摘要:

This study compares the structure of the limnetic zooplankton communities of four very different lakes ranging from an oligotrophic Great Lake to a small dystrophic reservoir. Only filter feeding cladocerans and calanoid copepods are considered. Using Levins' Theory of the Niche analysis, I measured habitat overlap values for each species pair to determine to what extent habitat selection can explain the number of limnetic zooplankton species coexisting in these lakes and to what extent other factors, such as resource allocation or differential predation, must be invoked to explain the observed diversity. A substantial portion of the ecological overlap of zooplankton populations can be alleviated by habitat selection. Zooplankton communities are dominated by two to three species, in both numerical abundance and grazing intensity. In Lake Michigan (large, oligotrophic), habitat selection was the major mechanism of competitive coexistence and it was achieved by extensive vertical migrations (mean a = .47). In Gull Lake (small, mesotrophic), habitat selection via vertical migration also predominated (mean a = .37) as the major mechanism of competitive coexistence. Three species, Diaptomus sp., Daphnia retrocurva, and Daphnia galeata exhibited high habitat overlap coefficients. In Cranberry Lake, a dystrophic reservoir, most species possessed high overlap coefficients (mean a = .62) and minimal migration amplitudes. Diaptomus spp. and Daphnia catawba predominated. In Lake George, a medium—sized oligotrophic lake, mean habitat overlap was also high (mean 2 = .65). The three dominant species were Diaptomus sp., Diaphanosoma leuchtenbergianum and Daphina galeata. For the latter two communities, which possessed similar high and mean overlap values, the basic equation was weighted by the standing crop and turnover rate of phytoplankton. The spatial–temporal positioning of the Lake George populations were correlated with algal standing crop and turnover patterns while the Cranberry Lake populations were not.

ISSN:0012-9615    

DOI:10.2307/1942410

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

The periodicity, associations, species diversity, and composition of the sublittoral, attached algal vegetation were studied at four ecologically distinct stations in southern Cape Cod and adjacent islands. In situ collections and observations of 142 species of attached sublittoral algae (Chlorophyta, Phaeophyta, and Rhodophyta) were made with the aid of diving equipment. These were categorized into four groups with respect to seasonal periodicity (seasonal annuals, aseasonal annuals, pseudoperennials, and perennials) based on thallus longevity, plant form during their adverse season, and the period in which each species population was present in the community. Using these criteria we recognize the influence of environmental change on algal development, and the plants' temporal influence on the community. Based on qualitative and quantitative sampling, 10 sublittoral alga associations have been recognized from 2 bottom substrata types. The distribution of sublittoral attached algae at our four stations was more complex than can be accounted for by the three sublittoral algal associations as described in studies of other geographic areas. This may be due to the different levels at which associations have been recognized, to the type and degree of substratum stability, and to whether consideration is given to algal seasonal periodicity. The concept of a universal tripartite zonation of sublittoral algae is unacceptable when these factors are considered in vegetational analyses. An in—habitat, seasonal comparison of species diversity and composition was based on adjacent quantitative collections from a uniform shell bottom at 6 and 12 m. Species diversity remained seasonally stable at these depths; species composition, however, changed dramatically throughout the year. At 6 m, brown algae dominated in winter and were replaced by red algae during summer. At 12 m, the seasonal floristic changes were less pronounced, and red algae dominated year—round. A between—habitat comparison of species numbers and composition was based on year—round qualitative collections and observations made at four regularly visited stations (5—22 m depth). Species numbers decreased with increased station depth from 5 to 22 m except for one heavily silted station. The seasonal change of floristic composition was most striking at West Chop (5—8 m) where short— and long—lived species were about equally represented. Seasonal differences in species composition between summer and winter were striking at a 5—8 m station but were less pronounced with increased station depth where long—lived species made up an increasingly larger percentage of the total vegetation. Floristic composition also differed among stations. About equal numbers of red and brown species occurred at 5—8 m in contrast to the occurrence of about 4 times more red than brown algal species at 22 m. The crust vegetation also made up an increasingly larger percentage of the deep vegetation when compared with the vegetation of shallower areas sampled. Species diversity at 20—22 m was much higher in southern Cape Cod than reported for similar depths to the north (Cape Ann, Massachusetts, New Hampshire; Halifax, Nova Scotia); this may be due in part to the intensive collecting of crustose forms during the present study.

ISSN:0012-9615    

DOI:10.2307/1942411

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Biotic and abiotic pathways of incident energy are rarely considered together in comparable detail, so little is known about whether energy or carbon relations remain constant in contrasting energy environments. An experiment to determine the fates of energy and carbon in cheatgrass (Bromus tectorum L.) was carried out on steep (40°) north— and south—facing slopes on a small earth mound, using many small lysimeters to emulate swards of cheatgrass. Meteorological conditions and energy fluxes that were measured included air and soil temperatures, relative humidity, wind speed, incoming and reflected shortwave radiation, net all—wave radiation, heat flux to the soil, and evaporation and transpiration separately. The fate of photosynthetically fixed carbon during spring growth (31 March to 31 May) was determined by separation of the plant tissues into mineral nutrients, ash, crude protein, crude fat, crude fiber, and nitrogen—free extract (NFE) (the proximate analysis scheme routinely used for feed analysis) for roots, shoots, and seeds separately. Temperatures and humidities were not notably different between slopes. However, all terms in the radiation balances were significantly larger on the south exposure, and the sensible heat flux to the atmosphere on the south exposure was much greater than on the north. The energy—saturated south exposure was subjected to water stresses much earlier than the north exposure. Total transpired water, the fraction of energy used to transpire water, and soil water content simultaneously showed an abrupt decrease in slope at about 10% soil water content (—14 bars average water potential) in the 60—cm lysimeter soil profile, indicating a rather sudden decrease in water availability to the plants. This change occurred near day 30 on the south exposure, and near day 45 on the north exposure, implying that water was readily available to the north exposure for about 2 wk longer than on the south exposure. Root and seed production were both about 30% greater on the north exposure than on the south, but shoot production was not different. Heats of combustion (caloric content) differed between roots, shoots, and seeds, but not between exposures. Total production and total energy fixation were not statistically different on the two exposures, because the greater biomass and variability of the shoots overwhelmed the statistically significant differences in roots and seeds. Incident shortwave radiation was very much greater on the south exposure than on the north, so the south exposure was much less efficient in converting solar energy into chemical energy stored in plant tissues. The clearest difference in carbon pathways occurred for crude protein in shoot tissues at the end of the growing season (23 g m—2on the north compared to 30 g m—2on the south). Since both exposures began the growing season with 33 g m—2, shoot protein was translocated to other tissues during the season, to a greater extent on the north exposure than on the south. The crude fiber and NFE (g m—2) in seed tissues were both greater on the north exposure, reflecting only a greater total seed biomass; but for roots and shoots there were no appreciable differences between exposures other than crude protein. The average individual seed weight was only about 10% heavier on the north exposure, although the north produced more than a third more total seed biomass. The composition of an average seed indicated that the difference in seed weight was caused primarily by about 8% more NFE in north exposure seeds, probably starches in the endosperm. By contrast, the crude protein in an average seed was almost identical between slopes–possibly a very important compensation for seedling size and vigor. Homeostasis (self—regulation) was evident as a gradation decreasing in the order seed viability (very strongly homeostatic)>seed composition (strong)>shoot and root composition (moderately strong)>average seed weight = shoot production (moderate)>seed number = mineral content (weak)>total root and seed production (no detectable homeostasis).

ISSN:0012-9615    

DOI:10.2307/1942412

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

The major upland plant community types of the Boundary Waters Canoe Area (BWCA) of northeastern Minnesota, identified by multivariate analyses (clustering and canonical and discriminant analysis) of 68 stands disturbed by logging and 106 stands undisturbed by logging, include the following: lichen, jack pine—oak, red pine, jack pine—black spruce, jack pine—fir, black spruce—feather moss, maple—oak, aspen—birch, aspen—birch—white pine, maple—aspen—birch, maple—aspen—birch—fir, fir—birch, and white cedar. Each of these types is based on a complex of 53 common species, through the name may incorrectly imply that one or two dominant overstory species are indicative of the type. Other forest stands from the BWCA are quantitatively related to the regional vegetation through discriminant analysis. Succession on the uplands in the area, without disturbance, leads to fir—birch and ultimately to the white—cedar community type. Whitetail deer may have had an impact on restricting the occurrence and reproduction of the white cedar type.

ISSN:0012-9615    

DOI:10.2307/1942413

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY