Flow-dependent competitive interactions between the invasive Gambusia geiseri and endangered Gambusia nobilis in a reconstructed desert wetland habitat: Implications for endangered species managment

Authors: Kelbi D. Delaune, Chad C. Hargrave

Year: 2013 (xlv)

Abstract

Habitat loss and invasive species are the principal cause for the current mass extinction on Earth, and aquatic species have experienced a disproportionately greater rate of extinction. This is because aquatic ecosystems have experienced increased rates of habitat loss and exotic invasions because of intense human interactions. This is especially true in areas where water is not abundant thus; desert systems have larger numbers of endangered taxa. The desert-spring fish, Gambusia nobilis was once distributed widely throughout the Pecos River drainage from Texas into southern New Mexico. However, water withdrawal and habitat alterations within this system have reduced the distribution of G. nobilis to ~6 regions throughout the drainage. Specifically, the San Solomon Spring complex near Balmorhea, Texas currently supports the largest natural G. nobilis population globally. However, populations in this area continue to face threats of extirpation because of habitat loss associated with agricultural irrigation in the area. Moreover, G. nobilis populations may be further stressed by the presence of an invasive congener (G. geiseri) that may compete with G. nobilis for resources. This is supported by recent population estimates showing that the invasive G. geiseri out numbers G. nobilis by as much as 20 to 1. Additionally, an analysis of gut content data and stable isotope data show high-diet overlap between G. nobilis and G. geiseri, suggesting that competitive dominance of G. geiseri may be a principal cause driving down G. nobilis population size in these localities. In order to test these predictions, a competition study with varying density treatments was conducted in the summer of 2012 but showed equal competitive interactions between the two congeners. However, we predict, based on field observations, flow may play an integral part in competitive interactions in this system. Herein, I will report on the results of a second competition study where flow was the key variable tested to determine competitive interactions. These data will be useful for modeling community dynamics necessary to adequately manage populations of the endangered G. nobilis throughout its range.