Propagule pressure and disturbance as drivers of invasive macrophyte abundance in public versus private lakes

Background/Question/Methods

Anthropogenic movement of aquatic invasive species (AIS) threatens to homogenize local and regional species richness.  It thus becomes important to understand mechanisms contributing to AIS establishment in novel water bodies.  This study investigated how propagule pressure and disturbance, two theoretical frameworks proposed to explain the likelihood of invasions, contributed to aquatic invasive macrophyte presence and abundance in a kettle lake district located in central New York State. 

This lake district is ideal to investigate propagule pressure and disturbance due to a gradient of outside boater/angling pressure (public vs. private lakes) and surrounding population density, which ranged from a minimum of 8 properties surrounding a single lake up to 144 properties.  We hypothesized that: (1) public lakes would have an increased abundance of invasive macrophytes, and (2) lakes with greater population densities would have increased invasive macrophyte abundances.

To quantify propagule pressure, questionnaires were administered to boaters at public launches.  In addition, homeowner questionnaires were given to property owners at each lake to further estimate propagule pressure and quantify disturbance (e.g., maintenance of septic systems).  Transect and quadrat sampling were conducted via snorkeling and SCUBA to determine macrophyte species richness and abundance (g/m²).      

Results/Conclusions

A total of 123 homeowner questionnaires were returned, with a response rate of 31%, in addition to 37 boat launch questionnaires.  Data obtained from boat launch and homeowner questionnaires revealed that public access lakes were associated with 36 and 29 other distinct water bodies, respectively, through outside boat traffic (mean = 32.5).  Private access lakes were associated with 31, 14, and 5 separate water bodies (mean = 16.6).  Public access lakes had an average invasive macrophyte abundance of 86 g/m² compared to 34 g/m² of invasive macrophyte abundance in private lakes.

Lake population densities were correlated with invasive macrophyte abundance; greater number of homes resulted in greater invasive macrophyte abundance.  For lakes with >100 properties, 22% of septic systems had not been serviced in more than six years, compared to only 8% of septic systems in lakes with fewer than 100 properties, suggesting lakes with increased population densities were more disturbed, promoting macrophyte invasions.

This study supports propagule pressure and disturbance as important drivers of native and invasive macrophyte community composition within a lake district.  These findings will assist the local lake associations with management goals, and will inform lake managers attempting to prevent the introduction and establishment of aquatic invasive macrophytes.