A logic for reasoning about time and reliability

We present a logic for stating properties such as, "after a request for service there is at least a 98\045 probability that the service will be carried out within 2 seconds". The logic extends the temporal logic CTL by Emerson, Clarke and Sistla with time and probabilities. Formulas are interpreted over discrete time Markov chains. We give algorithms for checking that a given Markov chain satis- fies a formula in the logic. The algorithms require a polynomial number of arithmetic operations, in size of both the formula and\003This research report is a revised and extended version of a paper that has appeared under the title "A Framework for Reasoning about Time and Reliability" in the Proceeding of the 10thIEEE Real-time Systems Symposium, Santa Monica CA, December 1989. This work was partially supported by the Swedish Board for Technical Development (STU) as part of Esprit BRA Project SPEC, and by the Swedish Telecommunication Administration.1the Markov chain. A simple example is included to illustrate the algorithms

The fate of the missing spores

It is well-known that many species with small diaspores can disperse far during extended temporal scales (many years). However, studies on short temporal scales usually only cover short distances (in, e.g., bryophytes up to 15 m). By using a novel experimental design, studying the realized dispersal, we extend this range by almost two orders of magnitude. We recorded establishment of the fast-growing moss Discelium nudum on introduced suitable substrates, placed around a translocated, sporulating mother colony. Around 2,000 pots with acidic clay were placed at different distances between 5 m and 600 m, in four directions, on a raised bog, with increased pot numbers with distance. The experiment was set up in April-May and the realized dispersal (number of colonized pots) was recorded in September. Close to the mother colony (up to 10 m), the mean colonization rates (ratio of colonized pots) exceeded 50%. At distances between 10 and 50 m colonization dropped sharply, but beyond 50 m the mean colonization rates stabilized and hardly changed (1-3%). The estimated density of spores causing establishments at the further distances (2-6 spores/m2) was realistic when compared to the estimated spore output from the central colonies. Our study supports calculations from earlier studies, limited to short distances, that a majority of the spores disperse beyond the nearest vicinity of a source. The even colonization pattern at further distances raises interesting questions about under what conditions spores are transported and deposited. However, it is clear that regular establishment is likely at the km-scale for this and many other species with similar spore output and dispersal mechanism

To what extent does surrounding landscape explain stand-level occurrence of conservation-relevant species in fragmented boreal and hemi-boreal forest?-a systematic review protocol

Background: Silviculture and land-use change has reduced the amount of natural forest worldwide and left what remains confined to isolated fragments or stands. To understand processes governing species occurrence in such stands, much attention has been given to stand-level factors such as size, structure, and deadwood amount. However, the surrounding matrix will directly impact species dispersal and persistence, and the link between the surrounding landscape configuration, composition and history, and stand-level species occurrence has received insufficient attention. Thus, to facilitate optimisation of forest management and species conservation, we propose a review addressing 'To what extent does surrounding landscape explain stand-level occurrence of conservation-relevant species in fragmented boreal and hemi-boreal forest?'.Methods: The proposed systematic review will identify and synthesise relevant articles following the CEE guidelines for evidence synthesis and the ROSES standards. A search for peer-reviewed and grey literature will be conducted using four databases, two online search engines, and 36 specialist websites. Identified articles will be screened for eligibility in a two-step process; first on title and abstract, and second on the full text. Screening will be based on predefined eligibility criteria related to a PECO-model; population being boreal and hemi-boreal forest, exposure being fragmentation, comparator being landscapes with alternative composition, configuration, or history, and outcome being occurrence (i.e., presence and/or abundance) of conservation-relevant species. All articles that pass the full-text screening will go through study validity assessment and data extraction, and be part of a narrative review. If enough studies prove comparable, quantitative meta-analyses will also be performed. The objective of the narrative review and the meta-analyses will be to address the primary question as well as six secondary questions, and to identify important knowledge gaps

A Fully Abstract Trace Model for Dataflow and Asynchronous Networks

A dataflow network consists of nodes that communicate over perfect unbounded FIFO channels. For dataflow networks containing only deterministic nodes, a simple and elegant semantic model has been presented by Kahn. However for nondeterministic networks the straight-forward generalization of Kahn's model is not compositional. We Present a compositional model for nondeterministic networks which is fully abstract, i.e. it has added the least amount of extra information to Kahn's model which is necessary for attaining compositionality. The model is based on traces. We also generalize our result, showing that the model is fully abstract also for classes of networks where nodes communicate over other types of asynchronous channels. Exaamples of such classes are networks with unordered channels, and networks with lossy channels

A hierarchy of compositional models of I/O-Automata

A semantic model of computer systems is compositional if it adequately represents the behavior of the modeled systems in a context of other systems. A compositional model is thus a good basis for specifying and reasoning about systems in a modular fashion. I/O-automata is a class of communicating system which can represent several types of asyncronously communicating systems, such as message-passing distributed systems, systems with broadcast communication, and systems with shared variables. In contrast to many other classes of communicating systems (e.g. in CCS or CSP), semantic models based only on traces(sequences of communication events) can in a compositional way represent safety, liveness and many other proportiesd of I/O-automata. In this paper, we investigate which semantic models of I/O-automata are compositional, and which are not. The investigation is confined to models based on traces. We study a number of trace-based semantical models of I/O-automata, which differ in their capability to represent safety, liveness, termination, and divergence properties. The defined models can be naturally ordered into a hierarchy, according to how much information they convey about the modeled systems. The main contribution of the paper is an investigation of whether there are other compositional models between adjacent compositional models in our hierarchy. Surprisingly enough, we can prove that for several pairs of adjacent models in the hierarchy, the gap between the two models contains no other compositional model. For instance, the nonexistence of a compositional model in the gap between a model that represents safety properties and a model that represents liveness properties means that liveness properties cannot be only partially represented if compositionality is desired. We indicate how our results can be applied to derive results about full abstraction

A fully abstract trace model for dataflow networks

A dataflow network consists of nodes that communicate over perfect FIFO channels. For dataflow networks containing only deterministic nodes, a simple and elegant semantic model has been presented by Kahn. However, for nondeterministic networks, the straight-forward generalization of Kahn's model is not compositional. We present a compositional model for nondeterministic networks, which is fully abstract, i.e., it has added the least amount of extra information to Kahn's model which is necessary for attaining compositionality. The model is based on traces

Protecting forest edges using trap logs - Limited effects of associated push-pull strategies targeting Ips typographus

Bark beetles can cause epidemic outbreaks and kill millions of cubic meters of economical and ecologically important forests around the world. It is well known what attracts and what repels different species of bark beetle, and these chemical cues can be used to protect trees and catch the beetles without using pesticides. Applying this knowledge, we investigated the use of push-pull strategies with trap logs along susceptible edges of a Swedish boreal spruce forest. The repellents (push) used were non-host volatiles (NHV) attached to tree trunks at the forest edge, and the attractants (pull) was a commercial aggregation pheromone attached to trap logs. The aim was to test whether the Ips typographus catch could be significantly increased by combining a push-pull system with traditional trap logs, thereby providing additional protection. The experiment was performed over two years and included the main flight period of I. typographus. The study sites were clear-cuts that had been harvested the preceding winter, and sun-exposed forest edges of mature spruce were targeted for protection. A full factorial setup was used comprising two treatments (repellent and attractant) and a control. Seven replicates of the trap logs were used, three during the first year and four during the second. The number of established I. typographus maternal galleries per square meter of log surface was used as the response variable. The trap logs captured large numbers of I. typographus, at an average density of 353 and 169 maternal galleries per m(2) during year 1 and year 2, respectively, over all treatments. Based on the catch data, with a sufficient number of trap logs, the risk of tree mortality at forest edges may be reduced and we recommend its general use. However, we did not see any significant effect of either the repellent or the attractant on the density of maternal galleries. Hence, we cannot recommend the addition of chemical cues to improve the efficiency of trap logs. Although trap logs are efficient in capturing bark beetles and hence may protect forest edges, it does not imply that they can provide protection on a larger scale. In line with other studies, we hence recommend that forest management to target nature-based solutions that strengthen the resilience of forest stands, by using mixed forest stands and resistant plant species, and nurture habitats for natural predators of I. typographus

Simulating perfect channels with probabilistic lossy channels

AbstractWe consider the problem of deciding whether an infinite-state system (expressed as a Markov chain) satisfies a correctness property with probability 1. This problem is, of course, undecidable for general infinite-state systems. We focus our attention on the model of probabilistic lossy channel systems consisting of finite-state processes that communicate over unbounded lossy FIFO channels. Abdulla and Jonsson have shown that safety properties are decidable while progress properties are undecidable for non-probabilistic lossy channel systems. Under assumptions of “sufficiently high” probability of loss, Baier and Engelen have shown how to check whether a property holds of probabilistic lossy channel system with probability 1. In this paper, we consider a model of probabilistic lossy channel systems, where messages can be lost only during send transitions. In contrast to the model of Baier and Engelen, once a message is successfully sent to channel, it can only be removed through a transition which receives the message. We show that checking whether safety properties hold with probability 1 is undecidable for this model. Our proof depends upon simulating a perfect channel, with a high degree of confidence, using lossy channels