Showing papers in "The American Naturalist in 1985"

Phylogenies and the Comparative Method

Abstract: Recent years have seen a growth in numerical studies using the comparative method. The method usually involves a comparison of two phenotypes across a range of species or higher taxa, or a comparison of one phenotype with an environmental variable. Objectives of such studies vary, and include assessing whether one variable is correlated with another and assessing whether the regression of one variable on another differs significantly from some expected value. Notable recent studies using statistical methods of this type include Pilbeam and Gould's (1974) regressions of tooth area on several size measurements in mammals; Sherman's (1979) test of the relation between insect chromosome numbers and social behavior; Damuth's (1981) investigation of population density and body size in mammals; Martin's (1981) regression of brain weight in mammals on body weight; Givnish's (1982) examination of traits associated with dioecy across the families of angiosperms; and Armstrong's (1983) regressions of brain weight on body weight and basal metabolism rate in mammals. My intention is to point out a serious statistical problem with this approach, a problem that affects all of these studies. It arises from the fact that species are part of a hierarchically structured phylogeny, and thus cannot be regarded for statistical purposes as if drawn independently from the same distribution. This problem has been noticed before, and previous suggestions of ways of coping with it are briefly discussed. The nonindependence can be circumvented in principle if adequate information on the phylogeny is available. The information needed to do so and the limitations on its use will be discussed. The problem will be discussed and illustrated with reference to continuous variables, but the same statistical issues arise when one or both of the variables are discrete, in which case the statistical methods involve contingency tables rather than regressions and correlations.

A Nutritional Explanation for Body-Size Patterns of Ruminant and Nonruminant Herbivores

Abstract: The gut capacity of mammalian herbivores increases linearly with body weight. This relationship, coupled with the change in basal metabolism with weight, produces an MR/GC ratio (metabolic requirement/gut capacity) that decreases with increasing body size. Since the retention of a food particle within the gut is proportional to this ratio, the extent to which food particles are digested will be related to body size. Plant material is made up of chemical components that react differently to digestive enzymes. The fiber fraction of plant material (i.e., cell wall) is digested slowly and exclusively by microbial symbiotes. A positive relationship probably exists between the fiber content of plant parts and their biomass in the environment. This relationship is used to describe a resource axis on which digestion rate is the scaling variable. In response to this resource axis and metabolic requirements, the fiber content of the diet of herbivores increases with body size. Ruminants are the predominant medium-s...

Conservation Strategy: The Effects of Fragmentation on Extinction

Abstract: La fragmentation de l'habitat (et des reserves) est la menace la plus serieuse pour la diversite biologique et est la cause fondamentale de la menace actuelle d'extinction

The resource-ratio hypothesis of plant succession

Abstract: The resource-ratio hypothesis assumes that each plant species is a superior competitor for a particular proportion of the limiting resources and predicts that community composition should change whenever the relative availability of two or more limiting resources changes. It is suggested that (1) the major limiting resources for mesic terrestrial habitats are often a soil resource, especially nitrogen, and light; (2) these resources are naturally inversely related, the habitats with poor soils having high-light availability and the habitats with rich soils having low-light availability; (3) the life history of a plant should depend on the point along the soil-resource: light gradient at which the plant is a superior competitor; and (4) primary succession and secondary succession on poor soils result from a temporal gradient in the relative availabilities of a limiting soil resource and light. If, as hypothesized, plants specialized on low-nutrient habitats are relatively short in height, short-lived, and ...

Coexistence mediated by recruitment fluctuations: a field guide to the storage effect

Abstract: For most species, a changeable environment creates a situation in which recruitment varies considerably from one breeding season to the next. If adults survive well, an occasional favorable recruitment can sustain population numbers over long periods. In effect, the gains made in favorable periods are stored in the adult population. Storage is particularly important when the species is at low densities, because then the potential population growth rate is very high if a favorable period occurs. Our past work showed that the storage mechanism could lead to coexistence of two species in lottery competition for space, as long as generations overlapped and there was sufficient variation in recruitment (Chesson and Warner 1981). This was true even if one species had an average competitive advantage. The storage model also operates when more than two species are competing, when resources renew independently of population sizes, and when not all the resource is used. It also operates in simple Lotka-Volterra sys...

The role of body size, phylogeny, and ambient noise in the evolution of bird song

Abstract: Morton found that the average emphasized frequency in the songs of tropical bird species in low-forest habitats was lower than that of species in grassland or edge habitats. He suggested that this was due to a frequency window around 1585-2500 Hz in low-forest habitats, and that there was selection to produce songs with frequencies in this window in order to increase the transmission distance of the songs. In the present study, we analyze the constraints of body size and evolutionary history on the ability of avian species to respond to this selection. Also, we examine the spectral distribution of ambient noise as an additional selective factor. There is a correlation between body size and the emphasized frequency of the song for the species analyzed by Morton. Larger birds produce songs with lower emphasized frequencies. Morton's demonstration that birds in the low forest produce songs with lower frequencies is confounded by the fact that larger birds also live in this habitat. Phylogeny is also a potent...

Ecosystem Behavior Under Stress

Abstract: The behavior of ecosystems under stress can be shown to be analogous to Selye's characterization (1973, 1974) of the response of higher organisms to stress. The ecosystem-level distress syndrome is manifest through changes in nutrient cycling, productivity, the size of dominant species, species diversity, and a shift in species dominance to opportunistic shorter-lived forms. These symptoms of ecosystem dysfunction are common in both terrestrial and aquatic systems under various stress impacts including harvesting, physical restructuring, pollutant discharges, introductions of exotic species, and extreme natural events (such as disastrous storms or volcanic activity). The progression of appearance of symptoms under intensifying stress levels may be interrupted temporarily as ecosystem homeostasis and homeorhetic mechanisms intercede. Inability to cope leads to further dysfunctions and, perhaps, to irreversible ecosystem breakdown.

Biological Control in Theory and Practice

Abstract: The conventional wisdom of biological control of insect pests, and its related ecological theory, is that successful natural enemies in long-lived ecosystems (1) impose a low, stable pest equilibrium, and (2) share the following properties: (a) host-specific; (b) synchronous with the pest; (c) can increase in density rapidly when the pest does; (d) need only one pest individual to complete their life cycle; (e) have a high search rate for the pest; (f) aggregate at areas of high pest density, which is thought to stabilize the interaction. These features are more characteristic of parasitoids than predators. We suggest that a stable pest equilibrium is neither a necessary nor a sufficient condition for control. We show that satisfactory control in model systems is compatible with both local extinction of the pest and polyphagy in the natural enemy. Only one of nine real examples of successful control is convincingly a stable interaction; the remainder show either strong evidence for instability and local e...

Seasonality, fasting endurance, and body size in mammals

Abstract: It has been argued that environmental seasonality selects for larger body size among homeotherms (Brodie 1975; Downhower 1976; Boyce 1979). Indeed, strong correlations between body size and seasonality have been shown both within species of mammals (Brodie 1975; Boyce 1978) and among mammal faunas (Zeveloff 1982). Because of high mortality during the season of resource shortage. surviving individuals enjoy low competition and abundant resources during the growth season. Selection then favors rapid growth and attainment of large size because of enhanced survivorship (Lindsey 1966; Peters 1983; Calder 1984) and, in some taxa, increased fecundity (Sadleir 1969; Boyce 1981). Large individuals experience higher survival rates during the period of resource shortage because they metabolize somatic stores at a lower weight-specific rate. Thus, seasonality may produce two consequences to body-size evolution: (1) reduced densitydependent competition because of seasonal high mortality, known as Ashmole's hypothesis (Ashmole 1963; Ricklefs 1980) and (2) increased fasting endurance for individuals of large size (Lindsey 1966; Calder 1974). In this note we analyze data that substantiate the significance of large body size for enhancing fasting endurance, responding to the challenge of Clutton-Brock and Harvey (1983), who claim that further knowledge of \"food reserves\" is necessary to evaluate the seasonality hypothesis. There are now sufficient data to predict fasting endurance in mammals by dividing energy reserves by their rate of use (after Stahl 1962). The result, the scaling of fasting endurance, favors large body size. This relationship will, of course, be of primary significance in seasonal environments where organisms must endure occasional periods of fasting.

Limiting Resources and Territoriality in Microtine Rodents

Abstract: Theories concerning microtine population dynamics have relied increasingly on assumptions about social structure, but there has been no underlying theory for social biology itself. Among microtine species, patterns of sex-specific territoriality, the best-studied aspect of their social organization, are diverse, setting the scene for the fragmented nature of the contemporary approaches to this issue. Here I propose a set of hypotheses to predict under what conditions females and males should defend territories. The argument has three main components. 1. Microtines have a low-quality diet and should tend to be food-limited. Females invest a large portion of their energy in reproductive physiology, and their reproductive success should be limited by their ability to acquire food and convert it into weaned offspring. Males invest much less energy in reproductive effort and do not participate in parental care; thus, their reproductive success should be limited by access to females. Therefore, territoriality i...

Body Size of Mammals on Islands: The Island Rule Reexamined

Abstract: On commente le tableau de frequence du nanisme et du gigantisme pour 365 populations insulaires de mammiferes terrestres; comparaison avec les mammiferes continentaux. Presentation d'un modele. Role de la limitation des ressources alimentaires de la reduction de la competition interspecifique et de la predation dans les communautes insulaires

The relative importance of behavioral and physiological adjustments controlling body temperature in terrestrial ectotherms

Abstract: A series of simple heat-transfer models (termed the basic, conduction, physiological, and wet-skin models) is derived to examine a large number of behavioral and physiological mechanisms that are used by terrestrial ectotherms to control body temperature (Tb; table 1). The models reaffirm two generalizations: (1) in environments where solar radiation is available, behavioral mechanisms may provide a range of Tb's that is many times greater than the range in Tb's that results from physiological adjustments; and (2) among behavioral mechanisms, the times of seasonal and daily activity appear to be the most critical in determining Tb. Furthermore, microhabitat selection is more important than postural adjustments for controlling Tb. The models calculate Tb as the sum of air temperature (Ta) and the body-air temperature difference (Td). The direct effects of body size are expressed as functions of Td. Many behavioral mechanisms can then be expressed as some fraction of Td. The models (eqs. 3, 6, 8, 11), in co...

Offensive-Defensive Interactions between Herbivores and Plants: Their Relevance in Herbivore Population Dynamics and Ecological Theory

Abstract: A consideration of known defensive attributes of plants, and others that can be reasonably postulated, leads to the description of two alternative offensive strategies of herbivores, termed stealth and opportunism, to counter plant defenses. Stealthy herbivores display adaptations to minimize damage and defensive responses of their food plants. On the other hand, opportunists take advantage of circumstances, such as physical stress and loss of acquired resistance of food plants, that impair plant defensive capability. In addition, opportunists themselves display adaptations, such as mass-attack behavior, that stress food plants. Herbivore species with low, relatively invariant population levels may be stealthy, whereas those with variable population levels may be opportunistic. Phase changes in herbivores may be the result of transition between the two strategies. It is suggested that interactions within and among species of herbivores are better understood in terms of interference and facilitation than i...

