Ecology of the burrowing amphipod, Pontoporeia affinis, in Lake Michigan / by Wayne Paul Alley.

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both intrinsic and extrinsic, that affect their behavior, performance,
and survival. The effects of these influencing factors are not independent but exhibit numerous complex interactions of which we know little.
Within any defined geographical area, some factors will remain constant
for a short time while others may vary. In general, the smaller the
area the greater will be the number of constant factors. In the shortterm investigation a small study area was sampled rapidly in order to
hold constant as many environmental factors as possible.
Populations are either randomly or nonrandomly distributed depending on the effects of the environmental factors. There are two categories
of departure from randomness: a contagious distribution where the individuals of the population tend to be aggregated, and a regular dispersal
where the individuals of the population tend to be uniformly space. It is
generally observed that practically all biological populations are contagiously dispersed and that few populations are randomly spaced.
Several theoretical distributions have been proposed to explain
these dispersal patterns and apparently some of these conform to empirical data. In the short-term study area two such theoretical distributions
were considered, the normal and the Poissono If members of a population conform to a Poisson distribution then the density of individuals is
low relative to the possible density that cound exist in that area, and the
members of the population are considered to be randomly spaced. If the
effects of all environmental factors are small or the environmental fac