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.

Keywords: Otis tarda; Tetrax tetrax; ecological release; kernel density estimators; species coexistence.

Figure 1

An example of a two‐dimensional kernel density estimator (KDE) procedure used to obtain the species' habitat niches from habitat data. Graph (a) KDEs were calculated from set coordinates in order to obtain comparable values for the analysis (cross points of dotted lines). The gray region reflects the 95% KDE volume of highest probability. This 95% KDE region is used for the analysis in order to avoid the influence of outlier observations. The white square represents niche position, where the KDE attained its highest density value. Niche breadth was estimated as the number of cells falling within the 95% KDE. Black dots are the values of each niche dimension for each bird observation. Graph (b) niche overlap was calculated as the volume under the area where two KDEs intersect. Brown and green lines delimitate two bivariate kernel density functions. The red surface reflects the region where both functions overlap