Effects of larval environment on metamorphic phenotype and performance: Disentangling emergent and nonlinear processes

Background/Question/Methods

For organisms with complex life cycles,  plastic responses to environmental variation in early life stages may have consequences for performance in subsequent life stages. In aquatic systems,  variation in intraspecific density dependence and hydroperiod can have dramatic effects on the timing of metamorphosis and post-metamorphic performance. Past studies conclude that hydroperiod and density interact in complex ways which are difficult to predict from their independent effects. However, the relationships between conspecific density and metamorphic phenotype and post-metamorphic performance are frequently nonlinear. In simple factorial experiments, nonlinear density dependence may create the appearance of emergent interactions, when in fact the combined effects are fundamentally predictable. Understanding how changes in hydroperiod and density affect metamorphosis requires and understanding of the functional form of larval density dependence. However very few studies have addressed this. Here we investigate the interactive effects of hydroperiod and larval density dependence on metamorphic phenotype and post-metamorphic performance in the Western Spadefoot toad (Pelobates cultripes) in Donana National Park, Spain. We conducted a factorial experiment in outdoor mesocosms in which we manipulated hydroperiod and five levels of larval density. This approach enabled us to quantify the functional relationship between larval stage density dependence on larval duration, metamorph size, fat stores, limb morphology and locomotor performance and test whether this functional form changes under different hydroperiod regimes.

Results/Conclusions

Relationships between larval density and most metamorphic endpoints were nonlinear and variation in hydroperiod altered this relationship in simple predictable ways. Metamorph mass, snout-vent and limb length decreased asymptotically with increasing density and frogs from drydown treatments were smaller and had shorter limbs than those from control treatments.  Time to metamorphosis increased asymptotically with increasing density with frogs from drydown treatments emerging earlier. Interestingly, fat stores and jumping performance only depended on hydroperiod, with frogs from drydown treatments characterized by reduced fat stores and jumping performance.  We demonstrate that the functional relationship between density and phenotype is consistent across hydroperiod environments.  Rather than modulating the dynamics between density and phenotype, hydroperiod shifts the relationship within the parameter space. We interpret these results in the context of past studies.