David G. Green

Open problems in artificial life

This article lists fourteen open problems in artificial life, each of which is a grand challenge requiring a major advance on a fundamental issue for its solution. Each problem is briefly explained, and, where deemed helpful, some promising paths to its solution are indicated.

Simulated effects of fire, dispersal and spatial pattern on competition within forest mosaics

Simulations representing tree locations on a rectangular grid (cellular automaton) imply that spatial patterns associated with fire, seed dispersal, and the distributions of plants and resources affect forest dynamics profoundly. Simulated fires ignited at random locations in a uniform environment create non-uniform habitats and lead to patches dominated by different vegetation types. Short-range seed dispersal promotes vegetation clumping; fires cause these clumps to coalesce into vegetation zones separated by sharp borders, especially across an environmental gradient. In simulation of competition within vegetation mosaics, tree populations with a competitive advantage still require the intervention of fire to eliminate rivals. Also, the availability of local seed sources enables established tree populations to exclude invaders, but fires can trigger sudden changes in the composition of such systems. In models of simple succession systems, ‘climax’ vegetation tends to displace ‘pioneer’ vegetation, even under harsh fire regimes.

Time series and postglacial forest ecology

Abstract Forest ecology suffers from a lack of long-term community records. Preserved pollen data are richer in such information than is generally realized. By applying suitable statistical techniques to pollen records, one can learn much about competition, succession, and population dynamics in past tree communities. In this study, preserved pollen records from Everitt Lake, Nova Scotia, are analyzed as time series. Time domain studies reveal the post-fire responses of individual tree taxa. Correlograms yield models of past forest succession patterns. The models explain some effects of changing fire frequency, thus suggesting mechanisms by which fire, competition, and climate combine to produce long-term forest composition changes. Frequency domain studies suggest relationships between disturbance cycles, stand composition, and forest mosaics. Fire frequencies are seen to be highest where fire-dependent species abound and most regular where tree stands have uniform, not mixed, composition.

Continental-scale interactions with temporary resources may explain the paradox of large populations of desert waterbirds in Australia

Arid Australia supports extraordinary numbers of waterbirds. We show that the solution to this seeming paradox lies in considering the availability of temporary wetland habitat in the context of the birds dispersal capability and fluctuations in the abundance of wetlands in time and space. For species with large dispersal capabilities, the Lake Eyre Basin of central Australia, amongst the driest regions on the continent, has the highest habitat availability for waterbirds. Analyses of landscape structure show that the wetlands of the Lake Eyre Basin are highly inter-connected and linked by broad pathways to wetter parts of south-eastern Australia. These analyses illustrate that organism traits and patch dynamics affect realised habitat availability and indicate that the processes that structure populations may operate at much larger spatial scales than those at which humans usually seek to manage the landscape.

Shapes of simulated fires in discrete fuels

Abstract Simulation models are used to examine the possible effects of discrete fuel distributions and of several fire-spread mechanisms on fire shapes. Two postulated fire spread mechanisms —heat accumulation and flame contact—are shown to yield near-ellipses in continuous fuels, but a wide range of shapes in discrete and very patchy fuels. The alternative shapes include ovoids, “tear-drop” (with the ignition point at varying positions on the major axis), and straight lines. Simulated fires in discrete, patchy fuels are less regular in shape than in uniform and continuous fuels and show little or no backburning. The results may explain certain observed differences between wildfire shapes that occur in different environments and at different burning intensities.

Risk and Social Work Practice

Abstract Risk assessment and risk management have emerged as central organising principles for an increasing number of health and welfare programs. As a consequence, the language, technologies and imperatives of risk have assumed considerable prominence in the practice of many social workers. The present paper addresses three significant and contemporary explanations for the rise of risk in the day-to-day work of many social workers. In the context of these approaches, the paper examines some of the challenges risk presents to professional and ethical social work practice. Four issues with particular meanings and impacts for social work are discussed, namely the priority of risk over need, changing professional roles, relocation of responsibility to individuals, and the culture of blame.

Fire shapes and the adequacy of fire-spread models

Abstract It has been suggested that shapes of burned areas resulting from fires spreading under uniform fuel and meteorological conditions may be described as ellipses, double ellipses, or ovoids. The adequacy of these shapes (together with simulation outputs) as bases for fire spread models was tested by finding the best fits of each shape to maps of experimental fires and comparing the results with fits given by a rectangle (an unlikely fire shape). Each of the models (ellipse, double ellipse, ovoid, simulation model, and even the rectangle) provided adequate approximations to the fire contours used in the tests. The parameter trends found implied that the fires examined tended to become more nearly elliptical in shape and to have higher eccentricity as they grew.

Complexity in landscape ecology

Foreword.- Preface.- 1. Complexity and ecology.- 2. Seeing the wood for the trees.- 3. Complexity in landscapes.- 4. Oh, what a tangled web.- 5. The imbalance of nature.- 6. Populations in landscapes.- 7. Living with the neighbours.- 8. Genetics and adaptation in landscapes.- 9. Virtual worlds.- 10. Digital ecology.- 11. The global picture.- References.- Index.

Interactive simulation of bushfires in heterogeneous fuels

The program IGNITE, developed by the authors, is a landscape fire modelling system that deals with fires in heterogeneous fuels. Landscapes are represented as cellular automata (grids of pixels) and fire spread is modelled as an epidemic process. An integrated geographic information system permits the importing and editing of maps from compatible sources, such as satellite imagery. Maps, models and other information are organized as scenarios; historical fires can be recorded and replayed. Modules are being developed for application to fire prevention, fire suppression, land-use management, and to training and education. An illustration of using the system to deal with heterogeneous fuel is its application to the problem of percolation in patchy fuel.

A fine-resolution palaeoecology and palaeoclimatology from south-eastern Australia

(1) This study attempts to bridge the gap in time-scale that has separated sedimentary pollen records from modern ecological and meteorological data. (2) Two pollen records were obtained, both at yearly intervals: a pollen-rain history (1975-84) and a record of past (1950-75) pollen accumulation, based on detailed 14C analyses of bomb carbon from finely-sectioned peat deposits. By matching these data with instrumental rainfall records at yearly intervals, recent pollen production and accumulation changes are related to short-term fluctuations in precipitation and fire occurrence. (3) The modern pollen-rain study demonstrates a close relationship between rainfall and the pollen production and accumulation of several taxa. (4) Comparisons between sedimentary pollen records and meteorological data show that the vegetation response, through pollen production and accumulation, was sufficiently sensitive to register short-term, low-amplitude changes in precipitation in the swamp sedimentary record. (5) Related studies include charcoal particle analyses, fire scar surveys, and process modelling. (6) The combined results provide a basis for recognizing short-term, low-amplitude changes in vegetation and precipitation, not only from contemporary sediments, but also from finely datable sediments of Holocene and late Pleistocene origin.