Daniel J. Isaak, Michael K. Young, Charles H. Luce, Steven W. Hostetler, Seth J. Wenger, Erin E. Peterson, Jay M. Ver Hoef, Matthew C. Groce, Dona L. Horan, and David E. Nagel

ECOLOGY

Significance

Many studies predict climate change will cause widespread extinctions of flora and fauna in mountain environments because of temperature increases, enhanced environmental variability, and invasions by nonnative species. Cold-water organisms are thought to be at particularly high risk, but most predictions are based on small datasets and imprecise surrogates for water temperature trends. Using large stream temperature and biological databases, we show that thermal habitat in mountain streams is highly resistant to temperature increases and that many populations of cold-water species exist where they are well-buffered from climate change. As a result, there is hope that many native species dependent on cold water can persist this century and mountain landscapes will play an important role in that preservation.

 Abstract

The imminent demise of montane species is a recurrent theme in the climate change literature, particularly for aquatic species that are constrained to networks and elevational rather than latitudinal retreat as temperatures increase. Predictions of widespread species losses, however, have yet to be fulfilled despite decades of climate change, suggesting that trends are much weaker than anticipated and may be too subtle for detection given the widespread use of sparse water temperature datasets or imprecise surrogates like elevation and air temperature. Through application of large water-temperature databases evaluated for sensitivity to historical air-temperature variability and computationally interpolated to provide high-resolution thermal habitat information for a 222,000-km network, we estimate a less dire thermal plight for cold-water species within mountains of the northwestern United States. Stream warming rates and climate velocities were both relatively low for 1968–2011 (average warming rate = 0.101 °C/decade; median velocity = 1.07 km/decade) when air temperatures warmed at 0.21 °C/decade. Many cold-water vertebrate species occurred in a subset of the network characterized by low climate velocities, and three native species of conservation concern occurred in extremely cold, slow velocity environments (0.33–0.48 km/decade). Examination of aggressive warming scenarios indicated that although network climate velocities could increase, they remain low in headwaters because of strong local temperature gradients associated with topographic controls. Better information about changing hydrology and disturbance regimes is needed to complement these results, but rather than being climatic cul-de-sacs, many mountain streams appear poised to be redoubts for cold-water biodiversity this century.

See http://www.pnas.org/content/113/16/4374.full

PNAS 19 April 2016; vol.113; no.16: 4374–4379

Fig. 2.

Stream climate velocity scenario showing isotherm shift rates based on the historical warming rate for 1968–2011 (A; X-HS scenario). Cumulative distribution curves summarizing network velocity profiles for six scenarios based on multiples of the historical warming rate (X, 2X, 3X scenarios) and assignment of stream sensitivities to historical values (HS scenarios) or supersensitive values (B; SS scenarios). Small images show climate velocities in black boxes (C and D) and August stream temperatures for the same locations (E and F).