Community effects of large-scale eradication of invasive watermilfoil

In ecosystems with a long history of invasion or a large-scale invasion that significantly changed the native community, radical removal of exotics may lead to unpredictable secondary effects.  Since macrophytes are important for community structure, it is natural to expect that changes in plant community may lead to cascading effects on invertebrates and fishes.  Eurasian watermilfoil (Myriophyllum spicatum) is an invasive macrophyte that creates dense monocultures and displaces native aquatic plants.  This long-term study was designed to evaluate effects of a lake-wide low-dose herbicide treatment to remove invasive watermilfoil in Minnesota lakes.  After eradication of milfoil, we observed a successful restoration of the native plant community: abundance and diversity of native plants increased in the two treated lakes as compared to the reference lakes.  However, a lag in reestablishment of native vegetation and/or a change in structural complexity of the habitat could have affected resident fish and invertebrate communities and their interactions.  Our results show that fish diversity and abundance did not change significantly in response to the herbicide treatment.  We discuss macroinvertebrate abundance and fish diet selectivity as a function of changing plant communities.  Understanding the long-term effects of radical changes in plant community is important for designing restoration strategies for removal of invasive species.