Assessing niche partitioning and competition between sympatric thrashers

Background/Question/Methods

Species with overlapping resource requirements frequently persist in the same region through the partitioning of ecological niches. Competing species often achieve stabilizing ecological isolation via differential responses to environmental variations, microsite selection or depressed abundances. The Crissal Thrasher (Toxostoma crissale) and Le Conte’s Thrasher (Toxostoma lecontei) are two congeneric species with a high potential for competitive exclusion, and occur sympatrically within arid regions of southwestern North America. To assess niche partitioning, we constructed species distribution models (SDMs) using a common set of environmental predictors hypothesized to influence these species. We inferred niche partitioning in these ‘baseline’ models when predictors were selected differentially or when discordant responses to common predictors were observed. To quantitatively explore competition, we used the predicted habitat of each species as an additional predictor to construct a set of competition scenario models. For these SDMs, we considered changes in predicted area of habitat and model composition as provisional affirmation for competitive exclusion.

Results/Conclusions

In the baseline models, strong niche partitioning was observed. Three vegetation predictors contributed substantially to the Crissal models, but not to those of the Le Conte’s Thrasher. Two hydrological predictors contributed substantially to the Le Conte’s models, but not to those of the Crissal Thrasher. Six predictors were common to models of both species, and the modeled responses to these predictors illustrated that despite partial niche overlap, our species occupied distinct habitats. In the competition scenarios, Crissal habitat was an important predictor of Le Conte’s habitat, but the converse was not true. The relative importance and modeled responses of the other predictors remained mostly unchanged. Spatial overlap of predicted habitat between the two species, increased by 14%, but there was little change in the area predicted for each species between scenarios. We believe the increase in overlap predominantly resulted from a distributional shift in predicted habitat for the Le Conte’s Thrasher towards that of the Crissal Thrasher. This may indicate that the Le Conte’s Thrasher would select somewhat different habitat in the absence of competition from its congener; however, further investigation is need to confirm this pattern.