Effect of Gambusia holbrooki on a Similar-Sized, Syntopic Poeciliid, Heterandria formosa: Competitor or Predator?


mosquitofish, killifish, body size, predator-prey model


We examined the effect of eastern mosquitofish, Gambusia holbrooki, on least killifish, Heterandria formosa, two similar-sized members of the family Poeciliidae, to determine which form of interaction was most important-competition or predation. Experimental populations were established in replicate mesocosms (wading pools), and growth and demographic characteristics of H. formosa populations were examined in the presence and absence of G. holbrooki. In pools with G. holbrooki, populations of H. formosa were small and significantly skewed toward large-bodied females, and relative abundance of males and juveniles was reduced. In populations of H. formosa alone, sex ratios and adult to juvenile ratios were about 1:1, individual adult female body sizes were smaller, and populations were large. This combination of population characteristics coincides with those predicted by a strong predator-prey model. We suggest that, even though these species are similar in adult size, diet, and habitat use, the dominant interaction between them, rather 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. formosa observed in this study.

Original Publication Citation

Belk, M.C., and C. Lydeard. 1994. Effect of Gambusia holbrooki on a similar-sized, syntopic poeciliid, Heterandria formosa: competitor or predator? Copeia, 1994:296-302.

Document Type

Peer-Reviewed Article

Publication Date


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Life Sciences



University Standing at Time of Publication

Full Professor