Papers by Jessica Gurevitch
Quality Standards for Research Syntheses
Handbook of Meta-analysis in Ecology and Evolution
This chapter presents guidelines to address the following questions: What makes a quantitative re... more This chapter presents guidelines to address the following questions: What makes a quantitative research synthesis good or flawed? How can authors improve the quality of their review at various stages in the process of planning and carrying out a research synthesis? What criteria can editors and reviewers use to assess whether a quantitative synthesis should be accepted for publication, revised, or rejected? How can readers of published syntheses determine how to evaluate the quality of what they are reading, and in doing so decide whether or not to trust its results and their interpretation? The guidelines are outlined in the order of the stages involved in conducting a synthesis. In addition to reviewing the questions that should be asked at each stage of the synthesis, the chapter also describes ways in which poor choices at each stage can compromise the integrity of the review; it also provides examples of good and poor practice.
The Interaction between Soil Nutrients and Leaf Loss during Early Establishment in Plant Invasion, 2004
Nitrogen availability is expected to affect both plant growth and the preferences of herbivores. ... more Nitrogen availability is expected to affect both plant growth and the preferences of herbivores. We hypothesized that an interaction between these two factors could affect the early establishment of native and exotic species differently, promoting invasion in natural systems. Taxonomically paired native and invasive species (Acer platanoides, Acer rubrum, Lonicera maackii, Diervilla lonicera, Celastrus orbiculatus, Celastrus scandens, Elaeagnus umbellata, Ceanothus americanus, Ampelopsis brevipedunculata, and Vitis riparia) were grown in relatively high-resource (hardwood forests) and low-resource (pine barrens) communities on Long Island, New York, USA for a period of 3 months, in 2004. Plants were grown in ambient and nitrogen-enhanced conditions in both communities. Nitrogen additions produced an average 12% initial increase in leaf number of all plants. By the end of the experiment, invasive species outperformed native species in nitrogen-enhanced plots in hardwood forests, wher...
Genetics, 1992
Achillea lanulosa has complex, highly dissected leaves that vary in shape and size along an altit... more Achillea lanulosa has complex, highly dissected leaves that vary in shape and size along an altitudinal gradient. Plants from a high and an intermediate altitude population were clonally replicated and grown in a controlled environment at warm and cool conditions under bright light. There were genetic differences among populations and among individuals within populations in leaf size and shape. Heritabilities for leaf size and shape characters were moderate. Leaves of the lower altitude population were larger and differed from the higher altitude plants in both coarse and fine shape. Plastic response to temperature of the growth environment paralleled the genetic differentiation between low and high altitude populations. There was no apparent trade-off between genetic control over morphology and the capacity for directional plastic response to the environment. Differences in leaf dissection and size at contrasting altitudes in this species are the result of both genetic divergence a...
Ecology and evolution, 2012
Biodiversity is the diversity of life at all scales, from genes to ecosystems. Predicting its pat... more Biodiversity is the diversity of life at all scales, from genes to ecosystems. Predicting its patterns of variation across the globe is a fundamental issue in ecology and evolution. Diversity within species, that is, genetic diversity, is of prime importance for understanding past and present evolutionary patterns, and highlighting areas where conservation might be a priority. Using published data on the genetic diversity of species whose populations occur in the Mediterranean basin, we calculated a coefficient of correlation between within-population genetic diversity indices and longitude. Using a meta-analysis framework, we estimated the role of biological, ecological, biogeographic, and marker type factors on the strength and magnitude of this correlation in six phylla. Overall, genetic diversity increases from west to east in the Mediterranean basin. This correlation is significant for both animals and plants, but is not uniformly expressed for all groups. It is stronger in the...
Trends in ecology & evolution, 2004
The link between species invasions and the extinction of natives is widely accepted by scientists... more The link between species invasions and the extinction of natives is widely accepted by scientists as well as conservationists, but available data supporting invasion as a cause of extinctions are, in many cases, anecdotal, speculative and based upon limited observation. We pose the question, are aliens generally responsible for widespread extinctions? Our goal is to prompt a more critical synthesis and evaluation of the available data, and to suggest ways to take a more scientific, evidence-based approach to understanding the impact of invasive species on extinctions. Greater clarity in our understanding of these patterns will help us to focus on the most effective ways to reduce or mitigate extinction threats from invasive species.
Ecology and evolution, 2012
Species introductions of anthropogenic origins are a major aspect of rapid ecological change glob... more Species introductions of anthropogenic origins are a major aspect of rapid ecological change globally. Research on biological invasions has generated a large literature on many different aspects of this phenomenon. Here, we describe and categorize some aspects of this literature, to better understand what has been studied and what we know, mapping well-studied areas and important gaps. To do so, we employ the techniques of systematic reviewing widely adopted in other scientific disciplines, to further the use of approaches in reviewing the literature that are as scientific, repeatable, and transparent as those employed in a primary study. We identified 2398 relevant studies in a field synopsis of the biological invasions literature. A majority of these studies (58%) were concerned with hypotheses for causes of biological invasions, while studies on impacts of invasions were the next most common (32% of the publications). We examined 1537 papers in greater detail in a systematic revi...
Experimental manipulation of natural plant communities
Trends in Ecology & Evolution, 1994
Attempts to elucidate the factors controlling the structure of plant communities have relied incr... more Attempts to elucidate the factors controlling the structure of plant communities have relied increasingly on field experiments. This is a powerful approach for testing theoretical predictions that offers important advantages over observational and comparative studies. However, field experiments suffer from intrinsic difficulties as well as more-easily remediable limitations. Recent progress has been made by new approaches including the use of multifactor experiments, and the development and dissemination of better statistical tools.
The Interaction between Competition and Predation: A Meta‐analysis of Field Experiments
The American Naturalist, 2000
Ecologists working with a range of organisms and environments have carried out manipulative field... more Ecologists working with a range of organisms and environments have carried out manipulative field experiments that enable us to ask questions about the interaction between competition and predation (including herbivory) and about the relative strength of competition and predation in the field. Evaluated together, such a collection of studies can offer insight into the importance and function of these factors in nature. Using a new factorial meta-analysis technique, we combined the results of 20 articles reporting on 39 published field experiments to ask whether the presence of predators affects the intensity of competitive effects and to compare the average effects of competition and predation. Across all studies, the effects of competition in the presence of predators were less than in the absence of predators, and the interaction between competition and predation for most response variables was statistically significant. Removal of competitors had much more positive effects on organisms' growth and mass than did exclusion of predators. Predator exclusion had much more beneficial effects on organisms' survival than did competition. The mean effects of competition and predation on density did not differ from one another. The results differed among trophic levels. Further understanding would benefit greatly from more field experiments that manipulate both competition and predation, that focus on a wider range of organisms and environments, that focus on population-level parameters such as density, and that report results more completely, including data such as sample sizes and variances.
Proceedings of the National Academy of Sciences, 2004
Ecological perturbations can either be necessary for maintaining tropical forest diversity or res... more Ecological perturbations can either be necessary for maintaining tropical forest diversity or responsible for its decline, depending on the scale, nature, and frequency of the disturbance. Anthropogenic disturbances such as logging and subsistence agriculture may promote the establishment of nonnative, invasive plant species, potentially affecting forest structure and diversity even long after the perturbation has ceased. We investigated the impacts of logging 50 and 150 years ago on tropical forest vegetation in Madagascar, a “hotspot” of biodiversity. Logging was the overriding factor influencing establishment of nonnative plants. Sites once logged never recovered native species diversity because of the dominance and persistence of invasive species.
Planta, 1988
Carbon-isotope ratios were examined as c513C values in several C3, C~, and C3-C4 F l a v e r i a ... more Carbon-isotope ratios were examined as c513C values in several C3, C~, and C3-C4 F l a v e r i a species, and compared to predicted ~3 C values generated from theoretical models. The measured cI13C values were within 4%~ of those predicted from the models. The models were used to identify factors that contribute to C3-1ike 5t3C values in C3-C4 species that exhibit considerable CA-cycle activity. Two of the factors contributing to C3-1ike fi13C values are high CO2 leakiness from the C4 pathway and pi/pa values that were higher than C4 congeners. A marked break occurred in the relationship between the percentage of atmospheric CO2 assimilated through the C4 cycle and the c5 13 C value. Below 50% C4-cycle assimilation there was no significant relationship between the variables, but above 50% the 613C values became less negative. These results demonstrate that the level of C4-cycle expression can increase from 0 to 50% with little integration of carbon transfer from the C4 to the C3 cycle. As expression increases above 50%, however, increased integration of C3-and C4-cycle co-function occurs.
