摘要:

Community types were described from 209 stands in the alpine zone of the North Cascades Range. The maritime to continental climatic change from west to east has a profound effect on regional vegetation patterns. Most of the community types in the eastern North Cascades are closely related to those in the Rocky Mountains, southern Alaska and the southern Yukon whereas communities in the western North Cascades are more similar to communities in other west coast ranges. Soils in the region include Entisols, Inceptisols, and Spodosols. Physical properties are quite similar in most soils. Organic matter, total cation exchange capacity, and pH generally decrease from west to east while exchangeable cations and nutrient levels are low throughout the region. A fellfield—dry graminoid—mesic environmental gradient was examined on Grouse Ridge, Mt. Baker. High soil temperature and low soil moisture regimes were typical of the ridgetop fellfield. During drought periods, on the vegetated portion of the slope, soil temperatures decreased and soil moisture stress increased with distance downslope; a reflection of increased plant cover and greater evapotranspiration towards the base of the slope. Species at the base of the slope had reduced vigor and much lower leaf water potential than those upslope. Phenological patterns were closely related to date of snowmelt and early—season temperature regimes along the gradient.

ISSN:0012-9615    

DOI:10.2307/1942614

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

Distribution, abundance, diet, and beak morphology of the six Geospiza finch species were studied at eight sites on seven Galapagos islands. The resulting information was used to test the theories of Lack and Bowman that interspecific competition (Lack) and floristic and food differences among islands (Bowman) determine the ecological and morphological characteristics of the finches. Both factors were found to be important in different aspects of finch ecology. Most of the 21 populations studied spent more time foraging for seeds and fruits than for insects. Most populations had generalized diets and nine out of 18 pairs of sympatric populations had diets overlapping by 50% or more. Diet breadth was positively correlated with variety of available foods (classified by an index comprising measures of size and hardness), but was not correlated with abundance or biomass of sympatric populations of finches (potential competitors). Competitive release, in the form of large population biomass on islands with few congeners, was not found. Nor was population biomass related to variety of available food. Overlap in diet between two species was greatest when the species were most similar in abundance and body size. Interspecific competition is indicated by the distribution and morphology of the finches. The absence of four combinations of species cannot be attributed to change: (1) G. conirostris and G. fortis, (2) G. conirostris and G. scandens, (3) G. difficilis and G. fuliginosa, and (4) G. scandens and G. difficilis. These absences cannot be attributed to the absence of suitable foods. and sites with similar floras and suites of food types do not necessarily have similar ground finch faunas. The ecological and morphological similarity of members of each of the four pairs suggests an inability to coexist for competitive reasons. In contrast, where only two species occur together on an island they tend to differ markedly in size (beak and body) and diet. Species with large beaks can eat larger and harder seeds and fruits than species with small beaks, and as a consequence they tend to have more diverse diets. Species with large beaks also eat medium—sized seeds and fruits quicker than species with small beaks. Species with small beaks may be able to deal with small soft seeds more efficiently than species with large beaks, but the data are not clear on this point. In addition, small species with small beaks have a metabolic advantage over large species with large beaks. Variety of available foods influences the large species more than the small species: the two largest species, G. magnirostris and G. conirostris, occur only on sites where plants producing large and hard seeds and fruits are plentiful. Food variety apparently does not influence the degree of diet overlap. The number of plant species on an island statistically determines the number of land—bird species in general, and Geospiza species in particular, which occur on that island. The influence is probably mediated through food supply, since 53% of the variation in Geospiza species diversity among the eight sites was accounted for in a multiple linear regression analysis by diversity of available seeds and fruits. We conclude that food supply and interspecific competition have jointly determined the ecological course of the radiation of Darwin's finches and the resulting pattern of species diversity. Interisland variation in vegetation favored the initial steps of differentiation. Competitive interactions among species influenced later stages by determining which ecological types could coexist on an island with a given array of foods.

ISSN:0012-9615    

DOI:10.2307/1942615

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

Solar radiation was measured within and above an east Tennessee deciduous forest over a 2—yr period. Diurnal patterns of within—forest radiation follow the temporal variation in incident radiation but become more irregular with depth in the forest because of the highly variable penetration of beam radiation in space and in time. Seasonally, radiation in the forest increases substantially from winter to its annual maximum in early spring as solar elevations increase. Although solar elevations continue to rise each day until the summer solstice, amounts of total radiation and its beam component drop sharply in the forest with the advent of leaf expansion. Diffuse radiation in the forest continues to increase slowly following the onset of leaf expansion but reverses as the forest approaches a fully leafed state. Following the summer solstice, forest structure remains essentially static until abscission while solar elevations decrease. In summer and early autumn, total radiation and both its beam and diffuse components decrease slowly in the forest to their annual minimum in autumn. With leaf abscission and subsequent fall, radiation increases slightly in the forest but soon declines again as solar elevations approach their annual minimum of the winter solstice.

ISSN:0012-9615    

DOI:10.2307/1942616

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY

摘要:

Larvae of Pieris rapae (Lepidoptera: Pieridae) were reared on several crucifer species and varieties which differed in their nitrogen content, either naturally or because they had been fertilized with NH4NO3. Growth rates, food consumption rates, and efficiencies of food utilization were determined for fifth instar larvae on all plants, using standard methods. Larvae on low—nitrogen plants consumed food faster, utilized it less efficiently, and utilized N more efficiently than larvae on high—nitrogen plants. As a result, growth rates and rates of accumulation of N into larval biomass were as high on plants of 1.5% (dry wt) N as on plants of 4.8% (dry wt) N. There was no correlation between larval growth rate and reported patterns of glucosinolate content in the food plants. We attribute low N accumulation rates and growth rates on Lepidium virginicum, Lunaria annua, and Thlaspi arvense to the presence in these plants of unique secondary compounds or atypical glucosinolate breakdown products. Larvae fed on Dentaria diphylla and on the most highly fertilized collards (6.1% dry wt N) accumulated N and grew at unusualy high rates. This may be because N in these plants was more digestible than in the other food plants. We suggest that larvae of P. rapae adjust their feeding rates to maximize the rate at which they can accumulate N, and thus the rate at which they can grow, on any given food plant. There is a limit to the rate at which N can be accumulated, because N is utilized less efficiently as it is consumed faster. We conclude that the growth of P. rapae larvae is limited by the availability of N in their food plants, that N budgets for such larvae are of greater ecological significance than energy budgets, and that, in this case at least, natural selection favors the rate (power output) rather than the efficiency of a biological process.

ISSN:0012-9615    

DOI:10.2307/1942617

出版商:Ecological Society of America    

年代:1977

数据来源: WILEY