Showing papers in "The American Naturalist in 1982"

Nutrient cycling and nutrient use efficiency

Abstract: Forest ecosystems systematically produce more litterfall dry mass per unit of nitrogen in sites with less aboveground nitrogen circulation. This pattern is observed both within and among tropical, temperate deciduous, coniferous, Mediterranean, and fertilized ecosystems. The differences among sites are probably related to differences in soil nitrogen availability. Patterns of nitrogen use for root and wood production probably reinforce the litterfall results. An examination of phosphorus and calcium use efficiency for litterfall production yields more ambiguous results. The pattern for nitrogen circulation and nitrogen use efficiency in forests has important implications for ecosystem-level properties, including the development of low nitrogen availability in soil.

Ejaculate Cost and Male Choice

Abstract: According to much evolutionary thinking, males of promiscuous species are able to produce quantities of sperm that are virtually unlimited and should be selected to mate indiscriminately with all available females. However, sperm are generally delivered in batches (ejaculates or spermatophores) that may include many millions of gametes. Males are limited with respect to the number of ejaculates they can deliver and the time required to restore depleted reserves. Because ejaculates are a limited resource, prudence would be expected in the allocation of ejaculates to females. Among the forces acting to limit the number of females with which a promiscuous male mates are high pregnancy initiation requirements, sperm competition, female choice and control, and the costs and risks of searching. Optimal strategies for ejaculate allocation will vary with the operational sex ratio. Males can and do discriminate among females as potential mating partners. The problems of limited ejaculatory capacity and male choice...

The Evolution of Helping. I. An Ecological Constraints Model

Abstract: The ecological factors underlying the evolution of helping behavior in birds and mammals are examined. I argue that a necessary first step for the evolution of cooperative breeding is a substructuring of the population into small, stable, social units; in most known cases these are extended-family units. The ecological conditions leading to the development of such units are explored, and a general model is presented that emphasizes ecological constraints that limit the possibility of personal, independent breeding. When severe constraints occur, selection will favor delayed dispersal and continued retention of grown offspring within their natal units. Differing proximate factors can be responsible for limiting the option of personal reproduction. In stable, predictable environments where marginal habitat is scarce, high population density and resulting habitat saturation can lead to a severe shortage of territory openings (Brown 1974; Koenig and Pitelka 1981). This decreases the chance for independent est...

Optimal life histories, optimal notation, and the value of reproductive value

Abstract: Reproductive value is the central construct in life history optimization. In the optimal life history, the reproductive value at every age represents the solution to a constrained maximization problem. This holds true regardless of trade-offs across age classes. The recent literature arguing to the contrary has misunderstood the principle and has neglected the constraints in the maximization. In reviewing the matter, this paper considers maximization over segments of the life history, maximization over segments taken in reverse sequence, maximization of terms of a decomposition into instantaneous contributions at a given age, and maximization of reproductive value over alternative states such as the sexes. To clarify the relation between the maximization procedure and the constraints, a simple iterative approach to achieving maximization and simultaneously satisfying the constraints is described. To arrive at the instantaneous decomposition, it is shown that reproductive value arises as an adjoint variabl...

Evolution in steppe with few large, hooved mammals

Abstract: Communities are organized through both the proximate and evolutionary constraints that shape the structure of populations and the interaction among species. Proximate constraints can be manipulated experimentally to assess the relative influence of a component species on the overall structure of a community, as in analyses of keystone (Paine 1969) or dominant species. These analyses are instructive in detecting how current selection pressures influence populations. Responses of species to current interspecific interactions, however, are not made in an evolutionary vacuum, but rather reflect the history of prior selection pressures that have molded the species' traits. Overall structure of some communities in turn may be influenced to a large extent by a few traits evolving in the most abundant species in response to continuous strong selection pressure. Assessment of the effect of these particular traits or suites of traits on community organization is difficult on an experimental basis as it necessitates removal of traits within species rather than species themselves (e.g., Windle and Franz 1979). Grasses with their basal intercalary meristems are cited commonly as having evolved with large herbivorous mammals (Osborn 1910; Love 1959; Barnard and Frankel 1964). Yet temperate grasslands and specifically steppes differ greatly in the extent of habitation by large mammals throughout the Neogene and hence the plants in these grasslands differ in the extent to which they have traits adapted to large mammalian grazers. In this paper we consider the structure of steppes on either side of the Rocky Mountains in an attempt to understand how differences in the history of acquisition of mammal-selected traits by plants and interactions with mammalian grazers can influence overall grassland structure. GRASSLANDS AND NATIVE LARGE HERBIVORES IN WESTERN NORTH AMERICA

On the adaptive significance of leaf height in forest herbs

Abstract: Herbaceous plants growing in temperate forests show a wide interspecific range in maximum foliage height (Whittaker 1956; Curtis 1959; Grime 1979). Among forest herbs of the northeastern United States, for example, leaf height varies from near ground level in genera like Mitchella, Goodyera, and Tipularia; through 10-100 cm in genera like Dicentra, Arisaema, and Caulophyllum; to over 150 cm in Heracleum, Impatiens, and Urtica. In view of the potential impact of leaf height on competitive ability for light in plants growing on dimly lit forest floors, this diversity in leaf height is a puzzling aspect of the great morphological variety seen in forest herbs. In this paper I outline a number of evolutionary constraints on leaf height in forest herbs, and use a game theoretic model to predict trends in this trait as a function of leaf phenology and the density of herbaceous competitors. Extensions of the model are discussed to include competition among relatives and the possible evolution of altruistic patterns in leaf height. Predictions of the model are compared with data on plant height and coverage by competitors along a forest gradient; the quantitative agreement between this pattern and the game theoretic model is checked using allometric data on 21 species of forest herbs. I conclude by analyzing the evolution of leaf height in orchid genera of the eastern United States in terms of the model. Throughout I assume that natural selection will favor plants whose form tends to maximize net carbon gain from a photosynthetic system of a given size, since such plants should have the greatest resources with which to compete for additional light, water, and nutrients (Givnish 1979).

What Should a Clever Ungulate Eat

Abstract: An optimal foraging model applicable to large herbivores is developed. This transforms primary data on vegetation composition, defined in terms of the standing biomasses, nutrient contents, and eating rates offered by a range of food types, into an expression for nutrient ingestion and assimilation rates. A "clever ungulate" is defined as a short-term optimizer for foraging performance alone. An optimal dietary range is identified, which varies for different target nutrients and for time-minimizer versus profit-maximizer tactics. Foraging performance is more sensitive to changes in food quality and in bite sizes than to differences in overall quantitative food abundance. Model predictions are compared with the observed foraging behavior of kudus under natural conditions. Real kudus are both selective and responsive in their food choice, and a rough correlation exists between the acceptances shown for particular plant species and their leaf crude protein contents. However, discrepancies suggest that bite-s...

Refuge design and island biogeographic theory: effects of fragmentation

Abstract: Cole's theoretical conclusion that one large site generally contains more species than several small ones of equal total area is falsified by data in the literature, as is his contention that exceptions will only occur when the species in the sites are but a small fraction of those in the species pool. For a variety of taxa, for a number of different habitat types, and for a wide range of sizes of the biota as a fraction of the pool, either there is no clear best strategy, or several small sites are better than one large site. Since there are numerous idiosyncratic biological considerations, plus a number of nonbiological ones that bear heavily on refuge design, it is unlikely that a general reductionist model can generate useful predictions or advice on this matter.

Stochasticity in Structural and Functional Characteristics of an Indiana Stream Fish Assemblage: A Test of Community Theory

Abstract: In general, ecological assemblages and communities appear to be regulated primarily by either deterministic or stochastic processes. It is currently important to quantify the relative frequencies of these two types of assemblages since most ecological theory is applicable only to deterministic systems. We attempted to distinguish the mechanism regulating species abundances and trophic structure in an Indiana stream fish assemblage. Samples collected over a 12-yr period were separated by season (spring, summer, and autumn) and species were then assigned to a trophic group based on published dietary data. Analyses showed a total lack of persistence for the ranks of species abundances and the ranks of trophic groups for all seasons. Consequently, we conclude that this assemblage is probably regulated by stochastic factors. This finding is concordant with many other studies which have documented the substantial effects of environmental unpredictability (i.e., floods and droughts) on other stream taxocenes.

Unsuccessful Predation and Evolution

Abstract: The theory of evolution by natural selection requires the recognition of aptations. A given genetic variant can be shown to have an advantage over another with respect to an individual's viability in a given environment if (1) some individuals in the population reproduce after an encounter with the agent of selection for which the variant is believed to be beneficial, and (2) the beneficial variant has a higher frequency among individuals which have survived encounters with the agent than among those which died as a result of the encounter or among those which did not encounter the agent. In the special case of evolution of antipredatory features, unsuccessful predation is a necessary condition. A literature survey of 60 predaceous species reveals that unsuccessful predation is widespread; only 19 of 100 prey species (19%) were attacked in one or more vulnerable size classes with an efficiency (measured after the prey was detected) of 90% or more. The nature and effectiveness of antipredatory defenses can...

Bat Predation and Sexual Advertisement in a Neotropical Anuran

Abstract: Many mating systems are characterized by conspicuous male sexual displays (Emlen and Oring 1977; Wells 1977). Predation and parasitism are thought to be important counter-selection forces in the evolution of acoustic sexual displays (Marler 1955; Moynihan 1970). However, only studies of crickets have shown that acoustically advertising males have a higher probability of attracting predators (Bell 1979; Walker 1964) or parasites (Cade 1975), and Cade (1979) showed that parasitism influences whether a male adopts a calling or a noncalling mating strategy. Recently we reported that the bat Trachops cirrhosus eats frogs and uses the frogs' advertisement calls for locational cues (Tuttle and Ryan 1981). Here, we investigate the role of predation in the evolution of the vocal repertoire of the frog Physalaetnus pustulosus. Male P. pustulosius produce calls of varying complexity (1 whine + 0-6 chucks; Rand and Ryan 1981) and chucks contain information about male body size (Ryan 1980). Soloing males produce the simple call (whine only) and only increase call complexity (i.e., the number of chucks) in response to other males (Rand and Ryan 1981). But females prefer calls that contain chucks (Rand and Ryan 1981). This creates a paradox: Why is it that males do not maximize their mate attraction ability by always producing calls more attractive to females? Rand and Ryan (1981) suggested that more complex calls might also be more attractive to acoustically foraging predators. We conducted a series of experiments in a flight cage and in the field to test the hypothesis that male P. pustulostus that produce calls more attractive to females (i.e., more complex calls) are also more prone to predation by T. cirrhosis. These experiments were conducted on Barro Colorado Island, Panama, from January to May, 1980. In the flight cage a T. cirrhosus was presented with a simple (whine only) and a complex (whine + 3 chucks) advertisement call. Calls were presented at a rate of one call/1.6 s. (about the normal P. pustulosus calling rate) and at an intensity of 75 dB SPL at 1 m from the speaker. The flight cage was 20 m2 and 2.1 m high. The observer was in one corner and the bat perched in the opposite corner. One speaker was located in each of the remaining corners. At the beginning of each trial the bat was about 4 m from each speaker. The stimuli were presented only when the bat was perched in the appropriate corner, and the stimuli were discontinued as soon as the bat flew from the perch to avoid habituation of the bat to the stimuli. (In other studies, bats almost invariably landed on speakers that continued to broadcast frog calls; Tuttle and Ryan 1981.) A response was recorded if a bat flew within 1 m of a speaker. In all responses the bat flew directly toward a speaker. The bats landed on a speaker in 44% of the responses and passed within 4

Seed Dispersal by Animals: Contrasts with Pollen Dispersal, Problems of Terminology, and Constraints on Coevolution

Abstract: Theoretical and empirical research on frugivory and seed dispersal has been influenced by concepts derived from the study of pollination. In particular, explicit and implicit analogies between seed dispersal and pollen dispersal have led to the expectation, under certain conditions, of the evolution of obligate, species-specific relationships between fruiting plants and the animals that disperse their seeds. The two systems differ in important respects, however. Plants benefit by directing pollen dispersers to a definite, recognizable "target," a conspecific flower, and they can provide incentives at flowers which serve to attract potential pollinators. In effect, there is "payment upon delivery" of the pollen. In contrast, for seeds the target (an appropriate site for germination and establishment) is seldom readily discernible, and dispersal beneath a conspecific plant may actually be undesirable. Another important difference is that frugivores are "paid in advance." Because of these differences and oth...

Functional responses of optimal foragers

Abstract: Functional responses are derived for several models in which a predator is able to vary the proportion of time that it spends foraging in an adaptive manner. Results suggest that: (1) the functional response of such predators is likely to be very plastic; (2) the traditional type 1, 2, 3 classification is not sufficient to describe the functional responses of such predators; and (3) variation in the proportion of time spent foraging can provide an alternative mechanism for traditional functional response forms.

Environs: Relativistic Elementary Particles for Ecology

Abstract: Corpuscular theories have a favored place in science. The power of physics, chemistry, and biology can be correlated with the extent of particulate conceptions in these subjects. Elementary particles, atoms, molecules, genes, cells, and organisms are all familiar examples of powerful particles. Ecology lacks an elementary particle of its own, with discoverable properties that can clarify the nature of environment and its relationship to life. The purpose of this paper is to suggest such a particle and indicate what is known about it and some directions for learning more. Ecology, the biological science of environment, has not produced a synthesis of environment from its broad technical knowledge of the influence of external parameters on organisms. Before Darwin (1859), environment was considered an organic whole. Everything in it made some contribution and had some meaning with respect to everything else. Darwin subscribed to this view, but his emphasis, and that of his followers, on the evolving organism struggling to survive, suppressed the exploration of holistic aspects of the origin of species that might have been developed. After Darwin, the organism came into great focus, first as a comparative anatomical entity, then later with physiological, cellular, molecular, behavioral, and genetic detail. In contrast, the organism's environment blurred through relative inattention into a fuzzy generality. The result was two distinct things (dualism), organism and environment, supplanting the original unified organismenvironment whole (synergism). So separated from environment, the organism had to adapt back in. Thus, the main activity of existence portrayed in evolutionary ecology literature, for a variety of improbable biological objects from genes (Waddington 1957; Dawkins 1978) to ecosystems (Odum 1969), is the pursuit of \"adaptive strategies.\" An overworked metaphor is the signal of a strained paradigm, and in this case it is clear that environment must become more than a nonspecific selector of traits according to advantages to individuals, or in response to strategies laid down by all sorts of implausible units. The leaving of offspring, Darwin's criterion of success in adaptation, came to be called \"fitness.\" This first principle of evolution has nothing to do with the degree of fit, in Leibniz' (1949) sense of mutual conformity, between many things in a large system of interrelationships. Fitness is a local

Nutrient spiralling in streams: implications for nutrient limitation and invertebrate activity

Abstract: Nutrient cycling in streams occurs in conjunction with downstream transport as spatially distributed process that has been termed spiralling. The intensity of reutilization of nutrients as they pas...

Defense of ripe fruit from pests: its significance in relation to plant-disperser interactions

Abstract: Ripe fruits of vertebrate-dispersed plants are susceptible to irreversible damage by pathogens and invertebrates while they are attached to the parent plant "waiting" for the visit of a dispersal agent. Adaptations of plants to decreasing fruit damage include temporal escape from pests, reducing exposure time, and decreasing fruit profitability to pests by means of secondary compounds and reduced and/or unbalanced nutrient content. A variety of secondary compounds has been reported from the ripe fruits of many vertebrate-dispersed plants, including alkaloids, saponins, volatile oils, resins and phenolics. Toxic fruits containing poisonous substances that potentially are defensive agents against pests may be very frequent, as revealed by an analysis of their occurrence among European plants. As judged from the literature, fruits containing secondary substances are at a competitive disadvantage relative to nontoxic ones. It is suggested that a hierarchy of fruit defense methods should exist, with the least ...

The relationship between adult size and brooding in marine invertebrates

Abstract: 1. Greater brood care is associated with smaller adult size in co-occurring benthic marine invertebrates of many but not all taxa. A review of the literature suggests that the reverse trend is rare. 2. Varied hypotheses can be advanced for the association of small adult size with greater brood care. The hypotheses concern allometry of egg production and brood capacity, longevity with repeated reproduction and variable recruitment, and dispersal. The hypotheses are not mutually exclusive and more than one explanation could apply to most of the reported cases of smaller adults associated with greater brood care. 3. None of the explanatory hypotheses which have been advanced fits all reported cases of greater brooding with smaller adult size, and life history correlates of both adult size and brood care are numerous. Thus the trend may result from selection for different life history traits in different taxa.

Beetle Horn Dimorphism: Making the Best of a Bad Lot

Abstract: There has been interest recently in the occurrence in a single species of more than one adaptive behavior or morphology. One kind of explanation for dimorphisms of this type is that they constitute frequency-dependent evolutionary stable strategies (Gadgil 1972; Maynard Smith 1976), with the reproductive payoffs for one type equal to those for the other because of the balances of costs and benefits for the two forms, and that they are thus maintained in a sort of balanced genetic polymorphism. A second possibility is that payoffs for the two forms are not equal but that the alternative forms represent facultative alternatives designed to function in different circumstances (different body size, season, etc.; see Warner et al. 1975; West-Eberhard 1979; Dawkins 1980 discusses both types of explanation). A classic example of such a dimorphism occurs in some species of homed beetles in which there are two more or less distinct forms: large individuals with well-developed horns (\"majors\"), and smaller ones which lack or have only poorly developed horns (\"minors\") (Arrow 1951). Evidence is accumulating (Eberhard 1977a; Palmer 1978; Eberhard 1979 and references; Eberhard 1980; M. Peinador, in prep.) that beetle horns commonly function as weapons in intraspecific battles. The fact that the horns of many species represent substantial proportions of the beetles' body weights argues that natural selection in these species has strongly favored increased fighting ability. Since beetles do not grow after their last moult (and minors are thus comdemned to remain minors for their entire adult lives), the continued existence of hornless or nearly hornless forms seems paradoxical since one might think that selection should eliminate those individuals poorly equipped to fight. This study of the dimorphic horned dynastine beetle Podischnus agenor Oliver shows that the behavior and ecology of minor males differs from that of major males in ways which suggest that the minor form is a facultative adaptation designed to reduce direct competition with major males, and thus to compensate partially for the competitive disadvantage resulting from the minors' smaller size.

Selection of Combined Versus Separate Sexes in Seed Plants

Abstract: In the majority of the populations of seed plants, there is a single morph, a "cosex," which combines maternal and paternal functions in the same individuals. Fewer than 10% of populations are dimorphic, with differentiated males and females specializing in one sexual function. Evolution of sex conditions has been principally from cosexuality to separate sexes. Conditions under which unisexual mutants invade cosexual populations are examined. The advantages of cosexuality include the facilitation of mating, reproductive economies, noninterchangeable diminishing returns imposed on paternal and maternal fitness by Bateman's Principle and the distinct spatial distribution of maternally and paternally derived offspring, and bet-hedging. Females and males share an ability to reproduce more efficiently than cosexes under certain conditions. Females (but not males) may gain benefits from increased outcrossing, or they may gain better seed dispersal of their more massive crops, while males can conceivably obtain ...

The evolution of helping. ii. the role of behavioral conflict

Abstract: In part I (Emlen 1982), an ecological constraints model was developed to predict the circumstances under which grown offspring would remain in familial units with their parents. Such retention was considered a necessary, but not sufficient, condition for the evolution of helping behavior. Whether or not nonbreeding auxiliaries will participate as helpers in such groups will depend upon the costs and benefits of such helping, measured from the viewpoints of both breeder and helper. In this paper, these costs and benefits are formalized, and the conclusion is reached that conflict frequently is to be expected between breeder and auxiliary over the behavioral role of the latter. When ecological constraints favor grown offspring remaining at home, but the retention of such individuals depresses the fitnesses of the breeders, helping by an auxiliary may evolve as a means of reducing the incremental cost to the breeder, and thus of minimizing the likelihood that the auxiliary will be expelled from the group. Mo...

Optimal giving-up times and the marginal value theorem

Abstract: Krebs et al. (1974) predicted that an optimal forager in a patchy habitat should apply the same giving-up time (GUT) to all patches, even if they differ in quality. This prediction has repeatedly appeared in the literature. However, it is based on a model which is not designed to predict GUT's. When an analogous model is derived which is designed to predict optimal GUT's, it is found that larger GUT's should be used in better quality patches. This result suggests a forager should be more persistent in patches it knows are good than in ones it knows are poor. Under the proposed definition of "better quality," it also follows that, on the average, more time will be spent in better quality patches, and a greater yield will be obtained. Increasing the mean interpatch travel time increases the optimal GUT's, as well as the mean patch yields and residence times. The theoretical development employed to predict optimal GUT's is not restricted to this strategy. Rather, by simply renaming certain quantities which a...

Avian Extinction on Barro Colorado Island, Panama: A Reassessment

Abstract: Barro Colorado Island (BCI) in Lake Gatun was formed early in the twentieth century when the Chagres River was dammed to form part of the Panama Canal. Since its isolation as an island, BCI has been an ideal laboratory for the study of extinction/colonization patterns on a recently isolated, continental island (Terborgh 1974; Willis 1974; Wilson and Willis 1975; Willis and Eisenmann 1979). The foundation for these studies was a list of birds observed on BCI. I analyze the BCI avifauna in a different way. First, I sampled birds of the forest undergrowth on BCI and in the Pipeline Road (PR) area of Parque Nacional

Multiple mating, sperm utilization, and social evolution

Abstract: Allozyme data show that multiply inseminated honey bee queens utilize the sperm of at least three males at any given time and that the sperm are, to some extent, mixing in the spermatheca. Statisti...

Dynamics and organization of a rocky intertidal fish assemblage: the persistence and resilience of taxocene structure

Abstract: This paper examines characteristics of a rocky intertidal fish taxocene in central California. Seasonal and annual changes in taxocene structure, diversity, and recruitment were described and attempts were made to correlate these parameters with environmental variables. In addition, the persistence and resilience of taxocene structure were examined to determine whether this assemblage was regulated through deterministic or stochastic processes. A series of tidepools were defaunated 15 times over 29 mo. Sampling did not grossly affect taxocene structure in the study site and recolonization from surrounding areas was rapid. Three types of abundance patterns were observed: (1) species present year round (residents), (2) species present seasonally, and (3) species occasionally present. Productivity appeared to be the main environmental factor affecting taxocene structure because there were significant correlations between it and: (1) diversity, (2) species richness, (3) numerical abundance, and (4) recruitmen...

Long-Distance Migration of Drosophila

Abstract: A recent paper by Jones et al. (1981) shows that Drosophila pseudoobscura flies released in the open desert can travel long distances. These investigators uggest that such migration can lead to considerable gene flow, and may account for the similarity of allozyme frequencies among populations of this species. It is possible, however, that the vagility of flies released in the desert is not their natural behavior. One would like to know if flies will leave favorable or already-populated areas and traverse those less favorable. We report here two release experiments in Death Valley, California, that demonstrate Drosophila will in fact leave oases and venture into the surrounding desert, and also travel from one oasis to another across many kilometers of desert. These results support the hypothesis that Drosophila migration may be more extensive than previously supposed. Our first experiment was at Travertine Springs in Death Valley (36? 26' N, 116? 49' W), an oasis 4.2 km southeast of Furnace Creek Ranch. (Furnace Creek Ranch is a commercial date grove and tourist complex with year-round irrigation.) The Travertine oasis area (fig. 1) forms an oval about 350 m long by 80 m wide (long axis NE to SW), and consists of a clump of date palms and several small springs. The water from these springs forms a net of rivulets traversing an area of grasses, mesquite (Prosopis), and tamarisk (Tamarix). To the southwest the oasis is bordered by a road, a dry wash about 100 m wide, and cliffs rising abruptly from the far side of the wash. To the northeast the oasis borders on open desert, which is an old alluvial fan. Here the vegetation is mainly saltbush (Atriplex) and creosote (Larrea), and the area is similar to the desert release site described by Jones et al. (1981). The release population consisted of about 30,000 Drosophila collected over 48 h at the Furnace Creek Ranch date grove. The sample consisted of about 20% D. pseudoobscura (\"black flies\"; identity confirmed by karyotyping of a sample) and the rest of roughly equal proportions of D. melanogaster and D. simulans (collectively, \"yellow flies\"): These flies were divided into three approximately equal groups, each of which was marked with a different color of micronized fluorescent dust. The groups were released shortly after sunset at 6:15 p.m., March 31, 1980. One group was released at the northeastern end of the oasis at the major spring outlet, one group at the base of the cliffs 450 m SW of the oasis release site, and the third group in the desert 350 m NE of the oasis release site. Flies were collected the next evening along a 1,600 m trapline extending from the base of the cliffs through the oasis and 1,150 m into the desert. The trapline contained 42 buckets of banana bait, spaced 33 m apart except for irregularities resulting from terrain and a few widely spaced buckets in the desert (fig. 1). Recaptured flies were scored the same evening under ultraviolet light. The results are given in table 1. These data are summarized by dividing the recapture trapline into three areas: the desert east of the oasis (23 baits), the dry

Vertical migration of zooplankton: a game between predator and prey.

Abstract: Many zooplankters in lakes and oceans assemble in the upper waters at night and sink to the lower layers in the day Planktivores also migrate following zooplankters This diel migration is studied by analyzing a habitat selection game between predators and prey, based on the predation hypothesis, ie, in the daytime zooplankton avoid predators (fish) that hunt by sight at the cost of reduced grazing on phytoplankton The equilibrium distribution of the game is as follows When the efficiency of predation by sight is low, both predator and prey concentrate in the upper layer (night phase), and, when it is high, most zooplankton stay in the lower layer and the fish population is distributed between both layers (day phase) The equilibrium distribution of zooplankton discontinuously changes with the predation efficiency at a threshold value Since the predation efficiency varies with light intensity, diel migration pattern is expected, and the following results are shown 1 The change in zooplankton distr

Paternity and Fruit Production in Trumpet Creeper (Campsis radicans)

Abstract: The effect of paternity on the likelihood of fruit production was determined by making 2,486 hand pollinations among 11 stems (9 individuals) of Campsis radicans. All individuals matured significantly different numbers of fruit from different pollen donors. Individuals that were favored as donors by one recipient were not necessarily those favored by other recipients. Selectivity increased slightly as more pollinations were made, and pollination by nonfavored donors was more successful in producing fruit during the first half of the pollination sequence than in the latter half. The pollen donors favored by particular recipients were usually those whose pollinations resulted in fruit with relatively many and large seeds. Fruit abortion seems to have been more important in donor selectivity than prezygotic phenomena, although parallels with multifactorial gametophytic incompatibility are evident. I suggest this selectivity by a pollen recipient with respect to pollen donors is an integrated response increas...

Community Diversity and Interactions Between the Size and Frequency of Disturbance

Abstract: It has recently been argued that, because of the influence of disturbance, few if any communities in nature achieve a competitive equilibrium (Drury and Nisbet 1973; Connell 1978). Disturbance, as well as other forms of harvesting such as predation and herbivory, may remove from the community individuals that would otherwise have utilized common resources (Yodzis 1978). While the importance of disturbance has been documented in many communities (see Connell 1978; White 1979; Pickett 1980), a complete understanding of its effects has yet to be achieved. A general hypothesis seen in the literature is that an intermediate rate of disturbance will produce the highest diversity of species if diversity is a measure of both species abundance and number (Paine and Vadas 1969; Horn 1975; Connell 1978). This \"intermediate disturbance hypothesis\" is predicated on disturbance initiating local seres, each having a progression of species invasion and replacement. Many discussions have used only the frequency of disturbance events or the total rate of disturbance to describe the disturbance regime in a community (Dayton 1973; Osman 1977; Sousa 1979a, 1979b). This rate can be quantified as a total amount of area disturbed per unit time or the probability that an area will be disturbed per unit time. However, actual agents of disturbance should be characterized not only by the frequency and timing of each disturbance event (see Abugov 1982), but also by the size of the disturbed areas or \"patches\" (Levin and Paine 1974) produced. Disturbance rate is thus equal to the sum of the size or magnitude of all disturbance events in a given area per unit time. That both size and frequency are important characteristics of disturbance has been noted before (e.g., Osman 1977; Connell 1978; Levin and Paine 1974; Denslow 1980b). However, the effect of the interaction between size and frequency on community diversity has not previously been discussed. Here I show that, as a result of differences in the dynamics of large and small disturbances, disturbance size will independently have a large role in structuring some communities. Empirical work on the effect of disturbance size is sparse and mostly recent. Denslow (1980b) has proposed that most species which utilize disturbance events should be adapted to the more common disturbance \"types' in a community; the major characteristics of type are size and frequency. Denslow (1980a, 1980b) and Hartshorn (1980) have both pointed out the influence of size of disturbance on the types of species found within tropical forests. Plants in temperate old-field communities have been shown to have differential success over a size gradient of artificially created openings (Davis and Cantlon 1969; Gross 1980). Colonizing

Nest parasitism in goldeneyes bucephala clangula: some evolutionary aspects

Abstract: Intraspecific nest parasitism, in which parasitic females lay eggs in the nests of host females of the same species, occurs in many birds and certain insects. Most known cases concern waterfowl (Anatidae). We examined host responses to experimental parasitism, adding eggs to nests of laying goldeneye ducks. Three different hypotheses on host responses yield mutually exclusive predictions. (1) The host is a determinate layer; her own clutch is unaffected by the number of parasitic eggs. (2) The host is an indeterminate layer and responds to parasitic eggs as to her own. She reduces her clutch by the number of eggs added by the parasite. (3) The host is an indeterminate layer and estimates the number of parasitic eggs. She responds in an optimal fashion and reduces her own clutch with approximately half the number of parasitic eggs, hence maximizing the production of host offspring. With 2 + 2 eggs added on adjacent days early during the host's laying period, prediction (2) is substantiated. Goldeneyes then...

Precise Sex Ratios in Highly Inbred Parasitic Wasps

Abstract: When Williams (1979) considered the question of adaptive sex ratios in out-crossed vertebrates he concluded that the evidence favored Mendelian (random, binomial) sex determination. This may be because vertebrates possess no efficient mechanism for regulating the sex of their offspring. Parasitic wasps do possess such a mechanism, namely arrhenotoky, and they show deviation from random sex determination. Outcrossed wasps may vary sex ratio with host size and this has been modeled by Charnov (1979) and Charnov et al. (1981). Hamilton (1967) has explained why highly inbred parasitic wasps should produce highly female-biased sex ratios. This paper shows that not only should the sex ratios be female biased, but they should also be precise. The selective advantage of a precise sex ratio can be calculated and is seen to be about 5%-30% for the cases considered here. Laboratory studies show that parasitic wasps do, in fact, produce precise sex ratios. Goniozus gordhi produces exactly one male as a rule in small ...