Context‐dependent expression of sexual dimorphism in island populations of the common wall lizard (Podarcis muralis)

The condition‐dependent sexual dimorphism model explains the evolution and maintenance of sexual dimorphism in traits targeted by sexual selection, and predicts that the magnitude of sexual dimorphism depends on the variability of individual condition, male traits being more variable than female corresponding traits. Most convincing examples concern insects, while studies among vertebrates are scanty because manipulating condition often is not possible, and the time to reach sexual maturity may be too long. Islands offer a unique opportunity to compare how the environment affects the expression of sexual dimorphism, since they represent ‘natural experimental sets’ in which different populations of the same species may experience alternative environmental constraints. We investigated the occurrence of context‐dependent expression in sexual dimorphism of head shape in insular populations of the common wall lizards (Podarcis muralis) inhabiting the Tuscan Archipelago (Tyrrhenian Sea). Alternative models were formulated: H₀ assumes that the sexual dimorphism is uninfluenced by islands, H₁ assumes the only effect of phylogeny, H_2A and H_2B account for the biogeography of the archipelago (island size and distance from the mainland), while H₃ assumes island‐specific effects on sexual dimorphism. Models were compared using Akaike's information criterion adjusted for multivariate analyses. All hypotheses performed better than H₀, but H₃ largely outperformed all other alternative hypotheses, indicating that environmental features of islands play an additive effect to ontogenetic, biogeographic and genetic factors in defining variation in head shape sexual dimorphism. Our results support the hypothesis of a context‐dependent sexual dimorphism in common wall lizards. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 552–565.     

Islands within islands: two montane palaeo‐endemic birds impacted by recent anthropogenic fragmentation

Anthropogenic habitat fragmentation of species that live in naturally patchy metapopulations such as mountaintops or sky islands experiences two levels of patchiness. Effects of such multilevel patchiness on species have rarely been examined. Metapopulation theory suggests that patchy habitats could have varied impacts on persistence, dependent on differential migration. It is not known whether montane endemic species, evolutionarily adapted to natural patchiness, are able to disperse between anthropogenic fragments at similar spatial scales as natural patches. We investigated historic and contemporary gene flow between natural and anthropogenic patches across the distribution range of a Western Ghats sky‐island‐endemic bird species complex. Data from 14 microsatellites for 218 individuals detected major genetic structuring by deep valleys, including one hitherto undescribed barrier. As expected, we found strong effects of historic genetic differentiation across natural patches, but not across anthropogenic fragments. Contrastingly, contemporary differentiation (D_PS) was higher relative to historic differentiation (F_ST) in anthropogenic fragments, despite the species’ ability to historically traverse shallow valleys. Simulations of recent isolation resulted in high D_PS/F_ST values, confirming recent isolation in Western Ghats anthropogenic fragments and also suggesting that this ratio can be used to identifying recent fragmentation in the context of historic connectedness. We suggest that in this landscape, in addition to natural patchiness affecting population connectivity, anthropogenic fragmentation additionally impacts connectivity, making anthropogenic fragments akin to islands within natural islands of montane habitat, a pattern that may be recovered in other sky‐island systems.     

Scale-dependent relationships between the spatial distribution of a limiting resource and plant species diversity in an African grassland ecosystem

One cornerstone of ecological theory is that nutrient availability limits the number of species that can inhabit a community. However, the relationship between the spatial distribution of limiting nutrients and species diversity is not well established because there is no single scale appropriate for measuring variation in resource distribution. Instead, the correct scale for analyzing resource variation depends on the range of species sizes within the community. To quantify the relationship between nutrient distribution and plant species diversity, we measured NO₃^- distribution and plant species diversity in 16 paired, modified Whittaker grassland plots in Serengeti National Park, Tanzania. Semivariograms were used to quantify the spatial structure of NO₃^- from scales of 0.4–26 m. Plant species diversity (Shannon-Weiner diversity index; H ) was quantified in 1-m² plots, while plant species richness was measured at multiple spatial scales between 1 and 1,000 m². Small-scale variation in NO₃^- (<0.4 m) was positively correlated with 1-m² H , while 1,000-m² species richness was a log-normal function of average NO₃^- patch size. Nine of the 16 grassland plots had a fractal (self-similar across scales) NO₃^- spatial distribution; of the nine fractal plots, five were adjacent to plots that had a non-fractal distribution of NO₃^-. This finding offered the unique opportunity to test predictions of Ritchie and Olff (1999): when the spatial distribution of limiting resources is fractal, communities should display a left-skewed log-size distribution and a log-normal relationship between net primary production and species richness. These predictions were supported by comparisons of plant size distributions and biomass-richness relationships in paired plots, one with a fractal and one with a non-fractal distribution of NO₃^-. In addition, fractal plots had greater large-scale richness than paired non-fractal plots (1,0–1000 m²), but neither species diversity (H ) nor richness was significantly different at small scales (1 m²). This result is most likely explained by differences in the scale of resource variation among plots: fractal and non-fractal plots had equivalent NO₃^- variation at small scales but differed in NO₃^- variation at large scales (as measured by the fractal dimension). We propose that small-scale variation in NO₃^- is largely due to the direct effects of plants on soil, while patterns of species richness at large scales is controlled by the patch size and fractal dimension of NO₃^- in the landscape. This study provides an important empirical step in understanding the relationship between the spatial distribution of resources and patterns of species diversity across multiple spatial scales.     

Predicting probability of occurrence and factors affecting distribution and abundance of three Ozark endemic crayfish species at multiple spatial scales

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Spatial scale dependence of the species diversityin Tianmu Mountain and its relationship with spatial patterns by using multifractal analysis

This study took the Tianmu Mountain National Natural Preservation Area in western Zhejiang Province as an example 1) to quantify the species diversity with selected species indices including Shannon-Wiener index (H), Margalef index (K) and Evenness index (E) at different spatial scales, 2) to analyze the spatial distribution patterns of the species diversity by multifractal parameters such as the singularity index α, its fractal dimension f(α), the f(α)–α spectrum range (SR) and its symmetry (Dist) using the multifractal theory, and 3) to determine their relationships. Results of nonlinear regression analysis with power functions showed that increasing spatial scale resulted in increasing H but decreasing E and K, indicating that the scale dependence of species diversity existed. By using the multifractal method, it was indicated that species spatial distribution had multifractal features. Moreover, strong linear relationships of the diversity indices H, E and K with α_min and clear nonlinear associations of the diversity indices H, E and K with power functions for SR and Dist were found. Since interactions of the species diversity and the spatial characteristics are very complex, the above mentioned relationships need further validation along with precise explanations of any correlations among their ecological processes.     

天目山物种多样性尺度依赖及其与空间格局关系的多重分形

以浙江省天目山国家级自然保护区为例,采用多尺度分析思想,利用多重分形分析方法,研究了不同尺度下物种多样性的变化、空间分布格局以及多样性与空间格局之间的关系。研究主要得到3方面的结论：（1）物种多样性具有尺度依赖性,随着空间尺度的增大,Shannon-Wiener多样性指数Ⅳ增大,Margalef多样性指数足和均匀度指数E减小;（2）多重分形参数αmin。多重分形谱的变化范围SR等能够定量反映物种的空间分布特征,空间大尺度越大,物种越聚集,空间分布越不均匀;（3）物种多样性与空间格局存在线性或幂函数关系。研究表明多重分形分析定量描述物种空间格局是有效性的,多重分形参数与生物多样性之间的定量关系为研究物种空间格局、生态属性与尺度之间的关系奠定了基础。因此,分形结合传统方法,在生物多样性方面的研究将有很大的潜在价值。     

Rapid Establishment of a Regular Distribution of Adult Tropical Drosophila Parasitoids in a Multi-Patch Environment by Patch Defence Behaviour

Females of the larval parasitoid of Drosophila, Asobara citri, from sub-Saharan Africa, defend patches with hosts by fighting and chasing conspecific females upon encounter. Females of the closely related, palearctic species Asobara tabida do not defend patches and often search simultaneously in the same patch. The effect of patch defence by A. citri females on their distribution in a multi-patch environment was investigated, and their distributions were compared with those of A. tabida. For both species 20 females were released from two release-points in replicate experiments. Females of A. citri quickly reached a regular distribution across 16 patches, with a small variance/mean ratio per patch. Conversely, A. tabida females initially showed a clumped distribution, and after gradual dispersion, a more Poisson-like distribution across patches resulted (variance/mean ratio was closer to 1 and higher than for A. citri). The dispersion of A. tabida was most probably an effect of exploitation: these parasitoids increasingly made shorter visits to already exploited patches. We briefly discuss hypotheses on the adaptive significance of patch defence behaviour or its absence in the light of differences in the natural history of both parasitoid species, notably the spatial distribution of their hosts.     

Genetic variability of the narrow endemic Rhamnus persicifolia Moris (Rhamnaceae) and its implications for conservation

Rhamnus persicifolia Moris is an endemic small tree belonging to the Rhamnus cathartica group, growing along mountainous streams of Central-Eastern Sardinia (Italy). ISSR markers were used to detect the genetic diversity within and among six populations representative of the species distribution range. In spite of the limited distribution of this endemic taxon, fairly high levels of genetic diversity were detected. Percentage of polymorphic bands (PPB), gene diversity (H_S and H_T) and Shannon information measure (Sh) were calculated both at population (PPB = 30.70%, H_S = 0.1105, Sh = 0.1646) and at species level (PPB = 68.42%, H_T = 0.2066, Sh = 0.3139).The existence of a spatial distribution of genetic diversity in R. persicifolia was revealed by a low gene flow, a fairly high level of genetic differentiation (G_ST = 0.4583) among populations and a positive correlation between genetic and geographic distances (Mantel test, r = 0.71, p = 0.016). The spatial genetic structure was also confirmed with BAPS analysis. Our results show that a certain level of isolation by distance and sex-ratio bias may explain the distribution of genetic diversity among populations.Conservation measures are suggested on the basis of the genetic diversity detected, by implementing an integrated in situ and ex situ conservation program for each population, in order to ensure effective protection for this endemic species.     

Similarity in occupancy of different-sized forest patches by small mammals on clearcuts: conservation implications for red-backed voles and small mustelids

Many mammalian species decline on forest sites that are harvested by clearcutting because of a loss of food, cover, and other components of stand structure. Small mustelids are impacted negatively as is the southern red-backed vole (Myodes gapperi), a principal prey species, that disappears from clearcuts within a year of harvest. These effects may be potentially ameliorated by aggregated retention harvests that leave unlogged patches on clearcuts. We tested three hypotheses (H) that (H₁) abundance, reproduction, and survival of M. gapperi populations, (H₂) total abundance, species richness, and diversity of the forest-floor small mammal community, and (H₃) the presence of small mustelids would be greater in large than small patches of retention forest on new clearcuts. We measured demographic responses of M. gapperi, total small mammals, and the presence of small mustelids (American marten, Martes americana, and small weasels (Mustela spp.)) from 2014 to 2016 in replicated treatments of four sizes (ha) of retention patches (means of 0.53, 1.50, 4.13, and 18.73) near Elkhart in south-central British Columbia, Canada. Mean abundance, reproduction, and survival attributes of M. gapperi were similar among treatment sites over the 3-year study. Overall mean abundance ranged from 3.5 to 5.3 voles per line in patches while this microtine was extirpated on clearcut sites (i.e., no forest patches). The similarity in population dynamics among the various forest patches across a gradient of increasing patch size of 4.5 to 35.3 times did not support H₁. Mean abundance, species richness, and diversity of total forest-floor small mammals were similar among treatment sites, and hence did not support H₂. Although not formally significant, mean species diversity did show a consistent increase from the largest (0.82) to the smallest (1.11) patch size, owing primarily to the presence of several generalist species such as Neotamias amoenus, Microtus, and Sorex in nearby early successional habitat. Small mustelids were present at similar levels among patch sizes, presumably in response to abundance of small mammal prey, and hence did not support H₃. Although our results were relatively short-term, the detailed assessment of population dynamics of M. gapperi indicated that habitat quality was sufficient to maintain this species regardless of patch sizes, ranging from 0.3 to 20.0 ha. Similarly, the total forest-floor small mammal community and presence of small mustelids also followed this pattern. All sizes of forest patches have conservation value and will help to maintain abundance and diversity of forest mammals, both predator and prey species, on clearcuts. Longer-term studies (e.g., at 5- to 10-year intervals) are essential to determine if our results are sustainable in augmenting forest restoration.

     

Low genetic diversity and weak population differentiation in Firmiana danxiaensis,a tree species endemic to Danxia landform in northern Guangdong,China

Danxia landform, a unique type of petrographic geomorphology, sporadically occurs in southern China. There are about 400 rare or threatened plants and animals in this landform, but little information on genetic aspects of these species has been available. Of them, Firmiana danxiaensis is a dominant tree species endemic to Mount Danxia, a typical Danxia landform in northern Guangdong, China. In this study, we examined genetic diversity and population structure of F. danxiaensis, in comparison with two other narrowly distributed Firmiana species, Firmiana hainanensis and Firmiana kwangsiensis, which grow in non-Danxia-landform regions. Nine microsatellite markers were used to investigate genetic diversity and differentiation in a total of 256 individuals from nine populations of F. danxiaensis, three of F. hainanensis, and one of F. kwangsiensis. The results showed that F. danxiaensis had the lowest genetic diversity (H_E = 0.364 ± 0.019) among the three species, but weak population structure similar to another Firmiana species (F_ST = 0.042 and 0.020, respectively). This low genetic diversity observed in F. danxiaensis is possibly due to confined natural distribution range and strong selective pressure in soils of Danxia landform, which may also be true for other endemic species in Danxia landform. For the long term survival of these endemic species, habitat conservation and anthropogenic introduction to similar habitats such as the adjacent Danxia landform areas should be two major strategies.     

Disturbance reinforces community assembly processes differentially across spatial scales

Background and AimsThere is a paucity of empirical research and a lack of predictive models concerning the interplay between spatial scale and disturbance as they affect the structure and assembly of plant communities. We proposed and tested a trait dispersion-based conceptual model hypothesizing that disturbance reinforces assembly processes differentially across spatial scales. Disturbance would reinforce functional divergence at the small scale (neighbourhood), would not affect functional dispersion at the intermediate scale (patch) and would reinforce functional convergence at the large scale (site). We also evaluated functional and species richness of native and exotic plants to infer underlying processes. Native and exotic species richness were expected to increase and decrease with disturbance, respectively, at the neighbourhood scale, and to show similar associations with disturbance at the patch (concave) and site (negative) scales.MethodsIn an arid shrubland, we estimated species richness and functional dispersion and richness within 1 m² quadrats (neighbourhood) nested within 100 m² plots (patch) along a small-scale natural disturbance gradient caused by an endemic fossorial rodent. Data for the site scale (2500 m² plots) were taken from a previous study. We also tested the conceptual model through a quantitative literature review and a meta-analysis.Key ResultsAs spatial scale increased, disturbance sequentially promoted functional divergence, random trait dispersion and functional convergence. Functional richness was unaffected by disturbance across spatial scales. Disturbance favoured natives over exotics at the neighbourhood scale, while both decreased under high disturbance at the patch and site scales.ConclusionsThe results supported the hypothesis that disturbance reinforces assembly processes differentially across scales and hampers plant invasion. The quantitative literature review and the meta-analysis supported most of the model predictions.     

The distributions of one invasive and two native crayfishes in relation to coarse‐scale natural and anthropogenic factors

1. Native crayfishes are often extirpated from portions of their range because of interactions with invasive species, anthropogenic alterations to environmental conditions or a combination of these factors. Our goal was to identify coarse‐scale natural and anthropogenic factors related to the current distributions of the invasive crayfish, Orconectes hylas, and two endemic crayfishes, Orconectes peruncus and Orconectes quadruncus in the St. Francis River drainage, Missouri, U.S.A. and to provide wider insights into the potential role of anthropogenic factors in facilitating species displacement. 2. We used classification trees to model coarse‐scale natural and anthropogenic environmental factors and their relation to the presence or absence of each species. Model results were then used to predict probability of presence for each species within each stream segment throughout the entire St. Francis River drainage. 3. Factors related to geology and soils were the best predictors of species distributions. A dichotomy of these factors explained much of the discrete distributions of the two native species. Agricultural‐related factors were identified as the most influential anthropogenic activity related to species distributions. All associations between the invasive species and anthropogenic factors were negative which suggested the invader was not likely to establish in heavily impacted areas. Overall, our models had high correct classification rates, and we were able to reliably predict the presence of the invader in the invaded drainage. 4. Given the negative associations of the invader with anthropogenic alterations at a coarse spatial scale, we believe other mechanisms are likely to be responsible for the widespread displacement of the two native species. These findings can be used to assist in conservation activities such as creation of refugia for native species and may direct future research to identify the mechanism(s) of species displacement.     

Latitudinal variation in global range-size of aquatic macrophyte species shows evidence for a Rapoport effect

1. To test hypotheses concerning the applicability of the Rapoport effect (RE: “species that occur at higher latitudes tend to have greater geographical range-size than species which have ranges limited to latitudes closer to the equator”) to aquatic macrophytes at global scale, we analysed the world latitudinal distribution and range-size of 1,083 vascular aquatic macrophyte species, from 91 genera in 11 families. We targeted macrophyte families strongly associated with inland aquatic habitats (i.e. with a zero, or only very low, proportion of constituent species which occur also in non-aquatic habitats), and which are distributed across a substantial latitudinal gradient, a necessary condition to test our hypotheses.
2. The macrophyte species present in these families include plants from all the normally accepted life form-defined functional groups of macrophytes, namely submerged, free-floating, floating-leaf rooted and emergent species, and represent the three major vascular taxonomic groups occurring as aquatic macrophytes (ferns/fern allies, monocots, and dicots). For the analysis, we used both latitude-only and areal measures of macrophyte species geographic range-size, within a 10 × 10° (latitude × longitude) grid of 238 grid cells, covering the six world ecozones (Palaearctic, Orient, Australasia, Nearctic, Neotropics, Afrotropics) that primarily contain inland freshwater and brackish macrophyte habitats.
3. The results provide new insight into the relationships between global range-size of macrophytes, latitude, and other potential spatio-environmental and anthropogenic drivers acting upon these plants at world scale. In particular, the outcomes indicated that: (1) the range-size versus latitude distribution of macrophytes shows evidence of a strong RE influence, with significantly greater species range-size at higher latitudes; and (2) the β-diversity pattern of species distribution along this latitudinal gradient is poorly explained by nestedness organisation, and species turnover is a more likely explanation of the observed changes in species distribution with latitude.
4. Spatio-environmental and anthropogenic variables other than latitude may also influence the observed global geographical pattern of macrophyte range-size, although their importance as predictors varies between individual families. Extent of agricultural land use, altitude, and historic (post-Quaternary) climate change velocity were all significant predictor variables for some families. However, interestingly, neither the area of land nor the area of waterbody present per grid cell were major influences on macrophyte range-size distribution.
5. Our finding of evidence for an RE, acting at global scale in aquatic macrophytes, contributes to increasing the generality of conclusions so far reached about the large-scale factors that drive patterns of species range-size at global scale. The study also provides a baseline for future macroecological work on aquatic plants, and potentially other freshwater organisms, particularly in the context of predicting how the world ranges of freshwater biota will respond to ongoing global environmental change.

     

Local contagion and regional compression: habitat selection drives spatially explicit,multiscale dynamics of colonisation in experimental metacommunities

Habitat selection, including oviposition site choice, is an important driver of community assembly in freshwater systems. Factors determining patch quality are assessed by many colonising organisms and affect colonisation rates, spatial distribution and community structure. For many species, the presence/absence of predators is the most important factor affecting female oviposition decisions. However, individual habitat patches exist in complex landscapes linked by processes of dispersal and colonisation, and spatial distribution of factors such as predators has potential effects beyond individual patches. Perceived patch quality and resulting colonisation rates depend both on risk conditions within a given patch and on spatial context. Here we experimentally confirm the role of one context‐dependent processes, spatial contagion, functioning at the local scale, and provide the first example of another context‐dependent process, habitat compression, functioning at the regional scale. Both processes affect colonisation rates and patterns of spatial distribution in naturally colonised experimental metacommunities.     

Global species richness of hydrobiid snails determined by climate and evolutionary history

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Spatial Genetic Structure of Campanula sabatia, a Threatened Narrow Endemic Species of the Mediterranean Basin

Campanula sabatia is endemic to NW Italy (Liguria) and it was included in the European Red list of endangered species due to the heavy human pressure on its habitat. AFLP markers were used to detect the genetic diversity within and among ten populations (totalling 83 individuals) representing the range of the species. In spite of the limited distribution of this endemic taxon, high levels in percentage of polymorphic bands (PPB), gene diversity (H _S and H _T) and Shannon??s information index (I) were detected both at population (PPB?=?60?%, H _S?=?0.1853, I?=?0.2836) and at species level (PPB?=?100?%, H _T?=?0.2415, I?=?0.3871). The coefficient of genetic differentiation among population (G??_ST) was 0.1935, while the level of gene flow (N?? _m) was estimated to be 2.0832. AMOVA analysis identified a genetic variation within populations of 83?% of the total. Bayesian clustering methods assigned individuals to two geographical groups partly found within the same population, probably due to a high rate of genetic exchange among its populations. Conservation measures are suggested on the basis of the genetic diversity detected to ensure an effective protection for this endemic species.     

Potential pitfalls in rescaling digital terrain model-derived attributes for ecological studies

Terrain attributes (e.g., slope, rugosity) derived in Geographic Information Systems (GIS) from digital terrain models (DTMs) are widely used in both terrestrial and marine ecological studies due to their potential to act as surrogates of species distribution. However, the spatial resolution of DTMs is often altered to match the scale at which species observations were collected. Here, we highlight the significance of adequately reporting the methods used to derive terrain attributes from DTMs and the consequences of their incorrect reporting in ecological studies. To ensure full repeatability of studies, they should report (i) the source and the resolution of the original DTM; (ii) the algorithm used to calculate terrain attributes; (iii) the method used for rescaling (e.g., aggregating or resampling, using the mean or maximum values); and (iv) the order in which these operations were performed. We contrast the effects of two common scale alteration approaches for the derivation of terrain attributes from DTMs. These two scale alteration methods differ in the step at which the change is performed: (i) the resolution alteration is performed after computing terrain attributes from the original DTM at the native resolution, or (ii) the resolution alteration is performed on the native DTM before computing terrain attributes. While these approaches conceptually do the same thing (i.e., change the resolution of the terrain attributes), we demonstrate that they produce two distinct sets of variables that are not interchangeable and describe different properties of the terrain. In a species distribution modelling (SDM) context, the first approach calculates terrain attribute values within the cell where a species is found, while the second approach calculates terrain attribute values with respect to neighbouring cells. A mutual substitution of the two approaches results in a decrease of models' discrimination ability and in misleading spatial predictions of species probability of occurrence. Regardless of the DTM-derived attribute, we argue that the choice of the approach should be carefully guided by both the ecological scale relevant to the question being asked and the performance of pre-analyses. We emphasize that selected methods be clearly described to encourage reproducibility and proper interpretation of results, thus enabling a better understanding of the role of scale in ecology.     

Fine‐scale genetic structure in an eastern Alpine black grouse Tetrao tetrix metapopulation

Understanding genetic consequences of habitat fragmentation is crucial for the management and conservation of wildlife populations, especially in case of species sensitive to environmental changes and landscape alteration. In central Europe, the Alps are the core area of black grouse Tetrao tetrix distribution. There, black grouse dispersal is limited by high altitude mountain ridges and recent black grouse habitats are known to show some degree of natural fragmentation. Additionally, substantial anthropogenic fragmentation has occurred within the past ninety years. Facing losses of peripheral subpopulations and ongoing range contractions, we explored genetic variability and the fine‐scale genetic structure of the Alpine black grouse metapopulation at the easternmost fringe of the species’ Alpine range. Two hundred and fifty tissue samples and non‐invasive faecal and feather samples of eleven a priori defined subpopulations were used for genetic analysis based on nine microsatellite loci. Overall, eastern Alpine black grouse show similar amounts of genetic variation (H_O = 0.65, H_E = 0.66) to those found in more continuous populations like in Scandinavia. Despite of naturally and anthropogenically fragmented landscapes, genetic structuring was weak (global F_ST < 0.05), suggesting that the actual intensity of habitat fragmentation does not completely hamper dispersal, but probably restricts it to some extent. The most peripheral subpopulations at the edge of the species range show signs of genetic differentiation. The present study gives new insights into the population genetic structure of black grouse in the eastern Alps and provides a more fine‐scale view of genetic structure than previously available. Our findings will contribute to monitor the current and future status of the population under human pressures and to support supra‐regional land use planning as well as decision making processes in responsibilities of public administration.     

Do spatial effects play a role in the spatial distribution of desert‐dwelling Acacia raddiana?

Abstract. We investigated the spatial pattern of A. raddiana in the Negev desert of Israel in order to gain insights into the factors and processes driving the dynamics of this species. Using a scale‐dependent measure, the ring statistic, we analysed both patterns observed in the field and time series of spatial tree distributions produced by a simulation model. In the field, random spacing was the predominant pattern observed. However seedlings were clumped on small scales. We ran the model under two contrasting scenarios representing hypotheses that explain the clumping of seedlings and the random distribution of trees. One hypothesis is that there is spatial heterogeneity in seed distribution, germination and seedling mortality, but that these heterogeneities are not correlated with each other in space. The second hypothesis assumes a correlation between these heterogeneities leading to areas suitable for establishment. However, the suitability of the sites is temporally variable. Furthermore, the second hypothesis assumes density‐dependent tree mortality due to competition. Both hypotheses lead to spatial distributions that are in qualitative agreement with the patterns observed in the field. Therefore, the classical view that a clumped seedling distribution and a random pattern of older trees is due to clumped regeneration and density‐dependent mortality may not hold for Acacia trees in the Negev.     

The distribution of hydroids (Cnidaria, Hydrozoa) from micro- to macro-scale: Spatial patterns on habitat-forming algae

Scaling up from local, short-term experiments to larger-area and longer-term ones is crucial to address the role of scale in ecology. Few studies, however, examined large-scale spatial variability in the distribution and abundance of marine organisms, with rare attempts to directly compare spatial variation at local (centimetres-metres) vs. regional (1000's of kilometres) scale. Here, we used a hierarchical design to describe the spatial distribution of the hydroids epiphitic of the brown alga Cystoseira amentacea, a habitat-forming species that provides a continuous, extensive settling substrate at regional scale along the rocky coasts in the Mediterranean Sea. This continuity provides the potential to deal with scale-related variability, increasing area of investigation without adding differences deriving from habitat heterogeneity or changes in topographic complexity. Hydroids were selected for their abundance and for their life cycle features (rapid growth, small body size, early sexual or asexual reproduction and short life span), allowing rapid responses to changes in environmental conditions. The aim of this study was to analyse whether the structure of hydroid assemblages living on C. amentacea had a consistent pattern of variation among three portions of the algal thallus (i.e., basal, middle, and distal) across a spectrum of scales and whether having or not a pelagic stage could exert a significant influence on the distribution patterns of the species. A total of 32 species were identified. Multivariate analyses showed that hydroid colonization of Cystoseira occurs differently along each thallus, with patterns of variation in the structure of assemblages differing at an even smaller spatial scale than that of single plants. However, such differences varied from patch to patch. Among the 14 species identified as “important” to define the hydroid assemblage inhabiting Cystoseira, only one (Clytia hemisphaerica) has free medusae, the other species reproducing by fixed gonophores or by short-lived medusoids. Univariate analysis showed significant differences among portions of thalli in terms of spatial variability at the various scales investigated, thus suggesting that patterns of multivariate variation along the three portions of thalli might vary across scale. Overall, our results suggest that patterns of distribution of hydroids along C. amentacea thalli significantly vary across spatial scales but that the observed differences can be hardly interpreted on the basis of life-cycle patterns.