It IS the aim of this section to demonstrate that
the five species of warbler are primarily regulated
by density dependent events, that is, that they
increase when rare and decrease when common
(relative to the supply of a limiting factor). The
strongest argument for this is the correlation of
abundances with limiting factors discussed later.
However, to avoid any risk of circularity, an independent partial demonstration will now be given.
If density independent events do not occur
randomly but have a periodic recurrence, then a
population controlled by these events could undergo a regular oscillation nearly indistinguishable
in form from that of a population regulated by
density dependent events. The distinction can be
made, however, by observing the effect of the
presence of an ecologically similar species. Here
it will first be shown that increases and decreases
are not random; then an argument will be given
which renders the density independent explanation
improbable.