Coexistence of Two Leaf-Beetles in the Same Fundamental Niche. Distribution, Adult Phenology and Oviposition

Abstract

This study investigated the coexistence of two leaf-beetles, Galerucella calmariensis and G. pusilla, on purple loosestrife (Lythrum salicaria). G. pusilla and G. calmariensis share their distribution on a continental, regional, and local scale. Models of stable coexistence of two competitors assume access of inferior species to resources or refuges that are temporarily or spatially unexploited by the superior species. Field and laboratory studies indicated that both species have inclusive fundamental niches on their host plant. Cytological studies and crossing experiments confirmed their status as true, reproductively isolated species. G. calmariensis and G. pusilla showed no differences in adult phenology, winter mortality, colonization pattern nor oviposition, thus no temporal variation in resource use. Adults aggregated at the same sites and on the same host plant individuals and their distributions are not independant. Aggregation or differences in resource use do not appear to play an important role in the coexistence of G. calmariensis and G. pusilla. This study indicated that both species have identical competitive abilities.