Habitat Shift After Experimental Removal of the Bigger Species in Sympatric Ctenomys Talarum and Ctenomys Australis (Rodentia: Octodontidae)

A removal trapping experiment was conducted in the field to investigate if microhabitat choice and population characteristics of Ctenomys talarum are affected by the bigger, dominant Ctenomys australis, in a zone where these subterranean rodent species are geographically sympatric. As reported previously by other authors, the species were distributed in a habitat segregation pattern which characterized the initial state, prior to C. australis removal. I detected a significant habitat shift-that is, changes in microhabitat choice by C. talarum individuals- in response to C. australis removal. However, it is remarkable that this only occurred after one year of continuous removal of C. australis, and was restricted to a very small expansion by C. talarum into the fringe of the area previously occupied by the dominant species. In addition, a slight increase of the C. talarum population density took place which was possibly due to the absence of the other species. It is suggested that the microhabitat typical of C. australis (with deeper, more friable soil and less vegetation) could be suboptimal and thus unpreferred for C. talarum. If the pre-experimental C. talarum population was satiated to its carrying capacity, it is possible that the small number of C. talarum individuals-mostly subadults-had invaded areas previously occupied by C. australis because they had no other choice than a suboptimal habitat. No substantial changes were detected in other population characteristics (sex ratio, age structure, distance between individuals) which were very similar to that in a control area. Interspecific interactions and displacement by the bigger C. australis could be significant but nonetheless marginal in sustaining the pattern of microhabitat segregation between the species. Difference in habitat preference, linked with species differences in body size and colour, are probably of much greater importance.