摘要:

The level of energy expenditure in eutherian mammals is correlated with the type of food consumed, especially in medium—sized to large species; some foods (including vertebrates, herbs, and nuts) permit high basal rates, while other foods (including invertebrates, fruit, and the leaves of woody plants) require low basal rates. Various properties of a food, including a low digestibility, a high content of plant secondary compounds, the inability to separate food from non—nutritive particles, and a seasonal reduction in availability, may require a consumer to have a low basal rate of metabolism; all of these factors are exaggerated in importance by the high food intake of large mammals. In small species most foods, except flying insects and seed crops in the desert, permit high basal rates and effective endothermy. Climatic factors modify these relationships to a limited extent, while a specialization in moderately—sized species to arboreal habits leads to reduced activity, smaller muscle mass, and lower basal rate than is found in terrestrial species using the same food resources. Mixed diets generally produce basal rates intermediate to those expected from the component specialist diets. The correlation of basal rate with food habits has the following demonstrable consequences for mammals. (1) The scaling of basal rate usually follows the Kleiber relation, unless food habits require low basal rates, or temperature regulation requires high rates. (2) Mammals that feed on foods associated with high basal rates have a higher reproductive output than mammals of similar size that use foods requiring low basal rates. (3) Predator avoidance is correlated with level of energy expenditure and food habits, terrestrial species with low rates of metabolism relying principally on burrows or passive integumental structures, such as shells, plates, or spines. (4) Most living "conservative" mammals use foods that require low basal rates of metabolism, a condition that protects conservative species from replacement by "advanced" eutherians, which are also required by these foods to have low basal rates.

ISSN:0012-9615    

DOI:10.2307/2937268

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

I studied the spring wildflower community of mesic deciduous forests in piedmont North Carolina to determine (a) the extent to which fecundity is pollination—limited in the community, (b) the importance of competition for pollination in affecting seed—set, and (c) the characteristics of plants and their floral visitors that most contribute to full pollination. Although inadequate pollination seems likely in the community, supplemental hand—pollination significantly improved fecundity in just 3 of the 12 species I examined. Pollination—limited reproductive success was evident only in a distinctive subset of the community, species pollinated primarily by queen bumble bees. The majority of wildflower species are pollinated by flies and solitary bees. Measurements of visitation rates and pollinator effectiveness on these plants confirmed that they are usually adequately pollinated in spite of a short blooming season, considerable overlap in flowering times, extensive pollinator sharing by concurrently blooming species, and inclement weather that frequently interrupts insect activity. Many of the flies and solitary bees are inconstant foragers, yet competition for pollination among wildflower species through differential pollinator attraction or interspecific pollinator movements usually does not significantly decrease the seed—set of plants with shared visitors. Competition may act with other causes of insufficient pollination, however, as a selective force to maintain a characteristic set of floral biology traits within the community, including autogamy and self—compatibility, extended receptivity, and pollination by a variety of visitor types. That these floral traits contribute significantly to the successful pollination of vernal herbs was demonstrated by observations of visitor behavior, plant caging experiments that excluded visitors or restricted their access to selected flowers, and measurements of floral lifetimes and seed—set for individual plants. These traits are effective regardless of the source of pollination—limited fecundity, and it is the prevalence of such traits, rather than floral specialization or character displacement, that distinguishes the forest spring wildflower community from other communities with potentially inadequate pollinator service.

ISSN:0012-9615    

DOI:10.2307/2937269

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

We have calculated a nitrogen cycle budget for West Africa south of the northern Sahara that quantifies biologically important pools of nitrogen in the region and major fluxes associated with these pools. Major compartments of our model include noncultivated systems broken down by vegetation zone, successional status, and plant components; annual and perennial crops each broken down by crop species and plant and harvest components; wetlands; anthropic systems; and soils and sediments. Base reference year is 1978. Biological nitrogen fixation and precipitation fluxes dominated nitrogen inputs to West Africa in 1978. Approximately 12 × 109kg were fixed in noncultivated systems; about half of this was fixed in early successional (0—6 yr) rain forests, and much of the rest in grazed or fallow savanna grasslands and woodlands. Less than 0.7 × 109kg were fixed in cultivated systems; legumes accounted for ≈25% of this. Total anthropic sources (fossil fuel combustion, fertilizer production, and agricultural commodity imports) were minor (<0.3 × 109kg). Precipitation inputs to the region were ≈0.4 × 109kg. Most nitrogen leaving West Africa did so volatilized by fire (≈8.3 × 109kg), principally in non—cropped systems. Major losses also occurred via hydrologic export to the Atlantic Ocean (1.5 × 109kg) and via denitrification (1.1 × 109kg). Total N losses from West Africa exceeded 11.0 × 109kg. The nitrogen immobilized in growing woody vegetation appeared to have amounted to ≈5.6 × 109kg. Most was immobilized in forested regions by vegetation regrowth during the fallow phase of the bush fallow crop rotation cycle, the dominant cropping system in West Africa. A very small amount was immobilized by perennial crops such as cacao and rubber. Net immobilization in soil was not estimated; total soil N was assumed to be in steady state for the region as a whole. Our overall budget balances within 1%, despite independent calculations of all major fluxes. The balance portrays a nitrogen cycle dominated by pools and fluxes in noncropped systems. In contrast to N balances in developed regions, direct anthropic fluxes are a minor part of the West Africa cycle. Indirect human influences, however, mainly through effects on vegetation cover, appear to be a major determinant of both the rates at which nitrogen is cycled in West Africa and the relative importance of most pools and fluxes.

ISSN:0012-9615    

DOI:10.2307/2937270

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Competition for substratum by encrusting marine organisms usually occurs by direct overgrowth at zones of contact. The documented mechanisms of competitive displacement are numerous, but the relative importance of species in a competing assemblage has been difficult to quantify. This study of the encrusting community on subtidal vertical rock walls in northern Massachusetts used 2 yr of photographic sampling of permanent quadrats to examine the competitive role of each common space occupier. The surface area gained or lost by individuals or colonies of each species in interactions with others was measured over intervals of ≥ 30 d. Four quantitative indices were used to describe competitive roles: the first measured the overgrowth activity of each species within a given assemblage (overgrowth index); the second assessed the relative importance of each species as an overgrowth competitor in the assemblage (weighted overgrowth index); the third described each species' ability to resist overgrowth (resistance index); and the fourth measured each species' growth rate when advancing over other organisms. Analysis of percent cover on subtidal rock walls over a 2—yr period showed that the ascidian Aplidium pallidium, the sponge Halichondria panicea, the octocoral Alcyonium siderium, a red fleshy crustose alga, and a mat of amphipod tubes covered most of the rock surface continuously, although the relative abundances of the species varied across sites and seasons. Several of the common species gained space by overgrowing other species directly or by lateral growth pushing the adjacent species away (lateral movement). This group included several colonial and solitary ascidian species, calcareous tubeworms, and encrusting bryozoans. A few species, including the fleshy red crustose alga and the crustose coralline alga Phymatolithon, gained space from others when an overlying organism receded. The various indices of spatial interaction showed that species which were common and frequent overgrowers in the system were usually not good at resisting overgrowth themselves. Species good at resisting overgrowth were generally large and slow—growing, and were not the most active overgrowers of other species. The interactions among species in this study were basically hierarchical, with the larger thicker colonies and individuals being the winners in competition for space. However, there were a number of reversals of competitive success, especially among the thick crustose forms, and there were numerous "standoffs" where borders did not change during the 30—d interval (19—79% of all encounters, depending on the species involved; 41% overall). The indices of spatial interaction (growth, overgrowth, resistance, overgrowth weighted by abundance) were significantly correlated between exposed and protected sites but were not well correlated between warm and cold seasons. Thus, species tended to have the same competitive roles in slightly different communities but their importance varied as their growth rate changed between warm and cold periods. Overgrowth did not always result in death of the organism overgrown. Species such as the fleshy red crustose alga and the coralline alga Phymatolithon were often encountered alive under invertebrates that had been in position for at least several months. Thus, poor competitors in this system were maintained by two processes: some are early recruits that grow rapidly but later get overgrown, while others are able to withstand overgrowth until predation or some other disturbance removes the overgrower.

ISSN:0012-9615    

DOI:10.2307/2937271

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY