More than 300 samples of Macrolepidoptera have been collected over 24
years at a site in southern England on field courses run for universit
y students. The samples were taken in mercury vapour light traps. They
show that numbers have fluctuated markedly between periods of high ab
undance and periods of low abundance. Species richness in the samples
is strongly affected by abundance. Evenness of distribution of numbers
between species is higher in samples from woodland than in samples co
llected over grass, and higher earlier than later in the season. For a
series of samples from the same population, MacArthur's overlapping n
iche and the broken stick resource apportionment models predict a weak
ly positive regression of the evenness J of a sample on species number
, whereas the sequential breakage model predicts a negative regression
. The latter implies the highest level of competitive interaction with
in the moth communities sampled. We find that the data agree with the
sequential breakage model, rather than the other two. A weak positive
regression was expected in view of the trapping method used but was no
t found. The fit of the sequential breakage model also implies that sp
ecies abundance is log normally distributed, which it may be for many
reasons. It is argued nevertheless that such comparisons may be of use
for detecting competitive interaction, and that it is important to do
so in order to judge the validity of predictions about effects of env
ironmental change or human interference on the structure of communitie
s. (C) 1996 The Linnean Society of London