EFFECT OF GAMBUSIA-HOLBROOKI ON A SIMILAR-SIZED, SYNTOPIC POECILIID, HETERANDRIA-FORMOSA - COMPETITOR OR PREDATOR

We examined the effect of eastern mosquitofish, Gambusia holbrooki, on least killifish, Heterandria formosa, two similar-sized members of th e family Poeciliidae, to determine which form of interaction was most important-competition or predation. Experimental populations were esta blished in replicate mesocosms (wading pools), and growth and demograp hic characteristics of H. formosa populations were examined in the pre sence and absence of G. holbrooki. In pools with G. holbrooki, populat ions of H. formosa were small and significantly skewed toward large-bo died females. and relative abundance of males and juveniles was reduce d. In populations of H. formosa alone, sex ratios and adult to juvenil e ratios were about 1:1, individual adult female body sizes were small er, and populations were large. This combination of population charact eristics coincides with those predicted by a strong predator-prey mode l. We suggest that, even though these species are similar in adult siz e, diet, and habitat use, the dominant interaction between them, rathe r than competition, is size-selective predation by large G. holbrooki on small H. formosa. However, in natural populations, predation on G. holbrooki by other species may ameliorate the strong effects on H. for mosa observed in this study.