A species may be regulated by density dependent
events and yet undergo dramatic changes in popu
lations due to changes in the limiting factor itself.
In this case tests by the theory of runs, used above,
are likely to be useless. However. if a correlation
can be made of the population with the environ
mental factor undergoing change, then not only
can density dependence, i. e. existence of a limiting
factor, be established, but also the nature of the
limiting factor. For, if an increase in one environ
mental variable can be established, an experiment
of the first type described above has been per
formed. That is, the habitat has been modified in
one factor and a resulting change in bird popula
tion has been observed. Therefore, because the
population changes, that one factor has been
limiting.