Showing papers in "Ecology in 1993"
TL;DR: The paper discusses first how autocorrelation in ecological variables can be described and measured, and ways are presented of explicitly introducing spatial structures into ecological models, and two approaches are proposed.
Abstract: ilbstract. Autocorrelation is a very general statistical property of ecological variables observed across geographic space; its most common forms are patches and gradients. Spatial autocorrelation. which comes either from the physical forcing of environmental variables or from community processes, presents a problem for statistical testing because autocorrelated data violate the assumption of independence of most standard statistical procedures. The paper discusses first how autocorrelation in ecological variables can be described and measured. with emphasis on mapping techniques. Then. proper statistical testing in the presence of autocorrelation is briefly discussed. Finally. ways are presented of explicitly introducing spatial structures into ecological models. Two approaches are proposed: in the raw-data approach, the spatial structure takes the form of a polynomial of the x and .v geographic coordinates of the sampling stations; in the matrix approach. the spatial structure is introduced in the form of a geographic distance matrix among locations. These two approaches are compared in the concluding section. A table provides a list of computer programs available for spatial analysis.
3,491 citations
TL;DR: In this article, the authors compared several approaches to determining the number of components to interpret from principal components analysis (PCA) using simulated data matrices of uniform correlation structure and data sets of lake morphometry, water chemistry, and benthic invertebrate abundance.
Abstract: Approaches to determining the number of components to interpret from principal components analysis were compared. Heuristic procedures included: retaining components with eigenvalues (Xs) > 1 (i.e., Kaiser-Guttman criterion); components with bootstrapped Xs > 1 (bootstrapped Kaiser-Guttman); the scree plot; the broken-stick model; and components with Xs totalling to a fixed amount of the total variance. Statistical ap- proaches included: Bartlett's test of sphericity; Bartlett's test of homogeneity of the cor- relation matrix, Lawley's test of the second X; bootstrapped confidence limits on successive Xs (i.e., significant differences between Xs); and bootstrapped confidence limits on eigen- vector coefficients (i.e., coefficients that differ significantly from zero). All methods were compared using simulated data matrices of uniform correlation structure, patterned ma- trices of varying correlation structure and data sets of lake morphometry, water chemistry, and benthic invertebrate abundance. The most consistent results were obtained from the broken-stick model and a combined measure using bootstrapped Xs and associated eigen- vector coefficients. The traditional and bootstrapped Kaiser-Guttman approaches over- estimated the number of nontrivial dimensions as did the fixed-amount-of-variance model. The scree plot consistently estimated one dimension more than the number of simulated dimensions. Bartlett's test of sphericity showed inconsistent results. Both Bartlett's test of homogeneity of the correlation matrix and Lawley's test are limited to testing for only one and two dimensions, respectively.
2,102 citations
TL;DR: In this paper, the authors advocate proportional habitat use by individual animals as a basis for analysis and use compositional analysis of such nonstandard multivariate data for analysis of habitat use based on radiotagged animals.
Abstract: Analysis of habitat use based on radio-tagged animals presents difficulties inadequately addressed by current methods. Areas of concern are sampling level, data pooling across individuals, non-independence of habitat proportions, differential habitat use by groups of animals, and arbitrary definition of habitat availability. We advocate proportional habitat use by individual animals as a basis for analysis. Hypothesis testing of such nonstandard multivariate data is done by compositional analysis, which encom- passes all MANOVA/MANCOVA-type linear models. The applications to habitat use range from testing for age class effects or seasonal differences, to examining relationships with food abundance or home range size. We take as an example the comparison of habitat use and availability. The concepts are explained and demonstrated on two data sets, illustrating different methods of treating missing values. We compare utilized with available habitats in two stages, examining home range selection within the overall study area first, then habitat use within the home range. At each stage, assuming that use differs from random, habitats can be ranked according to relative use, and significant between-rank differences located. Compositional analysis is also suited to the analysis of time budgets or diets.
1,961 citations
TL;DR: Canonical Correspondence Analysis (CCA) is quickly becoming the most widely used gradient analysis technique in ecology as discussed by the authors, and it has been shown to perform well with skewed species distributions, with quantitative noise in species abundance data, with samples taken from unusual sampling designs, with highly intercorrelated environmental variables and with situations where not all of the factors determining species composition are known.
Abstract: Canonical Correspondence Analysis (CCA) is quickly becoming the most widely used gradient analysis technique in ecology. The CCA algorithm is based upon Correspondence Analysis (CA), an indirect gradient analysis (ordination) technique. CA and a related ordination technique, Detrended Correspondence Analysis, have been crit- icized for a number of reasons. To test whether CCA suffers from the same defects, I simulated data sets with properties that usually cause problems for DCA. Results indicate that CCA performs quite well with skewed species distributions, with quantitative noise in species abundance data, with samples taken from unusual sampling designs, with highly intercorrelated environmental variables, and with situations where not all of the factors determining species composition are known. CCA is immune to most of the problems of DCA.
1,150 citations
TL;DR: Results suggest that diversity is lower in productive grasslands because accumulated litter, and possibly lower light penetration, inhibit germination and/or survival of seedlings, and thus decrease rates of establishment by new species.
Abstract: The biodiversity of a site should depend on the interplay of local colonization (gain) and extinction (loss) rates, but few theoretical explanations of diversity patterns on productivity gradients have included effects of productivity on colonization. In an 1 -yr study, experimental increases in productivity via nitrogen addition generally led to de- creased species richness in four grassland fields. Decreased diversity in productive plots was caused as much by lower rates of species gain as by greater rates of loss of existing species. Annual grasses and forbs had high gain and loss rates, but these were independent of productivity. In contrast, the rates of gain of perennial grass and forb species declined with productivity, and their rates of loss increased. Species richness was dependent on litter mass and on light penetration, but not on aboveground living plant mass, suggesting that there was no direct effect of productivity on diversity. Diversity in periodically burned prairie, which had low litter mass, was independent of productivity by the 10th and 11th yr of the study. Results suggest that diversity is lower in productive grasslands because accumulated litter, and possibly lower light penetration, inhibit germination and/or survival of seedlings, and thus decrease rates of establishment by new species. Higher productivity also leads to higher rates of loss, presumably via competitive displacement, of existing species. Results do not support the hypothesis that soil N heterogeneity controls diversity on the temporal and spatial scales of this study.
511 citations
TL;DR: Two less commonly used regression analyses that could find wide application in ecology, logistic regression and LOWESS regression are discussed.
Abstract: Least-squares linear regression and multiple regression are among the most commonly used analytical techniques of ecologists. However, these techniques only address a portion of the possible applications of regression methods. We discuss two less commonly used regression analyses that could find wide application in ecology, logistic regression and LOWESS regression. Logistic regression is appropriate in cases where the dependent vari- able is categorical, dichotomous, or polychotomous. It can be used with continuous and/ or discrete independent variables. Logistic regression is motivated by the underlying bi- nomial or multinomial distribution of dichotomous and polychotomous dependent vari- ables and transforms the data to explicitly model these distributions. Locally weighted regression scatterplot smoothing or LOWESS regression is used to model the relationship between a dependent variable and independent variable when no single functional form will do. LOWESS regression is motivated by the assumption that neighboring values of the independent variable are the best indicators of the dependent variable in that range of
511 citations
TL;DR: In this paper, the effect of moose browsing on the nutrient cycles of boreal forests was studied using three 40-yr-old enclosures on Isle Royale, Michigan, and the results suggest that the effects of herbivores on ecosystems may be amplified by positive feedbacks between plant litter and soil nutrient availability.
Abstract: Selective foraging by moose on hardwoods and avoidance of conifers alters community composition and structure, which in turn can affect nutrient cycles and pro- ductivity. The effect of moose browsing on the nutrient cycles of boreal forests was studied using three 40-yr-old enclosures on Isle Royale, Michigan. Two alternative mechanisms by which moose affect ecosystems were tested: (1) moose depress both the quantity and quality of litter return to the soil, and hence N mineralization and net primary productivity, by browsing on hardwoods and avoiding conifers; (2) moose stimulate N mineralization, and hence net primary productivity, by opening the canopy and by dropping fecal pellets. Soil nutrient availability and microbial activity, including exchangeable cations, total car- bon and nitrogen, nitrogen mineralization rates, and microbial respiration rates, were uniformly higher in exclosures than outside. These differences were more significant where browsing intensity was high and less often significant where browsing intensity was low. N mineralization in browsed plots declined with increasing moose consumption rates. Net primary production in enclosures and browsed plots was strongly correlated with N min- eralization. N mineralization in turn was positively correlated with litter N return and negatively correlated with litter cellulose content. These differences in litter quantity and quality were caused by an increased abundance of unbrowsed spruce outside the enclosures. Moose pellets alone mineralized less N but more C than soil alone, but pellets combined with soil stimulated N and C mineralization more than the sum of the two separately. However, this did not appear to be sufficient to offset the depression in nitrogen and carbon mineralization in soil resulting from the increased abundance of unbrowsed spruce. We conclude that, in the long term, high rates of moose browsing depress N mineralization and net primary production through the indirect effects on recruitment into the tree stratum, and subsequent depression of litter N return and litter quality. These results suggest that the effects of herbivores on ecosystems may be amplified by positive feedbacks between plant litter and soil nutrient availability.
507 citations
TL;DR: A study of one of the simplest systems incorporating both dispersal and local dynamics, coupling two discrete time logistic equations, demonstrates several surprising features, including passive dispersal can cause chaotic dynamics to be replaced by simple periodic dynamics.
Abstract: A study of one of the simplest systems incorporating both dispersal and local dynamics, coupling two discrete time logistic equations, demonstrates several surprising features. Passive dispersal can cause chaotic dynamics to be replaced by simple periodic dynamics. Thus passive movement can be stabilizing, even in a deterministic model without underlying spatial variation in the dynamics. The boundary between initial conditions leading to qualitatively different dynamics can be a fractal, so it is essentially impossible to specify the asymptotic behavior in terms of the initial conditions. In accord with several recent studies of arthropods and earlier theoretical work, density dependence may only be detectable at a small enough spatial scale, so efforts to uncover density dependence must include investigations of movement. 26 refs., 6 figs.
493 citations
TL;DR: The aims is to provide an overview of the consequences of lack of balance and to give some guidelines to analyzing unbalanced data for models involving fixed effects.
Abstract: Ecological studies typically involve comparison of biological responses among a variety of environmental conditions. When the response variables have continuous distributions and the conditions are discrete, whether inherently or by design, then it is appropriate to analyze the data using analysis of variance (ANOVA). When data conform to a complete, balanced design (equal numbers of observations in each experimental treatment), it is straightforward to conduct an ANOVA, particularly with the aid of the numerous statistical computing packages that are available. Interpretation of an ANOVA of balanced data is also unambiguous. Unfortunately, for a variety of reasons, it is rare that a practicing ecologist embarks on an analysis of data that are completely balanced. Regardless of its cause, lack of balance necessitates care in the analysis and interpretation. In this paper, our aims is to provide an overview of the consequences of lack of balance and to give some guidelines to analyzing unbalanced data for models involving fixed effects. Our treatment is necessarily cursory and will not substitute for training available from a sequence of courses in mathematical statistics and linear models. It is intended to introduce the reader to the main issues and to the extensive statistical literature that deals with them.
492 citations
TL;DR: The most dramatic density effects occurred where pollinators were induced to behave as generalists, suggesting that density—related declines in pollinator quality are more important than parallel declines in the quantity of visits.
Abstract: The local density of a plant population can influence both the number of pollinators attracted and the behavior (and thus pollination efficiency) of those that are. I present the results of a field experiment performed on Brassica kaber, a self—incompatible annual crucifer, designed to test for the existence of density effects in pollination and to separate the two major component effects: reductions in the quantity and in the quality of pollinator services. B. kaber plants were grown in fan—shaped density arrays in six floral backgrounds chosen to manipulate pollinator foraging behavior. Pollinators visiting focal plants behaved as generalists in backgrounds of the similar—flowered Brassica hirta, behaved as floral specialists where no alternative floral resources were provided (three treatments), and displayed a mixture of behaviors in two background plantings where dissimilar flowers were available. Pollinator visitation rates to focal B. kaber plants declined sharply at low density in all backgrounds except the similar flower (B. hirta) treatment. Widely spaced plants displayed significantly reduced seed—set in all backgrounds studied. The most dramatic density effects occurred where pollinators were induced to behave as generalists, suggesting that density—related declines in pollinator quality are more important than parallel declines in the quantity of visits.
