Showing papers in "The American Naturalist in 1991"

Energy and large-scale patterns of animal- and plant- species richness

Abstract: Many hypotheses have been proposed to explain the great variation among regions in species richness. These were tested by first examining patterns of species richness of birds, mammals, amphibians,...

Long-term experimental evolution in Escherichia coli. I. Adaptation and divergence during 2,000 generations.

Abstract: We assess the degree to which adaptation to a uniform environment among independently evolving asexual populations is associated with increasing divergence of those populations. In addition, we are concerned with the pattern of adaptation itself, particularly whether the rate of increase in mean fitness tends to decline with the number of generations of selection in a constant environment. The correspondence between the rate of increase in mean fitness and the within-population genetic variance of fitness, as expected from Fisher's fundamental theorem, is also addressed. Twelve Escherichia coli populations were founded from a single clonal ancestor and allowed to evolve for 2,000 generations. Mean fitness increased by about 37%. However, the rate of increase in mean fitness was slower in later generations. There was no statistically significant within-population genetic variance of fitness, but there was significant between-population variance. Although the estimated genetic variation in fitness within po...

Sources, Sinks, and Habitat Selection: A Landscape Perspective on Population Dynamics

Abstract: In the model described, we attempt to link breeding-site selection to population dynamics for situations in which there is more than one distinct type of habitat. The distribution of individuals between habitat types depends on the selective abilities of the species. This distribution, in turn, influences the population dynamics of the species as a whole. We show that the consequences of habitat selection on population dynamics for an ideal free distribution of individuals across habitats is predictably different from what would be predicted if habitat selection were preemptive, that is, if individuals, upon selecting a site, prevented others from sharing the site. If preemptive selection is ideal, average reproductive success declines with increasing density because each individual selects the best site available from those sites not yet occupied. The model allows us to compare the relative contribution of different types of habitats to a species' growth rate and population size. Furthermore, we can also...

Complex trophic interactions in deserts: an empirical critique of food-web theory

Abstract: Food webs in the real world are much more complex than food-web literature would have us believe. This is illustrated by the web of the sand community in the Coachella Valley desert. The biota include 174 species of vascular plants, 138 species of vertebrates, more than 55 species of arachnids, and an unknown (but great) number of microorganisms, insects (2,000-3,000 estimated species), acari, and nematodes. Trophic relations are presented in a series of nested subwebs and delineations of the community. Complexity arises from the large number of interactive species, the frequency of omnivory, age structure, looping, the lack of compartmentalization, and the complexity of the arthropod and soil faunas. Web features found in the Coachella also characterize other communities and should produce equivalently complex webs. If anything, diversity and complexity in most nondesert habitats are greater than those in deserts. Patterns from the Coachella web are compared with theoretical predictions and "empirical ge...

Physiological consequences of habitat selection

Abstract: By determining the microclimates that an animal experiences, habitats influence an animal's physiological capacities and ultimately its demographic and ecological performance. As a result, the ecology of organisms-especially of ectotherms-can be profoundly affected by the physiological consequences of habitat selection. Early ecologists such as Shelford and Chapman appreciated these issues, but most later ones tended to ignore physiology and instead focused on biotic interactions (e.g., competition). Recent technical and conceptual developments are now fostering a reintroduction of physiology into ecology. For issues relevant to thermal physiology, three steps are involved. First, the microclimates available in a habitat must be mapped. For ectotherms, this involves determining the operative environmental temperatures (Te)-that is, the potential body temperatures available in a habitat. Biophysical techniques can now generate Te maps with considerable accuracy. Second, the physiological effects of body temperature must be quantified. This requires laboratory studies of the effect of temperature on key performance traits. Third, the physiological suitability of habitats can be predicted by integrating the above environmental and physiological data. Analyses of the physiological conse- quences of habitat selection are exemplified in several case studies, and the importance of considering food and other factors in the analyses is stressed. An extension to endotherms is briefly discussed. The study of how and why organisms select particular habitats has long been central to ecology. In fact, the early literature of ecology was often dominated by discussions of habitat associations. Of special interest in the early days of ecology was the concept of limiting factors: which physical factors (e.g., tempera- ture, pH, salinity) limit the occurrence of organisms to particular habitats. Not surprisingly, early discussions often focused on interactions between physiology and the physical environment. Indeed, for many early ecologists, ecology and physiology were more or less synonomous concepts. Shelford, for example, de- fined ecology as "that branch of general physiology which deals with the organism as a whole . . and which also considers the organism with particular reference to its usual environment" (1913, p. 1). Similarly, Chapman argued: "The inter- relations of organisms in nature may be considered under the subject matter of ecology which is closely related to physiology, and in the minds of some biologists should be a subdivision of it" (1931, p. 3). Clearly, physiology was central to ecology at this time. In fact, physiological issues dominated nearly the first half of Chapman's (1931) book (Animal Ecology). Beginning with Elton (1927), however, ecological studies began to shift their focus from analyses of physiologically mediated interactions between organisms and their physical environments to analyses of interactions between individuals or between species, in other words, to analyses of population and community

