Intraspecific and interspecific competition induces density‐dependent habitat niche shifts in an endangered steppe bird

Abstract

Interspecific competition is a dominant force in animal communities that induces niche shifts in ecological and evolutionary time. If competition occurs, niche expansion can be expected when the competitor disappears because resources previously inaccessible due to competitive constraints can then be exploited (i.e., ecological release). Here, we aimed to determine the potential effects of interspecific competition between the little bustard (Tetrax tetrax ) and the great bustard (Otis tarda ) using a multidimensional niche approach with habitat distribution data. We explored whether the degree of niche overlap between the species was a density‐dependent function of interspecific competition. We then looked for evidences of ecological release by comparing measures of niche breadth and position of the little bustard between allopatric and sympatric situations. Furthermore, we evaluated whether niche shifts could depend not only on the presence of great bustard but also on the density of little and great bustards. The habitat niches of these bustard species partially overlapped when co‐occurring, but we found no relationship between degree of overlap and great bustard density. In the presence of the competitor, little bustard's niche was displaced toward increased use of the species' primary habitat. Little bustard's niche breadth decreased proportionally with great bustard density in sympatric sites, in consistence with theory. Overall, our results suggest that density‐dependent variation in little bustard's niche is the outcome of interspecific competition with the great bustard. The use of computational tools like kernel density estimators to obtain multidimensional niches should bring novel insights on how species' ecological niches behave under the effects of interspecific competition in ecological communities.

Belianes and Bellmunt fieldwork was financed by REGSEGA (Regs Sistema Segarra‐Garrigues), Departament de Medi Ambient i Habitatge (Generalitat de Catalunya). Funding was also provided by the Spanish Ministry of Science (CGL2009‐13029/BOS); the REMEDINAL2 and REMEDINAL3 networks of the CAM (S‐2009/AMB/1783 and S2013/MAE‐2719). R.T. was supported by a PhD grant from the Spanish Minister of Education (FPU grant no. AP2009‐0762) and a postdoctoral fellowship funded by REMEDINAL3 (S2013/MAE‐2719). FC was supported by the Andalucía Talent Hub Program launched by the Andalusian Knowledge Agency, cofounded by the European Union's Seventh Framework Program, Marie Skłodowska‐Curie actions (COFUND–Grant Agreement n° 291780) and the Ministry of Economy, Innovation, Science and Employment of the Junta de Andalucía.