Interpreting field measurements of juvenile growth and survival rates with population growth isoclines

Juvenile survival and growth rates are commonly studied demographic rates with consequences for population growth. For species that can achieve a size refuge from juvenile predators, the time spent at smaller vulnerable sizes is expected to affect population dynamics. But the interactive effects of juvenile growth and survival on population growth are rarely illustrated theoretically, and most studies of these concepts have been in experimental settings. The interactive effects of the two rates have applications to field studies of recruitment variation for a diversity of species that could be assessed with demographic models and isoclines. We conceptually illustrate the potential use of demographic isoclines for marine, terrestrial, and freshwater examples in the literature, and then demonstrate the use of a demographic isocline for an annual freshwater gastropod (Florida Apple Snail, Pomacea paludosa). Using a published size-indexed demographic model, we constructed a zero-population growth isocline for theoretical combinations of juvenile growth and survival rates. We then quantified daily juvenile survival and growth in two wetlands twice during the recruitment period, incorporating variable predator assemblages and seasonal environmental conditions (i.e., water depth and temperature). Daily juvenile survival rates were lower in the cooler dry season and  juvenile growth was faster in the warmer wet (rainy) season. Parameter combinations of juvenile growth and survival in the dry season predicted declining populations (λ < 1), while rates from the wet season predicted populations at replacement (λ = 1) or increasing. When parameters were combined for the full annual recruitment window, populations were projected to decline in both wetlands. The qualitative predictions were robust to variation in hydrologic conditions affecting reproductive rates, but with better hydrologic conditions, one population was near replacement. Our demographic isocline approach provided population-dynamic context to field-measured demographic rates, identified important temporal variation in survival and growth for the population, and generated new hypotheses for future investigation and management. We encourage others to consider developing demographic isoclines to interpret variation of early life stage demographic rates across spatially and temporally variable environmental conditions.