Divergence in the ecology of two species of Gambusia in secondary contact

Gambusia nobilis, a federally endangered species, and G. affinis (Poeciliidae) are small, livebearing fishes found in the southwestern U.S. The invasive G. affinis has been introduced globally to control mosquito populations. It is found in some populations of G. nobilis on Bitter Lake National Wildlife Refuge (BLNWR), the field site for this study. It is unclear to what extent the two species have diverged in behavior and ecology and how extensively they have hybridized on BLNWR, thereby threatening the endangered G. nobilis via genetic introgression. In this study I examined divergence in behavior and ecology between two species with four main objectives: (1) To determine if there is assortative mating between the two species; (2) To determine if there is divergence in life history characteristics between the two species; (3) To assess habitat and dietary differences between extant populations of the two species; and (4) To determine the degree to which the two species have genetically introgressed on BLNWR. I found these two fishes are markedly different in behavior and ecology. Males and females of both species show assortative preference in visual/olfactory tests. This assortative preference pattern was held during copulation, probably as a function of female choice. Data from field caught and lab breeding individuals show differences in key life history traits that reflect the trade-offs in their current environments. To assess ecological divergence I characterized the habitats of the two species. G. affinis persists in ephemeral environments and G. nobilis is restricted to spring-fed, stenohaline and stenothermal habitats. The two species also show differences in ecological niche as G. nobilis appears to feed at a higher trophic level. Finally, I characterized genetic patterns of hybridization. I found low genetic diversity for G. nobilis, probably a result of range contraction. Gene flow and rates of hybridization are low providing further evidence for divergence and reproductive isolation between these two species. The results presented here include characterization of the habitat requirements, heterospecific interactions, and population genetics of these two species on BLNWR and may be helpful to management of a sensitive species such as G. nobilis.