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171.
Structural properties of two types of mangrove stands on the northwestern coast of Sri Lanka 总被引:2,自引:1,他引:1
Mangrove stands in Puttalam lagoon and Dutch bay, two interconnected lagoons situated on the northwestern coast of Sri Lanka have been classified broadly into two groups, i.e., estuarine and island/mainland-fringing stands. Structural diversity of six mangrove stands, representing the two types was studied in terms of floristic composition, density, basal area, mean stand diameter, tree height, standing above-ground biomass and leaf-area index.Rhizophora mucronata andAvicennia marina were the dominant species. Higher mean stand diameters for the mangrove stands in Puttalam lagoon indicated greater maturity than the estuarine mangrove stands in Dutch bay. Nevertheless, estuarine stands in Dutch bay were structurally more complex (complexity indices 8.11–22.7) than the island/mainland-fringing mangrove stands (complexity indices 1.38–6.78). Higher number of species present in the estuarine mangrove stands is the major element that contributes to the higher values for the complexity indices for those stands. This appears to mask the contribution of stand-age to the complexity of a mangrove stand. Therefore complexity indices alone may not be used to explain adequately the structural diversity among mangrove stands. 相似文献
172.
The stomach contents were analyzed monthly for each year-class to elucidate the foraging pattern of bluegills in a small vegetated lake by the frequency occurrence and the points methods. Seasonal dietary changes of the year-classes were considered comparing the monthly fluctuations in abundance of major prey organisms. Though these bluegills are dietary generalists and opportunists like those in North America, their foraging pattern was characterized by a relatively clearer dietary shift during ontogeny and a wider food niche including piscivorous than those of bluegills with congeners in their home land. Therefore this finding provides evidence of the ecological release caused by the absence of congeners. 相似文献
173.
Eizi Kuno 《Population Ecology》1992,34(2):275-284
A simple differential equation model was developed to describe the competitive interaction that may occur between species through reproductive interference. The model has the form comparable to Volterra's competition equations, and the graphical analysis of the outcome of the two-species interaction based on its zero-growth isoclines proved that: (1) The possible outcome in this model, as in usual models of resource competition, is either stable coexistence of both species or gradual exclusion of one species by the other, depending critically upon the values of the activity overlapping coefficient cij; (2) but, for the same cij-values, competitive exclusion is much more ready to occur here than in resource competition; (3) and moreover, the final result of the competition is always dependent on the initial-condition due to its non-linear isoclines, i.e., even under the parameter condition that generally allows both species to coexist, an extreme bias in intial density to one species can readily cause subsequent complete exclusion of its counterparts. Thus, it may follow that the reproductive interference is likely to be working in nature as an efficient mechanism to bring about habitat partitioning in either time or space between some closely related species in insect communities, even though they inhabit heterogeneous habitats where resource competition rarely occurs so that they could otherwise attain steady coexistence. 相似文献
174.
Knowiedge on community structure of North Sea meiofauna has greatly increased recently. A quasisynoptic picture of meiofauna
densities and copepod community structure from 171 stations of the southern North Sea, sampled in April–May 1986, has been
obtained during the North Sea Benthos Survey. Latitudinal patterns in meiofauna abundance and copepod weight, abundance and
diversity exist in an area between 51°30′N and 58°30′N. Using TWINSPAN-classification five major groups of copepod species
can be recognized which are related to sediment type, latitude and depth. The part of the meiofauna in total benthic energy
flow, their role in the benthic food web and in biogeochemical cycles is discussed based on existing literature. There are
still considerable gaps in knowledge and the field is not progressing rapidly.
Publication no. 599 Netherlands Institute of Ecology, Centre for Estuarine and Coastal Ecology, Yerseke, The Netherlands. 相似文献
175.
Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: changes in nutrient concentrations,phytoplankton and zoobenthos 总被引:2,自引:2,他引:0
Iwakuma Toshio Hayashi Hidetake Yasuda Ikuko Hanazato Takayuki Takada Kaori 《Hydrobiologia》1990,(1):141-152
Large bag-type (75 m3) and tube-type (105 m3) enclosures were set up in the shallow eutrophic Lake Suwa and were each stocked with exotic planktivorous whitefish (Coregonus lavaretus maraena). The release of whitefish caused the increase in nutrient concentration in the tube-type enclosure whereas no such increase
was observed in the bag-type enclosure. Bottom sediment seemed to be an important source of chironomid food for whitefish.
The proportion of phytoplankton measuring<10μm and 20–40μm, which respectively corresponded toOchromonas spp. andCryptomonas sp., were lower in the fish enclosures than in the control, which might have been caused by high grazing pressure by rotifers.
The predation by whitefish might have affected the species composition of phytoplankton through reducing copepod predation
on rotifers, not through reducing the densities of cladocerans which directly feed on phytoplankton as many investigators
have reported. The phytoplankton biomass was not affected much by the release of fish. Possible reasons are that the increase
in density of rotifers reduced the biomass of available phytoplankton and also that inedible Cyanophyceae were in the decreasing
phase of their seasonal succession and could not increase successfully in spite of elevated nutrient levels. 相似文献
176.