Constraints on the Coevolution of Bacteria and Virulent Phage: A Model, Some Experiments, and Predictions for Natural Communities

Abstract: One view of the coevolution of parasites and their hosts is that of a gene-for-gene arms race between host defenses and parasite counterdefenses. We have incorporated mutations into a model of the ecological interactions between bacteria and virulent phage to determine rates of mutation that would be consistent with this scenario. The model assumes an open habitat (e.g., a chemostat) in which virulent phage and sensitive bacteria can coexist. Equilibrium densities of bacteria and phage are inversely proportional to the efficiency with which phage irreversibly adsorb to their hosts. The absolute rate at which mutations appear is proportional to the product of habitat size, population density, rate of increase, and mutation rate. The bacterium Escherichia coli B readily evolved resistance to virulent phage T4 in our chemostat experiments. Approximately 100 h was required for the appearance, establishment, and attainment of a resource-limited population of these T4-resistant mutants; this time period is clos...

Body Size and Limits to the Daily Range of Body Temperature in Terrestrial Ectotherms

Abstract: A nonequilibrium heat-transfer model is used to calculate the extreme range of body temperatures available to ectotherms of different masses. At about 1 kg a transition occurs in the amplitudes imposed on a terrestrial ectotherm's potential range in body temperature by the daily cycling of the thermal environment. Animals smaller than 1 kg can choose a wide range of temperatures, whereas animals larger than 1 kg experience smaller ranges in Tb. The range in Tb that small insects experience is limited by the range in Ta, which, however, can be large in the boundary layer of perching surfaces. Data from the literature indicate that ectotherms in the range of sizes from 0.01 to 1 kg have limited ranges of Tb (about 30⚬ C). The model predicts that it could be as large as 55⚬ C, 15⚬ C greater than the range of lethal temperatures of reptiles. That the observed range is only about half of that predicted reaffirms our understanding of the importance of behavioral thermoregulation for these animals. Body size app...

Asymmetrical Competition: Are Larger Animals Competitively Superior?

Abstract: Les animaux de plus grande taille ont tendance a etre superieurs aux moins gros dans la competition d'interference mais non dans la competition d'exploitation; dans cette derniere se sont les moins gros qui auraient tendance a etre superieurs

The evolutionary ecology of seed heteromorphism

Abstract: I develop a model of the ecology and evolution of seed heteromorphism. Heteromorphism is promoted over monomorphism by low correlation of the demographic success of the different morphs and high temporal variation in seed success. Available experimental data from two heteromorphic-seeded species satisfy the model conditions for heteromorphism and produce morph proportions similar to those predicted. Seed heteromorphism can arise when the evolution of morph behavior is constrained by a single concave fitness set or when different seeds on the same individual plant are constrained by different convex fitness sets. The fitness sets represent constraints on demographic success in different year types (trade-offs resulting from constraints in design, development, physiology, or genetics). Two kinds of heteromorphism may arise when the constraints differ for different seeds on the same plant. Whether a high-risk-low-risk heteromorphism or high-risk-high-risk heteromorphism evolves depends on the similarity of t...

Neighborhood Models of Plant Population Dynamics. I. Single-Species Models of Annuals

Abstract: We present tractable formulations for neighborhood models of annual plant population dynamic processes. These models are constructed from submodels, termed predictors, of individual plants. Fecundity and survivorship predictors give the fecundity and survivorship of an individual as a function of local population density. Dispersal predictors predict the dispersal pattern of a plant's maternal progeny and the survivorship of plants from seed to seedlings. We develop both computer models and analytically tractable models. Our computer models are designed to determine the population dynamic consequences of specific fecundity, survivorship and dispersal predictors. The analytical models are valid when dispersal is sufficiently large, and are used to explain the predictions of analogous computer models. We show through examples that the predictions of corresponding computer and analytical models may be virtually identical. Empirical tests of these models are practical because all model parameters and function...

