Showing papers in "The American Naturalist in 1998"

Why trees migrate so fast: confronting theory with dispersal biology and the paleorecord.

Abstract: Reid's paradox describes the fact that classical models cannot account for the rapid ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} ewcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} ormalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $$10^{2}\,- \,10^{3}\,\mathrm{m\,yr}\,^{- 1}$$ \end{document} ) spread of trees at the end of the Pleistocene. I use field estimates of seed dispersal with an integrodifference equation and simulation models of population growth to show that dispersal data are compatible with rapid spread. Dispersal estimates lay to rest the possibility that rapid spread occurred by diffusion. The integro...

The statistical inevitability of stability-diversity relationships in community ecology.

Abstract: In this article, we explain an often overlooked process has revitalized interest in stability-diversity relationships that may significantly contribute to positive correlations between and the ecological mechanisms that cause them. How- measures of species diversity and community stability. Empirical studies showing positive stability-diversity relationships have, for ever, while this and previous studies have supplied im- the most part, used a single class of stability (or, more accurately, portant documentation of stability-diversity correlations, instability) measures: the temporal variation in aggregate commu- the range of processes creating these relationships still re- nity properties such as biomass or productivity. We show that for mains unclear. In particular, from its beginnings until the these measures, stability will essentially always rise with species di- present, the ecological literature on stability-diversity versity because of the statistical averaging of the fluctuations in patterns has overlooked the possibility that random fluc- species' abundances. This simple probabilistic process will operate tuations in the abundances of individual species may be in the absence of any strong species interactions, although its strength is driven by the relative abundances of species, as well as as or more important than species interactions in creat- by the existence of positive or negative correlations in the fluctua- ing stability-diversity relationships. However, as we show tions of species. To explore the possible importance of this effect here, some stability-diversity correlations, including all in real communities, we fit a simple simulation model to Tilman's those clearly documented in the field, are at least partially grassland community. Our results indicate that statistical averaging explainable as the outcomes of probabilistic processes in- might play a substantial role in explaining stability-diversity corre- volving fluctuations in species abundances and would lations for this and other systems. Models of statistical averaging arise to greater or lesser extents even in the complete ab- can serve as a useful baseline for predictions of community stabil- ity, to which the influences of both negative and positive species sence of ecological interactions. An understanding of this interactions may then be added and tested. statistical averaging process is useful because it both makes the very general prediction that stability-diversity

Experimental Tests of the Dependence of Arthropod Diversity on Plant Diversity

Abstract: Because a diversity of resources should support a di- versity of consumers, most models predict that increasing plant di- 1998) have found that increasing plant diversity increases versity increases animal diversity. We report results of a direct ex- arthropod herbivore diversity. However, these studies are perimental test of the dependence of animal diversity on plant confounded by changes in plant community composition diversity. We sampled arthropods in a well-replicated grassland ex- that correlate with changes in plant diversity. The rela- periment in which plant species richness and plant functional rich- tionship between plant diversity and herbivore diversity ness were directly manipulated. In simple regressions, both the may be nonlinear because herbivore loads are often lower number of species planted (log2 transformed) and the number of in polycultures due to differences in the ability of herbi- functional groups planted significantly increased arthropod species richness but not arthropod abundance. However, the number of vores to locate host plants in mixed stands, suitability of species planted was the only significant predictor of arthropod smaller patches of host plants, and/or differences in the species richness when both predictor variables were included in effects of parasites and predators (reviewed in Andow ANOVAs or a MANOVA. Although highly significant, arthropod 1991). species richness regressions had low R 2

Allee Effects Limit Population Viability of an Annual Plant

Abstract: Allee effects may be experienced by plants when populations are too small or isolated to receive sufficient pollinator services to replace themselves. This article reports experimental data from an annual herb, Clarkia concinna, documenting that small patches suffered reproductive failure due to lack of effective pollination when critical thresholds of isolation were exceeded. In contrast, sufficiently large patches attracted pollinators regardless of their degree of isolation. These data accord with data on patch extinctions showing that small and isolated patches have a higher extinction rate than do large patches and with observations showing chronically low reproductive success in such patches prior to extinction. While not conclusively demonstrating that Allee effects cause extinction in small and isolated patches, the data are suggestive. Although threshold effects have been postulated in several mathematical models of population viability, this is the first report of data from natural populations that display the occurrence of such thresholds. These results have implications for the management of endangered plants, which often are restricted to isolated, small populations, as well as suggesting a potential limit to spatial spread in plant populations dependent on animal vectors for reproduction.

