Positive interactions among native and invasive vascular plants in Antarctica: assessing the "nurse effect" at different spatial scales.
Antarctica is a stressful ecosystem with few vascular plants, an ideal system to test positive interactions. Here, plants such as Deschampsia antarctica could generate more suitable micro-environmental conditions for the establishment of other plants (facilitation). We examined the co-occurrence of vascular plant species in the Antarctic Peninsula and assessed the potential nurse effect by D. antarctica on the native Colobanthus quitensis and the invasive Poa annua. We also measured the ecophysiological performance and survival of C. quitensis within and outside the canopy of D. antarctica in two study sites differing in stress levels. In addition, a survival experiment was conducted with the invasive Poa annua individuals within and outside D. antarctica individuals. In sites where present, target species co-occurred with D. antarctica in both Shetland Islands and Antarctic Peninsula. In agreement with the stress gradient hypothesis, we found evidence of facilitation between vascular Antarctic plant species. Specifically, we found that D. antarctica facilitates the native C. quitensis and the invasive P. annua and that the effect is stronger in more stressful sites. Additionally, C. quitensis distribution is compatible with an influence of either direct or indirect facilitation from D. antarctica. Facilitation between vascular plants may play a role structuring Antarctic plant communities. Thus, distribution of native species should be considered when assessing the introduction and spread of invasive species. Also, our results together with those from previous studies showed that the type and magnitude of biotic interactions may change with time and can depend on the plant traits considered.