Density-dependent Feedback Mechanisms Research Articles

An experimental analysis of density dependence in meadow voles: Within-season and delayed effects.

Wild mammal populations exhibit a variety of dynamics, ranging from fairly stable with little change in population size over time to high-amplitude cyclic or erratic fluctuations. A persistent question in population ecology is why populations fluctuate as they do. Answering this seemingly simple question has proven to be challenging. Broadly, density-dependent feedback mechanisms should allow populations to grow at low density and slow or halt growth at high density. However, experimental tests of what demographic processes result in density-dependent feedback and on what timescale have proven elusive. Here, we used replicated density perturbation experiments and capture-mark-recapture analyses to test density-dependent population growth in populations of meadow voles (Microtus pennsylvanicus) during the summer breeding season by manipulating founding population density and observing the pattern of survival, reproduction, and population growth. High population density had no consistent effect on survival rates but generally negatively influenced recruitment and population growth rates. However, these density-dependent effects varied within the breeding season and across years. Our study provides evidence that density-dependent feedback mechanisms operate at finer time scales than previously believed and that process, additively with delayed year effects, is key to understanding multiyear population demography.

Spatiotemporal outbreak dynamics of bark and wood-boring insects.

Bark and wood-boring insects (BWI) can cause important disturbances in forest ecosystems, and their impact depends on their spatiotemporal dynamics. Populations are usually at stable, low densities but can be disrupted by stochastic perturbations that trigger a transition to an epidemic phase. For less aggressive species, outbreaks die out quickly once the perturbation is removed, while aggressive species exhibit density-dependent feedback mechanisms facilitating persistent landscape-scale outbreaks. The interactions of attributes of tree, stand, landscape- and regional-scale stressors, and insect life history and behavior determine system-specific dynamics. However, most of our knowledge is based on a few species of mainly Holarctic bark beetles. With global change, it is becoming increasingly important to improve our understanding of the frequency and severity of BWI outbreaks.

Plant-herbivory feedbacks and selective allocation of a toxic metal are behind the stability of degraded covers dominated by Brachypodium pinnatum in acidic soils

Perennial tall-grasses have experienced a successful expansion in the last decades leading in some cases to extremely degraded communities, which remain in an alternate stable state. This research focuses on the mechanisms of persistence of the spreading native Brachypodium pinnatum in acidic soils. We hypothesize that plant-herbivory feedbacks and an adapted response to soil constraints, promote species cover stability. Eight on-site populations of the species growing in two contrasted situations (high-diversity grazed and low-diversity ungrazed communities) were sampled during a growing season and analysed for nutrient (N, P and K), metal (Al) and C allocation, biomass production and tissue quality. The species showed a high capacity for belowground accumulation of Al and an efficient reallocation and resorption of nutrients, which may explain its success in acidic, poor soils. The lack of attractiveness to herbivores of the species-poor stands prevented biomass removal, which strengthened some effects and enhanced the build-up of a thick layer of recalcitrant tissues, an effective barrier (aboveground and belowground) for the establishment of sympatric species. Density-dependent feedback mechanisms (herbivory attractiveness), and the particular attributes of B. pinnatum (i.e. powerful rhizomes with a selective performance for nutrients and aluminium) may lead to critical processes of community domination and degradation.

Complex dynamics generated by negative and positive feedback delays of a prey–predator system with prey refuge: Hopf bifurcation to Chaos

Various field and laboratory experiments show that prey refuge plays a significant role in the stability of prey–predator dynamics. On the other hand, theoretical studies show that delayed system exhibits a much more realistic dynamics than its non-delayed counterpart. In this paper, we study a multi-delayed prey–predator model with prey refuge. We consider modified Holling Type II response function that incorporates the effect of prey refuge and then introduce two discrete delays in the model system. A negative feedback delay is considered in the logistic prey growth rate to represent density dependent feedback mechanism and a positive feedback delay is considered to represent the gestation time of the predator. Our study reveals that the system exhibits different dynamical behaviors, viz., stable coexistence, periodic coexistence or chaos depending on the values of the delay parameters and degree of prey refuge. The interplay between two delays for a fixed value of prey refuge has also been determined. It is noticed that these delays work in a complementary fashion. In addition, using the normal form theory and center manifold argument, we derive the explicit formulae for determining the direction of the bifurcation, the stability and other properties of the bifurcating periodic solutions.

Intraspecific Intrusion at Bald Eagle Nests

Competition for nesting territory has been shown to act as a density-dependent feedback mechanism influencing population growth rate. However, little is known about the nature of territorial interactions between established breeders and floaters. We examined territorial intrusion rates and associated behaviours at active Bald Eagle Haliaeetus leucocephalus nests in the lower Chesapeake Bay in 2012 and 2013. The average intrusion rate experienced at study nests during the reproductive period was 0.28 ± 0.32 intrusions/h. Variance in intrusion rate was high and there was no apparent predictive pattern to these events. Juvenile intrusions occurred closer to the nest than adult intrusions, and breeders showed higher response rates toward adults, with 78% of adult intruders eliciting a response compared to 47% of juveniles. Breeding adults responded to intruders significantly more often and more aggressively when in the presence of their mate. Further research is necessary to broadly describe the relationship ...

DREISSENA POLYMORPHA AND CONCHOPHTHIRUS ACUMINATUS: WHAT CAN WE LEARN FROM HOST-COMMENSAL RELATIONSHIPS

Host specificity, extremely high prevalence and infection intensity, and easy sampling make the mantle-cavity ciliate Conchophthirus acuminatus a very convenient model to address numerous fundamental questions relating to symbiosis and commensalism. The acquisition of food by the ciliate as a result of Dreissena filtration activity is probably the basis of the symbiosis, with additional benefits to C. acuminatus being shelter, provision of oxygen, and dispersal. The number of C. acuminatus in a Dreissena population depends on the mussel's size-frequency distribution as there is a direct correlation between Dreissena size and infection intensity. Lack of a correlation between host density and commensal infection intensity may indicate that D. polymorpha and C. acuminatus have a different environmental optimum. Zebra mussels of a given length in each population may have their own carrying capacity of C. acuminatus infection intensity. Upon reaching this carrying capacity, a symbiont population may have density-dependent feedback mechanisms, which slow its reproduction rate within a host and/or increase its emigration from the mussel to maintain an optimal density within its host. Massive emergence of ciliates into open water may be synchronized with a mass occurrence of recently settled Dreissena juveniles to maximize the infection.

Control of Oligodendrocyte Precursor Proliferation Mediated by Density-Dependent Cell Cycle Protein Expression

The proliferation of oligodendrocyte precursors is closely regulated in the developing vertebrate CNS. In previous studies we have demonstrated a cell-type-specific density-dependent feedback mechanism that contributes to the control of oligodendrocyte progenitor cell clonal expansion in vitro. Here we demonstrate a density-dependent reduction in the proliferation of oligodendrocyte precursors. This inhibition of proliferation is correlated with increases in expression levels of the cell cycle inhibitor p27^Kip1, reductions in the expression of cyclin A and changes in the relative phosphorylation levels of Rb. These changes are reversible on replating at low density suggesting that additional signals are required for terminal oligodendrocyte differentiation.

Reshaping the Ecology of Invading Populations of Hemlock Woolly Adelgid, Adelges tsugae (Homoptera: Adelgidae), in Eastern North America

Hemlock woolly adelgid, Adelges tsugae Annand (Homoptera: Adelgidae), is native to Japan where it is an innocuous inhabitant of Tsuga diversifolia Masters and T. sieboldii Carriere throughout their natural growing areas. Native adelgid populations are regulated by host resistance and natural enemies, in particular the oribatid mite, Diapterobates humeralis (Hermann) and the coccinellid beetle, Pseudoscymnus tsugae Sasaji and McClure. Invading populations of A. tsugae in western North America on T. heterophylla Sargent and T. mertensiana Carriere are mainly regulated by host resistance. However, invading populations in eastern North America attain damaging levels on T. canadensis (L.) Carriere and T. caroliniana Engelmann and are regulated mainly by weather and negative density-dependent feedback mechanisms related to host deterioration. Although A. tsugae is only passively dispersed by wind, birds, forest-dwelling mammals and humans, it is spreading at an alarming rate and is sufficiently cold hardy to threaten the existence of the two eastern hemlock species throughout their natural ranges. The current hope for suppressing invading populations of hemlock woolly adelgid in eastern North America lies with the exotic predator, P. tsugae. Extensive laboratory studies of the biology and predatory ability of P. tsugae revealed that it feeds on all life stages of its prey, that its multivoltine life cycle is well synchronized with that of the adelgid, and that it has great potential for biological control. We have reared and released nearly 130,000 adults of P. tsugae in forests in Connecticut, New Jersey and Virginia during the past four years. P. tsugae has reproduced, dispersed, overwintered and reduced densities of hemlock woolly adelgid by 47–88% in only five months on release branches at these sites. Current studies are investigating the long-term ability of P. tsugae to regulate invading populations of A. tsugae in eastern North America.

Stochastic seasonality and nonlinear density-dependent factors regulate population size in an African rodent.

Ecology has long been troubled by the controversy over how populations are regulated. Some ecologists focus on the role of environmental effects, whereas others argue that density-dependent feedback mechanisms are central. The relative importance of both processes is still hotly debated, but clear examples of both processes acting in the same population are rare. Key-factor analysis (regression of population changes on possible causal factors) and time-series analysis are often used to investigate the presence of density dependence, but such approaches may be biased and provide no information on actual demographic rates. Here we report on both density-dependent and density-independent effects in a murid rodent pest species, the multimammate rat Mastomys natalensis (Smith, 1834), using statistical capture-recapture models. Both effects occur simultaneously, but we also demonstrate that they do not affect all demographic rates in the same way. We have incorporated the obtained estimates of demographic rates in a population dynamics model and show that the observed dynamics are affected by stabilizing nonlinear density-dependent components coupled with strong deterministic and stochastic seasonal components.

Stability in cyclic epidemic models

A general formulation for a family of cyclic epidemic models with density-dependent feedback mechanisms and removed classes is presented. A parameter, lambda, related to the basic reproductive rate determines the asymptotic behaviour of solutions of the model. It is shown that if lambda less than 1 the trivial solution is globally stable, and if lambda greater than 1 it is conditionally stable. The results are applied to a set of differential equations that has been used to model the life cycle of a parasite that has two hosts.