Reciprocal subsidies in ponds: does leaf input increase frog biomass export?

Leaf litter input to ponds can dramatically alter amphibian morphological phenotypes

Phenotypic plasticity in growth and development is commonly examined, but morphology can exhibit plasticity as well. Leg length plasticity is important, because it impacts mobility, which affects predator avoidance, prey capture, and seasonal movements. Differences in relative (i.e., body size adjusted) hind leg lengths > 5% in anurans affect jumping abilities, and resource levels and predation can generate these differences. Leaf litter input can alter larval growth and development and likely morphology as well. I show that relative leg length [leg length/snout-to-vent length (SVL) × 100%] can be quite variable, ranging from 44% of SVL to 120% of SVL across the following species: Hyla versicolor, Lithobates sylvaticus, L. sphenocephalus, and Anaxyrus americanus. Within species variability was highest in L. sylvaticus and almost as great as across species. I measured relative leg length for metamorphs from aquatic mesocosm studies examining the effects of plant litter type and quality. I also examined the relative importance of different environmental variables, including water quality, predation, resource level, and temperature. Good predictors were found only for the two ranids, where leaf litter input was the only variable found to affect relative leg length. Ranid frogs had longer legs when emerging from mesocosms with grass than mesocosms with no litter input, and deciduous leaves produced metamorphs intermediate in leg length. These results suggest that habitat changes in vegetation from land use change, invasive species, and climate change may affect the mobility and fitness of individuals through changes in metamorph relative leg length.

Range position and climate sensitivity: The structure of among-population demographic responses to climatic variation

Species' distributions will respond to climate change based on the relationship between local demographic processes and climate and how this relationship varies based on range position. A rarely tested demographic prediction is that populations at the extremes of a species' climate envelope (e.g., populations in areas with the highest mean annual temperature) will be most sensitive to local shifts in climate (i.e., warming). We tested this prediction using a dynamic species distribution model linking demographic rates to variation in temperature and precipitation for wood frogs (Lithobates sylvaticus) in North America. Using long-term monitoring data from 746 populations in 27 study areas, we determined how climatic variation affected population growth rates and how these relationships varied with respect to long-term climate. Some models supported the predicted pattern, with negative effects of extreme summer temperatures in hotter areas and positive effects on recruitment for summer water availability in drier areas. We also found evidence of interacting temperature and precipitation influencing population size, such as extreme heat having less of a negative effect in wetter areas. Other results were contrary to predictions, such as positive effects of summer water availability in wetter parts of the range and positive responses to winter warming especially in milder areas. In general, we found wood frogs were more sensitive to changes in temperature or temperature interacting with precipitation than to changes in precipitation alone. Our results suggest that sensitivity to changes in climate cannot be predicted simply by knowing locations within the species' climate envelope. Many climate processes did not affect population growth rates in the predicted direction based on range position. Processes such as species-interactions, local adaptation, and interactions with the physical landscape likely affect the responses we observed. Our work highlights the need to measure demographic responses to changing climate.

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

Single-site experiments have demonstrated detritus quality in wetlands can have strongly negative, neutral, and even positive influences on wildlife. However, an examination of the influence of detritus quality across several regions is lacking and can provide information on whether impacts from variation in detritus quality are consistent across species with wide ranges. To address this gap in regional studies we examined effects of emergent and allochthonous detritus of different nutrient qualities on amphibians and assessed a mechanism that may contribute to potential impacts. We used aquatic mesocosms to raise wood frogs (Rana sylvatica) from two regions of the United States with whole plants from purple loosestrife (Lythrum salicaria), leaf litter from native hardwood trees, and a mixture of both. We examined several metrics of amphibian fitness and life history, including survival, number of days to metamorphosis, and size at metamorphosis. Further, we quantified whether the effects of detritus type could translate to variation in anuran biomass or standing stock of nitrogen or phosphorus export. Our results show detritus with high nutrient quality (purple loosestrife) negatively influenced survival of wood frogs, but increased size of metamorphic individuals in two different regions of the United States. Despite the decrease in survival, the increase in size of post-metamorphic anurans raised with high quality detritus resulted in anuran biomass and standing stock of N and P export being similar across treatments at both locations. These results further demonstrate the role of plant quality in shaping wetland ecosystem dynamics, and represent the first demonstration that effects are consistent within species across ecoregional boundaries.

The effects of a fungicide and chytrid fungus on anuran larvae in aquatic mesocosms

The amphibian disease chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), has been linked to significant amphibian declines over the past three decades. The most severe effects of the pathogen have been primarily observed in relatively pristine areas that are not affected by many anthropogenic factors.One hypothesis concerning improved amphibian persistence with Bd in disturbed landscapes is that contaminants may abate the effects of Bd on amphibians. Recent laboratory studies have shown that pesticides, specifically the fungicide thiophanate-methyl (TM), can kill Bd outside of hosts and clear Bd infections within hosts. Using aquatic mesocosms, we tested the hypothesis that TM (0.43 mg/L) would alter growth and development of Lithobates sphenocephalus (southern leopard frog) tadpoles and Bd-infection loads in infected individuals. We hypothesized that the scope of such alterations and infection clearing would be affected by aquatic community variables, specifically zooplankton. TM altered zooplankton diversity (reduced cladoceran and increased copepod and ostracod abundances) and caused mortality to all tadpoles in TM-exposed tanks. In TM-free tanks, Bd-exposed tadpoles in high-density treatments metamorphosed smaller than Bd-unexposed, effects that were reversed in low-density treatments. Our study demonstrates the potential adverse effects of a fungicide and Bd on tadpoles and aquatic systems.

Carryover effects in amphibians: are characteristics of the larval habitat needed to predict juvenile survival?

Carryover effects occur when experiences early in life affect an individual's performance at a later stage. Many studies have shown carryover effects to be important for future performance. However, it is currently unclear whether variation in later environments could overwhelm factors from an earlier life stage. We were interested in whether similar patterns would emerge under the same experimental design with similar taxa. To examine this, we implemented a four-way factorial experimental design with different forestry practices on three species of anurans (each examined in different years) in the aquatic larval environment and terrestrial juvenile environment in outdoor mesocosms in central Missouri, USA. Using Cormack-Jolly-Seber mark-recapture models implemented in program MARK, we investigated whether one environment or both environments best predicted terrestrial juvenile survival. We found only limited evidence of carryover effects for one of three species in one time period. These were the effects of time to metamorphosis and body condition at metamorphosis predicting leopard frog (Lithobates sphenocephalus) survival. However, both effects were counterintuitive and/or very weak. For wood frogs (L. sylvaticus), all of the variables predicting survival had confidence intervals that included zero, but very low survival may have limited our ability to estimate parameters. The terrestrial environment was important for predicting survival in both American toads (Anaxyrus americanus) and southern leopard frogs. The partial harvest forest tended to increase survival relative to control forest and early-successional forest in American toads. Alternately, early-successional forest with downed wood removed increased survival for leopard frogs, but this treatment was no different from control forest for American toads. Previous studies have shown negative effects of recent clearcuts on terrestrial amphibians. It appears that vegetative regrowth after just a few years can mitigate these initial negative effects. Our study shows that variation in later environments probably can overwhelm variation from earlier environments. However, previous evidence of carryover effects suggests that more research is needed to predict when carryover effects are likely to occur.