DOI: 10.5061/dryad.2g8p1
title: Data from: Variation in thermal niche of a declining river-breeding frog: from counter-gradient responses to population distribution patterns

description: When dams or climate change alter the thermal regimes of rivers,
 conditions can shift outside optimal ranges for aquatic poikilothermic
 vertebrates. Plasticity in thermal performance and preference, however,
 may allow temperature-vulnerable fauna to persist under challenging
 conditions. To determine the effects of thermal regime on Rana boylii
 (Ranidae), a threatened frog species endemic to rivers of California and
 Oregon, we quantified tadpole thermal preferences and performance in
 relation to thermal conditions. We monitored temperature and censused
 populations across a coastal to inland cline in six catchments where dams
 have altered thermal environments in close proximity to river reaches with
 natural conditions. We found geographic variation in population
 distribution and abundance based on river size combined with water
 temperature. The large inland rivers that supported breeding frogs,
 although cooler in spring due to snowmelt, became warmer during the summer
 than occupied coastal sites. Inland populations were constrained to
 reaches where the average temperature over the warmest 30 days ranged from
 17.6 to 24.2°C, higher than coastal rainfall-driven systems where averages
 ranged from 15.7 to 22.0°C. Frogs in rivers with hypolimnetic-release dams
 bred in colder waters than they did in free-flowing rivers. Common-garden
 and field translocation experiments revealed local adaptations in larval
 growth and phenotypically plastic thermoregulatory behaviour. Tadpoles
 from all rivers had a positive linear growth response to temperature, but
 individuals from inland rivers displayed intrinsically higher growth
 rates. Consistent with a counter-gradient model of selection in which the
 response to temperature change is in the opposite direction of the change,
 individuals from cooler rivers selected warmer temperatures. When reared
 under common conditions, however, tadpoles showed similar temperature
 preferences regardless of source river. Our results suggest a role for
 local growth rate adaptation in structuring the distribution of Rana
 boylii. Plastic thermoregulatory behaviour by tadpoles may explain how
 small populations are able to persist where dams release cold water.
 Management of edgewater habitats to increase the availability of warm
 micro-sites may ameliorate this impact.
description: Thermal preference
 (Tsel) dataThermal preference
 (Tsel) data. We conducted three trials (each trial corresponds to a tab in
 the spreadsheet): (1) a single population source (full-sibling set) of
 tadpoles reared under different thermal regimes in four streams of the
 coastal SF Eel catchment (see article); (2) wild collected tadpoles from
 populations at thermal monitoring stations that represent three distinct
 clades within the species (Lind, Spinks, Fellers, & Shaffer, 2010) and
 three of the four niche clusters (SF Eel, Alameda Ck and NF Feather; see
 article); and (3) multiple population sources of tadpoles reared in a
 common thermal environment, the outdoor stream mesocosm experiment
 described in the article.Tsel
 data.xlsxEnvironmental monitoring
 data and frog abundanceWe calculated M30DAT for
 each river as the running mean of consecutive water temperature
 measurements from the previous 30 days for each day between May and
 October, averaging 2009, a cool year, and 2010, a warm year. We determined
 the temperature over the warmest 30 days of the summer, a period that
 coincides with rapid growth and development of tadpoles. We assessed water
 temperature from spring to autumn of 2009 and 2010 in 12 northern
 California river systems, pairing drainages affected by dams with
 drainages lacking dams (See in Article: Figure 1, and Appendices S1 and S2
 in Supporting Information). Each pair was in the same catchment, having
 similar geomorphology and habitats. We positioned data loggers (Thermocron
 iButtons DS1921G, 0.5°C accuracy, recording every 2 hr), in typical egg
 and tadpole habitats (Figure S2 in article). We obtained additional water
 temperature data collected by dam operators in our six catchments (Placer
 County Water Agency, Pacific Gas & Electric, and San Francisco Public
 Utilities Commission) for 63 monitoring stations, of which 51 were used by
 frogs for reproduction and comprise the sample size of our analyses. We
 characterised physical habitat of occupied sites by performing Principal
 Component Analysis (PCA) of the following variables at each monitoring
 station: mean of M30DAT for 2009 and 2010 (see above), mean annual
 discharge over the full period of record for the nearest stream gauge
 (United States Geological Survey,
 http://relicensing.pcwa.net/html/science/hydrology.php), elevation, and
 catchment area upstream of the station. Variables were scaled prior to PCA
 on a correlation matrix. Following PCA, we extracted the first two
 principal components, which accounted for 73% of the total
 variance.Discharge and drainage area loaded strongly on PC1, whereas
 M30DAT and elevation loaded strongly on PC2 (Table S2). We used k-means
 clustering of PC1 and PC2 to delineate the riverine environments occupied
