Larval dispersal dampens population fluctuation and shapes the interspecific spatial distribution patterns of rocky intertidal gastropods

Many marine benthic invertebrates pass through a planktonic larval stage whereas others spend their entire lifetimes in benthic habitats. Recent studies indicate that non‐planktonic species show relatively greater fine‐scale patchiness than do planktonic species, but the underlying mechanisms remain unknown. One hypothesis for such a difference is that larval dispersal enhances the connectivity of populations and buffers population fluctuations and reduces local extinction risk, consequently increasing patch occupancy rate and decreasing spatial patchiness. If this mechanism does indeed play a significant role, then the distribution of non‐planktonic species should be more aggregated – both temporally and spatially – than the distribution of species with a planktonic larval stage. To test this prediction, we compared 1) both the spatial and the temporal abundance–occupancy relationships and 2) both the spatial and the temporal mean–variance relationships of population size across species of rocky intertidal gastropods with differing dispersive traits from the Pacific coast of Japan. We found that, compared to planktonic species, non‐planktonic species exhibited 1) a smaller occupancy rate for any given level of mean population size and 2) greater variations in population size, both spatially and temporally. This suggests that the macroecological patterns observed in this study (i.e. the abundance–occupancy relationships and mean–variance relationships of population size across species) were shaped by the effect of larval dispersal dampening population fluctuation, which works over both space and time. While it has been widely assumed that larval dispersal enhances population fluctuations, larval dispersal may in fact enhance the connectively of populations and buffer population fluctuations and reduce local extinction risks.     

Hierarchical scales of spatial variation in the smaller surface and near-surface macrobenthos of a subtropical intertidal seagrass system in Moreton Bay,Queensland

Core samples were taken along a 4 km stretch of intertidal seagrass on North Stradbroke Island, eastern Australia, at nested scales of 1 m (stations), 150 m (sites), and 2 km (localities) to investigate the extent to which abundance, diversity, and assemblage composition of the dominant smaller members (<10 mm) of the intertidal seagrass macrobenthos vary spatially and over what scales. Gastropods and polychaetes dominated both the 91 species present and, together with decapods, also the numbers of individuals. Abundance was low (mean < 2000 individuals m⁻²) but species diversity was high (overall Simpson’s index of diversity 0.91), with 44% of species occurring only as one or two individuals, and with only two species contributing >10% to the total numbers (the microgastropod Calopia imitata and crab Enigmaplax littoralis, both little known, rarely recorded endemics). On average, a species only occurred at 6% of stations and only four occurred at >25%. Assemblages at the three localities did not vary significantly in gross ecological features (levels of species richness, faunal abundance and species diversity per component site) (ANOVA P ≫ 0.05), but did vary markedly in their composition at all spatial scales (PERMANOVA P < 0.05). Variance partitioning showed that components of total variance were least at the largest spatial scale (locality 15.9%) and greatest at the smallest scale (station 59.3%). The commoner individual species all showed random distributions at small spatial scales but clumped distributions at large spatial scales.     

Self-similar properties of the spatial structure of intertidal macro- and microbenthic communities

Spatial distribution (SD) of White Sea intertidal soft-bottom communities was studied at scales from decimetres to dozens of kilometres on the basis of an extensive dataset (464 samples of macrofauna, 349 samples of ciliates, and 333 samples of diatoms). We used the information index of structural heterogeneity D(I) (Azovsky et al., 2000 // Mar. Biol. 136 (3): 581-590) to characterize spatial variability in the species composition of the communities at different extent (total area surveyed) and grain (finest spatial resolution). The type of distribution was determined via the relation between D(I) and parameters of the spatial scale (extent and grain). At small scale (in terms of extent), all the communities were distributed randomly (mosaic SD). At larger scales, the estimated spatial variability depended neither on extent nor grain, exclusively on their ratio, i.e., was scale-invariant. This means that at some scale the spatial patterns of communities display self-similar properties (fractal SD). Such SD was found at a rather wide range of scales scales: 10(1)-10(4) m for the macrofauna, 10(0)-10(3) m for the ciliates, and 10(-1)-10(2) m for the diatoms. At still greater scales, patchy or gradient patters were observed. Thus, the ranges of fractal distribution were proportional to the average size of the organisms (approximately 10(4)-10(7) times the body size). We suppose that such spatial pattern reflects community self-organization in a relatively homogeneous environment and may be the most efficient way to realize the highest structural diversity on the basis of pre-formed complexes of predominant species. We also suppose that fractal-like patterns may be a general feature of the spatial organization of communities.     

Spatial variability of benthic-pelagic coupling in an estuary ecosystem: consequences for microphytobenthos resuspension phenomenon

The high degree of physical factors in intertidal estuarine ecosystem increases material processing between benthic and pelagic compartments. In these ecosystems, microphytobenthos resuspension is a major phenomenon since its contribution to higher trophic levels can be highly significant. Understanding the sediment and associated microphytobenthos resuspension and its fate in the water column is indispensable for measuring the food available to benthic and pelagic food webs. To identify and hierarchize the physical/biological factors potentially involved in MPB resuspension, the entire intertidal area and surrounding water column of an estuarine ecosystem, the Bay des Veys, was sampled during ebb tide. A wide range of physical parameters (hydrodynamic regime, grain size of the sediment, and suspended matter) and biological parameters (flora and fauna assemblages, chlorophyll) were analyzed to characterize benthic-pelagic coupling at the bay scale. Samples were collected in two contrasted periods, spring and late summer, to assess the impact of forcing variables on benthic-pelagic coupling. A mapping approach using kriging interpolation enabled us to overlay benthic and pelagic maps of physical and biological variables, for both hydrological conditions and trophic indicators. Pelagic Chl a concentration was the best predictor explaining the suspension-feeders spatial distribution. Our results also suggest a perennial spatio-temporal structure of both benthic and pelagic compartments in the ecosystem, at least when the system is not imposed to intense wind, with MPB distribution controlled by both grain size and bathymetry. The benthic component appeared to control the pelagic one via resuspension phenomena at the scale of the bay. Co-inertia analysis showed closer benthic-pelagic coupling between the variables in spring. The higher MPB biomass observed in summer suggests a higher contribution to filter-feeders diets, indicating a higher resuspension effect in summer than in spring, in turn suggesting an important role of macrofauna bioturbation and filter feeding (Cerastoderma edule).     

The Influence of Dry-season Conditions on the Bottom Dwelling Fauna of an East Australian Sub-tropical Estuary

Spatial patterns in benthic infaunal community structure of Port Curtis estuary (north-eastern Australia) were determined from quantitative grab samples and examined in relation to environmental variables. A total of 149 riverine, estuarine and open coastal stations were sampled during the winter survey, and 5744 individuals from 466 species identified. Filter-feeding organisms (primarily polychaetes, molluscs and crustaceans) dominated the benthos, and accounted for 50% of the total species abundance and 30% of the total species richness. Most taxa were uncommon, and 98% of species individually represented less than 2% of the total abundance. Distributional patterns in total species richness and abundance were highly correlated, and both parameters varied significantly with sediment grain size. Numbers of species and individuals were typically highest in coarse-sand and gravel sediments, and were lowest in fine, well-sorted, sands. Cluster analysis of species abundance data revealed nine community groupings characterised by small species sub-sets with restricted distributions. These groupings were primarily related to sediment grain size, sediment organic carbon content, salinity and depth. Not all variation in community structure was explained by these environmental variables, and it is likely that other unmeasured factors play an important role in determining benthic faunal composition in the estuary.     

生态学中的尺度问题：内涵与分析方法

尺度问题已成为现代生态学的核心问题之一.尺度问题主要涉及3个方面：尺度概念、尺度分析和尺度推绎.主要评述前两个方面.生态学尺度有三重概念：维数、种类和组分,其中每重概念又包含了多个定义,有必要进行澄清、分类和统一.尺度分析涉及尺度效应分析和多尺度空间格局分析.格局、过程及它们之间的关系,以及某些景观特性均表现出尺度效应,因此多尺度研究非常必要和重要.多尺度空间格局分析（尤其是特征尺度的识别）是进行尺度效应分析和跨尺度推绎的基础.多尺度分析需要特定的方法,景观指数法是最常用和最简单的方法,但也常产生误导;空间统计学方法（如半方差分析法、尺度方差分析法、空隙度指数法和小波分析法等）和分维分析法在最近十几年发展起来,并逐渐应用于生态学,在尺度分析上具有很大的应用潜力.各种方法在尺度分析上各有优势和不足,有必要同时使用两种或两种以上方法进行比较和评估.总之,有关尺度分析的研究需要进一步加强,从而为下一步的尺度推绎提供可靠的依据.     

Scale‐dependent determinants of plant species richness in a semi‐arid fragmented agro‐ecosystem

Aims: (1) Understanding how the relationship between species richness and its determinants depends on the interaction between scales at which the response and explanatory variables are measured. (2) Quantifying the relative contributions of local, intermediate and large‐scale determinants of species richness in a fragmented agro‐ecosystem. (3) Testing the hypothesis that the relative contribution of these determinants varies with the grain size at which species richness is measured. Location: A fragmented agro‐ecosystem in the Southern Judea Lowland, Israel, within a desert–Mediterranean transition zone. Methods: Plant species richness was estimated using hierarchical nested sampling in 81 plots, positioned in 38 natural vegetation patches within an agricultural matrix (mainly wheat fields) among three land units along a sharp precipitation gradient. Explanatory variables included position along that gradient, patch area, patch isolation, habitat heterogeneity and overall plant density. We used general linear models and hierarchical partitioning of variance to test and quantify the effect of each explanatory variable on species richness at four grain sizes (0.0625, 1, 25 and 225 m²). Results: Species richness was mainly affected by position along a precipitation gradient and overall plant density, and to a lesser extent by habitat heterogeneity. It was also significantly affected by patch area and patch isolation, but only for small grain sizes. The contribution of each explanatory variable to explained variance in species richness varied with grain size, i.e. scale‐dependent. The influence of geographic position and habitat heterogeneity on species richness increased with grain size, while the influence of plant density decreased with grain size. Main conclusions: Species richness is determined by the combined effect of several scale‐dependent determinants. Ability to detect an effect and effect size of each determinant varies with the scale (grain size) at which it is measured. The combination of a multi‐factorial approach and multi‐scale sampling reveals that conclusions drawn from studies that ignore these dimensions are restricted and potentially misleading.     

A balanced view of scale in spatial statistical analysis

Concepts of spatial scale, such as extent, grain, resolution, range, footprint, support and cartographic ratio are not interchangeable. Because of the potential confusion among the definitions of these terms, we suggest that authors avoid the term "scale" and instead refer to specific concepts. In particular, we are careful to discriminate between observation scales, scales of ecological phenomena and scales used in spatial statistical analysis . When scales of observation or analysis change, that is, when the unit size, shape, spacing or extent are altered, statistical results are expected to change. The kinds of results that may change include estimates of the population mean and variance, the strength and character of spatial autocorrelation and spatial anisotropy, patch and gap sizes and multivariate relationships. The first three of these results (precision of the mean, variance and spatial autocorrelation) can sometimes be estimated using geostatistical support-effect models. We present four case studies of organism abundance and cover illustrating some of these changes and how conclusions about ecological phenomena (process and structure) may be affected. We identify the influence of observational scale on statistical results as a subset of what geographers call the Modifiable Area Unit Problem (MAUP). The way to avoid the MAUP is by careful construction of sampling design and analysis. We recommend a set of considerations for sampling design to allow useful tests for specific scales of a phenomenon under study. We further recommend that ecological studies completely report all components of observation and analysis scales to increase the possibility of cross-study comparisons.     

空间尺度对地形异质性-物种多样性关系的影响:粒度和幅度

空间尺度是影响我们理解生态学格局和过程的关键因素.目前已有多种关于物种多样性分布格局形成机制的假说且研究者未达成共识,原因之一是空间尺度对物种多样性分布格局的环境影响因子的解释力和相对重要性有重要影响.地形异质性是物种多样性分布格局的重要影响因素.本文综述了在地形异质性-物种多样性关系的研究中,不同空间粒度和幅度对研究...     

Inconsistency in short-term temporal variability of microgastropods within and between two different intertidal habitats

Small-scale temporal variation in abundances of fauna in marine soft sediments has long been recognised. Many studies on rocky intertidal shores have, however, focused on larger fauna in single habitats and have primarily examined relatively long time-scales. The implications of small-scale variability are frequently not adequately addressed in the studies of changes in fauna over longer time-scales. Without knowledge of the magnitude of variation at smaller scales, comparisons across longer time-scales may be confounded. In this study, the temporal variability of a number of co-existing species of microgastropods in patches of two different intertidal habitats (coralline turf and sediment) in Botany Bay, New South Wales, Australia, was measured using a nested, hierarchical sampling design incorporating temporal scales of weeks, 1 and 3 months. In addition to habitats, there were also spatial scales of metres between plots and 100s of metres between the locations. There was generally a lack of consistency in the trends of variance for the three temporal scales at the smallest spatial scale of plots. In addition, the different species, including those that were closely related, showed different patterns of variation, depending on the habitat and site. These data show the importance of incorporating adequate scales of sampling in different habitats when analysing the distribution and abundance of microbenthos in intertidal habitats.     

Distribution, abundance and niche breadth of birds: scale matters

We used local habitat niche breadth, local abundance and body size of non-passerine afrotropical birds in Tsavo East National Park (Kenya) to predict species distributional ranges in Kenya and across Africa. Univariate analysis revealed a significant positive correlation between local abundance and distribution only on the scale of Kenya. Performing a multiple regression analysis, local abundance, local habitat niche breadth and body size explained a significant part of the variance in bird distribution, again only on the Kenyan scale. From these results, we speculate that on continental scales distributions may be more influenced by macroclimatic conditions and historical factors, whereas distributions on regional scales are predominantly influenced by ecological factors.     

Functional diversity and species turnover of benthic invertebrates along a local environmental gradient induced by an aquaculture unit: the contribution of species dispersal ability and rarity

The relation of macrobenthic species turnover (beta diversity) and species plylogenetic variation with functional diversity patterns, across an environmental gradient induced by an aquaculture unit, in a coastal area of the island of Lesvos (NE Aegean) has been investigated in this study. The contribution of rare species response and species dispersal ability in the variation of functional diversity patterns along the environmental gradient, on a spatio-temporal scale, has been also examined. Our results revealed that benthic functional diversity was decreasing monotonically with increasing species turnover rate and hence with increasing spatial variability along the environmental gradient. Increased environmental stress which was detected in the immediate vicinity of the fish cages resulted to low species functional redundancy, since different species didn’t perform the same functional role at the most disturbed part of the established gradient. Functional diversity patterns were found to be correlated with species population size, whereas a strong linear relationship was also detected with phylogenetic diversity patterns, thus supporting the claim that wider local taxonomic trees can support a wider range of species functions even in small spatial scales. Rare species loss seemed to be one of the dominant factors ruling functional diversity variation. Species with the minimum possible dispersal ability, which were mostly rare, tend to diminish both in species number and population size faster than species with wider dispersal ability towards the most disturbed areas. The aforementioned results indicate that rare species variation and endemic species loss are critical factors in determining functional diversity loss across a human-induced environmental gradient in soft bottom benthic communities.     

The role of spatial scale and the perception of large-scale species-richness patterns

Despite two centuries of exploration, our understanding of factors determining the distribution of life on Earth is in many ways still in its infancy. Much of the disagreement about governing processes of variation in species richness may be the result of differences in our perception of species‐richness patterns. Until recently, most studies of large‐scale species‐richness patterns assumed implicitly that patterns and mechanisms were scale invariant. Illustrated with examples and a quantitative analysis of published data on altitudinal gradients of species richness (n = 204), this review discusses how scale effects (extent and grain size) can influence our perception of patterns and processes. For example, a hump‐shaped altitudinal species‐richness pattern is the most typical (c. 50%), with a monotonic decreasing pattern (c. 25%) also frequently reported, but the relative distribution of patterns changes readily with spatial grain and extent. If we are to attribute relative impact to various factors influencing species richness and distribution and to decide at which point along a spatial and temporal continuum they act, we should not ask only how results vary as a function of scale but also search for consistent patterns in these scale effects. The review concludes with suggestions of potential routes for future analytical exploration of species‐richness patterns.     

The spatial scale of variability in small-mammal populations

We studied small-mammal populations across a range of spatial scales to determine which scales were relevant to demographic variability. We predicted that a scale of variability in population structure would occur at the scale of dispersal, which has previously been described as < 200 m for some small-mammal species. Systematic live-trapping surveys were conducted on nested grids at three scales: 1) extent = 4900 ha. grain =1000 m; 2) extent = 306 ha, grain = 250 m: and 3) extent = 31 ha. grain = 125 m. Prior to the present study, small-mammal populations had not been systematically live-trapped across a similar range of scales. From 1996 to 1998, surveys were conducted on a landscape intensively managed for timber, and on a reference landscape. Spatial analysis of abundance data demonstrated that small-mammal populations (four species) exhibited positive spatial autocorrelation at distances of between 133 and 533 m depending on the species and the landscape. No higher-order population structure was detected. Thus, within the range of scales we sampled, variability in small-mammal abundance occurred over short distances (i.e. 133–533 m).     

Spatial Patterns of Benthic Macroinvertebrates in Intertidal Areas of a Southern European Estuary: The Tagus, Portugal

This study characterizes the composition and spatial distribution patterns of the benthic macrofauna in the intertidal mudflats of the Tagus estuary, western Portugal. A total of 68 species, more than 226,000 specimens with a total wet weight biomass of approximately 1170 g were identified in 380 sites. The species Streblospio shrubsolii, Cyathura carinata, Tharyx sp., Hydrobia ulvae and Tubificids were the most common and abundant. Scrobicularia plana strongly dominated the biomass. The invertebrate macrofauna of the Tagus estuary shows similarities to what is known from other temperate mudflats. The diversity of species, their overall abundance and the ratio of Molluscs plus Crustaceans to Polychaete species corroborate the distinctiveness between temperate and tropical mudflats and sandflats. The spatial distribution of the fauna reflects the sediment characteristics but the relationship between the environmental and the biological data is not as strong as obtained for sublittoral areas. This relationship diminishes from the sublittoral shelf to sublittoral estuarine areas, showing minimum values in this study, suggesting that such a relationship is less straightforward as natural disturbance increases. Nevertheless, a mixture of grain-size, elevation (inundation time) and particular habitats (relic oyster beds) form the best explanatory factors for the spatial distribution patterns of the intertidal benthic macrofauna of the Tagus estuary.     

Biogeographical patterns of rocky intertidal communities along the Pacific coast of North America

Aim Our aim in this paper is to present the first broad‐scale quantification of species abundance for rocky intertidal communities along the Pacific coast of North America. Here we examine the community‐level marine biogeographical patterns in the context of formerly described biogeographical regions, and we evaluate the combined effects of geographical distance and environmental conditions on patterns of species similarity across this region. Location Pacific coast of North America. Methods Data on the percentage cover of benthic marine organisms were collected at 67 rocky intertidal sites from south‐eastern Alaska, USA, to central Baja California Sur, Mexico. Cluster analysis and non‐metric multidimensional scaling were used to evaluate the spatial patterns of species similarity among sites relative to those of previously defined biogeographical regions. Matrices of similarity in species composition among all sites were computed and analysed with respect to geographical distance and long‐term mean sea surface temperature (SST) as a measure of environmental conditions. Results We found a high degree of spatial structure in the similarity of intertidal communities along the coast. Cluster analysis identified 13 major community structure ‘groups’. Although breaks between clusters of sites generally occurred at major biogeographical boundaries, some of the larger biogeographical regions contained several clusters of sites that did not group according to spatial position or identifiable coastal features. Additionally, there were several outliers – sites that grouped alone or with sites outside their region – for which localized features may play an important role in driving community structure. Patterns of species similarity at the large scale were highly correlated with geographical distance among sites and with SST. Importantly, we found community similarity to be highly correlated with long‐term mean SST while controlling for the effects of geographical distance. Main conclusions These findings reveal a high degree of spatial structure in the similarity of rocky intertidal communities of the north‐east Pacific, and are generally consistent with those of previously described biogeographical regions, with some notable differences. Breaks in similarity among clusters are generally coincident with known biogeographical and oceanographic discontinuities. The strong correlations between species similarity and both geographical position and SST suggest that both geography and oceanographic conditions have a large influence on patterns of intertidal community structure along the Pacific coast of North America.     

