The historical role of agriculture and gardening in the introduction of alien plants in the western Mediterranean

The aim of this study is to clarify the role of gardening and agriculture in the introduction of alien vascular plants to the western Mediterranean Basin during the last century. The main country in which this study is based is Spain. However, other countries in the Western Mediterranean Basin also form part of it. These are Portugal, France, Italy and Morocco. Three percentages were calculated. Firstly, the percentage of alien flora was calculated. Then, we calculated the percentages of species introduced as either a consequence of gardening or of agriculture. The figures were based on thirty-four floristic studies carried out in the western Mediterranean Basin between 1904 and 2006. A regression model, in which the explanatory variable is time and the dependent variables are the three percentages mentioned above, was drawn up with the data obtained. The results show the existence of positive correlation between time and the percentages of total exotic flora and the species introduced through gardening (R = 0.64 and 0.56, respectively) and the negative correlation between time and the percentage of alien species introduced through agriculture (R = -0.43). During the last century, the roles of agriculture and gardening as sources of the introduction of exotic flora to the western Mediterranean Basin were reversed. However, in the areas where the socio-economic scene is more predominantly agrarian, the role of gardening is still limited.