Spatial and Temporal Scales in Habitat Selection

Abstract: Female yellow-headed blackbirds in eastern Washington State settle to nest at higher densities on marshes with higher emergence rates of odonates, the most important prey delivered to nestlings. However, settling densities of females were not correlated with odonate emergence rates on individual territories or on individual territories plus adjacent ones. Apparently, females assessed production of insects on breeding marshes at the time they settled, and they used this information when making settling decisions. However, they selected nest sites on the basis of vegetation density rather than food availability. The complexity of decision making by female yellowheads would not have been detected had our analysis been restricted to one spatial scale. Because interpretations of habitat selection behavior are scale-dependent, careful attention to scale and performing analyses on more than one spatial scale are essential in studies of habitat selection. Habitat selection, an almost universal activity among animals, affects nearly all of an individual's subsequent choices. As a result, considerable attention has been paid to both the theory of habitat selection and the ways in which organisms in different taxa actually evaluate and select from the different options available (Cody 1985). A general guiding theoretical principle is that preferences among environments should coevolve with the qualities of those environments. That is, organisms should respond positively to (prefer) environments in which their sur- vival and reproductive success is good (Levins 1968; Orians 1980a). Individuals that elect to settle in less suitable environments leave fewer surviving offspring than those choosing better habitats. Whereas the logic of this general approach is compelling, correlations between habitat features and success may be low or difficult to assess in real-world situa- tions. A normally good environment may be poor because other factors (diseases, predators) reduce quality below expected levels or because resources are tempo- rarily depressed. Ease of assessment may depend on the grain of environmental patches in relation to the scale of exploitation by the organism (Levins 1968). Moreover, individuals may have difficulty in assessing the actual qualities of environments. This is especially true when the value of the habitat depends on its ability to provide resources for some time into the future. Some of these resources may not be present or evident at the time the decision must be made. In this article, we explore stages in the habitat-selection process and consider different spatial scales of decisions with the objective of providing a more compre- hensive way of viewing details of habitat-selection processes.

Habitat Selection and Population Interactions: The Search for Mechanism

Abstract: The study of habitat selection and population interactions has a history about as long as any integrative topic in ecology. Its literature is classically scientific in the sense that it combines pattern discovery, theory, and empirical testing. It is, implicitly, one of the significant demonstrations that general ecological principles exist and can be discovered by hypotheticodeductive methodologies. I outline the development of the topic in the context of two questions, both of which take it as an assumption that habitat selection is an optimal-foraging process. How does habitat selection alter (or reinforce) the dynamics of population interactions? How do those interactions alter the optimal behavior? Competition within a single species forces the use of a wide variety of habitats. This has been shown in many taxa, and Fretwell's theory gives it a sound basis in evolutionary ecology. In other words, we ecologists know that the pattern exists, and we know why it exists. Ecologists are capitalizing on the...

The Role of Abiotic Factors in Community Organization

Abstract: Very few ecological studies have attempted to fully test the integrative role of abiotic and biotic factors in interspecific interactions. There is little if any dispute about the value of such an approach in community ecology. However, there is great disagreement over the present direction of the field; we advocate a reemphasis of the integrative role of abiotic and biotic factors. By using literature examples and data on the role of salinity in reversing the competitive relations between fish of the genus Lucania, we have suggested a reemphasis on these interactions. This represents a blend of the traditional fields of ecology, physiology, and toxicology and could facilitate progress in community ecology. As such it could also be a significant part of the new direction sought for the field of physiological ecology. Finally, we are not arguing that abiotic parameters are necessarily more important than any of a group of multiple factors that may be involved in regulating community structure, only that th...

Community-Assembly Mechanics and the Structure of an Experimental Species Ensemble

Abstract: It was relatively easy to produce alternative community states as a function of variability in a sequence of species invasions employed to assemble a community. Numerous mechanisms and processes are capable of producing both temporary and nonrecoverable differences in community structure. They include priority effects, intransitivities, emergent properties (e.g., vulnerability to invasion, specific topologies), and effects specific to differences in assembly sequences themselves. In ecological systems, the existence of alternative states presents a difficult comparative problem in the search for unifying principles that might underlie community-level organization. This problem can be solved only if we understand the cause of alternative states and determine whether such states are persistent or transient. Otherwise, ecological communities appear so variable that general principles and mechanisms elude detection, even if they exert a powerful influence over the structure of the system. Mechanisms that are ...

Homoplasy: the result of natural selection, or evidence of design limitations?

Abstract: Similarity in morphological form may arise from common ancestry (failure to evolve), from parallel evolution, from convergence, or from reversal to an apparently ancestral condition. Homoplasy from convergence, parallelism, and reversal is common, and its ubiquity creates difficulties in phylogenetic analysis. Convergent evolution often is considered one of the most powerful lines of evidence for adaptive evolution. But an alternative explanation for convergence and other evolved similarities is that limited developmental and structural options exist. Identical forms can be obtained when particular developmental phenomena are triggered by very different kinds of stimuli or when constraints exist that shape external form or limit morphological expression to a few options. Examples from plethodontid salamanders are used to illustrate an approach combining internalist and externalist analytical methods. In order to explain how morphologies evolve in lineages, both functionalist and structuralist approaches a...

A Cost-Benefit Analysis of Leaf Habit and Leaf Longevity of Trees and Their Geographical Pattern

Abstract: To maximize net gain of a tree, leaves must be replaced when net gain of a leaf per unit time over the leaf's life span is maximum. A model in which leaf longevity is determined to maximize the net gain of a leaf per unit time is constructed. The model predicts that leaf longevity is short when initial net photosynthetic rate of the leaf is large, long when the construction cost of the leaf is large, and short when the decrease in net photosynthetic rate with time is large. The model describes leaf habit (deciduousness and evergreenness) with the length of the favorable period for photosynthesis within a year and simulates distributional pattern of leaf habit along latitudes. The percentages of evergreenness decrease with decreasing favorable-period length and reach the minimum at an intermediate length of the favorable period but increase again with a decrease in the length of the favorable period. A bimodal distributional pattern with two peaks, one at lower and the other at higher latitudes, is observe...

Forage Quality and Aggregation by Large Herbivores

Abstract: Three general hypotheses have been proposed to explain why many large herbivores have highly aggregated patterns of distribution: dilution of predation risk, maintenance of forage in an immature bu...

On conditions for evolutionary stability for a continuously varying character

Abstract: The two conditions for stability of an evolutionary equilibrium, the m-stability and the δ-stability conditions, are discussed. The m-stability condition is a condition for the convergence of the population toward the equilibrium, and the δ-stability condition coresponds to a local version of the classic evolutionarily stable strategy (ESS) condition. Together the two conditions provide the condition for a continuous stable strategy. The convergence stability condition corresponds to the requirement for convergence due to initial increase of rare alleles in a monomorphic population, and the local ESS stability condition corresponds to the stability of a monomorphic population at the evolutionary equilibrium against the increase of rare alleles. In this way, an evolutionary equilibrium that is convergence stable, but not local ESS stable, will tend to become polymorphic. The local ESS stability condition therefore contributes more to a description of the dynamics of variation at an evolutionary equilibrium...

Determinants of Plant Distribution: Evidence from Pine Invasions

Abstract: The question of which factors limit the occurrence of a plant species to a particular site is addressed by considering 53 cases in which the distribution of pines (Pinus species; Pinaceae) has changed in the last century. We consider expansions of pines in and adjacent to their natural ranges in the Northern Hemisphere and the spread from sites of introduction in the Southern Hemisphere well outside the contemporary range of pines. We first consider a neutral hypothesis (with respect to climate or biological interactions as determinants of invasion): invasion simply requires that a species is present in sufficient numbers, with sufficient propagules over sufficient time to invade. We then explore the relative importance of climatic changes, disturbance, competition (including competition between seedlings and herbaceous plants during early establishment), herbivory, pathogens, and other agents that might influence pine membership in communities. Determinants of susceptibility to invasion often interact in...

Escalation of plant defense: do latex and resin canals spur plant diversification?

Abstract: Ehrlich and Raven's postulate that rapid diversification follows innovation in plant defense has often been invoked a posteriori for plant lineages of unusual diversity and chemical distinctiveness. The postulate can be more rigorously tested by defining a novel class of defense using chemical and/or anatomical criteria, independent of taxonomic lineage. If multiple plant lineages have evolved the new defense type, then according to the postulate they should be consistently more diverse than their sister groups (of equal age, by definition) when the latter retain the primitive defensive repertoire. Secretory canals are an independently defined, repeat- edly evolved feature that functions to protect plants from herbivores and pathogens. The canals might therefore be expected to allow plant radiation in an adaptive zone of reduced herbivory and disease. We have quantified the evidence for this hypothesis by comparing the diversities of lineages that have independently evolved canal systems with their sister groups for as many plant lineages as current taxonomic evidence allows. A sign test showed that canal-bearing lineages have consistently higher diversities than their sister groups (P = .0021). Explanations for this result, other than selective advantage conferred by secretory canals, are examined and provisionally rejected.

Intersexual Dietary Divergence and the Evolution of Sexual Dimorphism in Snakes

Abstract: Ecological causes for the evolution of sexual dimorphism can be confidently inferred only when the sexes differ in morphology or relative size of the feeding apparatus, in a direction inconsistent with that expected from sexual selection. Snakes are well suited for such an analysis because head sizes in this group are important for feeding but not for reproductive behavior. My data reveal significant sexual dimorphism in head size (relative to snout-vent length) in 47% of 114 species examined from seven families. Head size relative to body size is strongly correlated between males and females in comparisons among populations within species, among species within genera, and among genera within families. Hence, correlation between the sexes may powerfully constrain evolutionary shifts in dimorphism. Nonetheless, phylogenetic analysis identifies many independent origins and losses of the dimorphism. Geographic variation in relative head size and sexual dimorphism in head size are evident within wide-ranging ...

Metapopulation Models: The Rescue Effect, the Propagule Rain, and the Core-Satellite Hypothesis

Abstract: Metapopulation models are important ools for understanding distribution and abundance of organisms on large spatial scales (Levins 1969a; Hanski 1989). These models integrate local population dynamics with immigration and extinction events occurring between population sites (Levins 1969a, 1970; den Boer 1981; Hanski 1982, 1989). In this sense, they form a bridge between the traditionally separate domains of population ecology (local abundance) and biogeography (regional occurrence) (Andrewartha and Birch 1954; Hanski 1982). Metapopulation models provide a useful framework for understanding both correlative (Gill 1978; Hanski and Ranta 1983; Harrison et al. 1988) and experimental (Bengtsson 1989) data on distribution and abundance of natural populations. Extensions to the optimal design of subdivided nature reserves are also promising (Quinn and Hastings 1987). Levins (1969a, 1970) introduced an important class of metapopulation models of the following form:

Associations of Song Properties with Habitats for Territorial Oscine Birds of Eastern North America

Abstract: To investigate adaptations for long-range acoustic communication in birds, I analyzed associations between broad categories of habitats and properties of territorial songs for eastern North American oscines. From published recordings. I obtained three frequency properties (maximal, minimal, and dominant) and three temporal properties of songs (presence of sidebands, presence of buzzes, minimal period of repeated elements). Sidebands and buzzes indicated rapid amplitude modulation of a carrier frequency. Habitats occupied by territorial males were classified into six categories (broad-leaved or mixed forest, coniferous forest, parkland or forest edge, shrubland, grassland, and marshes). Frequencies in songs correlated strongly with body size, which varied among habitats. Analysis of covariance and phylogenetic regression, after controlling for body size, revealed an association of maximal but not dominant or minimal frequencies with habitat. In contrast, the temporal properties of song were all strongly as...

The Benefits-of-Philopatry Hypothesis for the Evolution of Cooperative Breeding: Variation in Territory Quality and Group Size Effects

Abstract: Cooperative breeding systems are characterized by the presence of individuals that remain as nonbreeders in their natal groups past the age of reproductive maturity. Currently, the most widely accepted explanation of this nondispersal is that immature members of cooperative species are faced with ecological constraints on independent breeding, such as habitat saturation. However, current evidence indicates that reproductive opportunities in many noncooperative breeders (where immature individuals always disperse) are just as limited as those in cooperative species. Thus ecological constraints alone cannot explain the differences in the two dispersal patterns. As an alternative, we describe a "benefits of philopatry" hypothesis, which proposes that nonbreeding helpers remain at home only when there is a net fitness benefit to doing so. This is most likely to occur either when territories in a local area vary greatly in quality or when group cooperation leads to high variance in fitness of individuals in di...

Genetic Variation for Habitat Preference: Evidence and Explanations

Abstract: Because adaptive shifts may often be initiated by evolutionary changes in behavior, it is of interest to determine the extent to which natural populations harbor genetic variation for ecologically important behaviors. Habitat preference is an especially significant behavior, be- cause it determines the regime of natural selection acting on loci that affect adaptation to the environment. A survey of the literature reveals that genetic variation for habitat selection is common, especially in arthropods and mollusks, the groups that have been studied most fre- quently. Possible adaptive mechanisms by which this variation could be maintained within populations include a genetic correlation between density-independent fitness in a habitat and a preference for it; and soft selection, whereby density-dependent population regulation occurs independently in separate habitats. Several studies have documented a phenotypic correlation between preference and performance, but as yet, no such genetic correlations have been un- equivocally demonstrated. We show theoretically that under hard selection, optimal habitat selection may often lower the probability of maintaining a polymorphism at a locus that affects adaptation to different habitats. Soft selection appears much more likely to promote variation for habitat preference. Mechanisms including resource competition and natural enemies whose numbers build up in a habitat-specific manner in response to host or prey density have the capacity to bring about selection favoring alleles whose carriers prefer relatively underused habitats. We believe that more progress in understanding the evolution of habitat preference will come from studies of these ecological mechanisms than from further demonstrations of the mere existence of genetic variation for such preferences.

Producers, Scroungers, and Group Foraging.

Abstract: Abstrac-t.We have developed a model that reconciles information-sharing and producerscrounger models of group foraging. Our model includes producers, scroungers, and an opportunistic forager that can both produce and scrounge but with reduced efficiency. We show that these three strategies can coexist only in the unlikely case that the opportunist's loss in searching ability is exactly equal to its gain in scrounging ability. However, all pairs of strategies can coexist. Three parameters control the proportions of coexisting strategists: the degree of compatibility between the opportunist's producing and scrounging activities; the proportion of food patches that are shared with scrounging individuals; and the effective group size. When there is little incompatibility between producing and scrounging, opportunists will always be present, unless the producer is able to consume most of the patch without sharing. The opportunist strategy is always excluded when there is a high degree of incompatibility between producing and scrounging. We consider the organismal and ecological factors that are likely to affect all three parameters. Our model predicts that scrounging behavior is likely to be selected in a wide range of foraging groups and that it may impose a considerable cost on sociality.

Spatial scaling of species composition: body masses of north american land mammals

Abstract: We describe the nonrandom assembly of the North American terrestrial mammalian fauna based on body size and spatial scale. The frequency distribution of body masses among species for the entire continental fauna was highly modal and right skewed, even on a logarithmic scale; the median size of the 465 species was approximately 45 g. In contrast, comparable frequency distributions for 24 small patches of relatively homogeneous habitat were essentially uniform, with approximately equal numbers of species in each logarithmic size class; the median sizes of the 19-37 species ranged from approximately 100 to 2,500 g. Frequency distributions for 21 biomes (large regions of relatively similar vegetation) were intermediate between the continental and local assemblages. This pattern of assembly indicates that species of modal size (20-250 g) tend not to coexist in local habitat patches and they replace each other more fre- quently from habitat to habitat across the landscape than species of relatively large or small size. We hypothesize that three mechanisms are necessary and possibly sufficient to produce this result: competitive exclusion of species of similar size within local habitats, differential extinction of species of large size with small geographic ranges, and greater specialization of modal-sized species owing to energetic and dietary constraints.

Habitat selection in plants

Abstract: Habitat-selection concepts have rarely been explicitly used for plants, perhaps because the majority of them are immobile. For plants, habitat selection results from evolutionary adjustment of species to environmental factors so that the species functions better in some habitats than in others. Habitat choice refers to the ability of a plant to disperse, in space or time, to preferred patches. Habitat specialization means that a species performs best in a small subset of patches in a given location. The modular structure of plants and their growth response to the patterns of resource availability allows them to occupy large areas, exposing them to spatial and temporal heterogeneity. Modular structure, to a large extent, determines the options available, as well as the constraints, for plants choosing habitats. Choice, however, may be made by the habitat rather than by the plant. That is, the characteristics of the habitat determine which species of the plants that disperse into the habitat become establis...

Evolution of colony characteristics in social insects. ii. number of reproductive individuals

Abstract: The inclusive-fitness models and single-locus gene frequency models suggest different thresholds for the cost allowing worker reproduction to evolve The advantage of the inclusive-fitness approach is that it allows combining the invasion and loss of worker reproduction in the same model According to the results derived here through this model, both polyandry and polygyny can restrict the conditions for the evolution of worker reproduction There is a conflict between queens and workers concerning both worker reproduction and sex allocation If a colony becomes either polygynous or polyandrous, the queens and workers can have similar interests concerning reproduction, and the colony could act in harmony to increase the inclusive fitnesses This requires that the workers can recognize the genetic heterogeneity within the colony and adjust their own behavior optimally If the workers can so behave, polyandry could evolve as the queens' way of reducing the queen-worker conflict and manipulating the workers

Occurrence patterns of bird species in habitat fragments : sampling, extinction, and nested species subsets

Abstract: Comparison of the species-area relationship in unfragmented chaparral habitat with that in urban chaparral fragments confirmed that rapid population extinction of resident bird species has occurred in these fragments. A strong positive correlation between the relative persistence ability of a species and its density remains even after correcting for the sampling effect of area. We conclude that this pattern is due to differences between species in extinction vulnerability attributable to density; the more abundant species persist longer in fragments. This differential vulnerability to extinction produces a pattern in which the bird species present in species-poor fragments are nested subsets of those in species-rich fragments.

The Developmental Basis of Worker Caste Polymorphism in Ants

Abstract: Among social Hymenoptera, the evolution of the worker caste has reached its apex in the ants, in which some taxa have evolved complex physical worker caste systems. Diverse worker caste systems can be generated through regulation of three aspects of larval growth: critical size, growth parameters, and reprogramming of these factors. Even the most complex caste systems could have evolved simply by the addition of revised programs to the end of an ancestral developmental pathway for workers. Worker castes in ants provide a system in which to study the evolution of reaction norms and developmental switches. In ants, the simplest developmental switch, revision of critical size alone, does not lead to discontinuous phenotypes. Only when changes in growth rules are tied to the decision to revise critical size are distinct phenotypes produced from the alternative developmental programs. The addition of ever more physical castes may be limited by both developmental and ecological factors.

Sperm Competition, Sperm Depletion, Paternal Care, and Relative Testis Size in Birds

Abstract: Testis mass in birds scales allometrically to body mass, the exponent being signifi- cantly smaller than unity. Two hypotheses were formulated in order to account for variation in relative testis size: (1) the sperm competition hypothesis, according to which males of taxa with a high intensity of sperm competition have larger testes than males of other taxa; and (2) the sperm depletion hypothesis, which proposes that taxa in which males copulate often also have large testes. Variation in testis mass after controlling for the effects of body mass and phylogeny was investigated in relation to copulation frequency per female (as a measure of the intensity of sperm competition) and per male (as a measure of the intensity of sperm depletion). Variation in relative testis mass was positively related to variation in copulation frequency per female, and the variation was significant; the sperm competition hypothesis was therefore supported by data. The sperm depletion hypothesis could not account for variation in testis size. Variation in relative testis mass was positively correlated with variation in social dispersion and was nega- tively correlated with variation in mate-guarding behavior.The extent of paternal care was re- lated to certainty of paternity as estimated from the presence of mate guarding, but not from relative testis mass.

Effects of Severe Defoliation on the Long-Term Resistance to Insect Attack and on Leaf Chemistry in Six Woody Species of the Southern African Savanna

Abstract: Severe defoliation as occurs in insect outbreaks can alter the chemistry and food value of woody-plant leaves for insects for several years after defoliation ceases. Two hypotheses, the active-defense hypothesis (AD) and the hypothesis of a carbon/nutrient balance (CNB), attempt to explain the mechanism of such responses to defoliation. We tested these hypotheses by studying the responses to severe defoliation by six southern African savanna woody species. Manual defoliation of the three fast-growing deciduous species we studied (Grewia flavescens, Acacia tortilis, Dichrostachys cinerea) resulted in increased resistance to insect attack in the next year. Concomitantly, nitrogen (N) and phosphorus (P) concentrations in leaves decreased and leaf total phenol and condensed tannin concentrations increased. The three slowly growing species that we studied (Burkea africana, Ochna pulchra, Euclea natalensis) responded differently to manual defoliation. In the year following defoliation, leaf resistance to insect...

Ecological aspects of the geographical distribution and diversity of mammalian species

Abstract: Using the comprehensive collection of range maps compiled for 679 species of North American mammals by Hall, we compute the distribution in the sizes of the geographical ranges of these mammalian species. The distribution is roughly lognormal. This corresponds to high skewing on a linear scale toward small geographical ranges, with a median geographical range of around 1% of the area of North America; only about 2% of the species have ranges exceeding 50% of this area. There are differences among taxa; skewing is high for bats, rodents, insecti- vores, and lagomorphs and less pronounced for carnivores and artiodactyls. In contrast to Rabinowitz et al.'s study of "seven forms of rarity," which found most of the British flora in the study to be widely distributed throughout Britain, we find that most mammalian species in North America have relatively restricted geographical ranges. This difference may derive from systematic differences between plants and animals, or from the small size and relative homoge- neity of Britain, or from other causes. The fact that half or more of North American mammals have ranges smaller than the combined size of Nicaragua and Honduras has implications for conservation biology, especially because such restricted patterns are more pronounced in the tropics, where deforestation is currently most severe. As mentioned by earlier authors, there are both latitudinal and longitudinal gradients in range size, with average ranges increasing to the North and to the East. At any one latitude, however, there is wide variability in range size. Corresponding to these gradients in range size are latitudinal and longitudinal gradients in species density, with density increasing as average range size decreases. These latitudinal gradients have long been appreciated, but we think that the longitudinal gradients may ultimately prove more illuminating because they are less confounded by the kinds of systematic changes (in temperature, etc.) that accompany latitudinal change. Following Aldrich and Loftas, we identify 23 distinct habitat types in North America and show that patterns both in geographical range and in species density are correlated with the number of different habitats a species range includes: high density of species and small geographical range sizes occur in areas of high habitat diversity. Correlations between the number of habitats (and the average geographical range of habitats) versus latitude and longitude explain some, but not all, of the geographic variation in species' range patterns noted in this article. Finally, we show that species are more habitat specific (having ranges embracing fewer distinct habitat types) toward more southern latitudes and toward more western longitudes. Much of what we know about continental variation in the number of different

Resource Competition in a Variable Environment: Phytoplankton Growing According to the Variable-Internal-Stores Model

Abstract: Many studies in population ecology and competition theory are based on models in which the consumption rate of a resource is some function of resource availability and in which the yield of new consumers is constantly proportional to the amount of resource consumed. For such models, the best competitors at equilibrium have low resource requirements, and the best competitors in nonequilibrium habitats have high maximal growth rates. In this study, I abandon the assumption of constant yield and allow consumption rates to be a function of the consumer's internal state with respect to a resource, as well as a function of external resource availability. Specifically, I study functions and parameters describing the competition of planktonic algae for a dissolved nutrient, phosphorus, that is supplied in periodic pulses. In this model, trade-offs between competitive abilities in equilibrium and nonequilibrium habitats can arise in several ways. The most important trade-offs are likely to involve the capacity to ...