Short and long-term impacts and recovery potential for aquatic plant and snail communities in lakes impacted by an invasive omnivore

Background/Question/Methods

Invasive species have both acute and chronic impacts on the recipient community. As omnivores, crayfish impact multiple levels of the food web through both direct and indirect interactions.  The invasion of rusty crayfish in the upper Midwest provides a unique opportunity to study how different taxonomic groups respond to the same invasive species over time.  Snails are a preferred prey item for rusty crayfish and a dominant component of aquatic invertebrate communities.  Rusty crayfish also severely reduce aquatic plant diversity and abundance, which may be an additional driver of change in invaded ecosystems. Thus, studies of these taxa can provide an indication of broader ecosystem impacts and the potential for ecosystem recovery.  We surveyed crayfish, snail, and plant communities in ten lakes in 1987, 2002, and 2011.  Some lakes exhibited dramatic increases in rusty crayfish populations, others saw substantial declines, and population levels remained roughly the same (either high or low) over the 24 years.   We hypothesized that lakes with declining crayfish populations would exhibit limited recovery potential, especially for snails.  We also predicted that the rate of loss of snails and macrophytes would be most pronounced in the acute stages, and slow down in the chronic stages.

Results/Conclusions

Both plants and snails exhibited signs of recovery in lakes experiencing declines in rusty crayfish.  Plant species increased significantly over time (mean species per sector per lake, 1987 < 2002 < 2011, P < 0.01).  Paired comparisons revealed that only 2011 and 1987 richness were significantly different, indicating the recovery of plant species to be very gradual.  Snail species richness exhibited very similar patterns (mean species richness per lake, 1987 < 2002 < 2011, P < 0.03).  For comparison, species richness remained the same in the reference lakes exhibiting no change in crayfish density over time (plants, P > 0.87; snails P > 0.5).  Loss of plant and snail species in lakes with increasing numbers of rusty crayfish was acute between 1987 and 2002, but then either slowed (for plants) or increased slightly (for snails) in 2011.  Lake wide averages for richness may be obscuring substrate and crayfish heterogeneity within lakes.  For this reason, future analyses will consider both substrate and spatial distribution of crayfish, snail, and plant species.  Our results suggest that impacts of invasive species change over time.  Understanding these dynamics help prioritize management activities and provide insight into how community interactions may temper impacts of invasive species.