A major focus of research on the dynamics of host-pathogen interactions has been the evolution of pathogen virulence, which is defined as the loss in host fitness due to infection. It is usually assumed that changes in pathogen virulence are the result of selection to increase pathogen fitness. However, in some cases, pathogens have acquired hypovirulence by themselves becoming infected with hyperparasites. For example, the chestnut blight fungus Cryphonectria parasitica has become hypovirulent in some areas by acquiring a double-stranded RNA hyperparasite that debilitates the pathogen, thereby reducing its virulence to the host. In this article, we develop and analyze a mathematical model of the dynamics of host-pathogen interactions with three trophic levels. The system may be dominated by either uninfected (virulent) or hyperparasitized (hypovirulent) pathogens, or by a mixture of the two. Hypovirulence may allow some recovery of the host population, but it can also harm the host population if the hyperparasite moves the transmission rate of the pathogen closer to its evolutionarily stable strategy. In the latter case, the hyperparasite is effectively a mutualist of the pathogen. Selection among hyperparasites will often minimize the deleterious effects, or maximize the beneficial effects, of the hyperparasite on the pathogen. Increasing the frequency of multiple infections of the same host individual promotes the acquisition of hypovirulence by increasing the opportunity for horizontal transmission of the hyperparasite. This effect opposes the usual theoretical expectation that multiple infections promote the evolution of more virulent pathogens via selection for rapid growth within hosts.
The concepts of the cities we know nowadays, and which we are accustomed to, change at a very rapid pace. The philosophy of their design is also changing. It will base on new standards, entering a completely different, futuristic dimension. This stage is related to changes in the perception of space, location and lack of belonging to definite, national or cultural structures. Cities of the future are cities primarily intelligent, zero-energetic, zero-waste, environmentally sustainable, self-sufficient in terms of both organic food production and symbiosis between the environment and industry. New cities will be able to have new organisational structures-either city states, or, apolitical, jigsaw-like structures that can change their position-like in the case of the city of Artisanopolis, designed as a floating city, close to the land, reminiscent of the legendary Atlantis. This paper is focused on the main issues connected with problems of the contemporary city planning. The purpose of the research was to identify existing technological solutions, whose aim is to use solar energy and urban greenery. The studies were based on literature related to future city development issues and futuristic projects of the architects and city planners. In the paper, the following issues have been verified: futuristic cities and districts, and original bionic buildings, both residential and industrial. The results of the analysis have been presented in a tabular form.
All over the world and for thousands of years, megaliths have been significant cultural elements, as well as sacred sites and places of power. Nowadays megaliths act as a strong magnet for tourists, who appreciate their history, esoterica and magic. Some megaliths were used for astronomical observations, so vital to maintain the continuity of harvest and crop. Other megalithic constructions were erected for funerary purposes, and served as individual or collective burial chambers. Megalithic structures are usually referred to as belonging to the European Neolithic but it has to be stressed that some megalithic constructions date back to the Bronze Age, and some were also built on other continents. Megaliths are a vital element of landscape and for historical reasons they are a sui generis monument, commemorating prehistorical cultures. At the same time, along with the remaining elements of the natural and cultural environment, they create a unique image of place identity, attracting large numbers of tourists. Interestingly, despite the strong attraction exercised by megaliths, there are still many places where tourism does not develop as rapidly as might be assumed. For the above-mentioned reasons, a comparative analysis of several megalithic sites has been conducted in Poland, Sweden, Portugal and Denmark. The following elements have been analysed: the megaliths immediate surroundings, the existing and planned or under-construction tourist and communication infrastructure, as well as architectural and spatial technical solutions and development. Also the key negative and positive elements have been defined which influence the tourist potential of the places in question, and constitute the tourism attractiveness factors of a region.
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