Kelp gull shell middens potentially facilitate root growth of non-native plants in Antarctica

Ecosystem engineering is an important factor influencing species establishment in novel environments. Here we tested how shell middens created by kelp gulls (Larus dominicanus) affect the growth of candidate non-native plants under simulated Antarctic conditions. Calcium compounds derived from the shells raised the organic Antarctic substrate pH from 6.0 to 7.3 and resulted in increased plant biomass (21–100%) over a 14-week experimental period compared to plants grown without shell-amended substrate. The increase in biomass was primarily driven by the development of longer and heavier roots. Plant calcium content (mg) doubled under the shell treatment while nitrogen content was unaffected and phosphorous (%) declined by 16–37%, although total accumulated plant phosphorous was unaffected. Our data suggest that, by transferring considerable quantities of marine-derived substrate (shells) into focal locations on land, kelp gulls can potentially act as ecosystem engineers by promoting non-native plant growth in Antarctica. These findings highlight the need for consideration of the potential suitability of marine vertebrate-influenced habitats for non-native plant species growth in remote and pristine environments such as Antarctica.