摘要:

Pinus contorta Dougl. (lodgepole pine) is the most important seral tree species in the Canadian Rocky Mountains, forming a complex mosaic of even— and several—aged forests of fire origin over a wide range of elevations and habitats. East of the Continental Divide in Alberta it is the most abundant tree of montane and lower subalpine forests, due to extensive and frequent wildfires before 1910. Sixty—three mature, undisturbed P. contorta forest stands were surveyed for physiognomy, vascular floristic composition, species cover, age structure, and physical site properties. A direct gradient ordination of stands was constructed using elevation corrected to 53°N as the first axis, and Synthetic Moisture Index derived from Moisture Indicator Values of species as the second axis. Stands were classified using minimum—variance cluster analysis with species cover data, error—sum—of—squares association analysis with species presence—absence data, and dominance analysis with species based on the leading species of shrub and herb—dwarf shrub strata. Vascular species richness per stand averaged 38. Species occurring in a significantly higher percentage of stands in one national park than the other were more numerous in Banff than Jasper. Most stands were<100 yr old. Prevailing soils were Eluviated Eutric Brunisols and Orthic Eutric Brunisols. Both vascular species and community classification (syntaxonomic) units showed ecologically meaningful distribution and abundance patterns on the ordination. Though based on different criteria, the four classifications were quite similar, permitting synthesis of five PINUS CONTORTA community types (CT's), these forming an ecocline from low—elevation dry to high—elevation moist: PINUS CONTORTA/SHEPHERDIA CANADENSIS/ARCTOSTAPHY—LOS UVA—URSI (PICO/SCHA/ARUV), P. CONTORTA/S.CANADENSIS/ELYMUS INNOVATUS (PICO/SHCA/ELIN), P. CONTORTA/ALNUS CRISPA/LINNAEA BOREALIS (PICO/ALCR/LIBO), P. CONTORTA/MENZIESIA GLABEL—LA/VACCINIUM SCOPARIUM (PICO/MEGL/VASC), P. CONTORTA/LEDUM GROENLANDICUM/V. SCOPARIUM (PICO/LEGR/VASC). Stands of PICO/SHCA/ARUV are mostly seral to Pseudotsuga menziesii, of PICO/SHCA/ELIN to either Picea glauca or Pseudotsuga menziesii, of PICO/ALCR/LIBO to Picea glauca—Abies lasiocarpa, of PICO/MEGL/VASC to Picea engelmannii—Abies lasiocarpa, and of PICO/LEGR/VASC mostly to Picea mariana. Pinus contorta may be climax on dry, oligotrophic sites above the cold elevational limit of Pseudotsuga. A stable species structure (dominance hierarchy) develops in subordinate vascular strata of seral Pinus contorta forests long before the pine is replaced by climax tree species. The P. contorta forests of Banff and Jasper are compared with those of adjoining regions in Alberta, British Columbia, and Montana.

ISSN:0012-9615    

DOI:10.2307/2937244

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

Nine species of sympatric rain forest tree squirrels were studied in the field and in captivity at Makokou, Gabon. The findings on resource partitioning among them include: (1) two species are restricted to special habitat types. The other seven which commonly co—occur in the mature rain forest formation represent a maximum of taxonomic diversity. (2) Differential use of the vertical vegetation column divides these seven species into four essentially arboreal and three ground—foraging forms. (3) The species occupying a given foraging level and habitat differ in body size by approximately Hutchinson's ratio. Body size can act to partition fruit resources by size and hardness. (4) The squirrels are frugivores or omnivores; there are quantitative differences in diet between some species. These differences vary seasonally and may only be important during the most difficult season. (5) The four largest species have short active periods. I suggest that the greater species richness in African compared to Nearctic forests is permitted by a more constant productivity of fruit and arthropods, and a higher plant species diversity in the African forest.

ISSN:0012-9615    

DOI:10.2307/2937245

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

Low species richness (five to nine species) and high population density (means of 0.2—8.6 individuals per square metre) characterize Conus assemblages on intertidal benches throughout the tropical Indo—West Pacific region. Data from 16 such habitats in Hawaii, Marshall Islands, Australia, Maldives, and Seychelles indicate that similarity of microhabitats between species is equal to or greater than random expectation. Significant between—species differences in zonation pattern occur across benches at a given time and place. The peak of C. ebraeus abundance typically occurs closest to shore; C. chaldaeus and C. sponsalis are usually most distant from shore. However, we found about as man significant within—species differences between censuses made at different times on the same bench as between—species differences within censuses. Co—occurring species thus tend not to use microhabitat resources differentially. Physical environmental variables including tide level, strength of water flow and time of day determine refuging and foraging activity patterns, and all species appear to respond similarly to these factors. The data thus do not support the hypothesis of temporal resource partitioning. We found evidence neither for homing, as mark—recapture results suggested that individuals occupy any convenient refuge after foraging, nor for interference competition for protected sites among Conus. species diversity is significantly correlated with (1) substrate topographic diversity measured either independently or as the diversity of microhabitats utilized by all species together, and (2) the proportion of individuals occupying protected sites. Where refuges (potholes and crevices) comprise only 2—9% of bench surface area, an average of 63% of Conus individuals occupy them, indicating strong selection for protected microhabitats. The presence and quality of these physical refuges are less important on benches with thick mats of algal turf binding sand. The gastropods burrow in these mats during periods of stress. A catastrophic rainstorm coinciding with low tide caused high mortality in an inner bench population at Enewetak, but more C. ebraeus survived (30%) than other species (8%). Tolerance to such unpredictable physical stress may explain the observed patterns of zonation of Conus of different species and sizes across benches.

ISSN:0012-9615    

DOI:10.2307/2937246

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

The forests of Maryland have been mapped at a scale of 1:250 000 on the basis of 15 regional associations: (1) tamarack, (2) bald cypress, (3) hemlock—yellow birch—black birch, (4) river birch—sycamore (5) sycamore—green ash—box elder—silver maple, (6) shingle oak, (7) chestnut oak—post oak—blackjack oak, (8) chestnut oak—bear oak, (9) chestnut oak, (10) loblolly pine, (11) basket oak—loblolly pine, (12) willow oak—loblolly pine, (13) basket oak, (14) sugar maple—basswood, and (15) tulip poplar. The associations differ from each other in species composition and in abundances of species common to many associations. Each was identified in the field by the presence of relatively few common discontinuous tree species referred to ascharacteristic species. Correlations between forest associations and geologic, topographic, and soils units mapped at a similar scale suggest that patterns of available water are important in controlling distribution of woody species throughout Maryland.

ISSN:0012-9615    

DOI:10.2307/2937247

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

The suggestion that in less stable environments resource limitation and subsequent interspecific competition may be relatively unimportant in determining bird community structure is explored by examining the dietary relationships within a guild of three ground—foraging passerine birds (Horned Lark, Sage Sparrow, and Western Meadowlark) in the shrubsteppe of southeastern Washington, USA, an area of severe, arid, unstable climate. General dietary analyses indicated a strong temporal component to the organization of bird diets: different species collected at the same time ate much the same things while the same species collected at different times ate different things. This pattern is reinforced by cluster analysis and stepwise discriminant analysis. Similarities in diet extended to other components as well. Dietary diversities tended to be the same for contemporaneous collections of birds, as did average prey sizes, although the latter evidenced a few statistically significant exceptions. Theoretically predicted relationships between diet and trophic structure morphology emerged only at the most general level, and even then were not always observed. In general, differences in body size or bill length were insufficient to account for variations in prey sizes, although meadowlarks did on occasion take significantly larger items than the other, smaller species. Average prey size was significantly correlated with the proportion of seeds in the diet and varied seasonally as seed consumption varied. Several aspects of this study indicate that shrubsteppe passerines are largely opportunistic in their foraging and diet selection, and that the apparent absence of fine tuning to their competitive milieu is most likely a function of the variable environment in which they coexist.

ISSN:0012-9615    

DOI:10.2307/2937248

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY

摘要:

Population densities of snowshoe hares in Alaska fluctuate on a 10—yr cycle. A snowshoe hare population was studied near Fairbanks, Alaska USA from 1971 through 1977 during the peak, decline, low, and early increase phases of the cycle. The mosaic of habitat types was utilized by hares according to the amounts of available food and cover. Habitats utilized were from the most dense to the least dense during the increase phase of the cycle and subsided in reverse order during the population decline. Presaturation dispersal was the causal force for habitat expansion during the increase phase of the cycle, while predation was the primary force for habitat contraction during the population decline. Snowshoe hares tend to move from winter to summer ranges in response to availability of food and cover. A patchy environment which provides refuges like dense black spruce or willow—alder thickets in winter and open summer range allow hares to shift their use seasonally in response to changes in diet and to take advantage of changing environmental conditions. During summer, hares feed on herbaceous material and low shrubs in open areas; during winter, when herbs and shrubs are covered with snow, they move into the dense spruce thickets to feed on spruce, willow, and alder. During periods of high hare densities, browsing intensity reached 100% in all habitat types and diameter at point of browsing exceeded 10 mm. Both indices suggest that a hare—vegetation interaction can initiate cyclic decline. These conclusions are consistent with Keith's model for the 10—yr cycle of snowshoe hares. Dense spruce or willow—alder thickets play an important role in snowshoe hare cycles in that they provide protective cover from predators during a population crash. Snowshoe hares, which are thus able to avoid local extinction by seeking these refuges, provide the stock for subsequent population increase. In the northern part of their range snowshoe hare populations are cyclic but in the southern part they are not. The cycle in the north is regulated in part by the presence of obligate hare predators which exhibit a delayed density—dependent cycle with snowshoe hares and the existence of a vegetational mosaic which provides both suboptimal and optimal habitats. The discontinuous coniferous forests, the facultative resident hare predators, and the presence of at least three other lagomorphs are all proximate factors which help to maintain a stable hare population in the southern portions of its range.

ISSN:0012-9615    

DOI:10.2307/2937249

出版商:Ecological Society of America    

年代:1980

数据来源: WILEY