Baseline phenological asynchrony alters vulnerability of interacting species to extreme climatic events

Background/Question/Methods

Changing climate can disrupt existing phenological relations between interacting species.  We might expect the historical baseline for these effects to be precise synchrony between the season at which a consumer most requires food and the time when its resources are most available. When this is the case, change in any direction would be detrimental to the consumer.  But is baseline synchrony the appropriate assumption?  Here, we develop the theme that the starting point for climate change impacts may often have been asynchrony or mismatch between consumer and resource.   To the extent that this has been true, assumptions of baseline synchrony risk mis-detection, mis-estimation, and mis-attribution of climate change impacts. Natural selection can result in asynchrony between exploiter and victim when victims successfully evolve to occupy enemy-free time. Asynchrony can also result from life-history tradedoffs.  If we can improve our understanding of the circumstances that might lead to the evolution of synchrony or asynchrony as a baseline, we can also improve our ability to interpret short-term data on species’ relationships, which ultimately will improve our long-term projections of future climate-change impacts.

Results/Conclusions

We illustrate asynchrony arising from tradeoffs for two species: Edith’s checkerspot butterfly and the winter moth. Initial observations of phenological mismatch in both systems were made prior to the onset of major impacts of anthropogenically-driven climate change.  Neither species can detect the phenological stage of its host plants with precision.  In both species, evolution of life history has involved compromise between maximizing fecundity and minimizing mortality, with the outcome being superficially maladaptive strategies in which many or even most individuals die of starvation through poor synchrony with their host plants. Both species have evolved high-risk life history strategies.  While winter moth eggs gamble with their own lives by hatching early, bay checkerspots gamble with the lives of their offspring by growing large and eclosing late as adults.  In both cases the result is the evolution of populations in which large numbers of individuals die because, as individuals, they fail to fit their life cycles into the available timespan.  Because such a population exists near the limits of its ecological tolerance, it is particularly vulnerable to impacts of climate change.  This vulnerability probably contributed to the skewed geographical pattern of population extinctions in the butterfly, which drove a northward and upward range shift in this species in the late 20^th century.