Showing papers in "Ecology in 1999"

The meta-analysis of response ratios in experimental ecology

Abstract: 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.

Global patterns of plant invasions and the concept of invasibility

Abstract: 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.

The ecological consequences of changes in biodiversity: a search for general principles101

Abstract: 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 com- munity 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.

Generality of leaf trait relationships: a test across six biomes

Abstract: 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 pho- tosynthetic 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.

PRIMARY CONSUMER δ13C AND δ15N AND THE TROPHIC POSITION OF AQUATIC CONSUMERS

Abstract: Stable nitrogen isotope signatures (δ15N) are increasingly used to infer the trophic position of consumers in food web studies. Interpreting the δ15N of consumers relative to the δ15N 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 δ15N values in 14 lakes in Ontario and Quebec. Values of δ15N 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 (δ13C) of primary consumers decreased along this same habitat gradient (mean littoral = −23.8‰, pelagic = −28.4‰, profundal = −30.5‰). Primary consumer δ13C and a categorical lake variable explained 72% of the variability in primary consumer δ15N. This relationship was corroborated by primary consumer δ15N and δ13C data from the literature, indicating that habitat-specific variation in ba...

Statistical issues in ecological meta‐analyses

Abstract: 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.

Seed Dispersal Near and Far: Patterns Across Temperate and Tropical Forests

Abstract: 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 f...

Climatic variability, plant phenology, and northern ungulates

Abstract: 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 seri...

The stoichiometry of consumer-driven nutrient recycling: theory, observations, and consequences

Abstract: Ecologists are increasingly recognizing the importance of consumers in reg- ulating ecosystem processes such as nutrient cycling. Ecologists have recently made con- siderable 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. Stoichi- ometric theory shows that both grazer and algal elemental composition are critical param- eters 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 eco- system 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 stoichi- ometry 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.

Biomass allocation in plants: ontogeny or optimality? a test along three resource gradients

Abstract: 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 field-gathered 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 ...

Microcosm experiments have limited relevance for community and ecosystem ecology: reply

Abstract: matures affects their adult fertility: population dynamics. American Naturalist 126:521-528. Ricklefs, R. E., and D. Schluter, editors. 1993. Species diversity in ecological communities. University of Chicago Press, Chicago, Illinois, USA. Robinson, J. V., and J. E. Dickerson. 1987. Does invasion sequence affect community structure? Ecology 68:587595. Robinson, J. V., and M. A. Edgemon. 1988. An experimental evaluation of the effect of invasion history on community structure. Ecology 69:1410-1417. Rose, K. A., G. L. Swartzman, A. C. Kindig, and F B. Taub. 1988. Stepwise iterative calibration of a multispecies phytoplankton-zooplankton simulation using laboratory data. Ecological Modelling 42:1-32. Schoener, T. W. 1971. Theory of feeding strategies. Annual Review of Ecology and Systematics 2:369-404. . 1983. Field experiments on interspecific competition. American Naturalist 122:240-285. Simberloff, D. 1981. Community effects of introduced species. Pages 55-81 in M. H. Nitecki, editor. Biotic crises in ecological and evolutionary time. Academic Press, New York, New York, USA. Sommer, U. 1988. Phytoplankton succession in microcosm experiments under simultaneous grazing pressure and resource limitation. Limnology and Oceanography 33:10371054. . 1991. Convergent succession of phytoplankton in microcosms with different inoculum species composition. Oecologia 87:171-179. Tilman, D. 1977. Resource competition between planktonic algae: an experimental and theoretical approach. Ecology 58:338-348. . 1982. Resource competition and community structure. Princeton University Press, Princeton, New Jersey, USA. Tilman, D., and R. W. Sterner. 1984. Invasions of equilibria: tests of resource competition using two species of algae. Oecologia 61:197-200. Tsuchiya, H. M, J. F Drake, J. L. Jost, and A. G. Fredrickson. 1972. Predator-prey interactions of Dictyostelum and Escherichia coli in continuous culture. Journal of Bacteriology 110:1147-1153. Underwood, J. 1986. What is a community? Pages 351-367 in D. M. Raup and D. Jablonski, editors. Patterns and processes in the history of life. Springer-Verlag, Berlin, Germany. Utida, S. 1953. Interspecific competition between two species of bean weevil. Ecology 34:301-307. . 1957. Cyclic fluctuations of population density intrinsic to the host-parasite system. Ecology 38:442-449. Vermeij, G. J. 1991. When biotas meet: understanding biotic interchange. Science 253:1099-1104. Vitousek, R M. 1990. Biological invasions and ecosystem processes: towards an integration of population biology and ecosystem studies. Oikos 57:7-13. Webb, S. D. 1991. Ecogeography and the great American interchange. Paleobiology 17:266-280. Wilbur, H. M., and R. A. Alford. 1985. Priority effects in experimental pond communities: responses of Hyla to Bufo and Rana. Ecology 66:1106-1114. Wilbur, H. M., and J. E. Fauth. 1990. Experimental aquatic food webs: interactions between two predators and two prey. American Naturalist 135:176-204. Wilson, D. S., and E. Sober. 1989. Reviving the superorganism. Journal of Theoretical Biology 136:337-356.

