Showing papers in "Ecological Monographs in 2010"

On the specification of structural equation models for ecological systems.

Abstract: The use of structural equation modeling (SEM) is often motivated by its utility for investigating complex networks of relationships, but also because of its promise as a means of representing theoretical concepts using latent variables. In this paper, we discuss characteristics of ecological theory and some of the challenges for proper specification of theoretical ideas in structural equation models (SE models). In our presentation, we describe some of the requirements for classical latent variable models in which observed variables (indicators) are interpreted as the effects of underlying causes. We also describe alternative model specifications in which indicators are interpreted as having causal influences on the theoretical concepts. We suggest that this latter nonclassical specification (which involves another variable type—the composite) will often be appropriate for ecological studies because of the multifaceted nature of our theoretical concepts. In this paper, we employ the use of meta-models to ...

Stoichiometric controls on carbon, nitrogen, and phosphorus dynamics in decomposing litter

Abstract: The mineralization of nitrogen and phosphorus from plant residues provides an important input of inorganic nutrients to the soil, which can be taken up by plants. The dynamics of nutrient mineralization or immobilization during decomposition are controlled by different biological and physical factors. Decomposers sequester carbon and nutrients from organic substrates and exchange inorganic nutrients with the environment to maintain their stoichiometric balance. Additionally, physical losses of organic compounds from leaching and other processes may alter the nutrient content of litter. In this work, we extend a stoichiometric model of litter nitrogen mineralization to include (1) phosphorus mineralization, (2) physical losses of organic nutrients, and (3) chemical heterogeneity of litter substrates. The enhanced model provides analytical mineralization curves for nitrogen and phosphorus as well as critical litter carbon : nutrient ratios (the carbon : nutrient ratios below which net nutrient release occurs) as a function of the elemental composition of the decomposers, their carbon-use efficiency, and the rate of physical loss of organic compounds. The model is used to infer the critical litter carbon : nutrient ratios from observed nitrogen and phosphorus dynamics in about 2600 litterbag samplings from 21 decomposition data sets spanning artic to tropical ecosystems. At the beginning of decomposition, nitrogen and phosphorus tend to be immobilized in boreal and temperate climates (i.e., both C:N and C:P critical ratios are lower than the initial ratios), while in tropical areas nitrogen is generally released and phosphorus may be either immobilized or released, regardless of the typically low phosphorus concentrations. The critical carbon : nutrient ratios we observed were found to increase with initial litter carbon : nutrient ratios, indicating that decomposers adapt to low-nutrient conditions by reducing their carbon-use efficiency. This stoichiometric control on nutrient dynamics appears ubiquitous across climatic regions and ecosystems, although other biological and physical processes also play important roles in litter decomposition. In tropical humid conditions, we found high critical C:P ratios likely due to high leaching and low decomposer phosphorus concentrations. In general, the compound effects of stoichiometric constraints and physical losses explain most of the variability in critical carbon : nutrient ratios and dynamics of nutrient immobilization and release at the global scale.

A user's guide to functional diversity indices

Abstract: Functional diversity is the diversity of species traits in ecosystems. This concept is increasingly used in ecological research, yet its formal definition and measurements are currently under discussion. As the overall behavior and consistency of functional diversity indices have not been described so far, the novice user risks choosing an inaccurate index or a set of redundant indices to represent functional diversity. In our study we closely examine functional diversity indices to clarify their accuracy, consistency, and independence. Following current theory, we categorize them into functional richness, evenness, or divergence indices. We considered existing indices as well as new indices developed in this study. The new indices aimed at remedying the weaknesses of currently used indices (e.g., by taking into account intraspecific variability). Using virtual data sets, we test (1) whether indices respond to community changes as expected from their category and (2) whether the indices within each category are consistent and independent of indices from other categories. We also test the accuracy of methods proposed for the use of categorical traits. Most classical functional richness indices either failed to describe functional richness or were correlated with functional divergence indices. We therefore recommend using the new functional richness indices that consider intraspecific variability and thus empty space in the functional niche space. In contrast, most functional evenness and divergence indices performed well with respect to all proposed tests. For categorical variables, we do not recommend blending discrete and real-valued traits (except for indices based on distance measures) since functional evenness and divergence have no transposable meaning for discrete traits. Nonetheless, species diversity indices can be applied to categorical traits (using trait levels instead of species) in order to describe functional richness and equitability.

Functional trait and phylogenetic tests of community assembly across spatial scales in an Amazonian forest

Abstract: Despite a long history of the study of tropical forests, uncertainty about the importance of different ecological processes in shaping tropical tree species distributions persists. Trait- and phylogenetic-based tests of community assembly provide a powerful way to detect community assembly processes but have seldom been applied to the same community. Both methods are well suited to testing how the relative importance of different ecological processes changes with spatial scale. Here we apply both methods to the Yasuni Forest Dynamics Plot, a 25-ha Amazonian forest with >1100 tree species. We found evidence for habitat filtering from both trait and phylogenetic methods from small (25 m2) to intermediate (10 000 m2) spatial scales. Trait-based methods detected even spacing of strategies, a pattern consistent with niche partitioning or enemy-mediated density dependence, at smaller spatial scales (25–400 m2). Simulation modeling of community assembly processes suggests that low statistical power to detect eve...

What a difference a species makes: a meta–analysis of dreissenid mussel impacts on freshwater ecosystems

Abstract: We performed a meta-analysis of published studies and long-term monitoring data sets to evaluate the effects of dreissenid mussels (Dreissena polymorpha and D. rostriformis bugensis), two of the world's most problematic biological invaders, on the biogeochemistry, flora, and fauna of lakes and rivers across North America and Eurasia. Dreissenid effects were structured along two distinct energy pathways. For the pelagic–profundal pathway, large mean reductions in phytoplankton (−35% to −78%) and zooplankton (−40% to −77%) biomass occurred and were dependent on habitat type. The largest effects were found in rivers, followed by littoral and pelagic habitats in lakes. In contrast, benthic energy pathways within littoral habitats of lakes and rivers showed dramatic increases in mean benthic algal and macrophyte biomass (+170% to +180%), sediment-associated bacteria (about +2000%), non-dreissenid zoobenthic biomass (+160% to +210%), and total zoobenthic biomass, which includes dreissenid mussel soft tissues (+...

Understanding ecosystem retrogression

Abstract: Over time scales of thousands to millions of years, and in the absence of rejuvenating disturbances that initiate primary or early secondary succession, ecosystem properties such as net primary productivity, decomposition, and rates of nutrient cycling undergo substantial declines termed ecosystem retrogression. Retrogression results from the depletion or reduction in the availability of nutrients, and can only be reversed through rejuvenating disturbance that resets the system; this differs from age-related declines in forest productivity that are driven by shorter-term depression of nutrient availability and plant ecophysiological process rates that occur during succession. Here we review and synthesize the findings from studies of long-term chronosequences that include retrogressive stages for systems spanning the boreal, temperate, and subtropical zones. Ecosystem retrogression has been described by ecologists, biogeochemists, geologists, and pedologists, each of which has developed somewhat independent conceptual frameworks; our review seeks to unify this literature in order to better understand the causes and consequences of retrogression. Studies of retrogression have improved our knowledge of how long-term pedogenic changes drive shorter-term biological processes, as well as the consequences of these changes for ecosystem development. Our synthesis also reveals that similar patterns of retrogression (involving reduced soil fertility, predictable shifts in organismic traits, and ecological processes) occur in systems with vastly different climatic regimes, geologic substrates, and vegetation types, even though the timescales and mechanisms driving retrogression may vary greatly among sites. Studies on retrogression also provide evidence that in many regions, high biomass or "climax" forests are often transient, and do not persist indefinitely in the absence of rejuvenating disturbance. Finally, our review highlights that studies on retrogressive chronosequences in contrasting regions provide unparalleled opportunities for developing general principles about the long-term feedbacks between biological communities and pedogenic processes, and how these control ecosystem development.

Recent advances in understanding migration systems of New World land birds

Abstract: Our understanding of migratory birds' year-round ecology and evolution remains patchy despite recent fundamental advances. Periodic reviews focus future research and inform conservation and management; here, we take advantage of our combined experiences working on Western Hemisphere avian migration systems to highlight recent lessons and critical gaps in knowledge. Among topics discussed are: (1) The pipeline from pure to applied researchers leaves room for improvement. (2) Population limitation and regulation includes both seasonal and between-season interactions. (3) The study of movements of small-bodied species remains a major research frontier. (4) We must increase our understanding of population connectivity. (5) With few exceptions, population regulation has barely been investigated. (6) We have increasingly integrated landscape configuration of habitats, large-scale habitat disturbances, and habitat quality impacts into models of seasonal and overall demographic success. (7) The post-breeding seas...

A Meta-Analysis Of Resource Pulse-Consumer Interactions

Abstract: Resource pulses are infrequent, large-magnitude, and short-duration events of increased resource availability. They include a diverse set of extreme events in a wide range of ecosystems, but identifying general patterns among the diversity of pulsed resource phenomena in nature remains an important challenge. Here we present a meta-analysis of resource pulse-consumer interactions that addresses four key questions: (1) Which characteristics of pulsed resources best predict their effects on consumers? (2) Which characteristics of consumers best predict their responses to resource pulses? (3) How do the effects of resource pulses differ in different ecosystems? (4) What are the indirect effects of resource pulses in communities? To investigate these questions, we built a data set of diverse pulsed resource-consumer interactions from around the world, developed metrics to compare the effects of resource pulses across disparate systems, and conducted multilevel regression analyses to examine the manner in which variation in the characteristics of resource pulse- consumer interactions affects important aspects of consumer responses. Resource pulse magnitude, resource trophic level, resource pulse duration, ecosystem type and subtype, consumer response mechanisms, and consumer body mass were found to be key explanatory factors predicting the magnitude, duration, and timing of consumer responses. Larger consumers showed more persistent responses to resource pulses, and reproductive responses were more persistent than aggregative responses. Aquatic systems showed shorter temporal lags between peaks of resource availability and consumer response compared to terrestrial systems, and temporal lags were also shorter for smaller consumers compared to larger consumers. The magnitude of consumer responses relative to their resource pulses was generally smaller for the direct consumers of primary resource pulses, compared to consumers at greater trophic distances from the initial resource pulse. In specific systems, this data set showed both attenuating and amplifying indirect effects. We consider the mechanistic processes behind these patterns and their implications for the ecology of resource pulses.

Hygroscopic particles on leaves: nutrients or desiccants?

Abstract: Aerosols have always been part of the atmosphere, and plant surfaces are a major aerosol sink. Given the nutrient content of aerosols and the natural stability of aerosol concentrations over evolutionary time, plants may have developed adaptations to aerosol input. Although little is known about such adaptations, leaf surface micro-roughness appears to play a key role. This review focuses on the deposition and fate of fine aerosols that are less than 2.5 μm in diameter. Most of these aerosols are hygroscopic, and they are often deliquescent (liquid) on transpiring leaves. Such concentrated solutions may be taken up by both the cuticle and stomata, contradicting previous concepts. The establishment of a continuous liquid water connection along stomatal walls affects individual stomata and is a new concept called “hydraulic activation of stomata” (HAS). HAS enables the efficient bidirectional transport of water and solutes between the leaf interior and leaf surface and makes stomatal transpiration partly in...

High‐dimensional coexistence based on individual variation: a synthesis of evidence

Abstract: High biodiversity of forests is not predicted by traditional models, and evidence for trade-offs those models require is limited. High-dimensional regulation (e.g., N factors to regulate N species) has long been recognized as a possible alternative explanation, but it has not be been seriously pursued, because only a few limiting resources are evident for trees, and analysis of multiple interactions is challenging. We develop a hierarchical model that allows us to synthesize data from long-term, experimental, data sets with processes that control growth, maturation, fecundity, and survival. We allow for uncertainty at all stages and variation among 26 000 individuals and over time, including 268 000 tree years, for dozens of tree species. We estimate population-level parameters that apply at the species level and the interactions among latent states, i.e., the demographic rates for each individual, every year. The former show that the traditional trade-offs used to explain diversity are not present. Demographic rates overlap among species, and they do not show trends consistent with maintenance of diversity by simple mechanisms (negative correlations and limiting similarity). However, estimates of latent states at the level of individuals and years demonstrate that species partition environmental variation. Correlations between responses to variation in time are high for individuals of the same species, but not for individuals of different species. We demonstrate that these relationships are pervasive, providing strong evidence that high- dimensional regulation is critical for biodiversity regulation.

Antarctic penguin response to habitat change as Earth's troposphere reaches 2°C above preindustrial levels

Abstract: We assess the response of pack ice penguins, Emperor (Aptenodytes forsteri) and Adelie (Pygoscelis adeliae), to habitat variability and, then, by modeling habitat alterations, the qualitative changes to their populations, size and distribution, as Earth's average tropospheric temperature reaches 28C above preindustrial levels (ca. 1860), the benchmark set by the European Union in efforts to reduce greenhouse gases. First, we assessed models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4) on penguin performance duplicating existing conditions in the Southern Ocean. We chose four models appropriate for gauging changes to penguin habitat: GFDL-CM2.1, GFDL-CM2.0, MIROC3.2(hi-res), and MRI-CGCM2.3.2a. Second, we analyzed the composited model ENSEMBLE to estimate the point of 28C warming (2025-2052) and the projected changes to sea ice coverage (extent, persistence, and concentration), sea ice thickness, wind speeds, precipitation, and air temperatures. Third, we considered studies of ancient colonies and sediment cores and some recent modeling, which indicate the (space/time) large/centennial- scale penguin response to habitat limits of all ice or no ice. Then we considered results of statistical modeling at the temporal interannual-decadal scale in regard to penguin response over a continuum of rather complex, meso- to large-scale habitat conditions, some of which have opposing and others interacting effects. The ENSEMBLE meso/decadal-scale output projects a marked narrowing of penguins' zoogeographic range at the 28C point. Colonies north of 708 S are projected to decrease or disappear: ;50% of Emperor colonies (40% of breeding population) and ;75% of Adelie colonies (70% of breeding population), but limited growth might occur south of 738 S. Net change would result largely from positive responses to increase in polynya persistence at high latitudes, overcome by decreases in pack ice cover at lower latitudes and, particularly for Emperors, ice thickness. Adelie Penguins might colonize new breeding habitat where concentrated pack ice diverges and/or disintegrating ice shelves expose coastline. Limiting increase will be decreased persistence of pack ice north of the Antarctic Circle, as this species requires daylight in its wintering areas. Adelies would be affected negatively by increasing snowfall, predicted to increase in certain areas owing to intrusions of warm, moist marine air due to changes in the Polar Jet Stream.

Morphological patterns and community organization in leaf-litter ant assemblages

Abstract: We present a new approach to determine the number and composition of guilds, using the hyperdiverse leaf-litter ant fauna as a model, based on appropriate morphological variables and species co-occurrence null models to describe the complex assemblages of interacting species community structure at the 1-m2 scale. We obtained 18 linear morphometric measures from 949 workers of 171 leaf-litter ant species (18 762 measurements) surveyed in four Atlantic Forest localities to test whether the assemblages are morphologically structured; the morphological characters were selected to indicate diet and foraging habits. Principal components analysis was used to characterize the morphospace and to describe the guild structure (number of species and composition). The guild proportionality assembly rule (significant tendency toward constant proportion of species in guilds) was assessed at the 1-m2 scale. Our analysis indicates that the division of leaf-litter ants into guilds is based mainly on microhabitat distribution in the leaf-litter, body size and shape, eye size, and phylogeny. The same guild scheme applied to four more sites shows that different Atlantic Forest areas have the same leaf-litter ant guilds. The guild proportionality assembly rule was confirmed for most guilds, suggesting that there are guild-specific limitations on species coexistence within assemblages; on the other hand, in a few cases the variance in guild proportion was greater than expected under the null assumptions. Other studies on ant functional group classification are partially supported by our quantitative morphological analysis. Our results, however, imply that there are more compartments than indicated in previous models, particularly among cryptic species (confined to soil and litter) and tropical climate specialists. We argue that a general null model for the analysis of species association based on morphology can reveal objectively defined groups and may thus contribute to a robust theory to explain community structure in general and have important consequences on studies of litter ant community ecology in particular.

Predator manipulation experiments: impacts on populations of terrestrial vertebrate prey

Abstract: Quantifying the relative impacts of top-down vs. bottom-up control of ecosystems remains a controversial issue, with debate often focusing on the perennial question of how predators affect prey densities. To assess predator impacts, we performed a worldwide meta-analysis of field experiments in which the densities of terrestrial vertebrate predators were manipulated and the responses of their terrestrial vertebrate prey were measured. Our results show that predation indeed limits prey populations, as prey densities change substantially after predator manipulations. The main determinant of the result of an experiment was the efficiency of predator manipulation. Positive impacts of predator manipulation appeared to increase with duration of the experiment for non-cyclic prey, while the opposite was true for cyclic prey. In addition, predator manipulation showed a large positive impact on cyclic prey at low prey densities, but had no obvious impact at peak prey densities. As prey population densities general...

Climate, snow, ice, crashes, and declines in populations of reindeer and caribou (Rangifer tarandus L.)

Abstract: Snow is a major determinant of forage availability for reindeer and caribou (Rangifer tarandus; hereafter Rangifer) in winter and is, consequently, a medium through which climate variation may influence population dynamics in this species. Periodic “icing” of winter ranges, where interludes of mild weather result in formation of crusted snow and basal ice that restrict access to forage, is held to be a cause of mass starvation, catastrophic declines in numbers, and even extirpation of local populations. It has been suggested that warming of the Arctic may result in increased frequency of winters with unfavorable snow and ice conditions, with serious consequences for Rangifer. This paper examines data on major declines in populations of Rangifer to determine the mechanism(s) of these events and the role of snow and ice conditions in them. Thirty-one declines, involving numerical decreases between 25% and 99%, were identified in 12 populations. Declines were of two types: the negative phase of irruptive osc...

Landscape matrix and species traits mediate responses of Neotropical resident birds to forest fragmentation in Jamaica

Abstract: Land cover and land use surrounding fragmented habitat can greatly impact species persistence by altering resource availability, edge effects, or the movement of individuals throughout a landscape. Despite the potential importance of the landscape matrix, ecologists still have limited understanding of the relative effects of different types of land cover and land uses on species patterns and processes in natural systems. Here we investigated whether Neotropical resident bird communities in limestone forest patches differed if they were embedded in three different human-dominated matrix types (agriculture, peri-urban development, and bauxite mining) relative to sites in continuous forest in central Jamaica. We found that species richness, community composition, and abundances were matrix-dependent, with agricultural landscapes supporting greater avian diversity and more intact community assemblages than either peri-urban or bauxite landscapes. Abundance of almost 70% of species differed in forest embedded in the different landscape matrix types. Traits related to resource use best predicted species responses, including diet guild, nest height, habitat association, and foraging strata. Insectivores, frugivores, canopy nesters, understory and canopy foragers, and forest-restricted species rarely observed in matrix habitats had lower abundances in forest fragments embedded in human-dominated matrix types than in continuous forest. In contrast, nectarivores, omnivores, granivores, ground and multi-strata nesters, ground foragers, and species regularly in matrix habitats were least sensitive to forest fragmentation. Results suggest that structure, composition, and land use disturbance regimes in matrix areas impact overall habitat quality in landscapes by potentially mediating resource availability inside as well as outside forest habitat. This study reinforces the importance of differentiating among land cover and land uses in fragmentation research and lends support to the hypothesis that resource availability may be a primary factor driving Neotropical bird responses to fragmentation.

Fossil insect folivory tracks paleotemperature for six million years

Abstract: Paleoecological studies enhance our understanding of biotic response to climate change because they consider timescales not accessible through laboratory or ecological studies. From 60 to 51 million years ago (Ma), global temperatures gradually warmed to the greatest sustained highs of the last 65 million years. Superimposed on this gradual warming is a transient spike of high temperature and pCO2 (partial pressure of carbon dioxide in the atmosphere; the Paleocene-Eocene Thermal Maximum 55.8 Ma) and a subsequent short-term cooling event (∼54 Ma). The highly resolved continental fossil record of the Bighorn Basin, Wyoming, USA, spans this interval and is therefore uniquely suited to examine the long-term effects of temperature change on the two dominant groups in terrestrial ecosystems, plants and insect herbivores. We sampled insect damage on fossil angiosperm leaves at nine well-dated localities that range in age from 52.7 to 59 Ma. A total of 9071 leaves belonging to 107 species were examined for the presence or absence of 71 insect-feeding damage types. Damage richness, frequency, and composition were analyzed on the bulk floras and individual host species. Overall, there was a strong positive correlation between changes in damage richness and changes in estimated temperature, a weak positive relationship for damage frequency and temperature, and no significant correlation for floral diversity. Thus, insect damage richness appears to be more sensitive to past climate change than to plant diversity, although plant diversity in our samples only ranges from 6 to 25 dicot species. The close tracking of the richness of herbivore damage, a presumed proxy for actual insect herbivore richness, to both warming and cooling over a finely divided, extended time interval has profound importance for interpreting the evolution of insects and plant–insect associations in the context of deep time. Our results also indicate that increased insect herbivory is likely to be a net long-term effect of anthropogenic warming.

Global contribution of echinoderms to the marine carbon cycle: CaCO3 budget and benthic compartments

Abstract: The contribution of carbonate-producing benthic organisms to the global marine carbon budget has been overlooked, the prevailing view being that calcium carbonate (CaCO3) is predominantly produced by marine plankton. Here, we provide the first estimation of the global contribution of echinoderms to the marine carbon cycle, based on organism-level measurements from species of the five echinoderm classes. Echinoderms global CaCO3 contribution amounts to ~0.861 Pg CaCO3 yr-1 (0.102 Pg C yr-1 of inorganic carbon) as a production rate, and ~2.11 Pg CaCO3 (0.25 Pg C of inorganic carbon) as a standing stock globally. Echinoderm inorganic carbon production (0.102 Pg C yr-1) is less than the global pelagic production (0.4-1.8 Pg C yr-1), and similar to the estimates for carbonate shelves globally (0.02-0.12 Pg C yr-1). Echinoderm CaCO3 production per unit area, is ~27.01 g CaCO3 m-2 yr-1 (3.24 g C m-2 yr-1 as inorganic carbon) on a global scale for all areas, with a standing stock of ~63.34 g CaCO3 m-2 (7.60 g C m-2 as inorganic carbon), and ~7.97 g C m-2 as organic carbon. The shelf production is 77.91 g CaCO3 m-2 yr-1 (9.35 g C m-2 yr-1 as inorganic carbon) in contrast to 2.05 g CaCO3 m-2 yr-1 (0.24 g C m-2 yr-1 as inorganic carbon) for the slope on a global scale. The biogeography of the CaCO3 standing stocks of echinoderms showed strong latitudinal variability. Roughly 80% of the global CaCO3 production from echinoderms occurs between 0 and 800 meters. The shelf and upper slope contribute the most. We provide a global distribution of echinoderm populations in the context of global calcite saturation horizons, since undersaturated waters with respect to mineral phases are surfacing. This shallowing is a direct consequence of ocean acidification, and in some places it may reach the shelf and upper slope permanently. These organism-level data contribute substantially to the assessment of global carbonate inventories, which at present are poorly estimated. Additionally, it is desirable to include these benthic compartments in coupled global biogeochemical models representing the "biological pump", since at present all efforts have focused on pelagic processes, dominated by coccolithophores.

Consequences of propagule dispersal and river fragmentation for riparian plant community diversity and turnover

Abstract: The spatial distribution and temporal availability of propagules fundamentally constrain plant community development. This study experimentally tested several hypotheses about the relative roles of wind and water dispersal in colonization and development of riparian communities along rivers. Through controlling the source of propagules (dispersed by wind, water, or both) reaching newly created, bare river margin sites, we isolated the relative roles of dispersal and other factors in plant community development over five years. Replicated treatments were established at 12 sites spanning 400 km along two adjacent rivers in northern Sweden, one fragmented by a series of dams, the other free-flowing. Bare river margins receiving only water-dispersed propagules had significantly higher species richness compared to plots receiving only wind-dispersed propagules during the initial two years of colonization. Species richness increased annually throughout the study along tranquil and turbulent reaches of the free-flowing river but reached an asymptote at comparatively low richness after a single year on the impounded river. Propagule source strongly influenced species richness during the initial establishment along both rivers, with richness being significantly higher in plots receiving water-dispersed seeds. This strong treatment effect continued to be important through time along the regulated river but diminished in importance along the free-flowing river where other factors such as soil moisture, light availability, and exposure of sites to fluvial disturbance overshadowed the influence of dispersal pathway in mediating species richness. This suggests that hydrochory (plant dispersal by water) may be more important for maintenance of diversity in regulated systems where long-distance dispersal is absent or negligible, but that the rich local propagule source along free-flowing rivers supports high species richness. The number of unique species was higher in water-dispersal plots along both the regulated and free-flowing rivers. This result suggests that hydrochory may contribute to temporal variability of sites, may enhance richness over time, and may have an important role in meta-population and meta-community dynamics of plant communities through long-distance (and local) dispersal and chance colonization. Our findings provide experimental evidence that water dispersal of plant propagules influences colonization dynamics and is important for long-term community development in riparian zones.

Interacting regime shifts in ecosystems: implication for early warnings

Abstract: Big ecological changes often involve regime shifts in which a critical threshold is crossed. Thresholds are often difficult to measure, and transgressions of thresholds come as surprises. If a critical threshold is approached gradually, however, there are early warnings of the impending regime shift. For example, in a one-dimensional ecosystem dynamics, autocorrelation approaches 1 from below, variance and skewness increase, and variance spectra shift to lower frequencies. Here we focus on variance, an indicator easily computed from monitoring data. There are two distinct sources of increased variance near a critical threshold. One is the amplification of small shocks that occurs as the square of the modulus of the leading eigenvalue (or leading pair of eigenvalues in the complex case) approaches 1 from below. This source, called “squealing,” is well-studied. The second source of variance, called “flickering,” involves brief excursions between attractors. Interacting regime shifts may muffle or magnify variance near critical thresholds. Whether muffling or magnification occurs, and the size of the effect, depend on the product of the feedback between the state variables times the correlation of these variables' responses to environmental shocks. If this product is positive, magnification of the variance will occur. If the product is negative, muffling or magnification can occur depending on the relative magnitudes of these and other effects. Therefore, monitoring programs should measure variates that have opposite responses to the critical transition. If the correlations to environmental shocks have the same sign, the variance of at least one variate will be magnified as the critical transition is approached. Simulation studies suggest that muffling may sometimes interfere with detection of early warning signals of regime shifts. However, more important effects of muffling and magnification may come from their effect on flickering, when random shocks trigger a state change in a system with low resilience. Muffling decreases the likelihood that a random shock will trigger a regime shift. Magnification has the opposite effect. Magnification is most likely when feedbacks are positive and state variables have positively correlated responses to environmental shocks. These results help delimit the conditions when regime shifts are more likely to cascade through complex systems.

Using DNA to assess errors in tropical tree identifications: How often are ecologists wrong and when does it matter?

Abstract: Ecological surveys of tropical tree communities have provided an important source of data to study the forces that generate and maintain tropical diversity. Accurate species identification is central to these studies. Incorrect lumping or splitting of species will distort results, which may in turn affect conclusions. Although ecologists often work with taxonomists, they likely make some identification errors. This is because most trees encountered in the field are not reproductive and must be identified using vegetative characters, while most species descriptions rely on fruit and flower characters. Because every tree has DNA, ecological surveys can incorporate molecular approaches to enhance accuracy. This study reports an extensive ecological and molecular survey of nearly 4000 trees belonging to 55 species in the tree genus Inga (Fabaceae). These trees were sampled in 25 community surveys in the southwestern Amazon. In a process of reciprocal illumination, trees were first identified to species using vegetative characters, and these identifications were revised using phylogenies derived from nuclear and chloroplast DNA sequences. We next evaluated the effects of these revised species counts upon analyses often used to assess ecological neutral theory. The most common morphological identification errors involved incorrectly splitting rare morphological variants of common species and incorrectly lumping geographically segregated, morphologically similar species. Total error rates were significant (6.8-7.6% of all individuals) and had a measurable impact on ecological analyses. The revised identifications increased support for spatially autocorrelated, potentially neutral factors in determining community composition. Nevertheless, the general conclusions of community-level ecological analyses were robust to misidentifications. Ecological factors, such as soil composition, and potentially neutral factors, such as dispersal limitation, both play important roles in the assembly of Inga communities. In contrast, species-level analyses of neutrality with respect to habitat were strongly impacted by identification errors. Although this study found errors in morphological identifications, there was also strong evidence that a purely molecular approach to species identification, such as DNA barcoding, would be prone to substantial errors. The greatest accuracy in ecological surveys will be obtained through a synthesis of traditional, morphological and modern, molecular approaches.

Decomposition of trait diversity among the nodes of a phylogenetic tree

Abstract: Biodiversity studies aim to explain spatiotemporal patterns of species distributions We propose a new methodology in which trait diversity is measured by the quadratic entropy index, with distances among species calculated from differences among trait states We show how this index of trait diversity can be decomposed among the nodes of a phylogenetic tree The contribution to trait diversity of a particular node is equal to the trait diversity among the n groups of species descending from it multiplied by an abundance weight (either proportional to the number of descendant species or to their relative abundance) We developed three tests to characterize the phylogenetic pattern of trait diversity and evaluated our methodology with seven evolutionary models The power of the tests was high and increased with the number of extant taxa and the number of traits analyzed The Type I error analyses (erroneous rejection of true null hypotheses) suggested that our tests are neither too liberal nor too conservative Species abundances were found to modify the phylogenetic signal in trait diversity if only a few species were abundant and if species abundances were correlated to their phylogenetic relatedness and/or their trait states By comparing phylogenetic signals in trait diversity from the local to the metacommunity level, we explored the factors that structure butterfly trait diversity in calcareous grasslands of northern France and southern Belgium We show that partial phylogenetic signal in traits combined with habitat filtering determined which species and lineages were able to co-occur locally Interestingly, no phylogenetic signal was detected when measures of abundance were included in our analyses For most species and clades, the abundance distribution among communities at the regional scale was random Overall, studying trait diversity in a phylogenetic context allows the link between current local ecological processes and lineage-dependent historical evolutionary factors to be thoroughly investigated

Hydrological change in lakes inferred from midge assemblages through use of an intralake calibration set

Abstract: Midge (Insecta: Diptera: Nematocera) assemblages were examined in surface sediment samples from Lake Pieni-Kauro and the Saavanjoki River, eastern Finland, using their sedimentary remains. The aim was first to identify the forcing factors behind midge distribution within an aquatic ecosystem and second to develop new midge-based calibration models for the reconstruction of past environmental conditions, with a special focus on paleohydrology. The third objective was to apply the models to a midge stratigraphy. The effect of sampling-point selection in paleolimnological studies and its possible influence on chironomid-inferred temperature reconstructions was also assessed. The species assemblages in Lake Pieni-Kauro showed high levels of heterogeneity within the surface sediment samples, signifying that the assemblages predominantly incorporate locally dwelling fauna, instead of integrating remains from a larger area. Stream flow and water depth were the most important factors explaining the midge distribu...

Linking process to pattern: estimating spatiotemporal dynamics of a wildlife epidemic from cross-sectional data

Abstract: Underlying dynamic event processes unfolding in continuous time give rise to spatiotemporal patterns that are sometimes observable at only a few discrete times. Such event processes may be modulated simultaneously over several spatial (e.g., latitude and longitude) and temporal (e.g., age, calendar time, and cohort) dimensions. The ecological challenge is to understand the dynamic latent processes that were integrated over several dimensions (space and time) to produce the observed pattern: a so-called inverse problem. An example of such a problem is characterizing epidemiological rate processes from spatially referenced age-specific prevalence data for a wildlife disease such as chronic wasting disease (CWD). With age-specific prevalence data, the exact infection times are not observed, which complicates the direct estimation of rates. However, the relationship between the observed data and the unobserved rate variables can be described with likelihood equations. Typically, for problems with multiple timescales, the likelihoods are integral equations without closed forms. The complexity of the likelihoods often makes traditional maximum-likelihood approaches untenable. Here, using seven years of hunter-harvest prevalence data from the CWD epidemic in white-tailed deer (Odocoileus virginianus) in Wisconsin, USA, we develop and explore a Bayesian approach that allows for a detailed examination of factors modulating the infection rates over space, age, and time, and their interactions. Our approach relies on the Bayesian ability to borrow strength from neighbors in both space and time. Synthesizing a number of areas of event time analysis (current-status data, age/period/cohort models, Bayesian spatial shared frailty models), our general framework has very broad ecological applicability beyond disease prevalence data to a number of important ecological event time analyses, including general survival studies with multiple time dimensions for which existing methodology is limited. We observed strong associations of infection rates with age, gender, and location. The infection rate appears to be increasing with time. We could not detect growth hotspots, or location by time interactions, which suggests that spatial variation in infection rates is determined primarily by when the disease arrives locally, rather than how fast it grows. We emphasize assumptions and the potential consequences of their violations.

Large herbivore responses to water and settlements in savannas

Abstract: Sustaining wildlife and pastoral communities in savannas worldwide depends on understanding how landscapes provide for their needs. The composite effects of multiple forces shape herbivore distribution in savannas. We propose a model describing the distribution of animal density along two resource utilization gradients. The model estimates where animals are most abundant in relation to water and pastoral settlements and how strongly they respond to these gradients, as indicated by the location of peak densities in landscape space and degree of attraction of animals to these locations. We use the model to show that distances to water and settlements interactively influence the distribution of wild herbivore and livestock densities in a semiarid protected and pastoral African savanna. The herbivores were distributed along distance from water and settlement gradients according to four distinct patterns, suggesting the preponderance of constrained foraging. The impact of distance to water and settlement on herbivore distribution was modified by land use type and temporal variation in rainfall, and also varied among different species of wildlife and between wildlife and livestock. Wild herbivores peaked in density farther from settlements than from water and were much more strongly attracted to their points of maximum density in the pastoral than in the protected land. The point of maximum density was farther removed from settlements in the protected than in the pastoral landscape where the wild herbivores were compressed into smaller suitable habitats. The interaction between distances from settlement and water produced discernible spatial segregation among species in terms of the locations of their points of maximum density, presumably to minimize interspecific competition for forage and water. Settlements exerted relatively stronger influence on livestock distribution than water, resulting in densities that declined exponentially away from settlements at all distances from water.

Fire, vital rates, and population viability: a hierarchical Bayesian analysis of the endangered Florida scrub mint

Abstract: Understanding and predicting changes in the abundance of natural populations is a central goal of ecology. These changes are influenced by a variety of exogenous processes (weather, floods, fire); variation in these processes leads to variation in vital rates (survival, fecundity) that may be positively or negatively correlated across the life cycle. We used 20 years of data and a hierarchical Bayesian model to estimate vital rates and their covariation in an endangered plant, Dicerandra frutescens ssp. frutescens (Lamiaceae), as a function of time since fire and random year effects. Germination and the number of flowering branches declined with time since fire, and all plants were increasingly likely to become nonreproductive with time since fire. Time since fire had negative effects on survival of seedlings, vegetative plants, and small flowering plants, and positive effects on survival of medium and large flowering plants. Model comparison strongly supported inclusion of time-since-fire effects and weakly supported inclusion of year effects influencing all vital rates (''model-wide'' year effects). We used samples from the joint posterior distribution of model parameters to simulate population dynamics as a function of fire regime and year-to-year environmental variation. These simulations suggest that populations of Dicerandra frutescens ssp. frutescens are least likely to go extinct if the average time between fires is ;24-30 years. The design of the simulations allowed us to distinguish variation in stochastic population growth associated with process variability (fire, year effects, and demographic stochasticity) from variation associated with parameter uncertainty (finite amounts of data). Even with 20 years of data, half or more of the uncertainty in population growth rates was due to parameter uncertainty. This hierarchical Bayesian population viability analysis illustrates a general analytical framework for (1) estimating vital rates as a function of an exogenous environmental factor, (2) accounting for covariation among vital rates, and (3) simulating population dynamics as a function of stochastic environmental processes while taking into account uncertainty about their effects. We discuss future areas of development for this approach.

Coupling between subtidal prey and consumers along a mesoscale upwelling gradient in the Galápagos Islands

Abstract: Our understanding of how communities are regulated diminishes with increasing spatial scale. Because oceanographic processes supply food, transport propagules, and set abiotic conditions across broad areas, they may explain metacommunity and food web functioning on large scales. Here we test for bottom-up manifestations of an oceanographic process, upwelling, through production of barnacle (Megabalanus spp.) prey to top-down control by whelks, fish, and urchins across a mesoscale (125 km) gradient of 28–68% upwelling at 12 subtidal rock wall sites in the Galapagos from 2002 to 2005. Multivariate analysis of in situ flow measurements and the occurrence of the Equatorial Undercurrent (EUC) distinguished weak (WUP), intermediate (IUP), and strong upwelling (SUP) site groups. Barnacle cover and recruitment were significantly higher at SUP than at WUP sites and decreased with depth from 6 to 15 m. High barnacle recruitment was a persistent feature of SUP sites from 2002 to 2004. This recruitment was significantly related to larval concentrations, vertical flow speeds, and to a lesser extent, onshore offshore flow speeds, while growth rates of barnacles were highest at three SUP and one IUP site. Three-dimensional (3-D) flow explained 39% of the variation in growth. Therefore, bottom-up effects of upwelling occurred via prey production as recruitment and growth of barnacles. Vertical flow was the most important component of the upwelling regime predicting these dynamics. Evidence of population level predator–prey coupling was provided by significant linear relationships between densities of the whelk, Hexaplex princeps, and both the cover and recruitment of barnacle prey. In a mesoscale predation intensity experiment 33–100% of barnacles were eaten by Hexaplex and fish, linking bottom-up and top-down effects. Significantly more barnacles were consumed at SUP than at WUP sites. The broader spatial context for upwelling and predator–prey coupling documented here was provided by the topography of the sea floor offshore, as the maximum depth and bottom slope within 15 km of the sites predicted 40% of upwelling duration. Eighty percent of the SUP sites were close to steep slopes and deep (>1100 m) water where upwelling from internal waves and the EUC is likely.

Nitrogen dynamics in a small arctic watershed: retention and downhill movement of 15N

Abstract: We examined short- and long-term nitrogen (N) dynamics and availability along an arctic hillslope in Alaska, USA, using a stable isotope of nitrogen ( 15 N), as a tracer. Tracer levels of 15 NH4 þ were sprayed once onto the tundra at six sites in four tundra types: heath (crest), tussock with high and low water flux (mid- and footslope), and wet sedge (riparian). 15 N in vegetation and soil was monitored to estimate retention and loss over a 3- year period. Nearly all 15 NH4 þ was immediately retained in the surface moss-detritus-plant layer, and .57% of the 15 N added remained in this layer at the end of the second year. Organic soil was the second largest 15 N sink. By the end of the third growing season, the moss-detritus-plant layer and organic soil combined retained � 87% of the 15 N added except at the Midslope site with high water flux, where recovery declined to 68%. At all sites, non-extractable and non- labile-N pools were the principal sinks for added 15 N in the organic soil. Hydrology played an important role in downslope movement of dissolved 15 N. Crest and Midslope with high-water-flux sites were most susceptible to 15 N losses via leaching, perhaps because of deep permeable mineral soil (crest) and high water flow (Midslope with high water flux). Late spring melt season also resulted in downslope dissolved- 15 N losses, perhaps because of an asynchrony between N release into melt water and soil immobilization capacity. We conclude that separation of the rooting zone from the strong sink for incoming N in the moss- detritus-plant layer, rapid incorporation of new N into relatively recalcitrant-soil-N pools within the rooting zone, and leaching loss from the upper hillslope would all contribute to the strong N-limitation of this ecosystem. An extended snow-free season and deeper depth of thaw under warmer climate may significantly alter current N dynamics in this arctic ecosystem.

Multiple Stressors and the cause of amphibian abnormalities

Abstract: The repeated occurrence of abnormal amphibians in nature points to ecological imbalance, yet identifying causes of these abnormalities has proved complex. Multiple studies have linked amphibian abnormalities to chemically contaminated areas, but inference about causal mechanisms is lacking. Here we use a high incidence of abnormalities in Alaskan wood frogs to strengthen inference about the mechanism for these abnormalities. We suggest that limb abnormalities are caused by a combination of multiple stressors. Specifically, toxicants lead to increased predation, resulting in more injuries to developing limbs and subsequent developmental malformations. We evaluated a variety of putative causes of frog abnormalities at 21 wetlands on the Kenai National Wildlife Refuge, south-central Alaska, USA, between 2004 and 2006. Variables investigated were organic and inorganic contaminants, parasite infection, abundance of predatory invertebrates, UVB, and temperature. Logistic regression and model comparison using the Akaike information criterion (AIC) identified dragonflies and both organic and inorganic contaminants as predictors of the frequency of skeletal abnormalities. We suggest that both predators and contaminants alter ecosystem dynamics to increase the frequency of amphibian abnormalities in contaminated habitat. Future experiments should test the causal mechanisms by which toxicants and predators may interact to cause amphibian limb abnormalities.

Stochasticity as an alternative to deterministic explanations for patterns of habitat use by birds

Abstract: Stochasticity is rarely explicitly investigated as a determinant of patterns of habitat use, even though evidence of its influence would undermine support for deterministic models of habitat selection. To assess the role of stochasticity in generating patterns in habitat use, we compared observed patterns of year-to-year variation in habitat use of 20 bird species over five years with patterns randomly generated from null models with different biological constraints (i.e., abundance, territoriality, exclusion of unused habitat, site fidelity, and habitat preference). We were able to recreate patterns of variability in spatial distributions with one or more null models for 18 species, suggesting an important role of stochasticity. Although year- to-year variability could be modeled as a purely stochastic process for only four species, null models in which random variability was constrained by the influences of vegetation or site fidelity collectively matched the observed patterns of variability of all 18 species. Support for these two models suggested that factors that increase the likelihood of returning to a previously used area and thus limit the spatial extent of random territory placement can generate patterns similar to those we observed. Greater support for models with preference constraints (i.e., models that included bird-vegetation associations) over models with avoidance constraints (i.e., models that excluded areas that were never occupied) indicated that preference was the primary mode of habitat selection. Stochastic patterns in habitat use may have resulted from a weak relationship between habitat selection and habitat quality, influence of non-vegetation habitat features (e.g., climatic fluctuations), nonequilibrium conditions, and a greater influence of stochastic variation at small spatial and temporal scales. Alternatively, responses to environmental cues may have been deterministic, but in summation, these responses may have created variability in spatial distribution patterns that was indistinguishable from random variation. In such cases, we suggest that it may be easier to model variation in habitat use as a random process rather than using deterministic models that account for a multitude of environmental factors influencing habitat selection of birds. Regardless, stochasticity may play a larger role in generating patterns of habitat use than is often considered, and it deserves wider attention.