摘要:

Helianthella quinquenervis (Asteraceae), the aspen sunflower, secretes sugar and amino acid rich nectar from involucral bracts during bud and flowering stages. The nectar is usually collected by ants as fast as it is secreted. H. quinquenervis is subject to predispersal seed predation by larvae of several insect species, primarily tephritid flies, an agromyzid fly, and 3 Lepidoptera. When present, ants interrupt oviposition efforts by the female flies but do not appear to be effective against lepidopteran or mammalian herbivores. The degree of protection by ants conferred upon plants was investigated by excluding ants from some plants and by correlating ant density with subsequent damage to ovules and developing seeds. At higher elevations (2896 and 3091 m) damage to plants with ants was significantly reduced compared with plants without ants. At a lower elevation (2734 m) where seed predator and ant densities were highest, the ants were less effective as deterrents and predation on ovules and seeds usually exceeded 60%. Flowers of some other composite species in the same habitats appear to be more effectively protected from predation by chemical deterrents, raising questions of the relative benefits of chemical deterrents compared to ants as a means of protection.

ISSN:0012-9658    

DOI:10.2307/1936460

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The influx of large numbers of alewife, Alosa pseudoharengus, into relatively small freshwater systems may have a considerable impact upon pre—established food chains and nutrient cycles. We estimate the total nutrient input to Pausacaco Pond, Rhode Island, USA, from alewives amounted to 0.43 g P, 2.7 g N, and 16.8 g/Cm2over a 2—mo period. This is largely through mortality of the spawning fish, and to a lesser extent through excretion. These inputs were much greater than the eventual nutrient loss to the system through emigration of juvenile fish. In tank experiments using pond microcosms, the initial response to the addition of the fish was a large phytoplankton bloom and an increase in litter respiration. The phytoplankton bloom was short—lived, and the most lasting effort was an increase in production and respiration in the leaf litter. This increased production in the litter community would support a long lasting supply of insect and benthic invertebrate food for young fish. The respiration rate of autumn leaves incubated in alewife streams during the migration was significantly higher than that of leaves incubated simultaneously in a stream which had no alewife run. Respiration rates of leaves incubated in the same streams before the arrival of alewives did not differ significantly. The increase in litter respiration, an indication of microbial and invertebrate activity on the leaf surface, was attributed to the additional nutrients supplied by the fish.

ISSN:0012-9658    

DOI:10.2307/1936461

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Leaf area index (LAI) and foliage—height profiles were determined for 13 northern hardwood stands ranging from 3 to 57 yr since clearcutting and for 1 uncut stand. Leaf litter collection and stratified clipping were used to measure LAI which showed no relationship to total canopy height or foliage—height distribution, and recovered quickly to near mature forest levels following cutting. A camera and telephoto lens were used to measure the distribution of heights to lowest leaf in the 12 oldest stands (7 / yr). Distributions were converted to foliage—height profiles using equations from MacArthur and Horn (1969). Profiles in the 2 youngest stands (3 and 4 yr) were determined by stratified clipping. Total canopy height varies predictably with age and is the major determinant of foliage—height diversity calculated using data in 1—m intervals by height. The concentration of foliage into a definable stratum occurs early in succession and persists through year 30. An associated lack of understory in this period results in a reduced foliage—height diversity using data normalized to a 4 strata per profile. Beyond year 30, canopy structure is variable but mature stands generally have leaves evenly distributed by height.

ISSN:0012-9658    

DOI:10.2307/1936462

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The pelagic environment is often regarded as rather homogeneous by comparison with shallow—water benthic environments. Species of krill of the pelagic genus Euphausia (Crustacea) display a trend in genetic variability from low in high latitudes to high in low latitudes, closely similar to trends displayed by shallow—water benthic invertebrates. We interpret the trends as genetic strategies, with few functionally—broad alleles in high latitude species or in species that range widely ecologically, and numerous functionally—narrow alleles in low latitude species that are narrowly restricted ecologically. The functionally—narrow alleles are maintained by forms of balancing selection and permit a high degree of specialization. Therefore the tropical kill species is highly sensitive to spatial heterogeneities within the pelagic zone, which does not appear homogeneous to selection.

ISSN:0012-9658    

DOI:10.2307/1936463

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Metabolic rates and water fluxes in winter—aggregated Yarrow's spiny lizards (Sceloporus jarrovi) were measured in the field using doubly—labeled water. Resting metabolic rates were estimated from measurements of microhabitat and lizard body temperatures. During the month (November) following aggregation at the hibernaculum and the month (April) before dispersal, lizards were relatively active and their metabolic rates averaged 0.071 ml CO2°g—1°h—1, °40% of their warm season rates. From December through March, lizards basked but were relatively inactive with metabolic rates averaging 0.031 ml CO2°g—1°h—1, °15% of warm season rates. Water influx rates averaged 10.3 ml°kg—1°day—1during the 120—day inactive period. Water loss was slightly higher than gain and lizards lost weight slowly while hibernating, having ended the 6—mo aggregation period with a 22% decline in body mass. Total energy expenditures during the aggregation period averaged °7.53 kJ,3 times as much energy during the aggregation period as would a lizard remaining inactive in a crevice at 10°C for the entire 6 mo. Possible benefits of winter basking are discussed.