Soft Selection, Hard Selection, Kin Selection, and Group Selection

Abstract: In this paper I illustrate the theoretical relationship among several different models of selection in structured populations, soft selection, hard selection, kin selection, and group selection, by using the covariance formulations ofLi (1967) and Price (1970, 1972) to partition the operation of selection into withinand between-group components. This partitioning ofcovariance can be directly extended from the single-locus population genetic models derived in this paper to the description of selection within and between groups for continuously distributed, quantitative traits of the type presently under investigation i several research programs of kin and group selection. In addition, because of the algebraic relationship between the covariance and the coefficient of linear regression, regression coefficients can be used to evaluate the relative importance of selection within and between groups in these models of subdivided populations. Price (1970, 1972) was the first o apply the covariance approach to the mathematics of group selection but only for \"the limiting case of reproductively isolated groups with no intergroup migration\" (Price 1972, p. 487). Because of this assumption concerning the complete isolation of groups, his major result (eq. [A17] of Price [1972]) did \"not depend upon any assumptions about mechanisms of heredity or anything else of that sort\" (Price 1972, p. 487). I will show that Price's equations for gene-frequency hange represent a special case of a general partitioning of covariance into withinand between-group components and, in addition, that it can be applied to cases with intergroup migration. Furthermore, the algebraic relationship between the covariance and the coefficient of linear regression will be used to illustrate how assumptions concerning hereditary mechanisms are implicit in the covariance formulations. In particular, the equations of Price (1970, 1972) completely describe selection in terms of the change in the mean value of a trait within a generation, but they require a hereditary mechanism in order to predict how much of this within-generation change is transmitted across generations ( ee below and also Arnold and Wade 1984a). The covariance formulation completely describes selection but it does not necessarily describe the response to selection. In the derivation presented below individuals will be assigned character values in such a way that the average value of the character is the

Patterns of Covariation in Life History Traits of Squamate Reptiles: The Effects of Size and Phylogeny Reconsidered

Abstract: Stearns' (1984) major conclusion was that patterns of covariation in life history traits within squamate reptiles are strongly influenced by classwide correlations with female body length. This res...

Rapidly Induced Defenses and Talking Trees: The Devil's Advocate Position

Abstract: We review the evidence that rapidly induced chemical changes in damaged leaves, for example, after attack by insects, (1) significantly influence the population dynamics of herbivorous insects and mites in the field, and (2) significantly reduce damage to plants by such herbivores. We also review the evidence for "talking trees"; that is, following experimental damage to tree foliage, chemical changes are induced in adjacent undamaged trees that mirror those in damaged trees. Most of the evidence for the effects of rapidly induced changes in damaged plants on herbivorous insects and mites is laboratory based, and often reveals only small effects (< 10%) on such things as larval development time or pupal weights. Many of the reported larger effects are statistically flawed. We argue that most induced responses may have negligible consequences for field populations. There is only one field study demonstrating significant effects of rapidly induced defenses on insect population dynamics. Evidence that foliag...

Infectious disease and species coexistence: a model of lotka-volterra form

Abstract: There is a growing body of evidence suggesting that infectious disease may influence the species composition of natural communities. We examine the effect of a shared infectious disease on species coexistence in a differential equation model that generalizes to two host species, a one-host one-disease model explored by Anderson and May (1979). In the Anderson-May model, the transmission rate is described by a "mass action" term; there is no acquired immunity; and the infectious disease is the only factor regulating population growth. These assumptions, which are generally more applicable to invertebrate than to vertebrate hosts, are carried over to our two-host model. We show that, just as in the familiar Lotka-Volterra model of direct competition, there are three possible outcomes to the interaction: (1) one host species may unilaterally exclude the other; (2) the two host species may coexist; or (3) either host may exclude the other, with the outcome depending on initial conditions. These outcomes are g...

Evolution of female choice and male parental investment in polygynous species: the demise of the "sexy son"

Abstract: Males of many polygynous animal species contribute nutrition, nest sites, parental care, or other material benefit to their mates in addition to their gametes Variation in female reproductive success may be caused by variation in the quantity or quality of the male contribution, and in such cases a female's mate choice affects her reproductive success I present here polygenic and haploid two-allele models for the joint evolution of a male secondary sexual trait and a female mating preference for that character The male trait affects male survivorship, mating success, and the number of offspring produced per mating A female's reproductive success is consequently affected by the type of male she mates This selection on fecundity is assumed to operate in one of two ways Under the assumption of limited male reproductive potential, the fecundity per mating is a declining function of the number of females to which a male is mated Under the alternative assumption of unlimited male reproductive potential,

Multiple Causes of Dispersal

Abstract: Recent interest in the evolution of the dispersal patterns of terrestrial vertebrates has emphasized functional explanations that invoke the influence of inbreeding avoidance (IA), competition for mates (CFM), and competition for environmental resources (CFR) (e.g., Packer 1979; Greenwood 1980, 1983; Dobson 1982; Greenwood and Harvey 1982). Studies of dispersal usually address two questions: why do individuals disperse, and why are dispersers often predominantly of one sex and age class? Direct evidence for the competitive hypotheses comes from behavioral interactions among individuals. Such evidence from primates, zebras, and wild dogs led Moore and Ali to conclude that \"the major cause of natal dispersal and transfer among mammals is competition, . . . and the 'inbreeding hypothesis' is both inadequate and unnecessary to explain general dispersal patterns\" (1984, p. 107). We disagree with these conclusions. Rather than attributing dispersal patterns to one cause, we contend that there may be multiple causes (Dobson 1982). Further, a broader review of the evidence among mammals indicates that CFM and CFR do not explain all dispersal patterns and that IA cannot be ruled out as a possible factor in some cases. In a review of several case histories of biological research, Hilborn and Stearns (1982) demonstrated that the assumption of single causation for a phenomenon, when in fact multiple causes are possible, often leads researchers to rule out important causal factors. Moore and Ali committed precisely this error when they concluded that, because CFM provides a better explanation than IA for the dispersal patterns they discussed, competitive hypotheses are probably sufficient explanations of general dispersal patterns in mammals. This logic portrays IA and CFM as competing hypotheses and thus presupposes that dispersal in mammals has a single cause (see Cohen and Nagel 1934; Hilborn and Stearns 1982). There is, however, no basis for this assumption; the IA and CFM hypotheses are not mutually exclusive. It is possible that dispersing individuals benefit from both increased access to unrelated mates and decreased intrasexual competition. Even if one cause is of primary importance in some species, other factors may contribute to the magnitude of dispersal and thus should not be excluded if a complete understanding is sought. Rather than trying to choose one cause over another, we should ask whether any of the three factors hypothesized to influence dispersal patterns (IA, CFM, and CFR) is a necessary and sufficient explanation for dis-

The Intrusion of Clonal Growth Patterns on Plant Breeding Systems

Abstract: As clonal plants get larger, individual flowers become more surrounded by other flowers borne on the same clone. With very restricted movement of pollen reported for both animal- and wind-pollinated species, the breeding system may change as a consequence of a clonal plant covering greater amounts of space. In Carex platyphylla, a wind-pollinated, self-compatible sedge, the absolute and relative amount of endogenous pollen per flowering culm increases sharply as a plant grows from one to about 10 culms, then increases at a slower rate, given a constant rain of exogenous pollen. In Trifolium repens, a bee-pollinated, self-incompatible, stoloniferous clover, a model based on known patterns of flower placement and stolon growth patterns, coupled with estimates of pollen-deposition and floret-visitation patterns by bees, suggests that the number and length of stolon internodes affect successful pollination. Also, the distribution of seeds among inflorescences is affected by clonal growth patterns and bee beha...

Persistence of stream fish assemblages: effects of environmental change

Abstract: Knowledge of temporal stability and persistence of ecological communities is critical to an understanding of the importance of biotic interactions in the maintenance of community structure. Herein we have considered temporal stability and persistence of two fish assemblages, one in Brier Creek, a harsh prairie stream in southern Oklahoma, and the other in Piney Creek, a more benign stream in the Arkansas Ozarks. Persistence, as indicated by presence data, was high for both stream assemblages over approximately one decade. Assemblage stability, as determined from measures of abundance, indicated greater change in the overall fish fauna of Brier Creek than that of Piney Creek. Southeastern Oklahoma had a severe summer drought in 1980 resulting in heat death of fishes in Brier Creek. Although some species were transiently affected, the harsh summer did not have a lasting effect on the Brier Creek fish assemblage overall. Upstream stations subjected to regular summer drying, however, showed greater variabilit...

Group selection, altruism, and structured-deme models

Abstract: Group selection is defined as a process by which traits advantageous to the group are favored because of the positive association of individuals exhibiting the traits. In addition, group selection acts to protect this positive association against cheats. This definition, unlike those in current use, incorporates the essential features of the traditional verbal arguments by excluding the effects of individual selection and incorporating the problem of cheating. Kin selection is considered an example of group selection in which the "groups" are associations of relatives and in which special mechanisms, such as individual recognition, maintain the integrity of the associations. The "groups" of group selection are quite different from the trait groups of structured-deme models, so that structured-deme models can be used to demonstrate individual or group selection. By analyzing the models in terms of group neighbors, it has been shown that whenever trait groups are formed at random only individual selection c...

Colors of Fruit Displays of Bird-Dispersed Plants in Two Tropical Forests

Abstract: Color is a key characteristic of fruits because it affects the probability that they will be noticed or selected and, consequently, that their seeds will be dispersed. This paper examines the colors of fruit displays of 383 bird-dispersed plant species in two diverse tropical forests in Costa Rica and Peru. We detail the frequency of ripe-fruit color displays and try to explain these patterns by considering a general model of selection on fruit colors. The generalization that "bird fruits tend to be red" is shown not to apply to the Neotropics-most ripe bird fruits in our sample are black, with red being the second most common color. The proportion of plant species bearing either black or red fruits is remarkably similar in Costa Rica, Peru, Europe, and Florida (62%-66%). Certain color combinations in fruit displays, formed by ripe fruits plus contrasting unripe fruits or accessory structures (bracts, peduncles, persistent calyces), are especially common. The colors black and red, for example, co-occur in...

Interpopulation Gene Flow by Pollen in Wild Radish, Raphanus sativus

Abstract: Although gene flow can act as an important evolutionary force, direct measurements of interpopulation gene flow remain sparse. A paternity exclusion procedure was used to compare multilocus progeny genotypes with those of their maternal parents and with all possible local paternal parents in three populations of wild radish, Raphanus sativus, isolated by 100 m to 1 km from other conspecifics. The comparison permitted identification of those progeny sired by paternal parents in other populations. Over all the seeds sampled, the rate of interpopulation gene flow by pollen varied twofold, from 8.2% to 17.9% per population per generation. Heterogeneity among plants varied considerably, from 0% to 44% of the seed set by an individual. The most important spatial correlate of successful interpopulation mating was the number of neighbors within 1 m, which had a significantly negative relationship with this parameter. The levels of gene flow by pollen reported here are sufficient to counterbalance selection at lev...

Species richness in streams of different size from the same drainage basin

Abstract: The species richness of stream benthic invertebrates was studied along a longitudinal profile of the Salmon River, Idaho, during spring, summer, and autumn. Sampling was done using replicate rocks and the analytical approach of Stout and Vandermeer (1975) was used to calculate theoretical number of species present, relative immigration rate, and relative spatial heterogeneity. Species richness varied with stream size, being highest in midorder streams and lower in headwater and high order streams. This downstream shift in species richness conforms to the river continuum concept (Vannote et al. 1980). Possible cause is associated with varying temperature regimes in the different-sized streams as well as other factors. Species richness also varied with season depending on stream size. Low order streams were more individualistic, probably because of a greater influence of local (terrestrial) environmental conditions, but showed a higher richness in summer than in autumn. Larger streams were more similar and ...