COS 261-2 - The influence of salinity on competition between invasive Gambusia holbrooki and native Aphanius fasciatus in the Mediterranean

Background/Question/Methods

Eastern mosquitofish (Gambusia holbrooki) are among the most invasive freshwater species globally. Competition from mosquitofish has been proposed as one of the main causes of displacement of many aquatic species such as the Mediterranean banded killifish (Aphanius fasciatus). Currently, A. fasciatus have disappeared from fresh and oligosaline waters, and their distribution appears to be largely restricted to higher salinity habitats. Hence, it has been suggested that salinity may limit mosquitofish invasiveness and provide refuge for native Aphanius. However, given the extreme salinity tolerance of mosquitofish, the exact mechanism behind this pattern is still unknown. To answer this question, we sampled multiple populations of G. holbrooki and A. fasciatus in Sardinia, Italy. We investigated differences in functional response between the species, sexes, and populations-within-species and the influence of changing salinity (15 and 30 ppt) on these. We further evaluated aggressive interactions between the species during direct food competition trials. We predicted that (i) mosquitofish would generally display higher functional responses than killifish but a decrease in feeding efficiency at high salinities, while the pattern for killifish should be reversed; (ii) mosquitofish would be more aggressive than killifish; and (iii) populations of both species would differ in functional response and aggression.

Results/Conclusions

Our analyses revealed that mosquitofish and killifish differ in their functional response, with mosquitofish showing a slightly higher functional response than their native counterparts. However, different populations of each species exhibited different functional responses, and functional response was also salinity-dependent. In the direct competition experiments, killifish were more voracious than mosquitofish, but mosquitofish were much more aggressive than killifish, resulting in severe injuries for killifish on multiple occasions. Again, we uncovered significant variation between populations for these species interactions, and mosquitofish aggressive behaviour was significantly reduced in higher salinity. However, we did not identify sex-specific differences in feeding patterns nor in aggressiveness. Our results indicate that aggressiveness but not a higher functional response facilitates the invasive success of mosquitofish in Europe by enhancing the ability to outcompete and displace native Aphanius fasciatus (and other species) from freshwater and oligosaline waters. Our results demonstrate that a focus on functional response only will fail to elucidate the drivers of invasion success, and thus, how invasive species impact ecosystems.