Oecologia, 1984
Three populations of the grass Danthonia spicata were observed to have different rates of biomass... more Three populations of the grass Danthonia spicata were observed to have different rates of biomass accumulation when grown in common environment treatments. The populations were native to adjacent sites of different successional age and different levels of shading. Twelve individuals from each population were clonally replicated and two replicates were grown in each of two light treatments, 100% and 22% of unshaded sunlight. Following growth in the treatments the populations all exhibited the same mean light-saturated photosynthetic rate of 11.7 ~tmol m-2s-1. This rate is intermediate for published values of sun and shade species and for species from along a successional gradient. There was no difference in photosynthetic rate among treatments. There was significant genetic variation for lightsaturated photosynthetic rate within populations but no significant differences among populations. The populations had similar leaf water potential values of -1.12 MPa in all treatments. There were significant differences among treatments and genotypes for specific leaf weight which resulted in significant differences among treatments and no significant differences among genotypes in light-saturated photosynthetic rate expressed on a leaf weight basis. Lightsaturated photosynthetic rate had a high heritability and low plasticity. We postulate that photosynthetic rate is under strong selection and that the observed rates permit populations of D. spicata to grow in a wide range of habitat light levels.
Oecologia, 1986
is one of only a few species in the Crassulaceae for which there is evidence for a high degree of... more is one of only a few species in the Crassulaceae for which there is evidence for a high degree of variability in the ratio of daytime to nighttime CO2 assimilation. There are both environmental and genetic components to this variability. S. wrightii grows over a wide altitudinal gradient. The purpose of this study was to compare low, intermediate, and high altitude populations with respect to the degree of CAM expression and the capability to tolerate limited water availability. We utilized clonallyreplicated genotypes of plants from each population in common environment greenhouse experiments. Genetic differences among the populations were found in long-term water use efficiency, in 24 hour CO2 exchange patterns, in biomass fi13C values, in carbon allocation, and in water status and ultimately survival during prolonged drought. The differences among the populations appear to be closely related to differences in the native habitats. The low altitude, desert plants had the greatest ability to grow and survive under conditions of limited water availability and appear to have the greatest shift to nighttime CO2 uptake during periods without water, while the high altitude plants had the poorest performance under these conditions and appear to shut down net carbon uptake when severely water limited.
Journal of Vegetation Science, 1995
population size . Consequently, the existence of size hierarchies has important implications for ... more population size . Consequently, the existence of size hierarchies has important implications for both ecological and evolutionary aspects of the structure and function of plant populations . Weiner & Thomas (1986) listed six possible causes of size hierarchies: age differences, genetic differences, environmental heterogeneity, maternal effects, competition, and differential effects of herbivory, parasitism, and disease. In a review of published experiments, they found that the most frequent explanation for size hierarchies is asymmetric competition in which small plants are overtopped by larger ones, leading to unequal access to light and progressively greater and greater inequality over time (until the onset of density-dependent mortality). While considerable work on this topic has been done on experimental populations of annuals in greenhouses, little is known about the relationship between size inequalities among individuals and the actual sizes of their neighbors in natural populations of annual species, making it difficult to assess the validity or generality of this explanation. Here we evaluate evidence for asymmetrical competition and other factors as causes of a size hierarchy within a natural population, using spatial analysis to take into account spatial patterns within the population. If asymmetrical intraspecific competition is an important cause of size hierarchies, then the data should demonstrate that large, dominant individuals are associated with small, suppressed neighbors. This critical information has been missing from previous studies. The goals of this study were to determine the degree of inequality among individual plant sizes in a dense natural stand of an annual, to examine spatial correlations between the sizes of individual plants and the sizes of their neighbors, and to analyze the effect of the presence, density and combined sizes of neighbors on focal plant size. We studied a population of a small, non-clonal annual plant, Myosotis micrantha (Boraginaceae). Plants germinate in the fall, overwinter as rosettes, flower in the early spring and die by late spring. Most vegetative Abstract. We examined spatial distributions and plant sizes along a transect through a natural population of a winter annual, Myosotis micrantha. A size hierarchy existed, as indicated by high values of Gini coefficients of inequality for plant mass and correlated measures. Plants with no immediate conspecific neighbors were larger than plants with one or more near neighbors, suggesting that competition from near neighbors depressed plant size. However, there was strong positive spatial autocorrelation in plant size: large plants were associated with large neighbors and small ones with small neighbors. Plant size was also positively correlated with the combined biomass of near neighbors. The population formed a two-phase mosaic of patches of relatively large plants alternating with patches of smaller plants. The data suggest that individual plants compete with conspecifics, but the effects of competition are symmetrical. The most likely explanations for this spatially structured size hierarchy are variation in plant density, patchy distribution of resources, or a combination of the two.
Journal of Ecology, 2005
1 Following severe, stand-replacing fires in 1995, we quantified emergence, growth and survival i... more 1 Following severe, stand-replacing fires in 1995, we quantified emergence, growth and survival in Pinus rigida seedlings in eight stands in three areas of the pine barrens of Long Island, New York, USA, and examined factors contributing to spatial and temporal variation in recruitment. 2 We followed 6431 marked seedlings in the first cohort following fire, as well as additional cohorts in subsequent years, for 7 years to assess the effects of fire intensity, soil characteristics, intraspecific density, interspecific competition and facilitation on variation in early demography at three different scales. We found substantial variation in many of the demographic parameters measured between plots within stands (10-50 m apart), among stands within areas (600 -1500 m apart), and among forest areas (5 -30 km apart), as well as among years. 3 Almost all adult P. rigida in most of the stands studied were killed by the 1995 fires and population recovery therefore depended upon recruitment from seed. Initial recruitment appears to be largely determined by seed limitation, which was affected by fire intensity combined with serotiny and other factors. 4 Subsequent seedling survival and growth were determined largely by inherent differences among areas and sites in factors including cover of the shrub Quercus ilicifolia and soil texture and composition. 5 Initial seedling density, and subsequent survival and growth, varied among plots within sites, among sites within areas, and among areas. Early survival was negatively correlated with intraspecific seedling density, but was enhanced by neighbouring Q. ilicifolia , although these shrubs inhibited later seedling growth. Early demographic variation may determine many of the differences observed among mature populations in this landscape. 6 Variation in demographic parameters across space and time, and over a hierarchy of scales, may have critical consequences at the population, community and landscape levels. A comprehensive evaluation of the nature and extent of such demographic variation across different systems would have major implications for understanding vegetation patterns at the population, community and landscape levels.
Journal of Biogeography, 2003
Differences in Photosynthetic Rate in Populations of Achillea lanulosa from Two Atitudes
Functional Ecology, 1992
1. Light-saturated photosynthetic rates were compared for plants collected from populations at tw... more 1. Light-saturated photosynthetic rates were compared for plants collected from populations at two elevations (1400 m and 3050 m) in the California Sierra. Plants were grown under warm and cool temperature regimes in a glasshouse at high photon flux densities. 2. There were large differences between the populations in photosynthetic carbon uptake, with the high-altitude plants exhibiting greater assimilation rates than those from the low-altitude population under both thermal regimes. Light-saturated photosynthetic rates did not differ ...
Forest Ecology and Management, 2006
We report the effects of an invasive tree (Syzygium jambos, Myrtaceace) on species composition, p... more We report the effects of an invasive tree (Syzygium jambos, Myrtaceace) on species composition, plant diversity patterns, and forest regeneration in primary and secondary forest in the Luquillo Mountains of northeastern Puerto Rico, including the area in and around the Caribbean National Forest (CNF) and the Luquillo Long Term Ecological Research site (Luquillo LTER). Land use history was reconstructed using aerial photographs from 1936 to 1989 and study sites were categorized into four groups that corresponded to their status in 1936: unforested, young secondary, mature secondary, and primary forests. In randomly selected forest stands in each forest type, we measured the abundance of invasive and native tree species, seedling recruitment for S. jambos as well as soil nutrient pools and tested for the effects of land use history on S. jambos density and diversity. A partial Mantel test was used to control for historical and elevational differences across study sites. The results indicate that S. jambos density was highest in habitats classified in 1936 as unforested, young, or mature secondary forests. Compared to all other forest classes, species diversity was significantly higher in primary forests. However, there was no statistically significant difference between observed and estimated species richness across the four forest types. S. jambos density and species diversity were strongly negatively correlated, even after controlling for land use history and elevation. There was significantly higher S. jambos seedling recruitment in areas that were either unforested or had young secondary forests in 1936. The results also indicate that S. jambos is able to establish viable populations in habitats with different soil nutrient status. S. jambos has also altered vegetation composition and diversity patterns in habitats where it is the dominant tree species. After nearly 185 years since its introduction to the island, S. jambos is not only well established within 30 m of stream channels, its presence does not appear to be limited by topographic, soil nutrient, or elevational conditions. This study suggests that land use change and subsequent plant invasions have produced a new vegetation assemblage that has led to potentially long-term changes in community structure, species composition, and successional trajectory in regenerating secondary forests in the Luquillo Mountains.
Ecology Letters, 2006
Invasion biologists often suggest that phenotypic plasticity plays an important role in successfu... more Invasion biologists often suggest that phenotypic plasticity plays an important role in successful plant invasions. Assuming that plasticity enhances ecological niche breadth and therefore confers a fitness advantage, recent studies have posed two main hypotheses: (1) invasive species are more plastic than non‐invasive or native ones; (2) populations in the introduced range of an invasive species have evolved greater plasticity than populations in the native range. These two hypotheses largely reflect the disparate interests of ecologists and evolutionary biologists. Because these sciences are typically interested in different temporal and spatial scales, we describe what is required to assess phenotypic plasticity at different levels. We explore the inevitable tradeoffs of experiments conducted at the genotype vs. species level, outline components of experimental design required to identify plasticity at different levels, and review some examples from the recent literature. Moreove...
Ecology, 1999
Meta-analysis provides formal statistical techniques for summarizing the results of independent e... more Meta-analysis provides formal statistical techniques for summarizing the results of independent experiments and is increasingly being used in ecology. The response ratio (the ratio of mean outcome in the experimental group to that in the control group) and closely related measures of proportionate change are often used as measures of effect magnitude in ecology. Using these metrics for meta-analysis requires knowledge of their statistical properties, but these have not been previously derived. We give the approximate sampling distribution of the log response ratio, discuss why it is a particularly useful metric for many applications in ecology, and demonstrate how to use it in meta-analysis. The meta-analysis of response-ratio data is illustrated using experimental data on the effects of increased atmospheric CO 2 on plant biomass responses.
Ecology, 1999
Quantitative synthesis across studies requires consistent measures of effect size among studies. ... more Quantitative synthesis across studies requires consistent measures of effect size among studies. In community ecology, these measures of effect size will often be some measure of the strength of interactions between taxa. However, indices of interaction strength vary greatly among both theoretical and empirical studies, and the connection between hypotheses about interaction strength and the metrics that are used to test these hypotheses are often not explicit. We describe criteria for choosing appropriate metrics and methods for comparing them among studies at three stages of designing a meta-analysis to test hypotheses about variation in interaction intensity: (1) the choice of response variable; (2) how effect size is calculated using the response in two treatments; and (3) whether there is a consistent quantitative effect across all taxa and systems studied or only qualitatively similar effects within each taxonsystem combination. The consequences of different choices at each of these stages are illustrated with a meta-analysis to examine the relationship between competition/facilitation intensity and productivity in plants. The analysis used a database of 296 cases in 14 studies. The results were unexpected and largely inconsistent with existing theory: competition intensity often significantly declined (rather than increased) with productivity, and facilitation was sometimes restricted to more productive (rather than less productive) sites. However, there was considerable variation in the pattern among response variables and measures of effect size. For example, on average, competitive effects on final biomass and survival decreased with standing crop, but competitive effects on growth rate did not. On the other hand, facilitative interactions were more common at low standing crop for final biomass and growth rate, but more common at high standing crop for survival. Results were more likely to be significant using the log response ratio (ln[removal/control]) as the effect size than using the relative competition intensity ([removal Ϫ control]/removal), although the trends for these conceptually similar indices did not differ. When all studies were grouped in a single meta-regression of interaction intensity on standing crop to test quantitative similarity among studies, survival showed the clearest negative relationship. However, when the same regressions were done for each unique combination of taxon and site within each study to test for qualitative similarity among studies, the slopes averaged over studies tended to be negative for biomass and growth rate, but not different from zero for survival. These results are subject to a number of caveats because of the limitations of the available data-most notably, the extension of effects of interactions on individual growth or survival to effects on population distribution and abundance or community structure is highly problematic. Nevertheless, the fact that none of the metaanalyses demonstrated a significant positive relationship between competition and standing crop but that we frequently found negative relationships is an important pattern that has not been apparent from qualitative surveys of individual studies, and it demonstrates the potential power of meta-analysis in ecology. We conclude with recommendations to overcome some of the limitations of the currently available data and meta-analytical procedures.
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Papers by Jessica Gurevitch