Showing papers in "Ecology in 2001"

Fitting multivariate models to community data: a comment on distance‐based redundancy analysis

Abstract: Nonparametric multivariate analysis of ecological data using permutation tests has two main challenges: (1) to partition the variability in the data according to a complex design or model, as is often required in ecological experiments, and (2) to base the analysis on a multivariate distance measure (such as the semimetric Bray-Curtis measure) that is reasonable for ecological data sets. Previous nonparametric methods have succeeded in one or other of these areas, but not in both. A recent contribution to Ecological Monographs by Legendre and Anderson, called distance-based redundancy analysis (db-RDA), does achieve both. It does this by calculating principal coordinates and subsequently correcting for negative eigenvalues, if they are present, by adding a constant to squared distances. We show here that such a correction is not necessary. Partitioning can be achieved directly from the distance matrix itself, with no corrections and no eigenanalysis, even if the distance measure used is semimetric. An ecological example is given to show the differences in these statistical methods. Empirical simulations, based on parameters estimated from real ecological species abundance data, showed that db-RDA done on multifactorial designs (using the correction) does not have type 1 error consistent with the significance level chosen for the analysis (i.e., does not provide an exact test), whereas the direct method described and advocated here does.

What is the observed relationship between species richness and productivity

Abstract: Understanding the relationship between species richness and productivity is fundamental to the management and preservation of biodiversity. Yet despite years of study and intense theoretical interest, this relationship remains controversial. Here, we present the results of a literature survey in which we examined the relationship between species richness and productivity in 171 published studies. We extracted the raw data from published tables and graphs and subjected these data to a standardized analysis, using ordinary least-squares (OLS) regression and generalized linear-model (GLIM) regression to test for significant positive, negative, or curvilinear relationships between productivity and species diversity. If the relationship was curvilinear, we tested whether the maximum (or minimum) of the curve occurred within the range of productivity values observed (i.e., was there evidence of a hump?). A meta-analysis conducted on the distribution of standardized quadratic regression coefficients showed that ...

Landscape connectivity: a graph‐theoretic perspective

Abstract: Ecologists are familiar with two data structures commonly used to represent landscapes. Vector-based maps delineate land cover types as polygons, while raster lattices represent the landscape as a grid. Here we adopt a third lattice data structure, the graph. A graph represents a landscape as a set of nodes (e.g., habitat patches) connected to some degree by edges that join pairs of nodes functionally (e.g., via dispersal). Graph theory is well developed in other fields, including geography (transportation networks, routing ap- plications, siting problems) and computer science (circuitry and network optimization). We present an overview of basic elements of graph theory as it might be applied to issues of connectivity in heterogeneous landscapes, focusing especially on applications of metapo- pulation theory in conservation biology. We develop a general set of analyses using a hypothetical landscape mosaic of habitat patches in a nonhabitat matrix. Our results suggest that a simple graph construct, the minimum spanning tree, can serve as a powerful guide to decisions about the relative importance of individual patches to overall landscape con- nectivity. We then apply this approach to an actual conservation scenario involving the

Mass/length residuals: measures of body condition or generators of spurious results?

Abstract: In studies of animal ecology, it is fashionable to use the residuals from an ordinary least squares (OLS) linear regression of body mass against a linear measure of size (the body size indicator, BSI) as an index of body condition. These residual indices are used to study the relationship between condition and reproductive investment, survi- vorship, habitat use, and other parameters. I identify a series of key assumptions underlying the use of this method, each of which is likely to be violated in some or all studies. These assumptions are: (1) that the functional relationship between mass and BSI is linear; (2) that condition is independent of BSI length; (3) that BSI length accurately indicates struc- tural size; (4) that there is no correlation between the size of BSI relative to other structural components (i.e., shape) and the parameter against which the residuals are analyzed; (5) that BSI length is strictly independent of mass; and (6) that BSI length is not subjected to error. Violations of these assumptions place the results of some studies in question and explain the poor relationship observed between OLS residuals and more direct measures of condition. I use avian morphometric data to illustrate how OLS methods can easily lead to Type I and Type II errors through violations of assumptions (5) and (6). Significant relationships reported between OLS residual indices and parameters correlating with body size (e.g., size of sexual ornaments or egg size) are at particular risk of being spurious when the correlation coefficient between mass and BSI is low. Residual indices of condition are often likely to be more reliable when calculated with alternative methods such as nonparametric or model II regression. However, whatever the method used to produce them, residual indices are not suitable for studying the heritability of condition.

Global-scale climatic controls of leaf dry mass per area, density, and thickness in trees and shrubs

Abstract: Leaf dry mass per unit area (LMA) is a product of leaf thickness (T) and of density (D). Greater T is associated with greater foliar photosynthetic rates per unit area because of accumulation of photosynthetic compounds; greater D results in decreased foliage photosynthetic potentials per unit dry mass because of lower concentrations of assimilative leaf compounds and decreases in intercellular transfer conductance to CO2. To understand the considerable variation in T and D at the global scale, literature data were analyzed for 558 broad-leaved and 39 needle-leaved shrubs and trees from 182 geographical locations distributed over all major earth biomes with woody vegetation. Site climatic data were interpolated from long-term world climatologies (monthly precipitation, surface temperature) or modeled using the Canadian Climate Center Model (monthly global solar radiation). Influences of total annual precipitation (WT), precipitation of the driest month (Wmin), monthly mean precipitation of the three dries...

