Showing papers in "The American Naturalist in 1976"

Species Diversity Gradients: Synthesis of the Roles of Predation, Competition, and Temporal Heterogeneity

Abstract: We suggest that the "predation" and "competition" hypotheses of community organization and species diversity are complementary. Maintenance of high diversity by competition appears to be relatively more important at higher trophic levels, while maintenance of high diversity by predation seems relatively more important at lower trophic levels. Further, predation is probably the dominant organizing interaction in trophically complex communities, while competition is probably the dominant organizing interaction in trophically simple communities. These hypotheses are supported on a local scale by experimental studies in the rocky intertidal communities of New England and the West Coast. A probable consequence of its greater temporal heterogeneity (i.e., a less stable, less predictable, and more stressful environment) is that the East Coast is trophically more simple and has an increased incidence of competitive exclusion. As a result, diversity is lower on the East Coast compared with the West Coast. A simila...

Bifurcations and Dynamic Complexity in Simple Ecological Models

Abstract: Many biological populations breed seasonally and have nonoverlapping generations, so that their dynamics are described by first-order difference equations, Nt+1 = F (Nt). In many cases, F(N) as a function of N will have a hump. We show, very generally, that as such a hump steepens, the dynamics goes from a stable point, to a bifurcating hierarchy of stable cycles of period 2n, into a region of chaotic behavior where the population exhibits an apparently random sequence of "outbreaks" followed by "crashes." We give a detailed account of the underlying mathematics of this process and review other situations (in two- and higher dimensional systems, or in differential equation systems) where apparently random dynamics can arise from bifurcation processes. This complicated behavior, in simple deterministic models, can have disturbing implications for the analysis and interpretation of biological data.

Optimal foraging: Attack strategy of a mantid

Abstract: Much recent theoretical work in foraging theory derives hypotheses from the general assumption that animals are "efficient" in their foraging activities. Useful reviews may be found in Krebs (1973), Schoener (1971), MacArthur (1972), and Charnov (1973). The problems considered usually relate to breadth of diet (Schoener 1969, 1971; Emlen 1966; MacArthur 1972; Marten 1973; Pulliam 1973; Werner and Hall 1974; Timin 1973; MacArthur and Pianka 1966; Pearson 1974; Rapport 1971), strategies of movement (Cody 1971; Pyke 1974a, 1974b; Smith 1974a, 1974b), or use of a patchy environment (Royama 1970; MacArthur and Pianka 1966; Pulliam 1973; Krebs et al. 1974; Smith 1974a, 1974b; Tullock 1970; Emlen 1973; Charnov, Orians, and Hyatt 1975). This list includes only the more theoretical contributions and is far from exhaustive. Several studies test the predictions of these formal models (Krebs et al. 1974; Pyke 1974a; Werner and Hall 1974; Smith 1974a, 1974b). In this paper I develop a simple model of breadth of diet for a randomencounter situation. Predictions made by the model will then be compared to the behavior of a real predator, the mantid, Hierodula crassa. I show that this mantid supports the predicted behavior. The final section argues that, even though the behavior is as predicted, several alternative explanations are not excluded. In fact, at least with this type of predator, the foraging behavior may reflect several ultimate factors, with energetic efficiency being only one.

Ecological Implications of Resource Depression

Abstract: A common but not universal consequence of the foraging activities of a predator is a lowering of capture rates with prey in its immediate vicinity. This may result from a number of different processes and need not require actual harvesting of any prey items by the predator. We term this phenomenon "depression" and its ecological consequences are the focus of this paper. A great deal of attention has been devoted to implications of changes in prey abundances as a result of the activity of predators. We will focus on changes in prey availability strictly in terms of the perspective of actual or potential predators. Potential here refers to predators who may visit the site shortly after "the prey have been depressed." Depression phenomena are familiar to most field ecologists but are seldom incorporated into formal ecological theory. While they are not really separate, it will be useful to divide our discussion into several sections, each focusing on a different question. (1) What are the processes by which prey might be depressed? (2) What are the characteristics of prey which affect their depressibility? Their recovery from the depression? (3) Are there prey that cannot be depressed? (4) What are the implications of depression from the viewpoint of a single predator individual? (5) Since prey depressed for one kind of predator are not necessarily depressed for another kind, what are the implications for competition theory? Here we are concerned with the possibility of enhancement of availability, as well as competition.

Life history patterns in zooplankton

Abstract: Variation in life history patterns has proven amenable to analysis based on the premise that such variation represents adaptive strategies (Cole 1954; Gadgil and Bossert 1970). Highly unpredictable or seasonal environments may favor opportunistic species with high intrinsic rates of increase ("r selected"), while more constant environments do not ("K selected" [MacArthur 1972]). Inspection of a natural community within a single environment reveals that the problem of optimally allocating resources among such conflicting demands as maximizing the potential for rapid increase, predator avoidance, and competitive ability may be met by a virtual continuum of compromises. Zooplankton communities illustrate this well (Hutchinson 1967) and are particularly amenable to quantitative analysis. Parthenogenetic and sexually reproducing zooplankton populations are both well represented in freshwater. Rotifers and cladocerans fall into the former category, have short multivoltine life cycles, are rather unspecialized feeders, and develop large, transitory populations. Other plankters, copepods for example, exhibit longer life cycles and fewer generations to the point of occasionally being biennial or triennial. Quantitative comparisons have not been made, however, beyond the reasonably obvious interpretation that r is likely to be higher in the former cases. This paper attempts to place the comparison of zooplankton life histories on a quantitative basis by estimating intrinsic rates of increase (r), thereby examining alternative life histories which apparently differ in their trade-offs among reproductive potential, predator avoidance, and competitive ability. The implications of this analysis will lend insight into seasonal abundance fluctuations in zooplankton and into certain biogeographic observations, including the composition of freshwater and marine zooplankton communities.

Sizes and Shapes of Liane Leaves

Abstract: We have developed evolutionary models to account for ecological patterns in the size and shape of vine leaves. Leaf size is predicted via an economical model in which photosynthetic gains due to enhanced gas exchange are balanced with the metabolic losses incurred in replacing the concomitant water loss. Mechanical efficiency and appropriate orientation in a light field form the basis of our model for leaf shape. Data from montane rain forest in Venezuela and from varied habitats in Costa Rica illustrate the predicted trends along vertical and horizontal microclimatic gradients. Large, cordate-based leaves with long petioles are favored in sunny situations, while small, narrow-based leaves with short petioles are favored in more shady environments. Compound or deeply lobed leaves of the smallest effective size are expected in the most exposed positions.

The Coevolution and Stability of Competing Species

Abstract: If two species compete for two renewable resources, will they coevolve so as to make continued coexistence more likely? This question is analyzed using the equations of MacArthur (1972) but allowing the constants in those equations to undergo evolutionary change, subject to constraints. Individual selection will cause these variables to take values which are evolutionarily stable, in the sense that if all individuals in a population have those values no mutant can invade it. It is then possible to compare the evolutionarily stable values for each species on its own with the values when they coexist. Two cases are considered: a coarse-grained case in which at any instant a consumer can search for one resource or the other but not for both, and a fine-grained case in which both resources can be sought simultaneously. In both cases, each consumer in the absence of its competitor would evolve as a generalist. In competition, each consumer will coevolve as a specialist on a different resource patch type in the...

Sexual selection and paternal investment in insects

Abstract: One way in which males offset the disparity in relative parental investment is to provide food to the female or offspring in the context of reproduction. In insects, nutritional paternal investment during reproduction may be of three major types: (1) females may receive nourishment from a glandular product of the male; (2) females received nourishment from food captured or collected by the male; and (3) the male himself may be eaten. I have shown elsewhere that the feeding of the female by the male during copulation in insects of the family Bittacidae probably evolved by female preference (i.e., intersexual selection) for males willing and capable of increasing their parental investment during reproduction. I hypothesize that female preference for greater male parental investment may have been the selective context for the evolution of all types of male investment patterns in insects. I also suggest some ways in which my argument might be tested.

Animal Behavior as a Strategy Optimizer: Evolution of Resource Assessment Strategies and Optimal Emigration Thresholds

Abstract: Models are proposed which examine emigration thresholds from resource patches encountered within a given search strategy. With one patch type and no competition during utilization, there is a single optimal investment duration (Iopt) which depends on the gain accumulated during investment in the patch, G(I), and on the mean search cost between patches. Solutions for Iopt depend on the form of G(I); various models are considered which offer diminishing returns with investment. With variation in the value of the patch type encountered, the animal should leave a patch when dG/dI becomes equal to SLmax, the average gain rate for the overall search strategy when the investment in each patch is optimal. For some resources, Iopt may equal 0. Selection should here favor more and more efficient "resource assessment" strategies which ensure the most efficient monitoring of cues correlating with G(I). These are all pure stratengies in relation to particular circumstances. Where there is resource sharing and systems ...

On the fitness of behavior sequences

Abstract: Our aim is to demonstrate a method of determining the extent to which behavior maximizes fitness. We believe the temporal organization of behavior to be in part dependent on the animal's genetic makeup and subject to natural selection and that behavioral strategies may be as adaptive as structural characters. The question of the adaptiveness of behavior has been found hard to study in the past (Tinbergen 1963) because of the difficulty of quantitative verification of hypotheses. The method presented here tests specifically whether deployment of behavioral options is optimally related to environmental conditions.

Competitive Interactions in Ecosystems

Abstract: An analysis was made of the indirect interactions between competing consumers. Two mechanisms for such interactions were investigated, interactions between the resource species and competition from a third consumer species. In both cases it was shown that the overall interaction between the consumers could be beneficial to both and that interaction between the species does not depend on direct niche overlap.

Autotrophy, Heterotrophy, and Resource Partitioning in Caribbean Reef-Building Corals

Abstract: Competition can be shown to occur between two species when the removal or thinning of one causes an increase in numbers of the other. This is due to an increase in availability of resources formerly used by both. Over evolutionary time, competition can lead to the extinction of one of the species or to specialization onto different resources by either or both species involved, that is, niche separation, with a concomitant reduction in the degree of competition. Resource partitioning often occurs by character displacement in feeding structures or in feeding behavior in animals or in plants by adaptation to different physical regimes (Schoener 1974). How scleractinian reef corals compete for space (Lang 1973) and how these competitive strategies affect he community structure of New World coral reefs (Porter 1974a) has been worked out in detail. This paper will examine the consequences of competition for energy resources within the Caribbean fauna. Reef-building (hermatypic) corals differ considerably from most animals in functioning at every trophic level of their ecosystem (Porter 1974b). They contain symbiotic algae and can in shallow water produce more oxygen than they consume, placing them among the reef's primary producers. The importance of this symbiosis lies in the tight chemical cycling it allows between plant and animal in response to the nutrient deficiency, particularly of nitrogen, in tropical waters (Lewis 1973). The importance of this symbiosis is also clearly reflected in the morphology of many species, a number of which are plantlike in their growth forms, maximizing their surface area and orienting the major axis of their growth plane toward the light (Goreau 1959; Roos 1967). Although incontrovertible vidence demonstrates the movement of photosynthetic products (primarily glycerol, and to a lesser extent glucose, alanine, and glycolic acid [Muscatine 1973]) from the associated symbiotic algae to the coral, the absolute value of this organic material has not been established. One of the principal theses of this paper will be that, arguing from morphological evidence alone, one can estimate the potential energetic ontribution of the zooxanthellae relative to other means of energy procurement by each species. Equally incontrovertible evidence exists that corals eat zooplankton (Yonge

Ecological Relationships of Co-Occurring Goldenrods (Solidago: Compositae)

Abstract: Comparisons are made of propagule characteristics and niche relationships among co-occurring species of goldenrods (Solidago) in a 23-yr-old field and a virgin tall-grass prairie. Five of the six goldenrod species from the prairie site are the same species as those found in the old field, allowing an empirical study of differences between "open" and "closed" communities postulated by earlier theoretical treatments. In general, each species produced fewer and heavier propagules with a reduced dispersal capacity in the prairie compared with the old-field populations. Under increasing competitive pressure, selection favors larger propagules. Assuming a species does not increase the absolute amount of resources allocated to propagule production, then heavier propagules should result in fewer propagules. Both changes result in decreased dispersal capabilities. The relationship between the annual number of propagules (N) per basal stem to the mean weight of an individual propagule (W) is N = kwb, where k and b ...

Succession: An Evolutionary Interpretation

Abstract: An evolutionary interpretation of succession includes the following points. First, in order for a population to be well adapted to a particular set of conditions, it cannot be adapted to many different sets. There is an apparent limit to the width of niche and of habitat that a population can exploit effectively. Selection preserves these unique adaptations and fosters population coupling with the environment. Second, succession provides a complex gradient of physical and biotic environments, analogous to spatial gradients, to which species respond in both ecological and evolutionary time. Selection adjusts the positions of populations on successional gradients in the same way they are adjusted on spatial gradients. Third, the evolutionary strategy, which involves life-cycle length, time of reproduction, reproductive output, and ultimately the degree of recombination, is a basic determinant of species position on both spatial and successional gradients. Fourth, patches of different successional environmen...

Sexual Selection and Nuptial Feeding Behavior in Bittacus apicalis (Insecta: Mecoptera)

Abstract: Field and laboratory studies of Bittacus apicalis reveal the probable occurrence and importance of intersexual selection (female choice) in these insects. Females select males on the basis of the a...

Seasonality, Resource Availability, and Community Diversity in Tropical Bird Communities

Abstract: Seasonal changes in the species composition and abundance of birds in seven habitats in central Panama are investigated to determine the extent to which equitable seasonal patterns in tropical regions are important in the evolution of species-rich tropical avifaunas. A brief review of the literature documents the distinct seasonality of rainfall and seasonal variation in the abundance of food resources (especially insects and fruits) exploited by the avifauna. Seasonal variation in avian community structure decreases with increasing vegetation complexity. This is apparently due to the increased buffering of the physical environment by the more complex vegetation. However, it is not sufficient to use a single index to examine whole avian communities. The impact. of seasonality varies among subsets of the avifauna. Insectivore species diversity and abundances generally vary more seasonally than do frugivore diversity and abundances in structurally mature habitats. Within shrub and forest habitats the degree...

The relation of reproductive effort to age

Abstract: The paper uses the reproductive effort model of Schaffer (1974) to provide conditions on the change in reproductive effort with age in evolutionarily stable life histories. It is shown that low adult mortality, continuing adult growth, a low rate of population increase, and a high sensitivity of survival and growth to reproduction all tend to favor increase in reproductive effort with age. Reproductive effort may decrease with age if the converse conditions hold.

Tautology in Evolution and Ecology

Abstract: Analysis of a number of popular ecological tenets, including natural selection, competitive exclusion, and parts of succession, species diversity, and spatial heterogeneity, reveals that they lack the predictive and operational qualities which define scientific theories. Instead they consist of the logical elaboration of certain axioms. Consequently, they must be termed tautologies. Tautologies may be useful logical aids, but they cannot replace true theories. Unless ecologists are careful to distinguish the two, their confusion may produce a body of thought resting on metaphysical rationale rather than empirical, predictive science.

Alkaloid-bearing plants: an ecogeographic perspective

Abstract: The incidence of alkaloid-bearing plants is dependent upon their habit and ecogeographical distribution. Among annual species the incidence of alkaloids is nearly twice that of perennials, among tropical floras nearly twice that of temperate floras, and a latitudinal cline is evident. In New Guinea, disparate communities differ in the incidence of alkaloid-bearing species and in the amount of alkaloid contained in their vegetative tissues. Families primarily distributed in the tropics have a higher percentage of alkaloid-bearing species than do those of temperate regions or those with cosmopolitan distributions. The most primitive orders, Magnoliales and Ranales, have a higher percentage of alkaloid-bearing species than the remainder of the dicots. These findings are discussed from the vantage point of coevolutionary theory. It is suggested that the ecogeographic patterns may be the result of differences in pest pressure, the alkaloids playing a defensive role in plants.

Resource Division in an Understory Herb Community: Responses to Temporal and Microtopographic Gradients

Abstract: Studies of niche division among forest herbs are relatively rare, yet in many temperate forests a majority of the vascular species are herbaceous. Although the distributions of herbaceous species often change along the major gradients provided by topography, moisture, or elevation (Davidson and Buell 1967; Bell 1974), the high within-plot diversities of some mesic herb communities, where a single square meter may contain more than a dozen species, imply that microgradients are equally important. Among the microenvironmental gradients already shown to affect he distributions of herbaceous understory species are: (1) microtopography, including tree fall mounds and local soil moisture gradients (Struick and Curtis 1962); (2) structure of the canopy, including the position and size of openings, throughfall, ight, and distance from trees (Moir 1966; Anderson et al. 1969; Eber 1972); (3) seasonal change, including differences in temperature and illumination (Vezina and Grandtner 1965; Bazzaz and Bliss 1971). This study investigates understory response to two types of microgradients-seasonal change and microtopography-in a species-rich cove hardwood forest.

The dorsal pigmentation pattern of snakes as an antipredator strategy: a multivariate approach

Abstract: Each species of the snake fauna of North America north of Mexico was qualitatively scored into one of five dorsal pattern classes and according to 16 behavioral-ecological variables that express th...

The optimal timing of reproduction

Abstract: The optimal timing of reproductive effort is determined in a model which takes into account (a) the physiological limits on the growth and biomass production of the individuals, (b) the survival function of the population, and (c) the reutilization of somatic biomass for reproduction at the end of the season. Total reproductive biomass is maximized by a step transition from somatic growth to reproductive production. Optimal transition is at a biomass which is much less than the maximally attainable biomass. Optimal transition time is an increasing function of the survival and of the reutilization conversion factor. Much higher optimal transition biomass is expected where reproductive success depends not only on the production of reproductive biomass but is an increasing function of somatic biomass as well.

Food plant specialization and environmental predictability in lepidoptera

Abstract: The expectation that the niche breadth of species in relatively unpredictable environments should be greater than that of comparable species in more reliable environments is violated by the larval feeding habits of temperate zone Macrolepidoptera. Species that feed primarily on herbaceous plants are more often restricted to a single plant family than are species which eat the leaves of trees or shrubs. This pattern appears attributable to the coevolution of oviposition preferences with the evolution of plant defense systems, which probably tend toward greater diversity in floristically diverse plant communities and toward uniformity in less diverse communities.

Niche Width in a Fluctuating Environment-Density Independent Model

Abstract: A model of the effect of density-independent optimizing selection on the distribution of a character is developed. Both the direct effect of the selection on the character and the secondary effect of the selection on a modifier allele which changes the distribution is analyzed. The results are that there would be selection for a lower variance in a constant environment and selection for a larger variance when the optimal value of the character is changing in time, only if the fluctuations in the optimum exceed a certain threshold amplitude. The application of the model to populations subject to different temporal patterns of fluctuating selection is discussed.

The Demographic Genetics of Liatris cylindracea Michx. (Compositae)

Abstract: The demographic genetics of a population of Liatris cylindracea were studied. The population has a nonstable age structure; there is a deficiency of young plants. Plants were divided into six age classes and the average heterozygosity of each age class measured at 27 allozyme loci. Average heterozygosity increases with increasing age class while FST, the standardized genetic variance, decreases. In addition, a positive relationship was found between individual heterozygosity and fecundity, longevity, and speed of development. Both age and reproductive potential are positively correlated with individual heterozygosity. The time of initial flowering is also related to heterozygosity; plants flowering after 2 seasons being more heterozygous than plants which do not flower. It is concluded that individual heterozygosity is being selected for.

What Determines the Rate of Evolution

Abstract: A new type of genetic load, the "lag load," is defined; it measures the extent to which a population is lagging behind the contemporary environment. An expression is obtained for the rate of evolution of a haploid sexual population as a function of the lag load, population size, per locus mutation rate, and selective advantage per favorable mutation.

The Ecological Factors Selecting for Polygyny in Altrical Birds

Abstract: Elaboration of the Organs-Werner natural selection model for the evolution of Podger leads to the expectation that the occurrence of territoriality in bachelor males will be indicative of the selective factors favoring Podger in alaxensis birds. Bachelor males should establish territories only if differential food availability between habitats has been an important factor favoring Podger. Bachelor males should not establish territories when differential protection from predation afforded by potential nesting substrates has been the primary selective factor. Previously published evidence is shown to be consistent with these predictions. It is suggested that vegetation structure as a defense against nest predation should be of paramount importance in determining habitat quality for colonial species, whereas food availability should be a more important factor for non colonial species. It was found that all species in which bachelor males defend territories are noncolonial, and nearly all species in which bac...

Synchronous Eruptions of Boreal Seed-Eating Birds

Abstract: Analysis of Audubon Society Christmas-count data (1962-1971) revealed a generally synchronous pattern of winter eruptions between eight species of seed-eating birds whose winter ranges normally include boreal forests. Species and populations occupying montane conifer forests in the West did not fit this pattern as well. Literature analysis revealed a circumboreally synchronized pattern of seed crop fluctuations in certain high-latitude tree species and an apparently resulting pattern of southward eruptions of birds dependent upon these foods. Between 1948 and 1971 eruptions were relatively large in 1949, 1955, 1957, 1959, 1961, 1963, 1965, 1968, 1969, and 1971.

Secondary compounds in plants: primary functions

Abstract: Many natural products supposed by ecologists to exist as secondary metabolic products for protection of plants exist in dynamic equilibrium, with rapid turnover rates, involving cycles that include primary products such as sugars and amino acids. Therefore, natural selection of secondary chemicals may be for their role in primary metabolic processes as well as in defense.

A community matrix analysis of heliconia insect communities

Abstract: Field manipulations were designed to determine interspecific interaction coefficients (α's) for insect species living in the water-holding bracts of Heliconia wagneriana and H. imbricata. Community stability was investigated by the analysis of species equilibrium densities and the matrix of regression coefficients of mortality rate on the density of each species. 1. Both positive and negative coefficients of interspecific interaction (α's) exist, meaning that both competitive and symbiotic effects occur. Some α's are not statistically significant, indicating no interspecies effects. 2. Occasional positive and nonsignificant intraspecies coefficients were found, meaning that over the densities studied intraspecific competition was not occurring for some species in one or the other Heliconia community. In the two cases of positive intraspecific coefficients, negative intraspecific coefficients occurred in the other Heliconia community. 3. Each community matrix is dominated by two α's which have high absolut...