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1. Hydrologic variability contributes to reduced survival through metamorphosis in a stream salamander

   https://doi.org/10.1073/pnas.1908057116  

   Lowe, Winsor H.; Swartz, Leah K.; Addis, Brett R.; Likens, Gene E. (September 2019, Proceedings of the National Academy of Sciences)

   Changes in the amount, intensity, and timing of precipitation are increasing hydrologic variability in many regions, but we have little understanding of how these changes are affecting freshwater species. Stream-breeding amphibians—a diverse group in North America—may be particularly sensitive to hydrologic variability during aquatic larval and metamorphic stages. Here, we tested the prediction that hydrologic variability in streams decreases survival through metamorphosis in the salamander Gyrinophilus porphyriticus , reducing recruitment to the adult stage. Using a 20-y dataset from Merrill Brook, a stream in northern New Hampshire, we show that abundance of G. porphyriticus adults has declined by ∼50% since 1999, but there has been no trend in larval abundance. We then tested whether hydrologic variability during summers influences survival through metamorphosis, using capture–mark–recapture data from Merrill Brook (1999 to 2004) and from 4 streams in the Hubbard Brook Experimental Forest (2012 to 2014), also in New Hampshire. At both sites, survival through metamorphosis declined with increasing variability of stream discharge. These results suggest that hydrologic variability reduces the demographic resilience and adaptive capacity of G. porphyriticus populations by decreasing recruitment of breeding adults. They also provide insight on how increasing hydrologic variability is affecting freshwater species, and on the broader effects of environmental variability on species with vulnerable metamorphic stages. 

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2. Individual growth rates and size at metamorphosis increase with watershed area in a stream salamander

   https://doi.org/10.1002/ecy.4217  

   Cochrane, Madaline M; Addis, Brett R; Swartz, Leah K; Lowe, Winsor H (February 2024, Ecology)

   Cottingham, Kathryn L (Ed.)

   Abstract A fundamental goal of ecology is to understand how the physical environment influences intraspecific variability in life history and, consequently, fitness. In streams, discharge and associated habitat conditions change along a continuum from intermittency to permanence: Headwater streams typically have smaller watersheds and are thus more prone to drying than higher‐order streams with larger watersheds and more consistent discharge. However, few empirical studies have assessed life history and associated population responses to this continuum in aquatic organisms. We tested the prediction that individual growth, rate of development, and population growth increase with watershed area in the long‐lived stream salamanderGyrinophilus porphyriticus, where we use watershed area as a proxy for hydrologic intermittence. To address this hypothesis, we used 8 years of mark–recapture data from 53 reaches across 10 headwater streams in New Hampshire, USA. Individual growth rates and mean size at metamorphosis increased with watershed area for watersheds from 0.12 to 1.66 km². Population growth rates increased with watershed area; however, this result was not statistically significant at our sample size. Mean age of metamorphosis did not vary across watershed areas. Lower individual growth rates and smaller sizes at metamorphosis likely contributed to reduced lifetime fecundity and population growth in reaches with the smallest watershed areas and highest vulnerability to drought. These responses suggest that as droughts increase due to climate change, headwater specialists in hydrologically intermittent environments will experience a reduction in fitness due to smaller body sizes or other growth‐related mechanisms. 

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3. Outbreeding reduces survival during metamorphosis in a headwater stream salamander

   https://doi.org/10.1111/mec.17375  

   Lowe, Winsor H; Addis, Brett R; Cochrane, Madaline M (June 2024, Molecular Ecology)

   Abstract Assessing direct fitness effects of individual genetic diversity is challenging due to the intensive and long‐term data needed to quantify survival and reproduction in the wild. But resolving these effects is necessary to determine how inbreeding and outbreeding influence eco‐evolutionary processes. We used 8 years of capture–recapture data and single nucleotide polymorphism genotypes for 1906 individuals to test for effects of individual heterozygosity on stage‐specific survival probabilities in the salamanderGyrinophilus porphyriticus. The life cycle ofG. porphyriticusincludes an aquatic larval stage followed by metamorphosis into a semi‐aquatic adult stage. In our study populations, the larval stage lasts 6–10 years, metamorphosis takes several months, and lifespan can reach 20 years. Previous studies showed that metamorphosis is a sensitive life stage, leading us to predict that fitness effects of individual heterozygosity would occur during metamorphosis. Consistent with this prediction, monthly probability of survival during metamorphosis declined with multi‐locus heterozygosity (MLH), from 0.38 at the lowest MLH (0.10) to 0.06 at the highest MLH (0.38), a reduction of 84%. Body condition of larvae also declined significantly with increasing MLH. These relationships were consistent in the three study streams. With evidence of localised inbreeding within streams, these results suggest that outbreeding disrupts adaptations in pre‐metamorphic and metamorphic individuals to environmental gradients along streams, adding to evidence that headwater streams are hotspots of microgeographic adaptation. Our results also underscore the importance of incorporating life history in analyses of the fitness effects of individual genetic diversity and suggest that metamorphosis and similar discrete life stage transitions may be critical periods of viability selection. 

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4. Headwater-stream salamander movements and home-range size increased under drought

   https://doi.org/10.1086/735624  

   Cochrane, Madaline M; Lowe, Winsor H (June 2025, Freshwater Science)

   Climate change is increasing the frequency and intensity of droughts and floods, which will likely have disproportionate effects on freshwater organisms. Stream salamanders are often top predators in small, fishless headwater streams that are highly responsive to hydrologic changes. Although these salamanders persist in dynamic stream environments, we lack the empirical understanding of how they respond to variable drought and flooding intensity that would allow us to predict future responses to climate-related changes in hydrology. We used 3 y (2019–2021) of data from passive integrated transponder (PIT)-tag surveys to characterize how movement distance, movement direction, and home-range size of the Northern Spring Salamander Gyrinophilus porphyriticus (Green, 1827) vary across a range of hydrologic conditions. Most notably, this PIT-tag data included a period of severe drought in the late summer of 2020. We found that total distance moved and home-range size increased with drought intensity. Total distance moved was not related to peak discharge at the range of flooding experienced across this study (<5-y flood recurrence intervals). Larval movements were skewed downstream after large floods but were skewed upstream across all other conditions. The direction of adult movements did not differ across extreme flood or drought conditions. Overall, adults moved greater distances and had larger home ranges than larvae (median movement = 1.9 m vs 1.6 m; median home range = 5 m2 vs 4 m2). We also found adults using moist terrestrial microhabitats for up to 14 consecutive days, even during moderate drought. Our results suggest that headwater salamanders will move more as drought intensity increases in the future. We interpret these movements as behavior used to find habitable, moist environments as stream reaches dry. These results suggest that adequate instream and riparian refuges will become more important for stream salamander survival as climate change progresses. 

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5. Post‐metamorphic growth partially compensates for the effects of climate‐driven stressors on juvenile frog performance

   https://doi.org/10.1002/ecs2.70177  

   Brannelly, Laura A; Ohmer, Michel_E B; Zimmerman, Lydia; Wantman, Trina M; Reuben, Phoebe L; Zegar, Jakub; Fontaine, Samantha S; Bletz, Molly C; LaBumbard, Brandon C; Venesky, Matthew D; et al (February 2025, Ecosphere)

   Abstract Human‐induced climate change, land use changes, and urbanization are predicted to dramatically impact landscape hydrology, which can have devastating impacts on aquatic organisms. For amphibians that rely on aquatic environments to breed and develop, it is essential to understand how the larval environment impacts development, condition, and performance later in life. Two important predicted impacts of climate change, urbanization, and land use changes are reduced hydroperiod and variable larval density. Here, we explored how larval density and hydroperiod affect development, morphology, physiology, and immune defenses at metamorphosis and 35 days post‐metamorphosis in the frogRana pipiens. We found that high‐density larval conditions had a large negative impact on development and morphology, which resulted in longer larval periods, reduced likelihood of metamorphosis, smaller size at metamorphosis, shorter femur to body length ratio, and reduced microbiome species evenness compared with animals that developed in low‐density conditions. However, animals from the high‐density treatment experienced compensatory growth post‐metamorphosis, demonstrating accelerated growth in body size and relative femur length compared with animals from the low‐density treatments, despite not “catching‐up” in size. We also observed an increase in relative gut length and relative liver size in animals that had developed in the high‐density treatment than those in the low‐density treatment, as well as higher bacterial killing ability, and greater jump distances relative to their leg length across different temperatures. Finally, metabolic rate was higher overall but especially at higher test temperatures for animals that developed under high‐density conditions, indicating that these animals may expend more energy in response to acute temperature changes. While the effects of climate change have direct negative effects on larval development and metamorphosis, animals can increase growth rate post‐metamorphosis; however, that compensatory growth might come at a cost and reduce their ability to cope with further environmental change such as increased temperatures. 

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