Experimental Evidence for the Population-Dynamic Mechanisms Underlying Extinction Cascades of Carnivores

SummarySpecies extinction rates due to human activities are high [1–3], and initial extinctions can trigger cascades of secondary extinctions, leading to further erosion of biodiversity [4]. A potential major mechanism for secondary extinction cascades is provided by the long-standing theory that the diversity of consumer species is maintained due to the positive indirect effects that these species have on each other by reducing competition among their respective resource species [5–7]. This means that the loss of one carnivore species could lead to competitive exclusion at the prey trophic level, leading to extinctions of further carnivore species. Evidence for these effects is difficult to obtain due to many confounding factors in natural systems, but extinction cascades that could be due to this mechanism have been demonstrated in simplified laboratory microcosms [8]. We established complex insect food webs in replicated field mesocosms and found that the overharvesting of one parasitoid wasp species caused increased extinction rates of other parasitoid species, compared to controls, but only when we manipulated the spatial distribution of herbivore species such that the potential for interspecific competition at this level was high. This provides clear evidence for horizontal extinction cascades at high trophic levels due to the proposed mechanism. Our results demonstrate that the loss of carnivores can have widespread effects on other species at the same trophic level due to indirect population-dynamic effects that are rarely considered in this context

Surfaces, Tree-Width, Clique-Minors, and Partitions

In 1971, Chartrand, Geller, and Hedetniemi conjectured that the edge set of a planar graph may be partitioned into two subsets, each of which induces an outerplanar graph. Some partial results towards this conjecture are presented. One such result, in which a planar graph may be thus edge partitioned into two series-parallel graphs, has nice generalizations for graphs embedded onto an arbitrary surface and graphs with no large clique-minor. Several open questions are raised. © 2000 Academic Press

The issue of parliamentary reform on England during the 1820s

This study is an attempt to determine how far parliamentary reform remained an important issue, and what arguments were offered for and against it, during a decade which did not produce the sort of major agitations in favour of the measure seen in 1816-19. Particular events and general trends characteristic of the decade are examined to see what effect they had on a reform debate which, though never the overriding obsession of the nation, did not disappear altogether. It is shown how the Queen Caroline affair, the largest mobilisation of anti-government opinion between the end of the Napoleonic Wars and the Reform Bill crisis, both provided a platform for reformist argument and to some extent directed attention away from purely political issues. Another section focuses on the effect of the severe agricultural distress of the early twenties on farming and landlord opinion and demonstrates that for a time at least reform was both widely discussed and widely supported in this sector of the community, in particular at the series of county meetings held in the first halves of 1821, 1822 and 1823. The attitude of the parliamentary Whig party to the issue is also examined, and their continuing difficulties over establishing a universally accepted party consensus on how, and even whether, parliamentary reform should be adopted as 'official' party policy are stressed. In a section dealing with the attitudes of the working classes and those who sought to influence them, the relationship of reform with such ideas and activities as Infidelity, Co-operation and trades unionism is looked at, and an attempt is made to gauge the extent to which Radicalism, or at least political feeling, revived during the severe slump in the textile-producing areas in 1826-7. Other important and interrelated facets of the period - the "liberalisation" of the Tory Government from 1822, the debate on Catholic Emancipation, the spread of education, the wider diffusion of general and political knowledge by mass print media expanding in size and sophistication, and the apparent increasing assertiveness of public opinion - are also dealt with, and the double-edged nature of their effect on the case for reform illustrated. The several attempts at partial representative or electoral change are described and their role in the contemporary reform debate is assessed, as are the initiatives on the closely related subjects of economical reform and retrenchment in government. The general conclusion of the study is that reform in the twenties by no means sank into oblivion. Conditions were against its assuming dominating importance, probably the most influential of those conditions being the comparative prosperity of the decade. However, several influential publicists for whom reform was "the one thing needful" continued to be active, and the mass enthusiasm of 1830-2 did not spring from nothing.<p

Analyzing and reconstructing reticulation networks under timing constraints

Reticulation networks are now frequently used to model the history of life for various groups of organisms whose evolutionary past is likely to include reticulation events like horizontal gene transfer or hybridization. However, the reconstructed networks are rarely guaranteed to be temporal. If a reticulation network is temporal, then it satisfies the two biologically motivated timing constraints of instantaneously occurring reticulation events and successively occurring speciation events. On the other hand, if a reticulation network is not temporal, it is always possible to resolve this issue by adding a number of additional unsampled or extinct taxa. In the first half of the paper, we show that deciding whether a given number of additional taxa is sufficient to transform a non-temporal reticulation network into a temporal one is an NP-complete problem. As one is often given a set of gene trees instead of a network in the context of hybridization, this motivates the second half of the paper which provides an algorithm for reconstructing a temporal hybridization network that simultaneously explains the ancestral history of two trees or indicates that no such network exists. We highlight two practical applications of this algorithm and illustrate the second application on a grass data set

Excluding any graph as a minor allows a low tree-width 2-coloring

This article proves the conjecture of Thomas that, for every graph G, there is an integer k such that every graph with no minor isomorphic to G has a 2-coloring of either its vertices or its edges where each color induces a graph of tree-width at most k. Some generalizations are also proved. © 2003 Elsevier Inc. All rights reserved

The K1 Protein of Kaposi's Sarcoma-Associated Herpesvirus Augments Viral Lytic Replication

ABSTRACT The K1 gene product of Kaposi's sarcoma-associated herpesvirus (KSHV) is encoded by the first open reading frame (ORF) of the viral genome. To investigate the role of the K1 gene during the KSHV life cycle, we constructed a set of recombinant viruses that contained either wild-type (WT) K1, a deleted K1 ORF (KSHVΔK1), stop codons within the K1 ORF (KSHV-K1 5×STOP ), or a revertant K1 virus (KSHV-K1 REV ). We report that the recombinant viruses KSHVΔK1 and KSHV-K1 5×STOP displayed significantly reduced lytic replication compared to WT KSHV and KSHV-K1 REV upon reactivation from latency. Additionally, cells infected with the recombinant viruses KSHVΔK1 and KSHV-K1 5×STOP also yielded smaller amounts of infectious progeny upon reactivation than did WT KSHV- and KSHV-K1 REV -infected cells. Upon reactivation from latency, WT KSHV- and KSHV-K1 REV -infected cells displayed activated Akt kinase, as evidenced by its phosphorylation, while cells infected with viruses deleted for K1 showed reduced phosphorylation and activation of Akt kinase. Overall, our results suggest that K1 plays an important role during the KSHV life cycle. IMPORTANCE Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of three human malignancies, and KSHV K1 is a signaling protein that has been shown to be involved in cellular transformation and to activate the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR pathway. In order to investigate the role of the K1 protein in the life cycle of KSHV, we constructed recombinant viruses that were deficient for K1. We found that K1 deletion viruses displayed reduced lytic replication compared to the WT virus and also yielded smaller numbers of infectious progeny. We report that K1 plays an important role in the life cycle of KSHV

Defensive insect symbiont leads to cascading extinctions and community collapse

Animals often engage in mutualistic associations with microorganisms that protect them from predation, parasitism or pathogen infection. Studies of these interactions in insects have mostly focussed on the direct effects of symbiont infection on natural enemies without studying community-wide effects. Here, we explore the effect of a defensive symbiont on population dynamics and species extinctions in an experimental community composed of three aphid species and their associated specialist parasitoids. We found that introducing a bacterial symbiont with a protective (but not a non-protective) phenotype into one aphid species led to it being able to escape from its natural enemy and increase in density. This changed the relative density of the three aphid species which resulted in the extinction of the two other parasitoid species. Our results show that defensive symbionts can cause extinction cascades in experimental communities and so may play a significant role in the stability of consumer-herbivore communities in the field. (Résumé d'auteur

Properties of star clusters - II. Scaleheight evolution of clusters

Until now, it has been impossible to observationally measure how star cluster scaleheight evolves beyond 1 Gyr as only small samples have been available. Here, we establish a novel method to determine the scaleheight of a cluster sample using modelled distributions and Kolmogorov-Smirnov tests. This allows us to determine the scaleheight with a 25 per cent accuracy for samples of 38 clusters or more. We apply our method to investigate the temporal evolution of cluster scaleheight, using homogeneously selected sub-samples of Kharchenko et al. (MWSC), Dias et al. (DAML02), WEBDA, and Froebrich et al. (FSR). We identify a linear relationship between scaleheight and log(age/yr) of clusters, considerably different from field stars. The scaleheight increases from about 40 pc at 1Myr to 75 pc at 1 Gyr,most likely due to internal evolution and external scattering events. After 1 Gyr, there is amarked change of the behaviour, with the scaleheight linearly increasing with log(age/yr) to about 550 pc at 3.5 Gyr. The most likely interpretation is that the surviving clusters are only observable because they have been scattered away from the mid-plane in their past. A detailed understanding of this observational evidence can only be achieved with numerical simulations of the evolution of cluster samples in the Galactic disc. Furthermore, we find a weak trend of an age-independent increase in scaleheight with Galactocentric distance. There are no significant temporal or spatial variations of the cluster distribution zero-point. We determine the Sun's vertical displacement from the Galactic plane as Z± = 18.5 ± 1.2 pc. ©2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society