When top-down is also bottom-up: Examining consumer-driven nutrient recycling in an arctic herbivore population

Herbivore consumers represent important top-down and bottom-up forces in ecosystem regulation and an understanding of the zoogeochemical mechanisms of their influences is needed to better understand ecosystem function. The consumer-driven nutrient recycling hypothesis (CNR) provides a framework for linking consumer populations and bottom-up regulation of ecosystem processes due to potential impacts of faecal nutrients on plant nutrient limitation. Here, we examine CNR by evaluating how changes in brown lemming density and foraging behaviour during different phases of their population cycle influence nutrient availability in an arctic tundra ecosystem. Our general objectives were to test whether CNR was supported in arctic tundra and whether CNR can be used to understand tundra ecosystem function. To achieve this, we (1) examined if faecal nutrient concentration and ratios varied between different phases of the lemming population cycle, (2) assessed whether lemming diets change seasonally, (3) examined whether faecal nutrients are affected by diet, (4) evaluated if changes in diet-caused changes in faecal quality influence plant nutrients and (5) examined the decomposition and nutrient loss from faeces. We found seasonal differences in faecal carbon (C) and phosphorus (P), with lower faecal C and P during late summer compared to mid-summer, but no differences in faecal N across seasons or changes in faecal nutrients across years. We also observed no differences in faecal nutrients (CNP) of lemmings fed different diets, but plants grown in faeces from an Eriophorum diet had greater biomass than that of plants grown with faeces from a Carex diet. Faeces persisted 1.5-4.4 years on the tundra depending on decomposition environment and while C and N were retained within decomposing faeces for several years, P was rapidly lost. Our data suggest tentative support for the CNR hypothesis in a tundra ecosystem; by providing limiting nutrients during the peak of the population cycle, faecal nutrients may influence ecosystem function and explain how the system recovers from cyclical disturbance regimes. Due to slow faecal decomposition, nutrients supplied during a population cycle peak may provide legacy effects on ecosystem function across multiple years.