Showing papers in "The American Naturalist in 1981"
TL;DR: A reasonable fit between predictions and observations is found, although the sparsity of data and methodological uncertainties weaken the corroboration in several cases.
Abstract: Based on the assumption that each trophic level acts as a single exploitative population, a model relating the trophic structure of ecosystems to their potential primary productivity is developed. According to the model, herbivory pressure should be most severe in relatively unproductive environments. With increased potential productivity, the role of predation in herbivore regulation should become more important and the impact of herbivory upon plant communities should decrease. In very productive environments, increase in herbivory pressure is again probable, at least in aquatic ecosystems. The predicted pattern of phytomass and predicted results of manipulations are compared with available data. A reasonable fit between predictions and observations is found, although the sparsity of data and methodological uncertainties weaken the corroboration in several cases. In terrestrial ecosystems, the present version of the model seems best applicable to the vertebrate branch of the grazing chain, whereas the a...
1,634 citations
TL;DR: The analysis confirms the view that coexistence can occur in a system where space is allocated largely at random, provided environmental variability is sufficiently great (Sale 1977); but the explanations and predictions differ in detail with those of Sale.
Abstract: In deterministic approaches to modeling, two species are generally regarded as capable of coexistence if the model has a stable equilibrium with both species in positive numbers. Temporal environmental variability is assumed to reduce the likelihood of coexistence by keeping species abundances away from equilibrium. Here we present a contrasting view based on a model of competition for space among coral reef fishes, or any similarly territorial animals. The model has no stable equilibrium point with both species in positive abundance, yet both species persist in the system provided environmental variability in birth rates is sufficiently high. In general the higher the environmental variability the more likely it is that coexistence will occur. This conclusion is not affected by one species having a mean advantage over the other. Not all kinds of environmental variability necessarily lead to coexistence, however, for when the death rates of the two species are highly variable and negatively correlated, th...
1,170 citations
TL;DR: If hand-pollinated plants produce more seeds than naturally pollinated controls, then reproduction is being limited, not by energy levels, but by pollinator activity; do pollinators limit the reproductive output of many plants?
Abstract: In many recent studies there have been attempts to describe and explain patterns of resource allocation in plants (e.g., Abrahamson and Gadgil 1973; Newell and Tramer 1978). Nearly all such studies are based on the unstated assumption that the reproductive output of a plant is resource limited. However, if hand-pollinated plants produce more seeds than naturally pollinated controls, then reproduction is being limited, not by energy levels, but by pollinator activity. As part of a study on the population biology of Arisaema triphyllurm, a herbaceous forest perennial (Bierzychudek 1981), I hand pollinated some plants in a natural population. The difference between hand-pollinated and natural seed production was remarkably large. Hand-pollinated plants produced over an order of magnitude more seeds (the mean for medium-sized plants was 43.2 seeds/plant, N = 12) than did controls of similar size (mean = 1.0, N = 20). While there was no relationship between the size of a plant and the number of seeds it produced when naturally pollinated (r = .11, N = 35), this same relationship was significant for hand-pollinated plants (r = .81, N = 33, P < .01), indicating that only the hand-pollinated individuals were resource limited. There is no reason to assume that this level of seed production is unusually low for Arisaema; I observed only slightly higher levels at several other sites and in other years (with a maximum of 10.3 seeds/plant). Arisaema's pollinators (mycetophilid and sciarid flies) seem abundant, if not particularly efficient. Others have observed similar low rates of seed production by Arisaema (Meehan 1886; Sakamoto 1961; Treiber 1980). Is this an isolated case, or do pollinators limit the reproductive output of many plants? The yield of many cultivated crop plants has been found to be pollinator limited (McGregor 1976), but this might be expected when large plantings are made of a single crop species. Information on native species under natural conditions is more difficult o find. Schemske et al. (1978) note that only 33% of naturally pollinated flowers of Eiythronium albidum set seed, compared with 78% of flowers outcrossed by hand. Willson et al. (1979) show that 82.3% of handoutcrossed Phlox divaricata produced some seed, compared to 58% of naturally pollinated plants at the same site. In Costa Rica 29.7% of hand-outcrossed flowers of Combretumfrtuticosurn set fruit, compared to 7% of naturally pollinated flowers (Schemske, in press). Brassavola nodosa, a Central American orchid, also seems to be pollinator limited: 67% of hand-outcrossed flowers set fruit, versus 12% of naturally pollinated flowers (Schemske 1980). Weller (1980) found the fecundity of hand-pollinated Lithospermum caroliniense averaged 17% compared to 9% for naturally pollinated individuals, a significant difference. Finally, Encyclia cordigera, another orchid, may also be pollinator limited: Among hand-pollinated inflorescences, the proportion producing no fruit ranged from 5% to 22%, depending on the identity of the pollen donors. Seventy-eight percent of naturally pollinated inflorescences set no fruit (Janzen et al. 1980). The results of some hand pollination experiments are quite different. Stephenson (1979) found that hand-pollinated Catalpa speciosa produced significantly
559 citations
TL;DR: Whether the predator under observation actually behaves optimally in accordance with the distribution of prey should be revealed by plotting the number of captures against the length of period for which the predator stayed in each patch.
Abstract: The optimal-patch-use problem in predation theory is investigated by use of a stochastic discrete model to match experimental situations when deterministic continuous models are inappropriate. We f...
531 citations
TL;DR: It is argued that parasite-host systems are ideal candidates for interdemic or "group" selection because of the potential for selection for avirulence based upon reduced host (hence group) survival.
Abstract: On theoretical grounds, we argue that parasite-host systems are ideal candidates for interdemic or "group" selection because of the potential for selection for avirulence based upon reduced host (hence group) survival. Such selection appears to have been an important ingredient in the stabilization of the myxomatosis-rabbit system in Australia, although clearly the evolution of resistance in the rabbit population also played a part. We present a simple mathematical model to demonstrate how easily group selection can (in theory) stabilize a parasite-host system. This model is not meant to be a literal translation of the myxomatosis-rabbit interaction, and in fact intentionally disregards host evolution; its purpose is to isolate the role of group selection in the parasite population. Fenner has established that both interdemic selection and host evolution were important in the stabilization of that system, although our model demonstrates that in theory interdemic selection alone could stabilize the interac...
509 citations
TL;DR: It is argued that classical, interspecific competition for food is weak and infrequent, and that it cannot fill the major ecological and evolutionary role favored for it by many workers on other organisms, and favor the view that most community patterns can be explained without invoking inter specific competition.
Abstract: Patterns in assemblages of phytophagous insects are interestingly similar to those displayed in many other groups of species. Specifically, we review eight such patterns. (1) Large areas support more species than small areas. (2) Complex habitats support more species than simple habitats. (3) Persistent habitats have more species, with narrower feeding niches, than ephemeral habitats. (4) Significant ecological (Hutchinsonian) distances exist between body sizes of successive instars. (5) Communities "saturate" with species in ecological time. (6) Density compensation occurs in depauperate guilds. (7) Species compete most strongly with close relatives. (8) Within habitats species show niche separation. In many other groups, such as vertebrates, these patterns are widely attributed to the effects of interspecific competition, either in whole or in part. For folivorous insects, in contrast, we argue that classical, interspecific competition for food is weak and infrequent, and that it cannot fill the major e...
498 citations
TL;DR: This paper summarizes data on development rate for 54 species of insects and data on annual temperature regimes on an elevational and a latitudinal gradient and points out several new evolutionary consequences of the position and shape of the development rate curve in relation to season and location.
Abstract: The fact that development rate of insects changes with temperature implies that the rate at which time passes for a developing insect depends upon the temperature regime it experiences. Understanding this so-called physiological time is of the utmost importance in explaining adaptations of insect life histories to resources that vary seasonally. This paper summarizes data on development rate for 54 species of insects and data on annual temperature regimes on an elevational and a latitudinal gradient. It then treats the curve representing temperature-dependent development as a filter of the temperature regimes during various seasons. The analysis points out several new evolutionary consequences of the position and shape of the development rate curve in relation to season and location. (1) If the criterion for maximizing fitness is fastest development accumulation, the optimal development curve can be specified. (2) In locations with high midsummer temperatures selection could favor different development cu...
412 citations
TL;DR: Habitat partitioning appears to be a difference in morphology whereby T. latifolia was prevented from growing in deep water because of the higher cost of producing broader leaves but better able to compete for light in shallowWater because of its greater leaf surface area.
Abstract: The aquatic plants Typha latifolia and T. angustifolia are observed to be strongly segregated along a gradient of increasing water depth with T. latifolia restricted to depths of less than 80 cm and T. angustifolia to depths greater than 15 cm. Transplantation of both species along the gradient in the absence of competitors showed that T. latifolia was little affected by the presence of T. angustifolia but T. angustifolia was capable of growing over the entire gradient. The loss of precompetitive distribution was not statistically significant for T. latifolia compared to a 39.6% loss for T. angustifolia. It was further observed that overlap was reduced by 43.5% during the course of the growing season. Rhizomes transplanted into natural stands failed to survive, further demonstrating that competition was actively operating to maintain zonation between species. The basis for habitat partitioning appears to be a difference in morphology whereby T. latifolia was prevented from growing in deep water because of...
