With a simple model, I show that comparisons of invasibility between regions are impossible to make unless one can control for all of the variables besides invasibility that influence exotic richness, including the rates of immigration of species and the characteristics of the invading species themselves. Using data from the literature for 184 sites around the world, I found that nature reserves had one-half of the exotic fraction of sites outside reserves, and island sites had nearly three times the exotic fraction of mainland sites. However, the exotic fraction and the number of exotics were also dependent on site area, and this had to be taken into account to make valid comparisons between sites. The number of native species was used as a surrogate for site area and habitat diversity. Nearly 70% of the variation in the number of exotic species was accounted for by a multiple regression containing the following predictors: the number of native species, whether the site was an island or on the mainland, and whether or not it was a nature reserve. After controlling for scale, there were significant differences among biomes, but not continents, in their level of invasion. Multiple biome regions and temperate agricultural or urban sites were among the most invaded biomes, and deserts and savannas were among the least. However, there was considerable within-group variation in the mean degree of invasion. Scale-controlled analysis also showed that the New World is significantly more invaded than the Old World, but only when site native richness (probably a surrogate for habitat diversity) is factored out. Contrary to expectation, communities richer in native species had more, not fewer, exotics. For mainland sites, the degree of invasion increased with latitude, but there was no such relationship for islands. Although islands are more invaded than mainland sites, this is apparently not because of low native species richness, as the islands in this data set were no less rich in native species than were mainland sites of similar area. The number of exotic species in nature reserves increases with the number of visitors. However, it is difficult to draw conclusions about relative invasibility, invasion potential, or the roles of dispersal and disturbance from any of these results. Most of the observed patterns here and in the literature could potentially be explained by differences between regions in species properties, ecosystem properties, or propagule pressure.
Meta-analysis provides formal statistical techniques for summarizing the results of independent experiments and is increasingly being used in ecology. The response ratio (the ratio of mean outcome in the experimental group to that in the control group) and closely related measures of proportionate change are often used as measures of effect magnitude in ecology. Using these metrics for meta-analysis requires knowledge of their statistical properties, but these have not been previously derived. We give the approximate sampling distribution of the log response ratio, discuss why it is a particularly useful metric for many applications in ecology, and demonstrate how to use it in meta-analysis. The meta-analysis of response-ratio data is illustrated using experimental data on the effects of increased atmospheric CO2 on plant biomass responses.
This paper uses theory and experiments to explore the effects of diversity on stability, productivity, and susceptibility to invasion. A model of resource competition predicts that increases in diversity cause community stability to increase, but population stability to decrease. These opposite effects are, to a great extent, explained by how temporal variances in species abundances scale with mean abundance, and by the differential impact of this scaling on population vs. community stability. Community stability also depends on a negative covariance effect (competitive compensation) and on overyielding (ecosystem productivity increasing with diversity). A long-term study in Minnesota grasslands supports these predictions. Models of competition predict, and field experiments confirm, that greater plant diversity leads to greater primary productivity. This diversity-productivity relationship results both from the greater chance that a more productive species would be present at higher diversity (the sampling effect) and from the better "coverage" of habitat heterogeneity caused by the broader range of species traits in a more diverse community (the niche differentiation effect). Both effects cause more complete utilization of limiting resources at higher diversity, which increases resource retention, further increasing productivity. Finally, lower levels of available limiting resources at higher diversity are predicted to decrease the susceptibility of an ecosystem to invasion, supporting the diversity-invasibility hypothesis. This mechanism provides rules for community assembly and invasion resistance. In total, biodiversity should be added to species composition, disturbance, nutrient supply, and climate as a major controller of population and ecosystem dynamics and structure. By their increasingly great directional impacts on all of these controllers, humans are likely to cause major long-term changes in the functioning of ecosystems worldwide. A better understanding of these ecosystem changes is needed if ecologists are to provide society with the knowledge essential for wise management of the earth and its biological resources.
Convergence in interspecific leaf trait relationships across diverse taxonomic groups and biomes would have important evolutionary and ecological implications. Such convergence has been hypothesized to result from trade-offs that limit the combination of plant traits for any species. Here we address this issue by testing for biome differences in the slope and intercept of interspecific relationships among leaf traits: longevity, net photosynthetic capacity (Amax) leaf diffusive conductance (Gs) specific leaf area (SLA), and nitrogen (N) status, for more than 100 species in six distinct biomes of the Americas. The six biomes were: alpine tundra-subalpine forest ecotone, cold temperate forest-prairie ecotone, montane cool temperate forest, desert shrubland, subtropical forest, and tropical rain forest. Despite large differences in climate and evolutionary history, in all biomes mass-based leaf N (Nmass), SLA, Gs, and Amax were positively related to one another and decreased with increasing leaf life span. The relationships between pairs of leaf traits exhibited similar slopes among biomes, suggesting a predictable set of scaling relationships among key leaf morphological, chemical, and metabolic traits that are replicated globally among terrestrial ecosystems regardless of biome or vegetation type. However, the intercept (i.e., the overall elevation of regression lines) of relationships between pairs of leaf traits usually differed among biomes. With increasing aridity across sites, species had greater Amax for a given level of Gs and lower SLA for any given leaf life span. Using principal components analysis, most variation among species was explained by an axis related to mass-based leaf traits (Amax, N, and SLA) while a second axis reflected climate, Gs, and other area-based leaf traits.
Stable nitrogen isotope signatures (delta(15)N) are increasingly used to infer the trophic position of consumers in food web studies. Interpreting the delta(15)N of consumers relative to the delta(15)N characterizing the base of the food web provides a time-integrated measure of trophic position. We use primary consumers (trophic level 2) as baseline indicator organisms and investigate the variation in baseline delta(15)N values in 14 lakes in Ontario and Quebec. Values of delta(15)N ranged from -2 to +9 parts per thousand and varied significantly as a function of lake habitat (mean littoral = 1.6 parts per thousand, pelagic = 3.1 parts per thousand, profundal = 5.2 parts per thousand). Stable carbon isotopic signatures (delta(13)C) Of primary consumers decreased along this same habitat gradient (mean littoral = -23.8 parts per thousand, pelagic = -28.4 parts per thousand, profundal = -30.5 parts per thousand). Primary consumer delta(13)C and a categorical lake variable explained 72% of the variability in primary consumer delta(15)N. This relationship was corroborated by primary consumer delta(15)N and delta(13)C data from the literature, indicating that habitat-specific variation in baseline delta(15)N and delta(13)C is a widespread phenomenon in freshwater systems. We present a method that uses the presented baseline delta(15)N-delta(13)C relationship and the delta(15)N and delta(13)C values of the consumer to estimate trophic position; it is a method that corrects for the described variation in baseline delta(15)N. These results emphasize the general importance of accounting for patterns in isotopic signatures characterizing the base of the food web when inferring trophic structure using stable isotopes.
Meta-analysis is the use of statistical methods to summarize research findings across studies. Special statistical methods are usually needed for meta-analysis, both because effect-size indexes are typically highly heteroscedastic and because it is desirable to be able to distinguish between-study variance from within-study sampling-error variance. We outline a number of considerations related to choosing methods for the meta-analysis of ecological data, including the choice of parametric vs. resampling methods, reasons for conducting weighted analyses where possible, and comparisons fixed vs. mixed models in categorical and regression-type analyses.
Dispersal affects community dynamics and vegetation response to global change. Understanding these effects requires descriptions of dispersal at local and regional scales and statistical models that permit estimation. Classical models of dispersal describe local or long-distance dispersal, but not both. The lack of statistical methods means that models have rarely been fitted to seed dispersal in closed forests. We present a mixture model of dispersal that assumes a range of disperal patterns, both local and long distance. The bivariate Student's t or "2Dt" follows from an assumption that the distance parameter in a Gaussian model varies randomly, thus having a density of its own. We use an inverse approach to "compete" our mixture model against classical alternatives, using seed rain databases from temperate broadleaf, temperate mixed-conifer, and tropical floodplain forests. For most species, the 2Dt model fits dispersal data better than do classical models. The superior fit results from the potential for a convex shape near the source tree and a "fat tail." Our parameter estimates have implications for community dynamics at local scales, for vegetation responses to global change at regional scales, and for differences in seed dispersal among biomes. The 2Dt model predicts that less seed travels beyond the immediate crown influence (30 m). Although Gaussian and exponential models predict slow population spread in the face of environmental change, our dispersal estimates suggest rapid spread. The preponderance of animal-dispersed and rare seed types in tropical forests results in noisier patterns of dispersal than occur in temperate hardwood and conifer stands.
