Increasing Resistance or Susceptibility to Exotic Invasion? - Contrasting Effects of Fish Barriers

The objective of this study is to assess the role of manmade vs. natural barriers in influencing the invasibility of certain exotic and nonnative fishes in northern boreal waters. Specifically, I examined both short and longer term effects of constructed and natural geomorphic barriers, on the invasive patterns of exotic brook trout (Salvelinus fontinalis) and nonnative pike (Esox lucius), within 10 northern Swedish river systems. The observed effects were further validated by studying removal effects of both natural and manmade barriers. I identified specific barrier characteristics in connectivity contexts that either increases susceptibility or resistance to invasive fishes.

Invasibility in this region is shown to be first and foremost determined by natural barriers, biotic resistance and/or manmade barriers in consecutive order. It is not uncommon that barriers are constructed in streams to prevent undesired species, thus protecting specific upstream habitats from invasion. In contrast, exotic invasion may be facilitated by stream barriers that reduce biotic resistance, thereby creating an empty niche. In addition, evidence of barrier dams functioning as stepping stones that facilitate further invasions upstream is discussed.