Native fish grows faster in the presence of a potential introduced competitor

Biotic resistance, through competition from native species, can limit the success of invasive species. Similarly, invasive species can impact natives when they reduce shared resources. Alternatively, some invaders may benefit natives if they either directly or indirectly increase resource acquisition by natives. I predicted that the native California killifish, Fundulus parvipinnis would reduce the fitness of the morphologically similar introduced sailfin molly, Poecilia latipinna in a California estuary. A field experiment found no effect of competition; increasing the density of the native killifish did not affect sailfin molly growth. However, there was an unanticipated positive association of female sailfin molly density on growth of native killifish. Unlike any other native fish in the estuary, the introduced molly is a live bearer and I hypothesize that the presence of female mollies may have generated a novel food source (newborn fish) for the native competitor. This potential food source could have overcompensated for any potential competition for other resources. The positive effect was not mutual, i.e., the presence of killifish did not increase the growth of adult mollies (killifish are oviparous and lay eggs on the substrate). While killifish may not limit mollies through competitive effects, they could provide partial resistance to invasion by preying on juvenile mollies. Additional experiments designed specifically to test whether juvenile mollies increase killifish growth would confirm the mechanism and potentially reveal the importance of this in nature. Considering the role of all life stages of an invader is a key step when evaluating interactions with natives and determining the success and impacts of introduced species.