Interactive abiotic and biotic stressor impacts on a stream-dwelling amphibian

Organisms within freshwater and marine environments are subject to a diverse range of often co-occurring abiotic and biotic stressors. Despite growing awareness of the complex multistress systems at play in aquatic ecosystems, many questions remain regarding how simultaneous stressors interact with one another and jointly impact aquatic species. We looked at multistress interactions in a protected stream ecosystem in Mendocino County, California. Specifically, we examined how diurnal temperature variation, turbidity, and predator cues altered the movement speed of larval Pacific giant salamanders (Dicamptodon tenebrosus). In a second experiment, we looked at how simulated low-flow summer conditions impact the expression of heat-shock proteins (HSPs) in the same species. Larvae moved almost one and a half times faster in the presence of chemical cues from trout and suspended sediment, and almost two times faster when both sediment and trout cues were present but were only marginally affected by temperature and visual cues from conspecifics. Interestingly, the order of stressor exposure also appeared to influence larval speed, where exposure to sediment and trout in earlier trials tended to lead to faster speeds in later trials. Additionally, larvae exposed to low-flow conditions had more variable, but not statistically significantly higher, expression of HSPs. Our findings highlight the potential interactive effects of an abiotic stressor, sedimentation, and a biotic stressor, and predator chemical cues on an ecologically important trait: movement speed. Our findings also demonstrate the likely role of HSPs in larval salamander survival in challenging summer conditions. Taken together, these findings show that larval D. tenebrosus responds behaviorally to biotic and abiotic stressors and suggests a possible pathway for physiological tolerance of environmental stress. Consideration of multistress systems and their effects is important for understanding the full effects of co-occurring stressors on aquatic organisms to guide appropriate conservation and management efforts based on ecologically relevant responses of organisms within an environment.

Fresh perspectives on the River Continuum Concept require trophic ecology approaches focussed on food web structure and energy mobilisation routes

Stream-dwelling communities are expected to show a gradual replacement of the dominant feeding types following the type of resources found along the river continuum. Yet, the underlying longitudinal gradients in food web structure and energy flow-paths remain poorly understood. Here, I synthesise novel research on the River Continuum Concept (RCC) and identify promising areas for future research linked to longitudinal changes in food-chain length and energy mobilisation routes. For example, feeding links and connectance should reach maximum values in mid-order rivers and then decrease to river mouths following uncovered longitudinal diversity patterns. Regarding energy mobilisation routes, a gradual replacement in the food web fuelling between allochthonous (leaf litter) and autochthonous (periphyton) resources should be expected. Beyond longitudinal changes in primary basal resource to consumer paths, other allochthonous (e.g. riparian arthropod inputs) and autochthonous (e.g. fish prey) inputs subsidising higher level consumers may show longitudinal changes, that is, terrestrial invertebrates decreasing but piscivory increasing downstream. However, the role of these inputs, that can alter predator niche variation and have indirect community-based effects, on both food web structure and energy flow-paths along the river continuum is not clear yet. Incorporating energy mobilisation and food web structure into RCC principles is necessary for a broad understanding of ecosystem functioning and trophic diversity in riverine systems, driving the emergence of novel insights. How function and structure of riverine food webs adapt to longitudinal changes in physical and biological environments represent a challenge for next generation of stream ecologists.