Positive abundance and negative distribution effects of a gastropod on an intertidal hermit crab

Summary Field experiments were used to determine the effect of a common intertidal snail (Nerita funiculata) on the use of space for foraging by the hermit crab Clibanarius digueti. Removals of Nerita resulted in an increased density of foraging Clibanarius, while additions of the gastropod had the opposite effect. The observed negative effect of the gastropod on individual hermit crabs appears to be food-related. Field surveys, however, suggested that the hermit crab population is limited by shell number, rather than food. Because Nerita contributes to the shell resource, its effect on the hermit crab population is positive. Nerita, therefore, has a negative effect on the distribution of foraging hermit crabs, but a positive effect on their abundance. Such decouplings of distribution and abundance effects are rare.     

A human-induced downward-skewed elevational abundance distribution of pteridophytes in the Bolivian Andes

Aim  To search for differences in the spatial variability of upper and lower elevational distribution limits of tropical ferns, based on the assumption that these are determined to different degrees by biotic and abiotic factors.
Location  The Yungas biogeographical region, in the Bolivian Andes.
Methods  From a data base of > 25,000 herbarium records, we analysed the skewness of the elevational distribution of 220 montane pteridophyte species, each with  15 records. Additionally, we compared the spatial variability of upper and lower elevational range limits of ferns in 351 plots of 400 m² each along four elevational transects separated by 15–450 km.
Results  Individual species showed variable elevational distribution patterns, ranging from symmetric to asymmetric, i.e. downward and upward skewed, but overall there was a statistically significant trend towards asymmetric distributions with abrupt upper limits and diffuse lower limits. This trend, however, was almost exclusively due to terrestrial species occurring at and above the current timberline. The analysis of the elevational transects revealed no significant trends.
Main conclusions  The downward-skewed distributional abundance of terrestrial, open-country ferns near the timberline appears to be a result of the extensive forest destruction that has lowered the timberline in the high Andes by 500–800 m, opening up habitats for a restricted suite of species. Our study shows that a limited number of species can cause a general trend in the overall data set, and that failure to extract these data may result in unsupported conclusions, in our case to assign a greater importance to biotic and abiotic factors in the elevational limitation of plants at lower and upper elevations, respectively.     

Distribution and abundance of butterflies in a mountain area in the northern Iberian peninsula

The effect of alutudinal range, distance to the latitudinal boundary of geographical range, body size and larval food plant on both the distribution and abundance of butterflies have been studied in a mountain area along a marked altitudinal gradient Multiple regression analysis revealed that distribution was positively related to altitudinal range and abundance Altitudinal range accounted for a great part of vanance in species distribution Altitudinal range increased as both distance to latitudinal boundary of geographical range and body size increased Abundance was not affected by altitudinal range, body size or distance to latitudinal boundary Larval food plants family was related to the abundance of butterflies, but not to distribution or altitudinal range These results suggest that regional distributions of butterflies are likely to be limited by climatic tolerances of species, while local abundance might be influenced by local resource levels     

Wrens on the edge: feeders predict Carolina wren Thryothorus ludovicianus abundance at the northern edge of their range

The marked range shifts seen with global warming suggest that ranges are limited by temperature. Other findings, however, suggest winter survival is directly tied to food availability. We studied Carolina wrens Thryothorus ludovicianus at the northern edge of their range to understand the roles of temperature and food in limiting this species. We established 21 transects over 3 habitats (residential, city park, and rural) with varying degrees of human influence on temperature and food supply. The three habitat types showed variations in wren density, temperature, and feeder presence. While wren densities showed similar seasonal patterns in all habitats, significantly higher densities of birds were observed in the city park and residential habitats. Post‐winter densities of Carolina wrens were predicted by the presence of bird feeders, and not by January mean minimum temperatures. Our findings suggest the winter range limits for endotherms is more directly related to food supply, and only indirectly related to temperature. Therefore supplemental feeding and other changes in food supply may modify the range shifts predicted from temperature changes alone.     

The effects of recombination rate on the distribution and abundance of transposable elements

Transposable elements (TEs) often accumulate in regions of the genome with suppressed recombination. But it is unclear whether this pattern reflects a reduction in the efficacy of selection against deleterious insertions or a relaxation of ectopic recombination. Discriminating between these two hypotheses has been difficult, because no formal model has investigated the effects of recombination under the deleterious insertion model. Here we take a simulation-based approach to analyze this scenario and determine the conditions under which element accumulation is expected in low recombination regions. We show that TEs become fixed as a result of Hill-Robertson effects in the form of Muller's ratchet, but only in regions of extremely low recombination when excision is effectively absent and synergism between elements is weak. These results have important implications for differentiating between the leading models of how selection acts on TEs and should help to interpret emerging population genetic and genomic data.     

Herbivory: effects on plant abundance, distribution and population growth

Plants are attacked by many different consumers. A critical question is how often, and under what conditions, common reductions in growth, fecundity or even survival that occur due to herbivory translate to meaningful impacts on abundance, distribution or dynamics of plant populations. Here, we review population-level studies of the effects of consumers on plant dynamics and evaluate: (i) whether particular consumers have predictably more or less influence on plant abundance, (ii) whether particular plant life-history types are predictably more vulnerable to herbivory at the population level, (iii) whether the strength of plant-consumer interactions shifts predictably across environmental gradients and (iv) the role of consumers in influencing plant distributional limits. Existing studies demonstrate numerous examples of consumers limiting local plant abundance and distribution. We found larger effects of consumers on grassland than woodland forbs, stronger effects of herbivory in areas with high versus low disturbance, but no systematic or unambiguous differences in the impact of consumers based on plant life-history or herbivore feeding mode. However, our ability to evaluate these and other patterns is limited by the small (but growing) number of studies in this area. As an impetus for further study, we review strengths and challenges of population-level studies, such as interpreting net impacts of consumers in the presence of density dependence and seed bank dynamics.     

The anomalous mole fraction effect in Chara: gating at the edge of temporal resolution

The anomalous mole fraction effect (AMFE) of the K(+) channel in excised patches of the tonoplast of Chara showed a minimum of apparent open-channel current at 20 mM Tl(+) and 230 mM K(+). Time series obtained at a sampling rate of 100 kHz (filter 25 kHz) were analyzed by three methods to find out whether the AMFE results from an effect on gating or on the conductivity of the open state. Fitting the amplitude histograms by a superposition of gaussians showed a broadening in the presence of Tl(+). Dwell-time analysis based on an O-O-C-C-C model failed to evaluate rate constants above the filter frequency. Thus, the absence of any reduction of apparent open-channel current in time series simulated with the evaluated rate constants could not be taken as evidence against the hypothesis of gating. Finally, a direct fit of the measured time series using five different 5-state Hidden Markov models revealed that the presence of Tl(+) changed the rate constants in such a way that the number of transitions into the short-lived open state (30 micros) increased strongly compared to those in the absence of Tl(+). These models explain 25% reduction of apparent single-channel current amplitude through a rapid gating mechanism.     

Positive and negative effects of estradiol-17 beta in the rat uterus

Estrogens could act as effectors or inhibitors of protein synthesis in the rat uterus, depending on the doses given to animals. A single injection of estradiol-17 beta to immature female rats led to the increase in protein synthesis and in enzyme activities involved in DNA synthesis. Four injections, given once daily, resulted in the inhibition of enzyme activity and synthesis of all proteins but one. The 105 kD protein which showed a gradual increase with the duration of estrogen treatment could be responsible for the negative action of estrogens on uterine growth.     

High genetic diversity of the endangered Iberian three‐spined stickleback (Gasterosteus aculeatus) at the Mediterranean edge of its range

1. The three‐spined stickleback (Gasterosteus aculeatus) on the Iberian Peninsula is only distributed in freshwater habitats and has completely disappeared from most of its range, mainly as a consequence of habitat degradation and invasive fish introductions. Genetic investigations have shown that Mediterranean‐Adriatic sticklebacks constitute an evolutionarily significant unit. Here, we present the first genetic data for Iberian populations living on the southern edge of the stickleback’s range. We used microsatellite markers to study gene diversity, population structure and genetic demography of stickleback populations. 2. High genetic differentiation among collections yielded a model of four genetically homogeneous units related to geography. The observed pattern of isolation by distance resulted mainly from the hydrographical pattern and limited gene flow among rivers. Moreover, low levels of gene diversity, high isolation and recent bottleneck events, which have led to small or even critical effective population sizes in several locations, could be explained by additional recent anthropogenic fragmentation. 3. We defined at least four evolutionarily significant units threatened by habitat fragmentation in north‐eastern Iberian sticklebacks. Because they retain long evolutionary histories, these populations should be considered of high conservation priority, and urgent management measures should be implemented.     

