摘要:

Several of Anisoptera maintain their body temperature more constant than ambient temperature. Species that spend most of their active period on perches (perchers) and make only short flights thermoregulate heliothermically, primarily by means of postural adjustments. Some of the species that fly continuously (fliers) are endothermic regulators during flight. They control body temperature by: (1) controlling metabolic heat production by alternately using gliding or powered flight; (2) controlling heat loss by altering circulation between thorax and abdomen. Small fliers are unable to regulate well because they are subject to high rates of convective heat loss. Cooling constants, as defined by Newton's law of cooling, are related to thoracic weight by a negative power function over a wide size range. Cooling constants are higher in live dragonflies than in dead ones, a difference that is accentuated at high ambient temperatures. High cooling constants at high temperatures probably result from increased rates of circulation. Most fliers, and some large perchers, are able to elevate their body temperature by "wing—whirring." In some species the rate of heat production apparently varies in response to ambient temperature. The body temperature at which flight begins after warm—up is positively correlated with body weight, probably because wing loading increases with increasing body size. The maximum voluntarily tolerated temperature and the threshold of heat torpor of several species were determined. Both parameters are correlated with habitat. The minimum temperature at which flight is possible is positively correlated with body weight and is slightly higher in tropical dragonflies than in temperate zone species. Body temperature is probably more variable relative to air temperature in tropical than in temperate zone species. The ability of dragonflies to maintain a relatively constant body temperature is determined primarily by climate, body size, and behavior.

ISSN:0012-9615    

DOI:10.2307/1942392

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

We studied the advantages and disadvantages of Bank Swallow (Riparia riparia) coloniality in 1972 and 1973 by examining 54 colonies, ranging in size from 2 to 451 active nests, near Ann Arbor, Michigan USA. Four disadvantages were investigated: (1) increased competition for nest burrows and nest building materials, (2) increased competition for mates and matings, (3) increased possibilities of misdirected parental care because of either brood parasitism or the mixing up of unrelated young, and (4) increased transmission of ectoparasites. Physical interference in reproductive functions and the possibility of intraspecific killing of offspring were also considered. The intensity of the various forms of competition increased with increasing colony size, though not always directly. Flea infestation also increased with increasing colony size. Intraspecific brood parasitism was not observed, and parent Bank Swallows began discriminating between their own and unrelated offspring at the time when the young first began to mix. Three hypotheses to explain the maintenance of coloniality were tested: (1) shortage of suitable nesting habitats, (2) advantages associated with social foraging, and (3) reduced predation on adults, young, or eggs. Shortage of suitable habitat could not be demonstrated. Parents did not appear to feed in groups, and survivorship of nestlings during cold weather and weight of nestlings at 10 days of age both suggested that competition for food increased with increasing colony size. Although there was no relationship between colony size and amount of nocturnal predation, adult birds mobbed diurnal predators. Using a stuffed weasel, we studied such mobbing responses. Our data suggest that diurnal predators at larger colonies are (1) detected more quickly, (2) mobbed by greater numbers of birds, and (3) subjected to more vocal commotion than are predators at smaller colonies. Further, we demonstrated that mobbing is at least sometimes effective in deterring avian predators. We suggest that reduced predation on eggs and young, resulting from both group defense and "selfish herd" effects, is an important advantage of Bank Swallow coloniality.

ISSN:0012-9615    

DOI:10.2307/1942393

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

The study area, bounded north and south by two large reservoirs, includes the most extensive remnant of floodplain forest in the Dakotas. Structure and composition of the forest overstory are strongly related to stand age and horizontal and vertical position on the floodplain. Populus deltoides Marsh. and Salix amygdaloides Anders. predominate in young stands which generally occur on low terraces near the center of the floodplain. Fraxinus pennsylvanica var. lanceolata (Borkh.) Sarg., Acer negundo L., Ulmus americana L., and Quercus macrocarpa Michx., which replace Populus and Salix through time, predominate in old stands on high terraces near the edge of the floodplain. Stands intermediate in composition are uncommon because of the discontinuous meandering pattern of the river across its floodplain. Surface soil environment and species diversity change markedly during the course of succession. The soils of young stands are generally sandy and low in organic matter. Soil nutrient content and available water capacity are generally higher in older stands because of higher organic matter content and repeated inputs of nutrient—rich silt from past floods. Tree species diversity (H') initially increases as stands age, reaches a maximum in stands with mixtures of both pioneer and terminal species, and declines slightly in the oldest stands. Both variety and evenness follow a similar pattern. Analyses of population structure indicate a recent decline in the establishment of small stems of Acer and Ulmus. Tree core analyses show a similar decline in diameter growth rate for Acer, Ulmus, and Fraxinus. Available data suggest that these changes can be attributed to the removal of periodic spring flooding caused by the presence and operation of the reservoirs. It is also hypothesized that the lack of seedling—sapling stands of Populus in the region is the result of a presumed reduction in the meandering rate of the river following reservoir construction and poor seedbed conditions in the absence of flooding.

ISSN:0012-9615    

DOI:10.2307/1942394

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Small mammals were live trapped on four grids in the taiga woods near Fairbanks, Alaska. Populations were censused at 2—wk intervals during the snow—free months and 4—wk intervals when snow was on the ground. One Microtus oeconomus population reached a peak of 73—83 animals/ha in 1969. Numbers declined over winter and no Microtus were captured until 1972 when the population density began to increase. A sympatric population of Clethrionomys rutilus did not show a 3—4 yr cycle and reached densities of 52—62 animals/ha in the autumn of each year of the study. Live trap survival data and sumilatneous climatological data indicate that overwinter survival was high for both species. Survival was good for Microtus ♀ ♀, while Clethrionomys ♀ ♀ had several periods of poor survival. In ♂ ♂, periods of low survival occurred during population increase and high density. These periods of low survival did not occur at the same time in both species. Nestling survival was poor in both microtines during population increase in 1969, and remained low for the whole breeding season for Microtus. Both species experienced weight losses during late summer and fall periods. Live trapping indicated that this decrease was due to individual weight losses that were as great as 35% over a 3—mo period. The C. rutilus population in the Fairbanks area had a broad niche in that it was found in many habitat types and morphological, physiological and ecological characteristics were variable. The M. oeconomus population, on the other hand, had a narrower niche and was more specialized than the C. rutilus population. It is hypothesized that niche breadth and variation in characteristics of the species are a determining factor for microtine cycling. Cyclic behavior should not be thought of as a product of latitude, or a universal characteristic of small microtines, but as an adaptation to a less variable niche.

ISSN:0012-9615    

DOI:10.2307/1942395

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY