There are few problems more compelling for ecological science than the recovery of threatened and endangered species.  The threats posed by invasive species can be particularly insidious for species vulnerable to extinction and it is imperative that we understand the competitive effects of invasive species so that effective management actions can be taken.  In this study, our goal is to understand the nature of the competition between a globally endemic and federally threatened species of terrestrial snail, the Chittenango Amber Ovate Snail (Novesuccinea chittenangoensis) (COAS), and a closely related invasive snail, Succinea spp. B. (Spp. B).  In 2008, we carried out a competition experiment in which we placed individual COAS in enclosures with varying densities (0, 10, 25, 50 individuals) of Spp B for a two-week period and recorded snail growth and mortality among the different densities.  We also performed in situ habitat use surveys to compare the spatial distribution of COAS and Spp. B and their use of available plants and substrates.  Multiple mark-recapture surveys each year since 2002 provide additional information on population size, size frequency distributions, and demographic rates of snails. 

Results/Conclusions

Results from the competition experiment indicated negative effects of Spp. B on COAS growth and survival.  COAS grew an average of 0.85 mm at the lowest density of Spp B and 0.15 mm at the highest density.  Similarly, percent mortality was about 15% lower at the lowest Spp. B density compared to the highest density. Habitat use surveys of each species showed coincident spatial distributions and overlapping use of plant species and substrates, which suggests a high probability of interaction between the species and some level of competition. Despite the apparent negative effect of competition, population estimates over the last six years suggest that COAS population size is variable but stable. A potential explanation for the coexistence of the two species is their differences in phenologies. Changes in size frequency distributions over time in 2008 showed that by mid-summer the largest and presumably most competitive Spp B. nearly disappeared while large COAS persisted.  This period of low-competition at the end of the summer may provide a window of growth that allows COAS to coexist with Spp B.  These preliminary results suggest that removal efforts of the invasive species may be unnecessary, although additional research is required to explore reproductive phenologies.