356 citations
TL;DR: It is found that r is maximized within the observed range in size, and variation in egg size between geographic strains should be positively correlated to variation in body size.
Abstract: Most examinations of body size center on a single factor. However, because body size is directly or indirectly linked to many, if not most, life history characters a more holistic approach is advocated. In this paper I present such an approach in the analysis of the optimum body size of Drosophila melanogaster. The basic life history parameters determining r are shown to be related to body size. Using these functions the relationship between r and body size is obtained. It is found that r is maximized within the observed range in size. A sensitivity analysis indicates that this result does not depend critically upon parameter estimation. This analysis also indicates that variation in egg size between geographic strains should be positively correlated to variation in body size. This prediction is shown to be correct. Reasonable variation in parameter values can account for much of the size range observed in the genus Drosophila. It does not appear to be possible to account for the very large size of certai...
339 citations
TL;DR: Adult male and female red-backed salamanders, Plethodon cinereus, employ "dear enemy" recognition, such that individuals are less aggressive and more submissive toward familiar territorial neighbors than toward strangers.
Abstract: Adult male and female red-backed salamanders, Plethodon cinereus, employ "dear enemy" recognition, such that individuals are less aggressive (threat displays and biting) and more submissive toward familiar territorial neighbors than toward strangers. Pheromones are used to distinguish familiar from unfamiliar conspecifics. This behavior reduces the likelihood of escalated contests between neighbors. When combat does occur, bites are usually directed at the two most vulnerable parts of an opponent's body. An attack to the tail might lead to autotomy of that organ, with resultant loss of fat reserves. Injury to the nasolabial grooves on the snout leads to impairment of their chemosensory function, resulting in reduced capture rate during foraging and perhaps in reduced ability to locate mates and competitors. Dear enemy recognition probably is an evolutionary response to the high cost and low payoff of escalated aggression among territorial neighbors.
336 citations
TL;DR: Snails exposed to the trematode parasite, Schistosoma mansoni, are shown to experience an early reproductive increase soon after exposure to S. mansoni whether or not they become parasitized, and snails which were exposed but not parasitized showed reduced future fecundity as a result of their high early reproductive output.
Abstract: The portion of an organism's energy budget that is allocated to reproduction at each age is selected to maximize overall lifetime reproductive success. Thus reproductive effort at each age should be increased until the benefit of current reproduction no longer outweighs future costs. A number of conditions should select for variations in levels of reproductive effort. High levels of reproductive effort are predicted in environments with constant and high juvenile survivorship, in years favorable for juvenile survivorship in variable environments, and when extrinsic adult mortality is high (Hirshfield and Tinkle 1975). High extrinsic mortality in adults may result from physical or biotic factors such as predation or parasitism. Biomphalaria glabrata snails exposed to the trematode parasite, Schistosoma mansoni, may or may not become infected. However, all snails exposed to these parasites appear to compensate for expected future loss in reproductive success by an increase in egg laying soon after exposure. Snails that become infected with schistosomes suffer increased mortality and show reduction or complete cessation of egg production usually beginning 4-6 wk after exposure. Increased reproductive effort early in life for an iteroparous species has been assumed to be associated with a cost to parents in terms of reduced survival and/or future reproduction, yet little data exist to substantiate this claim (Stearns 1976). In this paper, B. glabrata snails are shown to experience an early reproductive increase soon after exposure to S. mansoni whether or not they become parasitized. Our work further explores the consequences of fecundity compensation. Snails which were exposed but not parasitized showed reduced future fecundity as a result of their high early reproductive output.
TL;DR: In this paper, howler monkeys (Aloutta palliata) and spider monkeys (Ateles geoffroyi), both primary consumers, were investigated by carrying out feeding trials to determine food passage rates and by examining selected aspects of gut morphology.
Abstract: Critical determinants of dietary choice in animals may be internal rather than external and hence not readily detected by field observation. Digestive strategies of two sympatric primate species, howler monkeys (Aloutta palliata) and spider monkeys (Ateles geoffroyi), both primary consumers, were investigated by carrying out feeding trials to determine food passage rates and by examining selected aspects of gut morphology. On the average, a given marker took 20.4 + 3.5 h to first appear in the feces of howler monkeys as contrasted with a mean of 4.4 + 1.5 h for spider monkeys. Gut morphology showed that howlers had colons approximately double the size of those of spider monkeys. Howler monkeys are highly folivorous while spider monkeys are primarily frugivorous. Yet leaves are generally low in nonstructural carbohydrates while fruits are low in protein. Howlers, with their capacious hindguts and slow food passage rates, are able to ferment refractory plant parts more efficiently than spider monkeys and in...
TL;DR: In this article, 11 features of nitrogen metabolism in the plant and in the ecosystem are delineated, each appearing to have negative survival value, especially negative effects on yield, such as apparent failures in controls over N2 fixation and over nitrate uptake, preventable energy losses in soil microbial metabolism and in root reduction in nitrate; preventable mass losses of nitrogen via denitrification and ammonia volatilization, and insufficient partitioning of photosynthetic energy to support N 2 fixation in N-limited ecosystems.
Abstract: Eleven features of nitrogen metabolism in the plant and in the ecosystem are delineated, each appearing to have negative survival value, especially negative effects on yield These include apparent failures in controls over N2 fixation and over nitrate uptake; preventable energy losses in soil microbial metabolism and in root reduction in nitrate; preventable mass losses of nitrogen via denitrification and ammonia volatilization, and insufficient partitioning of photosynthetic energy to support N2 fixation in N-limited ecosystems A quantitative examination of the energetics of metabolizing N2, NH3, and NO3 - reveals behind often high costs some energy inefficiencies required for sufficiently fast kinetics and some compromises with other metabolisms Cost: benefit ratios can be defined for assimilation of the three forms of N, and each ratio may vary significantly according to other physiological demands, such as pH control at the rhizosphere or stomatal closure for efficient use of water For the individ
TL;DR: It is contention that, in the "battle between the sexes," the male sex is relatively victorious in the pied flycatcher as compared with most altricial bird species which are strictly or almost strictly monogamous.
Abstract: Some males of the pied flycatcher enhance their individual fitness by mating with two females, one of which receives their full support in caring for the brood, while the other is given very little or no aid and therefore, on average, produces a much reduced brood. These so-called secondary females thus have a low fitness. We refute the theories of differential territory quality and of sexy sons to explain the bigamous strategy in this species and argue that the male cheats some females into the role of being secondary. The trick used by the male is maintaining two spatially separate territories (polyterritorialism). We found that secondary females lay fewer eggs and take this as evidence that, at some point, they become "aware" of their true marital status and then, adaptively, reduce their clutch size. The alternative explanation, namely, that secondary females are intrinsically low-productive, for example, because of relatively younger age, can probably be refuted. Large males were found to be overrepr...
TL;DR: The new view that the temporary weight decrease in adults while rearing young may actually enhance parental inclusive fitness, i.e., it may be adaptive instead of purely detrimental, is suggested.
Abstract: The lowered weight in adult birds rearing young should result in energy savings during locomotion. Furthermore, the parents may live partly off the energy released during weight loss. For these reasons the parents could eat a smaller proportion of the food collected themselves and give a larger proportion to the young, potentially leading to more fledged young, faster growth, and/or better condition of the young than if the parents maintained prebreeding weight. I examine this possibility theoretically and estimate the potential number of extra young for various weight reductions and daily flight times in the parents. I estimate energy savings in flight only, since this is the most power-consuming type of locomotion. Because weight reduction in an individual bird lowers not only induced drag but also body drag, the power consumption in flight may vary approximately with the 1.5 power of the weight. Energy savings in flight resulting from weight loss may readily enable some birds to rear on average 1/4-1/2...
TL;DR: Using a simple model, it is proposed that the exclusion of random variations in predators can lead to greater extremes in the phytoplankton/herbivore populations in enclosures compared with those outside.
Abstract: Data on plankton ecosystems in large enclosures are used as a basis for consideration of the role of deterministic and random processes in these systems. Using a simple model, it is proposed that the exclusion of random variations in predators can lead to greater extremes in the phytoplankton/herbivore populations in enclosures compared with those outside. These results depend on the relative rates of internal and exogenous changes, and comparisons are made with results for a forest ecosystem.
TL;DR: Data do not support the need for alternative hypotheses centered on larval habitat preferences and stochastic recruitment, and broad geographic patterns of chaetodontids in the Pacific are similar to those seen in terrestrial vertebrates.
Abstract: Observations of the community of chaetodontid fishes at the northern end of the Great Barrier Reefs are consistent with the hypothesis that it is structured in ways similar to terrestrial vertebrate communities. Our data do not support the need for alternative hypotheses centered on larval habitat preferences and stochastic recruitment. Broad geographic patterns of chaetodontids in the Pacific are similar to those seen in terrestrial vertebrates.
TL;DR: Variation in the ratio of reproductive reserves to soma at adult eclosion among heliconiine species and sexes having relatively equivalent larval survivorship and availability of nutrients correlates with patterns of expected intake of nutrients and output of nutrients in the act of reproduction.
Abstract: In organisms with complex life cycles, potential reproductive effort at adult emergence may be defined as the sum of the proportion of the body nutrient content devoted to reserves earmarked for reproduction and the proportion of expected adult nutrient intake to be devoted to reproduction, and is proportional to expenditure in reproduction. For organisms with similar larval nutrition and survivorship, the ratio of reproductive reserves to soma at adult eclosion is predicted to vary inversely with expected adult nutrient intake and directly with expected reproductive output of nutrients. These predictions are supported by data from heliconiine butterflies. That is, variation in the ratio of reproductive reserves to soma at adult eclosion among heliconiine species and sexes having relatively equivalent larval survivorship and availability of nutrients correlates with patterns of expected intake of nutrients and output of nutrients in the act of reproduction. In general, changes in the ratio over adult life...
TL;DR: The model differs from previous optimization or maximization models for planktonic larvae by including onshore-offshore mixing and thereby linking selection on the precompetent and competent periods.
Abstract: A model of larval dispersal of coastal benthic invertebrates is proposed: Eddy mixing spreads larvae in the onshore-offshore direction during the period before competence to settle and continues to take larvae both onshore and offshore after they are competent to settle; predators kill larvae during both precompetent and competent periods. Distribution of larval stages of several species are consistent with the model. The model correctly predicts (1) increased competent period with increased precompetent period in comparisons among species, and (2) a competent period greater than or equal to the precompetent period. The model predicts a decrease in the ratio of competent to precompetent period as precompetent period increases, which is not apparent in available data. Also, some observed competent periods are longer than those predicted by the model. Reasons for these departures from predictions are suggested. The model differs from previous optimization or maximization models for planktonic larvae by incl...
TL;DR: Two alternative hypotheses for the evolution of sexual niche partitioning are discussed; the first suggests that niche differences are the result of selection against deleterious intersexual competition, and the second suggests that selection for interspecific competitive ability may lead to niche differences and may possibly be implicated in the Evolution of dioecism from a bisexual species.
Abstract: Evidence for sexual niche partitioning in Mercurialis perennis and Silene dioica in Wales and Trophis involucrata in Costa Rica is presented. The differences in M. perennis and T. involucrata are spatial in nature with males of M. perennis being found more frequently in patches of high soil pH while males of T. involucrata are found more frequently in patches of low total phosphorous content. Sexual niche partitioning in S. dioica was found to be temporal in nature, with females being precocious in their dominance of the leaf canopy. Sexual dimorphisms and the results of intersexual competition trials in S. dioica are discussed. Two alternative hypotheses for the evolution of sexual niche partitioning are discussed; the first of these suggests that niche differences are the result of selection against deleterious intersexual competition, and the second suggests that selection for interspecific competitive ability may lead to niche differences and may possibly be implicated in the evolution of dioecism fro...
TL;DR: The nutrient poor ecosystem is able to maintain its productivity under undisturbed conditions through a variety of nutrient conserving mechanisms, the most important of which are associated with the humus and root layer on top of the mineral soil.
Abstract: Nutrient poor forests are more common in the tropics than in the temperate regions, but most of the work on nutrient cycling in the tropics has been carried out on nutrient rich sites such as those in Puerto Rico and Costa Rica rather than nutrient poor sites such as those in central and eastern Amazonia. Consequently, there has been confusion as to whether nutrients are critical in tropical forests. Productivity and nutrient cycling in nutrient rich and nutrient poor forest ecosystems do not differ greatly as long as the ecosystems are undisturbed. However, when the forests are cleared for agricultural purposes, the nutrient poor systems quickly lose their productive potential, whereas the nutrient rich systems do not. The nutrient poor ecosystem is able to maintain its productivity under undisturbed conditions through a variety of nutrient conserving mechanisms, the most important of which are associated with the humus and root layer on top of the mineral soil. These mechanisms are destroyed when the nu...
TL;DR: It is concluded that the practice of multiple parasitoid introductions, and the use of parasitoids with high searching efficiency and a marked ability to seek out patches of high host density, are sound strategies for maximizing the depression in the host equilibrium populations and for ensuring that they remain locally stable.
Abstract: In this paper, we have explored the dynamics of some simple multiparasitoid models, concentrating on the factors affecting coexistence and equilibrium levels. As a prelude, a one parasitoid-one host system is examined since this provides the basic submodel for parasitism that is used throughout. This is followed by a system with two parasitoid species attacking the same host species, a system with facultative hyperparasitoids, and finally one with hosts, parasitoids, and hyperparasitoids. In each case, the conditions permitting the three species to coexist are examined and, by introducing a host rate of increase that is density dependent, we also show the extent to which parasitism can depress the host equilibrium below its carrying capacity. The results are discussed within the context of biological control. We conclude that the practice of multiple parasitoid introductions, and the use of parasitoids with high searching efficiency and a marked ability to seek out patches of high host density, are sound ...
TL;DR: Song repertoires of breeding male great tits are recorded during six successive seasons and correlations between repertoire size and reproductive success are tested, suggesting lifetime reproductive success appears to be highest for males with intermediate-sized repertoires.
Abstract: We recorded the song repertoires of breeding male great tits during six successive seasons and tested for correlations between repertoire size and reproductive success. Males with larger repertoires are more likely to survive to breed in a second season. They are also more likely to father offspring which survive to breed. Lifetime reproductive success appears to be highest for males with intermediate-sized repertoires. Males with larger repertoires produce heavier fledglings, and this may be related to territory quality.
TL;DR: It is hypothesized that in the absence of grazing, sand-plain species would competitively exclude reef-slope species from much of the subtidal reef, and reef hervibores, by limiting competitive dominants to refuge areas, are of primary importance in maintaining the large between-habitat diversity typical of shallow Caribbean reef systems.
Abstract: An extensive sand plain containing scattered hard substrates dominated by algae adjoins the shallower Galeta reef seaward from a depth of 11-14 m. This sand plain represents a physically marginal habitat where low light and lack of adequate attachment sites limit algal growth and where seasonal physical disturbances (turbidity and sand movement) cause large decreases in algal densities and cover. Light levels are higher and substrate more abundant on the nearby reef slope but species from the sand-plain assemblage rarely occur there. Algae from both reef-slope and sand-plain habitats become light saturated at approximately the same level (75-100 μE/m2 per s) but those from the sand plain were more productive (i.e., more efficient) throughout the range of light levels tested (13-170 μE/m2 per s). Efficient utilization of low light levels allows sand-plain species to survive on the dimly lit bottom but does not prohibit them from occupying areas of high light intensity. During two experimental periods of ap...
TL;DR: It is argued that total reproductive effort is a more realistic estimate of the resources actually committed to reproduction than is seed output, which conforms to the normally accepted meaning of the word "effort."
Abstract: MacArthur and Wilson (1967) recognized that the way in which organisms allocate resources to competing \"sinks\" could be used to determine the position of a species on a selective continuum. This recognition has stimulated a spate of experimental studies on resource allocation. The proportion of resources devoted to reproduction, as opposed to the development of a competitive growth form or defense against predators, has rightly been seen as the character of greatest importance. It is, in any case, the one which appears to be easiest to measure. Reproduction in this sense has usually been taken to mean sexual reproduction, especially since \"vegetative reproduction\" is an imprecise term capable of varied interpretations (Harper 1977). Herbaceous plants have proved to be ideal subjects for study in this respect and numerous attempts have been made to determine the reproductive effort of species and even of whole communities. An examination of table 1, which lists the important features of a number of such attempts, clearly reveals that no consensus of exactly what is being measured or how it is to be measured has yet been reached. There exist, therefore, both conceptual and methodological problems. Here we attempt to examine these problems and suggest possible ways in which they might be overcome. Perhaps the most striking conceptual dichotomy is that which divides the approach of Harper and Ogden (1970) from that of Gadgil and Solbrig (1972). The former have chosen to measure the proportion of resources devoted to seeds while the latter have measured allocation to \"all reproductive structures.\" These quantities will be referred to subsequently as seed output and total reproductive ffort, respectively. What makes this division so remarkable is that, with very few exceptions, no one on either side has felt it necessary to provide any satisfactory justification for his chosen method, nor does any other approach seem to have been seriously considered. This is not by any means a sterile academic argument; there are fundamental ecological differences between the quantities being measured by the two schools. We contend that total reproductive effort is a more realistic estimate of the resources actually committed to reproduction than is seed output. This usage conforms to the normally accepted meaning of the word \"effort.\" Production of the whole floral apparatus and perhaps supporting structures as well is all part of the \"effort\" involved in reproduction. Seed output, on the other hand, may be viewed as the result of the interaction of this effort with a number of environmental variables (pathogens, climate, predators, pollinators, etc.) over which the plant has little or no control. It should be made clear at this point that there is no objection to the measurement of seed output per se, but that this quantity should not be confused with reproductive effort. Unfortunately the conceptual problems do not end with the decision to measure total reproductive effort. Nearly all the workers who have used this approach have measured only the allocation to the actual flowers; all supporting structures
TL;DR: It is demonstrated that current knowledge of the taxonomic distributions of dioecy and self-incompatibility systems provides good reasons for assuming that selection for outcrossing may frequently be a sine qua non for the evolution of dIOecy.
Abstract: The evolution of dioecy in plants has traditionally been explained as a result of selection for outcrossing (Baker 1959; Carlquist 1966, 1974; Darlington 1958; Darwin 1877; Gilmartin 1968; Ho and Ross 1974; Lewis 1942; Lloyd 1972, 1975; Mather 1940; Maynard Smith 1978; Ross 1970, 1978; Ross and Shaw 1971; Ross and Weir 1976; Stebbins 1951). Population genetical models, notably those of Charlesworth and Charlesworth (1978, 1979) have indicated that inbreeding depression is usually necessary for the evolution of dioecy from hermaphroditism. Recently, the importance of outcrossing has been questioned independently by several authors who, following Bateman (1948), propose that sexual selection acting on the male and female components of hermaphrodites (see Charnov 1979; Charnov et al. 1976; Janzen 1977) and selection for optimal resource allocation can also explain the evolution of dioecy. Willson (1979, p. 779) presents her version of this sexual selection hypothesis to counter \"the kneejerk response that the advantage of dioecy and other functional differentiations of sexual roles in plants lies solely in the advantage of outcrossing.\" Givnish (1980) and Bawa (1980) also question the causal role of selection for outcrossing in promoting dioecy and offer alternative models based on the ecological roles of pollination and dispersal. Although we applaud the development of new theory, we fear that this recent cohort of \"alternative\" explanations may obscure some important reasons for retaining the outcrossing hypothesis. Here we demonstrate that current knowledge of the taxonomic distributions of dioecy and self-incompatibility systems provides good reasons for assuming that selection for outcrossing may frequently be a sine qua non for the evolution of dioecy. Willson (1979), Bawa (1980), and Givnish (1980) provide variously detailed scenarios in which differential male and female costs and success rates can lead to dioecy independently of inbreeding/outbreeding considerations (see also Charnov 1979). Bawa and Givnish go on to document new ecological correlates of dioecy which, they believe, provide evidence for the sexual selection explanations. In general, the explanations cannot easily be refuted, because they make few explicit assumptions about genetic mechanisms and because they depend on certain relationships between costs and benefits that strongly resist quantitative study because of difficulties in selecting and measuring a fitness-based cost/benefit currency. For this reason we do not wish to debate the relative merits of specific details of these various proposals, except to state that all of them seem to depend on a rather high predictability of success for a particular genotype and on a high degree of heritability of phenotypic gender (see Williams 1975, p. 130). Such dependence may be unwarranted, given the plasticity of individual development in plants (see Gottlieb 1977) and the great importance of spatial location in mating success. Instead, we try to assess the overall importance of any mechanisms for evolving dioecy which are independent of outbreeding considerations. We also
TL;DR: Cybernetic systems are systems with feedback in which input is determined, at least in part, by output and the portion of output that is returned to input is the feedback, and this may become the basis for feedback control.
Abstract: Cybernetic systems are systems with feedback (Wiener 1948). They are a special class of cause-and-effect (input-output) systems in which input is determined, at least in part, by output. The portion of output that is returned to input is the feedback, and this may become the basis for feedback control. Very small feedbacks may exert very large effects. Figure la illustrates a basic input-output system. Energy, matter, or information coming from the environment causes the system to respond; this reaction is transmitted as energy, matter, or information output back to the environment. Figure lb shows a feedback system consisting of two component subsystems. This system is determinate because its behavior is governed only by past causes. Its feedback structure may passively or emergently make its behavior stable, regular, or otherwise predictable, and may enable it to damp disturbance. Figure Ic illustrates a feedback control system in which the feedback subsystem is a controller through which information about desired output can be introduced. Actual output information is fed back to the controller, and the deviation of actual from desired becomes the basis for corrective action. This system is teleological because its behavior is guided by future or desired goals. The behavior is stable, regular, and purposeful since the feedback organization has been designed to actively achieve such characteristics. Feedback control systems may be manmade or natural, and may have living or nonliving components. Controllercontrolled system pairs that are common in experience include thermostatfurnace, guidance system-missile, driver-car, eye-hand, brain-body, etc. Both determinate and teleologic feedback systems are cybernetic because they contain feedback.
TL;DR: The methods used in this paper have generality and could be used to study the mating system in other mammals that are difficult or impossible to study by direct observation, as well as several other probable cases of monogamy among small rodents.
Abstract: Populations of Peromyscus polionotus, the oldfield mouse, are organized into family groups which are characterized by a high degree of monogamy. There is no evidence that females of this species ever mate with more than one male for each litter. Females that are collected in the field with males have the same mate for consecutive litters (the frequency of mate switching for these females does not differ significantly from zero). In addition, the male that is collected with the female is usually the father of her offspring (the frequency of nonpaternity is .117). Thus, oldfield mice are overwhelmingly monogamous. However, a few females may not form long-term monogamous relationships, as shown by the relatively high frequency of mate switching for females collected without males (.40). Recent research has uncovered several other probable cases of monogamy among small rodents. Any generalization about mating systems among mammals must consider the diversity of mating systems among small rodents. The methods ...
TL;DR: It is shown here that such a correlation does indeed exist when the effect of habitat on seed size is taken into account and the first statistical observation of a positive correlation between seed size and germination date is reported.
Abstract: Seed weights vary between species over a range of 10 orders of magnitude, but mean seed weight within species is one of the least plastic components of plant structure (Harper et al. 1970). Seed size appears to be so finely tuned to the ecological life history of species that it is generally conserved at the expense of seed number when environmental conditions necessitate a reduction in the reproductive output of a plant (e.g., Harper and Gajic 1961). Two major studies of seed' weight in relation to environmental conditions have shown shade and drought to be the most important correlates of seed size in the native floras of Britain and California, respectively (Salisbury 1942; Baker 1972). Seed size and lifespan are also correlated in the Californian flora but until now such a correlation has been reported absent from Britain (Salisbury 1942; Hart 1977). I show here that such a correlation does indeed exist when the effect of habitat on seed size is taken into account and I also report the first statistical observation of a positive correlation between seed size and germination date. All dissemules (in some cases actually fruits) are defined as \"seeds,\" following the convention of previous authors (Salisbury 1942; Baker 1972) and to allow comparison with other studies. By confining an analysis of seed size to the flora of a single habitat it is possible to uncover ecological correlates of this variable which would otherwise be masked by the strong effect of habitat differences on seed size. Calcareous grasslands (C. G.) offer a greater number of species for ecological comparison than any other habitat in the British Isles (Ratcliffe 1977). Mean seed weights of 75 dicot species obtained by weighing samples collected from this habitat in southern England (Silvertown 1979) and from published measurements (Salisbury 1942; Grubb 1976) show that biennials and perennials have similar ranges of seed weight while annuals have significantly lighter seeds than longer-lived herbs (Wilcoxon twosample test, P < .01; fig. 1). The low seed weight of annuals, in association with their characteristically greater reproductive effort (Harper 1977) maximizes seed number and opportunities for colonizing gaps in the vegetation by dispersal (Baker 1974; Rabinowitz 1978). Small size also favors the chances of a seed being incorporated in the pool of buried seed (Grime 1979)-an important strategy for persistence in species which have a short lifespan as reproducing individuals (Cohen 1966; Roberts and Feast 1973). The importance of habitat in determining seed size within a group of species of similar life span is emphasized by a comparison of the seed weights of annual arable weeds with the annuals of calcareous grassland in figure 1. The C. G. annuals are significantly lighter than the arable weeds (P < .01; there is no significant difference in seed weights between C. G. biennials, C. G. perennials, and arable annuals). Germination dates for C. G. plants were obtained by direct observation of permanent quadrats during a study of this habitat, and from literature sources
TL;DR: It is suggested that unapparent larval foods (those that are ephemeral and difficult to find) and unpredictable ant resources select against ant specialization, permitting only opportunistic use of protective ants.
Abstract: Many lycaenid butterflies counter attacks from natural enemies by associating with protective ants. The acquisition of this enemy-free space has strongly influenced lycaenid evolution. Four major adaptations are discussed: the thick larval cuticle, appeasement pheromones, the nectar gland with associated tentacles, and location of ants by ovipositing adults. The larval nectary is hypothesized to have a sequestering function when ants and larval food are not immediately coincident, and to release sheltering behavior by ants that reduces larval and pupal apparency. In facultative ant relationships the nectary may also function to appease "unknown" ants. When ants and larval food are coincident, the nectary is often absent or nonfunctional. Lycaenid larvae eat a diversity of unrelated foods, including flowering plants, fungi, lichens, cycads, ferns, conifers, homopterans, and ant larvae. They also "milk" homopterans and are fed orally by ants. These dietary shifts are apparently strongly conditioned by the d...