Environmental variability is ubiquitous, but its effects on populations and communities are neither fully understood nor readily predictable. This is partly because organismal traits change in response to shifting conditions, through evolution and phenotypic plasticity. Significant interactions can arise whenever environmental and phenotypic changes occur on similar timescales, as the properties of individuals and populations will depend on both their current and historic (i.e. prior) environments. While rapid evolution has received significant attention in this context, the dynamic consequences of plasticity for ecological processes are overlooked, yet are potentially as significant. Here, we experimentally investigate the impact of gradual plasticity on ecological processes, and develop a framework linking gradual plasticity and environmental variation to demographic outcomes. We focus specifically on the plastic responses of two freshwater phytoplankton species, the green algae Chlamydomonas reinhardtii and the cyanobacteria Microcystis aeruginosa to temperature variation at a range of scales. In doing so, we address the following questions: 1) Does an organism’s thermal history influence its performance under current temperature regimes? 2) If so, what are the domains over which this history matters? 3) Can a model that accounts for gradual plasticity increase the accuracy of ecological predictions in fluctuating environments?
Results/Conclusions:
Across a range of temperatures, the performance (population growth rate) of both Chlamydomonas reinhardtii and Microcystis aeruginosa populations depend substantially on their recent thermal history. Different combinations of past and present temperatures had both beneficial and detrimental effects relative to long term averages. Additionally, thermal history altered the boundaries of zero net growth isoclines, indicating that thermal history affects which temperatures are lethal over short-term exposures. We developed and parameterized a population growth model accounting for gradual thermal plasticity in complex fluctuating environments. Relative to standard models, our new model offered improved predictions of the growth of populations exposed to thermal fluctuations of varying amplitude, duration, and patterning. Improvements were especially notable given high amplitude fluctuations, and variation approaching the critical thermal maximum. Our results indicate that enhanced ecological prediction in variable environments requires integrating dynamic processes, such as gradual plasticity, that exist outside current paradigms into our thinking.