Abundant Centre Hypothesis Research Articles

Invasion of rocky shores by a mytilid mussel reveals an abundant‐centre distribution coupled with moderate increases in densities at its absolute range limits

AbstractSemimytilus patagonicus is an invasive mussel on the coast of southern Africa and has extended its range in recent years. We asked whether its distribution and abundance are consistent with the abundant‐centre hypothesis (ACH). Marginal populations were located by monitoring 33 rocky shore sites in South Africa and southern Namibia in 2021. This revealed no changes to its distributional limits since 2020. At nine of these sites, population demography was measured to allow a comparison of their densities and size structure. Four were central populations on the west coast of South Africa (including the site where the species was first detected in 2009). Four were marginal populations in South Africa: two towards the cold range edge in the north and two towards the warm range edge to the south. The ninth population was in southern Namibia, representing a recent invasion event first detected in 2014. Across the species' South African range, the distribution of its abundance was generally consistent with the ACH, with the greatest abundance at its range centre and a gradual decrease towards the range edges. However, the ultimate marginal population at both its cold and warm range edges showed moderate upticks in abundance compared to the penultimate marginal populations. Additionally, marginal populations in South Africa typically included a greater proportion of large individuals. Recruitment intensity was greater in warm range edge populations than cold range edge populations. The size structure of the population in Namibia resembled those of central populations in South Africa. Moderate increases in densities at the absolute range limits suggest that the species is currently undergoing spread into regions associated with moderately optimal environmental conditions (ultimate range edge sites) after encountering regions associated with suboptimal environmental conditions (penultimate range edge sites).

Using a robust multi‐settings inference framework on published datasets still reveals limited support for the abundant centre hypothesis: More testing needed on other datasets

AbstractAimThe abundant centre hypothesis (ACH) predicts a negative relationship between species abundance and the distance to the geographical range centre. Since its formulation, empirical tests of the ACH have involved different settings (e.g. the distance to the ecological niche or to the geographical range centre), but studies found contrasting support for this hypothesis. Here, we evaluate whether these discrepancies might stem from differences regarding the context in which the ACH is tested (geographical or environmental), how distances are measured, how species envelopes are delineated, how the relationship is evaluated and which data are used.LocationThe Americas.Time period1800–2017.Major taxa studiedMammals, birds, fish, and tree seedlings.MethodsUsing published abundance data for 801 species, together with species range maps, we tested the ACH using three distance metrics in both environmental and geographical spaces with range and niche envelopes delineated using two different algorithms, totalling 12 different settings. We then evaluated the distance–abundance relationship using correlation coefficients (traditional approach) and mixed‐effect models to reduce the effect of sampling noise on parameter estimates.ResultsSimilar to previous studies, correlation coefficients indicated an absence of effect of distance on abundance for all taxonomic groups and settings. In contrast, mixed‐effect models highlighted relationships of various strengths and shapes, with a tendency for more theoretically supported settings to provide stronger support for the ACH. The relationships were however not consistent across taxonomic groups and settings, and were sometimes even opposite to ACH expectations.Main conclusionsWe found mixed and inconclusive results regarding the ACH. These results corroborate recent findings, and suggest either that our ability to predict abundances from the location of populations within geographical or environmental spaces is low, or that the data used here have a poor signal‐to‐noise‐ratio. The latter calls for further testing on other datasets using the same range of settings and methodological framework.

Mechanisms underlying altitudinal and horizontal range contraction: The western black crested gibbon

