The authors provided theoretical expectations for immediate effects of herbivores on tundra-grassland plant-community nutrient levels throughout a single growing season and empirically evaluate these predictions. They established an experiment within two forb-dominated and two grass-dominated tundra-grassland communities.
They selected tundra-patches disturbed by small rodents during the previous winter, and neighbouring undisturbed tundra-patches. Within each tundra-patch, they set up a reindeer-open and a reindeer-exclusion plot.
Throughout the summer, they randomly collected over 2800 leaf samples from 34 vascular plant species/genera and analysed their nitrogen and phosphorus contents. Plant-community nutrient levels were consistently higher in tundra-patches affected by small rodents, both across tundra-grassland types and throughout the growing season. Forbs and grasses growing in small-rodent disturbed tundra-patches had 11% and 25% higher nutrient content, respectively, compared to undisturbed tundra-patches.
Reindeer affected only grasses growing in grass-dominated tundra-grasslands and the outcome was dependent on small-rodent winter disturbance. Reindeer increased grass nitrogen content in undisturbed tundra patches (+7%) and weakened the positive effects of small rodents in disturbed tundra-patches (from 25% to 15% higher nutrient content [both nitrogen and phosphorus]).
By enhancing plant nutrient levels throughout a single growing season, herbivores were key, immediate modifiers of plant-community nutrient dynamics in tundra grasslands. Higher nutrient contents still detected in senescent leaves at the end of the summer in herbivore-affected tundra suggest that herbivory is accelerating short-term tundra-grassland nutrient cycling rates.
The authors’ findings from tundra-grassland communities align with theoretical expectations of positive herbivore effects on nutrient cycling in relatively productive ecosystems.