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 investigat e if microhabitat choice and population characteristics of Ctenomys ta larum are affected by the bigger, dominant Ctenomys australis, in a zo ne where these subterranean rodent species are geographically sympatri c. As reported previously by other authors, the species were distribut ed in a habitat segregation pattern which characterized the initial st ate, prior to C. australis removal. I detected a significant habitat s hift-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. australi s, and was restricted to a very small expansion by C. talarum into the fringe of the area previously occupied by the dominant species. In ad dition, a slight increase of the C. talarum population density took pl ace 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 un preferred for C. talarum. If the pre-experimental C. talarum populatio n was satiated to its carrying capacity, it is possible that the small number of C. talarum individuals-mostly subadults-had invaded areas p reviously occupied by C. australis because they had no other choice th an a suboptimal habitat. No substantial changes were detected in other population characteristics (sex ratio, age structure, distance betwee n individuals) which were very similar to that in a control area. Inte rspecific interactions and displacement by the bigger C. australis cou ld be significant but nonetheless marginal in sustaining the pattern o f microhabitat segregation between the species. Difference in habitat preference, linked with species differences in body size and colour, a re probably of much greater importance.