Showing papers in "The American Naturalist in 1996"
TL;DR: It is concluded that trophic cascades and top-down community regulation as envisioned by trophIC-level theories are relatively uncommon in nature.
Abstract: Food webs in nature have multiple, reticulate connections between a diversity of consumers and resources. Such complexity affects web dynamics: it first spreads the direct effects of consumption and productivity throughout the web rather than focusing them at particular "trophic levels." Second, consumer densities are often donor controlled with food from across the trophic spectrum, the herbivore and detrital channels, other habitats, life-history omnivory, and even trophic mutualism. Although consumers usually do not affect these resources, increased numbers often allow consumers to depress other resources to levels lower than if donor-controlled resources were absent. We propose that such donor-controlled and "multichannel" omnivory is a general feature of consumer control and central to food web dynamics. This observation is contrary to the normal practice of inferring dynamics by simplifying webs into a few linear "trophic levels," as per "green world" theories. Such theories do not accommodate commo...
1,995 citations
TL;DR: It is proposed that such flow is often a key feature of the energetics, structure, and dynamics of populations, food webs, and communities whenever any two habitats, differing in productivity, are juxtaposed.
Abstract: This study quantifies the flow of energy and biomass from a productive marine system to a relatively unproductive terrestrial system. Biomass from marine food webs (here, the Gulf of California) enters the terrestrial webs of islands and coastal areas through two conduits: (1) shore drift of algal wrack and carrion and (2) colonies of seabirds. Both conduits support dense assemblages of consumers: arthropods are 85-560 times more abundant in the supralittoral than inland and 2.2 times more abundant on islands with seabird colonies than those without. Marine input (MI), not terrestrial primary productivity (TP) by land plants, provides most energy and biomass for terrestrial communities on 16 of 19 study islands. The ratio of perimeter to area (P/A) significantly predicts arthropod abundance on islands and is the major determinant of the relative importance of allochthonous flow; we expect P/A ratio to be important wherever transport of nutrients, detritus, and organisms among habitats occurs. Similar tran...
772 citations
TL;DR: Comparisons are analyzed to explore when trade-offs between resistance and fitness traits can be detected and reveal patterns that were most often associated with resistance to herbicides, followed by resistance to pathogens, and least often associated to herbivores.
Abstract: Despite the general belief that traits conferring resistance in plants to pests also confer costs, disagreement persists about the frequency of costs and the conditions under which they are most likely to be evident. In this article, we analyze 88 published comparisons to explore when trade-offs between resistance and fitness traits can be detected. Among the patterns revealed are that costs were most often associated with resistance to herbicides, followed by resistance to pathogens, and least often associated with resistance to herbivores; costs were more often found in crops versus wild species; greater control of the genetic background increased the probability of detecting costs of resistance; there was large variation in the cost associated with the same resistance trait in different genetic backgrounds; and many examples of costs of resistance appeared to be due to linkage rather than pleiotropic effects. We discuss these and other results and emphasize that dependencies among the data invalidate s...
603 citations
TL;DR: The adaptive value of plastic stem elongation in Impatiens capensis was tested by manipulating the controlling light cue, red to far red ratio, to produce elongated and nonelongated plants, which suggests an intrinsic cost of elongation independent of selection on morphology.
Abstract: In plants, stem elongation at high density in response to vegetation shade is hypothesized to be an example of adaptive phenotypic plasticity. Elongated stems may increase the light capture for plants in dense stands, while nonelongated stems may be favored for plants in low density. We tested the adaptive value of plastic stem elongation in Impatiens capensis by manipulating the controlling light cue, red to far red ratio, to produce elongated and nonelongated plants. These plants were then transplanted into high and low densities in a natural population. The results supported the adaptive plasticity hypothesis; elongated plants were more fit at high density, and suppressed plants were more fit at low density. Phenotypic selection analysis revealed selection for increased height in high density and for decreased height relative to leaf length in low density. Elongated plants showed less growth of the second internode at 2 wk after transplantation in both densities, which suggests a cost of elongation. Di...
568 citations
TL;DR: These models show the critical role of species compositional turnover in determining food web responses to bottom-up and top-down regulation by productivity and variation in predator death rates.
Abstract: I analyze a model of species interactions involving species that compete for a single resource and share a common "keystone predator" to study the "bottom-up" effects of productivity (potential car...
561 citations
TL;DR: To determine whether differences in levels of infection between host sexes are random (i.e., normally distributed around a mean of zero) or biased in favor of one sex, results of comparisons of helminth infections between female and male vertebrates are analyzed.
Abstract: Parasitism and its evolutionary relationship with host sex have received considerable attention in recent years. The role of parasites in maintaining host sexual reproduction (Hamilton 1980; Lively 1987; Ladle 1992) and in driving host sexual selection (Hamilton and Zuk 1982; Clayton 1991) has been explored theoretically and assessed empirically. One potential link between parasitism and host sex, however, has yet to be examined carefully. There are intrinsic biological differences between host sexes, differences that could lead to one sex being more prone to parasitic infections than the other. Although ost sex is often listed as a factor that can influence the parasite burden of individuals ( ee, e.g., Esch and Fern'andez 1993), statistically significant inequalities between female and male infection levels are not very common, and their direction is thought o depend on the particulars of the host-parasite system studied. There may be reasons (reviewed in Alexander and Stimson 1988; Bundy 1988; Zuk 1990) to expect one sex to be more heavily parasitized than the other. Physiological, morphological, and behavioral differences between females and males could operate to create a slight but consistent sexual bias in infection levels. For instance, high testosterone l vels can cause immunosuppression i males (Grossman 1985; Folstad and Karter 1992) and could lead to males in general suffering more from parasites than females do. Such a consistent bias may not be apparent from a scan of the literature on parasitic infections. Many parasite surveys have reported no significant differences between the prevalence or intensity of infection in female and male hosts, whereas others have found significant differences infavor of one or the other sex. However, many nonsignificant differences, if pointing in the same direction, can form significant trends when pooled and analyzed as a whole. Differences between the sexes may exist but be too marginal to be detected by examining a single host sample. The purpose of this note is to determine whether differences inlevels of infection between host sexes are random (i.e., normally distributed around a mean of zero) or biased in favor of one sex. To achieve this I analyze results of comparisons of helminth infections between female and male vertebrates, using data from the literature. Evidence of a bias in favor of one sex would suggest hat higher levels of parasitism may be a relative cost associated with that sex and could have a range of evolutionary implications.
500 citations
TL;DR: This work investigated three lekking birds at Nourages field station, French Guiana and found that Conspicuousness is a function of ambient light spectra during displays and the reflectance spectra of color pattern elements of the birds and their visual backgrounds.
Abstract: Forests exhibit a mosaic of different spectral environments that arise from forest geometry and weather If visual signals are used in mate choice, then forest geometry and weather will affect reproductive behavior because the appearance of a visual signal depends on the joint effects of ambient light and the animal's reflectance spectra We investigated three lekking birds at Nourages field station, French Guiana: Rupicola rupicola, Corapipo gutturalis, and Lepidothrix serena Conspicuousness is a function of ambient light spectra during displays and the reflectance spectra of color pattern elements of the birds and their visual backgrounds Each species places its lek and performs its lek displays in only one or two of the available light environments, and some may specialize in the more extreme spectra even within each light environment The color patterns and behavior of each species maximize its visual contrast during its display and reduce it off the lek or on the lek but not displaying Each specie
495 citations
TL;DR: Evidence for ecological speciation in sticklebacks is weakest, but there are several hints of its importance: speciation was rapid and accompanied by divergence into different ecological niches; selection against hybrids is stronger in the wild than in the laboratory, which suggests that hybrid fitness depends on ecological context.
Abstract: According to the naturalists of the first half of this century, adaptive radiation is the outcome of three ecological processes: phenotypic differentiation of populations by resource-based divergent natural selection, phenotypic differentiation through resource competition (ecological opportunity and divergent character displacement), and ecological speciation (speciation as a consequence of adaptation to different resource environments). Despite a recent surge of interest in the phenomenon, especially in phylogenetic histories of radiations, we know too little about the ecology of most radiations to assess the roles of the three processes. I summarize our own efforts to test the theory with a radiation of three-spined sticklebacks apparently still in its early stages. The role of divergent selection is supported by a strong relationship among populations and species in mean morphology, feeding performance, habitat use, and growth rate. Trade-offs in feeding performance and growth rate between habitats ar...
448 citations
TL;DR: The study focuses on the optimal responses of size, development time, and growth rate to changes in the amount of time available for completion of the life cycle, and shows that the optimal growth rate and size at maturity may respond in several different ways.
Abstract: The interrelationships among development time, growth rate, and adult size are investigated using simple optimization models of a seasonal life history in which larger adults have greater reproduct...
429 citations
404 citations
TL;DR: The minimum viable metapopulation (MVM) size is defined as the minimum number of interacting local populations necessary for long-term persistence of a metapoulation in a balance between local extinctions and recolonizations.
Abstract: We define the minimum viable metapopulation (MVM) size as the minimum number of interacting local populations necessary for long-term persistence of a metapopulation in a balance between local extinctions and recolonizations The minimum amount of suitable habitat (MASH) is defined as the minimum density (or number) of suitable habitat patches necessary for metapopulation persistence Levins's metapopulation model suggests that MASH can be estimated by the fraction of empty patches in a network in which the metapopulation occurs at a stochastic steady state We discuss three reasons why this rule of thumb is likely to give an underestimate, and possibly a severe underestimate, of MASH: the rescue effect, colonization-extinction stochasticity, and nonequilibrium (transient) metapopulation dynamics The assumption that metapopulations occur at a steady state, common to many models, may be frequently violated because of the high rate of habitat loss and fragmentation in many landscapes Scores of rare and en
TL;DR: It is shown that crossing of reaction norms for fitness is a sufficient condition for selection to favor specialized host choice, in the absence of search costs and other disadvantages of specialization.
Abstract: The idea that traits that adapt a population to one habitat or host are deleterious in other habitats or on other hosts is central to most theories for the evolution of ecological specialization. The trade-off concept has played an especially prominent role in discussions of the evolution of host specialization in phytophagous insects. The evidence for genetically based trade-offs in phytophagous insect populations is ambiguous, however. Cross-host genetic corre- lations for fitness traits are seldom negative, but this does not preclude the existence of trade-offs at a subset of loci controlling the fitness variation. One clear result is that cross-host genetic correlations for fitness traits are often less than one, which implies that genotypes have different fitness rankings on different hosts (i.e., that reaction norms for fitness cross). Using verbal arguments and a mathematical model, I show that crossing of reaction norms for fitness is a sufficient condition for selection to favor specialized host choice, in the absence of search costs and other disadvantages of specialization. In other words, selection will favor specialization if alleles that are positively selected on one host are less strongly positively selected, or neutral, on other hosts; it is not necessary for the alleles to be deleterious on the other hosts. Search costs and other factors may oppose the evolution of specialization, but introducing trade-offs into a model does not result in a quantum jump in the strength of selection favoring, specializa- tion, and thus in the likelihood of specialization evolving, compared to the situation in which alleles that affect fitness on one host are neutral on others. There is therefore no justification for focusing on the qualitative presence or absence of trade-offs as the critical issue in predicting or explaining the evolution of specialization. Furthermore, the quantitative genetic data, rather than give little information on why specialization evolves, indicate that the potential for selection to favor specialization exists in many phytophagous populations.
TL;DR: In this paper, the authors show that species with broader niche breadths have higher probabilities of fixing beneficial alleles, taking less time to do so; they have fewer deleterious alleles drifting to fixation (a lighter drift load) and a lower frequency of deleteriously alleles at mutation-selection equilibrium.
Abstract: The evolution of niche breadth or phenotypic plasticity is often assumed to be limited by negative genetic correlations among fitness in different environments, but these trade-offs are rarely observed. Many other constraints can reduce the mean niche breadth of species. This article discusses one of these limitations-that species with broader niche breadths can have a slower rate of evolutionary response. Species with narrower niche breadths have higher probabilities of fixing beneficial alleles, taking less time to do so; they have fewer deleterious alleles drifting to fixation (a lighter drift load) and a lower frequency of deleterious alleles at mutation-selection equilibrium (a lighter mutation load). These patterns are true even with no correlation at all between fitness in different environments; no trade-offs are assumed. Furthermore, niche breadth is likely to evolve to be more narrow, because of the association between location and alleles favored in local habitats for individuals with reduced m...
TL;DR: This article develops an integrated measure of fitness based on the philosophical underpinnings provided by the notion of propensity fitness, combined with classical demographic methods, and shows how life-history data can be formatted as an age-structured population projection matrix, A(m).
Abstract: Fitness is a vague, poorly developed concept in ecology and evolutionary biology. Although it is generally recognized that fitness is determined by the complete survival and reproductive schedules of individual organisms, experimental studies have rarely attempted to integrate these into a single measure of individual fitness. Instead, particularly in studies of natural selection, components of fitness are measured as surrogates for fitness. In this article, we develop an integrated measure of fitness based on the philosophical underpinnings provided by the notion of propensity fitness, combined with classical demographic methods. We show how life-history data, collected for an individual, can be formatted as an age-structured popula- tion projection matrix, A("?). The dominant eigenvalue, X('t), of this matrix is an estimate of that individual's propensity fitness. Using life-history data sets on European sparrowhawks and blue tits, we show that the interpretation of analyses of selection can shift radically using the inte- grated propensity fitness measure instead of components of fitness, such as lifetime reproductive output. Individual fitness, as formulated here, provides an integrated measure of performance that should prove useful in empirical studies in ecology and evolutionary biology.
TL;DR: A mathematical model is constructed that postulates that an Allee effect-disproportionately lowered fecundity below a critical threshold density of abundance-is the mechanism leading to a slower rate of spread in the early stages of the invasion.
Abstract: Since about 1940, when they were first released in the New York City area, house finches (Carpodacus mexicanus) have multiplied explosively and colonized much of eastern North America. We take advantage of the richly detailed documentation of this biological invasion to construct a mathematical model that predicts the rate of population spread on the basis of readily measurable demographic parameters. We seek to improve on previous models by predicting a rate of spread that accelerates following an initial period of slower growth, a pattern of spread followed by house finches as well as a variety of other invading species. We postulate that an Allee effect-disproportionately lowered fecundity below a critical threshold density of abundance-is the mechanism leading to a slower rate of spread in the early stages of the invasion. Our integrodifference equation model also emphasizes the link between long-distance dispersal and the rate of population spread.
TL;DR: Relationships between patterns of defense and attack were consistent with predictions based on optimal defense theory, and Roots were least likely to be attacked, while reproductive parts and leaves had high probabilities of attack.
Abstract: Optimal defense theory predicts that tissues that are unlikely to be attacked by herbivores should have low constitutive amounts of defense and high inducibility, while tissues that are likely to be attacked should have high levels of constitutive defense and low inducibility. We artificially damaged roots, leaves, and reproductive parts of wild parsnip (Pastinaca sativa L.) and found that these parts differ not only in constitutive levels of chemical defense but also in the degree to which these defenses are inducible. Reproductive parts contain the highest constitutive concentrations of a toxic furanocoumarin, xanthotoxin, but xanthotoxin production is not inducible in these parts. Roots contain the lowest constitutive levels of xanthotoxin and are highly inducible. Leaves are intermediate in both the constitutive amounts of xanthotoxin they contain and inducibility. To test the prediction of optimal defense theory, we surveyed three populations of wild parsnip for the presence of damage to roots, leave...
TL;DR: The impact of herbivory on the fitness of hermaphroditic plants requires knowledge of both male and female reproductive success, and how plants have evolved to respond to damage from herbivores in allocation patterns to floral resources may be influenced or constrained by floral adaptations.
Abstract: Plants are subject o selection exerted by both herbivores and pollinators simultaneously. Despite this fact, no previous tudy has directly linked foliar herbivory and pollination. A few studies have addressed how floral displays that attract pollinators may subsequently attract seed predators, when flowers mature to fruits (Campbell 1991; Brody 1992), but no one has examined how folivore-caused changes in floral characters, particularly corolla characters, affect pollinators. In the past, it has been assumed that floral characters, and particularly corolla characters, are relatively invariant because of the fitness consequences that reduced attractiveness to pollinators may have for self-incompatible plants (e.g., Harper 1977, p. 195). However, it is well known that natural variation in flower size, flower number, nectar production, and pollen production affects the attractiveness of plants to pollinators (e.g., Schemske 1980; Roubick and Buchmann 1984; Willmer 1986; Stanton and Preston 1988; Galen 1989). Effects of foliar herbivory on floral traits other than flower number are not well known (but see McKone 1989; Frazee and Marquis 1994; Quesada et al. 1995). Herbivore damage directly to corollas reduced the attractiveness of flowers to pollinators (Karban and Strauss 1993). The effects of foliar damage by herbivores on floral traits and, in turn, on the attractiveness of plants to pollinators have been generally ignored (Crawley 1989; Marquis 1992). The significance of floral responses to foliar herbivory is twofold. First, estimates of the effects of herbivory on plant fitness based on female fitness (seed production) may be misleading if plants suffer a reduction in male fitness (number of seeds sired on other plants through pollen). Herbivory affecting attractiveness to pollinators has the potential to strongly influence plant fitness through male function, since visitation rates can often affect male fitness (number of seeds sired by pollen) more strongly than female plant fitness (numbers of seeds set) (Bell 1985). Only a handful of studies have documented male and female plant fitness imultaneously, and at least three have shown that male and female plant fitness are either negatively or only weakly correlated (Bertin 1982; Ennos and Dodson 1987; Schlichting and Devlin 1989; Broyles and Wyatt 1990; Devlin and Ellstrand 1990). Thus, the impact of herbivory on the fitness of hermaphroditic plants requires knowledge of both male and female reproductive success. Second, how plants have evolved to respond to damage from herbivores in allocation patterns to floral resources may be influenced or constrained by floral adaptations
TL;DR: The hypothesis that sustained working capacity is related to basal metabolism is supported for mammals but not for birds, and residues of metabolic variables from allometric regressions on body mass were greater for mammals than for birds and suggest that mammals are more diversified in their energetic physiology.
Abstract: We examined the relationship between daily energy expenditure (DEE) and basal metabolic rate (BMR) in birds and mammals. Two models of the relationship between DEE and BMR were distinguished: a "shared pathways" model in which DEE replaces BMR in the active organism and a "partitioned pathways" model in which DEE includes BMR-that is, BMR is separate from the metabolic pathways that result in activity metabolism (ACT), and DEE = ACT + BMR. The appropriate null hypotheses for the relationship between basal and active metabolism are rDEE· BMR = 0 and rACT· BMR = 0, respectively. Correlations of the residuals (d and b) of the logarithms of DEE and BMR from their allometric regressions with the logarithm of body mass were tested against these null models. Using phylogenetically independent contrasts, we found no significant relationship between DEE and BMR in birds, but a strong relationship (rdb = 0.86) among mammals. Thus, the hypothesis that sustained working capacity is related to basal metabolism is supp...
TL;DR: A large number of natural comimunities have been investigated, especially by plant ecologists, and they found that different communities may exhibit different types of species-area relations.
Abstract: Species-area relationships have been of interest in ecology for a long time (de Candolle 1855; Jaccard 1902, 1908). They have been called \"one of community ecology's few genuine laws\" (Schoener 1976, p. 629). Plotting number of species (S) against sampling area (A), for a series of samples of increasing sizes, yields a monotonically increasing curve whose slope is steep at first but gradually becomes nearly flat. The shape of such curves has been used to help determine the area required to obtain an adequate sampling of the species in a particular community-the \"minimal area\" concept (Goodall 1952; Hopkins 1957; Cain and Castro 1959; Barkman 1989), to characterize community structure (Fisher et al. 1943; Goodall 1952; Preston 1962; May 1975), to estimate species richness (Evans et al. 1955; Kilburn 1966; Hubbell and Foster 1983; Palmer 1990; Baltanas 1992; Grassle and Maciolek 1992), to measure the effect of disturbance on communities (Lawrey 1991), and to define the appropriate size of reserves and natural areas in conservation biology (MacArthur and Wilson 1967; Soule et al. 1979; Williamson 1981). A large number of natural comimunities have been investigated, especially by plant ecologists. They found that different communities may exhibit different types of species-area relations. Among them, three expressions are most widely used: exponential curve (Gleason 1922, 1925),
TL;DR: Invasion of New Zealand by exotic birds was therefore primarily related to management, an outcome that has implications for conservation biology.
Abstract: Whether or not a bird species will establish a new population after invasion of uncolonized habitat depends, from theory, on its life-history attributes and initial population size. Data about initial population sizes are often unobtainable for natural and deliberate avian invasions. In New Zealand, however, contemporary documentation of introduction efforts allowed us to systematically compare unsuccessful and successful invaders without bias. We obtained data for 79 species involved in 496 introduction events and used the present-day status of each species as the dependent variable in fitting multiple logistic regression models. We found that introduction efforts for species that migrated within their endemic ranges were significantly less likely to be successful than those for nonmigratory species with similar introduction efforts. Initial population size, measured as number of releases and as the minimum number of propagules liberated in New Zealand, significantly increased the probability of transloc...
TL;DR: A weight-corrected measure of sexual dimorphism and a biologically realistic assay of mating competition, the operational sex ratio, are employed to reexamine the factors favoring the evolution of sexual sizeDimorphism in primates and produce results consistent with the sexual selection hypothesis.
Abstract: Male mating competition is generally regarded to account for sexual dimorphism in body size, but levels of sexual dimorphism do not appear to be associated with the intensity of intrasexual selection in polygynous mammals. In contrast, observations of accentuated dimorphism in certain taxa and in large species are consistent with nonadaptive explanations for the evolution of sexual size dimorphism based on phylogenetic inertia and allometry. Here we employ a weight-corrected measure of sexual dimorphism and a biologically realistic assay of mating competition, the operational sex ratio, to reexamine the factors favoring the evolution of sexual size dimorphism in primates. Independent contrasts that control for the effects of allometry and phylogeny produce results consistent with the sexual selection hypothesis; a strong relationship exists between sexual dimorphism in size and the intensity of male mating competition among polygynously mating primates. Increased sexual dimorphism in large primates may no...
TL;DR: Results are discussed in terms of the costs of producing longer sperm, the correlated evolution of sperm length and body size, the relationship between breeding system and sperm production patterns, and the nature of differences between vertebrates and invertebrates in sperm production and the size of testes.
Abstract: Relationships among body mass, testis mass, sperm length, and the number of sperm produced were examined among 11 Drosophila species, after controlling for phylogenetic ef- fects. This is the first study to examine many of these relationships comparatively in an inverte- brate taxon; patterns observed among these variables were fundamentally different from those consistently reported in studies of vertebrates. In regression analyses, testis mass increased with body mass with an exponent greater than one, which indicates that males of larger-bodied Drosophila species make a proportionately greater energetic investment in testes than do males of smaller-bodied species. The positive allometry of testis mass is hypothesized to be a combined consequence of the unusual positive relationship between body mass and sperm length and the positive relationship between sperm length and testis mass. Interspecific variation in testis mass was found to be a function of variation in sperm length rather than variation in the number of sperm produced. Significant trade-offs were identified between sperm length and the number of sperm produced and transferred per copulation. Results are discussed in terms of the costs of producing longer sperm, the correlated evolution of sperm length and body size, the relation- ship between breeding system and sperm production patterns, and the nature of differences between vertebrates and invertebrates in sperm production and the size of testes.
TL;DR: This work observed positive spatial associations between a rare endemic mustard, Lesquerella carinata var.
Abstract: Early ecologists emphasized the role of positive interactions inplant communities occurring in habitats characterized by intense abiotic stress (Clements et al. 1926; Allee et al. 1949). Until recently, however, positive interactions have been largely overlooked as important factors in community structure. In the last decade, renewed interest in facilitation has resulted in a number of studies that document he importance of this interaction i communities. Positive interactions have been described for sessile animals in intertidal zones (Witman 1985; Lively and Raimondi 1987) and for plants in a variety of environments such as deserts (e.g., McAuliffe 1988; Franco and Nobel 1989), salt marshes (Bertness 1991; Bertness and Shumway 1993; Callaway 1994), California oak woodlands (Callaway et al. 1991; Callaway 1992), and Great Lakes sand dunes (Kellman and Kading 1992; see review in Callaway 1995). Plants acting as facilitators may lower the soil surface temperature (Franco and Nobel 1989; Evans and Cabin 1995), decrease evaporation and hence soil salinity beneath their canopies (Bertness and Shumway 1993), offer protection from herbivores (McAuliffe 1984), or increase soil nutrients beneath their canopies (Callaway et al. 1991). Recent research presuming multiple, interacting causal mechanisms for community structure has demonstrated that facilitation and interference may operate simultaneously and that he overall effect of one species on another may vary among different microhabitats as mechanisms shift in relative importance (Callaway et al. 1991; Bertness and Hacker 1994; Bertness and Yeh 1994; Chapin et al. 1994). Complex combinations of negative and positive interactions operating simultaneously suggest hat current conceptual models of interplant interactions based on resource competition alone are limited in their potential for accurately depicting processes in natural plant communities. Inan attempt o integrate biotic and abiotic factors into one model, researchers have hypothesized (Bertness and Callaway 1994) that the relative importance of interference and facilitation may vary inversely along gradients of abiotic stress; the importance of facilitation is predicted to increase, and the importance of interference is predicted to decrease along a gradient of increasing abiotic stress. Here we provide the first est of this hypothesis. In 1993, we observed positive spatial associations between a rare endemic mustard, Lesquerella carinata var. languida (Brassicaceae), and bunchgrasses on the xeric, rocky slopes of the Garnet Range in western Montana; however, the association was evident at some sites and not at others. To expand our understanding of both the role of positive interactions in ameliorating abiotic stress in
TL;DR: It is demonstrated that interspecific interactions are extremely weak, relative to intraspecific ones, and that the spatial arrangement of species and individuals within them is critical to the observed dynamics.
Abstract: We develop statistical methods appropriate for the analysis of spatially structured population data. The methods are used to study the structure and dynamics of a four-species annual plant guild recorded in 1,000 permanent squares over a 10-yr period. We parameterize models that predict population density from one year to the next. In agreement with theoretical expectation all the models have locally stable equilibria, and overcompensation is rare. We demonstrate that interspecific interactions are extremely weak, relative to intraspecific ones, and that the spatial arrangement of species and individuals within them is critical to the observed dynamics. The impact of spatial density-dependent population growth on observed densities was calculated. In 52% of the cases population size would have been increased by at least a factor of 1.5 had there been no interactions between individuals, and in 9% of these it would have increased by a factor of four or more. This effect is shown to be largely a result of i...
TL;DR: A new framework for defining the conditions that sustain natural ecosystems is presented and principles to apply to sustainability of managed ecosystems are applied.
Abstract: Many natural ecosystems are self-sustaining, maintaining a characteristic mosaic of vegetation types for hundreds to thousands of years. In this article we present a new framework for defining the conditions that sustain natural ecosystems and apply these principles to sus- tainability of managed ecosystems. A sustainable ecosystem is one that, over the normal cycle of disturbance events, maintains its characteristic diversity of major functional groups, produc- tivity, and rates of biogeochemical cycling. These traits are determined by a set of four "interac- tive controls" (climate, soil resource supply, major functional groups of organisms, and distur- bance regime) that both govern and respond to ecosystem processes. Ecosystems cannot be sustained unless the interactive controls oscillate within stable bounds. This occurs when nega- tive feedbacks constrain changes in these controls. For example, negative feedbacks associated with food availability and predation often constrain changes in the population size of a species. Linkages among ecosystems in a landscape can contribute to sustainability by creating or ex- tending the feedback network beyond a single patch. The sustainability of managed systems can be increased by maintaining interactive controls so that they form negative feedbacks within ecosystems and by using laws and regulations to create negative feedbacks between ecosystems and human activities, such as between ocean ecosystems and marine fisheries. Degraded ecosys- tems can be restored through practices that enhance positive feedbacks to bring the ecosystem to a state where the interactive controls are commensurate with desired ecosystem characteris- tics. The possible combinations of interactive controls that govern ecosystem traits are limited by the environment, constraining the extent to which ecosystems can be managed sustainably for human purposes.
TL;DR: The life-history phenotypes of guppies from the north slope communities are compared and it is found that the high- and low-predation contrasts are remarkably similar to those reported earlier for the south slope communities.
Abstract: In earlier publications, we reported an association between the life-history patterns of guppies and the types of predators with which they co-occur. We contrasted guppies from high-predation sites (Crenicichla localities) with those from low-predation sites (Rivulus localities) found on the south slope of the Northern Range Mountains of Trinidad. Guppies from high-predation localities attain maturity at an earlier age and smaller size, produce more and smaller offspring per litter, and have higher reproductive allotments than their counterparts from low-predation sites. Here we present a parallel series of analyses for guppies from a new series of localities on the north slope of the Northern Range. These fish are also found in what appear to be high- and low-predation communities, but, with one exception, the species of predators are entirely different from those on the south slope. The larger predators are derived from marine families (gobies and mullets) that have invaded freshwater rivers; the south ...
TL;DR: Evaluated mating systems and effective pollen dispersal for three hermaphroditic, insect-pollinated Neotropical tree species, Calophyllum longifolium, Spondias mombin, and Turpinia occidentalis, all of which occurred at low adult densities at the study site.
Abstract: Studies of mating patterns of tropical trees, typically involving common species, have revealed that most species are outcrossed and that, in some cases, a significant fraction of outcross pollen moves long distances. We evaluated mating systems and effective pollen dispersal for three hermaphroditic, insect-pollinated Neotropical tree species, Calophyllum longifolium, Spondias mombin, and Turpinia occidentalis, all of which occurred at low adult densities at the study site. Mating patterns were estimated for each maternal tree within 84-ha populations of C. longifolium and S. mombin in 1992 and 1993 and within a 50-ha population of T. occidentalis in 1993. Each population was 100% outcrossed. Multilocus paternity exclusion analyses indicated that in C. longifolium, a minimum of 62% of effective pollen moved at least 210 m. For S. mombin, estimates of apparent pollen flow greater than 300 m were 5.2% and 2 5% in 1992 and 1993, respectively. For all species, pollen dispersal was strongly affected by the sp...
TL;DR: It is shown that temporal variation in density-independent growth rates, partially uncorrelated across patches, favored dispersal, and that dispersal polymorphisms may be maintained in temporally and spatially heterogeneous environments.
Abstract: Dispersal-movement between populations-is a central feature in the biology of most organisms. There is an enormous literature on the ecology and evolution of dispersal (e.g., Swingland and Greenwood 1986). Many theoretical studies have explored factors favoring the evolution of dispersal, including competition among kin and inbreeding effects (e.g., Hamilton and May 1977; Comins 1982; Frank 1986; Taylor 1988; Wiener and Feldman 1991), the influences of extrinsically generated, spatiotemporal heterogeneity (Gadgil 1971; Roff 1975; Metz et al. 1983; Levin et al. 1984; Cohen and Levin 1991), and the interplay of withinpopulation and between-population selection (Kuno 1981; Olivieri et al. 1995). It has been demonstrated (Hastings 1983; Holt 1985) that if individuals disperse at fixed per capita rates between sites with local density dependence, then, without emporal heterogeneity, spatial heterogeneity inabundance alone is unable to select for dispersal (see also Liberman and Feldman 1989). The reason is that if habitats vary in carrying capacity, K, there is an asymmetric flow of individuals from high-K to low-K patches (Holt 1985). Such flow, in turn, reduces density in high-K patches (increasing fitness there) while increasing density in low-K patches (depressing fitness there). Because dispersal is basically moving individuals down gradients in fitness, on average, dispersal is disfavored in spatially (but not temporally) heterogeneous environments. These theoretical results highlight the importance of temporal heterogeneity infavoring dispersal. In a metapopulation ofopen patches, if the rank order of fitness among patches varies through time, dispersal can be selectively advantageous (Bull et al. 1987). Dispersal in effect provides an evolutionary strategy that permits individuals to exploit spatiotemporal variation in fitness. The theoretical expectation of a relation between temporal variability and dispersal matches ome data from natural populations (Roff 1990). We previously examined a simple two-patch, discrete-generation model in which individuals dispersed at constant rates between two patches and experienced density dependence in each patch (McPeek and Holt 1992). We showed that temporal variation in density-independent growth rates, partially uncorrelated across patches, favored dispersal. Moreover, a polymorphism indispersal rates could be stably maintained if the two patches were heterogeneous (with different carrying capacities). Other authors have also shown that dispersal polymorphisms may be maintained in temporally and spatially heterogeneous environments (Frank 1986; Cohen and Levin 1991; Karlson and Taylor 1992). In this note, we demonstrate that chaotic population dynamics in otherwise
TL;DR: The relationship between sexual dichromatism and the use of carotenoids in plumage suggests that sexual selection may have promoted the expression of a condition-dependent honest indicator of phenotypic quality.
Abstract: Sexual dichromatism and the extent of male carotenoid-derived plumage coloration in breeding season North American passerine birds was analyzed by phylogenetic statistical methods. Passerines as a whole and five passerine subclades were analyzed by both independent contrasts and simulation methods. In passerines as a whole, carotenoids and sexual dichroma- tism are positively correlated irrespective of analysis method. In three of the subclades, the correlations are significant by a clear majority of analysis methods. Neither melanin nor structur- ally derived colors show similar significant increases with dichromatism. Carotenoids are ob- tained by animals solely through the diet, whereas both melanins and structurally derived colors can be synthesized. The relationship between sexual dichromatism and the use of carotenoids in plumage suggests that sexual selection may have promoted the expression of a condition- dependent honest indicator of phenotypic quality.
TL;DR: M moth and host biology is used in a phylogenetic framework to establish hypotheses for the evolution of active pollination and reciprocal specialization in the form of functional nectarlessness in yuccas.
Abstract: The obligate pollination mutualisms between yucca moths and yuccas are some of the most obvious cases of coevolution, but the phylogenetic origins and extent of coevolution in these interactions are little understood. Ecological and phylogenetic information from the yucca moth family, Prodoxidae, shows that pollination has evolved at least three times from separate moth behaviors. Passive pollination occurs in Greya during nectarine by one species and during oviposition by two other species. Active pollination among prodoxids has evolved only once, in the yucca moths. Several life-history traits necessary for the evolution of obligate mutualism are common traits within the Prodoxidae, and only active pollination and modified mouthparts appear to have been novel traits in the yucca moths. We use moth and host biology in a phylogenetic framework to establish hypotheses for the evolution of active pollination and reciprocal specialization in the form of functional nectarlessness in yuccas.