The effect of propagule pressure and disturbance on non-native abundance: A case study in a kettle lake district

Background/Question/Methods

Anthropogenic movement of non-native aquatic species threatens to change local and regional species richness; therefore it is important to understand factors that contribute to establishment of non-native species in waterbodies.  This study investigated how propagule pressure and disturbance, two mechanisms proposed to increase invasion success, influenced macrophyte communities in a kettle lake district located in central New York State.

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

To quantify propagule pressure, questionnaires were administered to boaters at public launches and homeowner questionnaires were given to property owners 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 in 2011, with a response rate of 31%, in addition to 37 boat launch questionnaires. Questionnaires revealed that public access lakes were connected with more outside waterbodies through boat traffic compared to private access lakes (mean = 32.5 and 16.6, respectively). 

Public lakes had an average non-native macrophyte biomass of 104.0 g/m² compared to 50.8 g/m² of non-native macrophyte biomass in private lakes.  Linear regression showed a non-significant relationship between total external waterbodies and non-native macrophyte biomass (p = 0.557; R² = 0.126).  However, one lake that is currently private has a past legacy of public use, and when removed from the regression analysis, a significant relationship between total external waterbodies and non-native macrophyte biomass was observed (p = 0.048; R²= 0.908).  This result suggests that past public access may confound current propagule pressure estimates. Additionally, percent native macrophyte richness (p = 0.01) was significantly greater in private lakes.  Disturbance variables including number of properties surrounding a lake and septic system maintenance were not significantly related to non-native macrophyte abundance.

This study supports propagule pressure, not disturbance, as an important driver of non-native macrophyte community composition.  Additionally, the results provide evidence that establishment of non-native species can reduce native species community diversity.