Stable nitrogen isotope signatures (δ 15 N) are increasingly used to infer the trophic position of consumers in food web studies. Interpreting the δ 15 N of consumers relative to the δ 15 N characterizing the base of the food web provides a time‐integrated measure of trophic position. We use primary consumers (trophic level 2) as baseline indicator organisms and investigate the variation in baseline δ 15 N values in 14 lakes in Ontario and Quebec. Values of δ 15 N ranged from −2 to +9‰ and varied significantly as a function of lake habitat (mean littoral = 1.6‰, pelagic = 3.1‰, profundal = 5.2 ‰). Stable carbon isotopic signatures (δ 13 C) of primary consumers decreased along this same habitat gradient (mean littoral = −23.8‰, pelagic = −28.4‰, profundal = −30.5‰). Primary consumer δ 13 C and a categorical lake variable explained 72% of the variability in primary consumer δ 15 N. This relationship was corroborated by primary consumer δ 15 N and δ 13 C data from the literature, indicating that habitat‐specific variation in baseline δ 15 N and δ 13 C is a widespread phenomenon in freshwater systems. We present a method that uses the presented baseline δ 15 N–δ 13 C relationship and the δ 15 N and δ 13 C values of the consumer to estimate trophic position; it is a method that corrects for the described variation in baseline δ 15 N. These results emphasize the general importance of accounting for patterns in isotopic signatures characterizing the base of the food web when inferring trophic structure using stable isotopes.
We examined biomass allocation patterns throughout the entire vegetative growth phase for three species of annual plants along three separate gradients of resource availability to determine whether observed patterns of allocational plasticity are consistent with optimal partitioning theory. Individuals of the annual plant species Abutilon theophrasti, Chenopodium album, and Polygonum pensylvanicum were grown from locally fieldgathered seed in controlled greenhouse conditions across gradients of light, nutrients, and water. Frequent harvests were used to determine the growth and allocation (root vs. shoot, and leaf area vs. biomass) responses of these plants over a 57-d period. Growth analysis revealed that each species displayed significant plasticity in growth rates and substantial amounts of ontogenetic drift in root: shoot biomass ratios and ratios of leaf area to biomass across each of the three resource gradients. Ontogenetically controlled comparisons of root: shoot and leaf area ratios across light and nutrient gradients were generally consistent with predictions based on optimal partitioning theory; allocation to roots decreased and leaf area increased under low light and high nutrient conditions. These trends were confirmed, though were less dramatic, in allometric plots of biomass allocation throughout ontogeny. These species did not alter biomass allocation (beyond ontogenetic drift) in response to the broadly varying water regimes. Furthermore, many of the observed differences in biomass allocation were limited to a given time during growth and development. We conclude that, for these rapidly growing annual species, plasticity in biomass allocation patterns is only partially consistent with optimal partitioning theory, and that these plastic responses are ontogenetically constrained. Further, while these species did adjust biomass allocation patterns in response to light and nutrient availability, they did not adjust biomass allocation in response to water availability, despite dramatic plasticity in growth rates along all three resource gradients. Our results support a developmentally explicit model of plasticity in biomass allocation in response to limiting resources.
Ecologists are increasingly recognizing the importance of consumers in regulating ecosystem processes such as nutrient cycling. Ecologists have recently made considerable progress in understanding nutrient cycling and trophic interactions in pelagic systems by application of a new concept, ecological stoichiometry, to consumer-driven processes. In this paper we synthesize these conceptual advances within pelagic ecology and attempt to illustrate how they may be usefully applied in other ecosystems. Stoichiometric theory shows that both grazer and algal elemental composition are critical parameters influencing rates and ratios of nutrient release. Thus, the stoichiometry of nutrient recycling is a feedback mechanism linking grazer dynamics and algal nutritional status. Incorporation of such effects into a fully dynamic stoichiometric model generates profound changes in the predicted dynamics of algae and grazers, suggesting that adoption of a stoichiometric view may substantively alter our view of the interaction between trophic dynamics and nutrient cycling. The basic predictions of stoichiometric models of nutrient release are generally supported by experimental data showing that N:P release ratios are primarily a function of algal N:P ratio and secondarily a function of grazer N:P ratio, and that rates of P release by grazers are also related to food P:C. Furthermore, evidence for effects of nutrient release stoichiometry on phytoplankton communities and pelagic ecosystem function is accumulating, including data showing consistent alterations in algal physiological status and ecosystem-scale changes in N fixation in response to altered grazer community structure and elemental composition. As the general features of the stoichiometry of algae-zooplankton interactions reflect fundamental biological processes linked to plant and animal mineral nutrition, the stoichiometric view of consumer-driven nutrient recycling can easily be applied to other ecosystems, including terrestrial and benthic food webs. A suite of potential applications of stoichiometric thinking to benthic and terrestrial habitats is suggested.
Models of climate change predict that global temperatures and precipitation will increase within the next century, with the most pronounced changes occurring in northern latitudes and during winter. A large-scale atmospheric phenomenon, the North Atlantic Oscillation (NAO), is a strong determinant of both interannual variation and decadal trends in temperatures and precipitation during winter in northern latitudes, and its recent persistence in one extreme phase may be a substantial component of increases in global temperatures. Hence, we investigated the influences of large-scale climatic variability on plant phenology and ungulate population ecology by incorporating the NAO in statistical analyses of previously published data on: (1) the timing of flowering by plants in Norway, and (2) phenotypic and demographic variation in populations of northern ungulates. We analyzed 137 time series on plant phenology for 13 species of plants in Norway spanning up to 50 yr (44 ± 0.5 yr, mean ± 1 SE) and 39 time series on phenotypic and demographic traits of 7 species of northern ungulates from 16 populations in North America and northern Europe spanning up to 30 yr (18 ± 2.0 yr). Plant phenology was significantly related to the NAO in 97 time series (71% of the total), in which dynamics of the NAO explained, on average, between 9% and 28% of the interannual variation in flowering dates. Following increasingly warm, wet winters, most plant species (9 of 13 species) bloomed earlier by an average of 13.0 ± 0.8 d to 26.4 ± 1.8 d, (mean ± 1 SE), one-third (4 of 11 species) bloomed longer by 13.4 ± 1.1 d to 18.8 ± 1.7 d, and there was an increase in spatial variability in timing of flowering across landscapes by five of six species. Woody plants displayed less sensitivity to climatic variability than did herbaceous species, and early-blooming plants were more strongly influenced by the NAO than were late-blooming plants. Ungulate phenotypic and demographic variables were significantly related to the NAO in 28 time series (72% of the total). Large-scale climatic variability influenced growth, development, fecundity, and demographic trends of all seven species of ungulates studied, and in some populations, the NAO acted together with density dependence. Individuals within mainland populations responded to winter warming with reduced body size and increased fecundity, whereas winter warming in maritime regions led to increased body size but reduced fecundity. Across sex and age classes, between 43% and 70% of the observed range in body mass among years was attributable to the dynamics of the NAO, and within cohorts of female red deer and reindeer, 47-70% of the observed range in fecundity was related to the NAO during the winter preceding cohort birth years. All but two populations of northern ungulates declined following increasingly warmer winters, and the NAO operated, in most instances, in concert with direct density dependence to limit populations. In comparison to the original analyses of these ungulate time series, in 10 of 19 series the NAO explained an equal or greater percentage of variation than that explained by local weather. These observations indicate that large-scale climatic variability has a detectable influence on the ecology of plants and animals in a variety of terrestrial ecosystems, and that the responses of plants to winter warming may, surprisingly, be more subtle than the responses of large herbivores.
