摘要:

The latitudinal variation in clutch size found in many animal species, including Pacific salmon, has been an enigmatic problem in ecology. We analyze egg number and egg size of 17 populations of coho salmon (Oncorhynchus kisutch) distributed over a latitudinal gradient in North America. These populations have a significant latitudinal increase in their egg number. But this increase is accompanied by a significant latitudinal decrease in their egg size. The total biomass of eggs produced also declines with latitude. Thus, the positive latitudinal trend in egg number cannot be explained by latitudinal variation in the total investment in eggs. This suggests that local optima in egg size may result in latitudinal clines in egg number. This reasoning, that egg number evolves around selection for egg size, is in fact predicted by life history theory and may explain the clutch size patterns observed in many organisms.

ISSN:0012-9658    

DOI:10.2307/1940241

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

In most populations of mammals and birds offspring live with (and initally may depend upon) their parents. Consequently, population size is strongly influenced by local fecundity. In contrast, most species of marine invertebrates have a dispersive larval stage, so that local fecundity has little or no effect on the establishment of new individuals. Similarly, many insects, amphibians, and fish have multiple life stages that live apart. Population regulation in such open systems is fundamentally different from more closed populations. I investigated the role of recruits in determining and regulating local population size of a sessile colonial invertebrate, Cellepora pumicosa (Bryozoa, Ascophora). Recruitment into a field population of C. pumicosa over an 18—mo period was less than post—recruitment mortality, causing a decline from 230 to 64 colonies (6.1 to 2.3% cover). The mortality rate was density independent. Colony growth rate declined among larger individuals, which usually regressed in size (shrank) before they died. The mechanics of recruitment limitation was further investigated using a simple demographic model. The model was designed to mimic an open population in which recruits were added to the local adult population at a rate independent of local fecundity or density. After they recruited, colonies could grow, shrink, or die at rates determined in the field. There were no density—dependent terms in the model, yet the population attained an equilibrium size and structure that was determined by a balance between rates of recruitment and size—specific mortality (i.e., storage) of recruited individuals. The equilibrium occurred because the per capita number of recruits declined as more and more individuals became established. Modelling based on the measured demography of C. pumicosa demonstrates that the observed levels of recruitment and mortality will limit the population size to very low levels, although relatively small increases in numbers of recruits would yield a substantially large population. In comparison, post—recruitment mortality would have to decrease greatly and remain density independent to achieve the same result. In contrast to the result for short—lived animals such as C. pumicosa, the population size of longer lived organisms is more sensitive to changes in mortality than recruitment, even when mortality is density independent and populations are recruitment regulated.

ISSN:0012-9658    

DOI:10.2307/1940242

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Recruitment to animal populations can occur through both immigration and in situ reproduction. These two components of recruitment are conceptually distinct and lead to different mechanistic models of population dynamics. We describe a capture—recapture design that can be used to obtain separate estimates of two recruitment components. We then illustrate the use of our method and estimators with capture—recapture data from a population of Microtus pennsylvanicus at the Patuxent Wildlife Research Center in Maryland.

ISSN:0012-9658    

DOI:10.2307/1940243

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

O. diadema is a simultaneously hermaphroditic polychaete worm with a brief protandrous phase. Pairs are formed preferentially between two simultaneous hermaphrodites, which reciprocally change sex roles and trade eggs. Allocation of reproductive effort during simultaneous hermaphroditic and protandrous phases was evaluated quantitatively, by microscopic examination of both fixed and living specimens. Ovarian tissue is four times as abundant as testicular tissue. The mean number of sperm counted in sectioned animals was taken as an approximation of the number of sperm offered per egg. As expected in species where the mating group always consists of two individuals, this number is low (50 sperm are offered per egg). Like eggs, sperm released at fertilization are replaced within 3 d. Such reduced allocation to testicular tissue allows saving in the physiological cost of sperm production. Efficiency of fertilization in the protandrous phase is low but positively correlated with body length (measured by the number of setigers). A fertilization ratio of 95% is obtained only at the simultaneous hermapohroditic stage. This explains why protandrous males are generally not selected as mates in the reproductive strategy of O. diadema. Self—fertilization is avoided by behavioral constraints. The observed female bias in reproductive effort in the mating system of O. diadema suggests that hermaphroditism should be evolutionarily stable, as the male investment in reproductive effort follows the law of diminishing returns.

