Cooperative Fish and Wildlife Research Units Program: Texas
Education, Research and Technical Assistance for Managing Our Natural Resources

Aquatic invasive species, such as the zebra mussel (Dreissena polymorpha), cause extensive ecological and economic damage, and pose an ongoing concern for potential introduction and establishment in the western United States. Our study applied habitat suitability indices and network analysis to identify the lakes most critical to the continued westward spread and establishment of zebra mussels from a key invasion front in Texas. We created multiple networks consisting of lake nodes and connecting roadway edges. Each network represented the potential connectivity of lakes for recreational users depending on the maximum roadway distance boaters were likely to travel. We evaluated three different maximum roadway distances based on boater movement surveys: 95% of boaters traveled within 363 km, 75% traveled within 125 km, and 50% traveled within 51 km. Using centrality measures of graph analysis, we identified lakes critical to the spread of zebra mussels by acting as hubs, stepping stones, or cutpoints. Water quality-based habitat suitability indices identified many lakes in eastern Texas with no or low suitability for mussel infestation. Most lakes in other regions of the study area were classified as having moderate to high suitability. In all networks, hubs were concentrated in northeastern Texas, with connectivity rapidly declining further west as lakes became more isolated. At the lowest maximum edge lengths, stepping stones were also concentrated in the northeast, but could be found in the western portions of the study area as maximum travel distance increased. No cutpoints were found in the 363 km maximum edge length network, indicating a highly connected network with the potential for further western spread facilitated by stepping stones in western Texas and New Mexico. Identifying critical lakes provides a predictive tool for resource managers to guide the allocation of limited time and resources for early detection and outreach campaigns. This novel use of network analysis via roadways is readily applicable to other invasive species such as aquatic plants that are spread between waterbodies by the overland transportation of boats and other recreational equipment.