Dynamics of headwater stream ecosystems are generally regarded as occurring at the interface of aquatic and terrestrial ecosystems. Terrestrial arthropod inputs can provide an energy subsidy and increase the abundance of predatory fish, and the ensuing effects potentially can cascade through the food web and ultimately affect primary producers. Nevertheless, the community-based effects of such inputs on stream food web dynamics are still poorly understood. We present experimental evidence that terrestrial arthropod inputs have an indirect but prominent effect on a stream benthic community by altering the intensity of fish predation in the food web. Two key elements of the stream food web, terrestrial arthropod inputs and the presence of predatory fish, were experimentally manipulated by using greenhouse-type covers and enclosures (or exclosures) in a forest stream located in northern Japan. When terrestrial arthropod inputs to the stream were experimentally reduced, fish predation pressure shifted dramatically from terrestrial to aquatic arthropods. The ensuing depletion of aquatic arthropods resulted in a subsequent increase in periphyton biomass. This field experiment revealed that terrestrial arthropod inputs were a primary factor controlling cascading trophic interactions among predatory fish, herbivorous aquatic arthropods, and benthic periphyton. These results provide empirical support for the perspective that transfers of energy and biomass from donor systems are frequently significant for the maintenance of biotic communities in recipient systems.
Few wetland studies from temperate North America have related either species richness or plant community composition to any direct measure of nutrient availability, or examined changes in species composition following experimental nutrient additions. Studies of wetlands in western Europe and of other terrestrial ecosystems in North America frequently show that nutrient enrichment leads to changes in species composition, declines in overall plant species diversity, and loss of rare and uncommon species. We therefore used an extensive literature survey and analysis of data on plant species composition, species richness, productivity or standing crop, and C:N:P stoichiometry in plant tissues and surface soils to draw conclusions about the nature of nutrient limitation in temperate North American bogs, fens, marshes, and swamps, and to infer their potential response to nutrient enrichment. We searched all major bibliographic data bases for studies containing such data (through March 1998) and added relevant data from our own ongoing research. We analyzed plant and soil data sets by wetland type and by wetland soil type, and the plant data set also by growth form. Existing studies appear to confirm common generalizations: (1) plant community type changes across broad nutrient gradients; (2) species richness declines as various indicators of nutrient availability increase beyond some threshold; and (3) rare and uncommon species are almost always associated with species-rich communities. However, (1) these generalizations do not always hold within community types; (2) for many community types, the threshold beyond which richness declines has not been established, and high or low diversity may occur below that threshold; and (3) the failure of many studies to include bryophytes precludes drawing strong conclusions about nutrient availability and diversity in peatlands. Marshes had significantly lower mean nitrogen: phosphorus (N:P) ratios in live tissue than other wetland types (bogs, fens, and swamps), which did not differ significantly from each other. Mean N:P ratios in live tissues were significantly higher in peatlands than in mineral-soil wetlands. Nitrogen: phosporus ratios in litter did not differ significantly between peatlands and mineral-soil wetlands but were higher than in live tissues. Among growth forms, the highest mean N:P ratios in live tissues occurred in bryophytes, and the lowest in vascular herbaceous species. Only bryophyte live tissues differed significantly from other growth forms; litter N:P ratios were not significantly different among growth forms. Average N:P ratios in surface soils were lower in marshes and swamps than in bogs and fens. Wetlands on mineral soils had lower average N:P ratios than wetlands on peat soils. Average surface soil N:P ratios rose sharply at high soil organic-matter contents (≥ 90%) and were generally greater than 16 at organic-matter concentrations above 20%. In combination, plant tissue and surface soil N:P ratios suggest that a large proportion of North American wetlands are either P limited or co-limited by N and P, especially those occurring on organic soils. Only marshes have N:P ratios in both live tissues and surface soils that consistently indicate N limitation, although soils data suggest that the majority of swamps are also N limited. Vascular herbaccous species appear to be N limited, but no pattern is apparent among other growth forms. Inferences drawn from N:P stoichiometry need to be verified by examining ecosystem response to experimental fertilization. High variances in plant and soil N:P ratios suggest that understanding nutrient limitation at both the community and individual-species level may be necessary to predict changes in species composition and richness with nutrient enrichment.
The Argentine ant (Linepithema humile) is a widespread invasive species that competitively displaces native ants throughout its introduced range. Although this pattern of displacement is well known, its underlying mechanisms remain little studied. To gain a more detailed understanding of this widespread competitive displacement, I compared the exploitative and interference abilities of the Argentine ant with those of seven species of native ants it displaces in riparian woodlands in northern California. I performed four different manipulative field experiments; each measured different aspects of the competitive ability of the eight species of ants in this study. The main goals of this study were to identify the mechanisms responsible for the Argentine ant's strong competitive ability, to determine if native ants are subject to species-specific trade-offs in exploitative and interference ability typically present among coexisting ants, and if so, to assess whether Argentine ants are subject to this trade-off as well. Argentine ants located and recruited to baits as quickly or more quickly than did native antsboth in areas where Argentine ants and native ants occurred together (i.e., at the edge of invasion fronts) and where they occurred separately (i.e., away from invasion fronts). Along the edge of invasion fronts, Argentine ants also controlled a greater proportion of baits than did native ants. In one-on-one interactions, individual Argentine ant workers experienced mixed success in overcoming individual workers of the seven native ant species. When fighting against native ants, Argentine ants used both physical aggression and chemical defensive compounds, although the latter mechanism was more often successful in deterring opponents. Chemical defensive compounds produced by Argentine ants were repellent but appeared no more so than those of native ants. Although Argentine ant workers were not able to overcome native ant workers consistently, Argentine ant colonies succeeded in displacing most native ant colonies from baits. The discrepancy between worker-level and colony-level interference ability suggests that numerical advantages are key to the Argentine ant's proficiency at interference competition. Like ants in other communities, the native ants in this study were subject to a competitive trade-off in which interference ability and exploitative ability were negatively correlated. In contrast, Argentine ants were proficient at both exploitative and interference competition relative to the native ants they displaced and are thus removed from this trade-off. These findings imply that Argentine ants secure a majority of available food resources where this species comes into contact with native ants. Argentine ants may be able to break the competitive trade-off constraining native ants because of their unique colony structure and because they have escaped their natural enemies. The observation that Argentine ants are uncoupled from the competitive trade-off constraining native ants may provide a general explanation for patterns of dominance within ant communities and for the success of other introduced species.
Tropical rain forests have the highest tree diversity on earth. Nonrandom spatial distributions of these species in relation to edaphic factors could be one mechanism responsible for maintaining this diversity. We examined the prevalence of nonrandom distributions of trees and palms in relation to soil type and topographic position ("edaphic biases") over a mesoscale (573 ha) old-growth tropical rain forest (TRF) landscape at the La Selva Biological Station, Costa Rica. All trees and palms ≥ 10 cm diameter were measured and identified in 1170 circular 0.01-ha plots centered on an existing 50× 100 m grid. Topographic position was classified for each plot, and slope and aspect were measured. Soil type data were taken from a previous study (Clark et al. 1998). A total of 5127 trees and palms were identified in 267 species. Detrended Correspondence Analysis and Canonical Correspondence Analysis showed that highly significant edaphic gradients were present, with swamp or highly fertile soils separated from the less fertile, well-drained upland soils. Species composition remained significantly related to topographic position when soil type was controlled for. The main floristic gradients were still significant when flooded sites were excluded from the analyses. Randomization tests on a weighted preference index were used to examine the relations of individual species to soil types and, within the dominant soil type, to topographic position. Of the 132 species with N≥ 5 individuals, 33 showed significant associations with soil type. Within the dominant soil type, 13 of 110 analyzable species were nonrandomly associated with one or more topographic positions. For a variety of reasons, including issues relating to sample size and adequate edaphic characterization of landscapes, we suggest that the ∼ 30% of species shown to be edaphically biased in this study is an underestimate of the true degree of edaphically related distributional biases. To evaluate this hypothesis will require mesoscale vegetation sampling combined with quantitative soil analyses at the same scale in a range of tropical rain forests. If edaphic distributional biases are shown to be common, this suggests that edaphically linked processes leading to differential recruitment are similarly common.
Variation in forest canopy structure influences both understory light availability and its spatial distribution. Because light is a major environmental factor limiting growth and survival of many forest species, its distribution may affect stand-level regeneration patterns. We examined spatial patterning in light availability and seedling regeneration in old-growth, second-growth, and selectively logged stands of tropical moist forest in northeastern Costa Rica. Our objectives were to determine how the frequency distribution and spatial pattern of understory light "microsites" differ among tropical wet forests; whether patterns of seedling regeneration are linked to spatial patterning of light availability; and whether these relationships differ among old-growth, second-growth, and selectively logged forest stands. We used both sensor-based and hemispherical photograph-based methods to measure light availability along three 130-160 m long transects in each of eight stands (three old-growth, three second-growth, and two selectively logged). Woody seedling abundance was assessed at 4 m2, 25 m2, and full-stand scales (430 m2), and species richness was computed at the 25-m2 and full-stand levels. Data were analyzed using both conventional parametric approaches and spatial statistics. Mean light availability did not differ markedly among stand types, but variance and frequency distributions of light availability did. Second-growth stands had significantly higher unweighted canopy openness along solar tracks and a higher frequency of microsites at intermediate light levels. Old-growth stands had greater representation of both low- and high-light microsites, and greater overall variance in light availability. Old-growth stands also had slightly higher abundance and species richness of woody seedlings. Light availability was significantly spatially autocorrelated in all stand types, but patch size (analogous to gap size) was twice as large in old-growth stands, based on sensor data. Seedling abundance was also spatially autocorrelated over greater distances in old-growth than in second-growth stands, often at similar spatial scales to light distribution. The selectively logged stands demonstrated spatial autocorrelation of light and seedling abundance over distances intermediate to the other two stand types. Despite the similarities in patterns of light and seedling distributions, relationships between woody seedling abundance, species richness, and the three light availability measures were not strong or consistently positive, regardless of whether standard regressions or partial Mantel tests were applied. Although seedling abundance is likely to be affected by a wide variety of factors, the similarities in the scales of spatial autocorrelation of light and seedling abundance suggest that current seedling abundance distributions may reflect past patterns of light distribution within the stands. Our results confirm the importance of examining spatial dependence of resource availability in studies of forest dynamics, but they also underscore the limitations of a single period of data collection. Long-term studies as well as experimental manipulations of resource availability are needed to establish causal relationships between resource availability and stand-level patterns of seedling regeneration.
Ecologists need a better understanding of how animals make decisions about moving across landscapes. To this end, we developed computer simulations that contrast the effectiveness of various search strategies at finding habitat patches in idealized landscapes (uniform, random, or clumped patches), where searchers have different energy reserves and face different mortality risks. Nearly straight correlated random walks always produced better dispersal success than relatively uncorrelated random walks. However, increasing patch density decreased the degree of correlation that maximized dispersal success. Only under high mortality and low energy reserves in a uniform landscape did absolutely straight-line search perform better than any random walk. With low mortality risks and high energy reserves, exhaustive systematic search was superior to the best correlated random walk; an increase in the perceptual range of the searcher (i.e., patch detectability) also favored exhaustive search over relatively straight random walks. For all conditions examined, the "average distance rule," a hybrid search rule incorporating both straightline and systematic search, was best. Overall, however, our results suggest that a simple and effective search rule for many landscape-explicit models would involve straight or nearly straight movements.
In grassland ecosystems, symbiotic associations between plants and mycorrhizal fungi are widespread and have important influences on the life histories, demography, and species interactions of plants, and on belowground ecosystem processes. To assess the consequences of the symbiosis at the plant community level, we conducted a 5-yr field experiment in tallgrass prairie to investigate the influence of arbuscular mycorrhizal fungi on plant species composition, relative abundances, and diversity. Replicate plots in which mycorrhizal fungi were suppressed with benomyl application every two weeks during each growing season, were compared to nontreated mycorrhizal control plots on six watershed units at the Konza Prairie in northeastern Kansas. Benomyl successfully reduced mycorrhizal colonization to <25% of mycorrhizal control plots. Mycorrhizal colonization of roots in control plots was inversely related to annual precipitation. Suppression of mycorrhizae resulted in decreases in abundances of the dominant, obligately mycotrophic C4 tall grasses, compensatory increases in abundances of many subordinate facultatively mycotrophic C3 grasses and forbs, but no change in total aboveground biomass, as estimated from canopy density. Suppression of mycorrhizal symbiosis resulted in a large increase in plant species diversity. Two possible mechanisms for mycorrhizal mediation of plant species composition and diversity are: (1) alterations in resource distribution among neighbors via hyphal connections, and (2) differential host species responses to mycorrhizal fungal colonization in communities in which the competitive dominants are more strongly or more weakly mycotrophic than their neighbors. The results of this study demonstrate that mycorrhizal symbiosis can have large effects on plant community structure, and that differential host species response to fungal colonization is a key factor explaining the dominance of warm-season C4 grasses in tallgrass prairie and limiting plant species evenness and diversity. The results also underscore the importance of above- and belowground linkages in tallgrass prairie and indicate that alterations in belowground fungi and rhizosphere processes can have large effects on aboveground floristic composition and diversity in grasslands.
Quantitative synthesis across studies requires consistent measures of effect size among studies. In community ecology, these measures of effect size will often be some measure of the strength of interactions between taxa. However, indices of interaction strength vary greatly among both theoretical and empirical studies, and the connection between hypotheses about interaction strength and the metrics that are used to test these hypotheses are often not explicit. We describe criteria for choosing appropriate metrics and methods for comparing them among studies at three stages of designing a meta-analysis to test hypotheses about variation in interaction intensity: (1) the choice of response variable; (2) how effect size is calculated using the response in two treatments; and (3) whether there is a consistent quantitative effect across all taxa and systems studied or only qualitatively similar effects within each taxon-system combination. The consequences of different choices at each of these stages are illustrated with a meta-analysis to examine the relationship between competition/facilitation intensity and productivity in plants. The analysis used a database of 296 cases in 14 studies. The results were unexpected and largely inconsistent with existing theory: competition intensity often significantly declined (rather than increased) with productivity, and facilitation was sometimes restricted to more productive (rather than less productive) sites. However, there was considerable variation in the pattern among response variables and measures of effect size. For example, on average, competitive effects on final biomass and survival decreased with standing crop, but competitive effects on growth rate did not. On the other hand, facilitative interactions were more common at low standing crop for final biomass and growth rate, but more common at high standing crop for survival. Results were more likely to be significant using the log response ratio (ln[removal/control]) as the effect size than using the relative competition intensity ([removal - control]/removal), although the trends for these conceptually similar indices did not differ. When all studies were grouped in a single meta-regression of interaction intensity on standing crop to test quantitative similarity among studies, survival showed the clearest negative relationship. However, when the same regressions were done for each unique combination of taxon and site within each study to test for qualitative similarity among studies, the slopes averaged over studies tended to be negative for biomass and growth rate, but not different from zero for survival. These results are subject to a number of caveats because of the limitations of the available datamost notably, the extension of effects of interactions on individual growth or survival to effects on population distribution and abundance or community structure is highly problematic. Nevertheless, the fact that none of the meta-analyses demonstrated a significant positive relationship between competition and standing crop but that we frequently found negative relationships is an important pattern that has not been apparent from qualitative surveys of individual studies, and it demonstrates the potential power of meta-analysis in ecology. We conclude with recommendations to overcome some of the limitations of the currently available data and meta-analytical procedures.
Methodological problems in describing patterns of senescence in wild populations have until recently impeded progress in understanding the evolution of a process that decreases individual fitness. We investigated age- and sex-specific survival in five populations of three species of ungulates (roe deer, Capreolus capreolus; bighorn sheep, Ovis canadensis; and isard, Rupicapra pyrenaica), using recent statistical developments of capture-mark-recapture models and long-term (12 to 22 yr) data on marked individuals. The yearly survival of females aged 2-7 yr was remarkably similar and very high (92-95%) in all five populations. Survival of adult males varied among species and populations. Survival decreased from 8 yr onward for both sexes in all populations, suggesting that senescence was a common phenomenon. Male survival was lower than female survival, and the gender difference increased with age. The extent of sex differences in survival was related neither to sexual dimorphism in mass nor to the level of polygyny, suggesting that species differences in social behavior, particularly mating system and the level of male-male aggression, may be more important than simply the level of polygyny in explaining sexual differences in survival. Our results underline the advantages of long-term monitoring of marked individuals for the study of evolutionary ecology.