Relationships between species richness and morphological diversity: insight from a geometric morphometric analysis of scarabs

Abstract Background Species richness (SR) and morphological diversity (MD) provide insights into biodiversity and have been studied extensively as important indicators in recent years. Most researches have been found a positive correlation between SR and MD in many groups, which reflects the influence of phylogenetic factors. However, if some ecological factors are considered, the relationship of SR and MD is not always consistent. In such cases it is interesting to know how phylogenetic factors and ecological factors affect the relations between SR and MD. Unfortunately, studies that examine both factors quantitatively, in the same group with large dataset are lacking. Results In this study, 5148 scarabs covering 92% families and 70% scarabaeoid subfamilies of Scarabaeoidea were selected as a test group. The taxonomic categories of family and subfamily were selected for phylogenetic representation and four feeding types (omnivory, phytophagy, coprophagy and non-feeding) were selected as ecological factors. A geometric morphometric analysis on the pronotum and elytron, which represent the major aspect of the morphology in dorsal view, were conducted. Correlations between MD and SR among sub-/families were always found to be consistent, except a genus-level test of the pronotum on the subfamily category and a species-level test of elytron on the family category. These phenomena reflected a strong phylogenetic influence. However, morphological diversities were not correlated significantly with species richness for any feeding type (ecological factors). In addition to this hypothesis test, the morphological diversity and known world species richness (~ 32000 species and 2453 genera of Scarabaeoidea) were estimated as well. Our global results were very similar to our sampling results. Conclusions Based on our analyses the hypothesis of ecological factor (represented by feeding habit) correlations was rejected in scarabs. On the contrary, phylogenetic factor correlations were found to be statistically significant. The morphological diversity of scarabs in most sub-/families fitted their species richness. At the same time, the correlation our test parameters revealed variation in both groups and characters, which might be caused by morphological changes under coevolution with different ecological factors.