Characterizing Natural Barriers to Nonnative Stream Fauna in Hawai‘i
Brendan Martin, Yin-Phan Tsang, Ralph W. Tingley III, Hannah Clilverd, and Dana M. Infante
Abstract.—Waterfalls, caused by the abrupt changes of elevation in streams, are natural barriers that influence the distribution and dispersion of aquatic species. The resulting habitat fragmentation has contributed to species specialization as well as barriers that inhibit passage of nonnative species upstream. In Hawai‘i, it is assumed that nonnative species are unable to pass waterfall barriers, yet they are present above some waterfalls, possibly facilitated by human introduction. In this study, we used a landscape approach to identify likely human introductions and examine the ability of nonnative stream fauna to bypass waterfalls. We identified the human activities associated with the high likelihood of species introduction. We found that when a local catchment has a population density >4.24 people/km2 or road length density is >0.01 km/km2, the presence of nonnative species in the stream is likely a result of human introduction. After filtering human facilitated introduction, we also assessed the potential waterfall climbing ability of 14 nonnative taxa. We found that 12 out of the 14 taxa were absent upstream of waterfalls, indicative of their inability to traverse waterfalls. Only two species, Tahitian prawn (also known as monkey river shrimp) Macrobrachium lar and American bullfrog Rana catesbiana, seem able to pass waterfalls. This study highlights the role that people play in facilitating species introductions in otherwise inaccessible habitats. Without human interference, waterfalls can be considered effective barriers to nonnative species and can be instrumental in supporting nonnative species eradication and control strategies.