Interspecific competition between two self-fertile and one related sexual species of tadpole shrimp Triops (Branchiopoda, Notostraca) with reproductive interference, Allee effect and environmental fluctuation

Abstract

Invasive alien species of tadpole shrimp Triops, originated from America, were first introduced into rice fields of Japan in the 1910s and have spread around Japan. In the 1950s, a European species of Triops also seems to have entered Japan. Originally, Japanese paddy fields had Asian Triops species as a prehistoric naturalised animal. A new invasive Triops from Western Australia has been reported in the south part of Japan, occasionally forming mixed populations. The expansion of Triops has resulted in negative ecological and potential economic impact. Longhursts revision did not contain any specimens of the richest collection in the world, i.e., Daday Collection in the Hungarian Natural History Museum. And therefore, Naganawa surveyed at least 20 times as many samples as those by Longhurst, and number the named Triops granarius s.l. species group from Africa-India to Siberia-Asia at eight species, organised in the same genus. As for the establishment or extinction patterns, based on Tilmans competition model, we built a simple mathematical competition model between two self-fertile and one related sexual species of Japan, incorporating reproductive interference. To make the mathematical model more natural, we also assumed environmental fluctuation and Allee effect, which reduces the population growth rate of sexual species with low density. Intense reproductive interference drove the competition to sexual exclusion of self-fertile species. However, even under intense reproductive interference, Allee effect allowed self-fertile species to survive when the competition started with the lower densities; contrary to the survival of sexual species when the competition started with the higher densities.