Long distance (> 20 km) downstream detection of endangered
stream frogs suggests an important role for eDNA in surveying for
remnant amphibian populations
Abstract
Globally, amphibian species have suffered drastic population declines
over the past 40 years. Hundreds of species are now listed as Critically
Endangered, with many of these considered “possibly extinct”. Most of
these species are stream-dwelling frogs inhabiting remote, montane
areas, where traditional surveys limit our ability to find remnant
populations. Environmental DNA (eDNA) monitoring could revolutionize
surveys for missing and endangered amphibian populations by enabling
sampling of downstream sections to screen the vast upstream catchments.
However, this is dependent on quantifying downstream detection
probability and distances. Here we tested this in two endangered stream
frogs (Litoria nannotis and L. lorica) that co-occur in a remote stream
catchment, and for which we know precise downstream distributional
limits from traditional surveys. Importantly, the last populations of L.
lorica persist in this catchment; one small (~1,000
frogs) and one very small (~100 frogs). We conducted
eDNA screening at set distances downstream from the populations (up to
22.8 km) using precipitation from two water volumes and via filtering,
during moderately high and low flow conditions. During high flow, we
reliably detected both species as far as 22.8 km downstream using the
larger water volumes and filtering techniques. We could only detect the
very small population of L. lorica immediately downstream. Detection
success was higher at the sites further downstream for both species
during high flow compared to low flow. Our downstream detection
distances (> 20 km) suggest eDNA is a valuable tool for
detecting rare stream amphibians. We provide recommendations on optimal
survey techniques.