Role Of Density Dependence Research Articles

Density Dependence in Three Snake River Sockeye Salmon Nursery Lakes in Central Idaho

AbstractSnake River Sockeye Salmon Oncorhynchus nerka, listed as an endangered species in 1991, currently inhabit three nursery lakes (Redfish, Pettit, and Alturas lakes) in the Sawtooth Valley, Idaho. Conspecific kokanee (lacustrine Sockeye Salmon) are also present in the lakes. Snake River Sockeye Salmon recovery efforts, initially focused on genetic conservation, are now attempting to rebuild naturally spawning populations using hatchery supplementation. However, in Sockeye Salmon nursery lakes, density dependence is frequently observed when elevated O. nerka abundance leads to declines in zooplankton biomass, body size, and shifts in community composition. In turn, these changes lead to reductions in juvenile O. nerka growth rates, survival, and adult returns. We examined a long‐term data set of O. nerka population metrics and associated zooplankton community metrics. We found evidence of density dependence within and among nursery lakes. We detected differences in zooplankton biomass, lengths of preferred zooplankton prey (Daphnia spp. and cyclopoid copepods), parr growth rates, and age‐1 smolt size among the three lakes. We found negative relationships between O. nerka density and zooplankton biomass and size. We identified positive relationships between zooplankton biomass and two response variables: smolt size at migration and growth rates of hatchery parr. The relationships were generally similar among lakes. Variable outcomes were a result of differences in O. nerka density (or zooplankton biomass), controlled primarily by the relative proportion of spawning and rearing habitat in each lake. Understanding unique lake habitats, ecological interactions, and the role of density dependence is germane to management of Snake River Sockeye Salmon populations.

Structural Uncertainty in Onchocerciasis Transmission Models Influences the Estimation of Elimination Thresholds and Selection of Age Groups for Seromonitoring.

BackgroundThe World Health Organization recommends monitoring Onchocerca volvulus Ov16 serology in children aged <10 years for stopping mass ivermectin administration. Transmission models can help to identify the most informative age groups for serological monitoring and investigate the discriminatory power of serology-based elimination thresholds. Model predictions depend on assumed age-exposure patterns and transmission efficiency at low infection levels.MethodsThe individual-based transmission model, EPIONCHO-IBM, was used to assess (1) the most informative age groups for serological monitoring using receiver operating characteristic curves for different elimination thresholds under various age-dependent exposure assumptions, including those of ONCHOSIM (another widely used model), and (2) the influence of within-human density-dependent parasite establishment (included in EPIONCHO-IBM but not ONCHOSIM) on positive predictive values for different serological thresholds.ResultsWhen assuming EPIONCHO-IBM exposure patterns, children aged <10 years are the most informative for seromonitoring; when assuming ONCHOSIM exposure patterns, 5–14 year olds are the most informative (as published elsewhere). Omitting density-dependent parasite establishment results in more lenient seroprevalence thresholds, even for higher baseline infection prevalence and shorter treatment durations.ConclusionsSelecting appropriate seromonitoring age groups depends critically on age-dependent exposure patterns. The role of density dependence on elimination thresholds largely explains differing EPIONCHO-IBM and ONCHOSIM elimination predictions.

Immature survival, fertility, and density dependence drive global population dynamics in a long‐lived species

Disentangling the influence of demographic parameters and the role of density dependence on species' population dynamics is a challenge, especially when fractions of the population are unobservable. Additionally, due to the difficulty of gathering data at large spatial scales, most studies ignore the global dynamic of a species, which would integrate local heterogeneity dynamics and remove the noise of dispersal. We developed an integrated population model (IPM) at a global scale to disentangle the main demographic drivers of population dynamics in a long-lived species. We used 28yr of Audouin's Gull demographic data encompassing 69 local patches (comprising 90% of the world population). Importantly, we took into account the unobservable fraction of non-breeders and also assessed the strength of density dependence for this fraction of the population. As predicted by life histories of long-lived organisms, temporal random variation in survival was highest for immature individuals (1.326, 95% credible interval [CRI] 1.290-1.940) and lowest for adults (0.499, 95% CRI 0.487-0.720). Large temporal fluctuations in the probability of taking a reproductive sabbatical would partly explain the consistency in adult survival, with individuals most likely refraining from breeding when environmental conditions were harsh. Immature survival and fertility were the main drivers of population dynamics during the study period (r2 =0.83, 0.77-0.87 and 0.73, 0.63-0.79, respectively). We found strong evidence of density dependence, not only due to the number of breeders (r2 =-0.34, -0.43 to -0.24) but also due to individuals on sabbatical (r2 =-0.18, -0.33 to -0.01). From a conservation point of view, the species shows a 5% annual global decrease during the last 10 years, and we propose an update of its conservation status. Even though population dynamics of long-lived organisms are very sensitive to changes in adult survival, we show here that, in the absence of strong environmental perturbations affecting this vital rate, fluctuations in population density are mainly driven by variations in survival of immature individuals and fertility. Integrated models based on long-term monitoring at a global scale may enhance our ecological and evolutionary understanding of how demographic drivers influence population dynamics.

