To what extent do natural disturbances contribute to Andean plant diversity? A theoretical outline from the wettest and driest parts of the tropical Andes.

This paper deals with natural disturbances and their impact on vascular plant enrichment at two climatically contrasting Andean ranges, i.e. the perhumid Cordillera Real in southern Ecuador and the arid Cordillera de Atacama in northern Chile. In the first case, main triggers for an additional input of pioneer species during succession stages initiated by perturbations are landslides, mudflows, and, to a lesser extent, cohort mortality, floods, and wildlife damages. Droughts and wind are stressors, which reduce plant growth but hardly plant diversity, in contrast to enhanced UV radiation with its mutagen effect. Though stress effects are similar in the Atacama, disturbance regimes differ considerably in this dry mountain environment. Here, most perturbations are of small dimension such as nitrogen inputs by feces of Lamoids and burrow activities of tuco-tuco mice, both of them fostering nitrophilous plant communities. Flooding, gelifluction, and other denudation processes such as sheet wash occur too, however, do not charge species enrichment in the dry Andes. Although the perhumid study site represents one of the world's plant diversity "hotspots" and, by contrast, the arid one a comparatively "coldspot", pioneer species during successive stages after natural disturbances contribute in a similar percentage to the total plant inventories (appr. 10% of the species numbers). Relatively seen, natural disturbances are most important for species enrichment in the Atacama (200-500 species per 10 000 km²), while most other ecological factors delimit plant survival. Instead, plant life at the Ecuadorian study area benefits from many climatic and edaphic site conditions, and consequently, disturbances are considered only one of many driving forces for its hotspot status (>5000 species per 10 000 km²). [ABSTRACT FROM AUTHOR]