A Quantitative Approach to the Study of Non-indigenous Plants: An Example from the Azores Archipelago

Abstract

A random survey of non-indigenous plants was performed on the nine islands of the Azores Archipelago, including 529 one square km plots, corresponding to about 20% of the land surface. The number of sampling plots varied from 12 for Corvo Island to 136 for Sao Miguel, while the number of sampled taxa per island varied between 152 and 365. The percentage of non-indigenous taxa was lowest for Corvo, Flores, and Sao Jorge (below 60%), and highest for Graciosa (above 70%), with intermediate values for the remaining islands (63–68%). The majority of the sampled taxa were Dicotyledoneae (above 60%) and Monocotyledoneae (20%) with lower percentages of Pteridophyta and Gymnospermae. Therophytes, hemicryptophytes and camaephytes were the most common life forms. The 30 most important non-indigenous plants in the Archipelago and per island showed some differences among islands. Their habitats ranged from anthropic to native vegetation. The proportion of non-indigenous taxa was positively correlated with several descriptors associated with human activities, but negatively correlated with altitude and slope.