Rapid species replacements between fishes of the North American plains: a case history from the Pecos River

A non-native population of plains minnow Hybognathus placitus in the Pecos River, New Mexico, USA, replaced the endemic, ecologically similar Rio Grande silvery minnow Hybognathus amarus in less than 10 years. Competitive exclusion is hypothesized as a mechanism of replacement. The following evidence was examined for potential support: (1) the historical population trend of H. amarus versus H. placitus; (2) relative suitability of the modified flow regime for H. amarus versus H. placitus; (3) potential for habitat overlap; and (4) spawning periodicity and body length in the non-native population of H. placitus. Historical data indicate H. amarus did not decline until H. placitus was present, after which it disappeared rapidly and H. placitus proliferated. The natural flow regime of the Pecos River was changed via base-flow reductions and capture of spring-snowmelt runoff, making it similar to flow regimes associated with likely source populations of non-native H. placitus. Spring runoff is an important spawning cue for H. amarus, but not H. placitus and H. placitus appears to be naturally more tolerant of diminished streamflow. Extant H. placitus were associated with a relatively pristine river reach that was most likely the stronghold for the replaced H. amarus population. Given high ecological similarity, habitat overlap was likely high during the period of replacement. Hybognathus placitus in the Pecos River had a protracted spawning season (April through September), whereas extant H. amarus in the Rio Grande had a short spawning season (a few days in May or June during snowmelt runoff). Most H. amarus die after their first spawning season and few exceed 60 mm standard length (SL), whereas quite a few H. placitus survive at least until their second spawning season and exceed 60 mm SL. Co-occurrence of multiple spawning cohorts may stabilize annual reproductive output of H. placitus and larger individuals may be more fecund and produce larger eggs that survive better. Spawning flexibility, greater size, and higher environmental tolerance likely gave H. placitus a reproductive and survival advantage over H. amarus, consistent with a deterministic pattern of rapid species replacements in the plains, in which tolerant, competitive species from the Red River drainage or Gulf of Mexico coast rapidly replace more sensitive, endemic congeners in disturbed, remnant habitats.