Apparent Density Dependence Research Articles

The albatross of assessing and managing risk for long-lived pelagic seabirds

Pelagic predators such as albatross have long been of conservation concern, but assessing their status poses numerous challenges. A standard monitoring method for albatross is colony-based nest counts to track numbers of breeders. However, a variable proportion of the population skips breeding in any given year and cannot be quantified by nest counts, creating several complications to efforts in understanding population dynamics. We used stochastic demographic matrix models for black-footed (Phoebastria nigripes) and Laysan (P. immutabilis) albatross to investigate: i) the potential for the skipping behavior of breeders to create apparent density dependence in nest counts, ii) the limitations to assessing population trends from nest counts and implications for evaluating impacts from fisheries bycatch, including calculating Potential Biological Removal values, and iii) the relative importance of at-sea versus on-island threats to population viability. We found the increased likelihood of these albatrosses skipping breeding following a successful season – a feature common to many seabirds and other taxa – results in substantial negative temporal auto-correlation in the observable population that can be misinterpreted as negative density dependence, with important implications for inferences about population viability. Black-footed albatross appear limited by fisheries bycatch, while Laysan albatross, which have low estimated bycatch mortality, are currently at greater risk from island-based threats. Our results suggest a cautionary approach to managing black-footed and Laysan albatross should be adopted because detecting population declines from nest counts could take decades. Ultimately, we highlight the inherent difficulties in assessing population status and trends in long-lived species such as albatross.

Interaction between colloidal particles on an oil–water interface in dilute and dense phases

The interaction between micron-sized charged colloidal particles at polar/non-polar liquid interfaces remains surprisingly poorly understood for a relatively simple physical chemistry system. By measuring the pair correlation function g(r) for different densities of polystyrene particles at the decane–water interface, and using a powerful predictor–corrector inversion scheme, effective pair-interaction potentials can be obtained up to fairly high densities, and these reproduce the experimental g(r) in forward simulations, so are self consistent. While at low densities these potentials agree with published dipole–dipole repulsion, measured by various methods, an apparent density dependence and long range attraction are obtained when the density is higher. This condition is thus explored in an alternative fashion, measuring the local mobility of colloids when confined by their neighbors. This method of extracting interaction potentials gives results that are consistent with dipolar repulsion throughout the concentration range, with the same magnitude as in the dilute limit. We are unable to rule out the density dependence based on the experimental accuracy of our data, but we show that incomplete equilibration of the experimental system, which would be possible despite long waiting times due to the very strong repulsions, is a possible cause of artefacts in the inverted potentials. We conclude that to within the precision of these measurements, the dilute pair potential remains valid at high density in this system.

Connecting Logistic Probability Models With Basic Dynamic Processes

The logistic distribution is a popular probability model yet it is usually not motivated with reference to the processes under study. While its popularity can be attributed to its simplicity, it can also be derived from basic contextual considerations. Although it has been shown that logistic growth is the limiting form of a class of Markov processes on a lattice, the connections of these microscopic models with statistics are less widely appreciated. We review some history of logistic distributions in classical models of infection, and describe how the apparent density dependence emerges as a consequence of a particular lattice embedding. We then review how logistic growth arises from microscopic random behavior. We also derive a “square-logistic” model from basic considerations. Finally, we describe how the underlying discrete model relates to ordinary logistic regression modeling of time dependence.

Long-term population demography of Trillium recurvatum on loess bluffs in western Tennessee, USA

Understanding the demography of long-lived clonal herbs, with their extreme modularity, requires knowledge of both their short- and long-term survival and ramet growth patterns. The primary objective of this study was to understand the dynamics of a clonal forest herb, Trillium recurvatum, by examining temporal and small-scale demographic patterns. We hypothesized: (i) there would be more variability in the juvenile age class compared with non-flowering adult and flowering adult classes due to year-to-year fluctuations in recruitment; (ii) rates of population growth (λ) and increase (r) would be highest in non-flowering ramets due to a combination of transitions from the juvenile stage and reversions from flowering adults; and (iii) inter-ramet distances would be most variable between flowering and juvenile ramets due to a combination of clonal growth, seed dispersal by ants and ramet death over time. Census data were collected on the total number of stems in the population from 1990 to 2007, and placed within one of three life stages (juvenile, three-leaf non-flowering and three-leaf flowering). Modified population viability equations were used to assess temporal population viability, and spatial structure was assessed using block krigging. Correlations were performed using current and prior season weather to current population demography. The first hypothesis was rejected. The second hypothesis was supported: population increase (r) and growth rate (λ) were highest in non-flowering ramets. Finally, the third hypothesis was rejected: there was no apparent density dependence within this population of Trillium and no apparent spatial structure among life stages. Overall population density fluctuated over time, possibly due to storms that move soil, and prior year's temperature and precipitation. However, density remained at some dynamic stable level. The juvenile age class had greater variability for the duration of this study and population growth rate was greatest for non-flowering ramets.

Partial seasonal migration in high-arctic Svalbard reindeer (Rangifer tarandus platyrhynchus)

We examined seasonal range use and calving success in wild Svalbard reindeer ( Rangifer tarandus platyrhynchus Vrolik, 1829) on two contrasting ranges separated by risky barriers (open sea, thin sea ice, and glaciers). One (“poor”) range had a depleted lichen resource and negative reindeer population trend, whereas the neighbouring (“rich”) range was recently occupied with initially high lichen abundance and positive population trend. Winter foraging conditions limit survival and reproduction in this predator-free system and lichens are the preferred winter food by reindeer. Accordingly, marked female reindeer that switched between the ranges (“migrants”) spent most winters on the lichen-rich range, yet most summers on the poor range (possibly owing to fidelity to calving area). While facing high mortality risk along the migration route, migrants spending the winter on the rich range and subsequent summer on the poor range had improved calving success compared with residents on the poor range. The partial seasonal migration pattern diminished as lichens were reduced and reindeer carrying capacity reached on the rich range. Besides this apparent density-dependence in migratory behaviour, spatial strategy seems shaped by past experience and trade-offs between current survival (safe habitat) and future reproduction (food-rich habitat).

Adult survival and microsatellite diversity in possums: effects of major histocompatibility complex-linked microsatellite diversity but not multilocus inbreeding estimators

Adult survival is perhaps the fitness parameter most important to population growth in long-lived species. Intrinsic and extrinsic covariates of survival are therefore likely to be important drivers of population dynamics. We used long-term mark-recapture data to identify genetic, individual and environmental covariates of local survival in a natural population of mountain brushtail possums (Trichosurus cunninghami). Rainfall and intra-individual diversity at microsatellite DNA markers were associated with increased local survival of adults and juveniles. We contrasted the performance of several microsatellite heterozygosity measures, including internal relatedness (IR), homozygosity by loci (HL) and the mean multilocus estimate of the squared difference in microsatellite allele sizes within an individual (mean d (2)). However, the strongest effect on survival was not associated with multilocus microsatellite diversity (which would indicate a genome-wide inbreeding effect), but a subset of two loci. This included a major histocompatibility complex (MHC)-linked marker and a putatively neutral microsatellite locus. For both loci, diversity measures incorporating allele size information had stronger associations with survival than measures based on heterozygosity, whether or not allele frequency information was included (such as IR). Increased survival was apparent among heterozygotes at the MHC-linked locus, but the benefits of heterozygosity to survival were reduced in heterozygotes with larger differences in allele size. The effect of heterozygosity on fitness-related traits was supported by data on endoparasites in a subset of the individuals studied in this population. There was no apparent density dependence in survival, nor an effect of sex, age or immigrant status. Our findings suggest that in the apparent absence of inbreeding, variation at specific loci can generate strong associations between fitness and diversity at linked markers.

Temporal trends in elephant Loxodonta africana numbers and densities in northern Botswana: is the population really increasing?

The apparent increase in elephant Loxodonta africana numbers in northern Botswana is of concern because it may affect other species. We compared changes in population growth rates based on elephant numbers and densities over 1973–2004. Population estimates and survey details extracted from published and unpublished sources allowed us to calculate growth rates. From 1973 to 1993 growth rate was positive when based on elephant numbers but did not differ from zero when calculated for densities. This discrepancy may be because of the significant increase in survey area during the same period. In contrast, none of the growth rates differed from zero for time series between 1996 and 2004, when the size of the survey area varied little. We propose two explanations for these results. The first suggests that the population did not grow, while the second proposes that the population expanded its range and increased in size. Notwithstanding, an equilibrium model best explained the variance in dry season estimates of elephant numbers for the complete time series. Such apparent density-dependence could be disrupted by any artificial reduction of numbers through culling as a management option in northern Botswana.

Impact of heat deposition profile on global confinement of NBI heated plasmas in the LHD**Original title in FEC2002, Lyon: response of temperature and density profiles to heat deposition profile and its impact on global scaling in LHD.

Energy confinement and heat transport of net-current-free NBI heated plasmas in the large helical device (LHD) are discussed with emphasis on density and power deposition profile dependences. Although the apparent density dependence of the energy confinement time has been demonstrated in a wide parameter range in LHD, the loss of this dependence has been observed in the high density regime under specific conditions. Broad heat deposition due to off-axis alignment and shallow penetration of neutral beams degrades the global energy confinement while the local heat transport maintains a clear temperature dependence, lying between Bohm and gyro-Bohm characteristics. The central heat deposition tends towards an intrinsic density dependence like τE∝(n̄e/P)0.6 from the state where density dependence is lost. The broadening of the temperature profile due to the broad heat deposition profile contrasts with the invariant property that has been observed widely as profile resilience or stiffness in tokamak experiments. The confinement improvement as a result of the inward shift of the magnetic axis is obvious in the core region, which emphasizes the improvement of transport because of the geometry being unfavourable for the central heating of NBI in this configuration. The edge pressure, clearly, does not depend on the magnetic axis position. Unlike a tokamak H-mode, the edge pressure is determined by transport and can be increased by increasing the heating power.

Variable age structure and apparent density dependence in survival of adult ungulates.

Large herbivores have strongly age-structured populations. Because recruitment often decreases as population density increases, in unexploited populations the proportion of older adults may increase with density. Because survival senescence is typical of ungulates, ignoring density-dependent changes in age structure could lead to apparent density-dependence in adult survival. To test for density dependence in adult survival, we used data from three populations that underwent considerable changes in density. Bighorn sheep (Ovis canadensis) on Ram Mountain, Alberta, ranged from 94 to 232, mountain goats (Oreamnos americanus) on Caw Ridge, Alberta, varied from 81 to 147, and estimates of roe deer (Capreolus capreolus) older than 1year at Chizé, France, ranged from 157 to 569. We used recent developments of capture-mark-recapture modelling to assess the response of adult survival to changes in density when age structure was and was not taken into account. Survival rates were 10-15% higher during the prime-age stage than during the senescent stage for all sex-species combinations. When adults were pooled into a single age class there was an apparent negative effect of density on female survival in bighorns and roe deer, and negative trends for female mountain goats, male roe deer and male bighorn sheep. When age class was taken into account, there were no significant effects of density on adult survival. Except for male mountain goats, the strength of density dependence was lower when age was taken into account. In ungulate populations, age structure is an important determinant of adult survival. Most reports of density dependence in adult survival may have been confounded by changes in age structure.

H mode transition threshold power scaling and its relation to the edge neutrals in JT-60U

As a result of an experiment at JT-60U on H mode transition power threshold scaling, which is one of the urgent ITER Physics R&D issues, it has been found that: (a) the derived scaling law of Pth(MW)=0.18ne0.5 (1019 m-3) BT1.0 (T) R1.5 (m) with presumed non-dimensional constraints predicts a threshold heating power of ⩽ 100 MW at 5 × 1019 m-3 for the ITER EDA design, and (b) an increase of the neutral particle density at the plasma edge results in a reduction of the edge ion collisionality just before the transition below unity and thereby an increase of the threshold heating power. It has also been found that both the low density boundary, below which the transition cannot occur, and the apparent density dependence of the threshold power scaling are related to the edge neutrals. It is suggested that a charge exchange process might well be the potential mechanism of the neutral effect

Measurement of Ascaris lumbricoides infection intensity and the dynamics of expulsion following treatment with mebendazole

Intensity of Ascaris lumbricoides infection was measured in terms of egg counts and worm burden in children 2-10 years of age. The expulsion of A. lumbricoides with a 3-day treatment of mebendazole occurred over 8 days, beginning on the second day of treatment. Ninety-seven percent of the worms were expelled between the second and seventh days. A rapid means of estimating eggs per gram (epg) by the Kato Katz technique correlated well with the method described by Martin & Beaver (1968). In spite of apparent density dependence in egg production, A. lumbricoides egg counts correlated well with worm burdens. It is concluded that, within the context of community surveys, epg is a reasonable means of identifying heavily infected individuals and that epg can be estimated rapidly by a slight modification to the standard Kato Katz technique.

Single Gaussian curve of growth abundance determinations from ultraviolet interstellar absorption-line data

view Abstract Citations (2) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Single Gaussian Curve of Growth Abundance Determinations from Ultraviolet Interstellar Absorption-Line Data Harris, A. W. Abstract Equivalent-width data sets used in published studies of interstellar element abundances in many sight lines are examined for evidence of saturation-related errors which give rise to, or enhance, the apparent correlations of depletion with mean hydrogen volume density, n-bar(H). It is shown that the same abundance data which exhibit a strong dependence on n-bar(H) show no tendency to correlate similarly with the strength of optical depth of the absorption lines, in contrast to the behavior expected if the apparent density dependence were due primarily to effects arising from line saturation. It is demonstrated that an apparent discrepancy between S II and P II b-values is essentially independent of the strength of the S II lines and n-bar(H). Publication: The Astrophysical Journal Pub Date: November 1987 DOI: 10.1086/165735 Bibcode: 1987ApJ...322..368H Keywords: Absorption Spectra; Abundance; Interstellar Gas; Iue; Ultraviolet Spectra; Depletion; Doppler Effect; Hydrogen; Astrophysics; INTERSTELLAR: ABUNDANCES full text sources ADS | data products SIMBAD (26) MAST (1)

Estimating the survival rate and mean longevity for adults in a field population of the green rice leafhopper, Nephotettix cincticepsUhler, by the application ofHokyo andKiritani's method

SummaryApplication of the female dissection method proposed byHokyo andKiritani (1967) was attempted in both 1968 and 1969 to estimate the daily survival rate and the mean longevity for the adult population of the green rice leafhopper, Nephotettix cincticeps, in a paddy field. The estimated mean longevity for females was far shorter than the physiological longevity of this species, ranging from 4 to 7 days with some variation between different generations. This could explain the remarkable discontinuity among successive generations which proved to form an important feature of the pattern of seasonal population changes of this insect. Between the two years, the estimated survival rate (and hence the mean longevity also) was negatively correlated to the estimated population size of adults that emerged in either of the two successive generations. This apparent density dependence suggests the possibility that the adult survival, including the effect of dispersal, plays some critical role in bringing about the remarkable population stability from year to year shown by the field population of N. cincticeps.