Search PubMed⌕ Search

Biomedical subjects

R M Nisbet

Publications and source records attributed to R M Nisbet.

At least 19 recordsLinked to original sources
PubMed2005

Persistence, spread and the drift paradox.

We derive conditions for persistence and spread of a population where individuals are either immobile or dispersing by advection and diffusion through a one-dimensional medium with a unidirectional flow. Reproduction occurs only in the stationary phase. Examples of such systems are found in rivers and streams, marine currents, and areas with prevalent wind direction. In streams, a long-standing question, dubbed 'the drift paradox', asks why aquatic insects faced with downstream drift are able to persist in upper stream reaches. For our two-phase model, persistence of the population is guaranteed if, at low population densities, the local growth rate of the stationary component of the population exceeds the rate of entry of individuals into the drift. Otherwise the persistence condition involves all the model parameters, and persistence requires a critical (minimum) domain size. We calculate the rate at which invasion fronts propagate up- and downstream, and show that persistence and ability to spread are closely connected: if the population cannot advance upstream against the flow, it also cannot persist on any finite spatial domain. By studying two limiting cases of our model, we show that residence in the immobile state always enhances population persistence. We use our findings to evaluate a number of mechanisms previously proposed in the ecological literature as resolutions of the drift paradox.

Models, Theoretical↗
PubMed2002

Single-species models for many-species food webs.

Most species live in species-rich food webs; yet, for a century, most mathematical models for population dynamics have included only one or two species. We ask whether such models are relevant to the real world. Two-species population models of an interacting consumer and resource collapse to one-species dynamics when recruitment to the resource population is unrelated to resource abundance, thereby weakening the coupling between consumer and resource. We predict that, in nature, generalist consumers that feed on many species should similarly show one-species dynamics. We test this prediction using cyclic populations, in which it is easier to infer underlying mechanisms, and which are widespread in nature. Here we show that one-species cycles can be distinguished from consumer resource cycles by their periods. We then analyse a large number of time series from cyclic populations in nature and show that almost all cycling, generalist consumers examined have periods that are consistent with one-species dynamics. Thus generalist consumers indeed behave as if they were one-species populations, and a one-species model is a valid representation for generalist population dynamics in many-species food webs.

Animals↗
PubMed2001

Habitat structure and population persistence in an experimental community.

Understanding spatial population dynamics is fundamental for many questions in ecology and conservation. Many theoretical mechanisms have been proposed whereby spatial structure can promote population persistence, in particular for exploiter-victim systems (host-parasite/pathogen, predator-prey) whose interactions are inherently oscillatory and therefore prone to extinction of local populations. Experiments have confirmed that spatial structure can extend persistence, but it has rarely been possible to identify the specific mechanisms involved. Here we use a model-based approach to identify the effects of spatial population processes in experimental systems of bean plants (Phaseolus lunatus), herbivorous mites (Tetranychus urticae) and predatory mites (Phytoseiulus persimilis). On isolated plants, and in a spatially undivided experimental system of 90 plants, prey and predator populations collapsed; however, introducing habitat structure allowed long-term persistence. Using mechanistic models, we determine that spatial population structure did not contribute to persistence, and spatially explicit models are not needed. Rather, habitat structure reduced the success of predators at locating prey outbreaks, allowing between-plant asynchrony of local population cycles due to random colonization events.

Animals↗
PubMed2000

Spatial structure and fluctuations in the contact process and related models.

The contact process is used as a simple spatial model in many disciplines, yet because of the buildup of spatial correlations, its dynamics remain difficult to capture analytically. We introduce an empirically based, approximate method of characterizing the spatial correlations with only a single adjustable parameter. This approximation allows us to recast the contact process in terms of a stochastic birth-death process, converting a spatiotemporal problem into a simpler temporal one. We obtain considerably more accurate predictions of equilibrium population than those given by pair approximations, as well as good predictions of population variance and first passage time distributions to a given (low) threshold. A similar approach is applicable to any model with a combination of global and nearest-neighbor interactions.

Animals↗
PubMed2000

Survival and production in variable resource environments.

A dynamic energy budget (DEB) model describes the rates at which organisms assimilate and utilize energy from food for maintenance, growth, reproduction and development. We study the dynamic behavior of one particular DEB model, Kooijman's kappa-rule model, whose key assumption is that somatic and reproductive tissues are competing for energy. We assume an environment in which the food density fluctuates either periodically or stochastically (pink noise). Both types of fluctuations stimulate growth; the magnitude of the (average) increase in size depends on both the strength and duration of the fluctuations. In a stochastic environment, the risk of mortality due to starvation increases with increasing fluctuation intensity. The mean lifespan is also a function of the model parameter kappa characterizing the partitioning of energy between somatic and reproductive tissues. Organisms committing a large fraction of resources to reproduction endure periods of food shortage relatively well. The effects of food fluctuations on reproduction are complex. With stochastic food, reproduction in survivors increases with increasing fluctuation intensities, but lifetime reproduction decreases. Periodic fluctuations may enhance reproduction, depending on the value of kappa. Thus, a variable food supply stimulates growth, increases mortality and may enhance reproduction, depending on life history.

Animals↗
PubMed2000

A dynamic energy budget model based on partitioning of net production.

We formulate a Dynamic Energy Budget (DEB) model for the growth and reproduction of individual organisms based on partitioning of net production (i.e. energy acquisition rate minus maintenance rate) between growth and energy reserves. Reproduction uses energy from reserves. The model describes both feeding and non-feeding stages, and hence is applicable to embryos (which neither feed nor reproduce), juveniles (which feed but do not reproduce), and adults (which commonly both feed and reproduce). Embryonic growth can have two forms depending on the assumptions for acquisition of energy from yolk. By default, when the energy acquisition rate exceeds the maintenance rate, a fixed proportion of the resulting net production is spent on growth (increase in structural biomass), and the remaining portion is channelled to the reserves. Feeding organisms, however, modulate their allocation of net production energy in response to their total energy content (energy in the reserves plus energy bounded to structural biomass). In variable food environment an organism alternates between periods of growth, no-growth, and balanced-growth. In the latter case the organism adopts an allocation strategy that keeps its total energy constant. Under constant environmental conditions. the growth of a juvenile is always of von Bertalanffy type. Depending on the values of model parameters there are two long-time possibilities for adults: (a) von Bertalanffy growth accompanied by reproduction at a rate that approaches zero as the organism approaches asymptotic size, or (b) abrupt cessation of growth at some finite time, following which, the rate of reproduction is constant. We illustrate the model's applicability in life history theory by studying the optimum values of the energy allocation parameters for constant environment and for each of the dynamic regimes described above.

Animals↗
PubMed1985

Stage-structure models of populations with distinct growth and development processes.

We extend the repertoire of stage-structure models which can be described in terms of delay-differential equations, by analysing models where the processes of growth and development within a stage are distinct. This permits the use of delay-differential equation models in situations where both population numbers and total biomass are dynamically significant.

Aging↗
PubMed1984

The dynamics of population models with distributed maturation periods.

An integro-differential equation for the dynamics of a subpopulation of adults in a closed system where only the adults compete and where there is a distribution of maturation periods is described. We show how the careful choice of a general weighting function based on the gamma distribution with a shift in origin enables us to characterize adequately some observed maturation-period distributions, and also makes local stability and numerical analyses straightforward. Using these results we examine the progression in the behavior of the distributed-delay model as the distribution is narrowed toward the limit of a discrete delay. We conclude that while local stability properties approach those of the limiting equation very rapidly, the persistent fluctuation behavior converges more slowly, with the dominant period and maximum amplitude being least affected by the details of the distribution, and the fine structure of solutions being most sensitive. Finally, we examine the consequences for population modeling, and using several examples of insect populations, conclude that although quite often a full maturation-period distribution should be incorporated in a given model, in many cases a discrete-delay approximation will suffice.

Age Factors↗
PubMed1981

Retinal branch vein occlusion.

Retinal branch vein occlusion is one of the most common diseases of the retinal vessels. In this retrospective study, 27 affected eyes were reviewed to determine the natural history, complications, and visual prognosis. Retinal branch vein occlusion most often involves temporal retinal veins at the arteriovenous crossing. The precise cause is unknown, but a high degree of association with systemic hypertension and diabetes mellitus is known. The clinical and fluorescein angiographic appearance of acute and chronic retinal branch vein occlusion is described and illustrated. The most common complications affecting visual acuity are macular edema and preretinal neovascularization. It is imperative that collateral channels be distinguished from neovascularization. If the patient's initial visual acuity is good, the prognosis is excellent, but if the initial acuity is poor, the visual outcome is less certain.

Adolescent↗
PubMed1981

Cyclodialysis with air injection in black patients.

Thirty-two cases are presented in which cyclodialysis with air injections was used to treat glaucoma in black patients. Nineteen of the 32 cases were of aphakic glaucoma and most represented failures of other glaucoma surgery. The success rate was found at 12 months postoperative follow-up to be 75%. Three of 13 eyes with phakic glaucoma developed cataracts. Cyclodialysis may be superior to trabeculectomy for surgical treatment of glaucoma in black patients.

Adult↗
PubMed1981

Retinopathy in the sexually abused battered child.

Two cases of Purtscher's retinopathy in sexually abused battered children were observed. The major ocular findings were severe diffuse subretinal, retinal, and preretinal hemorrhages. Sexual trauma in conjunction with shaking may have produced sufficient changes in intraabdominal and intrathoracic pressure to account for retinal and intracranial hemorrhage. A prompt examination by an ophthalmologist is recommended for all children who are victims of physical abuse.

Battered Child Syndrome↗
PubMed1979

Pediatric ocular photography platform.

We designed a rigid support platform to facilitate ocular photography of infants or small animals. The platform is versatile because it is compatible with either the Zeiss fundus camera or Zeiss Photo Slit Lamp.

Age Factors↗