Dissecting variation in juvenile fitness in a widespread but declining grassland perennial

Lea K Richardson and Drake E. Mullett, Chicago Botanic Garden, Glencoe, IL, Lea K Richardson and Stuart Wagenius, Plant Biology and Conservation, Northwestern University, Evanston, IL, Amy B. Dykstra, Bethel University, Erin Eichenberger, North Carolina State University, Gretel Kiefer, Chicago Botanic Garden, Scott Nordstrom, Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO, Ruth G. Shaw and Amy M. Waananen, Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN, Riley Thoen, University of Georgia

Background/Question/Methods
Patterns of survivorship and fitness within species affect population persistence and evolution. Factors that determine juvenile survival and mortality in long-lived perennials are often unknown because few studies track individuals through their lives. Understanding variation in juvenile fitness is especially critical when species are expected to have high juvenile mortality and persist in severely reduced and fragmented habitats where populations are declining. Juvenile fitness may vary based on broad scale properties such as population size, or based on fine scale factors like distance from maternal plant, or density of surrounding conspecifics. To better understand the factors affecting juvenile fitness in fragmented habitat, we conducted a 15-year study using a widespread but declining model prairie perennial, Echinacea angustifolia. We identified 917 seedlings in 14 remnant prairie sites between 2007 and 2013. Once identified, individual plants were tracked yearly through 2020 or until they died. Sites differed in their population size and degree of fragmentation. We used an aster life history modeling approach to quantify juvenile fitness in relation to their cohort year, age, population size, local density, and distance from maternal plant.
Results/Conclusions
80% of seedlings survived through the first summer and only 10% survived from birth through age six. Mean juvenile fitness differed among cohort years and tended to alternate higher then lower every other cohort year with the highest fitness for cohort year 2011 and lowest for cohort year 2013. Population size, defined as the maximum number of flowering adult plants per year at each site between 2006 and 2012, ranged from 7 to 591, and did not vary with juvenile fitness (p = 0.32). To measure density dependence of juvenile fitness, we averaged the weighted local density of conspecifics within 12 cm of plants throughout the study period. Juveniles surrounded by a higher density of conspecifics had higher predicted fitness than those surrounded by fewer conspecifics (p < 0.0001). The distance between juveniles and their maternal plants ranged from 2cm to 68cm. Predicted fitness increased with distance from maternal plant (p = 0.03). Our results suggest that conditions immediately surrounding juveniles, like higher density of other conspecifics and greater distance from the maternal plant, which may reflect high quality microsite characteristics, matter more to juvenile fitness than broad scale factors like population size.