The generic relations of species in small ecological communities

1. Evidence of conditions being more favourable or less favourable to species of the same genus, as compared with species on different genera (intrageneric versus inter-generic competition), can be found in the relative number of species and genera in small and in large natural communities of animals or plants. 2. It is, however, insufficient to show that the average number of species per genus is smaller in the smaller communities than in the larger as this is a mathematical result of taking a smaller sample from a larger group. It is necessary to show that the proportion of genera to species in the smaller communities is smaller or larger than would have been expected in a randomized sample of the same number of species, selected without reference to generic relationships from the larger fauna or flora. 3. Evidence had previously been brought forward to show that in large groups of animals or plants the number of genera with 1, 2, and with 3 and more species are closely represented by the mathematical `logarithmic series'. New evidence is here given to show that the same order exists in the genera and species of quite small ecological communities. 4. The logarithmic series has several mathematical properties of biological interest, including the possibility of calculating, for any population or sample, a factor known as the `Index of Diversity' which is common to all random samples from a single population. It is a measure of the extent to which the species are grouped into genera, and it is independent of the size of the sample. If the Index of Diversity is high, there are many genera in relation to the number of species; if the Index is low, there are fewer genera in relation to the number of species. 5. It thus becomes possible to compare the Index of Diversity in natural small or simple ecological communities, with that of the larger population from which these have been selected by nature. This has been done for a number of cases, including both animal and plant communities, and in every case from which significant results can be obtained the Index of Diversity in the small community is smaller than that of the larger fauna or flora. 6. The result therefore indicates that there are fewer genera in a small or simple community than would be expected in a sample of the same number of species selected at random--that is, independent of genera relationships--from the larger series. In other words, the evidence brought forward by this method indicates a selection by nature in favour of more than one species in the same genus rather than in favour of single species in different genera.