 by breeding frogs. We examined scatter plots of PC1 versus PC2 using k = 4
 and delineated distinct clusters of sites.Data on frog abundance refer to
 number of clutches per km of river reach. We conducted surveys of frog
 reproduction by searching for clutches of eggs which females attach to
 rocks in shallow water. Rana boylii has a lek mating system in which frogs
 congregate and oviposit at areas of coarse sediment deposition in a river,
 such as lateral cobble bars (Kupferberg, 1996). By searching several
 riffle pool sequences, approximately 500 m upstream and downstream of the
 water temperature monitoring stations, we covered multiple depositional
 environments where leks might occur. We repeated visits (≥29) throughout
 the breeding season to ensure high detectability of clutches and followed
 an established protocol (Kupferberg et al., 2012) searching both banks and
 marking clutches to prevent double counting. We standardised clutch
 abundance by the linear distance searched. Survey periods varied among
 rivers because frogs breed earlier in the spring (March–May) in coastal
 rain-driven systems, and later (May–June) in the inland rivers at the
 foothills of the Sierra Nevada mountains (henceforth, inland rivers),
 which receive rain and snowmelt.abundance_M30DAT.xlsxTadpole growth performance dataWe measured tadpoles’ growth in response to temperature conditions in a complementary set of manipulations conducted in three different experiments. First (Field Enclosures tab), within one catchment we translocated embryos from one source and reared them to metamorphosis in multiple streams with disparate thermal regimes (same experiment as Tsel Trial 1). Second (Common Garden Growth Chambers tab), we reared embryos from different catchments under controlled thermal conditions in the laboratory, and third (Common Garden tab), we used outdoor stream mesocosms to create a common-garden environment (corresponding to Tsel Trial 3). For the Field Enclosures experiment, in 2008 and repeated in 2009, we raised full-sibling tadpoles from shortly after fertilisation until front limb emergence in flowthrough stream enclosures placed in six locations in the catchment of the South Fork Eel River hereafter SF Eel, Appendix S2 in article). Details of animal husbandry methods are in Appendix S3 of article. For each replicate we measured body length and mass of tadpoles weekly and calculated the mean growth rate of all tadpoles in the enclosure (mg/day and mm/day). We used least squares linear regression to relate growth and size at metamorphosis to M30DAT. For the Common Garden Growth Chambers, we collected embryos from three of the coastal study rivers (two with dams) and four of the inland rivers (two with dams) from April to June 2010 (Table S1 of article). We transported embryos in chilled aerated river water to the laboratory and within 8 hr of collection placed them into temperature-controlled diurnal growth chambers (cycle set to 14.5 hr light and warm, 9.5 hr dark and cool). After hatching, we randomly chose five tadpoles from each clutch and placed them in 3.8-L aquaria with an aerated mixture of river water and de-chlorinated tap water (changed 29/week). To mimic warm summer conditions at locations with robust populations of frogs, one chamber was set at 18°C during the dark cycle, and 22°C during the light cycle. Daily mean temperature, calculated from 3 i-Button data loggers placed in randomly selected aquaria, was 19.5°C. To mimic conditions of the coolest occupied sites, the other incubator was set at 13°C dark cycle, 19°C light cycle, with a daily mean = 16.6°C. Each chamber held 35 aquaria (4–6 replicate clutches 9 7 rivers). We measured body and total length of tadpoles with calipers to the nearest 0.1 mm at weekly intervals after hatching. The response variable was body growth, mm/day. For the Common Garden experiment, we collected embryos in late May and early June 2010 from four rivers, and placed them in outdoor mesocosms at the University of California’s Richmond Field Station (37.913536°N, -122.333303°W). We captured tadpoles in four rivers: a coastal unregulated (South Fork Eel), a coastal regulated (Eel below Scott Dam), an inland unregulated (Middle Fork Feather) and an inland regulated (North Fork Feather). At each river, we obtained embryos from three different egg masses; furthermore, at the North Fork Feather we collected from two reaches (below the Cresta and Poe dams). After transferring embryos to Richmond and rearing them to tadpole stage, we placed 10 tadpoles in separate mesocosms consisting of re-circulating troughs (total of 15 troughs, 3 for each river except for the North Fork Feather which had 6 troughs). We mounted PVC pipe cut in half (2.2 m 9 15.3 cm diameter) on saw horses at a slight incline so water drained into a 946 L reservoir. Pumps and hoses continuously re-circulated water to the top of each trough lined with periphyton covered river rocks and loose algae. Tadpoles were reared indoors at 18°C until stage Gosner stage 25, and placed in the troughs on 21 July 2010. We measured and staged tadpoles weekly until front limb emergence (Gosner stage 42). We calculated a mean growth rate for each trough (mm of body length increase/day).Growth data.xlsx