Competitive mechanisms underlying the displacement of native ants by the invasive argentine ant

Abstract: 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 inter- ference ability typically present among coexisting ants, and if so, to assess whether Ar- gentine ants are subject to this trade-off as well. Argentine ants located and recruited to baits as quickly or more quickly than did native ants—both 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 chem- ical 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.

Terrestrial–aquatic linkages: riparian arthropod inputs alter trophic cascades in a stream food web

Abstract: 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

Patterns in nutrient availability and plant diversity of temperate north american wetlands

Abstract: 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 d...

Estimating effects of limiting factors with regression quantiles

Abstract: In a recent Concepts paper in Ecology, Thomson et al. emphasized that assumptions of conventional correlation and regression analyses fundamentally conflict with the ecological concept of limiting factors, and they called for new statistical procedures to address this problem. The analytical issue is that unmeasured factors may be the active limiting constraint and may induce a pattern of unequal variation in the biological response variable through an interaction with the measured factors. Consequently, changes near the maxima, rather than at the center of response distributions, are better estimates of the effects expected when the observed factor is the active limiting constraint. Regression quantiles provide estimates for linear models fit to any part of a response distribution, including near the upper bounds, and require minimal assumptions about the form of the error distribution. Regression quantiles extend the concept of one-sample quantiles to the linear model by solving an optimization problem ...

Edaphic factors and the landscape-scale distributions of tropical rain forest trees

Abstract: 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 dis- tributions 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 mea- sured and identified in 1170 circular 0.01-ha plots centered on an existing 50 3 100 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 Canon- ical 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 distri- butional 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.

Mycorrhizae influence plant community structure and diversity in tallgrass prairie

Abstract: In grassland ecosystems, symbiotic associations between plants and mycor- rhizal 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 mycor- rhizal 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 con- nections, and (2) differential host species responses to mycorrhizal fungal colonization in communities in which the competitive dominants are more strongly or more weakly my- cotrophic 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.

Search strategies for landscape-level interpatch movements

Abstract: 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 land- scapes (uniform, random, or clumped patches), where searchers have different energy re- serves 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 suc- cess. Only under high mortality and low energy reserves in a uniform landscape did ab- solutely 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 delectability) also favored exhaustive search over relatively straight random walks. For all conditions examined, the "average distance rule," a hybrid search rule incorporating both straight- line 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.

Empirical approaches to quantifying interaction intensity: competition and facilitation along productivity gradients

Abstract: 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 illu...

Age‐specific survival in five populations of ungulates: evidence of senescence

Abstract: Methodological problems in describing patterns of senescence in wild pop- ulations 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.

Spatial heterogeneity of light and woody seedling regeneration in tropical wet forests

Abstract: 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-g...

Trophic rank and the species-area relationship

Abstract: We acknowledge support by the National Science Foundation (to R. D. Holt and G. A. Polis), and by the National Environmental Research Council (R. D. Holt, J. H. Lawton, and N. D. Martinez). R. D. Holt and N. D. Martinez thank the NERC Centre for Population Biology, Imperial College at Silwood Park, for support and hospitality. We have profited from conversations on this and related topics with many scientists, and wish to thank in particular P. A. Abrams, W. B. Anderson, J. Bengtsson, W. J. O’Brien, Jr., M. Rosenzweig, T. Schoener, T. Tscharntke, and J. Wright.

Mycorrhizae indirectly enhance competitive effects of an invasive forb on a native bunchgrass

Abstract: Mycorrhizae are important mediators of plant competition, but little is known about the role of mycorrhizae in the intense competitive effects that exotic plants can have on native species. In the greenhouse, we tested the effect of arbuscular mycorrhizal (AM) fungi on interspecific competition between Cerztaur-ea maculosu and Fest~ccu idahoensis, on intraspecific competition between individuals of both species, and the growth of C. rnac~closawith either inorganic or organic phosphorus. Mycorrhizae had no direct effect on either species, but mycorrhizae increased C. maculosa's negative effect on F. iduhoe~zsis. When competing with C. rnac-ulosa, nonmycorrhizal F. idahoensis were 171% larger than they were when mycorrhizae were present. In a second experiment, C. maculosa grown with larger F. idahoe~zsis were 66% larger, in the presence of AM fungi, than when AM fungi were absent. Centaurea maculosu biomass was not affected by AM fungi, in either phosphorus treatment, in the absence of F. idahoensis. Root: shoot ratios differed between phosphorus treatments, but this difference seemed to be a result of slower growth in the organic phosphorus treatment. Our results were unusual in that the direct effects of my- corrhizae on both species were weak, but the indirect effect of AM fungi on the interactions between C. muc~tlosa and F. idahoerzsis was strong. Our results suggest that AM fungi strongly enhance the ability of C. ~naculosa to invade native grasslands of western North America. Key ~.ord.~: nrb~isclllcir rrrycoi-rhieal (AM) ,fuizgi; Centaurea maculosa; conzperition; exotic plants; Festuca idahoensis; interi7lor~tanr, gra.sslunris; iiz~,a.sion; mycorrhizae. ~~~~~~~~i~~~ between pairs of speciesare often me- diated by other species (Kareiva 1994, Miller 1994). Well documented indirect interactions include those be- tween sea otters, kelp and sea urchins 19781, starfish, molluscs and algae (Paine 1966),parasitic and autotrophic plants (Pennings and Callaway 1996), plants and soil microflora, (van der Putten et al, 1993. Bever 1994) and plants and mycorrhizal fungi (Grime

A Definitive Book on Seed Ecology@@@Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination

Abstract: Introduction. Ecologically Meaningful Germination Studies. Types of Seed Dormancy. Germination Ecology of Seeds with Nondeep Physiological Dormancy. Germination Ecology of Seeds with Morphophysiological Dormancy. Germination Ecology of Seeds with Physical Dormancy. Germination Ecology of Seeds in the Persistent Seed Bank. Causes of Within-Species Variations in Seed Dormancy and Germination Characteristics. A Geographical Perspective on Germination Ecology: Tropical and Subtropical Zones. A Geographical Perspective on Germination Ecology: Temperate and Arctic Zones. Germination Ecology of Plants with Specialized Life Cycles and/or Habitats. Biogeographical and Evolutionary Aspects of Seed Dormancy. Subject Index.

Avian habitat selection: pattern from process in nest-site use by ducks?

Abstract: Patterns of habitat use in animals presumably have evolved in response to diverse selective processes, so we first examined whether the theory of natural selection formed the conceptual framework for published studies (N = 270) of nest-site selection by birds. Most (61%) studies of nest-site selection tested for pattern arising from natural selection (whether used nest habitat differed from available nesting habitat), many (54%) tested for evidence of the process of natural selection (whether unsuccessful nests differed from successful nests), some (10%) tested whether the process of natural selection caused subsequent adaptation, but remarkably few conceptually linked these elements or used the theory of natural selection as the rationale for their questions. We then tested for evidence of natural (phenotypic) selection with data for six species of ducks. At nests, we used six variables to describe vegetation structure/nest position and categorized patch types (pond edge, native grass, planted cover, shrubs, or trees) in which nests were found; nest fates (abandoned, depredated, or successful) were also determined. For Blue-winged Teal (Anas discors), Northern Shoveler (A. clypeata), and Mallard (A. platyrhynchos), there were significant patterns of nonrandom nest-site placement within a gradient of vegetation structure/nest position. For Blue-winged Teal and Gadwall (A. strep- era), nest success varied within these gradients in a way that could exert directional se- lection. Several tests for adaptive nest-site choice were conducted. Nest fate did not influence fidelity of females to patch types. However, Mallards with previously unsuccessful nests dispersed farther than females with previously successful nests. Nonetheless, neither fidelity to patch type nor dispersal distance influenced subsequent nest success. In the long term (over 8 yr), there was a weak tendency within species for nest density to be higher among patch types where relative nest success was higher. In the short term (from year t to year t + 1), this pattern was not observed in a vegetation-structure/nest-position gradient for any species. The strongest evidence of adaptive response to nest fate was higher nest density on an island where nest success was relatively high.

Spatially explicit and stochastic simulation of forest- landscape fire disturbance and succession

Abstract: Understanding disturbance and recovery of forest landscapes is a challenge because of complex interactions over a range of temporal and spatial scales. Landscape simulation models offer an approach to studying such systems at broad scales. Fire can be simulated spatially using mechanistic or stochastic approaches. We describe the fire module in a spatially explicit, stochastic model of forest landscape dynamics (LANDIS) that in- corporates fire, windthrow, and harvest disturbance with species-level succession. A sto- chastic approach is suited to forest landscape models that are designed to simulate patterns over large spatial and time domains and are not used deterministically to predict individual events. We used the model to examine how disturbance regimes and species dynamics interact across a large (500 000 ha), heterogeneous landscape in northern Wisconsin, USA, with six land types having different species environments, and fire disturbance return intervals varying from 200 to 1000 yr. The model shows that there are feedbacks over time between species, disturbance, and environment, resulting in the re-emergence of patterns that char- acterized the landscape before extensive alteration. Landscape equilibrium of species com- position and age-class structure develops at three scales from the initial, disturbed landscape. Over 100-150 yr, fine-grained successional processes cause gradual disaggregation of the initial pattern of relatively homogenous composition and age classes. Species such as eastern hemlock ( Tsuga canadensis), largely removed from the landscape by past human activities, only slowly re-invade. Next, patterns on the various land types diverge, driven by different disturbance regimes and dominant species. Finally, aging of the landscape causes the prob- abilities of larger and more severe fires to increase, and a coarse-grained pattern develops from the disturbance patches. Influence of adjacent land types is shown as fires spread across land type boundaries, although modified in spread and severity. As found by others, altered landscapes are likely to retain their modified pattern for centuries, suggesting that nonequilibrium conditions between tree species and climate will persist under predicted rates of climate change. The results suggest that this modeling approach can be useful in examining species- level, broad-scale responses of heterogeneous landscapes to changes in landscape distur- bance, such as modified management or land-use scenarios, or effects of global change.

Bidirectional facilitation and interference between shrubs and annuals in the mojave desert

Abstract: Composition and structure of plant communities can be strongly influenced by plant interactions. Interactions among plants commonly comprise positive and negative effects operating simultaneously and bidirectionally. Thus, a thorough understanding of plant interactions requires experimental separation and quantitative assessment of the bidirectional positive and negative effects that add up to the net effects of plant interactions. Using the close spatial association of annual plants with a desert shrub (Ambrosia dumosa) in a sandy area of the Mojave Desert of California as a test system, we separated and quantified negative and positive effects of annuals on shrubs and of shrubs on annuals. We achieved the separation of negative and positive effects with an experimental design that included reciprocal removals of neighbors and simulations of physical effects of neighbors using artificial structures. All experimental manipulations were conducted on space originally occupied by Ambrosia shrubs to focus on immediate effects of neighbors on water availability rather than on long-term microenvironmental effects (e.g., nutrient accumulation). We quantified positive effects by calculating the difference between performance parameters of neighbors growing with artificial structures (thatch to mimic the physical effects of the presence of annuals and artificial canopies to mimic the physical effects of shrubs) and those of neighbors growing alone (removals). We estimated negative effects by calculating the difference between plant performance on control plots (shrubs and annuals growing together) and performances of plants growing with the artificial structures. Annuals had simultaneously strong negative and weak positive effects on shrub water status, growth, and reproductive output. Annuals also had an impact on the sex expression of shrubs by inducing shifts toward a higher male proportion in inflorescences of monoecious Ambrosia. In contrast, we found strong positive and weak or no negative effects of the shrubs on survival, biomass production, and seed production of the entire annual community and of selected annual species (the abundant native Chaenactis fremontii and the two dominant introduced annual species Bromus madritensis ssp. rubens and Schismus barbatus). Overall, in net effect, the interaction between shrubs and annuals can be described as facilitation or positive net effects of shrubs on annuals, and interference or negative net effects of annuals on shrubs. However, during the growing season, the ratios between positive and negative effects shifted. Annual plants benefited from the presence of shrubs to the greatest extent early in the growing season, and initial negative effects of annuals on shrubs declined as annuals senesced later in the season. Results of this study support the view that an experimental resolution of bidirectional positive and negative effects is necessary to achieve an accurate, mechanistic understanding of species interactions.

Resolving ecological questions through meta‐analysis: goals, metrics, and models

Abstract: We evaluate the goals of meta-analysis, critique its recent application in ecology, and highlight an approach that more explicitly links meta-analysis and ecological theory. One goal of meta-analysis is testing null hypotheses of no response to experimental manipulations. Many ecologists, however, are more interested in quantitatively measuring processes and examining their systematic variation across systems and conditions. This latter goal requires a suite of diverse, ecologically based metrics of effect size, with each appropriately matched to an ecological question of interest. By specifying ecological mod- els, we can develop metrics of effect size that quantify the underlying process or response of interest and are insensitive to extraneous factors irrelevant to the focal question. A model will also help to delineate the set of studies that fit the question addressed by the meta- analysis. We discuss factors that can give rise to heterogeneity in effect sizes (e.g., due to differences in experimental protocol, parameter values, or the structure of the models that describe system dynamics) and illustrate this variation using some simple models of plant competition. Variation in time scale will be one of the most common factors affecting a meta-analysis, by introducing heterogeneity in effect sizes. Different metrics will apply to different time scales, and time-series data will be vital in evaluating the appropriateness of different metrics to different collections of studies. We then illustrate the application of ecological models, and associated metrics of effect size, in meta-analysis by discussing and/or synthesizing data on species interactions, mutual interference between consumers, and individual physiology. We also examine the use of metrics when no single, specific model applies to the synthesized studies. These examples illustrate that the diversity of ecological questions demands a diversity of ecologically meaningful metrics of effect size. The successful application of meta-analysis in ecology will benefit by clear and explicit linkages among ecological theory, the questions being addressed, and the metrics used to summarize the available information.

Induced responses to herbivory in wild radish: effects on several herbivores and plant fitness

Abstract: Induced responses to herbivory are changes in plant quality following initial damage by herbivores. These changes can negatively affect subsequent herbivores. I studied induced responses in annual wild radish plants, Raphanus raphanistrum and R. sativus (Brassicaceae), which included increased density and total number of setose trichomes on newly formed leaves of previously damaged plants compared to undamaged controls. I also characterized the effects of induced responses on the preference and performance of several herbivores and the net consequences for plant performance in the field. Wild radish plants damaged by caterpillars or sprayed with a natural plant response elicitor, jasmonic acid, supported reduced growth of generalist noctuid larvae compared to unmanipulated control plants. Induced responses did not affect the growth of specialist Pieris rapae larvae. In choice and no-choice experiments, induction reduced the feeding by noctuid larvae but did not reduce gross growth efficiency, an indicator...