Deleting species from model food webs

We use food webs generated by a model to investigate the effects of deleting species on other species in the web and on the web as a whole. The model incorporates a realistic population dynamics, adaptive foragers and other features which allow for the construction of model webs which resemble empirical food webs. A large number of simulations were carried out to produce a substantial number of model webs on which deletion experiments could be performed. We deleted each species in four hundred distinct model webs and determined, on average, how many species were eliminated from the web as a result. Typically only a small number of species became extinct; in no instance was the web close to collapse. Next, we examined how the the probability of extinction of a species depended on its relationship with the deleted species. This involved the exploration of the concept of indirect predator and prey species and the extent that the probability of extinction depended on the trophic level of the two species. The effect of deletions on the web itself was studied by searching for keystone species, whose removal caused a major restructuring of the community, and also by looking at the correlation between a number of food web properties (number of species, linkage density, fraction of omnivores, degree of cycling and redundancy) and the stability of the web to deletions. With the exception of redundancy, we found little or no correlation. In particular, we found no evidence that complexity in terms of increased species number or links per species is destabilising.Comment: 30 pages, 9 figure

Coupling Fishery Dynamics, Human Health And Social Learning In A Model Of Fish-Borne Pollution Exposure

The final publication is available at Springer via http://dx.doi.org/10.1007/s11625-015-0317-5Pollution-induced illnesses are caused by toxicants that result from human activity and are often entirely preventable. However, where industrial priorities have undermined responsible governance, exposed populations must reduce their exposure by resorting to voluntary protective measures and demanding emissions abatement. This paper presents a coupled human-environment system model that represents the effects of water pollution on the health and livelihood of a fishing community. The model is motivated by an incident from 1949 to 1968 in Minamata, Japan, where methylmercury effluent from a local factory poisoned fish populations and humans who ate them. We model the critical role of risk perception in driving both social learning and the protective feedbacks against pollution exposure. These feedbacks are undermined in the presence of social misperceptions such as stigmatization of the injured. Through numerical simulation and scenario analysis, we compare our model results with historical datasets from Minamata, and find that the conditions for an ongoing pollution epidemic are highly unlikely without social misperception. We also find trade-offs between human health outcomes, the viability of the polluting industry and the survival of the fishery. We conclude that an understanding of human-environment interactions and misperception effects is highly relevant to the resolution of contemporary pollution problems, and merits further study.Ontario Graduate Scholarship progra

Topology Changes by Quantum Tunneling in Four Dimensions

We investigate topology-changing processes in 4-dimensional quantum gravity with a negative cosmological constant. By playing the ``gluing-polytope game" in hyperbolic geometry, we explicitly construct an instanton-like solution without singularity. Because of cusps, this solution is non-compact but has a finite volume. Then we evaluate a topology change amplitude in the WKB approximation in terms of the volume of this solution.Comment: 13 pages revtex.sty, 6 uuencoded figures contained, TIT/HEP-260/COSMO-4

Food web models: a plea for groups

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75012/1/j.1461-0248.2009.01321.x.pd

Competition Among Companies: Coexistence and Extinction

We study a spatially homogeneous model of a market where several agents or companies compete for a wealth resource. In analogy with ecological systems the simplest case of such models shows a kind of "competitive exclusion" principle. However, the inclusion of terms corresponding for instance to "company efficiency" or to (ecological) "intracompetition" shows that, if the associated parameter overcome certain threshold values, the meaning of "strong" and "weak" companies should be redefined. Also, by adequately adjusting such a parameter, a company can induce the "extinction" of one or more of its competitors.Comment: 5 pages, 3 figures include

Beyond connectedness: why pairwise metrics cannot capture community stability

The connectedness of species in a trophic web has long been a key structural characteristic for both theoreticians and empiricists in their understanding of community stability. In the past decades, there has been a shift from focussing on determining the number of interactions to taking into account their relative strengths. The question is: How do the strengths of the interactions determine the stability of a community? Recently, a metric has been proposed which compares the stability of observed communities in terms of the strength of three- and two-link feedback loops (cycles of interaction strengths). However, it has also been suggested that we do not need to go beyond the pairwise structure of interactions to capture stability. Here, we directly compare the performance of the feedback and pairwise metrics. Using observed food-web structures, we show that the pairwise metric does not work as a comparator of stability and is many orders of magnitude away from the actual stability values. We argue that metrics based on pairwise-strength information cannot capture the complex organization of strong and weak links in a community, which is essential for system stability

Physical nature of the central singularity in spherical collapse

We examine here the nature of the central singularity forming in the spherically symmetric collapse of a dust cloud and it is shown that this is always a strong curvature singularity where gravitational tidal forces diverge powerfully. An important consequence is that the nature of the naked singularity forming in the dust collapse turns out to be stable against the perturbations in the initial data from which the collapse commences.Comment: Latex file, 11 pages, 2 figures, Updated version to match the published version in PR

The color of environmental noise.

Abstract. Biological populations are strongly influenced by the random variation in their environment. The spectrum of frequencies in noise is particularly important to dynamics and persistence. Here we present an analysis of the variance spectra of a wide variety of long-term time series of environmental variables. Spectra were well approximated by the inverse power law 1/f ␤ within the appropriate range of frequencies f; however, the majority of spectra were ''flattened'' at low frequencies. With some qualification we found the spectral exponents (␤) to corroborate an earlier suggestion that terrestrial noise tends to be ''white'' (␤ Ͻ 0.5), while marine environments tend to be ''red'' (␤ ഠ 1) or ''brown'' (␤ ഠ 2). As well, we found a tendency for whiter noise in temperate latitudes than in either high or low latitudes. These results have wide-ranging consequences for ecosystem fragility and species conservation

Strengths of singularities in spherical symmetry

Covariant equations characterizing the strength of a singularity in spherical symmetry are derived and several models are investigated. The difference between central and non-central singularities is emphasised. A slight modification to the definition of singularity strength is suggested. The gravitational weakness of shell crossing singularities in collapsing spherical dust is proven for timelike geodesics, closing a gap in the proof.Comment: 16 pages, revtex. V2. Classification of irregular singular points completed, Comments and references on singularities with a continuous metric amende

Compact Three Dimensional Black Hole: Topology Change and Closed Timelike Curve (minor changes)

We present a compactified version of the 3-dimensional black hole recently found by considering extra identifications and determine the analytical continuation of the solution beyond its coordinate singularity by extending the identifications to the extended region of the spacetime. In the extended region of the spacetime, we find a topology change and non-trivial closed timelike curves both in the ordinary 3-dimensional black hole and in the compactified one. Especially, in the case of the compactified 3-dimensional black hole, we show an example of topology change from one double torus to eight spheres with three punctures.Comment: 20 pages revtex.sty 8 figures contained, TIT/HEP-245/COSMO-4