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Human activities, invasive species and climate change have altered the composition of native forest plant communities in North America. These changes have, in turn, altered the composition of leaf litter in woodland ponds with consequences for the organisms they harbour, such as amphibian larvae., We used mesocosms to assess how 10 species of allochthonous litter, which differ in their regional abundance and chemical composition, affect ephemeral ponds. We were particularly interested in effects on larval amphibian fitness, including metamorphic mass, larval period and survival., Litter had strong effects on the fitness of larval wood frogs ( Lithobates sylvaticus), water chemistry, and phytoplankton and periphyton biomass in the mesocosms. Litter from green ash ( Fraxinus pennsylvanica), a tree species in decline, enabled frog larvae to grow larger, develop faster and survive better than larvae from other litter treatments. Interestingly, three invasive wetland plant species produced heavy metamorphs, with shorter larval periods and higher survival relative to native wetland plants. Litter from red maple ( Acer rubrum, now among the most abundant native trees in the eastern United States), cattail ( Typha latifolia) and sedge ( Carex stricta) produced metamorphs that were small and survived poorly., Mass at metamorphosis was best explained by litter quality (carbon, nitrogen and phosphorus content) and primary producer biomass, while survival and larval period were best explained by a combination of litter quality, primary producer biomass and water chemistry (polyphenolics and pH). Additionally, litter quality and primary producer biomass explained differences in the elemental composition of metamorphs. Lastly, our data suggest that litter nitrogen may be a limiting resource that affects the export of metamorphic frog biomass from ephemeral ponds., Overall, our results indicate that anthropogenic changes in plant communities lead to changes in the quality of allochthonous litter input and strongly affect amphibian fitness. [ABSTRACT FROM AUTHOR]

1. Adult Pacific salmon ( Oncorhynchus spp.) transport marine nutrients to fresh waters and disturb sediments during spawning. The relative importance of nutrient fertilisation and benthic disturbance by salmon spawners can be modulated by environmental conditions (e.g. biological, chemical and physical conditions in the catchment, including human land use). 2. To determine the importance of the environmental context in modifying the uptake and incorporation of salmon-derived material into stream biota, we measured the nitrogen (δ15N) and carbon (δ13C) isotopic composition of benthic algae (i.e. epilithon) and juvenile coho salmon ( Oncorhynchus kisutch) in seven streams across a timber-harvest gradient (8-69% catchment area harvested), both before and during the salmon run. Conditional bootstrap modelling simulations were used to assess variability in the response of epilithon and juvenile coho salmon to spawning salmon. 3. In response to spawning salmon, epilithon exhibited enrichment in both δ15N (mean: 1.5‰) and δ13C (2.3‰). Juvenile coho were also enriched in both δ15N (0.7‰) and δ13C (1.4‰). Conditional bootstrap models indicate decreased variation in data as spatial replication increases, suggesting that the number of study sites can influence the results of Pacific salmon isotope studies. 4. Epilithon isotopic enrichment was predicted by environmental conditions, with δ15N enrichment predicted by stream temperature and timber harvest ( R2 = 0.87) and δ13C enrichment by discharge, sediment size, timber harvest and spawner density ( R2 = 0.96). Furthermore, we found evidence for a legacy effect of salmon spawners, with pre-spawner δ15N and δ13C of both epilithon and juvenile coho predicted by salmon run size in the previous year. 5. Our results show that the degree of incorporation of salmon-derived nitrogen and carbon differs among streams. Furthermore, the environmental context, including putative legacy effects of spawning salmon, can influence background isotopic concentrations and utilisation of salmon-derived materials in southeast Alaska salmon streams. Future studies should consider the variation in isotopic composition of stream biota when deciding on the number of study sites and samples needed to generate meaningful results. [ABSTRACT FROM AUTHOR]