Exploring the ecological consequences of evolutionary change by resurrecting centuries-old Daphnia resting eggs

Background/Question/Methods

Ecological theory is wrestling with the possibility that rapid evolutionary change can have considerable impacts on ecology. Ecological stoichiometry (ES) has been proposed as an ideal framework for integrating ecology and evolution. At the heart of ES lies the assumption that each species has a distinct and relatively fixed somatic stoichiometry, which manifest in ecological functions of species such as nutrient recycling. Little attention has been paid to intraspecific variation in nutrient excretion, and the potential for evolutionary shifts in excretion. Utilizing the dormant eggs of Daphnia pulicaria in the sediments of South Center Lake, Minnesota, we found striking shifts in population genetics over the past 1600 years. Moreover, individuals hatched from various sediment strata differed in nutrient use physiology. Specifically, rates of P-sequestration and the efficiency of P-use in genotypes resurrected from four sediment layers indicated that, ~700-yr-old (i.e., “older”) genotypes retained more of the ingested P, compared to ~20-yr-old (i.e., “younger”) descendants. Here, we tested whether such shifts in P sequestration and use impact P excretion, and algal quantity and quality in semi-continuous chemostats. We predicted that P inefficient younger genotypes would facilitate rapid growth, and higher P content of algae (Scenedesmus obliquus).

Results/Conclusions

We found strong support for our predictions. Specifically, after 24h, under P-replete (eutrophic) conditions, significantly more ³³P radioactivity was found in algae coexisting with P inefficient younger genotypes, indicating higher rates of consumer-driven P recycling. Moreover, higher rates of P supply translated to a steeper growth curve of the algal population (measured by flow cytometry) relative to the algal population coexisting with P efficient older (i.e., ancestral) genotypes. Although this general pattern was evident in P-limiting (oligotrophic) conditions, age of Daphnia genotypes did not significantly explain variation in algal P content and growth rate. Lower P concentration in oligotrophic conditions likely compromised the sensitivity of the radiotracer method employed (leading to higher measurement error in this treatment). Also, markedly slower algal population growth in low P conditions may require longer periods of observation for detection of treatment effects. Regardless, our study clearly indicates that: (i) there is intraspecific variation in nutrient excretion, (ii) evolution can act on this variation, and (iii) microevolutionary changes in a population has the potential to impact trophic interactions. This new view of ES, one that embraces intraspecific variation in stoichiometric traits has the potential to illuminate the ecological consequences of evolutionary change.