Spatial variation of reef fishes and the relative influence of biotic and abiotic habitat traits

Patterns of distribution of reef fishes were examined across three spatial scales and related to habitat traits along 25 km of the northern Portuguese coast. Response variables included the multivariate assemblage structure, the total number of taxa and individuals, and the abundance of single groups categorized according to their preference for the benthic, proximo-benthic or pelagic environment, feeding and reproductive behaviour. Habitat traits included topographic elements (small and large ‘drops’ like cracks and crevices) and the extent of dominant morpho-functional types of macroalgae (kelp, large foliose, small erect, turf-forming filamentous, and encrusting). All fish responses were characterized by the largest variance at the smallest scale (among transects tens m apart), followed by that among reefs (hundreds m to 1 km apart) and almost null variance among sites (some km apart). Small and large ‘drops’ of the substratum explained, respectively, considerable variation of assemblage structure and the total abundance of individuals, while the extent of bare rock influenced the richness of taxa and that of benthic fishes, fishes feeding on sessile invertebrates and fishes laying benthic eggs or having nesting behaviour. Combinations of abiotic and biotic structural attributes of reefs influenced proximo-benthic fishes, the predators of mobile animals and fishes releasing pelagic eggs. The here reported associations between patterns of distribution of reef fishes and habitat traits have implications for the design of future protection schemes suitable to guarantee the conservation of reef fish communities and of the processes responsible for their variation. Within the SLOSS (single-large vs. several-small) debate in the design of marine reserves, for example, effective protection to the studied reef fishes would be provided by a set of small reserves, rather than a single large which might be appropriate for fishes having wider home ranges.     

Scales of spatial patterns of distribution of intertidal invertebrates

Few comparative studies of spatial patterns at different scales have examined several species in the same habitat or the same species over a range of habitats. Therefore, variability in patterns among species or among habitats has seldom been documented. This study quantifies spatial patterns of a suite of intertidal snails and a species of barnacle using a range of statistical techniques. Variability in densities was quantified from the scale of adjacent quadrats (over a distance of centimeters) to tens of kilometers. Significant differences in abundances occurred primarily at two spatial scales. Small-scale differences were found at the scales of centimeters or 1–2 m and, for many species on many shores, these accounted for most of the variability in abundances from place to place. These are likely to be determined by behavioural responses to small-scale patches of microhabitat. Large-scale differences in abundance were also found in most species at the scale of hundreds of meters alongshore. These are likely to be due to variation in recruitment (and/or mortality) because of limited dispersal by adults of these species. There was little or no additional variation among shores, separated by tens of kilometers, than was shown among patches of shore separated by hundreds of meters. Identification of the scale(s) at which significant differences in abundance are found focus attention on the processes (and the scales at which these processes operate) that influence patterns of distribution and abundance. Some of the advantages and disadvantages of various procedures are discussed.     

Linking small desert mammal distribution to habitat structure in a protected and grazed landscape of the Monte, Argentina

Interpretation of landscape patterns from the perspective of different species allows knowing the way in which they perceive landscape, and how their perception varies with scale. We examined distribution of four small mammal species at different scales over a landscape including protected and grazed areas, and associated species distribution with landscape structure. The study was conducted in the central Monte Desert (Reserve of Ñacuñán). Trap grids were set in both areas at two scales, varying their grain and extent. To determine whether spatial patterns are random, clumped or regular, we used a point pattern analysis. Logistic regressions were performed to relate the presence-absence of small mammals to environmental variables. Intensity of the point pattern was not constant, either in the Reserve or the grazed area. Small mammal abundance exhibited a heterogeneous distribution, and the existence of a first-order effect was detected for all species. No second-order effects were detected, the point pattern was random for all species in both areas. Both areas were differently perceived by rodent species. Habitat structure in both conditions and its variations with scale appear to be important factors affecting distribution patterns.