Search results for "Density"
showing 10 items of 4402 documents
Seed dispersal, braeding system, tree density and the spatial pattern of trees – a simulation approach
2002
Summary Tropical tree populations with low densities and with clumped spatial distributions are at risk in the face of fragmentation. It is therefore important to understand factors driving spatial patterns of tree populations. An important set of factors which may influence the spatial pattern of plants, could be the spatial distribution of possible seed sources such as the breeding system (monoecious, dioecious), tree density and the spatial pattern of the parent population. Another set of important factors might be dispersal of seeds away from the parent plants characterised by the mean dispersal distance and by the distribution of dispersal distances (either negative exponential or logn…
Long-term changes in collembolan communities in grazed and non-grazed abandoned arable fields in Denmark
2004
Summary In order to explore long-term changes in microarthropod communities after introduction of livestock grazing in abandoned fields with herb–grass vegetation at Mols, E. Jutland, Denmark, soil and litter samples were collected from 7 pairs (blocks) of grazed and non-grazed plots over a period of 14 years. Sampling began just before fencing and initiation of cattle and sheep grazing in the spring of 1985. The total material included 76 collembolan species; 65 and 68 species were recorded in the grazed and non-grazed plots, respectively. The number of species recorded at individual sampling dates fluctuated considerably through the period. In the vegetation/litter layer the mean number o…
Curvilinear interspecific density-range size relationship in small mammals in Finland
2013
ABSTRACTAim Macroecological patterns have mainly been depicted as atemporal, withexisting research covering only short time periods. One fundamental pattern inmacroecology is the interspecific relationship between local abundance andregional range size, which is generally considered to be positively linear. Here,we examine structural details of the relationship between abundance and rangesize in cyclic populations of small mammals and its long-term temporal varia-tion.Location Finland.Methods We analysed 39 years of trapping data of Rodentia and Soricomor-pha collected in field and forest habitats across Finland. Abundance was mea-sured as the mean population density of individuals, and range…
Population dynamics of Cyclotella ocellata (Bacillariophyceae): endogenous and exogenous factors
1999
Population dynamics of Cyclotella ocellata PANTOCSEK were examined from 1991 to 1993 in an oligo-mesotrophic, gravel-pit lake near Madrid (Spain). Monthly and daily sampling was carried out to track seasonal- and auxosporulation dynamics, respectively. This species was the most abundant planktonic diatom in the lake. The population increased steadily in spring until the seasonal peak (3.8.10 6 ind/ L in April) and then abundance declined slowly through late winter. Time series techniques showed that the seasonal dynamics were driven by exogenous factors: there was a relationship between Cyclotella abundance and the chemical environment, both TP (synchronic) and SRSi (with delay). Auxosporul…
Competition for breeding sites and site-dependent population regulation in a highly colonial seabird, the common guillemot Uria aalge
2004
Summary 1. The hypothesis of site-dependent population regulation predicts that birds utilize available nesting sites in a pre-emptive (ideal despotic) manner, leading to density dependence in heterogeneous habitats as poorer sites are used at higher population densities. At small population sizes adaptive site choice protects populations against fluctuations (the buffer effect). 2. Common guillemots Uria aalge (Pontoppidan) breed at high density on sea-cliffs. The population breeding on the Isle of May, Scotland increased by 60% between 1981 and 2000. A good nest-site is a prerequisite for successful breeding and there is much competition for the best sites. Throughout this period, site us…
Optimal rates of bisexual reproduction in cyclical parthenogens with density‐dependent growth
1999
This work explores theoretical patterns of reproduction that maximize the production of resting eggs and the long-term fitness of genotypes in cyclical parthenogens. Our focus is on density-dependent reproduction as it influences the consequences of a trade-off between producing amictic daughters – which reproduce parthenogenetically and subitaneously – and producing mictic daughters – which undergo meiosis and bisexual reproduction. Amictic females increase competitive ability and allow the population to achieve a larger size; mictic females directly contribute to population survival through harsh periods by producing resting eggs. Although morphologically indistinguishable, the two types …
Continental-scale patterns of pathogen prevalence: a case study on the corncrake
2014
Pathogen infections can represent a substantial threat to wild populations, especially those already limited in size. To determine how much variation in the pathogens observed among fragmented populations is caused by ecological factors, one needs to examine systems where host genetic diversity is consistent among the populations, thus controlling for any potentially confounding genetic effects. Here, we report geographic variation in haemosporidian infection among European populations of corncrake. This species now occurs in fragmented populations, but there is little genetic structure and equally high levels of genetic diversity among these populations. We observed a longitudinal gradient…
Changing female spacing behaviour and demography in an enclosed breeding population of Clethrionomys glareolus
1988
The social organization of Clethrionomys glareolus Schreber was studied by live trapping in an enclosure of 0.8 ha in Central Finland between the years 1982–1986. The enclosure consisted of three habitats: abandoned field, brushwood, and moist spruce heath forest. The population density increased from 50 voles ha−1 in 1982 and 1983 to 150 ha−1 in 1984 and 1985. In the summer of 1986 the population crashed to almost zero. In the summer of 1984 after winter breeding the number of breeding females grew to 22 compared with 6–7 in previous years. In 1984 the mature females changed from territorial to group behaviour. During the high population density in 1984 and 1985 breeding stopped already in…
Larval Arrest in Development of Tribolium castaneum (Coleoptera: Tenebrionidae)
1986
Arrested larval development (in the last larval instar) of part of the total larval population has been detected in moderately crowded situations (40 larvae in 2 g food) in Tribolium castaneum (Herbst) cultures This phenomenon is the same found previously in highly crowded cultures of Drosophila melanogaster (Meigen) and other related species. The arrest may be viewed as a mechanism of physiological adaptation of organisms to competitive situations.
Electronic and magnetic structure ofLa0.875Sr0.125MnO3calculated by means of hybrid density-functional theory
2007
We present the results of ab initio calculations on magnetic and electronic structures of La1�xSrxMnO3 at low doping, x =1/8. Using the B3LYP hybrid exchange-correlation functional within the framework of densityfunctional theory, we predict a ferromagnetic ground state for La0.875Sr0.125MnO3 in both the low-temperature orthorhombic and the high-temperature pseudocubic phases. This is in contrast to its parent compound LaMnO3, for which we find in agreement with experiment the layered antiferromagnetic state to be the most stable one. The calculated density of states and bond population analysis suggest a tendency of formation of half-metallic spin states in the band gap of both structures.