摘要:

Forty—three of the 54 species of birds collected in or near tamarack bogs were predators of the larch sawfly. Birds feeding in the tree crowns utilized sawflies to a greater degree than birds feeding in the shrub layer or those that captured their prey while in flight. Predation varied within and between taxonomic groups of birds and important predators were found in every group collected. Some species popularly regarded as vegetarian (viz. sparrows) proved to prey heavily upon sawflies. The warblers, popularly regarded as important insect predators, were clearly of less importance as predators of the larch sawfly than the fringillids. A preference was indicated by many bird species for adult sawflies, even when they were relatively rarer than larvae. Larger and then smaller larvae followed in order of preference. Some species decapitated larvae and ate the body and either rejected or swallowed the head capsule. Positive functional responses were indicated at low and high prey densities for 22 predators, negative for four, and no response for two. The remaining predators were collected only at one of the two prey densities encountered. Population censuses provided data on 34 of the predator species in two plots. Numerical responses were suggested for all except those of sporadic occurrence. Nonresident species were prominent in the plot with a high prey density. Birds are likely to influence sawfly population trends at low insect densities, and perhaps at higher densities as well. The importance of predation on adult sawflies is stressed.

ISSN:0012-9658    

DOI:10.2307/1936326

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

The purpose of this study was to determine the nature of the movements of adult Rana pipiens in summer and the factors controlling them. Movements in a population in Cheboygan County, Michigan, were studied by means of a trailing device for following individual frogs, by weekly censuses of the study areas, and by direct observation. Trailing was accomplished by means of a spool of thread, mounted in a holding device on the backs of adult frogs; this unwound as the animals moved, marking their routes. One hundred thirty—six frogs were trailed for a total of 681 frog—days (one frog/one day); individuals were trailed continuously for periods up to 35 days. In fair weather in summer, leopard frogs on their home ranges in the fields typically spent more than 95% of a day's time sitting quietly in "forms," made by clearing the wet soil of dead vegetation. Several remained in the same form for more than 24 hr, one for more than five days. Travel on the home range occurred only infrequently during each day, and usually consisted of a shift from one resting spot (form) to another, seldom more than 5 to 10 m apart and usually much less. Such movement took place at all times of the diel period, but almost two—thirds of the total distance moved by frogs on their home range was covered between 8 pm and 8 am, presumably during the night. Movement within the home range formed complicated patterns of turns, doublings, and criss—crossed paths. Many frogs returned repeatedly to particular parts of their range, often to certain forms which seemed to have some significance as homes. Occasional brief trips into adjoining regions uninhabitable by frogs may have been exploratory, or attempts to reoccupy former parts of the home range which were no longer suitable for occupancy. Leopard frogs which had home ranges in wet woods seldom construct forms but instead, frequently used crevices and cavities as retreats. It is suggested that Rana pipiens may be a more common resident in wet forests than is generally realized, but is seldom seen in such places owing to its secretive habits. In nocturnal rains leopard frogs occasionally made extended excursions off their ranges. Such movement differed from home range movement in being direct, more or less continuous through the night, and often covering distances of 100 m or more; one trailed frog moved 159 m in a single night. These migratory movements stopped at daybreak, the frogs commonly remaining in the region they had reached for several days, unless forced by unfavorable moisture conditions to move to more moist regions. Occasionally the migration was continued on the night following the initial movement; one trailed frog traveled 240 m in two consecutive nights. Heavy rains of long duration in which almost the total population emigrated produced a marked, though temporary, effect on dispersion. Although leopard frogs were almost entirely confined to regions of moist soil in fair weather, during and immediately after heavy rains they scattered widely. Migratory frogs, even those which reached areas suitable for permanent occupancy, showed a strong tendency to return to their ranges following nocturnal excursions. Of 30 subadult and adult frogs captured off their range during or after rains, 25 were later taken on their home range or had mowed toward it. The function of the nocturnal migratory excursions is now known; they probably enable the animals to become acquainted with a large area around their home ranges. The most rapid rate of movement determined for a trailed frog during a migration was 46.6 m/hr. The distance traveled, hence rate of movement, showed a significant positive correlation with temperature. Rain appeared to be the environmental factor initiating migratory movement.

ISSN:0012-9658    

DOI:10.2307/1936327

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

A conspicuous band of bare sand averaging about 30 ft in width often separates reefs and beds of sea grasses (Thalassia and Cymodocea) in the Virgin Islands and other islands of the West Indies. This zone of sand appears to be the result of heavy grazing by parrotfishes (Scarus and Sparisoma) and surgeonfishes (Acanthurus) that stay close to reefs for shelter from predaceous fishes. Floating sea grass fragments are eaten by the halfbreak Hemiramphus brasiliensis and occasionally by the Bermuda chub Kyphosus sectatrix and the triggerfish Melichthys radula. Within the beds, the sea grasses are fed upon by the small resident parrotfish Sparisoma radians, the echinoids Lytechinus, Tripneustes, and Diadema, the green turtle Chelonia mydas, and in part by the queen conch Strombus gigas, and manatee Trichechus manatus. It is the author's opinion that if the pre—Columbian population of the green turtle could be restored and its fishery properly regulated, the enormous production of the sea grasses in the Caribbean region could be realized more fully for the benefit of man.

ISSN:0012-9658    

DOI:10.2307/1936328

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

The water economies of nine species of anuran amphibians have been examined for (1) response to controlled desiccation, (2) response to prolonged hydration, and (3) some physiological factors related to the observed interspecific differences in these responses. The nine species exhibited a marked variation in habitat preferences with regard to the availability of water, and correlations were observed between habitats and responses to experimental conditions. Species from more aquatic habitats exhibited lower tolerances to desiccation; however, they were able to endure hydration for significantly longer periods than terrestrial species. Physiological specialization was observed which may be primarily responsible for the observed interspecific differences in response to prolonged hydration. The skin of aquatic species was less permeable to water and the effective osmolarity of blood plasma was less than that of terrestrial species. It has been suggested that species from terrestrial habitats, especially those of the families Bufonidae and Hylidae, may be limited from wintering in water because of their intolerance to hydration.

ISSN:0012-9658    

DOI:10.2307/1936329

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

The phenotypic responses of suppressed trees were studied in a 60—year—old white pine stand. The first evidence of suppression is a tree—wide decrease in the production of secondary xylem. Height growth is relatively insensitive. During the latter 30 years of life suppressed trees made 50.7% of their total height growth but only 19.1% of their total diameter growth. Eventually the cambium fails to cut off xylem at the base of the tree. With time, the sheath of new—formed xylem retreats from the based giving rise to an increasingly large zone of missing rings. Girdling experiments indicate that diminished supplies of food and growth regulators move through the zone of missing rings to support primary root growth. Apparently the phloem remains functional in the zone of missing rings. Either the phloem can function for periods greater than 1 or 2 years, or the cambium cuts off phloem alone in the missing ring zone. In the normal cambial cycle, phloem alone is produced during the last part of the growing season. It is proposed that a physiological threshold exists below which growth conditions are inadequate for the production of secondary xylem but remain favorable for the production of primary root growth and possibly for secondary phloem. This physiological mechanism is of considerable ecological significance since, as suppression proceeds, the plant invests a higher and higher proportion of its decreasing energy supply in primary growth or in all tissues that require annual renewal thus prolonging its survival. Hence, the growth of white pine, while undergoing suppression, illustrates the principle of phenotypic adaptability.

ISSN:0012-9658    

DOI:10.2307/1936330

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

The harvest method was employed to study primary production in two plant communities, one of which had been continuously grazed for about 70 years while the other had been protected from grazing for a period of 11 years. Exclosures were constructed around both study areas to protect them from livestock grazing during the 2 years of the study. There was 45% more top growth in the protected area but only 68% as much root mass; consequently, average annual net production was at least 12% greater in the grazed area. Irrigation with 9 cm water increased top—growth yield 41.4%. Although over 30 species of plants were present in each community, two species contributed over half of the annual production in both areas. In the protected community 65% and in the grazed community 80% of the plant mass was underground. Proportion of plant mass underground ranged from 55% to over 98% in individual plants excavated whole. Following harvesting of the tops, dry weight of the roots decreased 1.6% during the first week, 22.1% during the first month, and 31.3% during the first 8 months. To measure seasonal trends in productivity, six plots, each 1 m2in area, were harvested every week during two growing seasons. Artemisia tridentata, the principal shrub present, seemed to be most productive during the fall and early spring while Stipa comata, the principal grass, was most productive during late spring and early summer.

ISSN:0012-9658    

DOI:10.2307/1936331

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

Shrub—carr vegetation over its full range of variation in southeastern Wisconsin was quantitatively analyzed and the influence of origin, environmental factors, and disturbance on the vegetation was investigated. Plant composition was sampled in 76 stands distributed over 13 counties. A list of common shrub—carr species was derived by combining presence with frequency or intercept data. Salix petiolaris and Cornus stolonifera were the most common of 38 shrub species. The vegetation had three distinct layers, an upper dominant shrub layer, an intermediate tall herb, grass, and sedge layer, and a low diminutive herb layer, but there was no stratification within the shrub layer. The pattern of plant distribution was very heterogeneous, due to irregularities in the soil surface and to disturbance. The response of common species to disturbance was indicated by arranging stands along a disturbance gradient. Most shrub—carrs in southeastern Wisconsin originated in the 1930's when shrubs colonized abandoned mowing meadows. The relative stability of the shrub—carr as a community appeared to be due to shrub resprouting after disturbance. Lowland forest undoubtedly invades the shrub—carr, although fire may kill seedlings and saplings and thus retard three invasion.

ISSN:0012-9658    

DOI:10.2307/1936332

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

Soil dilution plate counts on selective media for bacteria, streptomycetes, and fungi were made at intervals before and after two prescribed burns on 10—acre jack pine tracts. Carbon dioxide determinations were also made. These tracts were compared with similar tracts cut and unburned and uncut and unburned. Three—year results are given. The number and activity of most microorganisms decreased immediately after fire but rose abruptly to a very high level after the first rainfall following burning. This increase is believed caused by the leaching of ash minerals. Depth and extent of the effects were influenced by intensity of fire and moisture conditions. Number and activity of organisms were generally lower in burned soil the second growing season after fire. Some effects were still noticeable the third growing season. Streptomycete populations were greatly increased the third growing season. Rainfall also caused a fluctuation of microbial populations on the unburned tracts.

ISSN:0012-9658    

DOI:10.2307/1936333

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

Plant communities and some causal factors were studied on a stony soil, on pebble mounds on stony soil, and on an adjacent soil derived from the Pierre Shale in an area 14 miles northwest of Denver, Colorado, at approximately 6,000 ft altitude. Average annual precipitation is 15.2 inches. The three dominant perennial species on the soil derived from Pierre Shale were buffalograss, blue grama, and western wheatgrass, all of which are characteristic of the mixed prairie association of the Great Plains. True prairie species characteristic of prairies of the Midwest, big bluestem, little bluestem, Indiangrass, switchgrass, and prairie dropseed, were present in significant amounts on the stony soil; however, most of the other species were montane. Some of the montane species were mountain muhly, sandwort, and beardtongue. More than twice as many species occurred on the stony than on the other two soil conditions. The greater availability (lower tensions) of soil moisture in the stony soil appears to be the primary cause of the larger number of species and generally more mesic flora. Infiltration rates were highest on pebble mounds and stony soil. Although the stony soil contained less zinc, potassium, sodium, and sulfate, plant species from the stony soil contained larger quantities of these nutrients. The higher pH in the shale—derived soil (7.1 as compared to 6.0—6.3) and larger quantities of montmorillonitic clays may have reduced the availability of these nutrients to plants. The presence of vegetation on pebble mounds that indicates either disturbance or droughty conditions is attributed to the activities of the mountain pocket gopher. Species prominent on mounds were cheatgrass, sixweeks fescue, ragweed, and hairy goldaster. Similar soil moisture conditions in mounds and in stony soil indicate that the two soils should have the same kind of vegetation if disturbance is not a factor. Evidence suggests that pocket gophers may be the causal factor for both the mounds and the subclimax vegetation on mounds.

ISSN:0012-9658    

DOI:10.2307/1936334

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY

摘要:

Productivity studies have shown that plants with marked vertical extension of photosynthetic crown can be more productive per unit area of land or water occupied than plants whose photosynthetic surface is spread in a thin horizontal sheet on the earth's surface if environmental factors are not otherwise limiting. Geometric models including a flat disc and cones of several heights but constant base radius show that heightening cones intercept progressively more light. Amount of chlorophyll displayed per unit area of earth's surface can also increase greatly with vertical extension of aerial crown. These observations suggest that thickness, geometric configuration, and chlorophyll content of the photosynthetic portion of the vegetation per unit area of earth's surface, and light intensity incident on surfaces at right angles to sun's rays should be measured and described as basic data in primary productivity studies.

ISSN:0012-9658    

DOI:10.2307/1936335

出版商:Ecological Society of America    

年代:1965

数据来源: WILEY