Species diversity of herbivorous insects: a brief review to bridge the gap between theories focusing on the generation and maintenance of diversity

Abstract

Herbivorous insects are remarkably species-diverse, and the cause of such diversity remains a classical issue in the fields of ecology and evolution. The traditional explanation for the huge diversity of such insects is that repeated dietary changes over evolutionary time provided opportunities for speciation, thereby enhancing the diversification rate. A different view suggests that herbivore diversity became saturated over time, with factors affecting the points of dynamic equilibrium of species diversity within each lineage (and thus associated with maintenance of species diversity) being the determinants of the diversity evident today. Thus, both generation and maintenance processes, and their relative importance, are critical for understanding the diversity of herbivorous insects. Furthermore, the neutral theory of biodiversity and biogeography has recently gained attention as an alternative explanation for the generation and maintenance of diversity, as opposed to adaptive processes centred around host specificity. However, these possible routes toward herbivore diversity have rarely been evaluated in parallel, and the work of various groups has become both segmentalised and complicated, compromising any comprehensive understanding of the issue. Thus, in the present paper, I briefly review our knowledge of herbivore diversity and the major relevant studies. The aim was to share knowledge, creating a common starting point from which future discussions among researchers may be generated. It may be that no single approach can resolve the many remaining questions on herbivore diversity. However, an improved understanding of such diversity can be achieved by combining knowledge gained in studies of both the generation and maintenance of diversity.