Will increasing environmental variability amplify fitness tradeoffs across complex life cycles?

In the last three years, the Northeast experienced devastating floods from rainfall and urban wildfires from drought. These extremes are emblematic of changes in environmental variability worldwide – increasing fluctuations in temperature and precipitation. Environmental variability poses a particular challenge to animals that change environments during their lifetime, like insects and amphibians that live in water as juveniles and on land as adults. The best traits for surviving environmental fluctuations are different in water and on land, but individuals need to balance these conflicting pressures. One strategy is to spend less time in the environment where the risks are greatest. For example, many species mature early to escape dangerous juvenile environments, but maturing early can have consequences later in life. This research asks how maturing early in response to environmental variability affects traits of juveniles and adults, survival of each life stage, and population dynamics. Researchers will study a salamander that lives in headwater streams in New Hampshire, metamorphoses from an aquatic larva to an adult that lives on land, and shifts the timing of metamorphosis based on fluctuations in streamflow. This research will also identify headwater areas that provide the greatest services to humans. Headwater streams are the origins of freshwater resources humans rely on for survival, but with millions of kilometers of headwaters in the USA, it is difficult to identify the most critical streams. Researchers will develop mapping tools to identify these critical streams and share these tools with private landowners. Timing of metamorphosis determines how long each life stage spends in its environment, but may also influence how each stage adapts to its environment. Macroevolutionary patterns suggest that phenotypic evolution is fastest when one stage predominates the life cycle. This is hypothesized to result from relaxation of trait conflicts that occur when trait optimization at one stage produces fitness costs at another stage, which is common when stages occupy habitats with different trait optima (e.g., aquatic vs. terrestrial). If this hypothesis is correct, then changing the timing of metamorphosis should promote trait optimization in the longer stage at the expense of the shorter stage, amplifying fitness tradeoffs across stages and altering the contributions of stage-specific vital rates to population growth. This project uses the spring salamander (Gyrinophilus porphyriticus) to investigate how changing the timing of metamorphosis in response to environmental variability affects trait optimization within life stages, fitness tradeoffs across stages, and demography. G. porphyriticus metamorphoses four years earlier in hydrologically variable reaches than in hydrologically stable reaches, resulting in a 50% increase in the length of the adult stage. This life cycle variation appears to have a genetic basis, and there is selection against traits mismatched to hydrologic conditions. These attributes create an exciting opportunity to investigate the mechanisms and implications of fitness tradeoffs across complex life cycles exposed to increasing environmental variability. By linking results to ecosystem services in small watersheds, this project will generate knowledge and tools to identify headwater systems with the greatest value to society. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.