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Mean Abundance (mean + abundance)
Selected AbstractsEffects of gizzard shad on benthic communities in reservoirsJOURNAL OF FISH BIOLOGY, Issue 6 2003K. B. Gido Effects of gizzard shad Dorosoma cepedianum on benthic communities in a large southern reservoir (Lake Texoma, U.S.A.) were examined during two field enclosure and exclosure experiments in which enclosures were stocked at high and low densities in 1998 and 1999, respectively. In both years, chironomid abundance significantly increased in treatments that excluded large fishes from foraging on sediments. Mean abundance of chironomids and ostracods were significantly higher (P < 0·05) in exclosures than enclosures stocked with gizzard shad at 1140,1210 kg ha,1. In 1999, benthic invertebrate abundances did not differ (P > 0·08) between exclosure and enclosures stocked at 175,213 kg ha,1. Per cent organic matter, algal abundance and abundance of other macroinvertebrates in sediments did not differ significantly among treatments in either year. Although chironomid abundance was reduced in gizzard shad enclosures in 1998, food habits from this and other studies showed that adult gizzard shad in Lake Texoma only consumed detritus and algae. It is likely that high sedimentation rates in Lake Texoma limit the ability of gizzard shad to regulate algae and detritus in benthic sediments. Thus, it is concluded that disturbance of benthic sediments by gizzard shad caused the observed reduction in chironomid abundance, rather than through consumption or competition for resources. [source] Macrofauna Communities in the Eastern Mediterranean Deep SeaMARINE ECOLOGY, Issue 3 2003Ingrid Kröncke Abstract., During two expeditions with RV ,Meteor' in summer 1993 and winter 1997/98 the structural and functional diversity of the benthic system of the highly oligotrophic eastern Mediterranean deep sea was investigated. The macrofauna communities were dominated by polychaetes even at the deepest stations. The fauna at shallow stations was dominated by surface deposit feeders, whereas subsurface deposit feeders and predators generally increased with depth. A high percentage of suspension-feeding Porifera was found in the Levantine Basin. Mean abundance and number of taxa of both expeditions were significantly correlated to depth and distance to the nearest coast as well as to the total organic carbon (TOC) content in sediments. Numbers of taxa and abundance decreased generally with depth, although lowest numbers were not found at the deepest stations but in the extremely oligotrophic Levantine and Ierapetra Basin. Biomass measured during the second cruise was extremely low in the Ierapetra Basin and comparable to other extreme oligotrophic seas. The significant correlations found for TOC contents and macrofauna with distance to coast during both expeditions apparently reflect the role of hydrographically governed transport of organic matter produced in coastal regions into greater and extreme depths of the Mediterranean Sea. Seasonal differences in macrofauna communities due to seasonal differences in food supply were not found. However, recent large-scale hydrographic changes (Eastern Mediterranean Transient, EMT) might change the oligotrophy and, thus, the structure of the benthic communities in the Eastern Mediterranean deep sea. [source] Many zeros does not mean zero inflation: comparing the goodness-of-fit of parametric models to multivariate abundance dataENVIRONMETRICS, Issue 3 2005David I. Warton Abstract An important step in studying the ecology of a species is choosing a statistical model of abundance; however, there has been little general consideration of which statistical model to use. In particular, abundance data have many zeros (often 50,80 per cent of all values), and zero-inflated count distributions are often used to specifically model the high frequency of zeros in abundance data. However, in such cases it is often taken for granted that a zero-inflated model is required, and the goodness-of-fit to count distributions with and without zero inflation is not often compared for abundance data. In this article, the goodness-of-fit was compared for several marginal models of abundance in 20 multivariate datasets (a total of 1672 variables across all datasets) from different sources. Multivariate abundance data are quite commonly collected in applied ecology, and the properties of these data may differ from abundances collected in autecological studies. Goodness-of-fit was assessed using AIC values, graphs of observed vs expected proportion of zeros in a dataset, and graphs of the sample mean,variance relationship. The negative binomial model was the best fitting of the count distributions, without zero-inflation. The high frequency of zeros was well described by the systematic component of the model (i.e. at some places predicted abundance was high, while at others it was zero) and so it was rarely necessary to modify the random component of the model (i.e. fitting a zero-inflated distribution). A Gaussian model based on transformed abundances fitted data surprisingly well, and rescaled per cent cover was usually poorly fitted by a count distribution. In conclusion, results suggest that the high frequency of zeros commonly seen in multivariate abundance data is best considered to come from distributions where mean abundance is often very low (hence there are many zeros), as opposed to claiming that there are an unusually high number of zeros compared to common parametric distributions. Copyright © 2005 John Wiley & Sons, Ltd. [source] The pervasive influence of sampling and methodological artefacts on a macroecological pattern: the abundance,occupancy relationshipGLOBAL ECOLOGY, Issue 4 2008Peter D. Wilson ABSTRACT Aim To investigate the influence of sampling and methodological artefacts on the correlation between abundance and occupancy. Location Global scope. Methods A fixed effects weighted regression model was fitted to standardized effect size for 175 examples of correlations between abundance and occupancy. A regression tree model with standard effect size as the dependent variable was also fitted to the data. Results Standard effect size, and therefore the correlation between abundance and occupancy, was found to be strongly influenced by the type of abundance measure used to characterize the abundance,occupancy relationship. Local mean abundance (also referred to as ecological mean abundance) was primarily responsible for negative correlations. Negative correlations also resulted from a mismatch in the sampling extents of abundance and occupancy measures. Main conclusions The combination of abundance and occupancy measures selected to characterize the abundance,occupancy relationship for a given set of data has a profound impact on the sign of the correlation between the selected measures. Previous attempts to understand the processes giving rise to the pattern represented by the abundance,occupancy relationship have confounded sampling artefacts (e.g. spatial extent of abundance and occupancy information) and methodological artefacts (e.g. combining a truncated abundance measure such as local mean abundance with an untruncated occupancy measure such as proportion of occupied samples). Thus, a revision of the approach currently used to define and evaluate competing explanatory models of the abundance,occupancy relationship appears to be necessary. [source] Spatial variation in population growth rate and community structure affects local and regional dynamicsJOURNAL OF ANIMAL ECOLOGY, Issue 6 2008M. Kurtis Trzcinski Summary 1Theory predicting that populations with high maximum rates of increase (rmax) will be less stable, and that metapopulations with high average rmax will be less synchronous, was tested using a small protist, Bodo, that inhabits pitcher plant leaves (Sarracenia purpurea L.). The effects of predators and resources on these relationships were also determined. 2Abundance data collected for a total of 60 populations of Bodo, over a period of 3 months, at six sites in three bogs in eastern Canada, were used to test these predictions. Mosquitoes were manipulated in half the leaves partway through the season to increase the range of predation rates. 3Dynamics differed greatly among leaves and sites, but most populations exhibited one or more episodes of rapid increase followed by a population crash. Estimates of rmax obtained using a linear mixed-effects model, ranged from 1·5 to 2·7 per day. Resource levels (captured insect) and midge abundances affected rmax. 4Higher rmax was associated with greater temporal variability and lower synchrony as predicted. However, in contrast to expectations, populations with higher rmax also had lower mean abundance and were more suppressed by predators. 5This study demonstrates that the link between rmax and temporal variability is key to understanding the dynamics of populations that spend little time near equilibrium, and to predicting and interpreting the effects of community structure on the dynamics of such populations. [source] Responses of ants to selective logging of a central Amazonian forestJOURNAL OF APPLIED ECOLOGY, Issue 3 2000H.L. Vasconcelos Summary 1.,Relatively little information exists on the effects of logging on rain forest organisms, particularly in the Neotropics where logging operations have increased dramatically in recent years. In this study we determined experimentally the effects of selective logging of a central Amazonian forest on ground-living ants. 2.,The experimental design consisted of three 4-ha replicated plots representing control unlogged forest, forest logged 10 years prior to the start of the study (1987), and forest logged 4 years prior to the start of the study (1993). The logging operation removed 50% of the basal area of trees of commercial value, or about eight trees per hectare. This resulted in a significant decrease in canopy cover, and an increase in understorey vegetation density in logged plots relative to controls. 3.,Collection and identification of ants from a total of 360 1-m2 samples of leaf-litter revealed 143 ant species, of which 97 were found in the control plots, 97 in the plots logged in 1987, and 106 in those logged in 1993. Species richness, evenness and mean abundance (ants m,2) per plot did not vary among treatments. Most of the species found in the control plots were also present in the logged plots. However, population density of many species changed as a result of logging, an effect that persisted for at least 10 years after logging. Species commonly found in sites that were directly disturbed by logging (gaps and tracks) were rare in the undisturbed forest, as revealed by an additional collection of ants. 4.,These results suggest that the persistence of ant assemblages typical of undisturbed forest is likely to depend on the amount of structural damage incurred by logging. Thus management techniques that minimize logging impacts on forest structure are likely to help maintain the conservation value of logged forests for ground-dwelling ants. It is particularly important to minimize the extent of logging roads and tracks created by heavy machinery because these areas appear more prone to invasion by non-forest species. [source] The pathology and seawater performance of farmed Atlantic salmon infected with glochidia of Margaritifera margaritiferaJOURNAL OF FISH BIOLOGY, Issue 4 2000J. W. Treasure The pathology of glochidial infection of the freshwater mussel Margaritifera margaritifera was examined in farmed Atlantic salmon Salmo salar in fresh water and for 150 days after transfer of salmon to sea water. Prevalence of infection in fresh water was 95%, mean abundance 134 per fish and mean infection intensity 140. Prevalence in sea water was 80,94% in the first 7 weeks after transfer but glochidia were absent, apart from remains, after 50 days in sea water. Glochidia on salmon in fresh water were associated with localized hyperplasia and fusion of secondary gill lamellae. Focally extensive epithelial hyperplasia and fusion of secondary lamellae were present 4,10 days after transfer to sea water. Twenty-three days after transfer, small nodules with a more discrete appearance were present suggesting partial resolution of tissue response; hyperplastic responses associated with glochidia were much reduced after 50 days. Plasma chloride in infected fish 10 days after transfer was 153 mmol. 1,1, significantly higher than fish without infection, suggesting poorer adaptation to sea water. No mortalities due to glochidial infection in sea water were recorded and there was no significant difference in mean weight between infected and control fish. [source] Chagos feels the pinch: assessment of holothurian (sea cucumber) abundance, illegal harvesting and conservation prospects in British Indian Ocean TerritoryAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 1 2010A. R. G. Price Abstract 1.Data are analysed from visual censuses of shallow-water holothurians (sea cucumbers) in 72 shallow water transects 100,m×2,m within four atolls of Chagos. Mean holothurian abundance in Diego Garcia, where harvesting is absent, was 18.5 individuals/transect (all transects) and 55.4 individuals/transect (only those containing holothurians). In the three exploited atolls, mean abundance did not exceed 3.5 and 5.2 individuals/transect, respectively. 2.Comparison with data collected during this study and an earlier investigation reveals a marked decline over four years in both mean and maximum density of commercially valuable Stichopus chloronotus and Holothuria atra in Salomon and Peros Banhos, both exploited atolls, and also for Holothuria nobilis in the latter. 3.Holothurian counts were also made along an extensive transect (21,km×4,m) encircling Salomon atoll. Abundance showed highly significant negative correlation with fishing pressure, the latter estimated using an ordinal (0,3) scale (Rs=,0.605, P,0.01). Harvesting effects were not discernible using data from 200,m2 transects. 4.While recent studies have shown Chagos is virtually pristine regarding contaminant levels, its holothurian resources are under increasing pressure. Results from this study, and examination of Sri Lanka's fishing activity in distant waters, point to heavy and illegal harvesting. 5.Stronger measures are needed to control the illegal fishery, to prevent holothurian abundances falling to the non-sustainable levels now prevalent across much of the Indo-Pacific, and to ensure that Chagos remains a biodiversity hotspot and environment of international renown. Use of smaller surveillance vessels would facilitate this.Copyright © 2009 John Wiley & Sons, Ltd. [source] Distinctiveness of macroinvertebrate communities in turloughs (temporary ponds) and their response to environmental variablesAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 4 2009Gwendolin Porst Abstract 1.Turloughs are a prime example of a water body type that interfaces with both the European Habitats Directive (92/43/EEC) and Water Framework Directive (2000/60/EC) (WFD), highlighting the need for an integrated strategy to protect and manage surface waters and groundwaters. To date, research on turloughs, including their invertebrate communities is limited. 2.Eight turloughs were sampled for their macroinvertebrate communities and water chemistry in April 2007. Faunal samples were collected by means of a simple box sampler. 3.Replicate samples within each turlough clustered together, indicating that a single sample can provide a meaningful description of the turlough invertebrate community. Variation of invertebrate communities within turloughs was nested among turloughs. 4.Hydroperiod influenced mean abundance and taxon richness of macroinvertebrates, but no correlation was found between nutrient status and either mean abundance or taxon richness. 5.Turloughs are priority habitats under the EC Habitats Directive, requiring maintenance of ,favourable conservation status', which needs to be assessed through monitoring, and effected through appropriate management plans. While the distinctiveness of macroinvertebrate communities across turloughs is conducive to simple and cost-effective monitoring, this also challenges the applicability of the concept of type-specific communities across these highly dynamic ecosystems. Copyright © 2009 John Wiley & Sons Ltd. [source] The importance of experimental design in detecting the effects of protection measures on fish in Mediterranean MPAsAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 6 2002P. Guidetti Abstract 1.Marine Protected Areas (MPAs) are currently proliferating in the Mediterranean Sea. The assessment of their role in the protection and recovery of overexploited fish populations, however, is not yet completely supported by an adequate body of scientific evidence. This is in part because the number of studies that unambiguously assess changes induced by protection is limited. 2.Sampling to detect and measure the expected effects of protection on commercial fish (e.g. increase in mean abundance and size) is made difficult by intrinsic natural spatial and temporal variability. Ideally, temporal replication before and after the establishment of MPAs is necessary for estimating the average conditions in time. Spatial replication of control locations, on the other hand, is essential to prevent confusion in interpreting a difference between protected and unprotected locations. Until now, the basic requirements for appropriate experimental designs in MPAs were seldom satisfied, especially in the Mediterranean region. 3.In the present paper, an ideal experimental design aimed at overcoming many of the confounding influences that could result in misinterpretations of the results is considered, with a discussion of implications of applying less and less appropriate designs because of pragmatic constraints. Experimental design should thus be adapted to each individual MPA under study on the basis of the specific constraints that researchers face (e.g. ,before' (i.e. pre-designation) data are lacking; there is a single or more than one protected locations). 4.The issues related to MPAs, chiefly in the Mediterranean region, need to be considered to enable the identification of unambiguous hypotheses prior to sampling, based on clear logical structures. The importance of appropriate sampling and the subsequent interpretation of data would progressively reduce the degree of uncertainty of environmental analyses about the effects of MPAs on fish, with important implications for their management and further proliferation. Copyright © 2002 John Wiley & Sons, Ltd. [source] Fire resilience of ant assemblages in long-unburnt savanna of northern AustraliaAUSTRAL ECOLOGY, Issue 7 2008CATHERINE L. PARR Abstract Tropical savannas and rainforests contrast in their flammability and the fire resilience of their associated species. While savanna species generally exhibit high resilience to burning, there is much debate about the fire resilience of forest-associated species, and the persistence of forest patches in a flammable savanna matrix. Where fire has been excluded, savanna tends on a trajectory towards forest, with an increase in forest-associated plants and animal species. This study tested the idea that given the high proportion of forest-associated taxa in long-unburnt savanna, the fauna of these areas would be expected to exhibit less resilience to fire than the fauna in frequently burnt savannas. The study investigated the immediate and short-term effects on ant assemblages of re-introducing fire into long-unburnt savanna in northern Australia. The ant fauna exhibited high resistance to fires, with no significant short-term change in mean abundance or species richness; instead, seasonality had a far stronger influence on overall ant activity. Fire caused dramatic declines in dominance of the patchily distributed forest-associated species Oecophylla smaragdina and Papyrius sp., but had no effect on overall dominance by open savanna species of Iridomyrmex. Dominance by Iridomyrmex pallidus declined, but this was compensated for by increases in I. reburrus, while two other species of Iridomyrmex showed no change. This indicates a high level of functional redundancy among dominant species of Iridomyrmex, which universally dominate open savanna communities, but not of dominant forest-associated species. Overall, our findings demonstrate a high degree of fire-resilience of the long-unburnt savanna ant fauna. Despite the occurrence of forest-associated species, the high proportion of savanna species persisting in this habitat means that long-unburnt savanna retains the general response characteristics of frequently burnt savanna. [source] Lizard community structure along environmental gradientsJOURNAL OF ANIMAL ECOLOGY, Issue 2 2010Lauren B. Buckley Summary 1. ,How the total number of individuals in a community is divided among its species is governed by both the distribution of species along landscape-scale environmental gradients and by local resource partitioning. In vertebrate ectotherms, abiotic environmental conditions may constrain geographic distributions more strongly than local population densities due to thermal constraints on resource acquisition and due to behavioural thermoregulation. 2. ,We investigate whether local density and species richness are decoupled for lizard communities within the Southwest US by comparing 18 species-abundance distributions. 3. ,While species richness decreases strongly with decreasing temperature, there is no significant relationship between temperature or resource availability (net primary productivity) and the total number of individuals within a community. Consequently, in more species-rich communities species have lower mean abundances. 4. ,This suggestion that lizard species richness is not a function of an area's capacity to support more individuals questions for this group species diversity theories based on this assumption. [source] | ||||||||||