Positive and negative selection on the mitochondrial genome

Several recent studies have confirmed that mitochondrial DNA variation and evolution are not consistent with the neutral theory of molecular evolution and might be inappropriate for estimating effective population sizes. Evidence for the action of both positive and negative selection on mitochondrial genes has been put forward, and the complex genetics of mitochondrial DNA adds to the challenge of resolving this debate. The solution could lie in distinguishing genetic drift from 'genetic draft' and in dissecting the physiology of mitochondrial fitness.     

Impacts of climate on prey abundance account for fluctuations in a population of a northern wader at the southern edge of its range

Understanding the mechanisms by which climate change will affect animal populations is vital for adaptive management. Many studies have described changes in the timing of biological events, which can produce phenological mismatch. Direct effects on prey abundance might also be important, but have rarely been studied. We examine the likely importance of variation in prey abundance in driving the demographics of a European golden plover ( Pluvialis apricaria ) population at its southern range margin. Previous studies have correlated plover productivity with the abundance of their adult cranefly (Tipulidae) prey, and modelled the phenology of both plover breeding and cranefly emergence in relation to temperature. Our analyses demonstrate that abundance of adult craneflies is correlated with August temperature in the previous year. Correspondingly, changes in the golden plover population are negatively correlated with August temperature 2 years earlier. Predictions of annual productivity, based on temperature-mediated reductions in prey abundance, closely match observed trends. Modelled variation in annual productivity for a future scenario of increasing August temperatures predicts a significant risk of extinction of the golden plover population over the next 100 years, depending upon the magnitude of warming. Direct effects of climate warming upon cranefly populations may therefore cause northward range contractions of golden plovers, as predicted by climate envelope modelling. Craneflies are an important food source for many northern and upland birds, and our results are likely to have wide relevance to these other species. Research into the potential for habitat management to improve the resilience of cranefly populations to high temperature should be an urgent priority.     

Patch- and landscape-scale effects on howler monkey distribution and abundance in rainforest fragments

Variation at both the patch and landscape scale is known to influence the distribution and abundance of arboreal monkeys in rainforest fragments. However, few studies have examined the factors associated with these different scales of focus simultaneously. Using stepwise logistic-regression and generalized linear models (GLMs), howler monkey Alouatta palliata distribution and abundance were examined as a function of patch quality (fragment area, shape, tree DBH and canopy height) and landscape connectivity (isolation, total forest area, fragment and road abundance, corridor abundance and length) in 119 rainforest fragments in northern Chiapas, Mexico. The positive correlation observed between monkey distribution (presence/absence) and both fragment area and abundance may be explained by increased resources within larger fragments and those whose proximity allows greater exploration by monkeys. In contrast, GLM analysis indicated that monkey abundance in inhabited fragments was positively correlated with corridor abundance, canopy height and fragment area. These relationships could be explained by greater reproductive investments by monkeys in forest fragments whose size, degree of perturbation and degree of connectivity with other fragments suggest greater overall resource availability. Future studies should explicitly include a multi-scale approach to understanding the factors affecting patterns of monkey distribution and abundance, particularly as this relates to measures of and interactions between patch connectivity and resource availability.     

Evaluating the effects of climate change on tree species abundance and distribution in the Italian peninsula

Question: What is the effect of climate change on tree species abundance and distribution in the Italian peninsula? Location: Italian peninsula. Methods: Regression tree analysis, Random Forest, generalized additive model and geostatistical methods were compared to identify the best model for quantifying the effect of climate change on tree species distribution and abundance. Future potential species distribution, richness, local colonization, local extinction and species turnover were modelled according to two scenarios (A2 and B1) for 2050 and 2080. Results: Robust Random Forest proved to be the best statistical model to predict the potential distribution of tree species abundance. Climate change could lead to a shift in tree species distribution towards higher altitudes and a reduction of forest cover. Pinus sylvestris and Tilia cordata may be considered at risk of local extinction, while the other species could find potential suitable areas at the cost of a rearrangement of forest community composition and increasing competition. Conclusions: Geographical and topographical regional characteristics can have a noticeable influence on the impact of predicted climate change on forest ecosystems within the Mediterranean basin. It would be highly beneficial to create a standardized and harmonized European forest inventory in order to evaluate, at high resolution, the effect of climate change on forest ecosystems, identify regional differences and develop specific adaptive management strategies and plans.     

