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Threshold Density (threshold + density)

Distribution by Scientific Domains

Life Sciences	87%

Selected Abstracts

Exciton,Exciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals

ADVANCED MATERIALS, Issue 48 2009
Michele Saba
Exciton,exciton interaction in dot/rod CdSe/CdS nanocrystals has proved to be very sensitive to the shape of nanocrystals, due to the unique band alignment between CdSe and CdS. Repulsive exciton,exciton interaction is demonstrated, which makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, with projected pump threshold densities below 1 kW cm,2 for continuous wave lasing. [source]

Predator size, prey size and threshold food densities of diving ducks: does a common prey base support fewer large animals?

JOURNAL OF ANIMAL ECOLOGY, Issue 5 2009
Samantha E. Richman
Summary 1. Allometry predicts that a given habitat area or common prey biomass supports fewer numbers of larger than smaller predators; however, birds from related taxa or the same feeding guild often deviate from this pattern. In particular, foraging costs of birds may differ among locomotor modes, while intake rates vary with accessibility, handling times and energy content of different-sized prey. Such mechanisms might affect threshold prey densities needed for energy balance, and thus relative numbers of different-sized predators in habitats with varying prey patches. 2. We compared the foraging profitability (energy gain minus cost) of two diving ducks: smaller lesser scaup (Aythya affinis, 450,1090 g) and larger white-winged scoters (Melanitta fusca, 950,1800 g). Calculations were based on past measurements of dive costs with respirometry, and of intake rates of a common bivalve prey ranging in size, energy content and burial depth in sediments. 3. For scaup feeding on small prey <12 mm long, all clams buried deeper than 5 cm were unprofitable at realistic prey densities. For clams buried in the top 5 cm, the profitability threshold decreased from 216 to 34 clams m,2 as energy content increased from 50 to 300 J clam,1. 4. For larger scoters feeding on larger prey 18,24 mm long, foraging was profitable for clams buried deeper than 5 cm, with a threshold density of 147 m,2 for clams containing 380 J clam,1. For clams <5 cm deep, the threshold density decreased from 86 to 36 clams m,2 as energy content increased from 380 to 850 J clam,1. If scoters decreased dive costs by swimming with wings as well as feet (not an option for scaup), threshold prey densities were 11,12% lower. 5. Our results show that threshold densities of total prey numbers for different-sized ducks depend on prey size structure and depth in the sediments. Thus, heterogeneity in disturbance regimes and prey population dynamics can create a mosaic of patches favouring large or small predators. Whether a given area or total prey biomass will support greater numbers of larger or smaller predators will vary with these effects. [source]

Mutualism as a constraint on invasion success for legumes and rhizobia

DIVERSITY AND DISTRIBUTIONS, Issue 3 2001
Matthew A. Parker
Abstract Because hereditary symbiont transmission is normally absent in the mutualism of legume plants and root-nodule bacteria (rhizobia), dispersing plants may often arrive at new habitats where mutualist partners are too rare to provide full benefits. Factors governing invasion success were explored by analysing a system of two coupled pairwise competition models: a legume invader competing with a resident non-mutualistic plant, and a rhizobial population competing with a resident population of nonsymbiotic bacteria. The non-linear dependence of benefits on partner abundance in this mutualism creates the possibility of two alternative population size equilibria, so that a threshold density can exist for invasion. If legumes and rhizobia exceed a critical population size, both species achieve rapid population growth, while if initial densities of both species are below their respective thresholds, they remain rare and are thus vulnerable to extinction in the presence of competitors. Overall, the results indicate that legumes may often fail at colonization attempts within habitats where mutualist partners are scarce. Data on legume prevalence in island floras and rates of geographical spread by legume weeds are consistent with this inference. Predictive insights about invasiveness may emerge from comparative research on key traits identified by the model, especially the shape of the function determining the number of nodules formed at low rhizobial density. [source]

Density-mediated responses of bark beetles to host allelochemicals: a link between individual behaviour and population dynamics

ECOLOGICAL ENTOMOLOGY, Issue 4 2002
Kimberly F. Wallin
Abstract ,1. Bark beetles (Coleoptera: Scolytidae) accept or reject host conifers based partly on concentrations of phloem monoterpenes. They colonise trees in aggregations, in response to pheromones that attract flying beetles to trees undergoing colonisation. A series of entry and gallery construction assays was conducted to determine whether responses by individual beetles to monoterpenes are altered by pheromones and/or the presence of other beetles. 2. Entry into the amended media by Ips pini and the length of time until entry were not influenced by the presence of aggregation pheromones. 3. Entry into amended media was influenced by the presence of other beetles on the surface of, or constructing galleries in, the substrate. The effects of alpha-pinene and limonene on host entry behaviour were mediated by the density of beetles on the surface of the assay arena, and by the density of beetles constructing galleries within the medium. 4. The percentage of beetles entering medium amended with higher concentrations of monoterpenes increased with increased density of beetles on the surface of the assay arena, until a threshold density of three or four beetles per assay arena, after which entrance rate declined. 5. The presence of other beetles constructing galleries elicited more rapid entry by the test beetles. 6. Gallery lengths were generally higher in the presence of aggregation pheromones. 7. Gallery lengths increased with increased density of beetles within the assay arena. 8. These results suggest a link between the density of bark beetles and responses of individuals. This linkage may partially explain behavioural changes observed during population eruptions. [source]

Predator size, prey size and threshold food densities of diving ducks: does a common prey base support fewer large animals?

Contrasting interference functions and foraging dispersion in two species of shorebird (Charadrii)

JOURNAL OF ANIMAL ECOLOGY, Issue 2 2000
Michael G. Yates
Summary 1.,Above a threshold density of , 100 birds ha -1, strong interference occurred between redshank Tringa totanus (Linnaeus) feeding by sight on the amphipod crustacean Corophium volutator (Pallas). No aggressive interactions occurred between the birds and the probable cause was prey depression. 2.,Redshank fed in a square metre of mud that had recently been exploited by another redshank much less often than would be expected by chance. By avoiding areas where prey would have been recently exploited, the feeding rate of redshank was up to three times faster than it would have been had they not avoided other foraging redshank. 3.,Bar-tailed godwit fed in a square metre of mud that had been recently exploited by another godwit much more often than would be expected by chance in randomly moving birds. They tended to flock while foraging and showed no tendency to avoid areas where prey would have been recently exploited. 4.,There was no evidence that interference occurred between bar-tailed godwit Limosa lapponica (Linnaeus) feeding on the polychaete lugworm Arenicola marina (Linnaeus) at densities below 300 birds ha -1, even though aggressive interactions occurred between birds. [source]

Ecology of wildlife rabies in Europe

MAMMAL REVIEW, Issue 1 2006
KATJA HOLMALA
ABSTRACT 1The number of wildlife rabies cases has increased in Europe in recent years. We review the epizootiology of wildlife rabies in Europe, paying special attention to recent changes to the situation of two important vector species: the red fox and the raccoon dog. Red fox Vulpes vulpes has been the main vector of rabies since 1945, but the number and proportion of raccoon dog Nyctereutes procyonoides cases has rapidly increased during the past few years, particularly in north-eastern Europe. 2The transmission rate (average number of susceptible animals infected by each rabid animal) is critical for rabies spread and is partly determined by population density. Both raccoon dogs and foxes live in pairs. Foxes also live in family groups. Pairs and groups share their territories. Home range size usually correlates negatively with population density. Fox home ranges are 50,1500 ha, those of raccoon dogs 150,700 ha. The threshold value for rabies spread among foxes is estimated to be 0.63 individuals/km2. Although fox density in eastern and northern Europe may be lower than this, the pooled density of foxes and raccoon dogs exceeds the threshold density. 3Animal movements, especially dispersal of young, pose a risk for rabies spread. Although the likelihood of an epizootic is highest where fox and raccoon dog densities are highest, rabies may spread fastest where population densities are lower, because dispersal distances tend to correlate negatively with population density. 4Oral vaccinations have been more effective in rabies control than culling foxes. Where two vector species exist, vaccination should be conducted twice a year, because most raccoon dogs disperse in autumn but some foxes do not disperse before mid- or late winter. 5New rabies models, based on two vector species and their interaction, and which take into account the hibernation period of raccoon dogs, are needed for north-eastern Europe. [source]

Myths and facts on ratadas: Bamboo blooms, rainfall peaks and rodent outbreaks in South America

AUSTRAL ECOLOGY, Issue 3 2003
FABIAN M. JAKSIC
Abstract, ,Ratadas' are rodent irruptions or outbreaks that have been recorded in South America since the Spanish conquest in the 16th century. The notion that ratadas are associated with bamboo flowering and subsequent mast seeding at cycles of 30 years has appeared in the literature since the late 1800s. Based on 63 well-documented cases, we show that not only are ratadas associated with bamboo blooming, but also many are associated with rainfall peaks, and that these two outbreak types are geographically interspersed over South America. In addition, we dispel the notion that South American bamboo blooms occur every 30 years, which may only be the case for Merostachys fistulosa. For other species the modal cycles occur every 14 (Merostachys spp.), 12 (Chusquea quila and/or valdiviensis) or 14 years (Chusquea coleou). We also propose the hypothesis that rainfall-associated ratadas are ultimately caused by the occurrence of El Niño, and discuss the possible population dynamic mechanisms underlying rodent outbreaks in South America. Aiming at setting a standardized framework for spatial and temporal comparisons, we propose a trapping protocol and a threshold density for assigning the ,ratada' label. Several of the mice implicated in ratadas are reservoirs of emerging diseases, thus emphasizing the need for predictive power to forecast disease epidemics that affect human populations. Further, ratadas may be viewed as pulsed resources, thus enabling us to learn more of the ways communities respond to such intermittent inputs. [source]