Facilitation and competition on gradients in alpine plant communities

Abstract: We conducted a neighbor removal experiment in natural alpine plant com- munities of the southwestern Alps to test for the relative importance of competitive and facilitative interactions along elevational and topographical gradients. The experimental sites were chosen to encompass most of the floristic diversity observed along gradients of elevation and topography, which are the two main ecological gradients associated with alpine plant communities in the western Alps. The effects of neighbor removal on the survival, aboveground biomass, and reproduction of five target species were tested at each of six experimental sites. Using biomass data, we calculated relative competitive index (RCI) and log response ratio (LRR) as measures of interaction strength and direction. We found highly significant shifts from strong competitive effects in low and sheltered sites to strong facilitative responses in high and exposed sites. When experimental results were integrated with gradient analyses, we found that the responses of particular alpine plant species to neighbor removal generally depended on the species' position on elevational and topographical gradients. When neighbors were removed from around target species at ex- perimental sites that were lower in elevation than the distributional mean of the target species, biomass generally increased. When neighbors were removed from around target species at experimental sites that were higher in elevation than the distributional mean of the target species, biomass decreased. In other words, facilitation appeared to allow species from lower elevations to move up the gradient, but competition at low elevations appeared to restrict species from higher elevations from moving down the gradient. In high and exposed sites, experimental evidence for facilitation was coupled to small-scale spatial associations among species, but spatial disassociation was not coupled to experimental evidence for competition at any sites. We conclude that the distribution and abundance of many species in high-elevation communities of the western Alps appears to be enhanced by neighbors, and that species continua commonly observed along environmental gradients are the result of both negative and positive plant interactions.

Functional responses with predator interference: viable alternatives to the holling type ii model

Abstract: A predator's per capita feeding rate on prey, or its functional response, provides a foundation for predator-prey theory. Since 1959, Holling's prey-dependent Type II functional response, a model that is a function of prey abundance only, has served as the basis for a large literature on predator-prey theory. We present statistical evidence from 19 predator-prey systems that three predator-dependent functional responses (Beddington- DeAngelis, Crowley-Martin, and Hassell-Varley), i.e., models that are functions of both prey and predator abundance because of predator interference, can provide better descrip- tions of predator feeding over a range of predator-prey abundances. No single functional response best describes all of the data sets. Given these functional forms, we suggest use of the Beddington-DeAngelis or Hassell-Varley model when predator feeding rate becomes independent of predator density at high prey density and use of the Crowley-Martin model when predator feeding rate is decreased by higher predator density even when prey density is high.

Morphological and behavioral plasticity of larval anurans in response to different predators

Abstract: Many organisms can adjust to a changing environment by developing alter- native phenotypes that improve their fitness. Our understanding of phenotypic plasticity is largely based upon observations from single species responding to two different environ- ments and measuring a single plastic trait. In this study, I examine predator-induced phe- notypic plasticity in tadpoles by observing how six species of larval anurans respond to five different predator environments in 11 different traits (seven morphological traits, two behavioral traits, growth, and development). The results demonstrate that behavioral and morphological plasticity may be ubiquitous in larval anurans. The six prey species exhibited different responses to the same predator species, and each prey exhibited different responses to different predator species. This suggests that responses to a particular predator may not serve as general defense against all predators; rather, prey express predator-specific suites of responses. I also compared relative differences in plasticity among species and among traits. In contrast to earlier findings using only two predator environments, I found that different anurans possess similar degrees of plasticity for most of their traits when reared in a large number of environments. In addition, behavioral traits were always more plastic than morphological traits. Finally, I examined trait integration to address whether there were apparent trade-offs among traits and limits imposed by the abiotic environment. Trait integration, or the degree of correlated responses among traits across predator environments within a prey species, was very low. This further suggests that the suites of responses are predator specific and may be under independent directions of selection in different predator environments. Trait correlations across prey species indicated that there is an apparent trade-off between tail fin depth and body size. This relationship is supported by selection studies within prey species. Relating the responses to the pond permanence habitat gradient over which the amphibians exist, I found that species inhabiting more temporary ponds possess higher general activity levels, shallower tail fins, and larger bodies; these traits are known to result in more rapid growth. In a companion paper, I quantify the predation risk of the 30 predator- prey combinations and examine the relationships between prey response and predation risk.

Can plants stimulate soil microbes and their own nutrient supply? evidence from a grazing tolerant grass

Abstract: The primary source of mineral nutrients for plants is the decomposition of organic matter by soil microbes. Plants are traditionally viewed as largely passive participants in the decomposition process, incapable of directly affecting rates of decomposition significantly and primarily assimilating nutrients unused by the microbial pool. We performed a 13C pulse-chase experiment on a common grazing tolerant grass, Poa pratensis L., of Yellowstone National Park, to follow carbon flow into the soil rhizosphere and microbial biomass and the associated effects on soil N availability and plant N dynamics. Grazing promoted root exudation of carbon, which was quickly assimilated into a burgeoning microbial population in the rhizosphere of clipped plants. Moreover, these facilitating effects of defoliation on rhizospheric processes positively fed back on soil inorganic N pools, plant N uptake, leaf N content, and photosynthesis. Such findings are the first evidence, to our knowledge, that suggest (1) plants are cap...

Dominance and distribution of tree species in upper amazonian terra firme forests

Abstract: Amazonian forests are the largest and most diverse in the tropics, and much of the mystery surrounding their ecology can be traced to attempts to understand them through tiny local inventories. In this paper we bring together a large number of such inventories scattered across immense areas of western Amazonia in order to address simple questions about the distribution and abundance of tropical tree species in lowland terra firme forests there. The goal is to describe patterns of commonness and rarity at local (1 ha), landscape (∼104 km2), and regional (>106 km2) scales, and to fuse the results into a more complete picture of how tropical tree communities are structured. We present estimates of landscape-scale densities for ∼1400 taxa, based on data from tree plots scattered over large tracts of terra firme forest in eastern Ecuador and southeastern Peru. A database of morphological, ecological, and other traits of >1000 of these species compiled from the taxonomic literature is then used to explore how species that are common in the inventories differ from species that are rare. Although most species show landscape-scale densities of <1 individual/ha, most trees in both forests belong to a small set of ubiquitous common species. These common species combine high frequency with high local abundance, forming predictable oligarchies that dominate several thousand square kilometers of forest at each site. The common species comprising these oligarchies are a nonrandom subset of the two floras. At both sites a disproportionate number of common species are concentrated in the families Arecaceae, Moraceae, Myristicaceae, and Violaceae, and large-statured tree species are more likely to be common than small ones. Nearly a third of the 150 most common tree species in the Ecuadorean forest are also found among the 150 most common tree species in the Peruvian forest. For the 254 tree species shared by the two data sets, abundance in Ecuador is positively and significantly correlated with abundance ∼1400 km away in Peru. These findings challenge popular depictions of Amazonian vegetation as a small-scale mosaic of unpredictable composition and structure. Instead, they provide additional evidence that tropical tree communities are not qualitatively different from their temperate counterparts, where a few common species concentrated in a few higher taxa can dominate immense areas of forest. We hypothesize that most Amazonian forests are dominated at large scales by oligarchies similar in nature to the ones observed in Ecuador and Peru, and we argue that the patterns are more indicative of regulation of relative abundances by ecological factors than of nonequilibrium chance-based dynamics. The paper concludes with a discussion of the practical applications of predictable oligarchies over large areas of unexplored forest.

The diversity–disturbance relationship: is it generally strong and peaked?

Abstract: The contemporary literature accepts that disturbance strongly influences pat- terns of species diversity, and that the relationship is peaked, with a maximum at inter- mediate levels of disturbance. We tested this hypothesis using a compilation of published species diversity-disturbance relationships that were gleaned from a literature search of papers published from 1985 through 1996 and from references therein. We identified 116 species richness-, 53 diversity-, and 28 evenness-disturbance relationships in the literature, which we grouped according to shape of relationship (nonsignificant, peaked, negative monotonic, positive monotonic, or U-shaped). We tested the relationships between the strength and shapes of these relationships and attributes of the community, disturbance, and sampling and study design. Nonsignificant relationships were the most common, com- prising 35% of richness, 28% of diversity, and 50% of evenness studies. Peaked responses were reported in only 16% of richness, 19% of diversity, and 11% of evenness cases. Explained variation in the three measures of diversity was variable among studies but averaged -50%. It was higher when few samples and few disturbance levels were examined and when organisms within the samples were not exhaustively censused, suggesting that procedural artifact contributes to these relationships. Explained variation was also higher in studies in which disturbance was measured as a gradient of time passed since the last disturbance (mean r2 = 61%), vs. studies of spatial variation in richness (mean r2 = 42%). Peaked richness relationships had the greatest odds of being observed when sampled area and actual evapotranspiration were small, when disturbances were natural rather than an- thropogenic in origin, and when few disturbance levels were examined. Thus, on average, diversity-disturbance relationships do not have consistently high r2 and are not as consis- tently peaked as the contemporary consensus would suggest.

Wisdom and Wonder@@@2000. Precious Heritage: The Status of Biodiversity in the United States

Abstract: Foreword Preface Contributors Participating Institutions Acknowledgements 1. Biodiversity: Our Precious Heritage 2. Discovering Life in America: Tools and Techniques of Biodiversity Inventory 3. A Remarkable Array: Species Diversity in the United States 4. Conservation Status of U.S. Species 5. State of the States: Geographic Patterns of Diversity, Rarity, and Endemism 6. The Geography of Imperilment: Targeting Conservation towards Critical Biodiversity Areas 7. More than the Sum of the Parts: Diversity and Status of Ecological Systems 8. Leading Threats to U.S. Biodiversity: What's Threatening Imperiled Species 9. Strategies for Biodiversity Protection 10. Owning Up to Our Responsibilities: Who Owns Lands Important for Biodiversity? 11. Safeguarding Our Precious Heritage Appendix A: Extinct and missing species of the United States Appendix B: State Diversity, Endemism, and Rarity Appendix C: Kuchler Potential Natural Vegetation Types Appendix D: Principal Sources for the Natural Heritage Central Databases Literature Cited

Spatial controls of historical fire regimes: a multiscale example from the interior west, usa

Abstract: Our objective was to infer the controls of spatial variation in historical fire regimes. We reconstructed a multicentury history of fire frequency, size, season, and severity from fire scars and establishment dates of 1426 trees sampled on grids in four watersheds (-64 plots, over -1620 ha each) representative of the Blue Mountains, Oregon and Wash- ington, USA. The influence of regional climate, a top-down control, was inferred from among-watershed variation in fire regimes, while the influence of local topography, a bot- tom-up control, was inferred from within-watershed variation. Before about 1900, fire regimes varied among and within watersheds, suggesting that both top-down and bottom- up controls were important. At the regional scale, dry forests (dominated by ponderosa pine), burned twice as frequently and earlier in the growing season in southern watersheds than in northern watersheds, consistent with longer and drier fire seasons to the south. Mesic forests (dominated by subalpine fir or grand fir) probably also burned more frequently to the south. At the local scale, fire frequency varied with different parameters of topography in watersheds with steep terrain, but not in the watershed with gentle terrain. Frequency varied with aspect in watersheds where topographic facets are separated by significant barriers to fire spread, but not in watersheds where such facets interfinger without fire barriers. Frequency varied with elevation where elevation and aspect interact to create gradients in snow-cover duration and also where steep talus interrupts fuel continuity. Frequency did not vary with slope within any watershed. The presence of both regional- scale and local-scale variation in the Blue Mountains suggests that top-down and bottom- up controls were both important and acted simultaneously to influence fire regimes in the past. However, an abrupt decline in fire frequency around 1900 was much greater than any regional or local variation in the previous several centuries and indicates that 20th-century fire regimes in these watersheds were dramatically affected by additional controls such as livestock grazing and fire suppression. Our results demonstrate the usefulness of examining spatial variation in historical fire regimes across scales as a means for inferring their controls.

Biodiversity and the dilution effect in disease ecology

Abstract: Many infectious diseases of humans are caused by pathogens that reside in nonhuman animal reservoirs and are transmitted to humans via the bite of an arthropod vector. Most vectors feed from a variety of host species that differ dramatically in their reservoir competence; that is, their probability of transmitting the infection from host to vector. We explore a conceptual model of what we termed the “dilution effect,” whereby the presence of vertebrate hosts with a low capacity to infect feeding vectors (incompetent reservoirs) dilute the effect of highly competent reservoirs, thus reducing disease risk. Using Lyme disease as an example, we demonstrate the presence and estimate the magnitude of the dilution effect for local sites in eastern New York State. We found that the prevalence of Lyme disease spirochetes, Borrelia burgdorferi, in field-collected Ixodes ticks (37.6% and 70.5% for nymphal and adult stages, respectively) was dramatically lower than expected (∼90% and >95% for nymphal and adult stages, respectively) if ticks fed predominantly on highly competent reservoirs, white-footed mice (Peromyscus leucopus) and eastern chipmunks (Tamias striatus). We inferred the role of additional host species using an empirically based model that incorporated data on tick burdens per host, relative population densities of hosts, and reservoir competence of each host. Assuming an empirically realistic reservoir competence of 5% for non-mouse and non-chipmunk hosts, we determined that alternative hosts must provide 61% and 72% of larval and nymphal meals, respectively. Using computer simulations, we assembled simulated host communities that differed in species richness, evenness, and net interactions between alternative hosts and mice. We found that increasing species richness (but not evenness) reduced disease risk. Effects were most pronounced when the most competent disease reservoirs were community dominants and when alternative hosts had a net negative influence on the dominance of mice as a host for ticks. Our results highlight a critical role of biodiversity and host community ecology in the transmission of vector-borne zoonotic diseases that in turn has important consequences for human health.

Treefall gaps and the maintenance of species diversity in a tropical forest

Abstract: The maintenance of species diversity by treefall gaps is a long-standing paradigm in forest ecology. Gaps are presumed to provide an environment in which tree species of differing competitive abilities partition heterogeneous resources. The empirical evidence to support this paradigm, however, remains scarce, and some recent studies even suggest that gaps do not maintain the diversity of shade-tolerant species. Although there is evidence that gaps maintain the diversity of pioneer trees, most of this evidence comes from studies that did not make comparisons between gaps and intact forest sites (controls). Further, nearly all studies on the maintenance of diversity by gaps have ignored lianas, an important component of both old-world and neotropical forests. We tested the hypothesis that treefall gaps maintain shade-tolerant tree, pioneer tree, and liana species diversity in an old-growth forest on Barro Colorado Island (BCI), Panama. We compared the density and species richness of these guilds between paired gap and non-gap sites on both a per- area and a per-individual (per capita) basis. We found no difference in shade-tolerant tree density and species richness between the gap and non-gap sites. Both pioneer tree and liana density and species richness, however, were significantly higher in the gap than in the non- gap sites on both a per-area and a per-individual basis. These results suggest that gaps maintain liana species diversity and that this effect is not merely a consequence of increased density. Furthermore, our data confirm the long-held belief that gaps maintain pioneer tree species diversity. Because lianas and pioneer trees combined account for ;43% of the woody plant species on BCI, and in other forests, our results are likely to be broadly applicable and suggest that gaps play a strong role in the maintenance of woody species diversity.

Vegetation pattern formation in semi-arid grazing systems

Abstract: Hypotheses about the origin of vegetation pattern formation in semi-arid areas around the world almost all include a common feature of semi-arid areas: the presence of a positive feedback between plant density and water infiltration We investigate whether this positive feedback and the spatial redistribution of runoff water are sufficient to explain vegetation pattern formation For this purpose, we analyze a spatially explicit model con- sisting of partial differential equations using a method for demonstrating pattern formation (Turing analysis) Our analysis reveals that pattern formation can occur in semi-arid areas given only the positive feedback between plant density and local water infiltration coupled with the spatial redistribution of runoff water Thus, slope and underlying heterogeneity are not essential conditions Other factors in the model, such as herbivory, plant dispersal, rainfall, and drought tolerance of plants, appear to determine under what conditions pattern formation is likely but are not the primary factors that generate the patterns The model is in agreement with field observations and indicates the conditions for which vegetation pattern formation can be expected in arid and semi-arid grazing systems

Rain forest fragmentation and the structure of amazonian liana communities

Abstract: In tropical forests, lianas (woody vines) are important structural parasites of trees. We assessed the effects of forest fragmentation, treefall disturbance, soils, and stand attributes on liana communities in central Amazonian rain forests. Over 27 500 liana stems (≥2 cm diameter at breast height [dbh]) were recorded in 27 1-ha plots in continuous forest and 42 plots in 10 forest fragments ranging from 1 to 100 ha in area. For each plot, an index of forest disturbance was determined from a 20-yr study of tree-community dynamics, and 19 soil-texture and chemistry parameters were derived from soil surface samples (top 20 cm). Liana abundance was 187–701 stems/ha, and liana aboveground dry biomass varied from 3.7 to 12.3 Mg/ha. Liana abundance increased significantly near forest edges and was significantly positively associated with forest disturbance and significantly negatively associated with tree biomass. Liana biomass was similarly associated with disturbance and tree biomass but also increased signifi...

Small‐rodent dynamics and predation

Abstract: The hypothesis that the regular multiannual population oscillations of boreal and arctic small rodents (voles and lemmings) are driven by predation is as old as the scientific study of rodent cycles itself Subsequently, for several decades, the predation hypothesis fell into disrepute, possibly because the views about predation and rodent dy- namics were too simplistic Here we review the work that has been done on the predation hypothesis primarily in Fennoscandia over the past decade Models of predator-prey interaction have been constructed for the least weasel (Mustela nivalis) and the field vole (Microtus agrestis), which are considered to be the key specialist predator and the key prey species in the multispecies communities in the boreal forest region in Fennoscandia The basic model has been parameterized with independent field data, and it predicts well the main features of the observed dynamics An extension of the model also including generalist and nomadic avian predators predicts correctly the well- documented and striking geographic gradient in rodent oscillations in Fennoscandia, with the amplitude and cycle period decreasing from north to south These geographic changes are attributed to the observed latitudinal change in the density of generalist and nomadic predators, which are expected to have a stabilizing effect on rodent dynamics We review the other observational, modeling, and experimental results bearing on the predation hypothesis and conclude that it accounts well for the broad patterns in rodent oscillations in Fennoscandia We discuss the application of the predation hypothesis to other regions in the northern hemisphere The predation hypothesis does not make predictions about multiannual and latitudinal changes in body size, behavior, and demography of ro- dents, which may have some population-dynamic consequences With the current evidence, however, we consider it unlikely that the phenotypic and genotypic composition of pop- ulations would be instrumental for generating the broad patterns in rodent oscillations

The impact of herbivory on plants in different resource conditions: a meta-analysis

Abstract: Understanding how plant recovery from herbivory interacts with the resource environment is necessary to predict under what resource conditions plants are most affected by herbivory, and ultimately how herbivory impacts plant population dynamics. It has been commonly assumed that plants are generally best able to recover from herbivory when growing in high resource conditions, an assumption which is supported by some models (e.g., the continuum of responses model) but opposed by others (e.g., the growth rate model). The validity and generality of any effects of resources (light, nutrients, and water) on plant recovery from herbivory were tested with mixed-model, factorial meta-analyses using a log response ratio metric applied to plant growth and reproduction data from the ecological literature. In total, 81 records from 45 studies were included in the growth meta-analysis, and 24 records from 14 studies in the reproduction meta-analysis. High resource levels and the absence of herbivory both strongly increased plant growth and reproduction. There was no significant overall interaction between growth or reproduction after herbivory and re- source conditions, but the interaction terms were significant for each plant functional group in the growth meta-analysis. Basal meristem monocots grew significantly more after her- bivory in high resources, while both dicot herbs and woody plants grew significantly more after herbivory in low resources. A similar result was found in the 34.6% of growth records where exact- or overcompensation occurred. Overcompensation was more likely in high resources for monocots and in low resources for dicot herbs. The reproduction data set was too small to subdivide. These qualitative differences between monocot and dicot herbs and woody plants explain many of the contradictory results in the literature and show that no single current model can account for the responses of all plants to herbivory.

Effects of salmon-derived nitrogen on riparian forest growth and implications for stream productivity

Abstract: Anadromous Pacific salmon (Oncorhynchus spp.) transport marine-derived nitrogen (MDN) to the rivers in which they reproduce. Isotopic analyses indicate that trees and shrubs near spawning streams derive -22-24% of their foliar nitrogen (N) from spawn- ing salmon. As a consequence of this nutrient subsidy, growth rates are significantly in- creased in Sitka spruce (Picea sitchensis) near spawning streams. As riparian forests affect the quality of instream habitat through shading, sediment and nutrient filtration, and pro- duction of large woody debris (LWD), this fertilization process serves not only to enhance riparian production, but may also act as a positive feedback mechanism by which salmon- borne nutrients improve spawning and rearing habitat for subsequent salmon generations and maintain the long-term productivity of river corridors along the Pacific coast of North America.

Effects of soil nutrient availability on investment in acquisition of n and p in hawaiian rain forests

Abstract: We determined the influence of nutrient availability on the mechanisms used by plants to acquire nitrogen and phosphorus from the soil. Extracellular acid phosphatase production, mycorrhizal colonization, and N and P uptake capacities were measured in control, N-, and P-fertilized forests in three sites that varied in nutrient status from N limited to relatively fertile to P limited. Nitrogen fertilization increased extracellular phosphatase activity in all sites. Phosphorus additions consistently reduced phosphatase activity, mycorrhizal colonization, and P uptake capacity across sites. Our results indicate that these plants efficiently allocate resources to nutrient acquisition as suggested by an economic model. Investment in acquisition of a nutrient was greatest when that nutrient was limiting to growth, and plants appeared to allocate excess N to construction of extracellular phosphatases to acquire P. This increase in phosphatase production with N fertilization implies that even P-limited systems might respond to N deposition with greater productivity.

Effect of land disturbance and stress on species traits of ground beetle assemblages

Abstract: In this paper we test whether the morphology and life traits of species (in our case ground beetles of the family Carabidae) can be related to the main underlying axes of environmental variability of their habitats. Sites were selected a priori to maximize two gradients: land use as a general measure of disturbance characterized by an index of land management, and habitat adversity or stress as characterized by elevation and vegetation structure. The underlying environmental axes and the relationships of the morphology and life traits of the species with them were investigated using RLQ analysis, a multivariate ordination method able to relate a species trait table to a site characteristics table by way of a species abundance table. The first environmental axis was highly statistically significant and explained most of the variability. It was strongly negatively related to the intensity of land management, and positively related to increasing elevation and a set of variables reflecting vegetation stress. Two predictions were tested and found to be valid in the studied system: in highly managed lowland sites species were smaller, and the frequency of macropterous species (with better dispersal abilities) was higher. Other traits also showed significant relationships with the main environmental axis: in the intensively managed lowland sites species had broader bodies, longer trochanters, and wider femora (characters associated with plant eaters), were paler in color, overwintered only as adults, bred in spring or autumn, and were active in summer. We conclude that the ground beetle assemblages of the studied sites respond in a similar way to the same underlying environmental factors. This allows the precise definition of functional groups, which can be used to characterize functional diversity and its relationships with changes in land management.

Macroalgal canopies contribute to eelgrass (zostera marina) decline in temperate estuarine ecosystems

Abstract: Loss of eelgrass (Zostera marina) habitat from temperate estuaries worldwide often coincides with increased macroalgal accumulations resulting from increased delivery of anthropogenic nitrogen. We conducted macroalgal enclosure/exclosure experiments dur- ing summer 1998 within eelgrass populations in two estuaries of Waquoit Bay, Massachu- setts, USA, to evaluate how increased macroalgal biomass affects density, recruitment, growth rate, and production of eelgrass. One estuary featured a low nitrogen loading rate and sustained a relatively pristine eelgrass population with a 2 cm high macroalgal canopy. The other estuary had a sixfold higher nitrogen loading rate and a declining eelgrass population with a 9 cm high macroalgal canopy. Experimental units were 1 X 1 m plots of eelgrass fenced within 50 cm high plastic mesh that excluded or included macroalgae at canopy heights ranging from 0 to 25 cm. In both estuaries, rates of eelgrass loss increased, largely a result of decreased recruitment, and growth rates decreased (due to decreased rates of leaf appearance) with increasing macroalgal canopy height. Aboveground summer production in both estuaries decreased exponentially as macroalgal canopy heights in- creased. We conclude that macroalgal cover is a proximate cause for loss of eelgrass in the higher N estuary since, upon removal of macroalgae, we observed an increase in shoot density, a 55% increase in summer growth, and a 500% increase in summer aboveground net production. Based on summer growth data and density of shoots in our experimental plots the following spring, we suggest that the negative impacts of macroalgal canopies persist, but also that eelgrass recovery upon removal of macroalgae may be possible. To identify the mechanisms by which macroalgae potentially inhibit eelgrass production, we measured changes in nutrient and oxygen concentrations resulting from macroalgal canopies and estimated the relative importance of summer standing stocks of phytoplankton, epiphytes, and macroalgae to potential shading of eelgrass in both estuaries. We document both (1) unfavorable biogeochemical conditions (lowered redox conditions and potentially toxic concentrations of NH4+) imposed by the presence of macroalgal canopies and (2) potential light limitation of eelgrass by standing stocks of producers in the higher N estuary, with estimates of light reduction via macroalgae numerically more important than light sequestration by phytoplankton and epiphytes for newly recruiting shoots. Increased ma- croalgal biomass associated with increased nitrogen loading to estuaries can lead to eelgrass disappearance, and we identify an approximate 9-12-cm critical macroalgal canopy height at which eelgrass declines.

Stable-carbon-isotope ratios of river biota:implications for energy flow in lotic food webs

Abstract: Stable-isotope ratios of carbon ( 13 C/ 12 Co rd 13 C) have been widely used to determine the energy base of stream food webs, but such use is controversial due to un- explained variability in algal d 13 C. I used published d 13 C data from temperate headwater streams through medium-sized rivers (0.2-4000 km 2 watershed area) collected during sum- mer baseflows and original data from streams in northern California to analyze energy pathways through river food webs. The analyses showed three important results. First, epilithic algal d 13 C and watershed area are positively related, suggesting that effects of carbon limitation on algal carbon uptake result in 13 C enrichment of algal d 13 C in larger, more productive rivers. Second, epilithic algae and terrestrial detritus d 13 C values are often distinct in small shaded streams but overlap in some larger unshaded streams and rivers. Measurements of d 13 C values may be most useful in distinguishing algal and terrestrial energy sources in unproductive streams with supersaturated dissolved CO2 concentrations, and some productive rivers where CO2 concentrations are low relative to photosynthetic rates. Finally, consumer d 13 C values are more strongly related to algal d 13 C than terrestrial d 13 C. The relative contribution of terrestrial and algal carbon sources often varied by func- tional feeding group within and between sites. However, with the exception of shredders and scrapers, which respectively relied on terrestrial and algal carbon sources, patterns of consumer d 13 C clearly show a transition from terrestrial to algal carbon sources for many lotic food webs in streams with $10 km 2 watershed area. The observed transition to algal carbon sources is likely related to increasing primary production rates as forest canopy cover declines in larger streams, although decreasing retention or quality of terrestrial carbon may also play a role. Improved analyses of algal d 13 C and d 15 N combined with quantitative study of organic matter dynamics and food web structure should allow the relative im- portance of these factors to be distinguished in future food web studies.

Do arbuscular mycorrhizal fungi alter plant–pathogen relations?

Abstract: Arbuscular mycorrhizal (AM) fungi colonize roots of terrestrial plants and have been hypothesized to reduce susceptibility or to improve the vigor of hosts challenged by root pathogens. Meta-analysis was used to test whether a broad pattern exists in which AM fungi not only enhance plant growth but fundamentally alter plant-pathogen interac- tions. Data were gathered from studies published between 1970 and early 1998. In these studies nonmycorrhizal or mycorrhizal plants were untreated or challenged with root-in- fecting fungal pathogens or nematodes. Effect sizes for AM fungus treatment, pathogen treatment, and their interaction were calculated based on measures of average plant growth, pathogen growth, or AM colonization as reported by authors. AM fungus had a very large positive effect, and pathogens had a very large negative effect on plant growth. However, the interaction of AM fungi and pathogen depended upon the class of pathogen. AM fungi tended to decrease the harmful effects of fungal pathogens but to exacerbate the harmful effects of nematodes. Overall, inoculation with AM fungi had a large negative effect on growth of pathogens, but the outcome for nematodes depended upon mode of feeding. AM fungi harmed sedentary endoparasitic nematodes. AM fungi improved growth of migratory endoparasitic nematodes, but this outcome was not significant in an analysis limited to independent experiments. Reduced growth of pathogens in mycorrhizal plants may indicate increased host resistance. Alternatively, reduced AM colonization in pathogen-treated plants may suggest that root- infecting organisms compete for resources. Some experiments that did not meet criteria for meta-analysis were analyzed with x2 tests by calculating the proportion of experiments in which authors reported a significant pathogen effect in the absence vs. presence of AM fungi. AM fungi did not alter the effect of fungal pathogens but reduced the proportion of experiments in which nematodes de- pressed plant growth in a test limited to independent observations. Comparison of frequency distributions of effect sizes suggests that the difference in results for nematodes in the meta-analysis and the x2 tests stems from differences in the AM fungus-plant-pathogen systems examined in each approach rather than a statistical artifact. AM fungi reduced pathogen growth in -50% of studies included in x2 tests, and this effect was similar for fungal vs. nematode pathogens and sedentary vs. migratory nema- todes. Fungal pathogens reduced AM fungus growth more frequently than did nematodes. Both AM fungi and pathogens suffered reduced growth in 16% of experiments that assessed reciprocal effects. Reciprocal suppression was more common when the pathogen was a fungus, which may indicate that fungal pathogens are more likely than nematodes to compete with AM fungi. Almost all research on the effects of AM fungi on plant-pathogen relations has examined economically important species, in low-phosphorus soil, in greenhouse or microplot culture. Although these analyses revealed general patterns of the effects of AM fungi on plant- pathogen relations, research on a broader range of species over a wider range of environ- mental conditions is required to determine the domain for which these patterns apply.

Marked differences in survivorship among apple roots of different diameters

Abstract: Fine roots are responsible for a substantial fraction of terrestrial net primary productivity, and a better understanding of fine root production and turnover is crucial to improving global carbon and nutrient cycling models. In most studies, roots less than 1 or 2 mm in diameter (“fine roots”) have been treated as structurally and physiologically identical individuals. We used minirhizotron data from 16-yr-old apple (Malus domestica) trees to investigate differences in life span and life history among fine roots whose diameters differed by tenths of a millimeter. We also introduced the use of Cox proportional hazards regression models to assess the effects of multiple covariates on root mortality. Overwinter survivorship differed markedly among diameter classes in both years: 3–12% for roots 0.5 mm in diameter and brown, and these roots gave rise to new white laterals during the spring root flush. Based on the considerable differences in morphology and life history within the class of roots <1 mm in diameter, we advocate the use of a functional definition for the fine root whenever possible.

Dissimilatory nitrate reduction to ammonium in upland tropical forest soils

Abstract: The internal transformations of nitrogen in terrestrial ecosystems exert strong controls over nitrogen availability to net primary productivity, nitrate leaching into ground- water, and emissions of nitrogen-based greenhouse gas. Here we report a reductive pathway for nitrogen cycling in upland tropical forest soils that decreases the amount of nitrate susceptible to leaching and denitrification, thus conserving nitrogen in the ecosystem. Using '5N tracers we measured rates of dissimilatory nitrate reduction to ammonium (DNRA) in upland humid tropical forest soils averaging -0.6 p1g-g-'d-'. Rates of DNRA were three times greater than the combined N20 and N2 fluxes from nitrification and denitrification and accounted for 75% of the turnover of the nitrate pool. To determine the relative im- portance of ambient C, 02, and NO3 concentrations on rates of DNRA, we estimated rates of DNRA in laboratory assays using soils from three tropical forests (cloud forest, palm forest, and wet tropical forest) that differed in ambient C and 02 concentrations. Rates of DNRA measured in laboratory assays ranged from 0.5 to 9 (Lgg-'gd-' in soils from the three different forests and appeared to be primarily limited by the availability of NO3, as opposed to C Or 02. Tests of sterile soils indicated that the dominant reductive pathway for both NO2 and NO3 was biotic and not abiotic. Because NH4 is the form of N generally favored for assimilation by plants and microbes, and NO3 is easily lost from the ecosystem, the rapid and direct transformation of NO3 to NH4 via DNRA has the potential to play an important role in ecosystem N conservation.

Species composition interacts with fertilizer to control long-term change in tundra productivity

Abstract: Fifteen years of N and P fertilizer addition to an Alaskan moist tundra increased aboveground biomass and primary production by 2.5 times. Species composition of the fertilized vegetation also changed dramatically, from a mix of graminoid, evergreen, deciduous, and moss species to strong dominance by a single, deciduous shrub species, Betula nana. Analysis of these simultaneous changes allows insights into the interactions between changes in resource availability and changes in species composition in regulating vegetation biomass, production, and element use. By the 15th year (1995), both new leaf production and total leaf mass were lower in fertilized than in control plots, although leaf area in fertilized plots was twice that of controls. This occurred because Betula produced thinner leaves than other species, with a high specific leaf area (SLA, leaf area per unit leaf mass). Woody stem mass also increased dramatically in fertilized plots, with secondary growth accounting for over half of aboveground net primary production, NPP. The large increase in wood production was made possible, in part, by the low cost of production of Betula's thin leaves, allowing greater allocation to secondary growth. Wood also had lower N concentrations than leaves, allowing large accumulations of wood at low N cost. Overall, aboveground N concentration in Betula did not change in fertilized relative to control plots, because its low-N wood mass increased more than its high-N leaf mass (with high SLA). Because Betula was so strongly dominant on the fertilized plots and was better able to dilute its greater N supply with new growth, community production and biomass in fertilized plots were higher, and N concentration was lower, than would have been the case if species composition had not changed. Aboveground biomass and leaf area of individual species and functional types were predicted accurately by regression against the number of hits per point-frame pin across the full range of data, including both treatments. Changes in overall canopy structure and leaf display due to fertilization were thus due mainly to changes in species composition, with no detectable effect of treatment on size/structure relationships within species or functional types.

Soil fauna and plant litter decomposition in tropical and subalpine forests

Abstract: The decomposition of plant residues is influenced by their chemical com- position, the physical-chemical environment, and the decomposer organisms. Most studies interested in latitudinal gradients of decomposition have focused on substrate quality and climate effects on decomposition, and have excluded explicit recognition of the soil or- ganisms involved in the process. To test whether soil fauna exhibit different effects under diverse climates or substrates, we quantified decomposition rates and N fluxes in control and fauna-excluded treatments with litterbags containing relatively high-quality (Quercus gambehii) and low-quality (Cecropia scheberiana) litter in sites representing large differ- ences in climate as measured by AET (actual evapotranspiration). Two subtropical sites included a wet and a dry forest, and two temperate sites included north- and south-facing subalpine forests. We found that: (1) all three factors (climate, substrate quality, and soil fauna) independently influenced the decomposition rate of plant litter in the tropical and subalpine forests; (2) faunal effects on decomposition rates and N mineralization of Q. gambelii and C. scheberiana were consistently higher in the tropical wet forest than in the tropical dry and subalpine forests; (3) there was no significant interaction of fauna and litter species on the annual decay rates; and (4) the density (numbers per gram of dry litter) of total fauna was highest in the tropical wet forest, intermediate in the subalpine forests, and lowest in the tropical dry forest. Our results indicate that soil fauna have a dispro- portionately larger effect on litter decomposition in a tropical wet forest than in a tropical dry or a subalpine forest. The annual decay rates of Q. gambelii and C. scheberiana are not influenced differentially by the soil fauna in the tropical and subalpine forests.

Energy allocation strategy in young fish: allometry and survival

Abstract: We observed substantial variation in seasonal growth rates, autumn body size, and growing-season mortality among eight experimental cohorts of age-O rainbow trout, Oncorhynchus mykiss. Wet mass, water, lipids (storage), and lipid-free dry mass (structure) had biphasic allometries with inflexions at - 10 cm in length. Dry:wet mass and storage:structure ratios were positively related to fish length, indicating that the relative quantities of these constituents change with body size. Lipid concentration varied according to a sigmoid relationship with wet mass which also had a growth-rate dependence. Inde- pendent assessments of the allometry of growing-season survival and winter metabolism facilitated assessment of the costs and benefits of two alternate energy allocation strategies of young fish. For cohorts with low growth rates and small autumn body size, a somatic growth rate maximization strategy is optimum, producing a 5% net survival advantage over an energy storage maximization strategy. For cohorts with intermediate growth rates and autumn mass, somatic growth and energy storage strategies lead to similar first-year sur- vival. The fastest growing cohorts are estimated to have a net survival advantage of 7%, by adopting an energy storage maximization strategy over a growth rate maximization strategy.