Biomedical subjects

R Arditi

Publications and source records attributed to R Arditi.

7 recordsLinked to original sources

PubMed2005

Detection, identification and geographical distribution of European corn borer larval parasitoids using molecular markers.

Biological control requires specific tools for the accurate detection and identification of natural enemies, and to detect unusual variations in their density, which may follow changes in agricultural practices. Here we have developed specific molecular markers to detect Lydella thompsoni (Herting) and Pseudoperichaeta nigrolineata (Walker) (Diptera: Tachinidae) within the European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae). Primers amplifying fragments of the mitochondrial COI gene were designed following alignment of comparable sequences for a range of parasitoid and host species. Each of the primer pairs proved to be species specific to a tachinid species, amplifying DNA fragments of 191 and 91 bp in length for L. thompsoni and P. nigrolineata, respectively. This DNA-based technique allowed molecular evaluation of parasitism in O. nubilalis natural populations. In order to study the geographical distribution of both species in France, O. nubilalis diapausing larvae in maize stalks were collected from 12 locations over the whole country. The molecular evaluation of parasitism was compared with the traditional method of maintaining O. nubilalis populations in controlled conditions before breaking off the diapause. The percentage parasitism found in both species of tachinids was higher--approximately three times--using the molecular method, suggesting an underestimation by the traditional rearing protocol. Tachinid parasitism on O. nubilalis was not significantly different between geographical areas (south, central and north France) for both species. This study shows that molecular methods are very promising for the correct detection and identification of tachinid parasitoids in natural field populations.

Animals↗

PubMed2001

Directed movement of predators and the emergence of density-dependence in predator-prey models.

We consider a bitrophic spatially distributed community consisting of prey and actively moving predators. The model is based on the assumption that the spatial and temporal variations of the predators' velocity are determined by the prey gradient. Locally, the populations follow the simple Lotka-Volterra interaction. We also assume predator reproduction and mortality to be negligible in comparison with the time scale of migration. The model demonstrates heterogeneous oscillating distributions of both species, which occur because of the active movements of predators. One consequence of this heterogeneity is increased viability of the prey population, compared to the equivalent homogeneous model, and increased consumption. Further numerical analysis shows that, on the spatially aggregated scale, the average predator density adversely affects the individual consumption, leading to a nonlinear predator-dependent trophic function, completely different from the Lotka-Volterra rule assumed at the local scale.

Animals↗

PubMed2001

Parametric analysis of the ratio-dependent predator-prey model.

We present a complete parametric analysis of stability properties and dynamic regimes of an ODE model in which the functional response is a function of the ratio of prey and predator abundances. We show the existence of eight qualitatively different types of system behaviors realized for various parameter values. In particular, there exist areas of coexistence (which may be steady or oscillating), areas in which both populations become extinct, and areas of "conditional coexistence" depending on the initial values. One of the main mathematical features of ratio-dependent models, distinguishing this class from other predator-prey models, is that the Origin is a complicated equilibrium point, whose characteristics crucially determine the main properties of the model. This is the first demonstration of this phenomenon in an ecological model. The model is investigated with methods of the qualitative theory of ODEs and the theory of bifurcations. The biological relevance of the mathematical results is discussed both regarding conservation issues (for which coexistence is desired) and biological control (for which extinction is desired).

Animals↗

PubMed2000

Identifying predator-prey processes from time-series.

The functional response is a key element in predator-prey models as well as in food chains and food webs. Classical models consider it as a function of prey abundance only. However, many mechanisms can lead to predator dependence, and there is increasing evidence for the importance of this dependence. Identification of the mathematical form of the functional response from real data is therefore a challenging task. In this paper we apply model-fitting to test if typical ecological predator-prey time series data, which contain both observation error and process error, can give some information about the form of the functional response. Working with artificial data (for which the functional response is known) we will show that with moderate noise levels, identification of the model that generated the data is possible. However, the noise levels prevailing in real ecological time-series can give rise to wrong identifications. We will also discuss the quality of parameter estimation by fitting differential equations to such time-series.

Algorithms↗

PubMed1994

Maximum sustainable yield with continuous age structure and density-dependent recruitment.

The problem addressed is that of finding the harvesting policy that will maximize the yield and maintain a given population in a steady state. It is assumed that the equations describing the evolution of the population and the yield are linear but that the equation describing reproduction is nonlinear (density dependence). The optimal strategy consists in harvesting two age classes only: a younger age class is partially harvested, and an older age class is completely harvested as in the linear case. This problem is a particular case of a general abstract maximization problem in which, for example, nonlinear mortality or nonlinear value of the individuals can also be introduced. The appropriate compactness for this problem is obtained from the fact that maximizing sequences are decreasing.

Age Factors↗

PubMed1992

Maximum sustainable yield of populations with continuous age-structure.

We address the problem of finding the harvesting policy that will maximize the yield and maintain a population in a steady state. The population is characterized by continuous age classes and therefore follows differential equations. Here, we assume that the equations are linear (no density dependence). Two possible constraints are considered: either recruitment or total population are fixed to a constant. Under these conditions, the optimal policy is to harvest the fraction theta of a younger age class ã and to harvest totally an older age class b. The optimal solution (theta, ã, b) can be calculated explicitly if the fecundity and mortality schedules are given. The solution is compared to the simpler strategy of harvesting all individuals beyond a single age class â. It is shown that the latter strategy can be much less profitable than harvesting two age classes because it cannot take account of the different values of individuals according to their age.

Age Factors↗

PubMedDate not supplied

[Explicit model for searching behavior of predator].

The authors present an approach for explicit modeling of spatio-temporal dynamics of predator-prey community. This approach is based on a reaction-diffusion-adjection PD (prey dependent) system. Local kinetics of population is determined by logistic reproduction function of prey, constant natural mortality of predator and Holling type 2 trophic function. Searching behavior of predator is described by the advective term in predator balance equation assuming the predator acceleration to be proportional to the prey density gradient. The model was studied with zero-flux boundary conditions. The influence of predator searching activity on the community dynamics, in particular, on the emergence of spatial heterogeneity, has been investigated by linear analysis and numerical simulations. It has been shown how searching activity may effect the persistence of species, stabilizing predator-prey interactions at very low level of pest density. It has been demonstrated that obtaining of such dynamic regimes does not require the use of complex trophic functions.

Animals↗