much effort has been made to quantify how landscape composition influences the distribution of species, the possibility that geographical differences in species interactions might affect species distributions has received less attention. Investigating a predator-prey setting in a boreal forest ecosystem, we empirically show that large-scale differences in the predator community structure and small-scale competitive exclusion among predators affect the local distribution of a threatened forest specialist more than does landscape composition. Consequently, even though the landscape parameters affecting Siberian flying squirrel (Pteromys volans) distribution (prey) did not differ between nest sites of the predators Northern Goshawks (Accipiter gentilis) and Ural Owls (Strix uralensis), flying squirrels were heterospecifically attracted by goshawks in a region where both predator species were present. No such effect was found in another region where Ural Owls were absent. These results provide evidence that differences in species interactions over large spatial scales may be a major force influencing the distribution and abundance patterns of species. On the basis of these findings, we suspect that subtle species interactions might be a central reason why landscape models constructed to predict species distributions often fail when applied to wider geographical scales.     

Influence of seasonal food abundance and quality on the feeding habits of an opportunistic feeder,the intertidal crab <Emphasis Type="Italic">Pachygrapsus marmoratus</Emphasis>

The abundance of trophic sources on the intertidal zone is discontinuous and their supply can vary both in a predictable or unpredictable way. The Mediterranean semi-terrestrial crab Pachygrapsus marmoratus, is known, as adult, to entirely rely on the intertidal trophic sources, and, consequently, it faces the fluctuations of nutritional sources and quality. To clarify the relationships between the feeding habits of an Italian population of P. marmoratus and the temporal variation of its food sources, we carried out a 2-year sampling protocol. Data on seasonal variation in composition of intertidal food item assemblages, on the average content in N and C of the commonest algae, on seasonal changes in crabs feeding habits were collected and compared using a suite of multivariate and univariate techniques. Results showed that P. marmoratus takes advantage of the recruitment phase of the most common invertebrates, affecting and controlling the abundance even of those species whose adults are out of its reach. It can act both as a herbivore, and as a carnivore that shifts between bivalves, more abundant in spring/summer, and the periwinkle Melaraphe neritoides, during winter. In conclusion, we provide evidences on the food choice of this common rocky shore species, which is very plastic and capable of relying on many trophic sources, possibly influencing the abundance and/or the population structure of a number of intertidal populations.     

Detection of scale-specific community dynamics using wavelets

The response of ecological communities to anthropogenic disturbance is of both scientific and practical interest. Communities where all species respond to disturbance in a similar fashion (synchrony) will exhibit large fluctuations in total biomass and dramatic changes in ecosystem function. Communities where some species increase in abundance while others decrease after disturbance (compensation) can maintain total biomass and ecosystem function in the face of anthropogenic change. We examined dynamics of the Little Rock Lake (Wisconsin, USA) zooplankton community in the context of an experimental pH manipulation conducted in one basin of the lake. A novel application of wavelets was used to partition patterns of synchrony and compensation by time scale. We find interestingly that some time series show both patterns of synchrony and compensation depending on the scale of analysis. Within the unmanipulated basin, we found subtle patterns of synchrony and compensation within the community, largely at a one-year time scale corresponding to seasonal variation. Within the acidified lake basin, dynamics shifted to longer time scales corresponding to the pattern of pH manipulation. Comparisons between pairs of species in different functional groups showed both strong compensatory and synchronous responses to disturbance. The strongest compensatory signal was observed for two species of Daphnia whose life history traits lead to synchrony at annual time scales, but whose differential sensitivity to acidification led to compensation at multiannual time scales. The separation of time scales inherent in the wavelet method greatly facilitated interpretation as patterns resulting from seasonal drivers could be separated from patterns driven by pH manipulation.     

Fatty acid trophic markers elucidate resource partitioning within the demersal fish community of South Georgia and Shag Rocks (Southern Ocean)

Fatty acid analysis was used to study the trophic ecology of 10 demersal fish species in the South Georgia region. Principal component analysis grouped the species into three general clusters, revealing resource partitioning between species. Two groups were characterised by large proportions of either monounsaturated or polyunsaturated fatty acids, separating species according to their predominant feeding habitat. The third group showed fatty acid signatures overlapping with either or both of the previous two groups, suggesting a more opportunistic feeding behaviour for these species. Intraspecific comparisons furthermore revealed dietary variability with size, year and geographical location in several species. Mackerel icefish (Champsocephalus gunnari) in particular showed inter-annual differences in muscle lipid concentrations closely linked to prey availability with low lipid contents found in years of low krill (Euphausia superba) abundance. Despite the intraspecific differences the majority of species could be easily distinguished from each other, which indicates the utility of this method in the dietary analysis of higher predators.     

Mapping shifts in spatial synchrony in grassland birds to inform conservation planning

Spatial synchrony, defined as the correlated fluctuations in abundance of spatially separated populations, can be caused by regional fluctuations in natural and anthropogenic environmental population drivers. Investigations into the geography of synchrony can provide useful insight to inform conservation planning efforts by revealing regions of common population drivers and metapopulation extinction vulnerability. We examined the geography of spatial synchrony and decadal changes in these patterns for grassland birds in the United States and Canada, which are experiencing widespread and persistent population declines. We used Bayesian hierarchical models and over 50 years of abundance data from the North American Breeding Bird Survey to generate population indices within a 2° latitude by 2° longitude grid. We computed and mapped mean local spatial synchrony for each cell (mean detrended correlation of the index among neighboring cells), along with associated uncertainty, for 19 species in 2, 26-year periods, 1968–1993 and 1994–2019. Grassland birds were predicted to increase in spatial synchrony where agricultural intensification, climate change, or interactions between the 2 increased. We found no evidence of an overall increase in synchrony among grassland bird species. However, based on the geography of these changes, there was considerable spatial heterogeneity within species. Averaging across species, we identified clusters of increasing spatial synchrony in the Prairie Pothole and Shortgrass Prairie regions and a region of decreasing spatial synchrony in the eastern United States. Our approach has the potential to inform continental-scale conservation planning by adding an additional layer of relevant information to species status assessments and spatial prioritization of policy and management actions. Our work adds to a growing literature suggesting that global change may result in shifting patterns of spatial synchrony in population dynamics across taxa with broad implications for biodiversity conservation.