470 citations
TL;DR: Geostatistical techniques are used to quantify the scale and variability of soil nutrients at distances from 10 cm to 10 m in the field, with emphasis on the variability around individual perennial plants.
Abstract: Heterogeneity in the soil is regularly invoked as important for competitive interactions among plants (Chapin 1980), but surprisingly few attempts have been made to examine in situ soil heterogeneity in the context of individual plants (Snaydon 1962). Variability around individuals is of fundamental importance because current theories of plant competition (e.g., Grace and Tilman 1990) differ in their treatments of heterogeneity and scale. Although quantifying scale has been historically problematic, statistical advances of recent decades (Matheron 1963, Burgess and Webster 1980) now provide tools for ecologists to specifically address scale and heterogeneity in their experiments (Robertson et al. 1988, Rossi et al. 1992). In this study we use geostatistical techniques to quantify the scale and variability of soil nutrients at distances from 10 cm to 10 m in the field, with emphasis on the variability around individual perennial plants. The study area is 30 km south of Logan, Utah in a native sagebrush steppe (41029' N, 1 1047' W, 1575 m elevation); the soil is a silt loam formed from noncalcareous alluvial material. Average soil organic matter and pH are -2.7% (dry mass basis) and 6.3%, respectively. Prevalent at the site are the native shrub Artemisia tridentata (Rydb.) Beetle and the native tussock grass Pseudoroegneria spicata (Pursh) A. L6ve., both common Great Basin perennials. The sagebrush (Artemisia) plants tend to be relatively small (usually < 0.5 m tall with a fairly sparse canopy) for this species, but growth ring analysis of five plants showed several to be between 15 and 20 yr old. Neighbors of mature plants are sometimes closely spaced, in some cases within 0.5 m. Other plant genera present include Balsamorhiza, Zigadenus, Viola, Poa, and Lomatium.
TL;DR: This paper attempts to introduce some distribution-free and robust techniques to ecologists and to offer a critical appraisal of the potential advantages and drawbacks of these methods.
Abstract: After making a case for the prevalence of nonnormality, this paper attempts to introduce some distribution-free and robust techniques to ecologists and to offer a critical appraisal of the potential advantages and drawbacks of these methods. The techniques presented fall into two distinct categories, methods based on ranks and "computer-inten- sive" techniques. Distribution-free rank tests have features that can be recommended. They free the practitioner from concern about the underlying distribution and are very robust to outliers. If the distribution underlying the observations is other than normal, rank tests tend to be more efficient than their parametric counterparts. The absence, in computing packages, of rank procedures for complex designs may, however, severely limit their use for ecological data. An entire body of novel distribution-free methods has been developed in parallel with the increasing capacities of today's computers to process large quantities of data. These techniques either reshuffle or resample a data set (i.e., sample with replacement) in order to perform their analyses. The former we shall refer to as "permutation" or "randomiza- tion" methods and the latter as "bootstrap" techniques. These computer-intensive methods provide new alternatives for the problem of a small and/or unbalanced data set, and they may be the solution for parameter estimation when the sampling distribution cannot be derived analytically. Caution must be exercised in the interpretation of these estimates because confidence limits may be too small.
TL;DR: Nutrient addition positively affected snail growth in both experiments, indicating tight coupling between herbivore and algal growth (top-down ef? fects) and that bottom-up factors that directly affected plant growth could also indirectly affect consumers belonging to higher trophic levels.
Abstract: We conducted two experiments to determine the relative effects of herbivory and nutrients on an algal community in Walker Branch, a stream having effectively two trophic levels: primary producers and herbivorous snails. The first study (1989), performed in streamside channels, tested the effects of three factors: (1) stream water nitrogen (N), (2) phosphorus (P), and (3) snail grazing, on periphyton biomass, productivity, and community composition. The second study (1990), conducted in situ, tested the effects of snail grazing and nutrients (N + P). In the 1989 study, nutrients had positive effects, and herbivores had negative effects, on algal biomass (chlorophyll a, ash-free dry mass, total algal biovol? ume) and primary productivity (area- and chlorophyll-specific). Likewise, both nutrients and snail grazing exerted effects (+ and -, respectively) on biomass measured in the 1990 study (chlorophyll a, algal biovolume). Grazed communities were dominated by chloro- phytes and cyanophytes, which were overgrown by diatoms when herbivores were removed. Algal species that were reduced most by herbivores were increased most by nutrient ad? dition, and vice versa, suggesting a trade-off between resistance to herbivory and nutrient- saturated growth rates. Increases in algal biomass and productivity were slight with the addition of either N or P compared to responses observed when both nutrients were added together, suggesting that both nutrients were at growth-limiting levels. The greatest changes in periphyton structure or function were observed when both nutrients were added and simultaneously, grazers were removed, in contrast to lesser effects when nutrients were added under grazed conditions or grazers were removed at low nutrient levels, indicating dual control by both factors. Nutrient addition also positively affected snail growth in both experiments, indicating tight coupling between herbivore and algal growth (top-down ef? fects) and that bottom-up factors that directly affected plant growth could also indirectly affect consumers belonging to higher trophic levels. Indices quantifying the direct effects of top-down factors relative to bottom-up factors (top-down index, TDI) and the importance of interactions between these factors (interaction coefficient, IC) were computed. These indices showed that the relative strength of top-down and bottom-up factors varied among biomass and productivity parameters and that top-down and bottom-up effects, alone, were less important than their combined effects.
TL;DR: Measurements of light, soil nitrogen, and above- and be- lowground competition in vegetation that had received four combinations of fertilization and disturbance for 3 yr indicate variation in stress and disturbance.
Abstract: Variation in the intensities of above- and belowground competition is pro- posed to underlie changes in community structure associated with variable fertility and disturbance. Here we report measurements of light, soil nitrogen, and above- and be- lowground competition in vegetation that had received four combinations of fertilization and disturbance for 3 yr. Two levels of nitrogen addition (none and 17 g m-2 yr- ) and two of soil disturbance (none and annual tilling to remove all vegetation) were applied in a factorial design to 5 x 5 m plots (10 replicates) in a 31-yr-old field in southeastern Minnesota. During the 3rd yr of the experiment, community standing crop, light penetra- tion, and soil ammonium and nitrate concentrations were measured every 3-4 wk. Fer- tilization significantly increased productivity and tilling significantly decreased community biomass at the start of the growing season, indicating that the experimental treatments created variation in stress and disturbance. Transplants of Schizachyrium scoparium, a native perennial grass, were grown with no neighbors, or with the roots of neighbors, or with all neighbors in each combination of fertility and disturbance in order to examine competition intensity. In both undisturbed and tilled plots, belowground competition was the dominant interaction in unfertilized plots, whereas both below- and aboveground competition controlled growth in fertilized plots. The intensity of aboveground competition was greatest in plots with the lowest light penetration and declined significantly with increasing light supply. Similarly, belowground competition was most intense in plots with the lowest nitrogen availability and decreased significantly with increasing nitrogen availability. The intensities of above- and be- lowground competition were significantly negatively correlated. The total (above- and belowground) intensity of competition did not vary with fertility, but was significantly reduced by disturbance. Similarly, the effect of neighbor mass on transplant growth was significantly reduced by disturbance.
TL;DR: In this article, the authors applied a simple conceptual model to the investigation of sources of NO and N[sub 2]O and the environmental factors affecting fluxes in a drought-deciduous forest of Mexico.
Abstract: While much is known about control of production of NO and N[sub 2]O by nitrifying and denitrifying bacteria at the cellular level, application of this knowledge to field studies has not yielded unifying concepts that are widely applicable and that foster understanding of global sources of these atmospheric trace gases. We applied a simple conceptual model to the investigation of sources of NO and N[sub 2]O and the environmental factors affecting fluxes in a drought-deciduous forest of Mexico. Fluxes of NO and N[sub 2]O were higher in the wet season than the dry season, but addition of water to dry soil caused large pulses of CO[sub 2], NO, and N[sub 2]O emissions. Immediate increases of extractable soil NH[sub 4][sup +] and high rates of gross N mineralization and gross nitrification also were observed following wetting of dry soil. Soil NO[sub 2][sup [minus]] had accumulated during the dry season, and that NO[sub 2][sup [minus]] plus the pulse of increased soil NH[sub 4][sup +] were mostly consumed within 24 hours of wetting. This dynamic microbial processing of soil inorganic N coincided with the pulses of NO and N[sub 2]O production following wetting of dry soil. Acetylene inhibition experiments indicated that NO productionmore » was dependent on nitrification, that nitrification was the dominant source of N[sub 2]O when the soil was wetted at the end of the dry season, and that denitrification might be an important source of N[sub 2]O during the wet season. Post-wetting soil moisture was correlated negatively with NO fluxes and positively with N[sub O] fluxes. These results support a conceptual model in which N trace gas production is generally constrained by the rates of N mineralization and nitrification, while the specific ratios of NO and N[sub 2]O fluxes and the contributions from nitrifying and denitrifying bacteria are controlled largely by soil moisture. 42 refs., 5 figs., 4 tabs.« less
TL;DR: It is illustrated that tree productivity and chemistry, and the performance of associated insects, will change under CO2 atmospheres predicted for the next century, and relative increases in tree growth rates will be greatest for aspen and least for maple.
Abstract: Although rising levels of atmospheric carbon dioxide are expected to directly affect forest ecosystems, little is known of how specific ecological interactions will be modified. This research evaluated the effects of enriched CO2 on the productivity and phytochemistry of forest trees and performance of associated insects. Our experimental system consisted of three tree species (quaking aspen (Populus tre- muloides), red oak (Quercus rubra), sugar maple (Acer saccharum)) that span a range from fast to slow growing, and two species of leaf-feeding insects (gypsy moth (Lymantria dispar) and forest tent caterpillar (Malacosoma disstria)). Carbon-nutrient balance theory provided a framework for tests of three hypoth- eses; in response to enriched CO2: (1) relative increases in tree growth rates will be greatest for aspen and least for maple, (2) relative decreases in protein and increases in carbon-based compounds will be greatest for aspen and least for maple, and (3) relative reductions in performance will be greatest for insects fed aspen and least for insects fed maple. We grew 1-yr-old seedlings for 60 d under ambient (385 ? 5 AL/ L) or elevated (642 ? 2 pL/L) CO2 regimes at the University of Wisconsin Biotron. After 50 d, we conducted feeding trials with penultimate-instar gypsy moth and forest tent caterpillars. After 60 d, a second set of trees was harvested and partitioned into root, stem, and leaf tissues. We subsequently analyzed leaf material for a variety of compounds known to affect performance of insect herbivores. In terms of actual dry-matter production, aspen responded the most to enriched CO2 atmospheres whereas maple responded the least. Proportional growth increases (relative to ambient plants), however, were highest for oak and least for maple. Effects of elevated CO2 on biomass allocation patterns differed among the three species; root-to-shoot ratios increased in aspen, decreased in oak, and did not change in maple. Enriched CO2 altered concentrations of primary and secondary metabolites in leaves, but the magnitude and direction of effects were species-specific. Aspen showed the largest change in storage carbon compounds (starch), whereas maple experienced the largest change in defensive carbon compounds (con- densed and hydrolyzable tannins). Consumption rates of insects fed high-CO2 aspen increased dramatically, but growth rates declined. The two species of insects differed in response to oak and maple grown under enriched CO2. Gypsy moths grew better on high-CO2 oak, whereas forest tent caterpillars were unaffected; tent caterpillars tended to grow less on high-CO2 maple, whereas gypsy moths were unaffected. Changes in insect performance parameters were related to changes in foliar chemistry. Responses of plants and insects agreed with some, but not all, of the predictions of carbon-nutrient balance theory. This study illustrates that tree productivity and chemistry, and the performance of associated insects, will change under CO2 atmospheres predicted for the next century. Changes in higher level ecological processes, such as community structure and nutrient cycling, are also implicated.
TL;DR: The authors conducted an 8-year study of acorn production in three species of oak in east-central Missouri: white oak (Quercus alba), red oak (Q. velutina), and black oak (Black oak).
