Showing papers in "The American Naturalist in 1987"

Community regulation: variation in disturbance, competition, and predation in relation to environmental stress and recruitment

Abstract: We present a model of community regulation that incorporates the effects of abiotic disturbance, predation, competition, and recruitment density. We assume that mobile organisms (i.e., consumers) are more strongly affected by environmental stress than are sessile organisms and that food-web complexity decreases with increasing stress. The model makes three predictions under conditions of high recruitment. First, in stressful environments, consumers have no effect because they are absent or inactive, and competition for space is prevented. Both mobile and sessile organisms are regulated directly by environmental stress. Second, in moderate environments, consumers are still ineffective, but sessile organisms are less affected by stress and frequently attain high densities, leading to competition for space. Finally, in benign environments, consumers prevent competition for space unless the prey can escape a predation bottleneck and reach a high abundance. A reduction in recruitment density reduces the import...

The Relative Rates of Evolution of Sex Chromosomes and Autosomes

Abstract: We develop models of the rates of evolution at sex-linked and autosomal loci and of the rates of fixation of chromosomal rearrangements involving sex chromosomes and autosomes. We show that the substitution of selectively favorable mutations often proceeds more rapidly for X- or Y-linked loci than for the autosomes, provided that mutations are recessive or partially recessive on the average. Selection acting on a quantitative character is expected to result in similar long-term rates of gene substitution for X-linked and autosomal loci, unless there is strong directional dominance. Short-term responses to such selection often preferentially fix alleles at autosomal loci. The fixation of slightly deleterious alleles by random drift and the stochastic turnover of alleles at loci controlling quantitative characters under stabilizing selection usually proceed somewhat more slowly at sex-linked loci. In contrast, the fixation of underdominant chromosomal rearrangements by random genetic drift is faster with se...

Swarms of predators exhibit "preytaxis" if individual predators use area-restricted search

Abstract: We show that if individual predators restrict the area of their search following an encounter with prey, then this behavior translates into populations of predators flowing toward regions of high prey density. This result requires only that predators move at a constant speed but change their direction of movement more often when their stomachs are full and that increases in prey density increase the feeding rate and stomach fullness of predators. The partial differential equation that is derived by assuming such behavior includes terms representing both random motion and taxis on the part of the predator. The form and magnitude of these terms can be estimated by quantifying how prey density influences the frequency of directional changes in a foraging predator and by obtaining functional-response curves for predators that have been starved for different lengths of time. In general, the strength of a predator's taxis or aggregation response depends on its average velocity of search and on the sensitivity o...

Plant succession: life history and competition

Abstract: An approach based on competition among individual plants is presented as an explanation for species replacements during plant succession. Inverse correlations among life history and physiological traits that confer competitive ability under different environmental conditions are shown to be sufficient to produce successional replacements but not sufficient for understanding the complex variety of successional patterns unless they are applied at the individual level rather than at the population level or higher. With models based on competition among individual plants, various combinations of life history and physiological traits can produce the great variety of population dynamics found in natural successions. The classic successional pattern of species replacement results from a particular structure of correlations among life history and physiological characteristics. Atypical patterns of succession result when this correlation structure is altered. Both primary and secondary succession are modeled as no...

Patterns of mutualistic interactions in pollination and seed dispersal: connectance, dependence asymmetries, and coevolution

Abstract: Patterns of connectance and strength of mutual dependence in mutualisms have been examined by comparing the fraction of possible pairwise interactions established in a series of plant-pollinator and plant-seed disperser systems. As the number of species in the mutualistic system increases, the absolute number of interactions established increases, but connectance decreases exponentially. A given increase in diversity adds twice the number of interactions to dispersal systems as to pollination systems, suggesting a higher global specificity of the latter. Connectance patterns in mutualisms are analogous to some of those observed in complex food webs, suggesting a somewhat invariant structure in the relations between sets of interacting species. For seed-dispersal systems involving frugivorous birds, mutual dependence values are strongly skewed toward the low end and illustrate generally strong asymmetries in mutualistic interactions. These patterns may be expected by considering simple multiplicative effec...

Overcompensation in response to mammalian herbivory - the advantage of being eaten

Abstract: Plants of scarlet gilia, Ipomopsis aggregata, are exposed to high levels of mammalian herbivory (by mule deer, Odocoileus hemionus, and elk, Cervus elaphus) early in the season, before flowering. During this period of our study, up to 56% of all individuals experienced a 95% reduction in aboveground biomass. Browsed plants rapidly responded by producing new inflorescences and flowering within 3 wk. Unbrowsed plants produced only single inflorescences, whereas browsed plants produced multiple inflorescences. Field observations and experimental manipulations showed that plants with multiple inflorescences produced significantly greater numbers of flowers and fruits than unbrowsed individuals. Because there were no differences between browsed and unbrowsed individuals in the number of seeds produced per fruit, seed weight, subsequent germination success, and survival, browsed plants enjoyed a 2.4-fold increase in relative fitness. Consequently, there is an immediate reproductive advantage to being eaten. Und...

Extinction Thresholds in Demographic Models of Territorial Populations

Abstract: A basic demographic model is constructed for territorial species in a region where patches of habitat suitable for survival and reproduction are randomly (or evenly) interspersed with patches of unsuitable habitat. The model predicts the equilibrium occupancy of suitable habitat as a function of the proportion of the region composed of suitable habitat, h, and of the demographic potential of the population, k, which is determined by parameters of the life history and dispersal behavior of individuals. If 0 < k < 1, the demographic potential gives the equilibrium occupancy in a completely suitable region, and the population will become extinct in the region if h $\leq$ 1 - k. Difficulty in finding a mate, the finite extent of the region containing suitable habitat, and serially uncorrelated fluctuations in life history parameters all increase the minimum value of h necessary to sustain a population. Models of this type should be useful for predicting the effects of habitat destruction and fragmentation, or...

Parental investment in offspring in variable environments:theoretical and empirical considerations.

Abstract: It has been argued that environmental variability favors variation in offspring size. We have examined theoretically the effects of three types of environmental heterogeneity on parental investment: spatial variation, spatial variation with density-dependent offspring fitness, and temporal variability. We concluded that environmental heterogeneity rarely favors the production of variable offspring. With spatial heterogeneity and no density dependence in fitness, a single offspring size is always optimal. Variable offspring may be favored if density-dependent fitness loss to offspring is large and if parents can control the dispersal of offspring to the appropriate habitats. If environments vary temporally, the predicted optimal parental investment strategy depends on the measure of fitness used. If the arithmetic mean of fitness is used, a single offspring size is always favored. Variable offspring sizes are sometimes selected if fitness is measured by the arithmetic mean discounted for the variance or by...

The Effects of Edge Permeability and Habitat Geometry on Emigration from Patches of Habitat

Abstract: Using computer simulations, we investigated two factors that may affect emigration from insular patches of habitat, where emigration is the proportion of dispersing individuals that leave the habitat patch. The first factor, edge permeability, $\phi$, reflects the tendency of a disperser reaching the edge of a habitat patch to cross the boundary and emigrate; edge permeability is positively related to emigration for any given habitat patch. The second factor is the edge-to-size ratio (ESR), the proportion of home ranges at the edge of a habitat patch; for any degree of edge permeability greater than zero, the ESR is positively related to emigration. When habitats have relatively hard edges (e.g., 0 $\leq \phi \leq$ 0.1), edge permeability is a more important determinant of emigration than is the ESR. Conversely, when habitats have relatively soft edges ($\phi$ > 0.1), the ESR is the more important determinant of emigration. These interactions between the effects of edge permeability and patch size and sha...

Modeling animal guts as chemical reactors

Abstract: Chemical-reactor theory recognizes three ideal reactor types: batch reactors, which are filled with reactants, continuously stirred during the reaction, and then emptied of products after a given reaction period; plug-flow reactors (PFR's), in which reactants continuously enter and products continuously exit with no mixing along the flow path; and continuous-flow, stirred-tank reactors (CSTR's), in which reactants continuously enter and products continuously leave a stirred vessel. Performance equations for these reactors, together with kinetic models for simple enzymatic catalysis and microbially mediated (autocatalytic) digestive fermentation, reveal necessary functional relationships among initial concentrations of the limiting food component, gut volume, throughput time or gut holding time, and digestive reaction kinetics. We use these models to suggest optimization constraints for digestion, analogous to those of optimal foraging theory. Two general predictions are possible. To sustain the greatest d...

Evolution of species assemblages: effects of energetic constraints and species dynamics on the diversification of the north american avifauna

Abstract: Characteristics of the terrestrial avifauna of North America can be viewed as adaptations by a taxonomically, geographically, and ecologically defined assemblage of many species to the constraints imposed by its own biology and by the environment. We have identified distinctive patterns in the variation among species in population density, body size, area of geographical range, and trophic status. The patterns observed in bivariate plots of log-transformed variables can be characterized provisionally in terms of polygons that enclose combinations of the variables exhibited by species. The sides of these polygons may be either abrupt or indistinct. We suggest that sharp, clear-cut boundaries separating combinations of characteristics that species possess from those combinations that are not observed in any species are the result of absolute constraints. As a trivial example, the maximum size of the geographical range is determined by the size of the continent. A more interesting example of an apparently ab...

Costs and Benefits of Plant Resistance to Herbivory

Abstract: The cost-benefit theory of the evolution of plant resistance to herbivory assumes that the allocation of plant resources to defense against herbivores is costly. We present a graphical model, which...

Chloroplast DNA Evolution and Biosystematic Uses of Chloroplast DNA Variation

Abstract: Three major tasks confront the molecular-evolutionary biologist. The first and most central of these is to describe the natural history of a particular genome. This involves determining what kinds of mutations occur, what their frequencies are, and whether or not the mode and tempo of genome evolution change in different lineages. Armed with this basic information, one can then proceed in either of two divergent directions. One is to exploit the storehouse of DNA variation in an attempt o understand the genetic structure of populations and aspects of the speciation process and, at higher levels of genetic divergence, to uncover the pattern of species relationships. The other is to elucidate the biochemical and molecular mechanisms that underlie the describable record of DNA change. Thus, the molecular evolutionist must be able to work in several areas of biological investigation-natural history, evolutionary genetics, systematics, and molecular biology-and ideally should seek to achieve a synthesis of them. I review here the success in achieving these aims of researchers tudying the molecular evolution of chloroplasts. Although great progress has been made, there are two distinct limitations in our current understanding. First, essentially all of our knowledge of chloroplast DNA (cpDNA) evolution is in the realm of the natural history of the genome and in the application of cpDNA variation to questions of organismal evolution; we are woefully ignorant of the molecular bases of cpDNA change. Second, the progress that has been made in these areas is primarily based on studies with land-plant cpDNA's and, in particular, with the genomes of flowering plants. The study of algal cpDNA evolution is for the most part in its infancy. The organization of this article reflects these limitations in two ways. First, the great bulk of the discussion deals exclusively with land-plant chloroplast genomes; only near the end do I briefly discuss algal cpDNA's. Second, I consider land-plant chloroplast genomes in depth only with respect to their descriptive evolution and systematic utility. Information relating to molecular mechanisms of evolutionary change is touched on only in passing. To initiate readers completely unfamiliar with cpDNA, I begin by briefly summarizing certain basic facts concerning its structure, function, and expression.

The Importance of the Mechanisms of Interspecific Competition

Abstract: Schoener (1983, 1985) and Connell (1983a) summarized more than 150 field experiments designed to test for the existence of interspecific competition in natural communities. Although there are differences between their reviews (Schoener 1985), both concluded that interspecific competition was detectable in more than half of the species studied. That more than 150 such field experiments were available for review is a testimonial to the dissatisfaction with which ecologists have viewed traditional studies that attempted to infer the presence of competition using correlational or distributional data. The design of the experiments, though, is a memorial to the extent o which the often-criticized Lotka-Volterra competition equations still pervade ecological thought. The experiments used a nonmechanistic, Lotka-Volterra-based, phenomenological definition of competition: two species compete when an increase in the density of one species leads to a decrease in the density of the other, and vice versa. Schoener stated, \"I consider an interspecific competition experiment to be a manipulation of the abundances of one or more hypothetically competing species\" (1983, p. 241). I question whether such experiments actually measure the \"importance\" of interspecific competition in nature and whether the LotkaVolterra definition of competition, which is based on the interactions of a single pair of species, applies to species living in multispecies communities. I raise these questions and direct the comments that follow not as criticisms of Schoener's or Connell's reviews, but from concern about the pattern of thought in ecology that generated the more than 150 papers available for review. In a natural community, one species may influence a second species both directly and indirectly (Levine 1976; Lawlor 1979; Vandermeer 1980). For instance, Lawlor (1979) considered a theoretical case of four warbler species whose only direct interactions were competitive. Lawlor used MacArthur's (1968) field data on competition coefficients for these four species and had the warblers compete according to the Lotka-Volterra equations. In the four-species community, however, one of the species pairs behaved, in total, as if they were mutualists, and two other pairs behaved, in total, as if they barely interacted, even though they were strong direct competitors. This marked ifference between the direct process of pairwise interspecific competition and the total effect hat one species has on another in a multispecies community iscaused by indirect effects. As soon as there are more than two interacting species, one species can potentially influence a second species both directly and indirectly through another species.

Selection of Offspring Size at Independence and Other Size-Versus-Number Strategies

Abstract: A continuous, analytical model of the selection of size-number compromises is presented and applied to the selection of the amount of parental investment in each offspring until its independence. At the evolutionarily stable strategy, the proportional gain in the success of an offspring from an increment in the investment in the offspring equals the proportional loss in offspring numbers. A parallel marginal-value theorem applies to discontinuous variation in the amount of care. When selection acts directly on offspring size rather than on number, the evolutionarily stable level of care depends only on the fitness curve relating the fitness of single offspring to the amount of resources received. General conditions for brood reduction are described. A more-specific rapid-gain version of the continuous model suggests that the amount of parental care is influenced primarily by the level of care at which offspring success increases rapidly. The model can explain various observed differences in amounts of par...

A General Model for Parental Care

Abstract: This model provides a general framework for analyzing the effects of a relatively large number of variables on quantitative variation in parental care On the assumption that natural selection favors parents that maximize their reproductive value in each of many time intervals throughout a breeding attempt, the model predicts optimal parental responses to variation in age and number of offspring, time since the beginning of the breeding season, effort of the mate, and parental condition This list of independent variables can easily be expanded or restricted within the framework of the model Although the model is developed here for biparental monogamous species, it serves as the foundation for applications to other mating systems and for the addition or subtraction of variables to which the parents respond

Short-term apparent competition

Abstract: Interspecific interactions reflect the cumulative consequences of individual behavioral acts. The foraging decisions made by predators influence the way in which predation shapes the structure of prey communities. Alternative prey species co-occurring in a patch embedded in a matrix of many similar patches may interact through a shared mobile predator in two distinct ways. First, the functional response by an individual predator foraging in the patch to one prey species may be affected by the density of a second prey species in the patch (e.g., any time spent handling one prey reduces the time available for capturing other prey). Second, the presence of a second prey species may alter the propensity of predators to aggregate or remain in a given patch. We argue that this aggregative numerical response can in many circumstances generate -, - interactions (apparent competition) between prey species that otherwise would not interact. This is most likely if predators use a simple optimality criterion for prey...

Geographical Variation of Lodgepole Pine in Relation to Population History

Abstract: Relation des caracteristiques genetiques et morphologiques des populations de Pinus contorta ssp. latifolia depuis leur origine

Seed size and fruit-handling techniques of avian frugivores

Abstract: Whether a bird handles a fruit by crushing it with its bill or simply swallowing it whole has important ramifications for both the dispersal success of the plant and the feeding behavior of the bird. In cage experiments, birds that crushed fruits in their bills ("mashers") usually dropped many seeds without ingesting them. Large seeds were dropped more frequently than small seeds. The proportion of seeds dropped by five tanager species correlated negatively with their body weight. Birds that swallowed fruits whole ("gulpers") did not drop seeds before swallowing fruits. Gulpers regurgitated large seeds and defecated small seeds. Median regurgitation time was less than median defecation time. Gulpers had lower ingestion rates than mashers, probably because mashers did not ingest many seeds. Mashers spent more time than gulpers in handling fruits. Mashers may also be less restricted than gulpers of equivalent size in the sizes of fruits they can take. Plants may respond evolutionarily to the differences in ...

A method for analyzing selection in hierarchically structured populations

Abstract: Individual fitness depends on the particular ecological, genetic, and social contexts in which organisms are found. Variation in individual context among subunits of a population thus raises interesting questions about selection in nature but also complicates its study. We present a method for analyzing phenotypic selection in hierarchically structured populations. Applying this method, called contextual analysis, to the study of selection allows explicit answers to two frequently controversial questions. First, must group membership be taken into account in explaining differences in individual fitness? Second, what particular group properties are associated with observable group-level effects? Contextual analysis is a generalization to structured populations of the "selection gradient" method developed by Lande and Arnold (1983). The aim of the gradient method is to distinguish characters that have a causal relationship with fitness from others that do not but are still subject to selection as a result o...

Parental care and the evolution of egg size in fishes

Abstract: The quality of parental care appears to correlate positively with egg size, both among and within species of fishes. Past models of the trade-off between quantity and quality of offspring have been inadequate in explaining this correlation. Using features of models by Smith and Fretwell (1974), Shine (1978), and Taylor and Williams (1984), we constructed a model to explain continuous covariation between the quality of parental care and egg size. Our model contains three major assumptions about the dependence of offspring survival on egg size: offspring from larger eggs develop more slowly and take longer to resorb their yolk sacs and become juveniles; egg size determines initial juvenile size; and larger juveniles, which hatch from larger eggs, have lower mortalities, experience faster growth, and take less time to become adults. Under these assumptions, as parental care reduces instantaneous egg mortality, the optimal egg size increases. This increase is expected both among and within populations. Thus, ...

Reproductive Effort in Plants. 1. Carbon Allocation to Reproduction

Abstract: Reproductive effort, or the proportion of an organism's resources allocated to reproduction, is a crucial aspect of an organism's life history; the optimal allocation of resources to reproduction in different environments has been the subject of much theorizing Adequate tests of these theories have been hampered by the difficulties involved in assessing reproductive effort In this paper, we address the problem of determining which structures and activities should be considered part of reproduction, using Agropyron repens as the experimental material We approached the problem by first determining the structures and activities necessary for vegetative growth and then determining reproductive structures and activities by subtraction Using carbon as the currency of allocation, we defined vegetative growth as those structures directly involved in the capture of carbon (ie, leaves) plus all necessary support structures and activities The necessary support structures and activities were determined by comp

Territory Quality and Dispersal Options in the Acorn Woodpecker, and a Challenge to the Habitat-Saturation Model of Cooperative Breeding

Abstract: The central feature of cooperative breeding systems is the presence of one or more nonbreeding birds in a social group that devote time and energy to help raise the offspring of other group members. The most widely accepted model for the evolution of this behavior is that it arises when there is some ecological constraint to independent breeding by young birds that otherwise would disperse from their natal groups. Among territorial species, this constraint is usually hypothesized to be saturation of suitable habitat by sedentary established groups. We use data collected over a 10-yr period to test this model for a population of acorn woodpeckers (Melanerpes formicivorus) in central New Mexico. Acorn woodpeckers store large quantities of mast, and territories vary greatly in the amount of storage facilities present. Storage facilities significantly affect the survival and reproductive success of groups occupying those territories. Most helpers occur in high-quality territories; low-quality territories are ...

Putting Things in Order: A Critique of Detrended Correspondence Analysis

Abstract: Methods of ordination are multivariate statistical techniques designed to order individual entities on the basis of the similarities or differences for variables scored for each entity. These methods, most of which are based on linear-model theory, often fail to order entities correctly because of the nonlinear relationships among the observed variables. Under simple assumptions, the joint distribution of entities viewed in multidimensional variable space results in a curved structure that is called the circumplex in psychology, the horseshoe in archaeology, and the arch in ecology. We review the reasons for the formation of this structure and point out the limitations of traditional ordination methods for resolving data with this structure. We argue that detrended correspondence analysis, although designed as a new, improved ordination method, is no better and perhaps worse than traditional methods. All methods are influenced by data curvature and scaling. Until more-effective ordination methods can be d...

Offspring size and number: a life history model linking effort per offspring and total effort

Abstract: We develop an extension of Schaffer's (1972) model for a simple life history with constant annual adult and juvenile survival rates. In contrast with previous models, we distinguish between total effort and effort per offspring. We assume that offspring production is the product of the juvenile survival rate and brood size, that the juvenile survival rate is a function of effort per offspring, and that brood size is a function of effort per offspring and total effort. We further assume that the adult survival rate is a function of total effort and that optimal combinations of effort per offspring and total effort are those resulting in the highest population growth rate. Optimal effort per offspring depends only on the minimal effort per offspring and the rate at which the juvenile survival rate increases with increasing effort per offspring. Optimal total effort depends on effort per offspring, the ratio between the realized juvenile survival rate and the maximal adult survival rate, and the threshold at...

Cyclic and Stable Populations: Plankton as Paradigm

Abstract: We analyzed over 20 study-years of data from populations of Daphnia and algae in a wide variety of field situations. These systems display three types of dynamic behavior: both populations stable; both populations cyclic; and Daphnia cyclic but algae stable. The last pattern occurs whether we analyze the total amount of algae or only edible algae. There is evidence that this range of dynamics arises from the interaction between Daphnia and its food supply, occurring in systems that are structurally the same; that is, differences in biological rates or time delays, alone, can explain the existence of different dynamic classes. This is particularly the case when different classes occur in the same species in the same environment in different years, or in similar and adjacent habitats at the same time. The cycles thus appear to be internally driven, rather than resulting from external, cyclic, forcing factors. These findings support a basic premise of most mathematical models in ecology. The broad dynamic pa...

On the role of species in anagenesis

Abstract: Speciation is central to the hypothesis of punctuated equilibria s developed by Eldredge and Gould (1972; see also Gould and Eldredge 1977, 1986; Gould 1982) and by Stanley (1975, 1979). Without the claim that evolutionary change is associated with speciation, punctuated equilibria would consist merely of the statement (to which no one would take exception) that rates of evolution vary. Population geneticists have generally been skeptical about punctuated equilibria (e.g., Templeton 1980; Lande 1980a; Stebbins and Ayala 1981; Turner 1981; Charlesworth etal. 1982), in part because genetic theory and data appear to offer no support for the postulate that evolutionary change in, for example, morphological characters is contingent upon or associated with speciation (i.e., acquisition of reproductive isolation). These authors doubt that speciation should increase the rate of morphological evolution and are particularly skeptical of the punctuationists' supposition that lineages are static between speciation events because of an inability to respond to selection. Here I point out a consequence of speciation that supports a highly qualified version of punctuated equilibrium. None of the ideas explored below is in itself original, but I am not aware that they have been explicitly developed in the present context. In brief, I propose that because the spatial locations of habitats shift in time, extinction of and interbreeding among local populations makes much of the geographic differentiation of populations ephemeral, whereas reproductive isolation confers ufficient permanence on morphological changes for them to be discerned in the fossil record. Long-term anagenetic hange in some characters is then the consequence of a succession of speciation events. I have briefly referred to this argument before (Futuyma 1986a, pp. 404, 406; Futuyma 1986b, p. 377), without having developed the reasoning, evidence, and contrasts with alternative hypotheses presented here. Eldredge and Gould (1972) hypothesized that punctuated patterns follow from Mayr's (1954, 1963) neo-Darwinian but not universally accepted hypothesis of peripatric speciation, according to which rapid shifts in morphology occur in highly localized populations in concert with the acquisition of reproductive isolation from the main body of the species, which remains little changed. The morphological shifts envisioned by both Mayr and the punctuationists are typically not so pronounced as to mark the origin of higher taxa; they are of the magnitude that usually distinguish congeneric species; and it is shifts of this magnitude that I discuss in the first part of this note. Authors who have offered population-genetic explanations for rapid shifts between stable morphological states (e.g., Kirkpatrick 1982a; Newman et al. 1985)

Interacting selective pressures in conifer-bark beetle systems: a basis for reciprocal adaptations?

Abstract: Conifer-bark beetle interactions provide a useful model system for evaluating potentially reciprocal selective pressures between plants and insects. The phloem-feeding bark beetles that infest living conifer stems are a major source of host mortality, and their successful reproduction is usually contingent on the death of the tree. Trees respond to invasion by producing a series of localized secretions and biochemical alterations that can contain the insect and associated microorganisms. We describe the relative advantages and disadvantages of two beetle reproductive strategies: overwhelming trees with a synchronized mass attack; and selecting weakened trees that cannot offer strong resistance. Differences in the defensive physiology of grand fir, Abies grandis (Douglas) Lindley, and lodgepole pine, Pinus contorta var. latifolia Engelmann, may be partially responsible for differences in the behavior of the fir engraver beetle, Scolytus ventralis LeConte, and the mountain pine beetle, Dendroctonus ponderos...

Adaptive hypotheses of birth synchrony in ruminants : an interspecific test

Abstract: The ecological causes of birth synchrony in ruminants are examined. Measures of seasonality, including latitude and seasonal distribution of evapotranspiration, correlate well with birth-season length, explaining about half the variance of a 27-species sample. With one exception, the African buffalo, species with precocious "following" young display short birth seasons in all climates. It is inferred that predation on newborns plays an important role in imposing tight birth synchrony on following species, especially those occupying tropical and subtropical climates. A crude classification of species by diet (browsers vs. grazers) did not significantly improve predictions of birth-season length among sample species. I argue that, as a consequence of the diverse and complex mechanisms used to control the onset of breeding, birth-season lengths are extremely variable both between and within species. The onset of reproduction in species whose birth-season lengths are adapted to seasonal fluctuations in food s...

Why there are so few species of aphids, especially in the tropics

Abstract: It is possible to account for the distribution and low species diversity of aphids in terms of the constraints imposed by their way of life, namely, the short period for which they can survive without food, their high degree of host specificity, and the low efficiency with which they locate host plants. Ninety percent of plants are not used as hosts by aphids (Eastop 1973) mostly because these plants are just too rare.