摘要:

Spider species acceptance of, and segregation by, architectural configuration was investigated for the spiders of Green Canyon in northern Utah, USA. Modular habitat units consisting of 30.48—cm (1—foot) cubes of chicken wire supporting internal strands of macrame jute tied in different orthogonal configurations were used. Configurations including all three axes were tested at two strand densities. The null hypothesis, that spiders use structures independent of architecture, was rejected in favor of the alternate hypothesis that spider species differentially use structures dependent upon architecture. Of the eight most abundant species, two showed preferences for horizontal substrata and one chose vertical substrata. All spiders strongly responded to the amount of jute available in each module. The two most abundant jumping spiders were biased toward modules with widely spaced jute, while the two most abundant web—builders preferred closely spaced jute.

ISSN:0012-9658    

DOI:10.2307/1936670

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Interactions among bumblebees (Bombus terricola and B. vagans Smith) and small syrphid flies (Melanostoma mellinum L. and Toxomerus marginatus [Say]) were studied on pasture rose (Rosa carolina L.). Dominance interactions took the form: bumblebees>Melanostoma>Toxomerus. In the absence of bumblebees, Melanostoma foraged 41% longer, but the bees only reduced Toxomerus' foraging about 4%. The difference in bumblebee impact on the two syrphids resulted from Toxomerus rapidly returning after visits by bumblebees, while Melanostoma did not. Toxomerus used flowers most frequently when Melanostoma were not in them. Bumblebee foraging patterns appeared unaffected by either syrphid fly. Melanostoma may have a considerably greater impact than bumblebees on Toxomerus; a 14% reduction in foraging by Toxomerus was recorded in one test. This was because Melanostoma occupied flowers for several minutes at a time, while bumblebee visits lasted only several seconds. By affecting Melanostoma's activity, bumblebees inadvertently favored Toxomerus, because the latter returned quickly to flowers after bee visits; Melanostoma did not.

ISSN:0012-9658    

DOI:10.2307/1936671

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Bumblebees (Bombus terricola Kirby and B. vagans Smith) rapidly accumulated pollinia after contacting milkweed (Asclepias syriaca L.), with losses soon balancing gains. Vagans carried significantly more pollinia than did terricola. Often there were more pollinia on the mouthparts than on the legs, especially on vagans, but there were always more pollinia and corpuscula (combined) on the legs than on the mouthparts. Turnover of pollinia on bees ranged from ½ to 2 d. Proportions of pollinia on the mouthparts often increased over the season. Pollinia—bearing bumblebees foraging on cow vetch (Vicia cracca) carried fewer pollinia on their mouthparts and more pollinia on their feet than did bees captured on milkweed. Bees foraging on milkweed regularly lost claws or segments of their legs. Pollinia slowed the foraging speed of both bee species about 25%. Corpuscula alone had less effect on foraging than did pollinia. Difficulties in foraging included losing footing and extricating feet or mouthparts from flowers. Pollinia—bearing bumblebees moved to new plants more frequently than unimpeded individuals, enhancing the possibility of outcrossing.

ISSN:0012-9658    

DOI:10.2307/1936672

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The influence of grazing by the Snow Goose (Anser caerulescens subspecies atlantica) was studied in three distinctly different North Carolina salt marshes. One marsh was dominated by Spartina alterniflora, one by a mixture of Spartina patens and Distichlis spicata, and one by Scirpus robustus. Snow Goose grazing consists principally of removal of roots and rhizomes to a depth of 20—25 cm. Exclosures were utilized to compare (1) belowground biomass and net production, (2) aerial plant cover and (3) detritus availability between grazed and ungrazed plots. Ungrazed portions of all study had significantly more biomass (P<.001) than grazed areas in the year following grazing. Belowground net production was less in the grazed portions of two of the sites (P<.10). Grazing intensity, measured as percent biomass removed, was 58.1 ± 7.4% at all sites. Plant cover was reduced at all sites in the spring following grazing. The Scirpus robutus marsh recovered more rapidly than the other marshes and appears to be a more stable grazing system. Finally, at South Pond and Salt Flats, more uprooted root—rhizome material was found in grazed areas than in ungrazed (P<.001, P<.05 respectively). This strongly suggests that Snow Geese make belowground net production available for transport to nearby estuarine waters where it could be used by estuarine consumers.

ISSN:0012-9658    

DOI:10.2307/1936673

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Analysis of herbaceous vegetation and ant activity in random quadrats from 10 forest sites in West Virginia, USA, shows that ant—dispersed (myrmecochorous) plant species constitute about 30% of the herbaceous flora. Twenty—one such species were encountered in this study, three of which were not previously known myrmecochores: Carex jamesii, C. laxiculmis and Luzula echinata. Ant activity is a good predictor of the number of myrmecochorous species at a given site. There is also a weak correlation between ant activity and overall herbaceous species richness. A matrix of correlation coefficients and Wisconsin ordination suggest that the ant—dispersed species constitute a guild. The number of species in a guild for a given site may be limited by competition for ant diaspore vectors, and/or germination microsites. These resources are likely to be especially critical for shade—adapted forest herbs which experience nutrient stress. The ant—diaspore mutualism is viewed as a strategy to overcome nutrient scarcity, and the present study shows clearly that it is a major factor in forest herbaceous communities.

ISSN:0012-9658    

DOI:10.2307/1936674

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

This study integrates laboratory and field results to interpret habitat use and switching by individual bluegill sunfish (Lepomis macrochirus). Prey handling and search times for the bluegill were determined from laboratory feeding experiments at different prey sizes and densities in open water and sediment habitats. Relationships were then constructed to predict the expected return per unit time foraging in these habitats. Experience (learning) was found to increase foraging efficiency up to fourfold over the course of six to eight foraging bouts. A population of 225 bluegills was then introduced into a pond consisting predominantly of open water and sediment habitats. This population exhibited several habitat switches as resources declined during the 3—mo experiment; in addition, the population split over a 5—wk period, with individuals specializing on one habitat or the other. These specialists were considerably more successful than habitat generalists as measured by average amount of food in the stomachs. Return rates estimated from the laboratory experiments indicated that the two habitats were nearly equal in foraging profitability. The individual specialization indicated that learning and mutually interfering foraging mechanisms were involved in the species' use of the two habitats. The results suggest that learning and sampling are important aspects of both habitat use and the specialization of individuals within a population.

ISSN:0012-9658    

DOI:10.2307/1936675

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Litter and soil microarthropod populations were monitored following cable logging and clear—cutting of a forested watershed (WS 7) at the Coweeta Hydrologic Laboratory in the southern Appalachians of North Carolina. Annual mean densities of microarthropods in litter bags were reduced over 50% on the clear—cut watershed when compared with an adjacent forested watershed (WS 2), and averaged 8.4 individuals/g of litter on WS 7 vs. 20.4/g on WS 2 (P.001). In contrast, densities of microarthropods increased over 100% in deeper soil horizons (5—55 cm), averaging 89 800 microarthropods/m2on WS 7 vs. 43 700 microarthropods/m2on WS 2 (P<.001). Clear—cutting by methods that do not destroy the integrity of the litter—soil environment engenders two kinds of perturbations: (1) removal of the canopy, and (2) addition of organic matter inputs, primarily woody litter debris, to the forest floor. Without a protective canopy, temperatures at the litter—soil interface on WS 7 were increased; daily maximum summer temperatures averaged over 40°C. Thus, lethal or nearly lethal temperatures reduced microarthropod densities in the top 5 cm of litter and soil on WS 7; declines were attributed primarily to oribatid mite mortality. Increased densities below 5 cm were attributed primarily to increased prostigmatid mite abundance. The relationships between microarthropod densities, standing crops of organic matter and litter and microbial production were examined. The amount and vertical distribution and organic matter appears to be the best predictor of microarthropod densities and vertical distributions in most forest ecosystems.

ISSN:0012-9658    

DOI:10.2307/1936676

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Seed predation by small mammals has been proposed as one selective force in the evolution of ant—dispersal (myrmecochory) in many herbaceous plants of mesic deciduous forests. Three myrmecochorous species, Asarum canadense, Jeffersonia diphylla, and Sanguinaria canadensis, were studied in the Monongahela National Forest, West Virginia, USA, to (a) assess the impact of seed predators on seed mortality, (b) compare seed removal rates when ants were present vs. absent, and (c) evaluate the effects of elaiosome removal, seed burial, and doubling seed number per depot on rates of seed predation. Seed predators, especially rodents, cause more mortality among these woodland herbs than previously thought. Predispersal seed predation by rodents destroyed 24% and 47% of the seed crops of A. canadense and J. diphylla, respectively. The proportions of A. canadense seeds lost to predators varied among sites. Sanguinaria canadensis also suffered seed predation before capsules dehisced. Peromyscus leucopus was the most common seed predator. In exclosure experiments, seed predators removed 70% of the A. canadense seeds when dispersal by ants was prevented, but only 24—39% of the seeds suffered predation when ants were allowed access. Seed predators removed 84% of the S. canadensis seeds when ants were excluded, but 13—43% were lost when also exposed to ants. These estimated minimal proportions of seeds lost to predators when ants were allowed access differed between two study sites, but myrmecochory clearly reduced seed predation at parent plants. In laboratory experiments, P. leucopus located buried A. canadense and S. canadensis seeds less frequently than seeds on the surface (67.5% vs. 100%). Consumption of buried seeds by rodents was reduced when elaisomes were removed, as would be done by ants in natural conditions. Increasing the detectability of seeds through adding olfactory cues or doubling seed number per depot did not increase the frequency of predation, because mice were able to locate depots without these additional cues.

ISSN:0012-9658    

DOI:10.2307/1936677

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The limiting role of phosphorus on leaf decomposition and primary producers was investigated in a second—order woodland stream in Tennessee by experimentally enriching, for 95 d, adjacent reaches with an average of 60 and 450 mg PO4—P/L, respectively, over upstream control levels of °4 mg/L. Red oak (Quercus rubra) leaf packs in the enriched sections lost mass 24% faster than control packs (P<.05). Nitrogen content of the enriched packs increased 60% more, and P content increased 83% more than the respective increases in the control packs (P.05). Respiration rates of subsampled leaf discs were significantly higher than control rates only at the high level of enrichment. The increased respiration rates in the low and high enrichments accounted for 10 and 34% of the increased mass loss in the respective enriched sections, suggesting that the enrichment also produced increases in mechanical breakdown through faster microbial conditioning, increases in macroinvertebrate feeding, or both. Effects of the enrichment on aufwuchs initially consisted of increased chlorophyll a levels, followed by increased aufwuchs biomass levels. Dense growth of filamentous algae, including some Oscillatoria, which may be a nitrogen fixer, developed immediately downstream of P inputs. In addition, Nostoc, a known nitrogen—fixing blue—green alga, sampled after the enrichment, was significantly more abundant in the enriched sections than the control (P<.05). Densities of the snail, Goniobasis clavaeformis, a grazer—shredder sampled after the enrichment, also were significantly greater in the enriched reaches, suggesting that the lack of a sustained response of chlorophyll a to the enrichment may have been a result of increased grazing on algal biomass. These findings indicate that nutrient limitation of detrital processing is a significant factor in natural streams. The apparent increases in densities of benthic macroinvertebrates in the enriched sections, along with reported relationships between detrital food richness and macroinvertebrate growth and survivorship, suggest that nutrient limitation in streams also has ramifications on higher tropic levels.

ISSN:0012-9658    

DOI:10.2307/1936678

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Stomatal aperture and water balance in the field of eight Populus clones varying in growth rate were closely related to environmental factors and clonal differences were clearly expressed. Leaf water potential (ψl) was influenced by solar radiation, leaf conductance, evaporative demand, and soil moisture content. The effects of soil moisture on ψlwere greatly modified by atmospheric conditions and stomatal conductance. Several slow—growing clones exhibited extended periods of ψlbelow that of rapidly growing clones, despite high evaporative demand and the much greater transpiring surfaces of the fast—growing clones. Stomata of all clones responded to changes in light intensity and vapor pressure gradient (VPG). Pronounced stomatal sensitivity to VPG of two rapidly growing clones of common parentage, and the resultant capacity of these clones to moderate water deficits under high evaporative demand, were associated with drought resistance in one of the parents. Seasonal maximum leaf conductance was positively related to growth in several clones, suggesting that rapidly growing clones possess the capacity to carry on higher rates of gas exchange under favorable conditions. Analysis of changes in ψlwith changes in transpirational flux density (TFD) showed that for four clones, ψlchange per unit change in TFD decreased as TFD increased, indicating plant adaptation for prevention of damaging ψleven at high TFD. More rapidly growing clones exhibited a larger initial role of decline in ψlwith TFD, but reduced the rate of decline more than slow—growing clones as TFD increased.

ISSN:0012-9658    

DOI:10.2307/1936679

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY