Intrinsic and extrinsic sources of variation in the dynamics of large herbivore populations

2009 ◽  
Vol 87 (2) ◽  
pp. 103-111 ◽  
Author(s):  
J. P. Marshal ◽  
J. W. Cain ◽  
V. C. Bleich ◽  
S. S. Rosenstock
Keyword(s):  
Intraspecific Competition ◽  
Population Growth Rate ◽  
Ovis Canadensis ◽  
Equilibrium Density ◽  
Relative Importance ◽  
Large Herbivore ◽  
Sources Of Variation ◽  
Southwestern Usa ◽  
Lower Variability ◽  
Resource Variability

We evaluated the relative importance of intrinsic (density-dependent) and extrinsic (density-independent) sources of resource variability in the dynamics of bighorn sheep ( Ovis canadensis Shaw, 1804) populations in southwestern USA. We tested the hypothesis that populations experiencing greater variation in forage availability are less likely to be at equilibrium density with forage supplies, and thus, would demonstrate weaker evidence of intraspecific competition. We used regression to relate population growth rate to rainfall (forage conditions) and to abundance (intraspecific competition) for 20 sheep populations, and we estimated the strength of the effects of density and rainfall for each population. Then we compared among populations to look for relationships between the strengths of rainfall and density effects and the variability in rainfall experienced by each population. Populations demonstrating a density effect were associated with environments having lower variability than those that did not. Populations showing a rainfall effect were associated with environments having a variability higher than those that lacked a rainfall effect. These findings are consistent with the hypothesis that populations experiencing higher variation in forage resources are less likely to achieve an equilibrium density with forage supplies where intraspecific competition would be a large determinant to the dynamics of that population.

An outline of the dynamics of animal populations.

1954 ◽  
Vol 2 (1) ◽  
pp. 9 ◽  
Author(s):  
AJ Nicholson
Keyword(s):  
Stationary State ◽  
Natural Enemies ◽  
Environmental Conditions ◽  
Intraspecific Competition ◽  
Density Change ◽  
Equilibrium Density ◽  
Population Densities ◽  
Compensatory Reaction ◽  
Animal Populations ◽  
Intensity Of Competition

This article is itself a summarized statement concerning the various influences which affect population densities and the population systems these lead to. Known facts concerning animal populations have been analysed. systematized. and critically examined. using the experimental and mathematical approaches in the simpler situations. The more outstanding conclusions are listed below. Populations are self-governing systems. They regulate their densities in relation to their own properties and those of their environments. This they do by depleting and impairing essential things to the threshold of favourability, or by maintaining reactive inimical factors, such as the attack of natural enemies, at the limit of tolerance. The mechanism of density governance is almost always intraspecific competition, either amongst the animals for a critically important requisite, or amongst natural enemies for which the animals concerned are requisites. Governing reaction induced by density change holds populations in a state of balance in their environments. The characteristic of balance is sustained and effective compensatory reaction which maintains populations in being in spite of even violent changes in the environment, and which adjusts their densities in general conformity with prevailing conditions. Far from being a stationary state, balance is commonly a state of oscillation about the level of the equilibrium density which is for ever changing with environmental conditions. Destructive factors do not add to mortality when they continue to operate over long periods, but merely cause a redistribution of mortality, for the intensity of competition automatically relaxes sufficiently to make room for the destruction they cause. Such compensatory reaction causes the effect of destructive factors upon density to be much less when balance is reattained than that which they produce when they first operate.

Litter productivity in Large White pigs: 1. The relative importance of some sources of variation

1970 ◽  
Vol 12 (2) ◽  
pp. 225-233 ◽  
Author(s):  
G. S. Strang
Keyword(s):  
Litter Size ◽  
Relative Importance ◽  
Large White ◽  
Sources Of Variation ◽  
Size At Birth

SUMMARYData from 38000 Large White litters farrowed in 146 British herds were analysed to investigate the relative importance of various sources of variation affecting litter productivity. The effects of the season and the year of farrowing were very small relative to the effect of the herd of farrowing which was particularly large on the litter and average pig weights at three weeks. There was a small increase in the size of gilt litters as the age of the gilt at first farrowing increased. Litter size increased progressively with parity up to around the fourth litter and then declined gradually at higher pari-ties. There was a suggestion that the sire of a litter may hve a very small effect on litter size at birth but not at three weeks or eight weeks.

The distribution and estimation of jack pine budworm defoliation

1992 ◽  
Vol 22 (8) ◽  
pp. 1079-1088 ◽  
Author(s):  
W. Jan A. Volney
Keyword(s):  
Variance Components ◽  
Jack Pine ◽  
Relative Importance ◽  
Patchy Distribution ◽  
Random Source ◽  
Jack Pine Budworm ◽  
Pine Trees ◽  
Sources Of Variation ◽  
Free Feeding ◽  
Level Of Effort

A method to categorize defoliation on current year's shoots of jack pine (Pinusbanksiana Lamb.) was developed. The pattern of defoliation caused by the jack pine budworm (Choristoneurapinuspinus Free.) feeding on jack pine trees was studied in a variety of locations in Manitoba and Saskatchewan over several years. In addition to differences among locations and years, the crown level from which a sample was taken, the branch within a tree, the tree within a plot, and the plot within a study location were consistent sources of variation. In general, the upper crown was most heavily defoliated and the lower crown least so. Branches, trees, and plots are random sources of variation, and the variance components associated with each such source were analyzed to determine their contribution to the overall variance in the estimate of defoliation. The relative importance of each random source of variation varied with the percent defoliation. Variation among plots was significant at defoliation levels below 45% and peaked at 25%, indicating a patchy distribution of feeding within stands in this range of defoliation. Beyond 45% defoliation, feeding was uniformly distributed among plots within a stand. Variation in defoliation among branches and trees peaked in the 50–60% defoliation range and was negligible at the extremes. Advantage can be taken of the consistent patterns of defoliation among crown levels and knowledge of the variance components of the random sources of variation to design defoliation assessment protocols. These data were used to optimize the allocation of effort, among plots, trees, and branches, required to determine the level of defoliation in stands either with fixed level of effort or to a specified level of precision.

scholarly journals Sources of Variation in Shelling Percentage in Peanut Germplasm and Crop Improvement for Calcium Deficiency-Prone Soils1

1996 ◽  
Vol 23 (2) ◽  
pp. 76-81
Author(s):  
U. Hartmond ◽  
J. H. Williams ◽  
F. Lenz
Keyword(s):  
Seed Yield ◽  
Growth Habit ◽  
Crop Improvement ◽  
Physiological Model ◽  
Calcium Deficiency ◽  
Relative Importance ◽  
Ca Deficiency ◽  
Plant Growth Habit ◽  
Sources Of Variation ◽  
Calcium Availability

Abstract Calcium (Ca) deficiency causes peanut pegs and pods to abort, resulting in decreased shelling percentages and yields. Environmental factors influencing calcium availability include soil Ca content and soil moisture. Genetic attributes that influence the sensitivity of cultivars to soil Ca supply include pod size, soil volume per pod (varied by plant growth habit), and pod wall attributes. Where Ca fertilization is not possible, genetic solutions to Ca deficiency are important, and breeders need information on the relative importance of these attributes. The objective of this research was to quantify the relative importance of these three sources of variation. Data from three trials were used to evaluate the relative importance of these attributes. The trials, sited on Ca-deficient alfisols, used between four and 12 germplasm lines with varied Ca sensitivity- determining attributes. Lines differed in growth habit (spreading or bunch), pod volume, pod yield, shelling percentage, and seed yield. The trial treatments and environments (sites and seasons) also varied Ca supply through soil type, fertilization, and water supply. Assuming that Ca supply has little impact on crop growth rates (CGR), a physiological model was used to set aside the contributions of CGR to yield differences between treatments. The three trials were analyzed separately and then combined for further regression analysis by defining each site and treatment combination as an environment. Within trials, variations in shelling percentage accounted for up to half the variations in seed yield between lines. In the combined analysis, easily selected attributes—pod volume (58% of germplasm sums of squares) and plant habit (8%) and their interaction (14%)—accounted for much of the variation in shelling percentage. The interaction was due to shelling percentage being less influenced by pod volume in spreading than in the bunch types. Thus, in Ca-limiting situations, the spreading growth habit allowed larger seeded peanuts to be grown than the bunch growth habit because of the greater pod dispersal of this type. Assuming that the lines tested typified peanuts for their relation between attributes and Ca deficiency-based shelling percentage variations, breeders should place the greatest emphasis on small pod size to decrease peanut sensitivity to Ca deficiency. Increased soil available to each pod by pod dispersal decreases the need for small pods to decrease sensitivity to Ca-deficient soils.

The relative importance of cluster size, sowing depth, time of seedling emergence and between-plant spacing on variation in plant size in red beet (Beta vulgaris L.) crops

1987 ◽  
Vol 108 (1) ◽  
pp. 221-230 ◽  
Author(s):  
L. R. Benjamin
Keyword(s):  
Cluster Size ◽  
Seedling Emergence ◽  
Plant Size ◽  
Root Weight ◽  
Relative Importance ◽  
Plant Spacing ◽  
Sowing Depth ◽  
A Value ◽  
Sources Of Variation ◽  
Different Sources

SummaryBeets with a coefficient of variation (c.v.) of weight of only 36–46% were produced by reducing the variation in cluster size, sowing depth, time of seedling emergence, proximity and size of neighbours to as low a value as practicable. The relative importance of these different sources of variation in plant size was examined in two experiments sown in contrasting conditions.Variation in time of seedling emergence was a major source of variation in plant size and was partially caused by variation in the environment around the clusters. Variation in true seed size and between-plant spacing was achieved by use of clusters of different sizes, but these factors were associated with only a negligible variation in root size. Similarly, local interactions between plants, brought about by variation in plant-to-plant spacing, accounted for only a small proportion of the variation in root weight.

Relative Importance of Additive Genetic and Maternal Sources of Variation in Early Survival of Young Splake Hybrids (Salvelinus fontinalis × S. namaycush)

1974 ◽  
Vol 31 (9) ◽  
pp. 1499-1502 ◽  
Author(s):  
G. Burton Ayles
Keyword(s):  
Maternal Effects ◽  
Salvelinus Fontinalis ◽  
Genetic Effect ◽  
Genetic Effects ◽  
Additive Genetic Effect ◽  
Relative Importance ◽  
Egg Survival ◽  
Brood Stock ◽  
Sources Of Variation ◽  
Additive Genetic Effects

Additive genetic and maternal effects of survival of uneyed eggs, eyed eggs, and alevins were determined from five series of matings within a splake brood stock. Average values for family h2 (heritability) and family m (maternal variance/total variance) were estimated. There were additive genetic effects in alevin survival (h2 =.41 ±.18) but not in uneyed or eyed egg survival. Maternal effects were greatest within the uneyed stage (m =.78 ±.22), decreased during the eyed egg stage (m =.68 ±.24), and were least within the alevin stage (.40 ±.19). The additive genetic effect on alevin survival was attributable to genetic differences in the resistance of young splake to blue sac disease (h2 =.76 ±.28).

Sign in / Sign up

Export Citation Format

Share Document