Past disturbances and intraspecific competition as drivers of spatial pattern in primary spruce forests

AbstractThe role of density dependence in shaping spatial patterns in tree distributions presumably changes throughout stand development. However, empirical investigations into developmental processes are often limited by a lack of long‐term data on disturbance history, which further limits the ability to assess the role of spatial variation in site conditions (e.g., slope, aspect, mean annual temperature). This study included data from 289 plots within 26 primary forest stands of the Carpathian Mountains; stands were dominated by Norway spruce (Picea abies) and driven by mixed‐severity disturbance regimes. We assessed spatial patterns in living tree positions, tree diameters, and the relative position of living trees to dead trees. Random forest classification was used to discriminate between disturbance history, tree density, and site conditions and their effects on the observed spatial patterns. At the stand scale, distances between trees of equal diameter were more uniform that expected (tree diameter was showing repulsion), while tree positions and dead trees were mostly distributed randomly. The processes that best explained the spatial patterns were identified as self‐thinning mortality and past disturbances (100–150 yr). This study demonstrated that the plot and stand‐scale spatial patterns resulted from the combination of past disturbances and density‐dependent legacies derived from earlier forest development stages.

Sexual segregation in North American elk: the role of density dependence.

We investigated how density-dependent processes and subsequent variation in nutritional condition of individuals influenced both timing and duration of sexual segregation and selection of resources. During 1999–2001, we experimentally created two population densities of North American elk (Cervus elaphus), a high-density population at 20 elk/km2, and a low-density population at 4 elk/km2 to test hypotheses relative to timing and duration of sexual segregation and variation in selection of resources. We used multi-response permutation procedures to investigate patterns of sexual segregation, and resource selection functions to document differences in selection of resources by individuals in high- and low-density populations during sexual segregation and aggregation. The duration of sexual segregation was 2 months longer in the high-density population and likely was influenced by individuals in poorer nutritional condition, which corresponded with later conception and parturition, than at low density. Males and females in the high-density population overlapped in selection of resources to a greater extent than in the low-density population, probably resulting from density-dependent effects of increased intraspecific competition and lower availability of resources.

Crossing the Threshold: The Role of Density Dependence and Demographic Stochasticity in the Evolution of Cooperation

Crossing the Threshold: The Role of Density Dependence and Demographic Stochasticity in the Evolution of Cooperation

Crossing the Threshold: the role of density dependence and demographic stochasticity in the evolution of cooperation

We make two simplifications to a joint population model developed by M. Doebeli et al. of two populations whose growth rates depend on total population density and pay-offs governed by the Iterated Prisoner’s Dilemma. One population uses the ‘Always Defect’ strategy and the second uses the ‘Tit for Tat’ (TFT) strategy. In the deterministic model, there are two simple basins of attraction that lead to the extinction of one or the other population. In particular, a small TFT population cannot spread from rarity. We compute the boundary between these two regions. If, on the other hand, the growth rate of the TFT population is stochastic, then it is possible for the TFT population to become established if the growth rate at any given time is sufficiently large to allow the TFT population to cross the threshold computed in the deterministic model. We describe the factors that increase the likelihood of TFT establishment and explain why density dependence is an essential feature of the model. In particular, we ...

Anatomy of a population cycle: the role of density dependence and demographic variability on numerical instability and periodicity

Determining the causes of cyclic fluctuations in population size is a central tenet in population ecology and provides insights into population regulatory mechanisms. We have a firm understanding of how direct and delayed density dependence affects population stability and cyclic dynamics, but there remains considerable uncertainty in the specific processes contributing to demographic variability and consequent change in cyclic propensity. Spatiotemporal variability in cyclic propensity, including recent attenuation or loss of cyclicity among several temperate populations and the implications of habitat fragmentation and climate change on this pattern, highlights the heightened need to understand processes underlying cyclic variation. Because these stressors can differentially impact survival and productivity and thereby impose variable time delays in density dependence, there is a specific need to elucidate how demographic vital rates interact with the type and action of density dependence to contribute to population stability and cyclic variation. Here, we address this knowledge gap by comparing the stability of time series derived from general and species-specific (Canada lynx: Lynx canadensis; small rodents: Microtus, Lemmus and Clethrionomys spp.) matrix population models, which vary in their demographic rates and the direct action of density dependence. Our results reveal that density dependence acting exclusively on survival as opposed to productivity is destabilizing, suggesting that a shift in the action of population regulation toward reproductive output may decrease cyclic propensity and cycle amplitude. This result was the same whether delayed density dependence was pulsatile and acted on a single time period (e.g. t-1, t-2 or t-3) vs. more constant by affecting a successive range of years (e.g. t-1,…, t-3). Consistent with our general models, reductions in reproductive potential in both the lynx and small rodent systems led to notably large drops in cyclic propensity and amplitude, suggesting that changes in this vital rate may contribute to the spatial or temporal variability observed in the cyclic dynamics of both systems. Collectively, our results reveal that the type of density dependence and its effect on different demographic parameters can profoundly influence numeric stability and cyclic propensity and therefore may shift populations across the cyclic-to-noncyclic boundary.

Tests of density dependence using indices of relative abundance in a deer population

A major question in animal ecology is explaining the causes of population fluctuations. Consensus about the most reliable method to detect density dependence (DD) or environmental effects from time‐series data, however, has not yet been achieved. Times series analyses have been used with indices of relative abundance in numerous studies, although these indices are rarely validated. Here, we used three different time series of relative abundance (number of deer seen per hunter per day, hunting success and proportion of males in the harvest) to explore direct and delayed DD in a white‐tailed deer Odocoileus virginianus population on Anticosti Island, Québec, Canada. Three mathematical approaches were tested: linear models, autoregressive (AR) models, and total DD in life history. Tests of DD using different indices of abundance on the same population should lead to similar results if all indices exhibit similar behaviour. Indices of relative abundance correlated with each other, although sometimes weakly, such that we obtained similar DD estimates with each index using detrended non‐stationary series. In most time series, linear regression of Nt−1 and Nt and AR models did not detect DD, while we obtained strong evidence for DD from the life‐history approach. This meant that contrasting conclusions about the role of density dependence within this population were reached depending on which method was used. We conclude that the method that incorporates most biological realism, the life‐history approach, provided a different result than classical interpretation of autoregressive coefficients. Only the life‐history interpretation supported our a priori belief that density dependence operating through competition for food regulates the Anticosti deer population. Phenomenological analysis aiming to investigate changes in abundance should be carefully conducted as the use of inappropriate indices or methods could lead to inappropriate conclusions or management strategies. Preferably, the method used should match the time scale of the population sampling regime and species life history.

Long-term demographic analysis in goshawk Accipiter gentilis: the role of density dependence and stochasticity

Density dependence and environmental stochasticity are both potentially important processes influencing population demography and long-term population growth. Quantifying the importance of these two processes for population growth requires both long-term population as well as individual-based data. I use a 30-year data set on a goshawk Accipiter gentilis population from Eastern Westphalia, Germany, to describe the key vital rate elements to which the growth rate is most sensitive and test how environmental stochasticity and density dependence affect long-term population growth. The asymptotic growth rate of the fully age-structured mean matrix model was very similar to the observed one (0.7% vs. 0.3% per annum), and population growth was most elastic to changes in survival rate at age classes 1-3. Environmental stochasticity led only to a small change in the projected population growth rate (between -0.16% and 0.67%) and did not change the elasticities qualitatively, suggesting that the goshawk life history of early reproduction coupled with high annual fertility buffers against a variable environment. Age classes most crucial to population growth were those in which density dependence seemed to act most strongly. This emphasises the importance of density dependence as a regulatory mechanism in this goshawk population. It also provides a mechanism that might enable the population to recover from population lows, because a mean matrix model incorporating observed functional responses of both vital rates to population density coupled with environmental stochasticity reduced long-term extinction risk of 30% under density-independent environmental stochasticity and 60% under demographic stochasticity to zero.

The Role of Density Dependence in Growth Patterns of Ceded Territory Walleye Populations of Northern Wisconsin: Effects of Changing Management Regimes

Abstract We assessed density-related changes in growth of walleye Sander vitreus in the ceded territory of northern Wisconsin from 1977 to 1999. We used asymptotic length (L ∞), growth rate near t 0 (ω), and body condition as measures of walleye growth to determine the relationship between growth and density. Among lakes, there was weak evidence of density-dependent growth: Adult density explained only 0–6% of the variability in the growth metrics. Within lakes, growth was density dependent, L ∞, ω, and body condition of walleyes changing with density for 69, 28, and 62% of the populations examined, respectively. Our results suggest that walleye growth was density dependent within individual lakes. However, growth was not coherently density dependent among lakes, which was possibly due to inherent differences in the productivity, surface area, forage base, landscape position, species composition, and management regime of lakes in the ceded territory. Densities of adult walleyes averaged 8.3 fish/ha and di...

Further explorations of Skyrme–Hartree–Fock–Bogoliubov mass formulas: I: Role of density dependence in pairing force

The HFB-2 mass formula is generalized to make the δ-function pairing force density-dependent. It is shown that the mass data rule out the simple model of a pairing force that vanishes completely in the nuclear interior. Consistency with the mass data is found for a fairly wide range of δ-function pairing forces with a partial weakening in the nuclear interior. In particular, the form of density dependence determined by the realistic nuclear-matter calculations of Garrido et al. is shown to be compatible with the mass data, 2135 measured masses being fitted with an rms error of 0.656 MeV. On this basis we construct a new mass table, HFB-3, running from one drip line to the other. Shell quenching at the neutron-drip line is now somewhat stronger than before, but otherwise the new mass formula does not differ in any conspicuous way from the HFB-2 mass formula.

Dynamics of springtail and mite populations: the role of density dependence, predation, and weather

Abstract 1. Ecological theory suggests that density‐dependent regulation of organism abundance will vary from exogenous to endogenous factors depending on trophic structure. Changes in abundance of soil arthropods were investigated at three trophic levels, springtails (Collembola), predaceous mites (Acari), and macro‐arthropods (spider, adult and larval beetles, centipedes). Predictions were that springtails are predator regulated and mites are food limited according to the Hairston et al. (1960) model, which predicts alternating regulation by competition and predation from fungi to springtails to mites to macro‐arthropods. The alternate hypothesis was based on the bottom‐up model of trophic dynamics, which predicts that each trophic level is regulated by competition for resources.2. The relative contributions to springtail and mite population dynamics of endogenous (i.e. density‐dependent population growth related to food availability) and exogenous (i.e. predation and weather) factors were tested using time‐series analysis and experimental manipulation of water conditions. Box patterns were distributed within an aspen forest habitat located in the Canadian prairies and surveyed weekly from May to September 1997–1999. Each box depressed the leaf litter, creating a microhabitat island for soil arthropods that provided counts of invertebrates located immediately beneath the boxes.3. Strong evidence was found for endogenous control of springtail and mite numbers, indicated by a reduction in population growth related to density in the previous week. Contrary to predictions, no evidence was found for regulation of springtail numbers by mites, or for regulation of mite numbers by macro‐arthropods. Springtail population growth rate was related positively to current springtail density (8 and 23% variation explained) and related negatively to 1‐week lagged density (85 and 58%), and related negatively to temperature (5 and 5%) for time‐series data and for experimental addition of water respectively. Mite population growth rate was related positively to current mite density (54%) and temperature (4%), and negatively to 1‐week lagged mite density (20%) and precipitation (6%) for time‐series analysis. For experimental addition of water, mite growth rate was related positively to current mite density (44%) and temperature (5%), and negatively to 1‐week lagged density (11%). Results differed from the Hairston et al. (1960) model predictions but were consistent with a bottom‐up view that springtail and mite populations were regulated intrinsically by competition for food and secondarily by temperature as a function of reproduction.

Male Harvest in Relation to Female Removals in a Black-Tailed Deer Population

The role of density dependence in management of ungulate populations is a debated topic. I tested the effect of female removals on the yield of males in annual public hunting seasons for black-tailed deer (Odocoileus hemionus columbianus) in a 19-year study at Hopland Research and Extension Center, California. Separate treatment and control areas were not available, so pre- and posttreatment control periods were employed to examine for non-treatment effects (i.e., 6 yrs pretreatment, 7 yrs treatment, and 6 yrs posttreatment). During the treatment period, about 20 adult females/year for 3 years and 30/year for 4 years were removed. There was no difference in male harvests between pretreatment and posttreatment control periods (x = 28.5 pre- and 27.3 posttreatment, P = 0.44), so the controls were pooled for testing treatment effects. Male harvest during the treatment period (x = 34.9) was greater than the combined control periods (x = 27.9, P = 0.02). Increased male harvest occurred despite the treatment period coinciding with a 6-year drought. Neither rainfall nor acorn (Quercus spp.) crop was related to male harvest. At the end of the treatment period, combined male harvest and female removal comprised about 21 % of the estimated pre-hunt population. Males taken during the treatment period were younger and weighed less than those taken during control periods. However, 2-year-old males harvested during the treatment period had larger antlers. These results demonstrated that female harvests increased male yield apparently through lowering population density.

Artificial Reefs, the Attraction-production Issue, and Density Dependence in Marine Ornamental Fishes

Artificial reefs may provide a useful tool to enhance production of marine ornamentals and to divert detrimental harvesting activities from sensitive natural habitat. The efficacy of this strategy depends, in part, on the extent to which artificial reefs contribute to new production (vs. attract fishes from natural habitat) and therefore benefit harvested populations on a local and regional basis. Here the attraction and production hypotheses and their application to marine ornamentals are presented. Specifically, it is discussed how the strength and timing of density dependence can affect the response of fish population dynamics to artificial reefs. In addition to this discussion, examples of density dependence in marine ornamentals and related reef fishes are provided. Based on this information, a simple conceptual model is presented to clarify the role of density dependence, and this is followed by a discussion on the use of artificial reefs in the management and production of marine ornamentals. Finally, unresolved scientific issues that remain to be addressed are provided.

Predation of &lt;I&gt;Frankliniella occidentalis&lt;/I&gt; (Thysanoptera: Thripidae) in Field Peppers by &lt;I&gt;Orius insidiosus&lt;/I&gt; (Hemiptera: Anthocoridae)

Natural enemies have not been shown previously to play a significant role in regulating thrips in field crops. Most thrips are r-selected with population attributes that are believed to result in thrips outstripping the capacity of natural enemies to regulate thrips populations. In replicated field experiments conducted in 1995, insecticide applications to peppers were observed to increase rather than decrease populations of Frankliniella spp. thrips in flowers. We conducted experiments in 1996 and 1997 to determine if this phenomenon resulted from exclusion of predators. Species of thrips abundant in the pepper flowers were Frankliniella occidentalis (Pergande), F. tritici (Fitch), and F. bispinosa (Morgan). The only common predator in the flowers was Orius insidiosus (Say). Different treatments of biological and synthetic insecticides were used which altered the population densities of prey and predator. Covariate analyses over time were used each year to separate insecticide and predator effects on thrips populations. The predator significantly suppressed populations of all three thrips species. F. occidentalis and F. tritici reproduced in pepper flowers, but most of the larval thrips in untreated plots were F. occidentalis. Near extinction of F. occidentalis adults and larvae occurred within days after predator:prey ratios reached 1:40. Exclusion of the predator with synthetic insecticides resulted in continued abundance of thrips. We conclude that O. insidious was an effective predator that suppressed populations of Frankliniella spp. in field peppers during a period when thrips were rapidly colonizing and developing in the flowers. Species of Orius are abundant predators nearly worldwide, and the role of density dependence in regulation of thrips populations should be reevaluated.

