GRID CELL CONTOUR MAPPING OF POINT DENSITIES - BARK BEETLE ATTACKS, FALLEN PINE SHOOTS, AND INFESTED TREES

A method for calculating and displaying patterns of local density of plants and animals is presented for use with personal computers. The algorithm, coded in the BASIC programming language, uses x,y spatial point coordinates of organisms to calculate and display a coloured or shaded map of local densities within grid cells. The radius of the local areas about the grid points within which densities are calculated, the density-class interval boundaries when colouring the cells, and the grid resolution can all be varied to facilitate exploratory investigations. Contour density maps resulting from the method are shown for an attack distribution of the bark beetle Pityogenes chalcographus (Coleoptera: Scolytidae) on Norway spruce bark, Picea abies, and for a random distribution. Minimum allowed distance analysis of the attack distribution indicated a distance of behavioural avoidance of attack sites of about 1.6 cm for the bark beetle. Density maps of different local radii are also shown for Norway spruce trees killed by the bark beetle Ips typographus near Trebon, Czechoslovakia. Density maps using different local radii and density increments are presented for aggregated patterns of fallen Scots pine shoots, Pinus sylvestris, infested by Tomicus minor bark beetles in southern Sweden. A method is presented for statistical comparison of randomly-generated spatial point data with natural data by using Chi-square analysis of the histograms of differently coloured grid cells.