摘要:

Dynamic models of quantitative (polygenic) characters are more generally applicable in the analysis of life history evolution than are static optimization methods or one and two locus genetic models. A dynamic theory of life history evolution is derived by synthesizing population demography with quantitative genetics. In a population under weak selection with a nearly stable age distribution, the relative fitness of individuals with a particular life history phenotype can be approximated as an average of age—specific relative fecundity and mortality rates, weighted respectively by the present productivity and future reproductive value of each age—class. An adaptive topography is constructed showing that, with phenotype— and age—specific fecundity and mortality rates constant in time, evolution of the mean life history maximizes the intrinsic rate of increase of a population. However, the rate and direction of evolution in response to selection are strongly influenced by genetic correlations among characters. Negative genetic correlations among major components of fitness are often obscured phenotypically by positive environmental correlations, but commonly constitute the ultimate constraint on life history evolution, as illustrated by artificial selection experiments. Methods are suggested for measuring selective forces and evolutionary constraints that effect life history characters in natural populations.

ISSN:0012-9658    

DOI:10.2307/1936778

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

In the lace bug Gargaphia solani, the physiological costs of allocating energy to parental care at one time were measured in terms of females' subsequent abilities to survive to and reproduce at a later time. In the present of excess resources, three levels of energy expenditure were compared. (1) To prevent females from spending any energy on parental care, eggs were removed as they were laid. (2) Females were permitted to guard their first egg mass only until it hatched. (3) Females were allowed to rear their first clutch through maturation. The results suggest that there is a trade—off between energy used for maintenance and the energy used for reproduction. Increased egg production in young adults resulted in significantly decreased abilities to survive. Furthermore, there was a clear and significant trade—off between time invested in maternal care in G. solani and subsequent abilities to reproduce. Compared to females that were prevented from mothering, there was a significant decrease in subsequent fecundity and clutch number when parental care was extended to eggs and further significant decreases when the first clutch was reared to maturity. The adaptive value of maternal behavior in light of the potential costs to fitness is discussed.

ISSN:0012-9658    

DOI:10.2307/1936779

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The effects of initial nitrogen and lignin contents of six species of hardwood leaves on their decomposition dynamics were studied at the Hubbard Brook Experimental Forest. Rate constants (k) for annual leaf mass loss ranged from —0.08 to —0.47. The rate constants (k) had a negative linear correlation (r2= .89) with the ratio of initial lignin concentration to initial nitrogen concentration. Decomposition dynamics of the litter materials were described by inverse linear relationships between the percentage of original mass remaining and the nitrogen concentration in the residual material. Initial lignin concentration was highly correlated (r2= .93) with the slope of the inverse linear relationship for each of the litter types.

ISSN:0012-9658    

DOI:10.2307/1936780

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The winter diets of Rocky Mountain bighorn sheep in Banff National Park were studied to determine whether the age—sex classes have evolved differences in food exploitation patterns under selective pressure to reduce intrapopulation competition. Rams remained primarily on the north end of the study area while the "non—ram" superclass (ewes, lambs, yearlings) occupied primarily the south end. Vegetation analysis indicated that the southern part of the range was characterized by relatively more Bromus, Carex, Koeleria, Poa, Oxytropis, and Juniperus and less Festuca, Fragaria, Hedysarum, Arctostaphylos, and Rosa than the northern portion. In addition, the southern part of the range was typified by less soil, more xeric conditions, lower plant biomass and apparently overgrazed conditions relative to the more northern parts. Ewes, lambs, and yearlings were found to have diets more similar to each other than to that of adult rams. Class effects in multivariate statistical analysis were significant; the major univariate differences were that non—rams ate more Carex and less Festuca than did rams. This class effect was found to be wholly attributable to the fact that rams and non—rams segregate onto separate portions of the range which contain different availabilities of forage plants. It was considered to be unlikely that the spatial separation arose from diet preferences but rather from some unrelated factor. The observed differences should not be considered as an adaption for competition reduction.

ISSN:0012-9658    

DOI:10.2307/1936781

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Cluster analysis techniques are often used in preliminary studies of community organization to identify groups of associated species on the basis of distributional co—occurrence. Adequate statistical tests have not been available fore determining whether the associations recognized are sufficiently nonrandom to be considered significant, or might reasonably be expected on the basis of random distribution alone. For presence/absence occurrence data, an approximate test of significance may be preformed by generating random occurrence matrices within the constraints of a randomization model which treats as marginal constants both the observed number of occurrences of each species and the observed number of species present at each locality. Null (random) distributions of node values of dendrograms, derived from the randomly generated occurrence matrices, are used to assign approximate critical test values. These values may be applied directly to the dendrogram derived from the observed distribution data to determine which of the observed species clusters may be considered statistically significant. Groups of significantly associated species are reasonable entities within which to examine ecological relationships further.

ISSN:0012-9658    

DOI:10.2307/1936782

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The spatial distribution of roots of two alien grasses, Bromus tectorum and Poa pratensis, grown singly and in a mixture, was examined using a double—labelling radioisotope technique. Interactions between the root systems of these plants led to a restricted B. tectorum rooting volume in P. pratensis neighborhoods °30—d—old. The roots of B. tectorum failed to develop laterally. The altered B. tectorum root systems may contribute to its inability to persist in established P. pratensis swards.

ISSN:0012-9658    

DOI:10.2307/1936783

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Nutrient balance studies of mature ecosystems have shown that in many cases leaching losses are greater than atmospheric inputs. If the systems are not degrading, this means this means that the net losses must be compensated for by weathering of parent material. In contrast to ecosystems with rates of nutrient leaching that are higher than rates of atmospheric input, leaching of nutrients from an Amazonian rain forest ecosystem was less than or equal to input from the atmosphere every year between 1975 and 1980. If this forest is not aggrading this means that weathering of parent materials does not play an important role in the nutrient economy of the ecosystem. The forest apparently maintains itself on nutrients derived from the atmosphere.

ISSN:0012-9658    

DOI:10.2307/1936784

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Population growth rates can be estimated from sample cohort schedules of survivorship and fecundity, but the variation inherent in these estimates has received little attention. We define an ideal population such that it is completely described by the probabilities governing age—specific survival and reproduction. We define the lifetime contribution of an individual to population growth in a manner analogous to Fisher's (1931) reproductive values. The mean of these individual contributions is equal to the finite rate of increase for the population. We then investigate the properties of sample cohorts drawn from an ideal population. Estimates of population growth rates that are based on sample cohorts are shown to be biased. The magnitude of the bias decreases as the number of individuals used to construct the sample schedules of survivorship and fecundity is increased. This relationship conflicts with the statistical desirability of maximizing the number of estimates of the population growth rate. Bias can be reduced by pooling individual schedules to calculate a single estimate of the growth rate of the population within which individual contributions are defined. When cohort size is small, we recommend a modified jackknifing procedure for further reducing bias. To demonstrate the application of these methods, we obtain a 95% confidence interval for a rate of increased based on a sample cohort of aphids.

ISSN:0012-9658    

DOI:10.2307/1936785

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The leaves of Catalpa speciosa (Bignoniaceae) have extrafloral nectaries that are active from early June to September. Peak nectar secretion occurs in late June and early July when the eggs and young larvae of the first generation of the principal herbivore, Ceratomia catalpae (Sphingidae), are present. Leaves that have been damaged by this herbivore secrete significantly more nectar than undamaged leaves. The nectaries are visited by various species of ants, ladybird beetles, and a parasitoid. These predaceous insects attack and/or remove the eggs and young larvae of the herbivore. A series of ant exclusion experiments show that branches without ants have significantly fewer larvae removed, have significantly more leaf herbivory, and produce significantly fewer mature fruits than control branches (with ants). These data suggest that secretion of extrafloral nectar by C. speciosa and its subsequent harvesting by insects is mutually beneficial to both the plant and the predaceous insects.

ISSN:0012-9658    

DOI:10.2307/1936786

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Litter decomposition rates, measured by the litter bag technique, and nutrient dynamics were monitored for 2 yr in four plant communities in the seasonally flooded nonriverine Great Dismal Swamp. The cypress and maple—gum communities were more extensively flooded and had less acid soils than the cedar and mixed—hardwood communities. The highest decay rates for mixed litter, representing an average species composition on each site, were recorded on the cypress and maple—gum sites. Site controls indicated that litter quality, as affected by species importance, was primarily responsible for site variation in total litter decomposition rates. The flooded sites had a predominance of species that were more subject to rapid decay (higher P concentrations, low C:N ratios, and low lignin and tannic acid content). Nitrogen and phosphorous were accumulated, with a few exceptions, in the decomposing litter; calcium and magnesium were released; and potassium was accumulated following initial losses. Accumulations were probably a result of immobilization of external nutrient sources (e.g., throughfall) by microbes. In general, decomposition in the Great Dismal Swamp appears to be nitrogen and phosphorus limited.

ISSN:0012-9658    

DOI:10.2307/1936787

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY