Leaf litter resource quality induces morphological changes in wood frog (Lithobates sylvaticus) metamorphs

Abstract

For organisms that exhibit complex life cycles, resource conditions experienced by individuals before metamorphosis can strongly affect phenotypes later in life. Such resource-induced effects are known to arise from variation in resource quantity, yet little is known regarding effects stemming from variation in resource quality (e.g., chemistry). For larval anurans, we hypothesized that variation in resource quality will induce a gradient of effects on metamorph morphology. We conducted an outdoor mesocosm experiment in which we manipulated resource quality by rearing larval wood frogs (Lithobates sylvaticus) under 11 leaf litter treatments. The litter species represented plant species found in open- and closed-canopy wetlands and included many plant species of current conservation concern (e.g., green ash, common reed). Consistent with our hypothesis, we found a gradient of responses for nearly all mass-adjusted morphological dimensions. Hindlimb dimensions and gut mass were positively associated with litter nutrient content and decomposition rate. In contrast, forelimb length and head width were positively associated with concentrations of phenolic acids and dissolved organic carbon. Limb lengths and widths were positively related with the duration of larval period, and we discuss possible hormonal mechanisms underlying this relationship. There were very few, broad differences in morphological traits of metamorphs between open- and closed-canopy litter species or between litter and no-litter treatments. This suggests that the effects of litter on metamorph morphology are litter species-specific, indicating that the effects of changing plant community structure in and around wetlands will largely depend on plant species composition.

Detritus Quality and Locality Determines Survival and Mass, but Not Export, of Wood Frogs at Metamorphosis

“…Biomass export of anurans is driven by a combination of survival and growth, and detritus nutrient quality affects the influence of both of these drivers [3,23,25,29]. Do the factors that negatively influence metamorphic anuran export (i.e., secondary plant compounds), and those that positively influence it (i.e., high nutrient quality), offset one another?…”

“…Availability and quality of resources early in life can shape fitness [1,2] and a variety of life history traits of organisms, such as morphology [3,4] and size [5,6,7]. In particular, bottom-up effects can have measurable impacts on organisms with complex life histories, such as those that undergo metamorphosis.…”

“…Larval anurans are likely to be strongly affected by changes to resource quality in wetlands, as these organisms feed across trophic levels and are heavily reliant on allochthonous productivity in wetland ecosystems [22]. Recent studies have shown plant traits such as carbon-to-nitrogen ratios and variation in leaf litter species can impact development, morphology and fitness of larval anurans [3,4,6,23,24,25]. Martin et al [25] suggest lower leaf litter C:N contributes to faster decomposition and supports higher quality biofilms, which result in higher tadpole performance.…”

Reviewing the role of plant litter inputs to forested wetland ecosystems: leafing through the literature

“…For example, mesocosm studies reveal that tadpoles and zooplankton experience impaired predator recognition and high mortality when provided with phenolic‐rich litter inputs, even when nutrient content is high (Maerz et al. , b , , Burdett and Watts , Watkins et al. , Milanovich et al.…”

“…For example, tadpoles exposed to leaf litter of lower quality also developed into terrestrial amphibians with more massive intestines relative to body mass and longer limbs relative to body length (Stoler et al. ). It is unknown whether such post‐metamorphic changes offer any improvement in fitness.…”