Species richness is distinguished from the evenness of apportionment of individuals among species. Confusing these concepts in "diversity indices" is condemned. Diversity should be related to specified taxa and habitats. Two kinds of biological diversity are distinguished - gradient diversity and mosaic diversity. Of various proposed causes of high diversity, niche creation through physical and biological heterogeneity is emphasised. It is shown that when there is a mutualistic relationship between species higher species richness can be achieved without loss of stability. Examples of such high diversity ecosystems include the phytal community of temperate tidepool algae. Observations on the phytal community associated with Corallina officinalis indicate that increasing the abundance of a dominant epiphvtic serpulid, Spirorbis corallznae, correlates with higher numbers and greater species richness of the phytal. It is thought that the increase in the numbers and variety of spaces and surfaces created by settling spirorbids is the feature that offers more niches to the associated fauna.