The alien fern Azolla filiculoides (red-brown) grows among native Lemna duckweed in the Doñana marsh. Azolla appeared in Doñana in 2001 and has since expanded to cover around 2,000 ha. Its arrival coincided with an increase in phosphorus inputs to the marsh. Credit: Andy Green, EBD-CSIC

The alien fern Azolla filiculoides (red-brown) grows among native Lemna duckweed in the Doñana marsh. Azolla appeared in Doñana in 2001 and has since expanded to cover around 2,000 ha. Its arrival coincided with an increase in phosphorus inputs to the marsh. Credit: Andy Green, EBD-CSIC.