Abstract: We conducted an 8—yr study of flower and acorn production in three species of oak in east—central Missouri: white oak (Quercus alba), red oak (Q. rubra), and Black oak (Q. velutina). The goal was to evaluate whether mast—fruiting, the synchronous production of large seed crops followed by small seed crops, is simply a response to weather conditions or is actually an evolved reproductive strategy. In this paper, we address four specific questions: (1) Are annual fluctuations in acorn crop size the result of synchronous production of acorns by individuals? (2) Are large acorn crops the result of large flower crops and/or high survival of those flowers to mature fruit? (3) To what extent do weather variables account for variation in acorn production among years? (4) Does acorn crop size correlate negatively with prior acorn production? Red oak and white oak showed a greater degree of mast—fruiting than did black oak. Within a species, individuals tended to produce large acorn crops in the same years, but each species differed in which years they produced large crops. The size of a given acorn crop was determined by both flower abundance and survival of flowers to fruit. Principal components and multiple—regression analyses were used to describe the relationship between weather variables and acorn production. The first principal component explained the largest amount of the variation in black oak (R2 = 0.55) and the red oak (R2 = 0.89). In white oak, two principal components combined to explain 77% of the variation in acorn production. The weather variables that were associated with these principal components included spring temperature (positive effect) and summer drought (negative effect). Past acorn production had a major impact on the size of the current acorn crop, with each species showing a different pattern. In black oak, the current acorn crop was negatively correlated with the crop 3 yr prior but positively correlated with the crop 2 yr prior. In red oak, acorn crop size 1, 2, and 3 yr prior had negative correlations with current acorn crop, while acorn crop size 4 yr prior was positively correlated. In white oak, there were negative correlations between acorn crop size and crop size 1, 2, and 4 yr prior but a positive correlation with the acorn crop size 3 yr prior. These data are consistent with the hypothesis that mast—fruiting species must store resources during some years in order to produce a mast crop. We discuss the possibility that these three species may have inherent cycles of reproduction that are modified by the impact of weather conditions, black oak with 2—yr cycles, white oak with 3—yr cycles, and red oak with 4—yr cycles. We conclude that the patterns of acorn production for black oak, red oak, and white oak are not simply responses to weather events but are also a function of prior reproductive events. This suggests that masting is an evolved reproductive strategy.
TL;DR: The hypothesis that different traits are favored in different habitats, and that adaptation to one habitat has occurred at the expense of feeding rate in the other, is supported.
Abstract: I examined habitat use efficiencies of two morphologically distinct threespine sticklebacks (Gasterosteus spp.). The species make up one of several coexisting pairs that apparently formed in the past 13 000 yr in coastal British Columbia lakes. Previous com- parative work has shown that morphology and habitat use are correlated within and among populations. Sticklebacks occurring alone in lakes are intermediate in morphology and habitat use, suggesting that differences between the sympatric species result from ecological character displacement. I tested two hypotheses to explain these patterns, and which are implicit in general views of the causes of adaptive radiation: (1) Each resource (habitat) subjects its species to unique selection pressures, owing to the advantages of certain combinations of traits for foraging; (2) Phenotypes intermediate between habitat specialists suffer a competitive dis- advantage. These hypotheses were evaluated by measuring the foraging rates of the two species and their interspecific hybrids in the two main habitats (benthos and open water) provided in the laboratory. Efficiencies of habitat exploitation matched observed morphological differences. In the benthic habitat the larger, deeper bodies species (having also a wide mouth and few short gill rakers) was superior to the smaller, more slender species (having a narrow mouth and many long gill rakers). Success of hybrids was intermediate. This rank order of species efficiencies was reversed in the open water habitat. These results support the hypothesis that different traits are favored in different habitats, and that adaptation to one habitat has occurred at the expense of feeding rate in the other. Average foraging success of hybrids in the two habitats fell near or below the average of the two parental species, suggesting natural selection against intermediate phenotypes. Foraging efficiency in open water was greatest in the smaller size classes of fish, hinting that small size of the open water species is the result of selection for high foraging efficiency. However, size could not explain most differences in feeding efficiency between species, and it is likely that many traits contribute.
TL;DR: In this paper, the authors investigated the relationship between extracellular enzymes' activity and mass loss in a watershed in northern New York, and proposed an economic model that directly links N and P availability to litter decomposition rates on the basis of microbial allocation of resources.
Abstract: Because plant litter decomposition is directly mediated by extracellular enzymes (ectoenzymes), analyses of the dynamics of their activity may clarify the mechanisms that link decomposition rates to substrate quality and nutrient availability. We investigated this possibility by placing arrays of white birch sticks at eight upland, riparian, and lotic sites on a forested watershed in northern New York. For 3 yr, samples were analyzed for mass loss, protein, total Kjeldahl nitrogen (TKN), and total phosphorus (TP) accumulation, and the activity of 11 classes of extracellular enzymes involved in C, N, and P cycling. The relationship between lignocellulase activity and mass loss did not differ among sites. TKN and TP immobilization exhibited some spatial variation; rates of accumulation per 1% loss of initial mass, estimated from linear regressions, ranged from 2.2 to 4.4 mg/g OM for TP and from 43 to 139 mg/g OM for TKN, with maximum concentrations reached at °80% mass loss. The relationship between the activities of acid phosphatase (AcPase) and N—acetylglucosaminidase (NAGase), enzymes involved in the acquisition of P and N from organic sources, and mass loss displayed even greater variation among sites; the slopes of linear regressions relating mass loss and temporally integrated activity ranged from 0.019 to 0.135 activity—months per mass loss point and 0.107 to 0.775 activity—months per mass loss point, respectively, suggesting that edaphic rather than substrate quality factors were regulating activity. The extent of N limitation at each site was inferred by plotting TKN accumulation, defined as the slope of the linear regression TKN concentration vs. mass loss, in relation to NAGase activity accumulation, defined as the slope of the linear regression cumulative NAGase activity—months vs. mass loss. P limitation at each site was similarly assessed from an analogous plot of TP accumulation in relation to AcPase activity accumulation. Low N or P accumulation in conjunction with high acquisition activity was taken as an indication of nutrient limitation while the converse indicated surfeit. The diagrams suggested that decomposition at the upland hemlock and lotic sites, which displayed intermediate rates of OM loss (zero order k = 0.29 g/mo and 0.23 g/mo, respectively), was primarily N limited, while the riparian sites, which had the lowest rates of OM loss (k = 0.14 g/mo), appeared to be P limited. Relative to the others, OM loss at the upland deciduous sites (k = 0.38 g/mo) was not limited by either N or P. The concordance of field observations with predictions based on ectoenzyme regulation mechanisms suggest that enzyme activity assays in conjunction with nutrient concentration measurements may be a useful indicator of nutrient limitation. An economic model is proposed that directly links N and P availability to litter decomposition rates on the basis of microbial allocation of resources of extracellular enzyme production.
