摘要:

Current models of gas transport in aquatic plants hold that gases diffuse in an essentially static gas phase within the lacunae according to gradients in their individual partial pressures. This study introduces a new model of gases gas transport wherein the gases move through waterlilies in a mass flow. The internal gas spaces in Nuphar constitute a pressurized flow—through system, with ambient air entering the youngest emergent leaves against a small gradient in total gas pressure. This pressurization phenomenon is the result of purely physical processes driven by the gradients in temperature (thermal transpiration) and water vapor (hygrometric pressure) between the atmosphere and the lacunae of the youngest emergent leaves. The lacunae are continuous throughout the entire plant, and the older leaves vent the elevated pressure generated by the younger leaves. This arrangement gives rise to a flow—through pattern, accelerating both the rate of O2supply from the atmosphere to the roots, and the rate of CO2and CH4transport from the roots towards the atmosphere. This ventilation system represents an important adaptation by the waterlily to life in anaerobic sediments.

ISSN:0012-9658    

DOI:10.2307/1937277

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

We have examined stability at the carrying capacity for 25 genetically different populations of Drosophila melanogaster. In spite of their genetic heterogeneity, 20 of the populations yield stable equilibria and none have eigenvalues significantly greater than one. Computer simulations demonstrate how selection at the individual level may account for population stability (and, hence, that group selection is not necessary for the evolution of stability). Recent theoretical studies on density—dependent selection in random environments provide predictions consistent with our empirical findings.

ISSN:0012-9658    

DOI:10.2307/1937278

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

With the pressure chamber, the water potential of red spruce stems (Picea rubens) was measured in four altitudinal zones, including timberline, on Mount Monadnock, New Hampshire, during two winters. Average water potential over all zones and dates was —1.14 (sd @= 0.57, range —0.1 to —4.0) MPa for 545 stems from 91 trees. More variation in water potential occurred within single trees than between juvenile and adult trees, and than among altitudinal zones or among seasons. Temperature data from maximum—minimum thermometers and from thermistors implanted into trunks and soil show that bulk water flow periodically occurs in winter. The slow decline in water potential of branches excised in winter, and scanning electron micrographs of plugged stomates, indicate that high diffusion resistances maintain needles within the relatively high levels of water potential measured in this study. Our results disagree with traditional concepts, support the conclusions of Slatyer (1976) and Marchand and Chabot (1978), and indicate that the mechanisms behind the variability in water status of modular parts of single trees must be understood before the causes, timing, and consequences of winter death in timberline trees can be unraveled.

ISSN:0012-9658    

DOI:10.2307/1937279

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Populations of coral reef fishes are often thought to saturate habitable space and to be limited by its availability, particularly those fishes that defend space. In Panama, individual adults of the common Caribbean damselfish Eupomacentrus planifrons defend permanent, all—purpose territories . A territory contains a mass of coral from which the fish derives required substrate—based resources: shelter from predators, a nest site (males only), and food (from a distinct mat of algae). Two experiments tested whether an excess of suitable substrate was present in two of the main habitats of E. planifrons. (1) Coral substrate that contained algal mats was removed from small areas (up to 45 m2) in habitats composed of Agaricia and Acropora corals. After 3 mo new algal mats were established on previously mat—free parts of the experimental areas, and the density of adult E. platnifrons increased in the experimental areas. (2) The area of habitat of three small patch—reefs was reduced by one—half through the removal of coral substrate. One year later, the density of E. planifrons adults had increased on the remaining halves of those reefs. The results of both experiments indicated that a surplus of suitable substrate existed in the experimental areas and that E. planifrons was not generally limited by the availability of habitable space. Compared to control fish of the same length, experimental E. planifrons were similar in mass, in the amounts of fat reserves, and, in the case of females, in activity of their ovaries. Both experimental and control fish were breeding at the end of the experiment. Thus the experimental increase in population density did not appear to be detrimental to the physiological condition or reproductive capacity of the fish. We conclude that E. planifrons is probably not at the carrying capacity of the habitats it occupies

ISSN:0012-9658    

DOI:10.2307/1937280

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Heavy runoff from watersheds affects lowland streams in two general ways. Either a stream experiences spates (increased forward velocity and scouring of stream bottoms) or it becomes inundated by water from larger rivers, causing the stream to be backflooded. This study shows how these features affect the abundances and distributions of two Costa Rican species of stream—inhabiting predaceous waterbugs, Limnocoris insularis Champion and Cryphocricos latus Usinger (Naucoridae). Backflooding in the Sura was found to have no subsequent effect on the abundances or distribution of either species. But in the Sabalo, which spates, nymphal populations of L. insularis significantly decrease after a spate because of loss of numbers and loss of previous habitat. During severe spates, adults of both species can be washed out, causing abundances to remain low for several months to periods>1 yr. Abundances of nymphs and adults of C. latus are associated with fast current speeds, large rocks, and coarse—grained pebbles. These sorts of habitats are found in both streams. Even though C. latus is more common in the Sablo than is L. insularis, abundances of adults of the former species fluctuate more in the stream that spates than in the stream that does not, owing both to a high washout rate and to a slow recolonization rate. The basic pattern of distribution is explained by the difference between the flooding characteristics of the two streams.

ISSN:0012-9658    

DOI:10.2307/1937281

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Losses of mass in breeding passerines are commonly observed and presumed to reflect physiological stress from activity while feeding nestlings. Analyses of changes of body mass in House Wrens (Troglodytes aedon) show that: (1) females lose ° 13% of their body mass between onset of incubation and fledging, but reattain their original mass for second broods; (2) 50% of mass loss is achieved before hatching is completed; (3) loss of mass is virtually complete before food demands of the nestlings are greatest; (4) change in body mass during the period of highest food demand is independent of brood size; (5) males foraging at rates similar to females show no change in mass. These patterns do not reflect stress. Rather, the timing of mass losses of females may be adaptive by permitting a 23% reduction in power required to remain aloft in flight before the most demanding period of feeding nestlings.

ISSN:0012-9658    

DOI:10.2307/1937282

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Plants growing in dense stands of native vegetation are often less susceptible to herbivore attack than isolated plants or plants growing in monoculture. Three mechanisms have been suggested that may account for this phenomenon; (1) maintenance of a complex predator fauna in dense vegetation; (2) inhibition by complex vegetation of the ability of an herbivore to discover its host plants; and (3) decreased residence time of herbivores on plants growing in diverse vegetation. This study demonstrates that for both adults and larvae of the pipevine swallowtail butterfly, Battus philenor, inhibition of discovery is the primary cause of decreased susceptibility of its host plants, Aristolochia reticulata, to attack when grown amid native vegetation. In addition, this study demonstrates that a seasonal decrease in the susceptibility of host plants to discovery by adults is correlated with a seasonal increase in the height and density of the native vegetation. A similar inverse correlation between vegetation structure and susceptibility to discovery by larvae was detected. However, seasonal changes in vegetation structure do not influence susceptibility to larval discovery. This differential effect of changing vegetation structure on searching efficiency is probably due to differences in the way larvae and adults search.

ISSN:0012-9658    

DOI:10.2307/1937283

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Compositional patterns of forest layers (tree, shrub, herb, bryoid, and epiphyte) are weakly correlated in the Swan Valley. While one can roughly predict the composition of one layer based upon the composition of another layer, different strata do not change composition across environmental gradients at the samerate or in the same pattern. The relative position of two or more stands in species space is different for different layers. Evidence in support of this conclusion was derived from: (1) correlation of dissimilarity matrices, (2) correlation of stand placement on ordination axes, and (3) comparison of stand groups defined by cluster analysis.

ISSN:0012-9658    

DOI:10.2307/1937284

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Ovipositing females of the host—specific herbivorous moth Frumenta nundinella Zeller discriminated among horsenettle (Solanum carolinense L.) plants in the laboratory and at five field populations. In laboratory experiments, the number of larvae that developed in high—density plant groups was not significnatly greater than the number that developed in low—density groups. Consequently, individual plants in low—density groups were disproportionately affected; twice as many larvae developed on them as on individual plants in high—density groups. Moths also discriminated between groups of pure horsenettle (high relative abundance) and groups of horsenettle plus another naturally co—occurring species (low relative abundance). Individual plants in high relative abundance groups had a tenfold greater infestation than did plants in low relative abundance groups. Natural horsenettle populations varied in density, relative abundance, plant size, and fruit production. Plant size, fruit production, and population density were all positively correlated, so the populations with the greatest density had the largest plants, which produced the most fruits. Despite differences in these plant parameters among sites, the extent of infestation differed little, except for one site that lacked parasitoids that attack F. nundinella. Infestation within field sites was heterogeneous among plants. Virtually all large plants (those with @>15 fruits), but only about half the small plants (those with<15 fruits), became infested (one or more infested fruit per plant). However, a greater proportion of the fruit crop was infested on small plants than on large plants. Large plants remained in flower an average 2.5 wk longer than did small plants, so a smaller proportion of their flowers was available during the short period of high oviposition risk. Because of the short oviposition period, plants that flowered earlier or later than the mean flowering time of their populations appeared to have lower infestations than did plants that flowered close to the population means.

ISSN:0012-9658    

DOI:10.2307/1937285

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Three distinct population strategies were observed within the summer algal plankton of Lake Fryxell (Taylor Valley, South Victoria Land, 77°35'S, 163°15'E). Phytoplankton immediately under the ice (Ochromonas and Chlamydomonas) were adapted to relatively bright light but were limited by nitrogen availability. A deep maximum of Chroomonas and Pyramimonas was recorded at the bottom of the euphotic zone. This community did not respond to nitrogen or phosphorus enrichment. It was highly shade adapted but at this depth ambient irradiance was below that required to saturate photosynthesis. Net population increases in both the upper and lower euphotic communities occurred very early in the season. Flagellated algae in the middle of the oxygenated water column swam up to depths of greater light during the day and returned to lower depths of greater nutrient supply at night. These mid—euphotic populations continued to grow throughout midsummer. Comparisons with other Dry Valley lakes suggest that nutrient supply, rather than in situ light or temperature, determines the large lake—to—lake and depth variations in primary productivity. Nutrient availability appears to control algal biomass, but in contrast to arctic ecosystems, low light rather than low temperature dampens algal photosynthesis to cellular rates that are well below those recorded at lower latitudes. The complexity of production strategies in Lake Fryxell, and the occurrence of population maxima early in the season when sampling is logistically difficult, challenge the view that the desert lakes of Antarctica offer ideal and simple systems for limnological study.

ISSN:0012-9658    

DOI:10.2307/1937286

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY