In the western United States, the population of migratory monarch butterflies is on the brink of collapse, having dropped from several million butterflies at coastal overwintering sites in the 1980’s to about 2000 butterflies in the winter of 2020-21. At the same time, a resident (non-migratory) monarch butterfly population in urban gardens seems to be expanding northward. If anything, this urban population has been growing in recent years. We explore the meaning of these changes. The new resident population is not sufficient to make up for the loss of the migratory population; there are still orders of magnitude fewer butterflies now than in the recent past. The resident population also probably lacks the demographic capacity to expand its range inland during summer months, due to higher levels of infection by a protozoan parasite, and subsequently lower survival and fecundity. Nonetheless, the resident population may have the capacity to persist. This sudden change emphasizes the extent to which environmental change can have unexpected consequences. It also demonstrates how quickly these changes can happen. We hope it will provoke discussion about how we define resilience and viability in changing environments.
1. Behavior and organization of social groups is thought to be vital to the functioning of societies, yet the contributions of various roles within social groups towards population growth and dynamics have been difficult to quantify. A common approach to quantifying these role-based contributions is evaluating the number of individuals conducting certain roles, which ignores how behavior might scale up to effects at the population-level. Manipulative experiments are another common approach to determine population-level effects, but they often ignore potential feedbacks associated with these various roles. 2. Here, we evaluate the effects of worker size distribution in bumblebee colonies on worker production in 24 observational colonies across three environments, using functional linear models. Functional linear models are an underused correlative technique that has been used to assess lag effects of environmental drivers on plant performance. We demonstrate potential applications of this technique for exploring high-dimensional ecological systems, such as the contributions of individuals with different traits to colony dynamics. 3. We found that more larger workers had mostly positive effects and more smaller workers had negative effects on worker production. Most of these effects were only detected under low or fluctuating resource environments suggesting that the advantage of colonies with larger-bodied workers becomes more apparent under stressful conditions. 4. We also demonstrate the wider ecological application of functional linear models. We highlight the advantages and limitations when considering these models, and how they are a valuable complement to many of these performance-based and manipulative experiments.
Phenotypic plasticity can mask population genetic differentiation, reducing the predictability of trait-environment relationships. In short-lived plants, reproductive traits may be more genetically determined due to their direct impact on fitness, whereas vegetative traits may show higher plasticity to buffer short-term perturbations. Combining a multi-treatment greenhouse experiment with global field observations for the short-lived Plantago lanceolata, we 1) disentangled the genetic and plastic responses of functional traits to a set of environmental drivers and 2) assessed the utility of trait-environment relationshisps inferred from observational data for predicting genetic differentiation. Reproductive traits showed distinct genetic differentiation that was highly predictable from observational data, but only when correcting traits for differences in their (labile) biomass component. Vegetative traits showed higher plasticity and contrasting genetic and plastic responses, leading to unpredictable trait patterns. Our study suggests that genetic differentiation may be inferred from observational data only for the traits most closely related with fitness.
Phenological shifts are well-documented in the ecological literature. However, their significance for changes in demography and abundance is less clear. We used 27 years of citizen science monitoring to quantify trends in phenology and relative abundance across 89 butterfly species. We calculated shifts in phenology using quantile regression and shifts in relative abundance using list length analysis and counts from club trips. Elongated activity periods within a year were the strongest predictor of increases in relative abundance. These changes may be driven in part by changes in voltinism, as this association was stronger in multivoltine species. Some species appear to be adding a late-season generation while other species appear to be adding a spring generation, revealing a possible shift from vagrant to resident. Our results emphasize the importance of evaluating phenological changes throughout species' activity periods and understanding the consequences for such climate-related changes on viability or population dynamics.
Conditions experienced early in development can affect the future performance of individuals and populations. Demographic theories predict persistent population impacts of past resources, but few studies have experimentally tested such carry-over effects across generations or cohorts. We used bumble bees to test whether resource timing had persistent effects on within-colony dynamics over sequential cohorts of workers. We simulated a resource pulse for field colonies either early or late in colony development and estimated colony growth rates during pulse- and non-pulse periods. During periods when resources were not supplemented, early-pulse colonies grew faster than late-pulse colonies; early-pulse colonies grew larger as a result. These results reveal persistent effects of past resources on current growth and support the importance of transient dynamics in natural ecological systems. Early-pulse colonies also produced more queen offspring, highlighting the critical nature of resource timing for population, as well as colony, dynamics of a key pollinator.