TL;DR: Gender-specific physiological traits can help explain the maintenance of habitat-specific sex ratio biases in boxelder, Acer negundo var.
Abstract: In the semiarid Intermountain West, boxelder, Acer negundo var. interior, a deciduous, dioecious tree, exhibits significant habitat-specific sex ratio biases. Although the overall sex ratio (male/female) does not deviate significantly from one, the sex ratio is significantly male biased (1.62) in drought-prone habitats, while it is significantly female biased (0.65) in moist, streamside habitats. The causes underlying gender-specific habitat associations in this species are not known. We hypothesized that spatial segregation of the sexes is maintained by differences in gender-specific photosynthetic behavior, water rela- tions characteristics, and both instantaneous and integrated water-use efficiency. Gender- specific physiological characteristics were measured and related to growth, reproduction, population age structure, and habitat distribution of male and female trees. Under both field and controlled-environment conditions, males and females differed significantly in a number of physiological traits. Males maintained lower stomatal con- ductance to water vapor (g), transpiration (E), net carbon assimilation (A), leaf internal CO2 concentration (ci), carbon isotope discrimination (AL; an index of time-integrated ci and water-use efficiency), and higher instantaneous (A/E) and long-term (A) water-use efficiency than females. Furthermore, male trees exhibited greater stomatal sensitivity to both declining soil water content and increasing leaf-to-air vapor pressure gradients, a measure of evaporative demand. Higher rates of carbon fixation in female trees were correlated with higher g, higher leaf nitrogen concentrations, and greater stomatal densities. For females growing in both wet and dry habitats, vegetative shoots had higher growth rates than reproductive shoots, while for males, growth rates of the two shoot types did not differ. In streamside habitats, female trees exhibited significantly greater vegetative shoot growth when compared to male trees. In contrast, males showed slightly greater vegetative and much greater reproductive shoot growth in non-streamside habitats. Re- gardless of habitat or growing conditions, females allocated proportionately more of their aboveground biomass to reproduction than did males. These results suggest that (1) gender-specific physiological traits can help explain the maintenance of habitat-specific sex ratio biases in A. negundo along a soil moisture gradient, and (2) that the combination of the gender-specific physiology, growth, and allocation differences contribute to differences in the size (=age) structure of male and female plants within the population. Gender-specific physiological differences may have evolved as a product of selection to meet significantly different costs associated with reproduction in male and female plants.
TL;DR: It is investigated whether isolated trees modify the water balance of this ecosystem and if so, what implications this finding might have on models that assume homogeneity of soil water resources.
Abstract: Dehesa ecosystems of the southwestern Iberian Peninsula are characterized as a savanna—like rangeland dominated by scattered mediterranean evergreen oak trees. We investigated whether isolated trees modify the water balance of this ecosystem and if so, what implications this finding might have on models that assume homogeneity of soil water resources. The water balance of the two ecological components of the dehesas–(1) the tree—grass component, and (2) the open areas between the tree canopies with unshaded grass vegetation–was studied for three consecutive years in three locations in the Sierra Norte de Sevilla region of Andalusia in southern Spain. In this region, annual rainfall was generally between 600 and 800 mm, and the summer drought lasted °130 d. Soil water storage was measured with a neutron moisture gauge outside and under the tree canopy. Deep drainage between two consecutive census dates was calculated using field—measured drainage characteristics. Evapotranspiration (Ea) and surface runoff ...
TL;DR: In this paper, the influence of litter quality on the decomposition rate of leaves from nine Mediterranean shrubs and trees using litter bag methods for a 2-yr period at two ecosystems in southwest Spain.
Abstract: We studied the influence of litter quality on the decomposition rate of leaves from nine Mediterranean shrubs and trees using litter bag methods for a 2-yr period at two ecosystems in southwest Spain. Linear and nonlinear regressions were calculated be- tween mass loss and the concentrations of the major organic and inorganic constituents of leaves (lignin, cutin, cellulose, tannins, crude fat, soluble carbohydrates, nitrogen, and phosphorus) and between mass loss and leaf toughness in order to determine the best predictor of leaf litter decomposition in these ecosystems. In addition, ratios between some of the litter quality parameters were examined as mass loss predictors. Loss of soluble components from leaves was used to define two phases of decomposition: a leaching phase that lasted 2-4 mo and a postleaching phase. Leaf toughness and the ratio of toughness: phosphorus concentration were the best indicators of mass loss during the leaching phase in both ecosystems, and cutin: nitrogen or cutin: phosphorus ratios were the best predictors of mass loss in the postleaching phase, but only in the drier and more nutrient-poor ecosystem. When the two phases were combined, leaf toughness, toughness: nitrogen, and/or cutin: nitrogen significantly explained the mass loss in both ecosystems.
TL;DR: A nutrient amendment experiment was conducted for two growing seasons in two alpine tundra communities to test the hypotheses that: (1) primary production is limited by nutrient availability, and (2) physiological and developmental constraints act to limit the responses of plants from a nutrient-poor community more than plants from more nutrient-rich community to increases in nutrient availability as mentioned in this paper.
Abstract: A nutrient amendment experiment was conducted for two growing seasons in two alpine tundra communities to test the hypotheses that: (1) primary production is limited by nutrient availability, and (2) physiological and developmental constraints act to limit the responses of plants from a nutrient—poor community more than plants from a more nutrient—rich community to increases in nutrient availability. Experimental treatments consisted of N, P, and N + P amendments applied to plots in two physiognomically similar communities, dry and wet meadows. Extractable N and P from soils in nonfertilized control plots indicated that the wet meadow had higher N and P availability. Photosynthetic, nutrient uptake, and growth responses of the dominants in the two communities showed little difference in the relative capacity of these plants to respond to the nutrient additions. Aboveground production responses of the communities to the treatments indicated N availability was limiting to production in the dry meadow community while N and P availability colimited production in the wet meadow community. There was a greater production response to the N and N + P amendments in the dry meadow relative to the wet meadow, despite equivalent functional responses of the dominant species of both communities. The greater production response in the dry meadow was in part related to changes in community structure, with an increase in the proportion of graminoid and forb biomass, and a decrease in the proportion of community biomass made up by the dominant sedge Kobresia myosuroides. Species richness increased significantly in response to the N + P treatment in the dry meadow. Graminoid biomass increased significantly in the wet meadow N and N + P plots, while forb biomass decreased significantly, suggesting a competitive interaction for light. Thus, the difference in community response to nutrient amendments was not the result of functional changes at the leaf level of the dominant species, but rather was related to changes in community structure in the dry meadow, and to a shift from a nutrient to a light limitation of production in the wet meadow.
TL;DR: The ability of dace and trout to capture prey at varying velocities appears to be the dominant factor affecting microhabitat selection in these species.
Abstract: We constructed an energetic model to determine the optimal focal point current velocity (i.e., microhabitat) for rainbow trout (Oncorhynchus mykiss) and rosyside dace (Clinostomus funduloides) in Coweeta Creek, a fifth-order stream in North Carolina, USA. Energetic costs were evaluated by quantifying the metabolic expenditure associated with swimming at a given velocity. We estimated benefits by measuring potential energetic gains of feeding at a given velocity. This included estimates of the ability of the fish to capture prey at different current velocities as well as estimates of the frequency and the energy content of drifting prey at various velocities. We derived separate models for small (53-70 mm SL (standard length)) and medium (71-125 mm SL) trout, and medium (47- 52 mm SL) and large (53-70 mm SL) dace for all seasons, deriving net energy gain as a function of current velocity. We predicted fishes would occupy velocities at which net energy gain was maximized. Predicted velocities were compared with those utilized by fishes inhabiting Coweeta Creek. Optimal velocities predicted by energetic models ranged from 7.7 to 22.1 cm/s, and closely matched actual velocity use (average deviation = 2.6 cm/s). Prey capture success appeared to be the most important component in the models. Consequently, we constructed models based solely upon aspects of capture success; the average deviation from velocity use with these models was only 1.8 cm/s. Thus, the ability of dace and trout to capture prey at varying velocities appears to be the dominant factor affecting microhabitat selection in these species.
TL;DR: Strong community differences in the factors governing invasability by Carpobrotus edulis are demonstrated, showing invasion is expected to be slow due to harsh physical conditions that limit both survival and growth.
Abstract: The South African succulent Carpobrotus edulis has been invading native and non-native plant assemblages in California since its introduction in the early 1 900s. This study was designed to determine mechanisms controlling invasion by this species into three different communities: coastal grassland, backdune, and coastal scrub. The com- munities differed in several factors that influenced invasion by Carpobrotus: soil distur- bance, herbivory, and identity of competitors. Disturbance of the soil was required for establishment of Carpobrotus at the grassland site. Soil disturbance did not promote Car- pobrotus establishment in the coastal scrub or backdune. Establishment in these sites was restricted by herbivory. The influence of neighboring vegetation on Carpobrotus establish- ment was site dependent. At the grassland site, Carpobrotus seedling survival was reduced by grasses, and the only surviving seedlings were on gopher mounds. Growth of larger Carpobrotus plants, however, was unaffected by grasses. Proximity to shrubs, the dominant plant type at the backdune and coastal scrub sites, did not affect Carpobrotus seedling survival. It did, however, decrease both root and shoot biomass of seedlings and cuttings. Invasion is expected to occur most rapidly at the grassland site. Although germination and early survival there depend on the occurrence of rodent disturbances, these occur frequently, and seedling recruitment in the grassland was observed during this study. Once established, clonal growth is great and is not affected by herbivory or competition. In contrast, at the coastal scrub site, intense herbivory prevented successful establishment during this study. Cage experiments suggested that herbivory counteracted the high growth potential of Carpobrotus at this site. At the dune site, even caged plants showed very poor growth. Thus, although seedling recruitment does occur there, invasion is expected to be slow due to harsh physical conditions that limit both survival and growth. These findings demonstrate strong community differences in the factors governing invasability by Car- pobrotus edulis.
TL;DR: In this article, the authors investigated the response of the riverine food web to additions of the limiting nutrient, phosphorus, in a pristine tundra river for four consecutive sum-mers and found that the response was modified by a strong "top-down" feedback of insects grazing on epilithic algae and by competitive exclusion of black flies by caddisflies.
Abstract: Phosphorus fertilization of a pristine tundra river for four consecutive sum- mers dramatically changed biological processes and populations at all trophic levels. At the primary producer level, both algal biomass and productivity increased and chlorophyll accumulated on the river bottom in the first two summers. Diatom community composition changed little in spite of large chlorophyll changes. However, an increase in grazing insects prevented chlorophyll buildup in the third and fourth summers. Some microbial processes were also stimulated by the increased photosynthesis caused by fertilization. Total respiration of the epilithon, acetate uptake, and decomposition of lignin monomers were all stimulated but only in light-grown epilithon. When epilithon was grown in the dark in the fertilized region of the river, there was no increased respiration. Also, phosphorus did not stimulate the decomposition of Carex litter. Although insects grew more rapidly in the fertilized section of the river, there were community interactions that kept total insect production from appreciable change. The four most abundant large insects did increase their growth rates in response to phosphorus addition and there were increases in populations of Baetis lapponicus and Brachycentrus americanus. These increases were offset by the decline in abundance of the dominant species, the black fly Prosimulium martini, perhaps caused by competition for space from Brachycentrus. Growth of both young-of-the-year and adult grayling (Thymallus arcticus) was strongly stimulated by phosphorus addition in years 3 and 4 (not tested in years 1 and 2). Carbon and nitrogen stable isotope tracers indicated that the measured increases in insect and fish growth were largely attributable to increases in the production of epilithic algae. Overall, the results indicate a strong "bottom-up" response of the riverine food web to additions of the limiting nutrient, phosphorus. The response was modified in later years, however, by a strong "top-down" feedback of insects grazing on epilithic algae and by competitive exclusion of black flies by caddisflies.
TL;DR: Both hand—pollination and fertilizing during the blooming season increased total seed production, while watering alone has no effect on any component of reproductive success.
Abstract: Using experimental manipulations, we examined pollen and resource limitations to seed production and their interaction in a natural population of a monocarpic plant, Ipomopsis aggregata. Our design cross two factors, pollen level (hand—pollinated or control) and resource level (water added, water and fertilizer added, or control). Both hand—pollination and fertilizing during the blooming season increased total seed production, while watering alone has no effect on any component of reproductive success. Hand—pollination boosted number of seeds per flower, with no effect on flower number. In contrasts, fertilizing had its primary effect on the number of flowers produced, while also increasing the number of seeds per mature fruit in hand—pollinated plants. Fertilizing increased nectar volume, but path analysis detected no indirect effect of this increased reward rate on seed production, suggesting instead that fertilizing has a slight, but direct, effect on seeds per flower. These results argue against a strict dichotomy between pollen limitation and resource limitation of female reproductive success in plants.
TL;DR: Empirically derived fitness functions supported the hypothesis that population differences in life history were adaptive and demonstrated empirically how survival and growth rate can independently and inter- actively influence life history evolution.
Abstract: Life history data for three unexploited populations of brook trout, Salvelinus fontinalis, were used to test the predictions of life history theory that, relative to juveniles, (1) high adult survival favors low reproductive effort and delayed reproduction, and (2) increased juvenile growth rate favors high effort and early reproduction. Field data sup- ported both predictions. The population having the highest adult-to-juvenile survival ratio expended the least effort, reproduced latest in life, and experienced the lowest survival cost of reproduction. Among populations a high juvenile-to-adult growth rate was associated with early reproduction, high reproductive effort, and high reproductive cost. Early repro- duction was also associated with increased growth within populations. The adaptive significance of interpopulation variation in life history was assessed by comparing the fitness, r, of observed life histories with those of potentially alternative strategies. Empirically derived fitness functions supported the hypothesis that population differences in life history were adaptive. Observed combinations of age-specific survival and fecundity were those that maximized fitness. Within populations the fitness advantages associated with reproducing early in life favored reduced age at reproduction for the fastest- growing individuals. The results are consistent with the predictions of life history theory and demonstrate empirically how survival and growth rate can independently and inter- actively influence life history evolution.
TL;DR: Isotope ratios of soil organic C in forests appear to provide useful estimators of tree isotopic composition, offering the advantage of an easily obtained value that naturally integrates over plant material, time and, eventually, local area.
Abstract: In order to relate the isotopic composition of soil organic matter to parent vegetation and soil type, we measured the carbon stable isotope ratios of tree leaves and soil C at 14 locations in a temperate forest. Sites were selected over a wide variety of soil types and related vegetation associations, but within a single regional climate. The 613C of the bulk leaf material falling on the soil varied among sites, ranging from -29.5 to - 26'oo. Within this range, differences up to 1.5%oo were attributable to differences in tree species. Further differences were related to site. The 613C values of soil organic matter cover the range -29.8 to -24.30%oo and varied with both parent vegetation 613C and depth. Trans- formation and decay of organic C within the litter layers led to no isotope enrichment, irrespective of vegetation and humus type: mull, hydromull, moder, or anmoor. The dif- ference between woody and leaf material in litter was low, 0.23 ? 0.35 Too (mean ? 1 SD). On the other hand 613C in mineral horizons always increased with depth, reaching values richer than litter by 1.0 ? 0.5%oo at 20 cm and up to +1.5%oo at 1 m. The trend was independent of the soil physico-chemical characteristics, whether aerobic, anaerobic, eu- trophic, oligotrophic, or podzolic. Isotope ratios of soil organic C in forests appear to provide useful estimators of tree isotopic composition. When compared to the measure- ments of the vegetation, they offer the advantage of an easily obtained value that naturally integrates over plant material, time and, eventually, local area.
TL;DR: In this article, the Wequetequock-Pawcatuck system has been studied over the last four decades, and it has been found that the vegetation types in the high marsh are dominated by Triglochin maritima and forbs.
Abstract: Increasing rates of relative sea—level rise (RSL) have been linked to coastal wetland losses along the Gulf of Mexico and elsewhere. While such losses have yet to be reported for New England tidal marshes, rapidly rising RSL may still be affecting these systems. Studies of the Wequetequock—Pawcatuck tidal marshes over four decades have documented dramatic changes in vegetation that appear to be related primarily to differential rates of marsh accretion and sea—level rise. Other environmental factors such as sediment supply and anthropogenic modifications of the system may be involved as well. When initially studied in 1947—1948 the high marsh supported a Juncus gerardi—Spartina patens belting pattern typical of many New England salt marshes. On the most of the marsh complex the former Juncus belt has been replaced by forbs, primarily Triglochin maritima, while the former S. patens high marsh is now a complex of vegetation types–stunted Spartina alterniflora, Distichlis Spicata, forbs, and relic stands of S. patens. These changes are documented by vegetation sampling that closely followed the 1947—1948 methods and by peat core analysis. Marsh elevations were determined by leveling, and the mean surface elevation of areas where the vegetation has changed is significantly lower than that of areas still supporting the earlier pattern (4.6 vs. 13.9 cm above mean tide level). The differences in surface elevation reflect differences in accretion of marsh peat. Calculations based on sandy overwash layers deposited during historically recorded storms as well as on experimentally placed marker horizons of known age indicate that stable areas have been accreting at the rate of local sea—level rise, 2.0—2.5 mm/yr at least since 1938; changed areas have accreted at about one half that rate. Lower surface elevations result in greater frequency and duration of tidal flooding, and thus in increased peat saturation, salinity, and sulfide concentrations, and in decreased redox potential, as directly measured over the growing season at both changed and stable sites. It is proposed that these edaphic changes have combined to favor establishment of a wetter, more open vegetation type dominated by to distinctive communities–Stunted S. alterniflora and forbs. Changes documented on the Wequetequock—Pawcatuck system have been observed on the other Long Island Sound marshes and may serve as a model for the potential effects of seal—level rise on New England tidal salt marshes.