Ecology

Cockrum, E. Lendell

doi: 10.2307/1933968pmid: N/A

Kemp, Gerald A.; Keith, Lloyd B.

doi: 10.2307/1933969pmid: N/A

This paper describes red squirrel population dynamics on two intensive study areas (Camp and Main) in mixed—forest types near Rochester, Alberta. It also examines local and regional population fluctuations, and their relationship to cone crops and weather factors. Adult red squirrel numbers on our study areas varied little from 1967 to 1968. Yearly differences in reproductive output were caused principally by changing ovulation and pregnancy rates. Mean litter size increased significantly from 3.4 in 1967 to 4.3 in 1968; while the per cent adult females breeding increased significantly from 67 to 88. A life—table analysis of age—ratio data estimated mean annual mortality among juveniles (postweaning) at 67%; and an adult mortality rate of 34% for yearlings and 61% for older cohorts. A time—specific estimate of adult mortality on the Camp study area during the year starting summer 1967 was 21%. Red squirrel territories appeared to be of two distinct types: (1) defended winter food caches which were subsequently abandoned during the summer, and (2) "prime" territories in which a specific area was defended year round. During the summers of 1967 and 1968, 31% and 26% of study—area adult populations occupied prime territories. The distribution of prime territories chiefly reflected the presence of mature seed—producing conifers, and hence a potential year—round food supply. Deciduous areas were particularly important in overwintering the juvenile cohort. Fur returns were used as population indices in determining synchrony and periodicity of red squirrel fluctuations. Fluctuations tend to occur synchronously over much of the Prairie Provinces. Furthermore, mean intervals between peak years in Alberta (2.6) and Saskatchewan (2.8) were significantly shorter than in random series of comparable length, while the 2.9—year mean interval in Manitoba approached significance. A statistically significant correlation was found between white spruce cone crops and red squirrel populations in Alberta as indexed by annual fur harvests. The negative correlation between cone crops in late summer and rainfall during summer of the preceding year was almost significant. This supported the widely held view that weather factors influence bud differentiation and hence determine the size of the following year's cone crop. Our contention that cone production may in this way provide a vehicle through which weather affects squirrel populations was supported by a number of statistically significant negative correlations between Alberta and Saskatchewan fur returns and the preceding year's summer rainfall. Such a mechanism would explain the observed widespread synchrony of population fluctuations, since extensive regional weather patterns could be involved.

Shontz, John P.; Oosting, H. J.

doi: 10.2307/1933970pmid: N/A

Haplopappus divaricatus and Conyza canadensis commonly appear in first—year old fields of North Carolina. Haplopappus is very rare in fields of the North Carolina Piedmont, but occurs in varying densities on the coastal plain (0—83.8% relative density in 26 coastal plain old fields). Within the coastal plain, there is no east—west or north—south trend in the distribution of Haplopappus, and no correlation with the most recent grown crop is found. Fields disturbed in fall or winter, and fields with deep sandy soils and soils with low clay content, low moisture retention, and low available nutrients had high densities of Haplopappus and correspondingly low densities of Conyza. Haplopappus plants transplanted from the coastal plain to the Piedmont survived and produced large numbers of viable seeds, but no seedlings grew the following year around these transplants. Transplants from greenhouse to coastal plain and Piedmont fields survived and grew, but those in the coastal plain had greater dry weights. Haplopappus seeds planted in three plots in the Piedmont produced only one plant which set viable seeds. In greenhouse experiments both species were more affected by competition in sandy soil than in heavy soil. Haplopappus was more sensitive to interspecific competition, but decreased nutrients reduced the competitive ability to Conyza plants more than that of Haplopappus. Drought stress, which did not affect old or young Conyza plants, reduced the growth of young Haplopappus plants. Haplopappus showed less sensitivity to specific nutrient deficiencies than did Conyza. In the field and in greenhouse flats Haplopappus seeds did not germinate in heavy soil. The presence of added organic matter in sand reduced the germination percentage of Haplopappus, as did decay products of Conyza shoots. Haplopappus seeds in petri dishes on a temperature—gradient bar germinated at lower temperatures than did Conyza seeds. Germination percentages of Haplopappus were also greater when less water was available in the substrate. It is concluded that fall and winter disturbance of old fields kills many Conyza seedlings and opens bare soil. Because Haplopappus seeds germinate at low temperatures and in bare soil, this species is able to invade these areas. If no such disturbance occurs, few Haplopappus plants will mature, because Conyza plants are better competitors. Haplopappus apparently is prevented from becoming established in the Piedmont by the inability of its seeds to germinate in heavy soil and by the superior competitive ability to Conyza plants in the richer Piedmont soils.

Leak, William B.

doi: 10.2307/1933971pmid: N/A

Two simulation models were used to predict species changes in three deciduous forest cover types in New England. A fixed—rate simulator, employing differences among species in birth/death rates, was used for stands approaching a steady—state condition. A density—related simulator, using birth and death rates related to both species and population size, was applied to stands in dynamic condition. Predicted and actual changes over a 25—year period were reasonably consistent.

Cameron, Roy E.; Benoit, Robert E.

doi: 10.2307/1933972pmid: N/A

Cinder cones that arose December 1967 within Telefon Bay, Deception Island, Antarctica, were investigated 1 year later to determine the establishment of microorganisms and cryptogams. Culture media were inoculated to determine the presence and abundance of algae, fungi, heteorotrophic and chemoautotrophic, aerobic, microaerophilic, and anaerobic bacteria. No mosses or lichens had become established on the cones. Algae, fungi, and bacteria were generally most abundant around fumaroles emitting moisture and CO2. Several samples contained few or no culturable microorganisms. "Soil" properties of coarse—textured, relatively unweathered acid volcanic materials were unfavorable for growth, despite the presence of moisture. Microorganisms were identified from the cinder cones and included primarily soil diphtheroids and Bacillus spp., Chlorococcum humicola, and Penicillium spp. Most of the bacteria could grow at 2°C as well as at 20°C.

Maelzer, D. A.

doi: 10.2307/1933973pmid: 28976538

Four density—independent "populations" were constructed on a computer, and the simple regression of the log density in one generation (log Nn+1) on the log density in the previous generation (log Nn) was calculated for each of a number of replicates for each population. From the resultant sampling distribution of the regression coefficient for each population, it was found that (i) the mean slope was significantly less than the expected value of 1.0 for a density—independent population, and that (ii) the mean slope was influenced by sample size and the serial correlation between successive values of the rate of increase, ϒ. Despite its demonstrated bias the regression coefficient could still be useful if it were capable of distinguishing between populations with density—dependent processes and populations with no density—dependent processes. So data for a number of natural populations were analyzed by simple regression and each of the slopes so obtained was compared with the mean slopes (for simple regression) for the theoretical populations. These comparisons suggested that the simple regression coefficient was not a good criterion of density dependence.

St. Amant, J. L. S.

doi: 10.2307/1933974pmid: N/A

If the detection of regulation in a population is attempted by taking a linear regression of log population density against log previous population density, the slope, b, of the line thus obtained is supposed to be less than 1 only if regulatory tendencies are present in the population. However, a nonregulated population will probably give similar values of b < 1 provided only that the successive differences between log population values are not all the same. This spurious indication of regulation becomes more likely and more apparent if the number of census points is small, or if the variation in numbers of animals is large compared to the total population change over the period measured. Complexities which are almost certain to occur in natural situations and which make satisfactory models for regulated and nonregulated populations intractable or indistinguishable probably require that investigations of regulation either use more detailed information that census data alone, or that they adopt an experimental approach by testing for convergence of altered densities of subpopulations.

Swan, J. M. A.; Gill, A. M.

doi: 10.2307/1933975pmid: N/A

The mode of development of an extensive floating bog (400 m by 400 m) at the north end of Harvard Pond, an artificial lake, was studied by careful examination and local dissection of the bog mat within a 32—m by 16—m plot. Excavations were also made at the bog margin. The origins, spread, and consolidation of the floating mat are largely explained by the growth characteristics of one ericaceous shrub, Chamaedaphne calyculata (L.) var. angustifolia (Ait.) Rehd. A wetland forest was cut about 1830, and Chamaedaphne, first invaded the tree stumps between 1830 and 1890 (when they were located in a sedge meadow). Harvard Pond was formed about 1890, and the floating mat has developed between 1890 and 1968 (by the lateral spread of Chamaedaphne across the water surface to occupy the intervening spaces between stumps). Bog—advance rates were estimated from photographs and from annual growth increments for the shrub. Expansion appears to have been rapid (up to 6.3 cm/year or 2.5 inches/year).

Blackburn, Wilbert H.; Tueller, Paul T.

doi: 10.2307/1933976pmid: N/A

As a means of studying inter— and intrazonal invasion in black sagebrush (Artemisia nova A. Nels) communities six maturity classes were established for pinyon (Pinus monophylla Torr. and Frem.) and juniper (Juniperus osteosperma (Torr.) Little) in east—central Nevada. Pinyon and juniper invade and increase in black sagebrush communities until the understory, except for a few hardy plants, is eliminated. Juniper invades first and tends to be eventually replaced by pinyon. Accelerated invasion by both species started about 1921 and is closely related to overgrazing, fire suppression, and climatic change.

Gashwiler, Jay S.

doi: 10.2307/1933977pmid: N/A

The survival of naturally disseminated, filled, seed of Douglas—fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western redcedar (Thuja plicata) was studied from 1960 to 1967. Ten per cent of the Douglas—fir seed survived from the start of seed fall until the end of germination the following year. Mice and shrews, mostly deer mice (Peromyscus maniculatus), destroyed an estimated 41% of the seed; birds and chipmunks (mostly birds) took 24% and other factors (nonviability of filled seeds, invertebrates, disease, and others) accounted for 25%. Western hemlock seed survival was 22%. Mice and shrews (mostly deer mice) destroyed an estimated 22% of the seed, birds and chipmunks (mostly birds) 3% and other agents 53%. The sample of western redcedar seed was too small to be reliable. Douglas—fir seeds were preferred by ground—feeding birds and small mammals–less than half as many hemlock were taken. Most Douglas—fir and hemlock seed mortality occurred before the start of germination.