摘要:

Diseases and pathogens are receiving increasing recognition as sources of mortality in animal populations. Immune system strength is clearly important in fending off pathogen attack. Physical barriers to pathogen entry are also important. Various individual behaviors are efficacious in reducing contact with diseases and pests. This paper focuses on a fourth mode of defense: social barriers to transmission. Various social behaviors have pathogen transmission consequences. Selective pressures on these social behaviors may therefore exist. Effects on pathogen transmission of mating strategies, social avoidance, group size, group isolation, and other behaviors are explored. It is concluded that many of these behaviors may have been affected by selection pressures to reduce transmission of pathogens.

ISSN:0012-9658    

DOI:10.2307/1941192

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Classical food—web theory arises from Lotka—Volterra models. As an alternative, we develop a model from the logistic concept of demand and supply. We first extend the logistic to an arbitrary species in a trophic chain or stack by developing a simple equation for any population Xi: 1/XidXi/dt = ai— biXi— Xi/ciXi—1— diXi+1/Xi, which includes the effects of intra—specific competition for fixed resources (the term biXi), intra—specific competition for renewable resources in the lower trophic level (the term Xi/ciXi—1), and consumers in the upper trophic level (the term diXi+1/Xi). This equation emerges from the basic logistic concept of demand and supply, as captured by the consumer/resource ratios, and fulfills all the requirements for a plausible food—chain equation. We then generalize the equation to any population in a food web of arbitrary complexity 1/XidXi/dt = aibiXi— Xi/sjcijXjFrj(i)— skdikXkFkc(i)/Xi, where Frj(i)is the fraction of population Xjthat is a resource for i, and Fkc(i)is the fraction of population Xkthat consumes i. This equation meets all the requirements for a general food web model. Some properties of the model are discussed.

ISSN:0012-9658    

DOI:10.2307/1941193

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Seed dormancy, which is thought to have evolved in response to unpredictable environmental variability, has led to the existence of seed banks–populations of dormant, viable seeds in the soil. Seed banks are theoretically important to both the demography and genetic structure of plant populations. The presence of seed dormancy can also affect the evolution of traits not directly associated with dormancy and germination. Theoretical models have suggested that the existence of dormancy can influence the rate of evolution of post—germination traits. The eventual outcome (e.g., allele frequencies) may be influenced as well, leading to adaptive syndromes of germination and post—germination traits. Seeds that germinate in different conditions may experience different selective regimes for post—germination traits. If there are trade—offs between the fitness of post—germination traits in different environments, then seeds that germinate in the environment to which their post—germination traits are adapted will be at a selective advantage. If differences in germination and post—germination traits are genetically based, then potentially adaptive genetic correlations between germination and post—germination traits may evolve. We feel that investigating the ecological and evolutionary importance of these correlations requires an empirical approach. As a first step, here we ask whether the conditions necessary for such syndromes to arise exist in a particular plant population. We show that conditions favoring the joint evolution of dormancy and post—germination traits leading to adaptive syndromes exist in the mustard, Lesquerella fendleri, in central New Mexico. First, Lesquerella experiences the sort of variation in environmental conditions that would be expected to lead to adaptive trade—offs in the expression of post—germination traits for individuals that differ in germination traits. Annual precipitation varies greatly from year to year so that germination in drier years would be expected to select for more xerophytic traits. Within a year, microenvironmental spatial variation exists. Lesquerella growth and reproduction are sensitive to both year—to—year and microenvironmental variation. Second, the seed bank can affect both the demographic and genetic structure of the population. Dormant seeds remain viable for at least 3 yr and can mitigate the negative demographic effects of reproductive failure. Allozyme differences exist between seeds that germinate in the field and seeds that remain dormant, suggesting that the evolutionary potential of the aboveground population is influenced by dormancy. Finally, the necessary genetic and environmental variation is present. Both germination percentage and post—germination traits (e.g., leaf morphology) vary among and within populations as well as among environmental treatments. Thus, the potential exists for Lesquerella to respond to selective differences between temporal or spatial environments. We suggest experimental approaches for assessing the extent to which seed dormancy has affected or will affect the evolution of post—germination traits. The consequences of past evolution could be explored by asking "What genetic and phenotypic differences exist between individuals that germinate and those that remain dormant?", while controlling for factors that influence germination (maternal genotype, maternal environment, and germination environment). Exploring whether evolution is currently occurring would require an assessment of natural selection and the genetic potential for response to selection. Given the difficulty of such studies, a reasonable first step would be to explore how evolution can occur by performing an artificial selection experiment on dormancy or germination percentage. Then, correlated responses of post—germination traits could be examined. Empirical studies such as these are necessary in order to better understand the role of seed banks in plant ecology and evolution. Then, once associations that can be interpreted as adaptive syndromes are documented, questions about the ecological mechanisms (e.g., necessary frequencies of different year types) and genetic mechanisms (e.g., linkage disequilibrium vs. pleiotropy) can be explored. We hope to draw attention to seed dormancy, which is an often—ignored stage in the life history of plants, and to encourage empirical work, which lags far behind theory.

ISSN:0012-9658    

DOI:10.2307/1941194

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

When Leo Luckinbill (1973) grew Paramecium aurelia together with its predator Didinium nasutum in 6 mL of standard cerophyl medium, the Didinium consumed all the prey in a few hours. When the medium was thickened with methyl cellulose, the populations when through two or three diverging oscillations lasting several days before becoming extinct. When he used a half—strength cerophyl medium thickened with methyl cellulose, the populations maintained sustained oscillations for 33 d before the experiment was terminated. The data from this experiment provide a rare opportunity to test current predator—prey models. A standard differential equation predator—prey model with a carrying capacity for the prey and a saturating (Type 2) functional response predicts the outcome of Luckinbill's experiment qualitatively, but does not give a good quantitative fit to the data. Several modifications of this model are tested against the data for the populations grown in the medium thickened with methyl cellulose, using the Marquardt—Levenberg method to obtain the least squares best fit. Neither Leslie type models nor models with a ratio—dependent functional response do well, but adding either predator mutual interference or a sigmoid (Type 3) functional response improves the fit dramatically. Modeling the predator growth rate to depend on energy or nutrient storage instead of directly on the rate of consumption of prey, thus creating a delayed numerical response, along with predator mutual interference or a sigmoid functional response, produced the best models and gave excellent fits to the data. These models are further validated by the fact that changing only one or two parameter values to reflect the unthickened medium or the half—strength medium also gives reasonably good fits to the other data sets. The last model requires a more sigmoid functional response to fit the data in the thickened than in the unthickened medium, suggesting that an increase in the cost—benefit ratio of energy spent searching to energy gained capturing prey inhibits the predator searching at low prey densities.

ISSN:0012-9658    

DOI:10.2307/1941195

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

In eusocial Hymenoptera, relatedness asymmetries lead to conflict between parents and offspring over sex investment; workers strive for a 3:1 female—biased ratio of sex allocation, and queens strive for a 1:1 ratio. Many studies support this genetic relatedness hypothesis, but the variation in allocation ratios in natural populations is great and remains mostly unexplained. In this paper, we examined whether food supply determines sex investment of the ant Formica podzolica. We compare reproductive parameters of colonies and populations across habitats, and compare sex allocation of fed and unfed colonies. Nest density, worker size, sex ratio, and sex allocation were all greater along forest edges than in meadows, and these patterns were associated with natural food abundance. Furthermore, there was a strong tendency for individual colonies to produce either all—male or all—female sexuals, and worker size was greatest in colonies producing all females and smallest in those producing all males. Most important, sex investment was greatly affected by a supplemented diet, as the population investment ratio, R (i.e., ratio of males to males plus females), was female biased (0.36) for fed colonies and male biased (0.62) for unfed, control colonies. Finally, investment ratios were more male biased in polygynous than in monogynous colonies as predicted by a genetic relatedness hypothesis. These results demonstrate that food supply has an important proximate influence on sex investment, and may explain much of the natural variation in sex investment in populations of eusocial Hymenoptera.

ISSN:0012-9658    

DOI:10.2307/1941196

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Reductions in host survival due to parasitism have immediate effects on the population dynamics of both parasites and hosts. Despite its potential importance, parasite—induced host mortality has rarely been studied experimentally under natural conditions. We conducted a series of field releases of Drosophila putrida and Drosophila neotestacea that had been parasitized in the laboratory by the nematode Howardula aoronymphium. In 11 out of 12 independent tests, the proportion of flies that were parasitized declined significantly through time, suggesting that parasites cause host mortality. Independent tests showed that parasites have no detectable effect on dispersal of flies and that motherworms do not disappear from surviving hosts, thus supporting the conclusion that parasites cause significant host mortality in the field. Host mortality rate increased with worm burden, significantly so in four independent tests. This is a possible mechanism for density—dependent regulation of these parasite populations. Parasite—induced mortality was considerably greater among flies that had been released in the field than among those kept concurrently in the laboratory. Thus, laboratory or clinical traits may not provide an accurate assessment of parasite virulence in natural populations. These results support a central assumption of many recent ecological and evolutionary models, i.e., that parasites are harmful to their hosts under natural conditions.

ISSN:0012-9658    

DOI:10.2307/1941197

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Despite the blossoming interest in host—microparasite epidemiology, and in use of viruses in the biological control of insect pests, few empirical studies have attempted to quantify transmission and mortality rates under field conditions. We report a laboratory and field study in which the transmission parameter (u) and mortality rate (a) due to nuclear polyhedrosis virus (NPV) are measured in different larval instars of the cabbage moth, Mamestra brassicae (Lepidoptera: Noctuidae). Laboratory studies of food consumption and virus susceptibility were used to produce crude estimates of relative transmission rates in successive instars. Increased in the rate of feeding outstrip increases in virus resistance with instar, so we predict that transmission rates should increase in older larvae (assuming rate of intake of virus to be proportional to rate of feeding). This prediction was tested in a field experiment in which a constant initial density of susceptible and infected (moribund) larvae were reared together on cabbage plants for 2—8 d. Estimates of the linear transmission parameter (u) did not differ between instars and gave a mean value of 2.16 x 10—12for all instars and time points. Causes for the discrepancy between predictions based on laboratory data and field measurements are discussed. Differences were found between instars in the time from infection to death (?) (equivalent to 1/a, where @a is the rate of mortality due to viral infection). Second—instar larvae died more rapidly once infected than third instars, which, in turn, died more rapidly than fourth instars. The effect of host population stage structure on patterns of viral infection can be pronounced and if recognized, may significantly increase the accuracy and predictive value of models of host pathogen systems.

ISSN:0012-9658    

DOI:10.2307/1941198

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Non—genetic effects of maternal environment often have large influences on offspring phenotypes. Some maternal effects may be adaptive in heterogeneous environments, whereby mothers can program a developmental switch in their offspring in response to a predictive environmental cue. For herbivorous insets, maternal rearing hosts (i.e., maternal environment) may have large effects on offspring, including adaptive effects on patterns of host suitability. For example, if maternal rearing host is predictive of local host availability, then it is advantageous for mothers reared on one host to produce offspring that are physiologically 'acclimated' to that host. Previous research has demonstrated that seed beetles, Stator limbatus, collected from Cercidium floridum survived better and developed faster on this host than beetles collected from Acacia greggii or C. microphyllum at either the same site or from a different locality. These results indicate either within— and among—population host—associated genetic differentiation or a non—genetic maternal host effect, in which offspring survive better and develop faster on the host their mother was reared on. We examine the influence of maternal and paternal rearing hosts on the survivorship, development time, and body size in offspring in S. limbatus. We find no evidence that beetles are acclimated to perform better on the host their mother was reared on via non—genetic maternal effects. However, maternal rearing host did affect offspring in other ways; mothers reared on C. floridum produced larger offspring that developed faster than offspring of mothers reared on A. greggii, due to a non—genetic maternal effect (paternal rearing host had no effect). In addition, both maternal and paternal host had a large effect on offspring survivorship; offspring of parents reared on C. floridum survived better than those of parents reared on A. greggii. The symmetry between the maternal and paternal host effects on survivorship suggests that they represent a response to intense selection during the experiment, and suggest the potential for rapid adaptation to this host. Alternatively, non—genetic effects of both maternal and paternal rearing host may simultaneously influence survivorship of S. limbatus offspring. Although paternal effects are generally disregarded as an important source of environmental variation, they are potentially important for S. limbatus; non—genetic paternal effects were detected in this experiment for development time of offspring: fathers reared on C. floridum produced offspring that developed slower on C. floridum, and offspring that developed faster on A. greggii, than fathers reared on A. greggii.

ISSN:0012-9658    

DOI:10.2307/1941199

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The hypothesis that fitness following dispersal from the natal host increases with body size was tested under field conditions in the egg parasitoid Trichogramma pretiosum. In one experiment, size distributions of males and females at emergence were compared to those of males successful in locating mates and females successful in locating hosts. Mate and host location success increased with size in smaller parasitoids but were constant with respect to size in larger parasitoids. Selection intensities were 3.5—5.4 times greater in males than females possibly reflecting the small contribution of postdispersal matings to total male fitness in this partially sibmating species. In a second experiment using field releases of genetically marked lines, the number of trap hosts parasitized by "large" females reared from Helicoverpa zea eggs and "small" females reared from Sitotroga cerealella eggs was compared. Large females parasitized more hosts than small females in one trial but the reverse was observed in a second trial. Acceptance and suitability of the trap host species were not influenced by marker line or rearing host. We hypothesize that a genotype—environment interaction based on host age, host species, and wasp genotype accounts for the heterogenous results of the second experiment. Our main conclusion is that average postdispersal fitness of adult T. pretiosum increases with size in smaller wasps and is constant with respect to size in larger wasps. Contrary to general expectation, average wasp fitness does not increase linearly with size. Most importantly, wasp size is not a reliable predictor of individual or average cohort fitness; consistent size—fitness relationships emerge only when fitness is averaged over many genotypes and environments. Systematic factors such as host age and wasp genotype can have a strong influence on wasp fitness without strongly affecting size. The implications of these results for progeny and sex allocation in Trichogramma and for the mass production of parasitoids in biological control programs are discussed.

ISSN:0012-9658    

DOI:10.2307/1941200

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

A plant's grazing history can influence the nutritional quality of its leaves and directly affect herbivore performance. In addition, leaf nutritional quality can indirectly affect herbivore performance by influencing food consumption. We examined the direct and indirect influences of prior grazing of Solidago missouriensis upon leaf nutritional quality and the growth and fecundity of Trirhabda canadensis (Coleoptera: Chrysomelidae). We fed leaves from both previously grazed (during 1988 and 1989) and ungrazed goldenrod plots derived from a single S. missouriensis clone to larvae and adults of T. canadensis in laboratory performance trials. We computed standard insect performance indices and used structural equation modeling and path analysis to interpret the results of these trials. Prior grazing of goldenrod had a direct negative impact upon the growth of T. canadensis larvae in spring of 1990; a concomitant reduction in foliage consumption accentuated this effect. Larvae fed leaves from grazed plots accumulated biomass and N more slowly, used them less efficiently, and reached a lower final mass than did larvae fed leaves from plots without prior folivory. Goldenrods produced a new flush of leaves following larval grazing in the spring of 1990. Therefore, adults feeding in plots defoliated by larvae ate younger leaves than those feeding in undefoliated plots. Leaves from defoliated plots had direct positive impacts upon growth and egg production of adult T. canadensis. Adults converted these young leaves into biomass with greater efficiency and preferred them to older leaves from ungrazed plots in choice tests. However, negative indirect effects of prior grazing upon initial adult mass and total foliage consumption negated any positive influence that these more nutritious leaves had upon growth, and made the overall impact of prior grazing upon egg production negative. These results indicate that impacts of food availability and quality must be considered jointly to understand how prior grazing influences herbivore performance.

ISSN:0012-9658    

DOI:10.2307/1941201

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY