The assembly of forest communities according to maximum species height along resource and disturbance gradients

An ongoing debate in ecology concerns the relative importance of competition in driving community patterns, especially along gradients of disturbance and productivity. We used a null model to address this question by testing for non‐random organization of forest species according to maximum height, a trait associated with competitive ability for light. Specifically, we compared the species present in 409 different temperate forest plots with the pool of potentially present species (n=639 species), spanning a 50 000 km² area of southern Ontario, Canada. In contrast to current theory, coexisting forest species were neither more similar (i.e. convergent), nor more different (i.e. limiting similarity) in maximum height than expected by chance. However, coexisting forest species had larger maximum heights on average, and their maximum heights were more symmetrically distributed than expected by chance, suggesting that competition has reduced the representation of smaller plant species within plots (i.e. higher turn‐over of species with smaller maximum heights among forest plots). We explored the possibility that our findings resulted from smaller species having relatively narrower niches; however, a conclusive test of this explanation will require knowledge of fundamental, rather than realized niche breadth. We also tested the prediction that the influence of competition changes along gradients of productivity and disturbance by examining how the effect size of our null model tests changed along these gradients. We observed that species with smaller maximum heights were increasingly under‐represented in more productive forest communities, suggesting an increased role for competition in determining species membership in more productive communities. In contrast to theory, however, the effect size of our tests did not significantly change along a gradient of forest disturbance. In summary, we found evidence that maximum species height plays a significant role in driving the non‐random organization of plant species among hundreds of mature forest plots, and that this role is more pronounced in more productive forest plots. 1     

Phylogenetic alpha and beta diversity in tropical tree assemblages along regional-scale environmental gradients in northwest South America

Aims Environmental gradients are drivers of species diversity; however, we know relatively little about the evolutionary processes underlying these relationships. A potentially powerful approach to studying diversity gradients is to quantify the phylogenetic structure within and between assemblages arrayed along broad spatial and environmental gradients. Here, we evaluate the phylogenetic structure of plant assemblages along an environmental gradient with the expectation that the habitat specialization of entire lineages is an important evolutionary pattern influencing the structure of tree communities along environmental gradients.Methods We evaluated the effect of several environmental variables on the phylogenetic structure of plant assemblages in 145 plots distributed in northwestern South America that cover a broad environmental gradient. The phylogenetic alpha diversity was quantified for each plot and the phylogenetic beta diversity between each pair of plots was also quantified. Both the alpha and beta diversity measures were then related to spatial and environmental gradients in the study system.Important findings We found that gradients in temperature and potential evapotranspiration have a strong relationship with the phylogenetic alpha diversity in our study system, with phylogenetic overdispersion in low temperatures and phylogenetic clustering at higher temperatures. Further, the phylogenetic beta diversity between two plots increases with an increasing difference in temperature, whereas annual precipitation was not a significant predictor of community phylogenetic turnover. We also found that the phylogenetic structure of the plots in our study system was related to the degree of seasonal flooding and seasonality in precipitation. In particular, more stressful environments such as dry forests and flooded forests showed phylogenetic clustering. Finally, in contrast with previous studies, we find that phylogenetic beta diversity was not strongly related to the spatial distance separating two forest plots, which may be the result of the importance of the three independent mountain ranges in our study system, which generate a high degree of environmental variation over very short distances. In conclusion, we found that environmental gradients are important drivers of both phylogenetic alpha and phylogenetic beta diversities in these forests over spatial distance.     

Integrating large‐scale geographic patterns in flight morphology,flight characteristics and sexual selection in a range‐expanding damselfly

While geographic trait variation along environmental clines is widespread, associated patterns in sexual selection remain largely unexplored. Geographic patterns in sexual selection may be expected if 1) phenotypes vary geographically and sexual selection is dependent on the local phenotypes in the population, and if 2) sexual selection is influenced by geographically structured environmental conditions. We quantified geographic variation in flight‐related traits and flight performance in mated and unmated males and tested for geographic variation in sexual selection on these traits in the poleward range‐expanding damselfly Coenagrion scitulum across a set of eleven core and edge populations ordered along thermal gradients in the larval and in the adult stage. We found little support for trait differentiation between core and edge populations, instead we found considerable geographic trait variation along the larval and adult thermal gradients. As expected under time constraints, body mass decreased with shorter larval growth seasons. Lower temperatures during the adult flight period were associated with a higher body mass, a higher flight speed and a higher fat content; these traits likely evolved to buffer flight ability at suboptimal temperatures and to optimize starvation resistance. Across the large geographic scale, we found a consistent higher flight duration in mated males. Instead, sexual selection for higher fat content was stronger in populations with lower adult flight temperatures and sexual selection for lower body mass acted only in edge populations. Our results indicate sexual selection on flight performance to be consistent over a large geographic scale and this despite the clear geographic patterns in sexual selection on the underlying morphological traits. Our results highlight that to fully understand the fitness implications of geographically changing trait patterns, researchers should consider the entire phenotype–performance–fitness axis and incorporate effects of geographically structured life‐stage specific environmental conditions on this axis.     

Performance and reference data in the jump squat at different relative loads in elite sprinters,rugby players,and soccer players

The aims of this study were to compare the outcomes and provide reference data for a set of barbell mechanical parameters collected via a linear velocity transducer in 126 male sprinters (n = 62), rugby players (n = 32), and soccer players (n = 32). Bar-velocity, bar-force, and bar-power outputs were assessed in the jump-squat exercise with jump-squat height determined from bar-peak velocity. The test started at a load of 40% of the athletes’ body mass (BM), and a load of 10% of BM was gradually added until a clear decrement in the bar power was observed. Comparisons of bar variables among the three sports were performed using a one-way analysis of variance. Relative measures of bar velocity, force, and power, and jump-squat height were significantly higher in sprinters than in rugby (difference ranging between 5 and 35%) and soccer (difference ranging between 5 and 60%) players across all loads (40–110% of BM). Rugby players exhibited higher absolute bar-power (mean difference = 22%) and bar-force (mean difference = 16%) values than soccer players, but these differences no longer existed when the data were adjusted for BM (mean difference = 2.5%). Sprinters optimized their bar-power production at significantly greater relative loads (%BM) than rugby (mean difference = 22%) and soccer players (mean difference = 25%); nonetheless, all groups generated their maximum bar-power outputs at similar bar velocities. For the first time, we provided reference values for the jump-squat exercise for three different bar-velocity measures (i.e., mean, mean propulsive, and peak velocity) for sprinters, rugby players, and soccer players, over a wide range of relative loads. Practitioners can use these reference values to monitor their athletes and compare them with top-level sprinters and team-sport players.     

ICE-BASED CENSUS OF BOWHEAD WHALES MIGRATING PAST POINT BARROW, ALASKA, 1978-1983

Bowhead whale ( Balaena mysticetus ) census data obtained during the northward spring migration are summarized for 1978–1983. Population size estimates are derived from counts made by observers standing on the seaward edge of shorefast ice in the vicinity of Point Barrow, Alaska, from mid-April to early June. The research design utilized two counting stations: South Perch, the primary counting station, and North Perch, used to determine the number of whales missed by South Perch observers. The percentage missed is estimated for each visibility category and used here to correct the census counts. Each season's population estimate is calculated as the sum of the number of trials of several independent multinomial distributions representing different visibility conditions. Corrections are applied for unwatched hours and hours with inferior visibility. A mean estimate of the number of whales passing within view of the census station was computed as 3,674 ± 299. This estimate was based on data collected in 1978, 1981, 1982, and 1983, years with the least apparent biases. Aerial survey data provide estimates of the proportion of whales passing at various distances seaward of the census sites as follows: 0.58 from the ice edge to 2 km, 0.76 to 3 km, and 0.80 to 4 km. Correcting for whales too far offshore to be seen by the ice-based observers results in a population estimate of over 4,200 bowheads.     

Classification of macroinvertebrate communities across drainage basins in Victoria, Australia: consequences of sampling on a broad spatial scale for predictive modelling

1. Spatial scale may influence the interpretation of environmental gradients that underlie classification and ordination analyses of lotic macroinvertebrate communities. This could have important consequences for the spatial scale over which predictive models derived from these multivariate analyses can be applied. 2. Macroinvertebrate community data (identified to genus or species) from edge and main-channel habitats were obtained for sites on rivers from 25 of the 29 drainage basins in Victoria. Trends in community similarity were analysed by carrying out separate multivariate analyses on data from the edge habitats (199 sites) and the main-channel habitats (163 sites). 3. Hierarchical classification (UPGMA) showed that the edge data could be placed into 11 site groups and the main-channel data into 12 site groups. 4. Ordination analysis (hybrid multidimensional scaling) showed no sharp disjunctions between site groups in either habitat; overlap was frequent. Correlation of the ordination patterns with environmental variables showed that edge communities varied longitudinally within a drainage basin and from the east to the west of Victoria. These two trends were superimposed on one another to form a single gradient on the ordination. The taxon richness of edge communities was also related to the species richness of macrophytes at a site. Main-channel communities also displayed a longitudinal and a geographic gradient, but these two gradients were uncorrelated on the ordination. 5. Community similarity only weakly reflected geographic proximity in either habitat. A preliminary subdivision of Victoria into a series of biogeographic regions did not match the pattern of distribution of site groups for the edge habitat, illustrating the difficulties of applying to lotic communities a priori regionalizations based on terrestrial features of the landscape. 6. The longitudinal gradients in the two data sets were commonly observed in data gathered at smaller spatial scales in Victoria. The other gradients (geographic, macrophyte), however, were either not consistently repeated or not evident at smaller spatial scales. At small spatial scales (i.e. within a single drainage basin) gradients were related to variables that varied over restricted ranges, e.g. mean particle size of the substratum. 7. Species richness was very variable when plotted against river slope or distance of site from source; both of these are measures of position on the longitudinal gradients. In contrast to suggestions in the literature, species richness did not show a unimodal trend on these gradients, or any other trend. 8. Environmental gradients (apart from longitudinal gradients) that underlie predictive models of macroinvertebrate distribution are reflections of the spatial scale on which the model has been constructed and cannot be extrapolated to different scales. Models must be suited to the spatial scale over which predictions are required.     

Using genetic variation to infer associations with climate in the common frog,Rana temporaria

Recent and historical species' associations with climate can be inferred using molecular markers. This knowledge of population and species‐level responses to climatic variables can then be used to predict the potential consequences of ongoing climate change. The aim of this study was to predict responses of Rana temporaria to environmental change in Scotland by inferring historical and contemporary patterns of gene flow in relation to current variation in local thermal conditions. We first inferred colonization patterns within Europe following the last glacial maximum by combining new and previously published mitochondrial DNA sequences. We found that sequences from our Scottish samples were identical to (92%), or clustered with, the common haplotype previously identified from Western Europe. This clade showed very low mitochondrial variation, which did not allow inference of historical colonization routes but did allow interpretation of patterns of current fine‐scale population structure without consideration of confounding historical variation. Second, we assessed fine‐scale microsatellite‐based patterns of genetic variation in relation to current altitudinal temperature gradients. No population structure was found within altitudinal gradients (average F_ST = 0.02), despite a mean annual temperature difference of 4.5 °C between low‐ and high‐altitude sites. Levels of genetic diversity were considerable and did not vary between sites. The panmictic population structure observed, even along temperature gradients, is a potentially positive sign for R. temporaria persistence in Scotland in the face of a changing climate. This study demonstrates that within taxonomic groups, thought to be at high risk from environmental change, levels of vulnerability can vary, even within species.     

Disentangling the relative importance of climate,size and competition on tree growth in Iberian forests: implications for forest management under global change

Most large‐scale multispecies studies of tree growth have been conducted in tropical and cool temperate forests, whereas Mediterranean water‐limited ecosystems have received much less attention. This limits our understanding of how growth of coexisting tree species varies along environmental gradients in these forests, and the implications for species interactions and community assembly under current and future climatic conditions. Here, we quantify the absolute effect and relative importance of climate, tree size and competition as determinants of tree growth patterns in Iberian forests, and explore interspecific differences in the two components of competitive ability (competitive response and effect) along climatic and size gradients. Spatially explicit neighborhood models were developed to predict tree growth for the 15 most abundant Iberian tree species using permanent‐plot data from the Spanish Second and Third National Forest Inventory (IFN). Our neighborhood analyses showed a climatic and size effect on tree growth, but also revealed that competition from neighbors has a comparatively much larger impact on growth in Iberian forests. Moreover, the sensitivity to competition (i.e. competitive response) of target trees varied markedly along climatic gradients causing significant rank reversals in species performance, particularly under xeric conditions. We also found compelling evidence for strong species‐specific competitive effects in these forests. Altogether, these results constitute critical new information which not only furthers our understanding of important theoretical questions about the assembly of Mediterranean forests, but will also be of help in developing new guidelines for adapting forests in this climatic boundary to global change. If we consider the climatic gradients of this study as a surrogate for future climatic conditions, then we should expect absolute growth rates to decrease and sensitivity to competition to increase in most forests of the Iberian Peninsula (in all but the northern Atlantic forests), making these management considerations even more important in the future.     

Top-predator abundance and chaos in tritrophic food chains

Nonlinear models and laboratory experiments suggest that populations can be chaotic, whereas field data show that a fair proportion of observed populations are not too far from being chaotic. Thus, a natural question arises: do ecosystems enjoy special properties at the edge of chaos? By limiting the analysis to three classes of tritrophic food chains and to the role played by the nutrient available to the bottom of the ecosystem, we show that top-predator mean abundance is maximum at the edge of chaos. The geographical variability of species abundance along nutrient gradients and the dynamic complexity of observed natural populations are consistent with our findings. Effective guidelines for the sustainability of exploited ecosystems are also derived.     

Fine-scale spatial and temporal variations in diets of the pipefish Stigmatopora nigra within seagrass patches

Diets of the pipefish Stigmatopora nigra were analysed to determine if food availability was causing S. nigra to distribute according to habitat edge effects. Gut analysis found little difference in the diets of S. nigra at the edge and interior of seagrass patches, regardless of time of day or season. Fish diets did, however, vary with seagrass density: S. nigra in denser seagrass consumed more harpacticoid copepods and fewer planktonic copepods. The lack of difference in prey eaten by S. nigra at the edge and interior of patches suggests either that food was not determining S. nigra distribution patterns within patches or that differences in fish densities across patches meant that relative fish-prey densities were similar at edge and interior positions. Alternatively, any edge effects in diet might be masked by gradients in seagrass structure.     

Adding energy gradients and long‐distance dispersal to a neutral model improves predictions of Madagascan bird diversity

Macroecological patterns are likely the result of both stochastically neutral mechanisms and deterministic differences between species. In Madagascar, the simplest stochastically neutral hypothesis – the mid‐domain effects (MDE) hypothesis – has already been rejected. However, rejecting the MDE hypothesis does not necessarily refute the existence of all other neutral mechanisms. Here, we test whether adding complexity to a basic neutral model improves predictions of biodiversity patterns. The simplest MDE model assumes that: (1) species' ranges are continuous and unfragmented, (2) are randomly located throughout the landscape, and (3) can be stacked independently and indefinitely. We designed a simulation based on neutral theory that allowed us to weaken each of these assumptions incrementally by adjusting the habitat capacity as well as the likelihood of short‐ and long‐distance dispersal. Simulated outputs were compared to four empirical patterns of bird diversity: the frequency distributions of species richness and range size, the within‐island latitudinal diversity gradient, and the distance‐decay of species compositional similarity. Neutral models emulated empirical diversity patterns for Madagascan birds accurately. The frequency distribution of range size, latitudinal diversity gradient, and the distance‐decay of species compositional similarity could be attributed to stochastic long‐distance migration events and zero‐sum population dynamics. However, heterogenous environmental gradients improved predictions of the frequency distribution of species richness. Patterns of bird diversity in Madagascar can broadly be attributed to stochastic long‐distance migration events and zero‐sum population dynamics. This implies that rejecting simple hypotheses, such as MDE, does not serve as evidence against stochastic processes in general. However, environmental gradients were necessary to explain patterns of species richness and deterministic differences between species are probably important for explaining the distributions of narrow‐range and endemic species.     

Diel variation in the difference in air temperature between the forest edge and interior in the Usambara Mountains, Tanzania

The difference in air temperature between the forest edge and interior was simultaneously compared at ten sites at 2‐h intervals over a 229‐day period in the East and West Usambara Mountains in Tanzania. Measurements were recorded with data loggers placed 1.5 m above the ground on the shaded side of trees and positioned 1–2 and 100 m from the forest edge. The difference in air temperature between the forest edge and interior varied regularly over a 24‐h period with mean maximum difference in air temperatures of 0.85°C occurring at 15:00 h and mean minimum diel difference in air temperature of 0.02°C occurring at 21:00 h. Diel variation in the difference in air temperature between the forest edge and interior varied little by season and was unrelated to elevation.     

Spatial patterning of metabolism by mitochondria, oxygen, and energy sinks in a model cytoplasm

Metabolite gradients might guide mitochondrial localization in cells and angiogenesis in tissues. It is unclear whether they can exist in single cells, because the length scale of most cells is small compared to the expected diffusion times of metabolites. For investigation of metabolic gradients, we need experimental systems in which spatial patterns of metabolism can be systematically measured and manipulated. We used concentrated cytoplasmic extracts from Xenopus eggs as a model cytoplasm, and visualized metabolic gradients formed in response to spatial stimuli. Restriction of oxygen supply to the edge of a drop mimicked distance to the surface of a single cell, or distance from a blood vessel in tissue. We imaged a step-like increase of Nicotinamide adenine dinucleotide (NAD) reduction approximately 600 microm distant from the oxygen source. This oxic-anoxic switch was preceded on the oxic side by a gradual rise of mitochondrial transmembrane potential (Deltapsi) and reactive oxygen species (ROS) production, extending over approximately 600 microm and approximately 300 microm, respectively. Addition of Adenosine triphosphate (ATP)-consuming beads mimicked local energy sinks in the cell. We imaged Deltapsi gradients with a decay length of approximately 50-300 microm around these beads, in the first visualization of an energy demand signaling gradient. Our study demonstrates that mitochondria can pattern the cytoplasm over length scales that are suited to convey morphogenetic information in large cells and tissues and provides a versatile model system for probing of the formation and function of metabolic gradients.     

Chemotactic Sensing towards Ambient and Secreted Attractant Drives Collective Behaviour of E. coli

We simulate the dynamics of a suspension of bacterial swimmers, which chemotactically sense gradients in either ambient or self-secreted attractants (e.g. nutrient or aspartate respectively), or in both. Unlike previous mean field models based on a set of continuum partial differential equations, our model resolves single swimmers and therefore incorporates stochasticity and effects due to fluctuations in the bacterial density field. The algorithm we use is simple enough that we can follow the evolution of colonies of up to over a million bacteria for timescales relevant to pattern formation for E. coli growing in semisolid medium such as agar, or in confined geometries. Our results confirm previous mean field results that the patterns observed experimentally can be reproduced with a model incorporating chemoattractant secretion, chemotaxis (towards gradients in the chemoattractant field), and bacterial reproduction. They also suggest that further experiments with bacterial strains chemotactically moving up both nutrient and secreted attractant field may yield yet more dynamical patterns.     

Directional Bleb Formation in Spherical Cells under Temperature Gradient

Living cells sense absolute temperature and temporal changes in temperature using biological thermosensors such as ion channels. Here, we reveal, to our knowledge, a novel mechanism of sensing spatial temperature gradients within single cells. Spherical mitotic cells form directional membrane extensions (polar blebs) under sharp temperature gradients (≥∼0.065°C μm⁻¹; 1.3°C temperature difference within a cell), which are created by local heating with a focused 1455-nm laser beam under an optical microscope. On the other hand, multiple nondirectional blebs are formed under gradual temperature gradients or uniform heating. During heating, the distribution of actomyosin complexes becomes inhomogeneous due to a break in the symmetry of its contractile force, highlighting the role of the actomyosin complex as a sensor of local temperature gradients.     

Vegetation patterns,microtopography, and soils on a Chihuahuan desert playa

Abstract. Spatial patterns of vegetation on an ephemeral lake bed (playa) in the northern Chihuahuan desert were closely correlated with elevation. Distributions of species along a 265 m transect from the center to the edge of the playa showed abrupt boundaries at 110 m and 220 m. These boundaries seem related to the maximum elevation covered with standing water following complete flooding, and to an edaphic boundary between heavy clay soils at the center of the playa and adjacent soils. The complete turnover in species composition across this transect, with an elevation change of only 85 cm, indicates that environmental gradients resulting from flooding and soil development are steep. Vegetative patterns within the lowest part of the playa were patchy, with patch diameters ranging from 2 to 5 m. These patches appear to be produced by differences in the duration and frequency of flooding between small knolls and depressions resulting from gilgai microtopography.     

油菜光合器官面积与导数光谱特征的相关关系

 运用导数光谱分析技术,研究了不同氮肥水平下不同品种油菜(Brassica napus)的 叶面积指数（Leaf area index, LAI）及角果皮面积指数 （Pod area index,PAI ）与冠层导数光谱及其衍生参数的定量关系。结果表明,油菜导数光谱与花前LAI和花后PAI均有良好的相关关系,在 750 nm附近相关关系最好,相关系数达到0.9左右。三边参数与油菜LAI和PAI的相关性顺序为：红边＞黄边＞蓝边,面积参数＞振幅参数＞位置 参数。油菜红边导数光谱的双峰现象降低了红边位置对油菜LAI和PAI的敏感程度,利用线性外推法拟合红边位置能 提高其对油菜LAI和PAI的敏 感程度。在三边参数及其衍生参数中,红边面积及其与蓝边面积的差与LAI及PAI的相关性最好,且适用于该研究中使用的不同品种。因此,750 nm 处的一阶导数光谱、红边面积及其与蓝边面积的差可用于有效地监测油菜的光合器官面积。     

The influence of habitat heterogeneity on freshwater bacterial community composition and dynamics

Multiple forces structure natural microbial communities, but the relative roles and interactions of these drivers are poorly understood. Gradients of physical and chemical parameters can be especially influential. In traditional ecological theory, variability in environmental conditions across space and time represents habitat heterogeneity, which may shape communities. Here we used aquatic microbial communities as a model to investigate the relationship between habitat heterogeneity and community composition and dynamics. We defined spatial habitat heterogeneity as vertical temperature and dissolved oxygen (DO) gradients in the water column, and temporal habitat heterogeneity as variation throughout the open-water season in these environmental parameters. Seasonal lake mixing events contribute to temporal habitat heterogeneity by destroying and re-creating these gradients. Because of this, we selected three lakes along a range of annual mixing frequency (polymictic, dimictic, meromictic) for our study. We found that bacterial community composition (BCC) was distinct between the epilimnion and hypolimnion within stratified lakes, and also more variable within the epilimnia through time. We found stark differences in patterns of epilimnion and hypolimnion dynamics over time and across lakes, suggesting that specific drivers have distinct relative importance for each community.