A comparison of the establishment success, response to competition, and community impact of invasive and non-invasive Gambusia species

Abstract

How an invader responds to the novel biotic elements of a new community will affect its ability to invade. Species that are able to cope well with novel competitors might be expected to achieve greater establishment success. We compared the population-level responses of two mosquitofish species, the widespread invader Gambusia affinis and non-invasive G. geiseri to competition from each other and a non-invasive competitor. We simulated the invasion of a simplified pond community by introducing different combinations of the Gambusia spp. into communities already inhabited by a novel competitor, the red shiner Cyprenella lutrensis. We measured the effect of competition on establishment success by comparing population abundances achieved by each Gambusia species in all treatments, and examined whether the invasive and non-invasive Gambusia differed in their community impact by comparing their effects on the abundances of pond fauna. We also used N and C stable isotope analysis to compare their trophic roles. Both novel and intrageneric competition negatively affected both Gambusia spp.’s abundances, but the invasive G. affinis managed to remain more abundant than G. geiseri regardless of competition treatment. Stable isotope analysis revealed the Gambusia spp. to have similar trophic placement and showed competition to cause significant trophic shifts only in G. geiseri. A cascading effect (reduced phytoplankton abundances) was detected only when G. affinis was present. These results suggest that a higher vulnerability to novel competition along with life history traits contributes to the limited spread of G. geiseri in comparison to the widespread G. affinis.