Patch-level facilitation fosters high-Andean plant diversity at regional scales

Abstract

Aim: Local effects of ecosystem engineers on biodiversity can scale up to the landscape level, stressing the importance of ecological processes as determinants of species richness at larger spatial scales. In harsh environments, cushion plants often act as ecosystem engineers given their ability to buffer extreme abiotic conditions, thus providing unique and more favorable niches for the establishment of less stress-tolerant plant species. We assessed if facilitation by cushion plants influences patterns of plant diversity at increasing spatial scales. Location: Northern Patagonian Andes, Argentina. Methods: Based on plant species records within and outside cushions, we compared plant diversity in the presence and absence of cushions at the community, mountain and regional scale. Specifically, observed and estimated numbers of species occurring in either cushion plants or in the surrounding open areas were compared with the species numbers of the open areas. Results: The presence of cushion plants significantly increased species richness at all spatial levels analyzed. At the patch level, a higher number of species was recorded growing within cushion plants than in open area plots of similar size. Consistently, hypothetical communities lacking cushion plants showed significantly lower species richness than observed communities with cushion plants. These differences in species richness at patch and community scales increased with altitude, hence facilitation by cushion plants became more important at higher elevations. Moreover, according to asymptotic non-parametric estimators, cushion plants could increase overall regional species richness up to 40%. Conclusions: Our findings suggest that about one third of the high-Andean flora of the region owes its presence to the existence of nursing plants, defying the view that abiotic factors alone are enough to fully determine species occurrences at large spatial scales. This study provides evidence that local-scale ecological processes can be crucial in promoting and maintaining biodiversity at any spatial scale.