Why More Productive Sites Have More Species: An Experimental Test of Theory Using Tree‐Hole Communities

Abstract: One of the most common explanations for an increase in species richness with productivity is what we have dubbed the "More Individuals Hypothesis." According to this hypothesis, more productive sites can support higher total abundances and, since species richness is an increasing function of total abundance, so will it be of productivity. This hypothesis assumes that communities are limited by productivity. We tested the More Individuals Hypothesis using the detritivorous aquatic insect communities of tree holes. When tree holes with varying levels of productivity (debris amount) were allowed to be colonized (through oviposition), more productive tree holes did have more species but not more individuals. Neither was total energy use strictly proportional to productivity. Only in communities forced to disassemble through productivity reductions were the predictions of the More Individuals Hypothesis satisfied. Ovipositing adults may prefer productive tree holes not because they contain more resources but because they are anticipated to be less likely to dry out. In tree holes, and more generally, the More Individuals Hypothesis is an insufficient explanation for increases in species richness with productivity because it neither accounts for the different processes of local colonization and extinction nor allows body size to correlate with extinction risk.

Diversity-stability relationships: statistical inevitability or ecological consequence?

Abstract: Department of Ecology, Evolution, and Behavior, 1987 Upper logical interactions, and for previous data (in Tilman Buford Circle, University of Minnesota, St. Paul, Minnesota 1996), the relationship between variance in species abun55108 dances and mean species abundances is such that there is little or no net effect of diversity on stability via statistical Submitted June 16, 1997; Accepted October 7, 1997 averaging. However, interspecific interactions, such as competition, that cause compensatory changes in the abundances of species can stabilize community biomass in these cases.

Bilateral symmetry and sexual selection: a meta-analysis.

Abstract: A considerable body of primary research has accumulated over the last 10 yr testing the relationship between developmental instability in the form of fluctuating asymmetry and performance of individuals in mating success itself or sexual attractiveness. This research comprises 146 samples from 65 studies of 42 species of four major taxa. We present the results of a meta-analysis of these studies, which demonstrates that there is indeed an overall significant, moderate negative relationship: for studies, the overall mean Pearson's r or effect size = -.42, P <.0005; for species, the overall mean r = -.34, .01 < P < .025. Based on calculated fail-safe numbers, the effect-size estimates are highly robust against any publication or reporting bias that may exist. There is considerable evidence that the magnitude of the negative correlation between fluctuating asymmetry and success related to sexual selection is greater for males than for females, when a secondary sexual trait rather than an ordinary trait is studied, with experimentation compared with observation, and for traits not involved with mobility compared with traits affecting mobility. There is also limited evidence that higher taxa may differ in effect size and that intensity of sexual selection negatively correlates with effect size.

Annual variability in seed production by woody plants and the masting concept: reassessment of principles and relationship to pollination and seed dispersal.

Abstract: By analyzing 296 published and unpublished data sets describing annual variation in seed output by 144 species of woody plants, this article addresses the following questions. Do plant species naturally fall into distinct groups corresponding to masting and nonmasting habits? Do plant populations generally exhibit significant bimodality in annual seed output? Are there significant relationships between annual variability in seed production and pollination and seed dispersal modes, as predicted from economy of scale considerations? We failed to identify distinct groups of species with contrasting levels of annual variability in seed output but did find evidence that most polycarpic woody plants seem to adhere to alternating supra-annual schedules consisting of either high or low reproduction years. Seed production was weakly more variable among wind-pollinated taxa than animal-pollinated ones. Plants dispersed by mutualistic frugivores were less variable than those dispersed by either inanimate means or animals that predominantly behave as seed predators. We conclude that there are no objective reasons to perpetuate the concept of mast fruiting in the ecological literature as a shorthand to designate a distinct biological phenomenon. Associations between supra-annual variability in seed output and pollination and seed dispersal methods suggest the existence of important reproductive correlates that demand further investigation.

Predicting Population Trends from Size Distributions: A Direct Test in a Tropical Tree Community

Abstract: Forest ecologists often evaluate how well the species composition of saplings in the understory matches that of the canopy: absence of juveniles suggests that a tree species is suffering population decline. Here we offer a theoretical and empirical test of this assertion using data from a 50-ha census plot in panama. Theory indicates that higher rates of population change, lambda, lead to more steeply declining size distributions (more juveniles relative to adults). But other parameters also affect the size distribution: lower growth rate of juveniles and lower survival at any size produce more steeply declining size distributions as well. Empirical evaluation of 216 tree populations showed that juvenile growth was the strongest predictor of size distribution, in the direction predicted by theory. Size distribution did correlate with population growth, but weakly and only in understory species, not canopy species. Size distribution did not correlate with the growth rate of larger individuals nor with survival. Results suggest that static information on the size distribution is not a good predictor of future population trends, while demographic information is. Fast-growing species will have fewer juveniles in the understory than slow-growing species, even when population growth is equal.

Mass‐ and Density‐Dependent Reproductive Success and Reproductive Costs in a Capital Breeder

Abstract: For capital breeders, mass may affect reproductive potential. Reproductive expenditure may reduce future reproductive potential, particularly when resources are scarce. To test the hypothesis that reproductive success and the costs of reproduction vary according to mass and population density, we analyzed 25 yr of data on bighorn ewes (Ovis canadensis). The number of adult females was first limited by yearly removals, then allowed to triple. We found no survival costs of reproduction for ewes aged 4–7 yr. For ewes aged 8–14 yr, survival was density dependent for barren ewes but not for ewes that weaned lambs. Failure to lamb was rare and negatively correlated with fertility the following year. At low population density, lactation had a negative effect on mass gain but had a limited reproductive cost. At high density, heavy ewes had higher reproductive success than light ewes, and the reproductive cost and somatic costs of reproduction increased. The cost of reproduction was greater for light than...

Costs of resistance to natural enemies in field populations of the annual plant Arabidopsis thaliana.

Abstract: The annual plant Arabidopsis thaliana is widely used as a model system in molecular genetics, but little is known about populations in the field. In this experimental field study of natural populations of Arabidopsis, I tested the assumption that plant resistance has fitness costs. Models of the evolution of resistance assume a cost, which is envisioned as a reduction in fitness in the absence of natural enemies, such as insect herbivores and pathogens. The presumed basis of this cost is the diversion of limiting resources away from present and future growth and reproduction. Recent failures to detect allocation costs of resistance to herbivores have raised questions about whether costs exist and, thus, about the appropriateness of theories that postulate such costs. I found genetic variation for two traits commonly thought to function as resistance characters: trichome density and total glucosinolate concentration. Under field conditions, these characters both reduced damage by the natural assem...

Direct and indirect effects of predation and predation risk in old-field interaction webs.

Abstract: Indirect effects emerge when a change in the abundance of one species indirectly affects another by changing the abundances of intermediate species-called density-mediated indirect effects-or they arise when one species modifies how two other species interact-called trait-mediated indirect effects. I report on field experiments that evaluated how grass and herb biomass in old-field interaction webs was influenced indirectly by a spider carnivore through its interactions with a generalist and a grass-specialist grasshopper species. I manipulated interaction pathways between the spider and the plants using different combinations of the grasshopper species. I changed the modality of predator-prey interactions to isolate density-mediated from trait-mediated effects using natural spiders (predation spiders) or spiders that were prevented from subduing prey by mouthpart manipulation (risk spiders). I found that indirect effects were stronger in speciose, reticulate food webs than in linear food chains owing to a trait-mediated effect, a diet shift by herbivores in response to predation risk. Spiders alone did not have significant effects on grasshopper densities in the field experiments, removing any possibility of density-mediated indirect effects. The study illustrates that ecologists should not underestimate the importance of behavioral ecology in determining community-level interactions.

Sexual Conflict and the Energetic Costs of Mating and Mate Choice in Water Striders

Abstract: Analyses of intersexual conflicts of interest over courtship, mating, or mate guarding require an understanding of the physiological costs of sexual interaction. Repeated respirometric mea...

Food web stability: the influence of trophic flows across habitats.

Abstract: In nature, fluxes across habitats often bring both nu- ory, which plays a central role in consumer-resource in- trient and energetic resources into areas of low productivity from areas of higher productivity. These inputs can alter consumption teractions and food web dynamics. They further rates of consumer and predator species in the recipient food webs, suggested that multichannel omnivory can dampen or fa- thereby influencing food web stability. Starting from a well-studied cilitate trophic cascades. McCann and Hastings (1997) tritrophic food chain model, we investigated the impact of alloch- recently found that food web dynamics were stabilized by thonous inputs on the stability of a simple food web model. We weak to moderate amounts of trophic omnivory, one considered the effects of allochthonous inputs on stability of the component of multichannel omnivory. They did not, model using four sets of biologically plausible parameters that rep- however, examine the influence of other types of multi- resent different dynamical outcomes. We found that low levels of allochthonous inputs stabilize food web dynamics when species channel omnivory on food web dynamics. In this article, preferentially feed on the autochthonous sources, while either in- we address this problem by extending a simple food creasing the input level or changing the feeding preference to favor chain to include a different component of multichannel allochthonous inputs, or both, led to a decoupling of the food omnivory: allochthonous inputs (inputs entering from chain that could result in the loss of one or all species. We argue another habitat). We demonstrate that allochthonous that allochthonous inputs are important sources of productivity in sources can also stabilize food web dynamics, further many food webs and their influence needs to be studied further. suggesting that donor control may be an important fac- This is especially important in the various systems, such as caves, headwater streams, and some small marine islands, in which more tor in community dynamics and that trophic cascades energy enters the food web from allochthonous inputs than from may be weakened in systems that have relatively large al- autochthonous inputs. lochthonous inputs. The movement of resources across habitat boundaries

Evolutionary theories of aging: confirmation of a fundamental prediction, with implications for the genetic basis and evolution of life span.

Abstract: Evolutionary considerations predict that rate of aging should vary in direct relation to the mortality rate of presenescent young adults (extrinsic mortality rate) independently of differences in physiology, such as rate of metabolism. This prediction emerges from theory irrespective of the particular genetic mechanisms responsible for variation in aging. Yet this critical relationship has not been confirmed in comparative studies of natural populations. In the present analysis, rate of aging is estimated by the rate of increase in mortality rate (mx) with age (x). Comparisons between natural and captive populations of birds suggest that the Weibull model (mx = m0 + alphaxbeta) provides a better description of aging than the Gompertz model (mx = m0esigmax). Rate of aging is quantified by the parameter omega (dimension: 1/time), which is calculated from the Weibull parameters alpha and beta (omega = alpha1/(beta+1)). In this analysis, rate of aging in birds and mammals is directly related to extrinsic mortality (estimated by the initial mortality rate, m0) independently of taxonomic group and of variation in body size and, by implication, metabolic rate. When time is expressed in years, rate of senescence is related to initial mortality rate by omega = 0.294m0(0.367). This result implies that natural selection in response to variation among taxa in m0 has resulted in the evolutionary modification of factors that influence the rate of aging in natural populations. The potential strength of selection on factors that could further reduce rate of aging is indicated by the proportion of deaths due to aging-related causes. Although species with low initial mortality rates also exhibit reduced rates of increase in mortality rate with age (i.e., delayed senescence), the relatively high proportion of aging-related deaths in such species suggests that further evolutionary responses leading to long life are severely constrained. This argues against mutation accumulation and antagonistic pleiotropy as genetic mechanisms underlying senescence and suggests, instead, that rate of aging represents a balance between wear and tear, on the one hand, and genetically controlled mechanisms of prevention and repair, on the other. Evidently, remedies for extreme physiological deterioration in old age either are not within the range of genetic variation or are too costly to be favored by selection.

Leaf size, sapling allometry, and Corner's rules: phylogeny and correlated evolution in maples (Acer).

Abstract: We studied the evolution of leaf size, sapling canopy allometry, and related traits in 17 Acer species growing in the understory of temperate deciduous forests, using parsimony methods, randomization tests, and independent contrasts calculated on a phylogeny inferred from nuclear ribosomal internal transcribed spacer (ITS) sequences. Bivariate correlations and multivariate analyses indicated two independent suites of coevolving traits, and the results were robust over a range of alternative phylogenies. The first suite consisted of strong positive correlations among twig thickness, leaf size, inflorescence length, and branch spacing (Corner's rules). Seed size and mature height were also weakly correlated with these traits. The second suite reflected aspects of sapling crown allometry, including crown size, stem diameter, and total leaf area, which appear to be related to shade tolerance. There was a weak negative correlation between sapling crown size and mature height, but no correlation with leaf or seed size. Most correlations were similar in magnitude for ahistorical and independent contrasts analyses, and discrepancies between these two measures were greater in traits with lower levels of convergent evolution. The evolutionary correlations among twig, leaf, seed, inflorescence, and canopy dimensions emphasize the need for integrated theories of evolution and function of these disparate traits.

Predator-mediated plasticity in morphology, life history, and behavior of Daphnia: the uncoupling of responses

Abstract: We studied the way 12 traits responded to fish kairo- mones in a set of 16 Daphnia magna clones derived from four dif- history traits, and behavior of prey species (reviewed by ferent habitats—two where daphnids co-occur with fish and two Harvell 1990; Larsson and Dodson 1993). These preda- without fish. These clones differed widely in their response to tor-prey communication links have been found through- predator kairomones, with none of the clones showing a signifi- out aquatic food webs, including the relationship be- cant response in all traits and all clones showing a response for at tween piscivorous fish and planktivorous fish (e.g., least one trait. Most of the clones showed a significant response in Gliwicz and Jachner 1993; Nilsson et al. 1995), between one to four traits, with no evidence for an association between dif- planktivorous fish and zooplankton (e.g., Stibor 1992;

Optimal foraging, specialization, and a solution to Liem's paradox.

Abstract: Species that appear highly specialized on the basis of their phenotype (e.g., morphology, behavior, and physiology) also sometimes act as ecological generalists. This apparent paradox has been used to argue against the importance of competition as a diversifying evolutionary force. We provide an alternative explanation based on optimal foraging theory. Some resources are intrinsically easy to use and are widely preferred, while others require specialized phenotypic traits on the part of the consumer. This asymmetry allows optimally foraging consumers to evolve phenotypic specializations on nonpreferred resources without greatly compromising their ability to use preferred resources. The evolution of phenotypic specialization on nonpreferred resources can be driven by competition, but the specialists act as ecological generalists whenever their preferred resources are available. Our model identifies at least three different concepts of specialization that need to be distinguished, based on diet, prey utilization efficiencies, and phenotypic adaptations. The relationships among these concepts are complex and often counterintuitive. Specialists should often reject the very resources that they have evolved traits to use. The most extreme phenotypic specializations should occur in the absence of a trade-off between using preferred and nonpreferred resources. Our model may explain why extreme phenotypic-specializations evolve more often in fish communities than in terrestrial vertebrate communities and provides a mechanism whereby species can coexist in stable communities despite common preferences for some resources.

Ecological biogeography of southern ocean islands: species-area relationships, human impacts, and conservation.

Abstract: Previous studies have concluded that southern ocean islands are anomalous because past glacial extent and current temperature apparently explain most variance in their species richness. Here, the relationships between physical variables and species richness of vascular plants, insects, land and seabirds, and mammals were reexamined for these islands. Indigenous and introduced species were distinguished, and relationships between the latter and human occupancy variables were investigated. Most variance in indigenous species richness was explained by combinations of area and temperature (56%)—vascular plants; distance (nearest continent) and vascular plant species richness (75%)—insects; area and chlorophyll concentration (65%)—seabirds; and indigenous insect species richness and age (73%)—land birds. Indigenous insects and plants, along with distance (closest continent), explained most variance (70%) in introduced land bird species richness. A combination of area and temperature explained most var...

The Influence of Dispersal and Diet Breadth on Patterns of Genetic Isolation by Distance in Phytophagous Insects

Abstract: To determine the effects of dispersal ability and diet with widespread gene flow (Slatkin 1973; Endler 1979; breadth on population-genetic structure, we reviewed the allozyme Hanks and Denno 1994; but see Mopper 1996). Simi- literature and estimated genetic isolation by distance (IBD) for 43 species/host races of phytophagous insects. Subsequently, we larly, it is typically argued that factors diminishing gene tested two opposing hypotheses regarding the influence of dis- flow among populations should facilitate the process of persal ability on IBD: that IBD slopes do not vary with mobility, speciation, since through their effect on gene flow, these but that intercepts increase with mobility, and, alternatively, that factors should permit the divergence of populations IBD slopes vary with dispersal ability. We found that from tens of (Mayr 1963; Bush 1975; Futuyma and Mayer 1980; Rice kilometers to more than 1,000 km, IBD is weak in sedentary and and Hostert 1993; Wiegmann et al. 1993). highly mobile species but pronounced in moderately mobile spe- A key to understanding the degree to which gene flow cies. We attribute the weak IBD in strong dispersers to the homog- enizing effects of gene flow, whereas in sedentary species, limited limits both the evolution of local adaptations and the gene flow allows nearly all populations to diverge. In intermediate formation of new species lies in understanding not only dispersers, genetic homogeneity is achieved at small spatial scales, the amount of gene flow typically occurring among natu- but limited dispersal promotes genetic divergence over long dis- ral populations but also what factors govern levels of tances. We also tested the hypothesis that IBD increases with de- gene flow. The list of factors that have been hypothesized creasing diet breadth. We discovered no such pattern, casting to influence gene flow is extensive and includes geo- doubt on the supposition that specialization promotes speciation graphic distance (Wright 1943; Kimura and Weiss 1964; by influencing population-genetic subdivision. Finally, we found that the number of populations is a more important consideration Maruyama 1971; Nagylaki 1976; Slatkin 1993; Hellberg than the number of polymorphic loci in studies of IBD. 1994; Peterson and Denno 1998), dispersal capability (Waples 1987; Govindaraju 1988; Palumbi 1992; Wil-

Models suggesting field experiments to test two hypotheses explaining successional diversity.

Abstract: A simple mathematical model of competition is devel- oped that includes two alternative mechanisms promoting succes- turbed sites before the dominant competitors arrive. In sional diversity. The first underpins the competition-colonization addition, rapid growth under resource-rich conditions hypothesis in which early successional species are able to persist allows them temporarily to outperform late successional because they colonize disturbed habitats before the arrival of late species even if both are present in a recently disturbed successional dominant competitors. The second underpins the site. We refer to the first mechanism as the ''colonizing niche hypothesis, in which early successional species are able to advantage'' and the second as the ''successional niche.'' persist, even with unlimited colonization by late successional dom- These mechanisms often operate together but do not inants, because they specialize on the resource-rich conditions typ- ical of recently disturbed sites. We modify the widely studied necessarily do so. competition-colonization model so that it also includes the mech- For example, after ploughing at Cedar Creek (Minne- anism behind the niche hypothesis. Analysis of this model suggests sota), old fields are initially dominated by annuals with simple experiments that determine whether the successional diver- high fecundity and rapid growth such as Ambrosia artem- sity of a field system is maintained primarily by the competition- isiifolia. These species are subsequently displaced by pe- colonization mechanism, primarily by the niche mechanism, by rennials, such as Schizachyrium scoparium, that have slow neither, or by both. We develop quantitative metrics of the relative growth and low fecundity and are capable of creating and importance of the two mechanisms. We also discuss the implica- tions for the management of biodiversity in communities struc- tolerating low levels of soil nitrogen (species with low R*; tured by the two mechanisms. Tilman 1982). Ambrosia tends to arrive before Schizachy- rium and initially outgrows it (Tilman 1994), but Ambro-

Allee effects in metapopulation dynamics.

Abstract: Department of Ecology and Evolutionary Biology, University of where p is the fraction of habitats occupied by the species California, Irvine, California 92697-2525 in question, m is the colonization rate of empty patches, and e is the extinction rate of occupied patches. The Submitted November 4, 1997; Accepted February 17, 1998 nontrivial equilibrium p* 5 1 2 (e/m) is globally stable as long as m . e. The Levins model is directly analogous to the logistic model dN/dt 5 rN[1 2 (N/K)] (Gilpin and Hanski

The ghost of genetic diversity past : historical dna analysis of the greater prairie chicken

Abstract: Most, if not all, of the ''classic,'' often-cited examples illustrating the genetic effects of a population bottleneck are open population can be directly linked to a population bottle- to alternative explanations due to the lack of adequate control neck by comparing levels of genetic variability prior to populations, that is, low levels of genetic variability are often as- and following a demographic contraction. sumed to be the result of a past population bottleneck without The greater prairie chicken is a grassland/prairie spe- having any prebottleneck measures. Here we provide the first clear cies with limited dispersal and a lek mating system. Dur- case history where both prebottleneck and postbottleneck mea- ing the last century, populations have become increas- sures of genetic variability have been collected from a natural sys- ingly affected by the loss of natural habitats because of tem. Analysis of DNA from museum specimens of the greater prairie chicken Tympanuchus cupido from central Illinois revealed human activities. In the state of Illinois, greater prairie the loss of specific alleles (known to have been present earlier in chicken populations, estimated to include millions of in- this century) following a demographic contraction. Lost alleles in- dividuals in the 1860s, began to decline dramatically cluded common ones present in all other populations sampled and from an estimated 25,000 birds in 1933 to 2,000 in 1962, others unique to the Illinois population. 500 in 1972, 76 in 1990, and to less than 50 in 1993

Long-term dynamics in a metapopulation of the American pika.

Abstract: A 20‐yr study of a metapopulation of the American pika revealed a regional decline in occupancy in one part of a large network of habitat patches. We analyze the possible causes of this decline using a spatially realistic metapopulation model, the incidence function model. The pika metapopulation is the best‐known mammalian example of a classical metapopulation with significant population turnover, and it satisfies closely the assumptions of the incidence function model, which was parameterized with data on patch occupancy. The model‐predicted incidences of patch occupancy are consistent with observed incidences, and the model predicts well the observed turnover rate between four metapopulation censuses. According to model predictions, the part of the metapopulation where the decline has been observed is relatively unstable and prone to large oscillations in patch occupancy, whereas the other part of the metapopulation is predicted to be persistent. These results demonstrate how extinction‐coloni...

A latitudinal gradient in northeast Pacific intertidal community structure: evidence for an oceanographically based synthesis of marine community theory.

Abstract: Intertidal systems have been models for the study of the roles of competition, predation, and disturbance in determining community structure. These systems exhibit considerable regional va...

Differentiation of allelic frequencies at quantitative trait loci affecting locally adaptive traits.

Abstract: σ 2 t 5 (1 1 Fst)σ 0 , Direct comparisons of population differentiation obwhere σ w, σ b, and σ t are the within-population, beserved in neutral loci with that seen in quantitative traits tween-population, and total genetic variances in the trait, have revealed discordant patterns, most commonly that respectively, and σ 0 is the genetic variance for the trait quantitative traits are more differentiated than single expected if the populations formed a single panmictic neutral loci. For example, body size has a greater prounit (Wright 1951, 1952; Lande 1992). Thus an estimate portion of genetic variation attributable to amongof population differentiation for quantitative traits population differences than do presumed neutral allo(termed Qst by Spitze 1993) can be computed as zymes in both Drosophila buzzatii (Prout and Barker

Levels of Mate Recognition Within and Between Two Drosophila Species and Their Hybrids

Abstract: If sexual selection is to result in speciation, traits involved in mate choice within species need to be capable of producing sexual isolation between species. We investigated the association between mate choice and sexual isolation using interspecific hybrids between two sibling species, Drosophila serrata and Drosophila birchii. A perfuming experiment demonstrated that olfaction was involved in the sexual isolation between the two species. A quantitative genetic analysis using 30 populations of hybrids between the two species indicated that mating success in hybrid individuals was predominately determined by cuticular hydrocarbons; the average genetic correlation between mating success and cuticular hydrocarbon profile was 0.84, and in some instances exceeded 0.95. Multivariate analysis of the cuticular hydrocarbons of the two species revealed that there were three independent blends of cuticular hydrocarbons that separated three levels of organization: species, sex, and sex within species. The hydrocarbons used by hybrids in mate choice included those that separated the two species, demonstrating that species-specific characters may be used in mate choice within populations. The interspecific reciprocal cross had major effect on which cuticular hydrocarbons were associated with mating success, indicating that the expression of the cuticular hydrocarbons was strongly sex linked.

Effects of disturbance on species diversity: a multitrophic perspective.

Abstract: Models of the effects of disturbance on ecological mediate intensity enhance diversity by reducing competi- communities have largely considered communities of competing tive exclusion because they prevent competitively supe- species at a single trophic level. In contrast, most real communities have multiple interacting trophic levels. I explored several versions rior species from attaining population sizes that are large of simple single- and multitrophic models to determine whether enough to monopolize all of the limiting resources. At predictions of the intermediate disturbance hypothesis (IDH), de- low levels of disturbance, one species can effectively mo- rived from considering only a single trophic level, apply to multi- nopolize limiting resources, leading to reduced diversity trophic situations. The IDH was predicted by models of competing because of competitive exclusion. At high levels of distur- species at a single trophic level but did not hold in many situations bance, species diversity declines because some species with more natural trophic structure. In general, basal species in a cannot reproduce fast enough to compensate for the in- food web tended to follow the IDH, whereas competitors at top trophic levels did not. Additional analyses indicated that outside creased mortality imposed by the disturbance. immigration interacted with trophic structure to produce widely Many studies now suggest that intermediate distur- differing predictions about the consequences of disturbance and bance levels can enhance diversity. For example, interme- that density-dependent disturbance events could recapture the diate levels of windfalls, fire, and animal digging in the IDH in some multiple trophic level situations. Model predictions soil enhance terrestrial plant diversity (Platt 1975; Keeley matched the results of empirical studies to date: the IDH has gen- et al. 1981; Sprugel and Bormann 1981; Sousa 1984; erally been supported for species competing for nondynamic basal Denslow 1985). Similarly, wave disturbance, rolling boul- resources but not for mobile aquatic invertebrates at higher tro- phic levels. The model analysis also verified basic predictions of ders, and consumers enhance the diversity of sessile in- verbal models addressing the effects of physical stress. Three differ- tertidal organisms (Paine 1966; Paine and Vadas 1969; ent aspects of disturbance and their contributions to species coex- Dayton 1971; Lubchenco 1978; Sousa 1979; Paine and istence were identified: changes in average mortality rates, changes Levin 1981; Dethier 1984). Indeed, the intermediate dis- in temporal variability, and changes in spatial heterogeneity. The turbance hypothesis has become sufficiently established results indicate that the IDH should be applied with caution to in ecological thought that it now exerts a major influence real multitrophic communities. on many management plans (e.g., Yellowstone National

The Evolution of Coloniality in Birds in Relation to Food, Habitat, Predation, and Life‐History Traits: A Comparative Analysis

Abstract: Coloniality in birds has been intensively studied under the cost and benefit approach, but no general conclusion can be given concerning its evolutionary function. Here, we report on a comparative analysis carried out on 320 species of birds using the general method of comparative analysis for discrete variables and the contrast method to analyze the evolution of coloniality. Showing a mean of 23 convergences and 10 reversals, coloniality appears to be a rather labile trait. Colonial breeding appears strongly correlated with the absence of feeding territory, the aquatic habitat, and nest exposure to predators but was not correlated with changes in life‐history traits (body mass and clutch size). The correlation of coloniality with the aquatic habitat is in fact explained by a strong correlation with the marine habitat. Unexpectedly, we found that the evolution toward a marine habitat in birds was contingent on coloniality and that coloniality evolved before the passage to a marine life. These res...

Paternity and multiple signaling: effects of a secondary sexual character and song on paternity in the barn swallow.

Abstract: Multiple signals may evolve because they provide independent information on the condition of a signaler. Females should pay attention to male characters relative to their reliability as signals of male attractiveness or quality. Since behavioral traits are flexible and, therefore, subject to strong environmental influences, females should weigh stable morphological signals higher in their choice of mates for genetic benefits than flexible behavioral traits, for example, by paying particular attention to phenotypically plastic traits when produced in combination with an exaggerated morphological signal. Consistent with this prediction, female barn swallows Hirundo rustica, which are known to prefer males with the longest tail feathers (a secondary sexual character), also preferred males with extreme expressions of a behavioral trait (song rate), as determined from patterns of paternity assessed by microsatellites. However, a statistical interaction between tail length and song rate implied that so...