Predation ofFrankliniella occidentalis(Thysanoptera: Thripidae) in Field Peppers byOrius insidiosus(Hemiptera: Anthocoridae)

Natural enemies have not been shown previously to play a significant role in regulating thrips in field crops. Most thrips are r-selected with population attributes that are believed to result in thrips outstripping the capacity of natural enemies to regulate thrips populations. In replicated field experiments conducted in 1995, insecticide applications to peppers were observed to increase rather than decrease populations of Frankliniella spp. thrips in flowers. We conducted experiments in 1996 and 1997 to determine if this phenomenon resulted from exclusion of predators. Species of thrips abundant in the pepper flowers were Frankliniella occidentalis (Pergande), F. tritici (Fitch), and F. bispinosa (Morgan). The only common predator in the flowers was Orius insidiosus (Say). Different treatments of biological and synthetic insecticides were used which altered the population densities of prey and predator. Covariate analyses over time were used each year to separate insecticide and predator effects on thrips populations. The predator significantly suppressed populations of all three thrips species. F. occidentalis and F. tritici reproduced in pepper flowers, but most of the larval thrips in untreated plots were F. occidentalis. Near extinction of F. occidentalis adults and larvae occurred within days after predator:prey ratios reached 1:40. Exclusion of the predator with synthetic insecticides resulted in continued abundance of thrips. We conclude that O. insidious was an effective predator that suppressed populations of Frankliniella spp. in field peppers during a period when thrips were rapidly colonizing and developing in the flowers. Species of Orius are abundant predators nearly worldwide, and the role of density dependence in regulation of thrips populations should be reevaluated.

The Role of Density Dependence in the Population Dynamics of a Tropical Palm

The Role of Density Dependence in the Population Dynamics of a Tropical Palm

THE ROLE OF DENSITY DEPENDENCE IN THE POPULATION DYNAMICS OF A TROPICAL PALM

The role of density dependence in the population dynamics of tropical trees has been a subject of considerable debate. Here, we present data on the demography of the edible palm Euterpe edulis, classified into seven size categories and monitored over three years. On average, each adult palm contributed 98 seedling recruits per year into the population. The pattern of mortality was similar to that of other palms, with mortality being highest among the smallest plants. Those plants with a diameter at soil level >20 mm had an annual mortality <7%. Density dependence was found to act only on the seedling stage of the life cycle. The probability of survival and transition of seedlings to the next size class were affected both by the density of seedlings and the presence of conspecific adults. Matrix modeling indicated that the true finite rate of population increase (λ) was 1.28 and that the observed reverse “J”-shaped size distribution of plants was a consequence of the density dependence operating in the population. Elasticity analysis showed that the survival elements in the matrix contributed most to the value of λ, and that the position of the transition matrix in growth–survival–fecundity (G–L–F) space was influenced by density. The matrix model incorporating density dependence predicted size distributions and densities approximating the maximum observed in the field. Spatial simulations indicated that the predictions from the matrix model relating to the size structure of plants are robust, but that the predictions of densities are sensitive to the precise spatial dynamics of the population.

Could asynchrony in activity between the sexes cause intersexual social segregation in ruminants?

In many sexually dimorphic mammal species, the sexes live outside the mating season in separate social groups ('social segregation'). Social segregation occurs in a wide range of environmental conditions, but its cause in unknown. I suggest that social segregation is caused by a lower level of activity synchrony between individuals in mixed-sex groups than in single-sex groups, owing to sex differences in activity rhythm. As a consequence, mixed-sex groups are more likely to break up than single-sex groups, resulting in a predominance of single-sex groups at equilibrium. To test this hypothesis in red deer (Cervus elaphus L.), I developed an index of activity synchronization and showed that deer in mixed-sex groups were significantly less synchronized in their activity than deer in single-sex groups. Thus, low intersexual synchrony in activity can lead to social segregation. However, a lower level of intrasexual (female-female and male-male) activity synchrony within mixed-sex than within single-sex groups implies that additional factors (other than sex differences in foraging rhythm) contribute to the higher degree of instability of mixed-sex groups.