Effects of the invasive riparian plant Arundo donax on riverine fish: a call for action?

Studies that investigate the effects of a given invasive exotic species within real-world contexts are essential for informing effective management decisions. Here, we used extensive surveys of stream reaches in north-eastern Spain to test whether the presence of the widely distributed invasive exotic riparian plant Arundo donax is detrimental to the conservation of riverine fishes. To explore potential impacts of A. donax at different temporal scales, we examined fish responses using indicators of relatively recent effects (body-condition index) and longer lasting effects (functional diversity indices, taxonomic richness, and the relative abundance of 12 fish species). We found correlative evidence for net positive effects of the presence of A. donax on native fish richness and body condition but negative effects on the overall functional diversity of fish assemblages, including invasive exotic and translocated native fish species. Our analyses also revealed significant interactions between A. donax and local stream conditions, suggesting that these net effects of the plant on fish might shift in direction in some circumstances. In particular, several indicators at the fish community scale support that the presence of A. donax may be beneficial to fishes in streams with high Strahler order values, in-stream habitat diversity and lack of channelization. The plant may provide shelter and refugia to fish in these medium, low-land Mediterranean streams with typically poor cover of high riparian vegetation. Therefore, without overlooking the serious threats caused by biological invasions, this study illustrates that, at least under specific environmental conditions, the local removal of A. donax alone may not achieve the desired ecological outcomes if pre-existing fish-habitat relationships are not taken into account.