Estimating predation risk in zooplankton communities: the importance of vertical overlap 总被引:1,自引:1,他引:0
In order to estimate predation risk in nature, two basic components of predation need to be quantified: prey vulnerability, and density risk. Prey vulnerability can be estimated from clearance rates obtained from enclosure experiments with and without predators. Density risk is a function of predator density, and the spatial and temporal overlap of the predator and prey populations. In the current study we examine the importance of the vertical component of overlap in making accurate estimates of predation risk from the invertebrate predator Mesocyclops edax on rotifer versus crustacean prey. The results indicate that assumptions of uniform predator and prey densities cause a significant underestimation of predation risk for many crustacean prey due to the coincident vertical migration of these prey with the predator. The assumption of uniformity is more reasonable for estimating predation risk for most rotifer prey. 相似文献
177.
Macroinvertebrate density, biomass and drift were studied from moss-covered and moss-free channels in the South Fork Salmon River, Idaho. Insect densities were compared for 10 different substrate types and locations involving moss (Fontinalis neo-mexicana), sand, pebbles and cobbles. An ANOVA test demonstrated that insect densities varied significantly with substrate type (P < 0.05), and that total insect density in moss clumps differed significantly from densities in mineral substrates. Insect densities were 4–18 times greater in moss clumps than in mineral substrates under and adjacent to moss; sands under moss supported the lowest densities. During most tests, densities in pebble and cobble substrates adjacent to moss clumps were not significantly different from those found in similar substrates in the moss-free channel. The 20% moss-covered channel had 1.6 to 7.2 greater insect density and 1.4 to 6.1 greater biomass than did the moss-free channel for the tests conducted. Generally, midges (Chironomidae) made up over 50% of the insect community; annelids were the principal non-insect invertebrates.In spite of greater insect density and biomass in a moss-covered than in the moss-free channel, we did not demonstrate universally increased drift of the immature stages from the moss-covered channel, at least during daylight hours. As a consequence, we infer that salmonid fishes, feeding primarily on drifting insects during the daytime, may not derive increased caloric benefit from moss habitats until the insects emerge as adults. 相似文献
178.
Bag-type enclosures (75 m3) with bottom sheets and tube-type enclosures (105 m3) open to the bottom sediment were stocked with exotic whitefish (Coregonus lavaretus maraena) to study their predation effects on the plankton community. The fish fed mainly on adult chironomids during the period of
their emergence (earlier part of the experimental period). Thereafter, the food preference was shifted to larvae of chironomids
and crustacean zooplankters. The predation effects on the plankton community were not evident in the bag-type enclosures where
zooplankton densities were consistently low. The fish reduced the crustacean populations composed ofBosmina fatalis, B. longirostris andCyclops vicinus in the tube-type enclosures where the prey density was high (above ca. 50 individuals 1−1). The results suggested that the intensity of predation depended on the prey density. Rotifers increased in the fish enclosure,
probably becauseCoregonus reduced the predation pressure byCyclops vicinus on rotifers and allowed the latter to increase. In the fish enclosures, no marked changes in species composition were observed.
Zooplankton predated by the fish seemed to be distributed near the walls of the enclosures. Problems of enclosure experiments
for examining the effects of fish predation on pelagic zooplankton communities are discussed. 相似文献
179.
Gray A. Williams 《Hydrobiologia》1990,193(1):139-146
The role of littorinids in structuring communities is discussed. On hard substrates preferential grazing of the dominant algae often enables competitively inferior species to utilize the rock substrate. Investigations of these systems has led to the proposal of a number of factors that should be analyzed in order to assess the effect of herbivores on community structure. Some of these factors have been investigated for Littorina mariae Sacchi & Rastelli. L. mariae is a micro-epiphytic grazer browsing the surface covering of epiphytes off the alga Fucus serratus (L.). The relationship between the host alga, F. serratus, and L. mariae is far more intricate than that between winkles and hard substrates as the alga itself is a dynamic resource. L. mariae is spatially and temporally linked to F. serratus. L. mariae is found almost exclusively on the alga and is positively attracted by extracts of the alga. The life history of the winkle is closely synchronized with that of the alga (the winter decrease of L. mariae populations is associated with the seasonal die-back of the host alga). On analysis of the factors considered important to assess herbivory, and examples of the effects of other epiphytic herbivores on algal success, it is suggested that L. mariae could potentially play an important role in structuring the community of it's host fucoid and that this may influence larger scale community structure. Experimental manipulations are proposed to evaluate these hypotheses. 相似文献
180.
McQueen Donald J. Johannes Mark R. S. Lafontaine Nathalie R. Young Andrew S. Longbotham Eric Lean David R. S. 《Hydrobiologia》1990,(1):337-341
We used two analyses to test the hypothesis that planktivore abundances contribute to the residual variations of Secchi depth
or chlorophyll-a plotted with respect to mean summer epilimnetic total phosphorus. The first analysis involved 15 lake years of data from
six lakes. The data set comprised mark-recapture assessments of piscivore and planktivore numbers and estimates of mean summer
chlorophyll-a, total phosphorus and Secchi depth. We found that residual chlorophyll-a variation was not significantly (p>0.05) correlated with planktivore densities, but that planktivore densities did contribute (p<0.02) to the residual variation of Secchi depth on mean total phosphorus. The second analysis included all of the data used
in the first plus an additional 13 lake years of data from the literature. These data showed that the percentage of the total
fish community comprising planktivores did not significantly (p>0.05) contribute to the residual variation in chlorophyll-a with respect to mean summer total phosphorus. Together, our results suggest that planktivore abundance has a significant
cascading impact on water clarity, but no long term statistically significant impact on mean summer chlorophyll-a concentration. 相似文献