Historical disturbances to rivers appear to constrain contemporary distribution of a river-dependent frog

Abstract

Frogs dependent on lotic environments are sensitive to disturbances that alter the hydrology (e.g., water impoundments), substrate (e.g., debris torrents), and riparian vegetation (e.g., wildfires) of river ecosystems. Although rivers are often very dynamic, disturbances can push environmental baselines outside of narrowly defined ecological tolerances under which a species evolved. Short-lived lotic-dependent organisms, restricted to movements within the water or the riparian corridor, are at risk of local extirpations owing to such disturbances if they fragment and isolate affected populations from recolonizing source populations. In Oregon, USA, the foothill yellow-legged frog (Rana boylii) is at its northernmost range margin and has experienced an approximately 41% range contraction compared to their historical distribution. To inform conservation and management, we used species distribution models to identify environmentally suitable watersheds based on intrinsic baseline environmental variables, and then examined potential effects of human-caused alterations to rivers, including splash dams used to ferry timber downstream prior to 1957, large water impoundments, and adjacency to agricultural croplands. We used machine-learning in program Maxent and three different river layers that varied in extent and location of mapped rivers but contained distinct information to produce species distribution models which we then combined into a single ensemble model. Stream order, annual precipitation, and precipitation frequency were the highest ranked baseline environmental variables in most models. Watersheds with highly suitable baseline conditions in our ensemble model were negatively correlated with anthropogenic disturbances to rivers. Foothill yellow-legged frogs appeared to be sensitive to human-caused disturbances to rivers, perhaps indicative of their narrow ecological tolerance to in-river conditions. We do not anticipate variables in our model to change much through time. Rather, for conservation we identified potential legacy (spash dams) and ongoing human-caused disturbances that are more likely to change conditions for the species in the short- and long-term.

Identifying drivers of population dynamics for a stream breeding amphibian using time series of egg mass counts

“…Summer water temperatures can be colder in regulated streams because of hypolimnetic releases of water from deep reservoirs, slowing growth and development of tadpoles (Catenazzi & Kupferberg, 2013, 2017; Wheeler et al, 2015). In addition to altered flow regimes, the absence of R. boylii from sites within its historical range has been linked to habitat destruction, disease, introduced species, agricultural land use and pesticide spraying upwind from streams, and the use of splash dams to move timber (Adams et al, 2017; Davidson et al, 2002; Linnell & Davis, 2021; Moyle, 1973). Given the observed declines and extirpations, and the variation in population density among extant populations, there is a need for a quantitative assessment of how population growth is related to stream and watershed characteristics.…”

“…Endangered Species Act (U.S. Fish and Wildlife Service, 2021a). Even within the North Coast region where the species does not appear to be under threat of regional extirpation, occupancy and population density vary greatly among stream reaches both within and between watersheds (Catenazzi & Kupferberg, 2017;Linnell & Davis, 2021;U.S. Fish and Wildlife Service, 2021b).…”