ISSN:0012-9658    

DOI:10.2307/1936464

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Precipitation and streamwater from the Bowl, a watershed in central New Hampshire, were analyzed chemically during 1973 and 1974. The Bowl, covered by a northern hardwood forest with spruce and fir at higher elevations, has never been logged or otherwise disturbed by humans. Calcium, sulfate, and nitrogen ions dominated stream chemistry. The biogeochemistry of the cations seemed to be regulated more by precipitation, soil—water movement, and chemical weathering reactions than by forest succession. Nitrate concentrations remained nearly constant, at °2.5 mg/l throughout the study, with no apparent seasonal fluctuations; in contrast, nitrate concentrations in the stream draining a nearby 55—yr—old forest definitely declined during the growing season. Nitrate budgets indicated a net loss of this important plant nutrient from both watersheds. However, a net accumulation of ammonium was sufficient to give a net increase of total N in both watersheds. These data do not support the hypothesis of Vitousek and Reiners that old—growth forests may reach a point of no net growth and no net uptake of nutrients. Mixed deciduous—coniferous forests in New England, free from human disturbances, may reach an age where they become prone to natural disturbances that create a mosaic of similar—aged group of trees, each group having differing abilities to accumulate nutrients.

ISSN:0012-9658    

DOI:10.2307/1936465

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

There are only 2 species of canids in mainland Chile: the foxes Dusicyon culpaeus and Dusicyon griseus. These 2 species are shown to diverge in body size with increasing latitude: body size of D. culpaeus increases whereas the size of D. griseus decreases south of 33° south latitude (SL). The variables latitude, temperature, annual actual evapotranspiration, and mean size of potential prey species have been previously proposed as correlates of body size in carnivores, but they do not explain the size trends exhibited by the 2 Dusicyon. Instead we found that constancy of mean size of potential prey species throughout the range, and increased habitat overlap between D. culpaeus and D. griseus south of 33° SL, are most likely to explain the size trends of these congeners. Our results suggest a trade—off between habitat and food overlap: progressive increments of interspecific habitat overlap south of 33° SL are compensated by progressively larger differences in body size and in mean prey size.

ISSN:0012-9658    

DOI:10.2307/1936466

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The complex nature of, and importance of detritus consumption in, estuarine food webs make analysis of specific pathways of carbon in estuaries very difficult. We have evaluated the use of the natural abundances of the stable carbon isotopes in estuarine flora and fauna as a qualitative indicator of food sources in a Georgia salt marsh estuary. The salt marsh ecosystem is particularly amenable to this technique because of the wide span of distinctive13C/12C ratios of the dominant plants °13C of ‐12 to ‐26%). Laboratory experiments showed negligible further fractionation of the characteristic °13C of marsh plants during microbial degradation of dead plant material, and during assimilation of ingested detritus by estuarine invertebrates. Invertebrates having a variety of feeding habits were collected in monospecific and mixed species stands of marsh plants. Marsh snails and insects had °13C values close to those of the associated vascular plants. Deposit—feeding crabs had isotope ratios biased away from the marsh plant °13C values, likely a result of feeding on benthic diatoms and out—of—stand foraging. Mud snails and filter—feeding bivalves showed13C/12C ratios similar to those of benthic and pelagic algae. The °13C values of marsh fiddler crabs varied from ‐12 to ‐24% in various habitats having predominately C4or C3food source plants. Individuals collected from populations of 3 crab species in a Spartina alterniflora marsh had13C/12C ratios with a standard deviation of only about 1% from the mean°13C. Stable carbon isotope ratio analysis appears to be a powerful tool in the evaluation of carbon pathways in estuarine food webs.

ISSN:0012-9658    

DOI:10.2307/1936467

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Laboratory—determined larval growth rates of the detritivore (collector—gatherer) Paratendipes albimanus (Chironomidae) responded proportionally to the microbial densities of 4 food sources. Substrates with higher microbial activities and biomasses produced greater growth rates in the order: pignut hickory (Carya glabra) leaves>white oak (Quercus alba)>insect feces>natural stream detritus. Laboratory growth rates of P. albimanus were linearly related to quantitative estimates of food quality based on substrate adenosine triphosphate (ATP) and respiration rates but were not statistically related to total N or C. Although P. albimanus is univoltine in Augusta Creek, Michigan, an experimental laboratory population of first—instar larvae completed a 2nd generation during the summer when fed detritus generated from hickory leaves. A second experimental population failed to develop past the first instar when fed natural detritus. The natural growth pattern of P. albimanus involves the interaction of temperature and food quality.

ISSN:0012-9658    

DOI:10.2307/1936468

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Baboon dung was collected in the Shai Hills, Ghana at 4—wk intervals from November 1975 to January 1977, and was planted in sterilized soil in order to estimate the viable seed content of the dung. A total of 6,465 seedlings germinated, belonging to 59 species, although 70% of the seedlings belonged to 2 fleshy—fruited species. The species composition varied from month to month as did the relative importance of dry fruits. Germination of ingested seeds (removed from dung) was significantly improved over that of fresh seeds in 3 of 4 species tested (Securinega virosa, Azadirachta indica, and Nauclea latifolia). The 4th species, Diospyros mespiliformis, showed no significant improvement.

ISSN:0012-9658    

DOI:10.2307/1936469

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY