Unveiling the factors determining herbivore species richness is a major challenge in community ecology. Although herbivore species is closely associated with plant species, the degree of the correlation between plant and herbivore species richness considerably varies among the previous studies. Some studies detected the correlation, while others did not and instead concluded that abiotic factors determined the herbivore species richness.
Two hypotheses are proposed to explain such variation of the degree of correlation. First, if some plant species are not utilized by herbivore species, the correlation would become weak. Second, if niches of herbivore species overlap with each other, the correlation is expected to be weak.
In 19 semi-natural grasslands, plant and herbivore insect species and their relationships (herbivory and staying on the plant) were surveyed. Herbivores' niches were measured from the direct observation of herbivory or examined by the statistical method detecting the preferred stays.
Herbivore species richness was better explained by the utilized plant species richness than by the total plant species richness, supporting the first hypothesis. Furthermore, the degree of niche overlap between herbivore species (Horn's index) negatively affected the correlation, supporting the second hypothesis as well.
To our best knowledge, this is the first empirical study that explained the variation of the correlation between plant and herbivore species richness, and would contribute to further understanding of the determinants of local herbivore species richness.