AbstractAimSpecies ranges in mountain areas may shift both horizontally and altitudinally, resulting from climate change and anthropogenic impact. Two hypotheses (the abundant centre hypothesis and the contagion hypothesis) have been proposed to account for patterns of horizontal range contraction. However, undulating topograph causes a mosaic of unsuitable habitats, which may complicate the spatial pattern of range contraction. We develop a framework incorporating horizontal and altitudinal range contraction patterns of a species living in mountain areas, to better understand the underlying mechanisms of species range contraction.LocationChina, Northern Laos and Northern VietnamTaxonWestern black crested gibbon, Nomascus concolorMethodsWe collected occurrence data of the gibbons from various sources and modelled their potential distribution range in the 1950s, 1980s and 2010s, using ecological niche modelling. We compared distances from the centre point of the potential range in the 1950s to centre points of the largest 100 patches in the 1950s and the 2010s to understand the patterns of horizontal range contraction. We also calculated potential distributions within different altitudinal ranges for six populations in each period to understand the patterns of altitudinal range contraction.ResultsPotential horizontal distribution of the gibbons decreased by 69% from the 1950s to the 2010s. The centre point of the 100 largest patches in the 2010s was further apart than in the 1950s, supporting the contagion hypothesis. No populations extended their range to higher altitude, suggesting climate change did not have a profound effect on altitudinal shifts in gibbon range. All populations lost a substantial proportion of their ranges at lower altitudes (500–1,500 m) but to different degrees, suggesting that populations experienced different anthropogenic pressures.Main conclusionsAnthropogenic threats including human population increase, agricultural expansion and hunting, were more likely than climate change to have caused range contraction in western black‐crested gibbons. This study highlights the importance of studying horizontal and altitudinal range contraction simultaneously.

The influence of stochasticity, landscape structure and species traits on abundant–centre relationships

Species have been commonly hypothesized to have high population densities in geographic areas which correspond to either the centre of the species geographic range or climatic niche (abundant–centre hypothesis). However, there is mixed empirical support for this relationship, and little theoretical underpinning. We simulate a species spreading across a set of replicated artificial landscapes to examine the expected level of support for abundant–centre relationships in geographic and niche space. Species niche constraints were modeled as a single axis which was related directly to population growth rates. We found strong evidence for abundant–centre relationships when populations follow deterministic growth, dispersal is high, environmental noise is absent and intraspecific competition is low. However, the incorporation of ecological realism reduced the detectability of abundant–centre relationships considerably. Our results suggest that even in carefully constructed artificial landscapes designed to demonstrate abundant–centre dynamics, the incorporation of small amounts of demographic stochasticity, environmental heterogeneity or landscape structure can strongly influence the relationship between species population density and distance to species geographic range or niche centre. While some simulated relationships were of comparable strength to common empirical support for abundant–centre relationships, our results suggest that these relationships are expected to be fairly variable and weak.

Rejection of the genetic implications of the \u201cAbundant Centre Hypothesis\u201d in marine mussels

The ‘Abundant-Centre Hypothesis’ is a well-established but controversial hypothesis stating that the abundance of a species is highest at the centre of its range and decreases towards the edges, where conditions are unfavourable. As genetic diversity depends on population size, edge populations are expected to show lower intra-population genetic diversity than core populations, while showing high inter-population genetic divergence. Here, the genetic implications of the Abundant-Centre Hypothesis were tested on two coastal mussels from South Africa that disperse by means of planktonic larvae, the native Perna perna and the invasive Mytilus galloprovincialis. Genetic structure was found within P. perna, which, together with evidence from Lagrangian particle simulations, points to significant reductions in gene flow between sites. Despite this, the expected diversity pattern between centre and edge populations was not found for either species. We conclude that the genetic predictions of the Abundant-Centre Hypothesis are unlikely to be met by high-dispersal species with large population sizes, and may only become evident in species with much lower levels of connectivity.

Density of intertidal barnacles along their full elevational range of distribution conforms to the abundant-centre hypothesis.

The abundant-centre hypothesis (ACH) predicts that the density of a species should peak at its distribution centre and decrease similarly towards distribution margins. The ACH has been deduced from a theory that postulates that environmental conditions should be most favourable for a species at the centre of its distribution. This idealised density pattern, however, has been supported by limited field studies, as natural patterns are often more complex. It is thus of interest to examine under what conditions compliance with the ACH could be favoured. Such conditions could be smooth environmental gradients with limited habitat patchiness throughout the distribution range of a species. Thus, we tested the ACH by measuring the density of an intertidal barnacle (Semibalanus balanoides) across its full vertical distribution range (from low to high elevations) on a rocky shore with similar substrate properties across elevations. To do a reliable test, we surveyed eight elevation zones applying an equal sampling effort in each zone. Average barnacle density conformed to the ACH, as it peaked at the middle of the vertical distribution range of this species. The same underlying theory predicts a similar unimodal pattern for maximum body size, but this trait was decoupled from density, as maximum barnacle size increased from low to high elevations. Overall, although the ACH is not a universal predictive tool as once envisioned, it may predict some cases well, as shown by this study. Therefore, the ACH should not be discarded completely, and its domain of application should be further evaluated.

The shape of abundance distributions across temperature gradients in reef fishes

Improving predictions of ecological responses to climate change requires understanding how local abundance relates to temperature gradients, yet many factors influence local abundance in wild populations. We evaluated the shape of thermal‐abundance distributions using 98 422 abundance estimates of 702 reef fish species worldwide. We found that curved ceilings in local abundance related to sea temperatures for most species, where local abundance declined from realised thermal ‘optima’ towards warmer and cooler environments. Although generally supporting the abundant‐centre hypothesis, many species also displayed asymmetrical thermal‐abundance distributions. For many tropical species, abundances did not decline at warm distribution edges due to an unavailability of warmer environments at the equator. Habitat transitions from coral to macroalgal dominance in subtropical zones also influenced abundance distribution shapes. By quantifying the factors constraining species’ abundance, we provide an important empirical basis for improving predictions of community re‐structuring in a warmer world.

Limited support for the “abundant centre” hypothesis in birds along a tropical elevational gradient: implications for the fate of lowland tropical species in a warmer future

AbstractAimSince Darwin, ecologists have assumed that species tend to be abundant in the centre of their geographic distributions and rare at their range edges. We test two predictions of this abundant centre hypothesis: (a) that species are common in the centre of their range and rare at the edges and (b) that empirical distributions along environmental gradients are well described by symmetric bell‐shaped curves.LocationA reef‐to‐ridgetop elevational gradient in the Yopno–Urawa–Som Conservation Area (YUS CA) on the Huon Peninsula in Papua New Guinea.TaxonBirds.MethodsWe use mist net capture rates to quantify species’ abundances as a function of range position, then fit a series of models with different shapes, including symmetric curves, to species’ empirical abundance distributions.ResultsSpecies peak abundances were located near species’ low elevation range edges, high elevation range edges and everywhere in between. Real abundance distributions were fit well by symmetric curves in fewer than half of the species we analysed. Most lowland species have high abundances at their low elevation margin near sea level.Main conclusionsWe find only limited support for the abundant centre hypothesis. Most lowland species appear to currently occupy a “truncated” realized niche, suggesting that they will be able to persist at low elevations even as temperatures increase and that lowland biotic attrition is unlikely. In addition to analyses, we present (a) a full species list for the 256 species recorded thus far within the YUS CA, with elevational limits for all forest species and (b) the raw data from our extensive mist net surveys. This information provides valuable baseline data that can be used by future biodiversity scientists to evaluate the changes in the YUS avifauna and will be generally useful to biodiversity scientists studying tropical elevational gradients.

Distribution of the isopod Excirolana braziliensis on sandy beaches of the Atlantic and Pacific Oceans

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 583:137-148 (2017) - DOI: https://doi.org/10.3354/meps12333 Distribution of the isopod Excirolana braziliensis on sandy beaches of the Atlantic and Pacific Oceans Gastón Martínez1,2,*, Matías Arim1,2, Omar Defeo1,2 1Centro Universitario de la Región Este (CURE), Universidad de la República, Ruta 9 y Ruta 15, PC 27000 Rocha, Uruguay 2Departamento de Ecología y Evolución, Facultad de Ciencias, Universidad de la República, PC 11400 Montevideo, Uruguay *Corresponding author: gmartinez@cure.edu.uy ABSTRACT: We assessed geographic patterns of abundance of the isopod Excirolana braziliensis across its whole distribution range in the context of the abundant centre hypothesis (ACH). We also evaluated auxiliary hypotheses to the ACH involving habitat availability and suitability. We compiled species abundance and occurrence information on 139 Pacific and Atlantic sandy beaches of the Americas. Abundance patterns were contrasted against 5 hypothetical distribution models. Habitat availability (sandy beach along the coast), and the role played by grain size on isopod abundance were analysed. Maximum entropy niche modelling based on primary production, salinity, water temperature and tidal range data was used to estimate trends in habitat suitability. E. braziliensis abundance peaked at the 2 edges of its range on the Atlantic coast and towards the centre of its range on the Pacific coast. Congruently, the niche model predicted very low habitat suitability at the centre of the species’ range on the Atlantic coast. Primary production was the main contribution to the model (74.8%). The highest abundances were found in fine sediments (0.20 mm). Support for the ACH for E. braziliensis was found only for the Pacific coast, whereas habitat suitability and availability together with local in-beach morphodynamics accounted for deviations from ACH predictions in the Atlantic. The highest abundances registered in upwelling areas and on beaches with fine sands highlight the primary role played by regional and local conditions over geographic location. KEY WORDS: Abundant-centre hypothesis · Habitat availability · Primary production · Salinity · Sandy beaches · Surface water temperature Full text in pdf format Supplementary materialSee Comment and Reply Comment on this article PreviousNextCite this article as: Martínez G, Arim M, Defeo O (2017) Distribution of the isopod Excirolana braziliensis on sandy beaches of the Atlantic and Pacific Oceans. Mar Ecol Prog Ser 583:137-148. https://doi.org/10.3354/meps12333 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 583. Online publication date: November 16, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Campanula lingulata populations on Mt. Olympus, Greece: where\u2019s the \u201cabundant centre\u201d?

BackgroundThe abundant-centre hypothesis (ACH) assumes that a species becomes more abundant at the centre of its range, where the environmental conditions are most favorable. As we move away from this centre, abundance and occupancy decline. Although this is obvious intuitively, efforts to confirm the hypothesis have often failed. We investigated the abundance patterns of Campanula lingulata across its altitudinal range on Mt. Olympus, Greece, in order to evaluate the “abundant centre” hypothesis along an elevation gradient. Furthermore, we explored the species’ presence and dynamics at multiple spatial scales.MethodsWe recorded flowering individuals during the summer months of 2012 and 2013 along a series of transects defined by paths. We investigated whether the probability of acquiring a larger number of individuals is larger toward the centre of its altitudinal distribution. We also calculated mean presence and turnover at different spatial scales that ranged from quadrats of 10 × 10 m2 to about 10 × 10 km2.ResultsWe were able to identify an abundant centre but only for one of the years of sampling. During the second year, we noted a two-peak abundance pattern; with the first peak occurring at 650–750 m and the second at 1100–1300 m. Variability in the species-presence pattern is observed across a wide range of spatial scales. The pattern along the transect displays fractal characteristics, consistent with a dimension of 0.24–0.29. We found substantial changes of state between the 2 years at all resolutions.ConclusionsOur results do not contradict the ACH, but indicate that ecological distributions exhibit types of variability that make the detection of abundant centres more difficult than expected. When a random fractal disturbance is superimposed upon an abundant centre, we can expect a pattern in which the centre is difficult to discern from a single instance. A multi-resolution or fractal approach to environmental variability is a promising approach for describing this phenomenon.

Demographic structure and genetic variability throughout the distribution of Platte thistle (Cirsium canescens Asteraceae)

AbstractAimUnderstanding spatial variation in the demographic and genetic structure of populations is central to explaining causes of species range limits and to species conservation. The Abundant Centre Hypothesis (ACH) predicts that as one moves away from the centre of a species’ biogeographical range, populations become less frequent and more isolated, as well as exhibiting decreasing within‐population density. This increased isolation may lead to reduced genetic variability in peripheral populations by limiting gene flow. In this study, we asked whether the frequency, within‐population density and genetic diversity of Cirsium canescens (Platte thistle) populations decreased from the range centre to the edge, as predicted by the ACH.LocationCentral United States, including portions of the Great Plains and Rocky Mountains.MethodsFrequency of population occurrence at regional and landscape scales, within‐population density, and within‐population genetic variation were evaluated along eight centre‐edge transects within the species’ distribution. Leaf tissue samples were collected from each population to establish genetic variability using six simple sequence repeat loci.ResultsConsistent with the ACH, peripheral regions of C. canescens’ range were less likely to contain populations than central regions. In regions where C. canescens did occur, however, frequency of populations at a landscape scale peaked at intermediate distances from centre and within‐population density was unrelated to distance. Populations exhibited reduced genetic variability towards range edges.Main conclusionsThe ACH underestimates the complexity of the relationship between variation in abundance and genetic diversity with distance from C. canescens’ range centre. Decreases in Platte thistle population frequency combined with no decreases in within‐population density near range edges suggest that quality habitat exists in the peripheral range, but these patches are rarer. Although genetic variability was reduced at the distribution edges, this decline was stronger towards the western edge, associated with increased topographic complexity.

Climate and local abundance in freshwater fishes.

Identifying factors regulating variation in numbers of individuals among populations across a species' distribution is a fundamental goal in ecology. A common prediction, often referred to as the abundant-centre hypothesis, suggests that abundance is highest near the centre of a species' range. However, because of the primary focus on the geographical position of a population, this framework provides little insight into the environmental factors regulating local abundance. While range-wide variation in population abundance associated with environmental conditions has been investigated in terrestrial species, the relationship between climate and local abundance in freshwater taxa across species' distributions is not well understood. We used GIS-based temperature and precipitation data to determine the relationships between climatic conditions and range-wide variation in local abundance for 19 species of North American freshwater fishes. Climate predicted a portion of the variation in local abundance among populations for 18 species. In addition, the relationship between climatic conditions and local abundance varied among species, which is expected as lineages partition the environment across geographical space. The influence of local habitat quality on species persistence is well documented; however, our results also indicate the importance of climate in regulating population sizes across a species geographical range, even in aquatic taxa.

High genetic diversity declines towards the geographic range periphery of Adonis vernalis, a Eurasian dry grassland plant.

Genetic diversity is important for species' fitness and evolutionary processes but our knowledge on how it varies across a species' distribution range is limited. The abundant centre hypothesis (ACH) predicts that populations become smaller and more isolated towards the geographic range periphery - a pattern that in turn should be associated with decreasing genetic diversity and increasing genetic differentiation. We tested this hypothesis in Adonis vernalis, a dry grassland plant with an extensive Eurasian distribution. Its life-history traits and distribution characteristics suggest a low genetic diversity that decreases and a high genetic differentiation that increases towards the range edge. We analysed AFLP fingerprints in 28 populations along a 4698-km transect from the geographic range core in Russia to the western range periphery in Central and Western Europe. Contrary to our expectation, our analysis revealed high genetic diversity (range of proportion of polymorphic bands = 56-81%, He = 0.168-0.238) and low genetic differentiation across populations (Φ(ST) = 0.18). However, in congruence with the genetic predictions of the ACH, genetic diversity decreased and genetic differentiation increased towards the range periphery. Spanish populations were genetically distinct, suggesting a divergent post-glacial history in this region. The high genetic diversity and low genetic differentiation in the remaining A. vernalis populations is surprising given the species' life-history traits and points to the possibility that the species has been widely distributed in the studied region or that it has migrated from a diverse source in an East-West direction, in the past.

The abundance structure of Azorella selago Hook. f. on sub-Antarctic Marion Island: testing the peak and tail hypothesis

Understanding the spatial distribution of organisms and the factors underlying it are key questions in ecology. Two competing hypotheses exist about the form of spatial variation in abundance. The abundant centre hypothesis suggests that abundance is highest in the centre of a species’ range and declines towards the range margins. By contrast, the peak and tail spatial pattern in abundance posits that several high abundance areas exist across a species range. Here, we test these competing hypotheses by surveying the abundance of the keystone plant species Azorella selago Hook. f. (Apiaceae) across sub-Antarctic Marion Island on a regular spatial grid. We also examine several factors that might explain variation in abundance. Azorella selago occurs between ca. 30 and 850 m above sea level, with sharp discontinuities in abundance at ca. 30 m and at 667 m a.s.l. The survey and analyses revealed a complex abundance structure with patches of high abundance alternating with areas of low abundance or absence, providing support for the peak and tail hypothesis, but with some support for the abundant centre idea too. Variation in abundance was best explained by a model including the negative effects of elevation and of closed vegetation. Our work provides support for the peak and tail pattern of spatial variation in abundance, which has profound importance for understanding the mechanisms underlying the spatial distribution of abundance and other macroecological regularities.

Fragmented host distribution and trombiculid parasitic load: Eutrombicula araucanensis and Liolaemus pictus in Chile

Habitat fragmentation produced by human impacts can generate changes in the distribution and abundance of populations across their geographic ranges. The chigger mites are ectoparasites that are widely distributed on a wide range of species. The distribution depends of the characteristics of the host and of the habitat where said species is found. As the host distribution is highly dependent on environmental factors, chigger mites are good study models to test geographical distribution patterns. The main objective of this study was to determine variation in abundance of Eutrombicula araucanensis, parasite on Liolaemus pictus, by testing three hypotheses: abundant centre hypothesis, abundance optimum hypothesis and latitudinal distribution pattern. For this purpose, mites from 147 individuals from 23 localities along the distribution of L. pictus were extracted. The three distribution models evaluated in the present study were not adequate to represent the geographical structure of parasitic mites on L. pictus. No association between the distance from the area of greatest species abundance and relative intensity, or association with the distance to the mite’s distribution centre was observed. In addition, no latitudinal distribution pattern was observed. Temperate forest fragmentation may influence the population density of mites, with each forest patch having different optimal conditions for development. The heterogeneity makes it difficult to find a clear distribution pattern.

Effects of marginality on plant population performance

AbstractAimPopulations at the edge of a species' distribution range may differ substantially from central populations. Peripheral populations may have either a high evolutionary potential or be prone to extinction, but the processes driving these outcomes are still unclear. Peripheral plant populations have been the subject of numerous studies and reviews, with many focusing on their genetic characteristics. In this review, we consider the effect of marginality on demographic species‐specific traits.LocationWorld‐wide.MethodsWe reviewed the literature based on direct comparisons between central and peripheral plant populations. Strict inclusion criteria were applied to avoid biased analysis that may arise as a result of inaccurate boundary considerations or inappropriate comparisons. We inferred from the published data whether a certain trait had a better performance in central or peripheral populations (reliability of the abundant centre hypothesis,ACH).ResultsThere have not been enough studies on plant performance to allow for generalizations on the effects of marginality on plants.ACHexpectations were not met in most cases and specific responses to marginality were observed at the species and population levels. Population and plant size more often met theACHassumptions, suggesting that most geographically peripheral populations are also ecologically marginal. The availability of resources, the reproductive strategy, the level of ploidy and the ability to cope with interspecific competitors seem to drive the numerous exceptions to theACHexpectations.Main conclusionsThe large numbers of exceptions to theACHexpectations suggest that a new comprehensive theory is needed to explain the effects of marginality in plants and to identify any general patterns. From the theoretical point of view, we propose that population history and dynamics should be considered when attempting to explain the processes that occur in peripheral plant populations.

Demographic and population‐genetic tests provide mixed support for the abundant centre hypothesis in the endemic plant Leavenworthia stylosa

The abundant centre hypothesis (ACH) assumes that population abundance, population size, density and per-capita reproductive output should peak at the centre of a species' geographic range and decline towards the periphery. Increased isolation among and decreased reproductive output within edge populations should reduce within-population genetic diversity and increase genetic differentiation among edge relative to central populations. The ACH also predicts asymmetrical gene flow, with net movement of migrants from the centre to edges. We evaluated these ecological assumptions and population-genetic predictions in the endemic flowering plant Leavenworthia stylosa. Although populations were more spatially isolated near range edges, the geographic centre was surrounded by and not coincident with areas of peak population abundance, and plant density increased towards range edges. Per-capita seed number was not associated with distance to the range centre, but seed number/m(2) increased near range edges. In support of ACH predictions, allelic diversity at 12 microsatellite loci declined with distance from the range centre, and pairwise FST values were higher between edge populations than between central populations. Coalescent analyses confirmed that gene flow was most infrequent between edge populations, but there was not an asymmetric pattern of gene flow predicted by the ACH. This study shows that among-population demographic variability largely did not support the ACH, while patterns of genetic diversity, differentiation and gene flow were generally consistent with its predictions. Such mixed support has frequently been observed in tests of the ACH and raises concerns regarding the generality of this hypothesis for species range limits.

Environmental Domains and Range-Limiting Mechanisms: Testing the Abundant Centre Hypothesis Using Southern African Sandhoppers

Predicting shifts of species geographical ranges is a fundamental challenge for conservation ecologists given the great complexity of factors involved in setting range limits. Distributional patterns are frequently modelled to “simplify” species responses to the environment, yet the central mechanisms that drive a particular pattern are rarely understood. We evaluated the distributions of two sandhopper species (Crustacea, Amphipoda, Talitridae), Talorchestia capensis and Africorchestia quadrispinosa along the Namibian and South African coasts, encompassing three biogeographic regions influenced by two different oceanographic systems, the Benguela and Agulhas currents. We aimed to test whether the Abundant Centre Hypothesis (ACH) can explain the distributions of these species’ abundances, sizes and sex ratios and examined which environmental parameters influence/drive these distributions. Animals were collected during a once-off survey at 29 sites over c.3500 km of coastline. The ACH was tested using a non-parametric constraint space analysis of the goodness of fit of five hypothetical models. Distance Based Linear Modelling (DistLM) was performed to evaluate which environmental traits influenced the distribution data. Abundance, size and sex ratio showed different patterns of distribution. A ramped model fitted the abundance (Ramped North) and size (Ramped South) distribution for A. quadrispinosa. The Inverse Quadratic model fitted the size distribution of T. capensis. Beach slope, salinity, sand temperature and percentage of detritus found on the shore at the time of collection played important roles in driving the abundance of A. quadrispinosa. T. capensis was mainly affected by salinity and the morphodynamic state of the beach. Our results provided only some support for the ACH predictions. The DistLM confirmed that the physical state of the beach is an important factor for sandy beach organisms. The effect of salinity and temperature suggest metabolic responses to local conditions and a role in small to mesoscale shifts in the range of these populations.

Range limits and geographic patterns of abundance of the rocky intertidal owl limpet, Lottia gigantea

Aim? We evaluate the stability of the range limits of the rocky intertidal limpet, Lottia gigantea, over the last c. 140 years, test the validity of the abundant centre hypothesis, and test indirectly the roles played by recruitment limitation and habitat availability in controlling the range limits. Because this species is size-selectively harvested, our results also allow us to assess conservation implications. Location? The Pacific coast of North America, from northern California to southern Baja California (41.74° N–23.37° N), encompassing the entire range of L. gigantea. Methods? The historical and modern distributions of L. gigantea were established using museum data and field observations. Overall and juvenile abundances of local populations were estimated at 25 field sites. The spatial distribution of abundance was evaluated statistically against the predictions of five hypothetical models. The availability of habitat was estimated by measuring the percentage of unavailable sandy beach within cumulative bins of coast across the range of L. gigantea. Results? The northern limit of L. gigantea has contracted by c. 2.4° of latitude over recent decades (after 1963), while the southern limit has remained stable. The highest abundances of L. gigantea occur in the centre of its geographic range. Habitat availability is ample in the centre and northern portions of its range, but is generally lacking in the southern range. The northern range is only sparsely populated by adults, with sharp declines occurring north of Monterey Bay (36.80° N). In the southern range, abundance drops precipitously south of Punta Eugenia (27.82° N), coinciding with the region where suitable habitat becomes sparse. Main conclusions? Support for the abundant centre hypothesis was found for L. gigantea. Northern populations are characterized as being recruitment-limited, demographically unstable and prone to local extinctions, while southern populations are suggested to be habitat-limited. The abundant centre is suggested to result partly from a combination of the indirect effects of human harvesting, generating denser populations of smaller individuals, and larval recruitment from well-protected offshore rocky islands primarily found in the range centre.

Body size clines in the European badger and the abundant centre hypothesis

Aim To test the abundant centre hypothesis by analysing the physical and climatic factors that influence body size variation in the European badger (Meles meles). Location Data were compiled from 35 locations across Europe. Methods We used body mass, body length and condylo-basal length (CBL) as surrogates of size. We also compiled data on latitude, several climatic variables, habitat type and site position relative to the range edge. We collapsed all continuous climatic variables into independent vectors using principal components analysis (PCA), and used a general linear model to explain the morphometric variation in badger populations across the species’ range. Results Body mass and body length were nonlinearly and significantly related to latitude. In contrast, CBL was linearly related to latitude. Body mass changed nonlinearly along the temperature (PC1) gradient, with the highest values observed at mid-range. Furthermore, body mass, body length and CBL differed significantly among habitats, with badgers showing larger size in temperate habitats and core areas relative to peripheral zones. Main conclusions Our analysis supports the nonlinear pattern predicted by the abundant centre hypothesis only for body mass and body length. These results imply that individuals are largest and heaviest at the centre of the climatic range of badger distribution. Variation of CBL with latitude follows a linear trend, consistent with Bergmann’s rule. Our results provide mixed support for the abundant centre hypothesis, and suggest food availability/quality to be the main mechanism underlying body size clines in this species.