ISSN:0012-9658    

DOI:10.2307/1934959

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Analyses of the movements and web—site characteristics of the desert spider Agelenopsis apera (Gertsch) demonstrate that web locations offering the following habitat features are actively selected: shrubs, depressions, litter, and flowering herbs. A model of the effects of the thermal environment and prey avalability on the reproductive success of spiders occupying various web—site types is developed. The estimated productivity for an excellent web site (grassland depression with attractants) is 13x that determined for a poor site (larva surface). Model results suggest that more energy is to be obtained from selection of a favorable thermal environment (eight—fold difference) than from a site offering greater numbers of prey (two—fold difference). The presence of flowers at web sites increases the probability of receiving on occasional high prey density, whereas litter and habitat features providing shade (shrubs and depressions) allow increased spider activity through limitation of body temperature.

ISSN:0012-9658    

DOI:10.2307/1934960

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Nine hummingbird—pollinated species of Heliconia occur together at Finca La Selva, in the wet Caribbean lowlands of Costa Rica. In forest habitats, Heliconia clumps (clones) are typically small; in more open areas, many clumps attain large size. This probably reflects differences in light intensity and degree of vegetative competition in these habitats. Nine species of hummingbirds regularly visit Heliconia flowers at La Selva. The four hermits are nonterritorial, traplining foragers with long, curved bills. Non—hermits frequently hold territories at Heliconia clumps, and have short, straight bills. Pollination by hermits tends to produce more cross—pollination; territorial hummingbirds increase self—pollination. Different Heliconia species appear to be specialized for pollination by either hermits on non—hermits, largely through components of the caloric phenotype: amount and timing of nectar production, rate of inflorescene and flower production, and morphological paramerters that affect the energetic efficiency of nectar—harvesting hummingbirds. Habitat may influence pollination systems through its effects on clump size and thus on the number of flowers a clump can have at any one time. Ultimately, specialization for hermits or non—hermits may depend on the degree of self—compatibility of the different Heliconia species. Hermit—pollinated Heliconia mostly show sequential and nonoverlapping flowering peaks, probably resulting from competition for pollinators and/or selection against hybridization. Two hermit—pollinated species bloom simultanesoulsy, thereby inducing the birds to utilize an other—wise little—used microhabitat. Heliconia species pollinated by non—hermits bloom in the early to middle rainy season, and are mostly separated by habitat. Isolating mechanisms among sympatric Heliconia species involve both spatial and temporal patterns of partioning available pollinators. Floral parameters include mechanical (different site of pollen deposition on the bird) and ethological (caloric and visual factors affecting flower choice) mechanisms. Selection for pollinator specificity may result in convergence of blooming peaks, provided that other isolating mechanisms are present. Human activity has broken down some habitat barriers by producing large areas of second growth.

ISSN:0012-9658    

DOI:10.2307/1934961

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Several species of coexisting dragonfly larvae were studied for four consecutive years in a 1—ha old farm pond. Larval development, emergence patterns, and adult flight patterns showed that the most abundant species were univoltine and developed relatively synchronously (i.e., like a cohort). Three of the common species emerged in early spring, and the others emerged later in the summer. The common genera with the most similar microhabitat had a distinct temporal separation that may serve in reducing interspecific competition. However, coexisting congeneric species had almost identical life histories, supporting the hypothesis that ecological homologues can coexist because of "errors of exploitation" of the dominant species. The larval population dynamics of each dominant species (Ladona deplanata, Epitheca spp., and Celithemis fasciata) was characterized by a constant percentage numerical decline, coupled with a dramatic biomass increase from time of hatching to final instar. Larval mortality during this period averaged 92% per annum, but population biomass increased at least tenfold during the same period for each species. Comparison of larval estimates with emergence data revealed that at least 80% of the final instars die just before leaving the water to emerge. Life history variations among species smoothed out composite density and biomass trends which averaged about 1,000 individuals/m2and 2 g dry wt/m2, respectively. In general, population dynamics from year to year were quite consistent, indicating a relatively high degree of stability in terms of species composition and densities.

ISSN:0012-9658    

DOI:10.2307/1934962

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Distribution of nutrient elements (N, P, K, Ca, Mg, S, Fe, and Na) varied substantially among the tissues and species of a late successional oak—pine forest at Brook—haven, New York. Mean nutrient concentration in the biomass was 2.3 times higher in Vaccinium angustifolium than in Pinus rigida; but in net production mnean nutrient concentrations were high in Quercus alba and Q. ilicifolia, intermediate in Q. coccinea, Gaylussacia baccata, Vaccinium angustifolium, and V. vacillans, and low in P. rigida and Kalmia angustifolia. With the exception of Kalmia the species of the lower strata and earlier woody successional stages of the forest higher mean nutrient concentrations in the biomass than did the upper strata and climax tree species. Among tissues flowers, leaves, and fruits contained the highest concentrations; heartwood, the lowest. Weighted mean nutrient concentrations were higher for net primary production than for biomass by "enrichment ratios" mostly between 1.5 and 3.0 in the Bookhaven forest and comparable forests for which data are available. Nutrient element profiles showed the contrats in concentrations of different elements in species as deviations from the means for the forest as a whole. The contrasting nutrient profiles may express niche difference in nutrient use among the plant species of the community.

ISSN:0012-9658    

DOI:10.2307/1934963

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Feeding territories of Golden—winged Sunbirds contain enough energy to support an individual's daily energy requirements, and the amount of nectar per flower inside a territory tends to average higher than in adjacent undefended flowers. When undefended nectar levels are low (especially below 2 @ml per flower) the costs of territorial defense can easily be offset by energy saved from shortened foraging time budgets made possible by feeding at the higher average nectar levels. At higher undefended nectar levels the costs of territorial defense should no be recoverable. The balance between these costs and gains appears to define the conditions when territorial defense in this species is advantageous.

ISSN:0012-9658    

DOI:10.2307/1934964

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Interactions among the predatory rotifer Asplanchna and alternative prey Paramecium and Euglena are examined in a laboratory environment to determine whether the addition of an alternative, less desirable prey stabilizes a simple predator—prey interaction. Rates of increase, carrying capacities, and dynamics of predator resting egg production are determined by both short—term and long—term experiments with partial medium renewal. Isoclines and isoplanes are constructed; relative stability of the Asplanchna—Paramecium and the Asplanchna—Paramecium—Euglena interactions is determined. According to a mathematical analysis of deviations from equilibrium, stability properties change only slightly with addition of Euglena. Although Asplanchna rate of increase is reduced by addition of Euglena, feeding rate on Paramecium remains high, and Asplanchna cannot survive on Euglena alone. The biology of the organisms used in this study affect stability properties of the system more than any other factor.

ISSN:0012-9658    

DOI:10.2307/1934965

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

A laboratory study of energy flow in different—aged pea aphids was accomplished by the balance sheet method. Growth, reproduction, molted exoskeletons, oxygen consumption, and honeydew production were monitored daily. Absolute amounts of energy expended on reproduction, growth, maintenance, and rejecta all varied with age of the aphid. Efficiencies of production and respiration also varied with the life history stage. Assimilation efficiency did not vary with age. Gross productions efficiency (° = 49%) and assimilation efficiency (°=83%) are higher in this insect than in most other insects reported. The reason for such high efficiency may be the unusually high amino acid content of pea phloem sap. Rates of exudation of amino acids and sugars from pea phloem through severed aphid stylets and greater than twice the total daily energy requirement of continuously feeding aphids. This would be true even for passively feedings aphids. Under the study conditions the daily ingestions of one average adult aphid is approximately 8% of the daily net primary production of a 0.15 a (dry wt) pea seedling.

ISSN:0012-9658    

DOI:10.2307/1934966

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

The purpose of this study was to measure some aspects of the water relations of selected plant species in the alpine tundra, Colarado, in order to assess the possible role of water limitation on primary production. Leaf water potentials were measured in Kobresia myosuroides, Geum rossii, Bistorta bistortoides, Deschampsia caespitosa, and Caltha leptosepela. Leaf resistances to water loss were measured in Bistorta and Caltha. Measurements were made at dawn and midday throughout the season and several times through selected days in plants along a moisture gradient. Leaf resistance was related to leaf water potential, light, and temperature. Leaf water potentials decreased throughout the season and were higher on the wetter site. Minimum water potentials of the species in the order listed above were —40, —23, —17, —31, and —17 bars. Minimum leaf resistances of Bistorta and Caltha were 0.8 and 1.6 s.cm—1, respectively, and were lower on the wetter site. Both Bistorta and Caltha on the wet meadow showed partial midday stomatal closure, but Bistorta on the dry meadow did not At —16 bars leaf resistance increased abruptly in Caltha, but was still low in Bistorta. Leaf resistances were low at incident solar radiation levels of 0.2 ly ° min—1and at temperatures near 20°C. Leaf resistances increased with temperatures below 15°C. Root resistances for Bistorta were 0.9 x 104s ° cm—1° bar—1at the dry site and 0.8 x 104at the wet site, and for Caltha were 0.6 x 104s ° cm—1° bar—1at the wet site. The changing daily pattern of leaf water potential and leaf resistance is shown to be related to the diurnal environmental pattern, to root resistances, and to the relationships between leaf resistance and leaf water potential and between leaf water potential and leaf relative saturation deficit.

ISSN:0012-9658    

DOI:10.2307/1934967

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

By means of mathematical models and an experimental system we argue that density—dependent genetic feedback has the potential to regulate the amount of energy taken by a predator (parasite and herbivore) population from the prey or host population. The experimental system was a simulation using the housefly, Musca domestica, with artificial reproduction and prey, thereby restricting predator population feeding and adjusting it to surplus energy of the prey or host population. The experimental predator population, controlled by genetic feedback, was adjusted to a mean density of about 53 animals and in turn was removing less than the total amount of energy available from the prey population. The control predator population, regulated only by competition, fluctuated wildly about a mean density of about 147 animals, and consumed about 78% of the energy available from its prey population. Fluctuations existed in the experimental predator population, but were significantly damped compared with those in the control. These results generally complement those of earlier mathematical models, and support the idea that prey evolution can enhance predator stability and lead to a balanced supply—demand economy between predator and prey. In nature, the genetic feedback mechanism must function interdependently with other population control mechanisms, but the attempt here is to isolate the mechanism and assess its effect.

ISSN:0012-9658    

DOI:10.2307/1934968

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY