Global seagrass distribution and diversity: A bioregional model

Seagrasses, marine flowering plants, are widely distributed along temperate and tropical coastlines of the world. Seagrasses have key ecological roles in coastal ecosystems and can form extensive meadows supporting high biodiversity. The global species diversity of seagrasses is low (< 60 species), but species can have ranges that extend for thousands of kilometers of coastline. Seagrass bioregions are defined here, based on species assemblages, species distributional ranges, and tropical and temperate influences. Six global bioregions are presented: four temperate and two tropical. The temperate bioregions include the Temperate North Atlantic, the Temperate North Pacific, the Mediterranean, and the Temperate Southern Oceans. The Temperate North Atlantic has low seagrass diversity, the major species being Zostera marina, typically occurring in estuaries and lagoons. The Temperate North Pacific has high seagrass diversity with Zostera spp. in estuaries and lagoons as well as Phyllospadix spp. in the surf zone. The Mediterranean region has clear water with vast meadows of moderate diversity of both temperate and tropical seagrasses, dominated by deep-growing Posidonia oceanica. The Temperate Southern Oceans bioregion includes the temperate southern coastlines of Australia, Africa and South America. Extensive meadows of low-to-high diversity temperate seagrasses are found in this bioregion, dominated by various species of Posidonia and Zostera. The tropical bioregions are the Tropical Atlantic and the Tropical Indo-Pacific, both supporting mega-herbivore grazers, including sea turtles and sirenia. The Tropical Atlantic bioregion has clear water with a high diversity of seagrasses on reefs and shallow banks, dominated by Thalassia testudinum. The vast Tropical Indo-Pacific has the highest seagrass diversity in the world, with as many as 14 species growing together on reef flats although seagrasses also occur in very deep waters. The global distribution of seagrass genera is remarkably consistent north and south of the equator; the northern and southern hemispheres share ten seagrass genera and only have one unique genus each. Some genera are much more speciose than others, with the genus Halophila having the most seagrass species. There are roughly the same number of temperate and tropical seagrass genera as well as species. The most widely distributed seagrass is Ruppia maritima, which occurs in tropical and temperate zones in a wide variety of habitats. Seagrass bioregions at the scale of ocean basins are identified based on species distributions which are supported by genetic patterns of diversity. Seagrass bioregions provide a useful framework for interpreting ecological, physiological and genetic results collected in specific locations or from particular species.     

Detecting and managing an overgrazing-drought synergism in the threatened <Emphasis Type="Italic">Echeveria longissima</Emphasis> (Crassulaceae): the role of retrospective demographic analysis

Echeveria longissima, a threatened herb whose habitat has been severely overgrazed and eroded, was studied for three years in a currently grazed and a fenced area. Matrix population models were used to assess if livestock elimination provides a proper management strategy. The merits of retrospective perturbation analyses in terms of management planning have been debated. Nevertheless, they may prove useful when applied in combination with exclosures because they may detect the effects of anthropogenic disturbance on population dynamics. Thus, the results of retrospective and prospective methods were compared. A rapid decrease in population size was projected in both areas, even though it was faster in the exposed one. The demographic processes that were favourable or detrimental in a given year were magnified outside of the fence, but buffered in the exclosure, showing a strong drought-disturbance synergism. Thus, the largest difference in the population growth rate λ between areas was observed in the driest year. Higher nurse-plant density inside the fence seems to alleviate drought effects. The use of prospective analysis alone may lead to erroneous management decisions, since the highest elasticities corresponded to transitions that were favoured by human activities. While allowing for an increased λ in the short term, intervention aimed at increasing these transitions further without attending others that are lessened by disturbance may introduce large changes in the population dynamics, with negative long-term consequences. Retrospective methods can detect which processes have been altered by disturbance and its synergisms, so we may more efficiently restore healthy population dynamics.     

Effect of reproductive modes and environmental heterogeneity in the population dynamics of a geographically widespread clonal desert cactus

The dynamics of plant populations in arid environments are largely affected by the unpredictable environmental conditions and are fine-tuned by biotic factors, such as modes of recruitment. A single species must cope with both spatial and temporal heterogeneity that trigger pulses of sexual and clonal establishment throughout its distributional range. We studied two populations of the clonal, purple prickly pear cactus, Opuntia macrocentra, in order to contrast the factors responsible for the population dynamics of a common, widely distributed species. The study sites were located in protected areas that correspond to extreme latitudinal locations for this species within the Chihuahuan Desert. We studied both populations for four consecutive years and determined the demographic consequences of environmental variability and the mode of reproduction using matrix population models, life table response experiments (LTREs), and loop and perturbation analyses. Although both populations seemed fairly stable (population growth rate, λ∼1), different demographic parameters and different life cycle routes were responsible for this stability in each population. In the southernmost population (MBR) LTRE and loop and elasticity analyses showed that stasis is the demographic process with the highest contributions to λ, followed by sexual reproduction, and clonal propagation contributed the least. The northern population (CR) had both higher elasticities and larger contributions of stasis, followed by clonal propagation and sexual recruitment. Loop analysis also showed that individuals in CR have more paths to complete a life cycle than those in MBR. As a consequence, each population differed in life history traits (e.g., size class structure, size at sexual maturity, and reproductive value). Numerical perturbation analyses showed a small effect of the seed bank on the λ of both populations, while the transition from seeds to seedlings had an important effect mainly in the northern population. Clonal propagation (higher survival and higher contributions to vital rates) seems to be more important for maintaining populations over long time periods than sexual reproduction.     

Spatial and temporal variations in species occurrence rate affect the accuracy of occurrence models

Aim Predictive models of species occurrence have potential for prioritizing areas for competing land uses. Before widespread application, however, it is necessary to evaluate performance using independent data and effective accuracy measures. The objectives of this study were to (1) compare the effects of species occurrence rate on model accuracy, (2) assess the effects of spatial and temporal variation in occurrence rate on model accuracy, and (3) determine if the number of predictor variables affected model accuracy. Location We predicted the distributions of breeding birds in three adjacent mountain ranges in the Great Basin (Nevada, USA). Methods For each of 18 species, we developed separate models using five different data sets — one set for each of 2 years (to address the effects of temporal variation), and one set for each of three possible pairs of mountain ranges (to address the effects of spatial variation). We evaluated each model with an independent data set using four accuracy measures: discrimination ability [area under a receiver operating characteristic curve (AUC)], correct classification rate (CCR), proportion of presences correctly classified (sensitivity), and proportion of absences correctly classified (specificity). Results Discrimination ability was not affected by occurrence rate, whereas the other three accuracy measures were significantly affected. CCR, sensitivity and specificity were affected by species occurrence rate in the evaluation data sets to a greater extent than in the model‐building data sets. Discrimination ability was the only accuracy measure affected by the number of variables in a model. Main conclusions Temporal variation in species occurrence appeared to have a greater impact than did spatial variation. When temporal variation in species distributions is great, the relative costs of omission and commission errors should be assessed and long‐term census data should be examined before using predictive models of occurrence in a management setting.     

Five (or so) challenges for species distribution modelling

Species distribution modelling is central to both fundamental and applied research in biogeography. Despite widespread use of models, there are still important conceptual ambiguities as well as biotic and algorithmic uncertainties that need to be investigated in order to increase confidence in model results. We identify and discuss five areas of enquiry that are of high importance for species distribution modelling: (1) clarification of the niche concept; (2) improved designs for sampling data for building models; (3) improved parameterization; (4) improved model selection and predictor contribution; and (5) improved model evaluation. The challenges discussed in this essay do not preclude the need for developments of other areas of research in this field. However, they are critical for allowing the science of species distribution modelling to move forward.     

Conformational investigation of alpha,beta-dehydropeptides. XV: N-acetyl-alpha,beta-dehydroamino acid N 'N '-dimethylamides: conformational properties from infrared and theoretical studies.

The FTIR spectra were analysed in the region of the nu(s)(N-H), AI(C=O) and nu(s)(Calpha=Cbeta) bands for a series of Ac-DeltaXaa-NMe2, where DeltaXaa = DeltaAla, (Z)-DeltaAbu, (Z)-DeltaLeu, (Z)-DeltaPhe and DeltaVal, to determine a predominant solution conformation of these alpha,beta-dehydropeptide-related molecules. Measurements were taken in CCl4, DCM and MeCN solutions. In the same way, spectra of saturated analogues Ac-Xaa-NMe2, where Xaa = Ala, Abu, Leu, Phe and Val, were investigated. To help interpret the spectroscopic results, conformational maps were calculated by the B3LYP/6-31+G** method. Also, the relative energies of all conformers of the dehydro compounds in vacuo as well as in the studied solvents in addition to the theoretical IR frequencies of these conformers were calculated. For comparison, molecules of two saturated analogues, Ac-L-Ala-NMe2 and Ac-L-Phe-NMe2, were calculated in a similar way. Both unsaturated and saturated compounds, which have an aliphatic side chain, occur in CCl4 and DCM mainly as a mixture of extended conformers with the C5 H-bond and open conformers. As solvent polarity increases, participation of the open conformers also increases, and in MeCN, the model amides are almost exclusively in the open form, except Ac-DeltaAla-NMe2, which shows a small amount of the H-bonded conformer. Ac-DeltaAla-NMe2 and Ac-DeltaAbu-NMe2 have stronger C5 hydrogen bonds than those of their saturated counterparts. As the calculations indicate, the open conformation of the unsaturated amides is conformer H/F with phi, psi -44 +/- 5 degrees, 127 +/- 4 degrees. This is the second lowest in energy conformer in vacuo and in CCl4 and the lowest one in more polar solvents. The open conformation of Ac-L-Ala-NMe2 constitutes conformer C with phi, psi -101.5 degrees, 112.7 degrees. For Ac-DeltaAla-NMe2 and Ac-DeltaAbu-NMe2, FTIR also reveals the presence of a third conformer. Calculations indicate that is the semiextended conformer D with the N1-H1...N2 hydrogen bond/contact. In all solvents, Ac-L-Phe-NMe2 and Ac-(Z)-DeltaPhe-NMe2 show only the extended E and the open H/F, respectively. In both there is an amide/pi(Ph) interaction.     

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.     

Dispersal and influences on movement for Anoplophora glabripennis calculated from individual mark-recapture

We conducted an individual mark‐release‐recapture experiment on the beetle, Anoplophora glabripennis Motchulsky (Coleoptera: Cerambycidae). This invasive beetle has been introduced from Asia to Europe and North America and poses a serious threat to several important species of tree. Eradication efforts may benefit from knowledge of dispersal behaviour. Trees were cut and held to determine emergence rate of A. glabripennis. Unique marks were painted onto 912 beetles released into a group of 165 trees in Gansu, China. Data on subsequent sightings of beetles were used in a truncated diffusion model to calculate flight distances. Characteristics of the trees and climatic information were used in statistical tests for influence on movement. A total of 2245 sightings of beetles were observed and 29% of marked beetles were resighted. The scanning technique using binoculars was 90% effective in finding beetles and provided 81% accuracy for determining the sex of the beetles. Experimental manipulation of density quantified how A. glabripennis congregated on unoccupied trees and were repulsed from crowded hosts. The seasonal emergence rate of adults declined exponentially from July 20 to August 5. The results suggested A. glabripennis fly to nearby host trees at a rate of 34% per day. Median flight distance was estimated at 20 m per day. Statistical analysis with a generalized linear model tested the beetle's propensity to leave a tree and distance of flight. Generally, beetle movement showed a significant response to beetle density, weather conditions, beetle size, and tree size, in that order. The techniques developed here improve on previous recapture techniques to quantify dispersal and can be useful for analysing populations of other organisms.