The cascading effects of birch on heather moorland: a test for the top-down control of an ecosystem engineer

1. Single species can have a disproportionate effect on ecosystem function and diversity, yet our understanding of the importance of single species in driving terrestrial ecosystems during succession remains poor. 2. Utilizing a long-term experiment, where birch was planted on heather moorland 20 years ago, the cascading effects of a single tree species (Betula pubescens) on ecosystem characteristics (plant species richness, soil chemistry, soil fauna and decomposition rates) were tested. 3. Under the birch, plant species richness decreased and the vegetation composition changed, with lower cover of grasses and Vaccinium myrtillus. The depth of the soil organic horizon, its moisture content and percentage carbon were all smaller under the birch than under the heather. Concentrations of available phosphorus and mineralizable-N were significantly greater in the soil under birch than under the heather plots. Decomposition was faster in the birch than in the heather plots. The abundance and species richness of collembola and oribatid, mesostigmatid and prostigmatid mites were all significantly greater under the birch than under the heather. 4. The durability of the engineering effects of the birch was studied in a second experiment. Plots were established in first generation birch woodland that had developed on Calluna-dominanted moorland. The plots were cleared of birch and planted with heather. After 20 years soil chemical properties, microarthropod communities and decomposition rates were not significantly different between plots with and without the birch. However, the mass of the soil O-horizon was significantly greater in the felled birch plots than in the control birch plots, providing the first indication of a change towards soil properties more typical of a Calluna moorland. Thus for most of the birch engineering effects measured here their durability in the absence of the engineering species is at least 20 years. 5. This work has provided experimental evidence that birch acts as a top-down engineer, driving cascading effects on both above- and below-ground communities, soil chemical and physical properties and ecosystem processes. The work also shows that the role of birch in driving changes in the ecosystem is durable 20 years after the removal of the birch