Biomedical subjects

M P Hassell

Publications and source records attributed to M P Hassell.

11 recordsLinked to original sources

PubMed1998

Top-down versus bottom-up and the Ruritanian bean bug.

In a recent article, Hunter uses the late George Varley and George Gradwell's long-term data on the winter moth (Operophtera brumata) and green tortrix (Tortrix viridana) populations to propose a method of quantifying the relative importance of top-down effects (because of natural enemies) and bottom-up effects (because of resource competition) in influencing population dynamics. We believe this approach is deeply flawed. Using Varley and Gradwell's winter moth study, we show that the problems with Hunter's analysis lie in his misinterpretation of the population dynamics and his inappropriate use of statistical techniques. We also emphasize the importance of distinguishing clearly between two quite different things: firstly, top-down and bottom-up regulation of populations and secondly, the much simpler task of categorizing factors affecting changes in population density as either top-down or bottom-up processes.

Journal Article↗

PubMed1997

The effects of a pool of dispersers on host-parasitoid systems.

When individuals migrate in a multi-patch environment, a considerable proportion of their lifetime might be spent in transit between patches. We investigate the effects such a pool of dispersers can have on local stability and dynamics for a variety of multi-patch host-parasitoid models. When an arbitrary number of patches with internal Lotka-Volterra dynamics is coupled via a global pool of dispersers, the equilibrium is globally stable. The global pool is stabilising if dispersal is by hosts only, by parasitoids only, or by both hosts and parasitoids. If dispersal is local such that individuals first enter a pool close to the patch where they originate and then disperse to adjacent pools, the equilibrium is locally stable. We also analyse the situation where the functional response of parasitoids within a patch is Holling type II which is known to destabilise host-parasitoid systems. Coupling this single patch to a pool of dispersers can produce a locally stable interaction, provided the handling time of hosts is not too long. However, the pool provides a biologically realistic example of an interaction that is locally stable but not permanent. The longer the handling time, the smaller the region of population densities within which populations converge to the equilibrium state. In a multi-patch environment with a global disperser pool, the dynamics of the system are not qualitatively different from the single patch case (i.e. the equilibrium can be locally stable but the system is not permanent). In a multi-patch environment with local disperser pools, true spatial interactions between patches can develop. In contrast to the global pool, local pools can destabilise the stable equilibrium of the single patch case. Limit cycles develop around this unstable equilibrium that lead to extremely complicated dynamics. In contrast to the global pool, a system of local pools can exhibit bounded fluctuations so that populations do not go extinct.

Animals↗

PubMed1996

Persistence of multispecies host-parasitoid interactions in spatially distributed models with local dispersal.

Recent theoretical studies have shown that dispersal between neighbouring local populations can promote the persistence of interacting metapopulations, even when the local dynamics are unstable and the environment is uniform. This persistence is associated with striking and self-organized spatial patterns in the densities of the local populations. Here we extend previous work on spatially distributed host-parasitoid interactions to wider questions of community structure, by considering various three-species systems: two parasitoid species attacking a common host species; two host species attacked by a single parasitoid species; or a host-parasitoid-hyperparasitoid interaction. In each of these cases, multispecies coexistence of the total populations can occur, even though the local population dynamics are unstable. Furthermore, co-existence tends to be accompanied by some degree of persistent spatial segregation of the competing species, despite the completely uniform environment. At its most extreme, this results in one species being confined to small, relatively static, "islands" within the habitat, giving the appearance of isolated pockets of favourable habitat. That dynamics can impose and maintain such "self-organizing" spatial segregation of competing species, has interesting implications for understanding the local abundance of natural populations.

Animals↗

PubMed1996

Metapopulations and equilibrium stability: the effects of spatial structure.

Recently, there has been a great deal of interest in the dynamics of metapopulations, where a number of local populations are coupled via dispersal. The importance of movement for the persistence of an ensemble of locally unstable patches has been established in many studies. In this paper, we present analytical and simulation results concerning the effects of spatial structure on the equilibrium stability of individual populations. We conclude that for general single-species and two-species competition models, the introduction of the spatial dimension in a biologically sensible way has no effect on the overall stability properties. In host-parasitoid models, however, strong host or parasitoid over-dispersal may be destabilizing.

Animals↗

PubMed1990

Host-parasitoid associations in patchy environments.

Studies of insect host-parasitoid interactions have contributed much to the consensus that spatial patchiness is important in the regulation of natural populations. A variety of theoretical models predict that host and parasitoid populations, although unstable in the absence of environmental heterogeneity, may persist at roughly steady overall densities in a patchy environment owing to variation in levels of parasitism from patch to patch. Observed patterns of parasitism, however, have a variety of forms (with variation in attack rates among patches depending directly or indirectly on host density, or showing variation uncorrelated with host density). There is some confusion about the dynamical consequences of these different forms. Here we first show how the dynamical effects of all these forms of environmental heterogeneity can be assessed by a common criterion. This 'CV2 greater than 1 rule' states that the overall population densities will remain roughly steady from generation to generation if the coefficient of variation squared (CV2) of the density of searching parasitoids in the vicinity of each host exceeds approximately unity. By partitioning CV2 into components, we show that both direct and inverse patterns of dependence on host density, and density-independent patterns, all contribute to population regulation in the same way. Second, we show how a maximum-likelihood method can be applied to the kind of field data that are usually available (that is, percentage parasitism versus local host density) to estimate the components of CV2. This analysis indicates that heterogeneity is large enough to stabilize dynamics in 9 of 34 published studies, and that density-independent heterogeneity is the main factor in most cases.

Algorithms↗

PubMed1984

Parasitism in patchy environments: inverse density dependence can be stabilizing.

There are now many examples in the literature where the spatial distribution of per cent parasitism by insect parasitoids is either directly or inversely dependent on host density per patch. While it is well known that direct density dependent relationships can contribute markedly to the stability of a host-parasitoid interaction, inverse relationships have been more-or-less ignored. Using difference equation models, the dynamics of host-parasitoid interactions are described where parasitism per patch varies across the range from direct to inversely density dependent. These models demonstrate for a variety of host distributions that inverse relationships can also strongly promote stability.

Animals↗

PubMed1978