such as storms, severe winters, some predation,
and some disease. The second type of event de
pends upon the density of the population for both
its occurrence and strength. Examples are short
ages of food and nesting holes. Both types seem
to be important for all well-studied species. The
first kind will be called density independent and
the second density dependent. This is slightly
different from the usual definitions of these terms
which require the effects upon the population and
not the occurrence to be density independent or
dependent (Andrewartha and Birch 1954)