Understanding the patterns and drivers of variation in species’ responses under various environmental contexts is useful in determining the extent of their adaptive capacities to the consequences of ongoing climate change. For many species, a well-documented response to global environmental change is a shift in various aspects of its life-history, including its timing or phenology. Often, variation in phenological patterns is associated with changes in abiotic factors used as proxies for resource availability or other suitable conditions. However, its association with changes in biotic factors that can drive limitations to resource access is largely unknown. Using a long-term experimental dataset on small mammals monitored in the southwestern United States, we fit generalized additive models (GAMs) to characterize seasonal patterns and drivers of variation in breeding phenology under different competitive landscapes. We described patterns and drivers of phenological variation for both sexes of representative congeneric species (Chaetodipus penicillatus and C. baileyi) that were found in plots that allowed access to the behaviorally dominant species in the system, Dipodomys spp. (controls), and in plots where these were excluded (exclosures). We found that altered competitive landscapes affect the breeding patterns of congeneric species unequally. Female C. baileyi advanced their breeding in the presence of kangaroo rats, while C. penicillatus did not exhibit quantitatively significant phenological shifts in altered competitive landscapes. However, there were subtle differences in the breeding patterns of C. penicillatus, including higher breeding proportions of females, and shorter duration of peak breeding periods of males in plots without kangaroo rats. We also documented less specific responses to abiotic factors in both pocket mice species, relative to Dipodomys spp. These results suggest that plasticity of phenological responses, that is often described in the context of annual variation in abiotic factors, can occur in response to biotic context as well. Additionally, in response to competitive pressures, species may employ adaptive strategies, such as low phenological synchrony, to compensate for the adverse demographic impacts of mismatched resource and species’ phenology. Variation in phenological responses under different biotic conditions shown here further demonstrate that shifting biotic interactions are rampantly occurring in many natural systems and a more nuanced understanding of its impacts on population dynamics is useful to gain better insights on biodiversity patterns in a changing world.
