Evenness and species number in some moth populations

More than 300 samples of Macrolepidoptera have been collected over 24 years at a site in southern England on field courses run for university students. The samples were taken in mercury vapour light traps. They show that numbers have fluctuated markedly between periods of high abundance 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 collected over grass, and higher earlier than later in the season. For a series of samples from the same population, MacArthur's overlapping niche and the broken stick resource apportionment models predict a weakly 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 within 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 not found. The fit of the sequential breakage model also implies that species 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 environmental change or human interference on the structure of communities.