Measuring Spatial Distribution (Download the Lab Quicklink to satellite image, Quicklink to Excel Sheet)
NOTE: Do NOT print out the image. It is VERY large and it is easiest to work on the computer.
Much of the research in ecology is ultimately tied to the problems of species distributions and abundance. In some cases, only a single species is of interest. Many of the earlier investigators were content to quantify population parameters for their favorite species in a variety of habitats. Recent research is more often concerned with assemblages of species (e.g. description at the community level). This research attempts to determine the environmental factors (abiotic factors) and species interactions (biotic factors) that may explain the observed patterns
Dispersion patterns are described as regular, random, or clumped. Truly random distribution are uncommon in nature since organisms tend to interact with both their environment and nearby organisms (con- or heterospecific). Regular patterns are often seen in response to dispersed resources (such as telephone poles used as perching sites for birds) or through behavioral interactions (birds on a wire; or penguins in a rookery). Clumped distributions are most commonly seen (paper birch) and may be due to social behavior or habitat structure (for example, paper birch is found near water).
To calculate the dispersion pattern
the study area is divided into quadrats. The number of individuals in each
quadrat are then counted and the mean number of organisms per quadrat and the
variance among quadrats is calculated. Variance (abbreviated "s2")
is a measure of the amount of divergence about the average (or mean; abbreviated
"
"). A sample with widely fluctuating differences in the number of plants in
each quadrat will have a large variance while one with a homogeneous data set
will have a low variance. If there is little variation from one quadrat to
another (more or less the same number of organisms in each plot) then the
organisms are said to have a "regular" or "dispersed" distribution and the
variance to mean ratio (s2:) will be less than 1.0. If many of the
plots are uninhabited, while other have a large number of organisms, the
variance will be much greater than the mean and the organisms are said to
exhibit a "clumped" or "contagious" distribution. Clumped distributions have a
variance to mean ratio considerably greater than 1.0. Finally, if the variance
is very close to the value of the mean, then the population is said to exhibit a
random distribution (s2:~ 1.0).
More on the variance to mean ration or Coefficient of Dispersion and other methods used to calculate the distribution of organisms: