Different ammonia tolerances may facilitate spatial coexistence of Gammarus roeselii and the strong invader Dikerogammarus villosus

Abstract

The introduction of invasive species that can replace native species is one of the most critical threats to the biodiversity of aquatic systems. Here we investigated the potential contribution of one factor to the coexistence of the indigenous amphipod Gammarus roeselii and the invasive amphipod Dikerogammarus villosus in the same ecosystem (Lake Constance) within different microhabitats. We quantitatively studied the influence of ambient ammonia concentrations on the distributions of the two amphipod species. We also assessed the ammonia tolerance ranges of both species in laboratory experiments by measuring mortality rate, precopula disruption, egg mortality, and microhabitat choice. The proportion of G. roeselii among the two amphipod species was significantly positively related to the ammonia concentration in the water, which indicated that the distribution of the invasive D. villosus was limited at high ammonia concentrations. Although the mortality rates of the two species did not significantly differ, G. roeselii was more tolerant to ammonia with regard to precopula disruption, egg mortality, and microhabitat choice. The effective ammonia concentrations that led to a significantly reduced direct reproductive success in D. villosus were within the range of the highest field concentrations measured, where only G. roeselii occurred. D. villosus may have a smaller range than the indigenous G. roeselii partially because of its lower tolerance to higher ammonia concentrations, which lead to reduced reproductive success. Beside other habitat parameters differences on ammonia tolerance between the two amphipods might allow their coexistence along a gradient of microhabitats in Lake Constance.