Figure 1: A female foothill yellow-legged frog (Rana boylii) waiting to lay eggs (gravid).

By Ryan Peek, Helen Dahlke, and Sarah Yarnell

An organism’s success relies on responding to environmental cues that trigger activities such as breeding, migration, feeding, predator evasion, etc. Responses can be finely tuned to specific cues, or may require multiple triggers. For example, changes in day length and air temperature cue many bird migrations over thousands of miles between breeding and wintering grounds. Some catfish species spawn immediately after heavy rains, sensing the increase in water levels and decreases in water temperature. Mosquitoes can smell increased carbon dioxide levels from mammals as far as 50m away and use these “invisible plumes” to guide them towards food. In dynamic environments like rivers, environmental cues may help organisms forecast stable conditions suitable for activities such as laying eggs or rearing larvae, especially if these cues reliably indicate good conditions.

Human activities can disrupt or eliminate environmental cues that keep species successful. A recent example is endocrine disrupting chemicals which can block or disrupt hormones needed for breeding, migration, feeding (Wingfield and Mukai 2009, Rogers et al. 2013, Kabir et al. 2015). In many species, we only have some understanding of environmental cues and what disrupts them. Better understanding cues can help managers implement restoration and protection actions.

We have been exploring environmental cues for the foothill yellow-legged frog [FYLF] (Rana boylii) in rivers in the Sierra Nevada. Significant work has identified suitable breeding and rearing habitat conditions for these river-breeding frogs, which have adapted to California’s seasons and are genetically wired to lay eggs during the spring snowmelt when river flows recede and water temperatures increase.

Figure 2:  Foothill yellow-legged frog breeding timing over three years in the North Fork American River.

How do frogs detect this cue in flow patterns? Frogs can detect fine changes in the nutrients in the water, or more specifically, compounds like potassium and sodium. Streamflows have different chemical signatures depending on their water source. Snowmelt, groundwater, and rainwater have different chemical signatures. With climate warming, there will be less snow, more rain, more frequent high intensity storms, and longer periods of low flows. This may be important for species which use environmental cues based on water chemistry to trigger breeding. As these cues shift in the future, understanding which specific cue is driving the response (breeding) will important for managing rivers in the Sierra Nevada.

We’ve been monitoring foothill yellow-legged frog populations in Sierra rivers since 2011. To determine how water chemistry might be a breeding cue, we examined daily water samples along the North Yuba River during spring of 2014. These samples were analyzed for isotopes of oxygen and hydrogen to determine the water source. Isotopes in water samples can reveal the shares of snowmelt, rainfall, and groundwater in streamflow. Preliminary results indicate that the frogs didn’t begin breeding until after the water was completely groundwater driven, when the proportion of rain and snow isotopes were zero. Using a water chemistry cue that links to California’s seasons (i.e., wet winters=rain and snow, dry summers=little rain or snow, just groundwater) would reliably allow organisms to focus on stable hydrology periods, even though these times can shift from year to year with droughts, El Nino, etc. By cueing to the beginning of the low flow period, frogs (and other species) may reduce the risk of breeding during storm events or during unstable flow periods that can scour egg masses from rocks, or strand and dry egg masses.

Breeding was only seen in the mainstem North Yuba (green line in plots below). Pauley Creek and Haypress Creek (the blue and pink respectively) were likely too cold for frogs to successfully breed and develop over the summer, but these sites illustrate important elevational gradients in source water (See Plots).

Figure 3: Hourly water temperatures in 3 study sites in the North Yuba watershed. FYLF breeding was only observed at mainstem North Yuba site (green).

Figure 4: Proportion of storm event water that was rain vs. groundwater at three sites in North Yuba. Average daily stage plotted for each site (dashed line).

Isotope analysis has helped us understand water sources for assessing food web connectivity and species. Our new work shows that water chemistry cues seem particularly effective for frogs, particularly FYLF, in Sierra streams. Understanding these cues may help in developing more effective flow releases below dams or other actions that support needed flow, chemical, or temperature cues.

We continue to monitor our long term sites and collect additional water samples at some sites to further assess how water chemistry may act as a breeding cue for these sensitive amphibians in regulated and unregulated systems.

Further Reading

Foothill yellow-legged frogs. Ecology, river regulation, and conservation. Pacific Southwest Research Station, USFS.

Kabir, E.R., M.S. Rahman, I. Rahman. 2015. A review on endocrine disruptors and their possible impacts on human health. Environmental Toxicology and Pharmacology. Vol. 40 (1), 241-258.

Peek, R., H. Dahlke, S. Yarnell. 2016. Linking water source signatures with native amphibian breeding timing in a Northern Sierra Nevada watershed. Hydroecology C10. Presentation for Annual Meeting at Society for Freshwater Science, Sacramento CA.

Rogers, J.A., L. Metz, V. W. Yong. 2013. Review: Endocrine disrupting chemicals and immune responses: A focus on bisphenol-A and its potential mechanisms. Molecular Immunology. Vol 53 (4) 421-430.

Wingfield, J. C. and M. Mukai (2009). Endocrine disruption in the context of life cycles: perception and transduction of environmental cues. General and Comparative Endocrinology 163(1-2): 92-96.

van Breugel, F., J. Riffell, A. Fairhall and M.H. Dickinson (2015). Mosquitoes Use Vision to Associate Odor Plumes with Thermal Targets. Current Biology 25(16):2123-2129.

Yarnell S.M., R.A. Peek, D.E. Rheinheimer, A.J. Lind, J.H. Viers. 2013. Management of the Snowmelt Recession. 137.