Showing papers by "University of California, Santa Barbara published in 2003"

Epitaxial BiFeO3 multiferroic thin film heterostructures.

Abstract: Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO3, is reported. Structure analysis indicates that the crystal structure of film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The observed enhancement is corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters.

Biogeochemical Hot Spots and Hot Moments at the Interface of Terrestrial and Aquatic Ecosystems

Abstract: Rates and reactions of biogeochemical processes vary in space and time to produce both hot spots and hot moments of elemental cycling. We define biogeochemical hot spots as patches that show disproportionately high reaction rates relative to the surrounding matrix, whereas hot moments are defined as short periods of time that exhibit disproportionately high reaction rates relative to longer intervening time periods. As has been appreciated by ecologists for decades, hot spot and hot moment activity is often enhanced at terrestrial-aquatic interfaces. Using examples from the carbon (C) and nitrogen (N) cycles, we show that hot spots occur where hydrological flowpaths converge with substrates or other flowpaths containing complementary or missing reactants. Hot moments occur when episodic hydrological flowpaths reactivate and/or mobilize accumulated reactants. By focusing on the delivery of specific missing reactants via hydrologic flowpaths, we can forge a better mechanistic understanding of the factors that create hot spots and hot moments. Such a mechanistic understanding is necessary so that biogeochemical hot spots can be identified at broader spatiotemporal scales and factored into quantitative models. We specifically recommend that resource managers incorporate both natural and artificially created biogeochemical hot spots into their plans for water quality management. Finally, we emphasize the needs for further research to assess the potential importance of hot spot and hot moment phenomena in the cycling of different bioactive elements, improve our ability to predict their occurrence, assess their importance in landscape biogeochemistry, and evaluate their utility as tools for resource management.

Global Trajectories of the Long-Term Decline of Coral Reef Ecosystems

Abstract: Degradation of coral reef ecosystems began centuries ago, but there is no global summary of the magnitude of change. We compiled records, extending back thousands of years, of the status and trends of seven major guilds of carnivores, herbivores, and architectural species from 14 regions. Large animals declined before small animals and architectural species, and Atlantic reefs declined before reefs in the Red Sea and Australia, but the trajectories of decline were markedly similar worldwide. All reefs were substantially degraded long before outbreaks of coral disease and bleaching. Regardless of these new threats, reefs will not survive without immediate protection from human exploitation over large spatial scales.

LATITUDINAL GRADIENTS OF BIODIVERSITY:Pattern,Process,Scale,and Synthesis

Abstract: ▪ Abstract The latitudinal gradient of decreasing richness from tropical to extratropical areas is ecology's longest recognized pattern. Nonetheless, notable exceptions to the general pattern exist, and it is well recognized that patterns may be dependent on characteristics of spatial scale and taxonomic hierarchy. We conducted an extensive survey of the literature and provide a synthetic assessment of the degree to which variation in patterns (positive linear, negative linear, modal, or nonsignificant) is a consequence of characteristics of scale (extent or focus) or taxon. In addition, we considered latitudinal gradients with respect to generic and familial richness, as well as species evenness and diversity. We provide a classification of the over 30 hypotheses advanced to account for the latitudinal gradient, and we discuss seven hypotheses with most promise for advancing ecological, biogeographic, and evolutionary understanding. We conclude with a forward-looking synthesis and list of fertile areas f...

Variations in microbial community composition through two soil depth profiles

Abstract: Soil profiles are often many meters deep, but with the majority of studies in soil microbiology focusing exclusively on the soil surface, we know very little about the nature of the microbial communities inhabiting the deeper soil horizons. We used phospholipid fatty acid (PLFA) analysis to examine the vertical distribution of specific microbial groups and to identify the patterns of microbial abundance and communitylevel diversity within the soil profile. Samples were collected from the soil surface down to 2 m in depth from two unsaturated Mollisol profiles located near Santa Barbara, CA, USA. While the densities of microorganisms were generally one to two orders of magnitude lower in the deeper horizons of both profiles than at the soil surface, approximately 35% of the total quantity of microbial biomass found in the top 2 m of soil is found below a depth of 25 cm. Principal components analysis of the PLFA signatures indicates that the composition of the soil microbial communities changes significantly with soil depth. The differentiation of microbial communities within the two profiles coincides with an overall decline in microbial diversity. The number of individual PLFAs detected in soil samples decreased by about a third from the soil surface down to 2 m. The ratios of cyclopropyl/monoenoic precursors and total saturated/total monounsaturated fatty acids increased with soil depth, suggesting that the microbes inhabiting the deeper soil horizons are more carbon limited than surface-dwelling microbes. Using PLFAs as biomarkers, we show that Gram-positive bacteria and actinomycetes tended to increase in proportional abundance with increasing soil depth, while the abundances of Gram-negative bacteria, fungi, and protozoa were highest at the soil surface and substantially lower in the subsurface. The vertical distribution of these specific microbial groups can largely be attributed to the decline in carbon availability with soil depth. q 2003 Elsevier Science Ltd. All rights reserved.

AIRS/AMSU/HSB on the Aqua mission: design, science objectives, data products, and processing systems

Abstract: The Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU), and the Humidity Sounder for Brazil (HSB) form an integrated cross-track scanning temperature and humidity sounding system on the Aqua satellite of the Earth Observing System (EOS). AIRS is an infrared spectrometer/radiometer that covers the 3.7-15.4-/spl mu/m spectral range with 2378 spectral channels. AMSU is a 15-channel microwave radiometer operating between 23 and 89 GHz. HSB is a four-channel microwave radiometer that makes measurements between 150 and 190 GHz. In addition to supporting the National Aeronautics and Space Administration's interest in process study and climate research, AIRS is the first hyperspectral infrared radiometer designed to support the operational requirements for medium-range weather forecasting of the National Ocean and Atmospheric Administration's National Centers for Environmental Prediction (NCEP) and other numerical weather forecasting centers. AIRS, together with the AMSU and HSB microwave radiometers, will achieve global retrieval accuracy of better than 1 K in the lower troposphere under clear and partly cloudy conditions. This paper presents an overview of the science objectives, AIRS/AMSU/HSB data products, retrieval algorithms, and the ground-data processing concepts. The EOS Aqua was launched on May 4, 2002 from Vandenberg AFB, CA, into a 705-km-high, sun-synchronous orbit. Based on the excellent radiometric and spectral performance demonstrated by AIRS during prelaunch testing, which has by now been verified during on-orbit testing, we expect the assimilation of AIRS data into the numerical weather forecast to result in significant forecast range and reliability improvements.

The impact of marine reserves: do reserves work and does reserve size matter?

Abstract: Marine reserves are quickly gaining popularity as a management option for marine conservation, fisheries, and other human uses of the oceans. Despite the popularity of marine reserves as a management tool, few reserves appear to have been created or designed with an understanding of how reserves affect biological factors or how reserves can be designed to meet biological goals more effectively (e.g., attaining sustainable fish populations). This shortcoming occurs in part because the many studies that have examined the impacts of reserves on marine organisms remain isolated examples or anecdotes; the results of these many studies have not yet been synthesized. Here, I review the empirical work and discuss the theoretical literature to assess the impacts of marine reserves on several biological measures (density, biomass, size of organisms, and diversity), paying particular attention to the role reserve size has in determining those impacts. The results of 89 separate studies show that, on average, with the exception of invertebrate biomass and size, values for all four biological measures are significantly higher inside reserves compared to outside (or after reserve establishment vs. before) when evaluated for both the overall communities and by each functional group within these communities (carniv- orous fishes, herbivorous fishes, planktivorous fishes/invertebrate eaters, and invertebrates). Surprisingly, results also show that the relative impacts of reserves, such as the proportional differences in density or biomass, are independent of reserve size, suggesting that the effects of marine reserves increase directly rather than proportionally with the size of a reserve. However, equal relative differences in biological measures between small and large reserves nearly always translate into greater absolute differences for larger reserves, and so larger reserves may be necessary to meet the goals set for marine reserves. The quality of the data in the reviewed studies varied greatly. To improve data quality in the future, whenever possible, studies should take measurements before and after the creation of a reserve, replicate sampling, and include a suite of representative species. Despite the variable quality of the data, the results from this review suggest that nearly any marine habitat can benefit from the implementation of a reserve. Success of a marine reserve, however, will always be judged against the expectations for that reserve, and so we must keep in mind the goals of a reserve in its design, management, and evaluation.

The implications of exoenzyme activity on microbial carbon and nitrogen limitation in soil: a theoretical model

Abstract: Traditional models of soil organic matter (SOM) decomposition are all based on first order kinetics in which the decomposition rate of a particular C pool is proportional to the size of the pool and a simple decomposition constant (dC/dt=kC). In fact, SOM decomposition is catalyzed by extracellular enzymes that are produced by microorganisms. We built a simple theoretical model to explore the behavior of the decomposition–microbial growth system when the fundamental kinetic assumption is changed from first order kinetics to exoenzymes catalyzed decomposition (dC/dt=KC×Enzymes). An analysis of the enzyme kinetics showed that there must be some mechanism to produce a non-linear response of decomposition rates to enzyme concentration—the most likely is competition for enzyme binding on solid substrates as predicted by Langmuir adsorption isotherm theory. This non-linearity also induces C limitation, regardless of the potential supply of C. The linked C and N version of the model showed that actual polymer breakdown and microbial use of the released monomers can be disconnected, and that it requires relatively little N to maintain the maximal rate of decomposition, regardless of the microbial biomass’ ability to use the breakdown products. In this model, adding a pulse of C to an N limited system increases respiration, while adding N actually decreases respiration (as C is redirected from waste respiration to microbial growth). For many years, researchers have argued that the lack of a respiratory response by soil microbes to added N indicates that they are not N limited. This model suggests that conclusion may be wrong. While total C flow may be limited by the functioning of the exoenzyme system, actual microbial growth may be N limited.

Introduced species and their missing parasites

Abstract: Damage caused by introduced species results from the high population densities and large body sizes that they attain in their new location1,2,3,4. Escape from the effects of natural enemies is a frequent explanation given for the success of introduced species5,6. Because some parasites can reduce host density7,8,9,10,11,12,13 and decrease body size14, an invader that leaves parasites behind and encounters few new parasites can experience a demographic release and become a pest4,15. To test whether introduced species are less parasitized, we have compared the parasites of exotic species in their native and introduced ranges, using 26 host species of molluscs, crustaceans, fishes, birds, mammals, amphibians and reptiles. Here we report that the number of parasite species found in native populations is twice that found in exotic populations. In addition, introduced populations are less heavily parasitized (in terms of percentage infected) than are native populations. Reduced parasitization of introduced species has several causes, including reduced probability of the introduction of parasites with exotic species (or early extinction after host establishment), absence of other required hosts in the new location, and the host-specific limitations of native parasites adapting to new hosts.

A reversibly switching surface.

Abstract: We report the design of surfaces that exhibit dynamic changes in interfacial properties, such as wettability, in response to an electrical potential. The change in wetting behavior was caused by surface-confined, single-layered molecules undergoing conformational transitions between a hydrophilic and a moderately hydrophobic state. Reversible conformational transitions were confirmed at a molecular level with the use of sum-frequency generation spectroscopy and at a macroscopic level with the use of contact angle measurements. This type of surface design enables amplification of molecular-level conformational transitions to macroscopic changes in surface properties without altering the chemical identity of the surface. Such reversibly switching surfaces may open previously unknown opportunities in interfacial engineering.

Dense packing and symmetry in small clusters of microspheres.

Abstract: When small numbers of colloidal microspheres are attached to the surfaces of liquid emulsion droplets, removing fluid from the droplets leads to packings of spheres that minimize the second moment of the mass distribution. The structures of the packings range from sphere doublets, triangles, and tetrahedra to exotic polyhedra not found in infinite lattice packings, molecules, or minimum-potential energy clusters. The emulsion system presents a route to produce new colloidal structures and a means to study how different physical constraints affect symmetry in small parcels of matter.

Propagule dispersal in marine and terrestrial environments: a community perspective

Abstract: Studies in terrestrial systems suggest that long-distance propagule dispersal is important for landscape pattern and dynamics, but largely inconsequential for local demography. By contrast, in marine systems, dispersal at regional scales may drive local dynamics, because many species may have large mean dispersal distances. To assess var- iation in marine dispersal scales, we estimated mean dispersal distances from genetic iso- lation-by-distance slopes. Estimates ranged widely, from a few meters to hundreds of kilometers. Dispersal differed among taxonomic groups (macroalgae, invertebrates, and fish) and among species in different functional groups (e.g., producers and herbivores). Differences in dispersal scale have important implications for marine community dynamics, reserve design, responses to large-scale perturbations, and evolution of interacting species. To place genetic estimates of marine dispersal in context, we compared them to other measures of dispersal in the ocean and to estimates of dispersal on land. Maximum scales of dispersal by sedentary marine species exceeded maximum estimates of terrestrial plant dispersal by at least one to two orders of magnitude. Direct and genetic estimates of terrestrial plant dispersal were comparable to estimates of marine plant dispersal. Rates of marine macroalgal range expansion, however, far exceeded spread rates of terrestrial plants. Terrestrial plant spread rates were more similar to those of short-dispersing marine organ- isms that lack secondary dispersal by drifting adults. Genetic estimates of dispersal by different functional groups suggest that herbivores typically disperse much farther than their plant resources both on land and in the sea, although the timing, frequency, and consequences of dispersal may differ in the two systems. Terrestrial herbivores have more flexible dispersal behavior than marine organisms that disperse each generation by plank- tonic transport of larvae. Our results validate some long-standing views about the greater dispersal potential of species in the ocean, but also highlight the extreme heterogeneity in dispersal scale among marine species. As a result, development of a community perspective on marine connectivity will require consideration of multiple dispersal mechanisms and scales.

Ultrahigh-Density Nanowire Lattices and Circuits

Abstract: We describe a general method for producing ultrahigh-density arrays of aligned metal and semiconductor nanowires and nanowire circuits. The technique is based on translating thin film growth thickness control into planar wire arrays. Nanowires were fabricated with diameters and pitches (center-to-center distances) as small as 8 nanometers and 16 nanometers, respectively. The nanowires have high aspect ratios (up to 106), and the process can be carried out multiple times to produce simple circuits of crossed nanowires with a nanowire junction density in excess of 1011 per square centimeter. The nanowires can also be used in nanomechanical devices; a high-frequency nanomechanical resonator is demonstrated.

Electrochemical interrogation of conformational changes as a reagentless method for the sequence-specific detection of DNA.

Abstract: We report a strategy for the reagentless transduction of DNA hybridization into a readily detectable electrochemical signal by means of a conformational change analogous to the optical molecular beacon approach. The strategy involves an electroactive, ferrocene-tagged DNA stem-loop structure that self-assembles onto a gold electrode by means of facile gold-thiol chemistry. Hybridization induces a large conformational change in this surface-confined DNA structure, which in turn significantly alters the electron-transfer tunneling distance between the electrode and the redoxable label. The resulting change in electron transfer efficiency is readily measured by cyclic voltammetry at target DNA concentrations as low as 10 pM. In contrast to existing optical approaches, an electrochemical DNA (E-DNA) sensor built on this strategy can detect femtomoles of target DNA without employing cumbersome and expensive optics, light sources, or photodetectors. In contrast to previously reported electrochemical approaches, the E-DNA sensor achieves this impressive sensitivity without the use of exogenous reagents and without sacrificing selectivity or reusability. The E-DNA sensor thus offers the promise of convenient, reusable detection of picomolar DNA.

The Influence of Media Violence on Youth

Abstract: Research on violent television and films, video games, and music reveals unequivocal evidence that media violence increases the likelihood of aggressive and violent behavior in both immediate and long-term contexts. The effects appear larger for milder than for more severe forms of aggression, but the effects on severe forms of violence are also substantial (r = .13 to .32) when compared with effects of other violence risk factors or medical effects deemed important by the medical community (e.g., effect of aspirin on heart attacks). The research base is large; diverse in methods, samples, and media genres; and consistent in overall findings. The evidence is clearest within the most extensively researched domain, television and film violence. The growing body of video-game research yields essentially the same conclusions. Short-term exposure increases the likelihood of physically and verbally aggressive behavior, aggressive thoughts, and aggressive emotions. Recent large-scale longitudinal studies provide converging evidence linking frequent exposure to violent media in childhood with aggression later in life, including physical assaults and spouse abuse. Because extremely violent criminal behaviors (e.g., forcible rape, aggravated assault, homicide) are rare, new longitudinal studies with larger samples are needed to estimate accurately how much habitual childhood exposure to media violence increases the risk for extreme violence. Well-supported theory delineates why and when exposure to media violence increases aggression and violence. Media violence produces short-term increases by priming existing aggressive scripts and cognitions, increasing physiological arousal, and triggering an automatic tendency to imitate observed behaviors. Media violence produces long-term effects via several types of learning processes leading to the acquisition of lasting (and automatically accessible) aggressive scripts, interpretational schemas, and aggression-supporting beliefs about social behavior, and by reducing individuals' normal negative emotional responses to violence (i.e., desensitization). Certain characteristics of viewers (e.g., identification with aggressive characters), social environments (e.g., parental influences), and media content (e.g., attractiveness of the perpetrator) can influence the degree to which media violence affects aggression, but there are some inconsistencies in research results. This research also suggests some avenues for preventive intervention (e.g., parental supervision, interpretation, and control of children's media use). However, extant research on moderators suggests that no one is wholly immune to the effects of media violence. Recent surveys reveal an extensive presence of violence in modern media. Furthermore, many children and youth spend an inordinate amount of time consuming violent media. Although it is clear that reducing exposure to media violence will reduce aggression and violence, it is less clear what sorts of interventions will produce a reduction in exposure. The sparse research literature suggests that counterattitudinal and parental-mediation interventions are likely to yield beneficial effects, but that media literacy interventions by themselves are unsuccessful. Though the scientific debate over whether media violence increases aggression and violence is essentially over, several critical tasks remain. Additional laboratory and field studies are needed for a better understanding of underlying psychological processes, which eventually should lead to more effective interventions. Large-scale longitudinal studies would help specify the magnitude of media-violence effects on the most severe types of violence. Meeting the larger societal challenge of providing children and youth with a much healthier media diet may prove to be more difficult and costly, especially if the scientific, news, public policy, and entertainment communities fail to educate the general public about the real risks of media-violence exposure to children and youth.

Materials selection guidelines for low thermal conductivity thermal barrier coatings

Abstract: Materials selection guidelines are desirable in identifying and developing alternative materials for higher-temperature capability thermal barrier coatings. Some relate to identifying candidate materials that exhibit particularly low values of thermal conductivity at high temperatures and others relate to thermodynamic stability in contact with the thermally grown oxides formed on bond-coat alloys and superalloys. By using existing theories of the minimum thermal conductivity, a materials parameter is developed that can be used to identify candidate alternatives to yttria-stabilized zirconia for high-temperature applications.

The use of newspaper data in the study of collective action

Abstract: Studying collective action with newspaper accounts of protest events, rare only 20 years ago, has become commonplace in the past decade. A critical literature has accompanied the growth of protest event analysis. The literature has focused on selection bias—particularly which subset of events are covered—and description bias—notably, the veracity of the coverage. The “hard news” of the event, if it is reported, tends to be relatively accurate. However, a newspaper's decision to cover an event at all is influenced by the type of event, the news agency, and the issue involved. In this review, we discuss approaches to detecting bias, as well as ways to factor knowledge about bias into interpretations of protest event data.

Plant ecotypes: genetic differentiation in the age of ecological restoration

Abstract: Recent studies illustrate the emerging field of restoration genetics, which is a synthesis of restoration ecology and population genetics. The translocation of organisms during the restoration of native ecosystems has provoked new questions concerning the consequences of sampling protocols and of intraspecific hybridization between locally adapted and transplanted genotypes. Studies are now underway to determine both the extent of local adaptation among focal populations and the potential risks of introducing foreign genotypes, including founder effects, genetic swamping and outbreeding depression. Data are needed to delineate ‘seed transfer zones', or regions within which plants can be moved with little or no consequences for population fitness. Here, we address the revival of transplant and common garden studies, the use of novel molecular markers to predict population genetic consequences of translocation, and their combined power for determining appropriate seed transfer zones in restoration planning for native plant populations.

Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H 2 O contents

Abstract: [1] We present a new compilation of physical properties of minerals relevant to subduction zones and new phase diagrams for mid-ocean ridge basalt, lherzolite, depleted lherzolite, harzburgite, and serpentinite. We use these data to calculate H2O content, density and seismic wave speeds of subduction zone rocks. These calculations provide a new basis for evaluating the subduction factory, including (1) the presence of hydrous phases and the distribution of H2O within a subduction zone; (2) the densification of the subducting slab and resultant effects on measured gravity and slab shape; and (3) the variations in seismic wave speeds resulting from thermal and metamorphic processes at depth. In considering specific examples, we find that for ocean basins worldwide the lower oceanic crust is partially hydrated (<1.3 wt % H2O), and the uppermost mantle ranges from unhydrated to � 20% serpentinized (� 2.4 wt % H2O). Anhydrous eclogite cannot be distinguished from harzburgite on the basis of wave speeds, but its � 6% greater density may render it detectable through gravity measurements. Subducted hydrous crust in cold slabs can persist to several gigapascals at seismic velocities that are several percent slower than the surrounding mantle. Seismic velocities and VP/VS ratios indicate that mantle wedges locally reach 60–80% hydration. INDEX TERMS: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 3660 Mineralogy and Petrology: Metamorphic petrology; 3919 Mineral Physics: Equations of state; 5199 Physical Properties of Rocks: General or miscellaneous; 8123 Tectonophysics: Dynamics, seismotectonics; KEYWORDS: subduction, seismic velocities, mineral physics, H2O

Towards Quantum Superpositions of a Mirror

Abstract: We propose an experiment for creating quantum superposition states involving of the order of 10(14) atoms via the interaction of a single photon with a tiny mirror. This mirror, mounted on a high-quality mechanical oscillator, is part of a high-finesse optical cavity which forms one arm of a Michelson interferometer. By observing the interference of the photon only, one can study the creation and decoherence of superpositions involving the mirror. A detailed analysis of the requirements shows that the experiment is within reach using a combination of state-of-the-art technologies.

Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?

Abstract: [1] New thermal-petrologic models of subduction zones are used to test the hypothesis that intermediate-depth intraslab earthquakes are linked to metamorphic dehydration reactions in the subducting oceanic crust and mantle. We show that there is a correlation between the patterns of intermediate-depth seismicity and the locations of predicted hydrous minerals: Earthquakes occur in subducting slabs where dehydration is expected, and they are absent from parts of slabs predicted to be anhydrous. We propose that a subductingoceanicplatecanconsistoffourpetrologicallyandseismicallydistinctlayers:(1) hydrated, fine-grained basaltic upper crust dehydrating under equilibrium conditions and producing earthquakes facilitated by dehydration embrittlement; (2) coarse-grained, locally hydrated gabbroic lower crust that produces some earthquakes during dehydration but transformschieflyaseismicallytoeclogiteatdepthsbeyondequilibrium;(3)locallyhydrated uppermost mantle dehydrating under equilibrium conditions and producing earthquakes; and (4) anhydrous mantle lithosphere transforming sluggishly and aseismically to denser minerals. Fluid generated through dehydration reactions can move via at least three distinct flowpaths:percolationthroughlocal,transient,reaction-generatedhigh-permeabilityzones; flow through mode I cracks produced by the local stress state; and postseismic flow through fault zones. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7230 Seismology: Seismicity and seismotectonics; 8123 Tectonophysics: Dynamics, seismotectonics; 8135 Tectonophysics: Evolution of the Earth: Hydrothermalsystems (8424); 3660 Mineralogyand Petrology: Metamorphicpetrology;

Species diversity: from global decreases to local increases

Abstract: Current patterns of global change can strongly affect biodiversity at global, regional and local scales. At global scales, habitat destruction and the introduction of exotic species are contributing to declines in species diversity. At regional and local scales, evidence for declines in diversity is mixed, and recent work suggests that diversity might commonly be increasing. In spite of these trends, considerable research continues to consider explicitly the effects of declines in diversity on processes that operate at regional and local scales (such as ecosystem functioning), without explicitly considering the converse set of questions, namely the effects of increases in diversity. Here, we examine evidence that indicates how species diversity is changing across spatial scales and argue that global decreases in diversity are commonly contrasted by increases in diversity at regional and local scales.

The Ecology and Evolution of Seed Dispersal: A Theoretical Perspective

Abstract: ▪ Abstract Models of seed dispersal—a key process in plant spatial dynamics—have played a fundamental role in representing dispersal patterns, investigating dispersal processes, elucidating the consequences of dispersal for populations and communities, and explaining dispersal evolution. Mechanistic models of seed dispersal have explained seed dispersion patterns expected under different conditions, and illuminated the circumstances that lead to long-distance dispersal in particular. Phenomenological models have allowed us to describe dispersal pattern and can be incorporated into models of the implications of dispersal. Perhaps most notably, population and community models have shown that not only mean dispersal distances but also the entire distribution of dispersal distances are critical to range expansion rates, recruitment patterns, genetic structure, metapopulation dynamics, and ultimately community diversity at different scales. Here, we review these developments, and provide suggestions for furthe...

Toward an Understanding of Definitions and Measures of School Engagement and Related Terms

Abstract: This article provides an overview of definitions and measures related to school engagement. The intent herein, is to explore the construct and measurement of school engagement and related terms and provide a summary of previous literature, in an effort to offer a foundation to advance related scholarship and practice. Previous articles exploring school engagement, school bonding, and other associated terms (e.g., school attachment, school commitment, motivation) include a variety of definitions and measures. Items used in previous research addressing school engagement and related terms were classified into five contexts: a) academic performance, b) classroom behavior, c) extracurricular involvement, d) interpersonal relationships, and e) school community. Based on this review, it is suggested that school engagement is a multifaceted construct that includes affective, behavioral, and cognitive dimensions. Conceptualizing school engagement as a multifaceted construct has implications for both research and practice.