Positive and negative inotropic effects of elevated extracellular potassium level on mammalian ventricular muscle

The effect of moderate elevation in extracellular potassium concentration (up to 12 mM) on contraction of cat ventricular muscle was examined. Isometric force development was recorded from eight excised trabeculae and from six coronary-perfused in situ papillary muscle preparations. Contraction in the steady state was variably affected, sometimes decreasing monotonically, sometimes remaining unchanged, with increasing potassium level. In 11 of these 14 preparations, the steady state was preceded by a transient period in which the contraction was augmented. In addition, eight excised trabeculae were used in an experimental arrangement designed to distinguish between inotropic effects caused by potassium-induced alterations in the action potential and other, more direct, effects of this ion on contraction. The negative inotropic effect is attributable to a potassium-induced reduction in the amplitude and/or duration of the action potential plateau. The positive inotropic effect was found in experimental arrangements where effects of the potassium-rich medium on action potential time-course were effectively "buffered." The positive inotropic effect thus depends on the presence of the elevated potassium concentration and can occur independently of effects on the action potential time-course.     

Modelling spatial and environmental effects at the edge of the distribution: the red-backed shrike Lanius collurio in Northern Portugal

This study examined the interplay of spatial and environmental effects shaping the range margin of the red‐backed shrike (Lanius collurio) in northern Portugal. The occurrence of shrikes in 10 × 10 km UTM squares was related to three sets of explanatory variables, reflecting environmental effects (climate and habitat), large‐scale spatial trends, and neighbourhood influences (considering an autologistic term); spatial variables were used as surrogates for historical and demographic factors. Multiple logistic regression models were built for each set, and then variation partitioning based on partial regressions isolated the unique and shared components of explained variation. The environmental model revealed a dominant influence of climate effects, with the occurrence of shrikes increasing with frost and thermal amplitude, declining with insolation, and responding unimodally to rainfall. There was a weaker influence of habitat conditions, though shrikes were more likely with increasing cover by annual crops and pastures, and decreasing forest cover. Only a relatively small proportion of explained variation was due to a ‘pure’ environmental component (10.4%), as most variation explained by environmental factors appeared spatially structured (51.9%). The unique contributions of spatial variables to the overall model were also small, though the neighbourhood effects appeared relatively stronger than large‐scale trends. Taken together, results suggested that the south‐western range margin of the red‐backed shrike was largely determined by spatially structured environmental factors. Nevertheless, there were also ‘pure’ environmental factors determining some isolate occurrences irrespective of any spatial structure, and ‘pure’ spatial factors that appeared to favour the occupation of squares surrounding the core distribution areas irrespective of environmental conditions. These results add to the growing evidence that both environmental and spatial factors need to be considered in predictive modelling of species range margins.     

Influence of edge effects on common vole population abundance in an agricultural landscape of eastern France

Vole demographics are often modified close to wooded environments. Population monitoring in such settings reveals substantial edge effects, although these have seldom been quantified. This study compares the abundance of common voleMicrotus arvalis (Pallas, 1778) populations at various distances from three types of wooded environment (hedgerow, copse and forest) and in two types of habitat (temporary and permanent grassland). In temporary grassland, substantial edge effects were detected along the boundaries of forests and hedgerows. At 100 m from the forest edge, voles were twice more abundant than at 50 m, four times more than at 25 m and eight times more than at 5 m. In permanent grassland, the edge effect was such that voles were practically absent up to 100 m from the forest edge. For both types of grassland, the edge effect was more pronounced for forest habitat than for hedgerows, that is, for elements with a large surface area of wooded environment. Our results suggest that landscape context may greatly affect the local abundance of voles. Given the considerable damage done byM. arvalis, the presence of wooded patches in grassland areas leads to lower vole densities in adjacent areas and this might be an alternative method to controlM. arvalis as a pest species.