ISSN:0012-9658    

DOI:10.2307/1940244

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

The signs and strengths of spatial correlations between densities of predators and their prey were scale dependent both in simulations and when calculated from acoustic transects across nearshore distributions of Atlantic cod (Gadus morhua), and its prey, capelin (Mallotus villosus). Spectral analyses of 1985 and 1986 transects indicated that spatial correlation (coherence) varied relative to: (1) the occupation of thermal refuge areas by capelin (sea temperatures4—10 km) densities were in phase (positively correlated), but at scales less than aggregation dimensions (3.5 km), densities were out of phase (negatively correlated). At the smallest scales measured (2—3 km), densities exhibited maximum negative correlation in both years. When capelin were aggregated within thermal refuges, predator—prey densities were coherent (in phase) only at scales larger than those of the refuges. When actually feeding, cod achieved coherent, in—phase distributions with capelin at scales as small as 3.5 m.

ISSN:0012-9658    

DOI:10.2307/1940245

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

In size—structured populations interspecific competitors often have been suggested to have their greatest impact on the juvenile life stage. We hypothesized that roach (Rutilus rutilus), by virtue of their superior foraging ability on zooplankton, would cause juvenile perch (Perca fluviatilis) to shift from feeding on zooplankton to macroinvertebrate prey, the prey consumed by adult perch. This should potentially increase size—class competition in perch, producing a competitive bottleneck at the macroinvertebrate feeding stage. To test this, we experimentally examined how roach density affected both the diet and growth of young—of—the—year (0+) and 1—yr—old (1+) perch as well as how it affected the zooplankton and benthic macroinvertebrate resource levels in 10 enclosures in a pond over a 3—mo period. The densities of 0+ and 1+ perch were held constant between enclosures, where the density of roach was varied over five density levels. A significant effect of roach density on zooplankton (cladoceran) biomass was observed over the experimental period. By the end of the experiment, the biomass of zooplankton was low in all enclosures except those without roach, and the mean size of two zooplankters, Bosmina and cyclopoid copepods, decreased with increasing roach density. In contrast, the total biomass of macroinvertebrates in the enclosures did not change with increasing roach density. There was, however, evidence that chironomids were released from predation by Sialis due to an increased consumption of Sialis by 1+ perch. As predicted from our hypothesis, 0+ perch shifted from pelagic zooplankton to benthic macroinvertebrates as roach density increased. The roach density at which 0+ perch shifted to feed on benthic prey decreased over the experimental period, but occurred at the same density of pelagic prey. The proportion of zooplankton in the diet of 1+ perch and roach also decreased with increasing roach density, with 1+ perch totally dropping zooplankton from their diet and roach shifting to plant to detrital material. Over time we observed that roach increased its utilization of plant and detrital material. We found density dependence in growth rates of roach and 0+ perch as well as 1+ perch. The growth data suggested that the main competitive effect of roach occurred during the first half of the experiment. This was related to roach's high consumption of plant material at all roach densities at the end of the experiment and to the low water temperature during the second half of the experiment.

ISSN:0012-9658    

DOI:10.2307/1940246

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Glacial lakes, turbid (@>5 nephleometric turbidity units) with suspended particles (1—30 @mm), have both lower May—November levels of chlorophyll a and temperatures compared to nonglacial systems. Macro—zooplankton densities are also lower and dominated by Cyclops and Diaptomus. Extensive surveys showed that, regardless of the presence or absence of planktivorous fish, filter—feeding caldocerans (e.g., Bosmina, Daphnia, and Holopedium) were only absent from the limnetic zooplankton community of the glacial lakes. Both laboratory and in situ biochamber experiments demonstrated that turbidity reduced Daphnia survival and recruitment. Nondiscriminating filter feeders ingest glacial silt (average diameter of °11 @mm) because the size—range overlaps that of the phytoplankton. We speculate that such an inefficient foraging strategy, especially when silt levels are high and algal numbers low, lowers energy extractable from ingested food below maintenance levels. Thus, the limnetic macro—zooplankton community of most Alaskan glacial lakes is restricted to either the selective herbivore Diaptomus and the raptorial feeding Cyclops, or to just Cyclops.

ISSN:0012-9658    

DOI:10.2307/1940247

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

In a population convergence experiment, the initial densities of adults of the predatory backswimming bug Notonecta hoffmanni were set above and below a putative equilibrium density in stock tanks. The experiment was done at two constant rates of food supply (wingless Drosophila) for the larger instars (in natural pools Notonecta feed mainly on terrestrial arthropods that fall on the water surface). It was predicted that the densities of the resulting populations would converge on an equilibrium set by the rate of food supply for the larger instars. The tanks also contained zooplankton (mainly Daphnia), which were the main food supply for the smaller instars of Notonecta. The resulting overwintering populations converged towards the appropriate equilibrium densities, via density—dependent and food—dependent fecundity and then cannibalism. However, the populations overshot their equilibria, producing overconvergence. In natural populations such overconvergence might tend to produce 2—yr cycles in abundance. Overconvergence resulted from the insensitivity of the survivorship of the original adults (at least over the short term) to differences in food supply between treatments, allowing them to continue to affect (via cannibalism and reproduction) the eventual density of the new overwintering population. Because Notonecta population density was determined by the externally supplied, locally uncoupled food supply, even though the early instars depended for food largely upon dynamic populations of zooplankton, the dynamics of the Notonecta population were simpler than a description of the food web might suggest.

ISSN:0012-9658    

DOI:10.2307/1940248

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

In this study I investigated the important ecological interactions that contribute to maintaining a striking species distributional pattern for the Enallagma damselflies (Odonata: Coenagrionidae) among lakes that do and do not support fish populations. One group of Enallagma species is found as larvae only in lakes containing fish, while the remaining Enallagma species are found as larvae only in fishless lakes. I performed observational and experimental studies to evaluate the importance of water chemistry, competition among the Enallagma species, and predators of the Enallagma species in maintaining the two discrete assemblages. The results of observational studies suggested that water chemistry could not account for the pattern of species distributions, but that differences in predator compositions and competition among the Enallagma species may. Quantitative sampling indicated that dragonfly larvae were the most abundant invertebrate predator group in both lake types, with each lake type supporting a characteristic set of dragonfly species, as in the damselflies. Fish were also potentially major predators in lakes containing fish. Also, Enallagma densities in both lake types were among the highest reported in the literature, suggesting that density—dependent competitive interactions may potentially contribute to maintaining the Enallagma species distributions. The results of laboratory and field experiments demonstrated that predation by large dragonflies in fishless lakes and predation by fish in fish—containing lakes are two major environmental factors maintaining the Enallagma species distributions. When simultaneously offered two Enallagma species in the laboratory, one from the fishless lake group and one from the fish—containing lake group, fish consumed more individuals of species from the fishless lake group, but all dragonfly species, regardless of lake type affiliation, consumed more individuals of species from the fish—containing lake group. The results of field experiments in which I transplanted Enallagma species between the lake types showed that large dragonflies in fishless lakes and fish impose much greater mortality on Enallagma species with which they do not coexist than on those with which they do. Other field experimental results indicated that density—dependent competitive interactions among the Enallagma species were only apparent in the fishless lakes, affected species from both groups similarly, and altered their growth but not their survival. The results of this study are discussed in the context of overall community organization and the consequences to the evolution of species.

ISSN:0012-9658    

DOI:10.2307/1940249

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

We performed a manipulative field experiment to test for competitive effects of the stem galling sawfly (Euura lasiolepis) on three other gall—forming sawflies on arroyo willow (Salix lasiolepis). Within a willow clone, three branches were left untreated, three branches had E. lasiolepis females placed on them in nylon screen bags during that species' oviposition period, and three branches were bagged during that same period but had no sawflies introduced. When oviposition by E. lasiolepis was completed, bags were removed, and three other sawflies were permitted to oviposit on the branches. We found no significant interspecific competitive effects on the densities of three species in response to 9— and 17—fold increase in densities of galls of the stem—galling sawfly on branches of field plants in 1985 and 1986, respectively. The densities of stem gallers on the experimental branches were high, but were near densities observed on some unmanipulated field plants. The density of the petiole galler in 1985 was higher on branches with the stem galler than was the density of the leaf galler. This suggested a facilitation effect, which was not repeated in 1986. The leaf folder had significantly greater densities than the leaf galler or petiole galler. However, bagging of branches resulted in greatly reduced densities of the leaf folder compared to unbagged control branches in both years. The petiole galler and leaf galler were not affected by bagging in either year, probably due to their later oviposition phenology. Within treatment branches, where stem gall densities were high, these three sawfly species did not significantly avoid oviposition on shoots with stem galls, and in 1986 there were generally positive co—occurrence patterns of each of these species with stem galls on shoots. We conclude that interspecific competitive effects on gall density was not important under these field conditions for the leaf galler and the petiole galler, but bagging effects on the leaf folder density could have reduced the likelihood of detecting competitive effects by the stem galler on this species. Differences in the results of this study with an earlier experiment may be related to compensatory ability of the larger field plants and differences in stem gall densities.

ISSN:0012-9658    

DOI:10.2307/1940250

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY