Long-term monitoring indicates shifting fall migration timing in monarch butterflies (Danaus plexippus).

Eastern North American monarch butterflies (Danaus plexippus) embark on a yearly migration between summer breeding grounds in the northern United States and southern Canada and overwintering sites in central Mexico, traveling up to 4300 km. This annual multi-generational migration cycle, like many seasonal natural phenomena, may be impacted by recent changes in climate and habitat. We investigated long-term trends in monarch abundance and fall migration timing over a 29-year period in Cape May, New Jersey, using data collected from daily population surveys designed to track patterns of occurrence during the fall migration period through Cape May (1 Sept-31 Oct). Between 1992 and 2020, the migration midpoint, average peak migration day and first peak migration day shifted between 16 and 19 days later in the season, an average of approximately six days per decade. This observed shift in migration timing is correlated with increasing temperatures in September and October in northeastern monarch breeding grounds over the study period. Our data do not demonstrate a significant directional trend in monarch abundance over the study period, yet population data collected at overwintering sites in Mexico indicate a substantial decline over the same period. Further postponement of fall migration may lead to lower migration success and exacerbate the overall decline of this iconic species.

Asymmetric interference competition and niche partitioning between native and invasive Anolis lizards.

Species can compete both directly via aggressive encounters (interference) and indirectly through their shared use of a limited resource (exploitation). Depending on the circumstances interference, exploitation, and their interplay can either lead to competitive exclusion or drive niche partitioning to maintain species coexistence. Thus, understanding species coexistence in nature requires accurately identifying the mechanisms that contribute to competition among the species in question. In the southern United States, the native lizard Anolis carolinensis becomes more arboreal in the presence of the invasive Anolis sagrei, resulting in highly consistent vertical habitat partitioning where the species co-occur. These species have been thought to largely ignore each other and engage only in exploitative competition for shared arthropod prey. To test for the presence and consequences of direct interference, we conducted behavioral trials in the field, introducing a heterospecific male intruder to individuals of both species. We find that interference competition is asymmetric in favor of A. sagrei, which are more likely to display and less likely to retreat than A. carolinensis. Concordant with their arboreal tendencies, male A. carolinensis also trend toward retreating upward more often than expected by chance. These asymmetries are prevalent despite the almost complete absence of physical attacks, suggesting that interspecific signaling and avoidance behavior by A. carolinensis resolve most potential conflicts before they escalate to combat. Our results highlight the potential for direct interference more subtle than frequent outright combat to structure communities, and Anolis assemblages in particular.