Invasive exotic species can be detrimental to native populations due to effects from predators and diseases with which the native populations have not co-evolved and the introduction of new competitors which may lead to reduced numbers, or to competitive exclusion. These effects can be stronger on islands, and island systems with many endemic species appear to be especially vulnerable to invasion. The Cuatro Ciénegas spring system, a desert oasis in the state of Coahuila, México, is an example of a highly endemic island system. Hemichromis guttatus, a West African jewel cichlid and popular aquarium trade fish, has been introduced into the Cuatro Ciénegas valley. It is present in a few locations and appears to be spreading. Once introduced to an island system, the spread of an invader among islands depends on many factors including environmental conditions, biotic interactions, and island geography (particularly inter-island distances). Observations of H. guttatus in the wild suggest that temperature is a primary influence on its distribution. To test this hypothesis, I am conducting critical thermal maximum/minimum experiments in a controlled laboratory environment to determine the temperature tolerance range of H. guttatus.
Preliminary data indicate that the average upper tolerance limit for H. guttatus is 39.7°C (n=10). Values ranged from 37.2°C – 40.6°C. The standard lengths of the fish tested ranged from 2.9cm to 5.2cm. These preliminary results suggest that the maximum temperature tolerance limit lies outside the upper limit of the range of observed water temperatures in the valley (warmest springs – ca. 34.5°C). Thus, the absolute maximum tolerable temperature may not be important in determining distribution of this fish in the wild. Lower limit temperature tolerance is also a candidate as a dispersal limitation. Experiments are underway to determine the lower temperature limit, and results to date will be presented at this meeting. An alternate hypothesis, however, is that optimal or preferred temperature is more important than the upper or lower temperature tolerance limits. Future directions include determining optimal temperatures for H. guttatus in Cuatro Ciénegas.