Phenological changes are some of the most widely documented responses to climate change but both the magnitude and direction of such changes vary greatly among species. These taxonomic idiosyncrasies can alter the strength of species interactions and thereby influence ecosystem functioning. Elucidating the causes of these idiosyncrasies is crucial as we strive to preserve ecosystem services in a rapidly changing world. This study uses weekly historic (1959 – 1960) and recent (2006-2014) surveys of grasshopper abundance across an elevation gradient in the Front Range of Colorado to investigate how the degree of phenological overlap between co-occurring species has been influenced by 50 years of climate warming. Temperature clines are known to influence developmental rates along elevation gradients. Previous work in this system has implicated life history strategy as an important mediator of phenological response; developmental rates at high elevation have been delayed in taxa with earlier adult emergence but have changed little in late-season species. This study asks whether differences in phenological response lead to increased phenological overlap between early and late-season species and if this pattern is consistent across three sites differing in elevation (1752m, 2591m, and 3048m).
Across elevations, we detected larger advancements in phenology of late-season species, which led to increased phenological overlap with early-season species. The magnitude of overlap increase was influenced by the alignment of the seasonal timing of warming with the phenology of the species involved. Warming tended to occur later in the season and therefore had a larger impact on the developmental rates of late-season species. Increasing phenological overlap of these generalist herbivores is likely to enhance competition among them, which may have significant consequences for the composition and stability of this assemblage. Our findings highlight the importance of considering phenological shifts in a community context.