摘要:

A population of Great Britain pocket mice, Perognathus parvus, was live—trapped in 1967—72 on a 2.7—ha study area in south—central Washington to determine their density, reproductive performance, longevity, movements, and home range, as well as their functional role in northern shrub—steppe habitat. During 41,310 trap—nights we captured 15,386 mice representing seven species: Perognathus parvus, Peromyscus maniculatus, Onychomys leucogaster, Reighrodontomys megalotis, Lagurus curtatus, Microtus montanus, and Sylvilagus nuttallii. Pocket mice accounted for 91%, deer mice 8%, and the remaining species only 1% of the captures. Most pocket mice remained inactive underground between December and February. Adult ♂ ♂became trappable at an average date of 26 March; ♀ ♀ emerged about 18 April. Once active, adults remained trappable for an average of 60 days in years of average—to—excellent food production, and for 90 days in years of poor production. Sex ratios of trappable animals differed significantly from 1:1 in about half the sampling periods, and usually favored adult ♂ ♂. Males had scrotal testes for an average of 4 mo, with the highest proportion in April. The first signs of estrus were observed in April, the first palpable pregnancies in May, and the last pregnant ♀ ♀ usually in July. In years of high food availability adult ♀ ♀ averaged 2 litters; in average years they had 1.1 litters; and in poor years, only one ♀ in three became pregnant. The average size of three litters found in live—traps was 3.7. In 1969, a year of excellent food production, juveniles from early litters bred during late summer. Subadults were first trapped in June for an average of 40—70 days. Survival from weaning to the following breeding season ranged between 56%—58% (1967, 1970) and 80% (1968); 17%—19% survived to the 3rd yr; and 2%—3% survived to the 4th yr. The highest overwintering success was in juveniles born during the year of lowest precipitation, poorest food supply, and reduced reproduction. Differential survival affected the age distribution of animals during the breeding season. One—year—olds dominated the breeding populations in 1968 and 1970; but in 1969 only 55% of the breeders were born the preceding year. The age distribution had little apparent effect on the reproductive success of the population. Perognathus parvus ceased aboveground activities between late summer and autumn depending upon environmental conditions, sex, and age. Population size was estimated by means of the Jolly—Seber stochastic model which helped interpret population responses when components of the population were not trappable. The peak autumnal population varied about a mean of 320 animals with only a twofold variation between the estimates for the lowest (206, 1968) and the highest (437, 1969) years. Within years the greatest variation was a fivefold increase in numbers observed between the early breeding season and fall population peak of 1969. The estimated summer population size showed a high degree of correlation with the amount of precipitation falling between October and April, precipitation that largely stimulated growth and reproduction in vegetation, which in turn affected the small—mammal population. Adult ♂ ♂ had larger home ranges (1,560—4,005 m2) than ♀ ♀ (508—2,301 m2); adults had larger home ranges than subadults. Home ranges were largest in years of average—to—excellent food production and moderate population densities; they were smallest either in years of poor resources and moderate population densities or in years of average resources but high densities. Centers of home ranges shifted little between years, indicating a high degree of stability and spatial partitioning of the area. This population of Perognathus parvus displayed a regular sex—related timing of commencement of activities, a delay in appearance of subadults, and a relatively brief duration of annual activity aboveground. Collation of the data emphashized that the seemingly dense populations of pocket mice were in fact well spaced temporally throughout the year, a well as spatially, with a minimum of overlap between sexes and, more importantly, between age groups.

ISSN:0012-9615    

DOI:10.2307/1942329

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

In the first few years following fire, burned chaparral areas are typified by a flush of seed germination and seedling growth not seen in unburned chaparral. The effects of fire on factors of potential importance to this plant response were examined in recently burned and long unburned chaparral, dominated by Adenostoma fasciculatum, in the Santa Ynez Mountains near Santa Barbara, California. Measurement of soil moisture, soil texture, and light indicated that changes in these factors following fire had little direct effect on postfire germination and growth. Bacteria and fungi were more abundant in burned than in unburned chaparral soil. Thus, the hypothesis that microbial depletion of soil oxygen in unburned chaparral inhibits seed germination appears untenable. Changes in organic matter content, pH, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium were examined in the upper soil layers of both burned and unburned chaparral during the first 18 mo following fire. Mineral addition as ash—fall was also evaluated. Burning increased levels of most mineral elements significantly. Furthermore, a large reservoir of readily available organic nutrients was added in the ash. Causes of low nutrient levels in unburned chaparral were also investigated. Additions of nutrients to unburned chaparral soil in the greenhouse and field resulted in increased growth of species common in burned chaparral areas. Bioassays of aqueous A. fasciculatum leaf washings were performed against 10 plant species common in burned chaparral areas. Significant depression of germination and growth was observed. Rain throughfall collected under the shrubs was also inhibitory. Efforts to isolate and identify the toxins are described. Numbers of small herbivorous mammals were greatly reduced in recently burned chaparral. This factor is shown to have a marked effect on seedling survival. Applications of various heat treatments to unburned chaparral soil and to fresh seeds of species common to burned chaparral indicated that the seed of several species are released from dormancy by heat. Insolational heating of soil in cleared, but unburned, chaparral is sufficient to stimulate germination.

ISSN:0012-9615    

DOI:10.2307/1942330

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

The dynamics and energetics of subpopulations of a numerically dominant deposit—feeding polychaete species, Pectinaria californiensis Hartman, were studied and compared with crude determinations of the same for the larger but rarer coexisting species of the same feeding mode, the heart urchin Brisaster latifrons (Agassiz) and the sea cucumber Molpadia intermedia (Ludwig). Monthly samples, taken for 1 yr at five stations in Puget Sound representing different habitats, were used in conjunction with laboratory measurements of respiration to assess the effects of seasonal and spatial variation in growth, mortality, and respiration on estimates of energy flow through these species populations. Pectinaria larval settlement (2,900—24,000 animals/m2) occurred at all locations in June 1970. Two of three age classes or cohorts were present simultaneously. Pectinaria represented 4%—26% of macrofaunal (>1 mm) biomass, and 9%—47% of numbers at the five locations, based on the mean of four seasonal estimates. At the two stations where Brisaster and Molpadia coexisted with Pectinaria they contributed, respectively, 79% and 4% of macrofaunal biomass at one station and 13% and 63% at the other. Recruitment and growth of the two echinoderms appeared negligible, as neither numbers nor mean size changed during the study period. Annual production of Pectinaria, not including excretion or mucus production, varied 1.4—48 g C/m2°yr (14—49 kcal/m2°yr). The ratio of annual production to mean annual biomass, varying in the study area 3.3—5.5m provided a better estimate of turnover than the more commonly used ratio based on the lifetime of a cohort because of the difficulty of determining lifespan, a problem with most long—lived organisms. Pectinaria contributed 14%—42% of macrofaunal respiration in the area studied. But these numbers were shown to be affected by the failure to reproduce in the laboratory in situ oxygen—tension conditions. Such overestimates of population respiration from laboratory measurements were most marked for Brisaster and Molpadia. These latter estimates, while reflecting biomass data, unrealistically overshadowed the respiration of all other organisms. The sum of Pectinaria production and respiration (corrected for in situ oxygen tension) varied 2.6—9.2 g C/m2°yr (27—98 kcal/m2°yr), reflecting differences in rates of growth and mortality among stations. At two stations where primary production data were available, Pectinaria assimilated at least 1.3% and 3.3% of the carbon fixed by phytoplankton. Subsequently, 0.6% and 1.7%, respectively, were made available to predators and decomposers in the form of Pectinaria flesh. Because of its greater turnover of assimilated energy, Pectinaria contributed more to metabolic processes and to foodchain dynamics of the seabed than did the coexisting echinoderms. The echinoderms, on the other hand, may exert important influences on the structure of the community not accounted for in normal energetic assessments. Spatial and temporal variations in energy flow through species subpopulations can be large, and thus may limit the usefulness of a stability assumption in the development of predictive models for organic matter budgets.

ISSN:0012-9615    

DOI:10.2307/1942331

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Net primary productivity and distribution of mixed grassland plant communities in western North Dakota were studied. The semiarid continental climate supports several grassland types, delimited on the bases of topography and substratum. The dominant species and herbage (shoot) production (mean of 2 yr) of stands located on what were classified as silty range sites included, on rolling upland, (1) Stipa viridula (128 g m—2), Stipa comata (128 g m—2), and (3) Bouteloua gracilis (110 g m—2); on ravine slopes, (4) Stipa spartea—forbs (135 g m—2) and (5) Andropogon spp. (126 g m—2). On sandy range sites were found, in shallower ravine bottoms and at bases of slopes, (6) Sporobolus heterolepis—Stipa spartea (160 g m—2) and (7) Sporobolus heterolepis (185 g m—2); on slopes with very sandy soils, (8) A. scoparius—Stipa spartea (132 g m—2), (9) A. scoparius—forbs (107 g m—2), and (10) A. scoparius (119 g m—2). The highest annual herbage production was measured in Stand 7 (200 g m—2) and the lowest in Stand 3 (77 g m—2). Root mass ranged from 6230 g m—2(Stand 7) to 3030 g m—2(Stand 9). Annual root production could not be measured but was estimated to be equivalent to herbage production. Variation in herbage production between sites and between years was attributed to differences in soil moisture and fertility (especially the latter in sites with sandy soils). Fresh mulch was highest in the Stipa—dominated sites (288—320 g m—2) and lowest in Andropogon— and Bouteloua—dominated stands (212—234 g m—2). The highest fresh mulch decay constant and shortest equilibrium time were determined to occur in sites dominated by Sporobolus heterolepis, and the converse for Stipa—dominated sites. Chlorophyll values ranged from 78 mg m—2(Stand 3) to 542 mg m—2(Stand 7); variation depended on plant morphology and phenology as well as biomass. Maximum efficiency in herbage production occurred later in the year for stands with a large warm—season component (Sporobolus heterolepis an Andropogon spp.) than in sites dominated by cool—season grasses (Stipa spp.). On an annual basis, Stand 7 was most efficient (0.12%) and Stand 3 least (0.06%). The interrelationships between drought tolerance, phenology (cool— vs. warm—season development), physiology, and production are discussed.

ISSN:0012-9615    

DOI:10.2307/1942332

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY