Atmospheric invasion of non-native pollen in the Mediterranean region.

Most research on the impacts of plant invasion focuses on native plant performance, community structure, and ecosystem functioning. Some non-native species can also pose a risk to human health. One such risk is the allergenic nature of the pollen of some introduced plants. We examined whether patterns of airborne pollen differed between non-native and native taxa by summarizing data from seven Spanish Mediterranean localities monitored over 13 yr. The pollen spectra contained 27 native pollen taxa and 18 non-native taxa. Even though pollen from native taxa were more diverse and were present longer in the atmosphere than the non-native, in some years neither the prevalence of the two nor their weekly maximum pollen values differed significantly. However, maximum values for non-native taxa were found earlier in the season than for native pollen. A small percentage of non-native pollen includes pollen from introduced taxa that have not invaded natural habitats (e.g., ornamental plants). Non-native pollen has a larger proportion of allergenic pollen than native pollen. Therefore, the results reveal that the presence of non-native airborne pollen from naturalized and non-naturalized plant species increases the total amount of airborne allergenic pollen grains and the period of allergenic susceptibility.