The wood frog (Rana sylvatica = Lithobates sylvaticus) is a common, early-spring breeding anuran species in the United States and Canada. Females typically lay their egg masses in concentrated areas of a few meters over several days. Most female wood frogs mature after two years. Each female lays one egg mass in a given year, and most show high (~100%) site fidelity after first breeding, although a small portion of juveniles disperse up to 2000 m away from their natal site before their first breeding season. The lifespan of wood frogs depends on latitude, but they rarely live longer than five years. From 2000 to 2020 we conducted wood frog egg mass counts in 64 freshwater nonpermanent wetlands in the 3212 hectare Yale-Myers Forest in northeastern Connecticut, USA. The wetlands varied in surface area (average = 2642 m², range = 24–41361 m², CV = 252), canopy closure (i.e., global site factor; average = 52%, range = 0–98%, CV = 68), depth (average = 52 cm, range = 22–118, CV = 46), and egg mass counts (average = 71, range = 0–1113, CV = 130). As each female only lays one egg mass per year (i.e., only produces one clutch) and site fidelity is high, egg mass counts offer an accurate proxy for the number of breeding females within a pond in a given year. Previous work indicates egg mass counts are an accurate and precise technique for monitoring wood frog populations.

Pond-level variables

Attributes of ponds in our models included maximum pond depth and canopy closure. Depth was recorded at the time of egg mass surveys. Most ponds have a permanent depth gauge so measurements are standardized across years, otherwise depth was recorded as the deepest point in the pond. Pond canopy closure was measured as in Arietta et al. (2020) by using five hemispherical photographs taken along the shore at each cardinal point and at the center of each pond during leaf-off and leaf-on seasons. We estimated average leaf-on and leaf-off global site factor (GSF; the ratio of above-canopy radiation to under-canopy radiation) (Anderson 1964) and used a weighted GSF value integrated over the duration of wood frog embryonic and larval life cycle (Halverson et al. 2003). GSF is scaled between 0–1, and we report it here as a percentage.

Regional-level variables

We included air temperature and Palmer Drought Severity Index as regional-scale variables. Here we define regional factors as those affecting multiple breeding populations simultaneously. We downloaded daily temperature records from the National Climatic Data Center of the National Oceanic and Atmospheric Administration (NOAA) observing station at the Windham Airport in Willimantic, Connecticut, approximately 19 km south of the study area. We estimated winter thaw as the number of days between 1 October and 30 March above freezing in the winter prior to breeding (i.e., winter thaw_(t-1)). This date range gives an estimate of the fall and winter conditions for juveniles and adults and aligns winter temperature with the hydrologic water year that begins 1 October each year.

The Palmer Drought Severity Index (hereafter drought severity) uses temperature, precipitation, and soil information to estimate the departure of moisture supply from the norm. We downloaded historical monthly drought severity data for Connecticut from the National Centers for Environmental Information division of NOAA (available at https://www.ncdc.noaa.gov/temp-and-precip/drought/historical-palmers/psi/200011-202010). Drought severity typically ranges between -4 and 4, although more extreme values are possible. A drought severity value around zero indicates normal conditions, whereas a value ≤ -4 indicates extreme drought and a value ≥ 4 extremely wet conditions. We used an average monthly drought severity value from 1 March to 30 September to represent the moisture conditions that breeding adults, tadpoles, and new metamorphs would experience from the highest pond levels (early spring) to lowest levels in late summer and early fall. To test if larval and juvenile conditions affected females during their first breeding year, we also tested a two-year lag in drought severity (i.e., drought severity_(t-2)) corresponding to the first year of maturity.

Estimating density dependence measures

Females typically take two years to reach sexual maturity, so the effect of larval intraspecific competition within a focal pond on a breeding female was defined as the density of egg masses (i.e., egg masses / pond area) two years prior.

The effect of neighboring ponds (a proxy for terrestrial density dependence) was estimated using a summed function of egg mass data from neighboring ponds (i.e., within 500 m) weighted by inverse distance. Closer ponds are given greater weight in generating the estimate.

Population growth rate

We defined population growth rate (hereafter growth rate) as the number of breeding females over a generation time of two years:

ln((egg masses_t + 1)/egg masses_(t-2) +1))/2 
Where t is the number of egg masses in the survey year, t-2 is the number egg masses two years prior, and the entire function is divided by the number of years between measurements. This approach normalizes high and low values for better comparison across ponds.

ReadMe_file.txt information

woodfrogdata.csv

This is the main data file with all years of data from all ponds. This dataset contains 14 columns x 1130 rows

Pond.ID = pond name

Year = year of data collection

Depth = depth (cm) of water within a pond during egg mass surveys at depth gauge or in deepest point of pond (if no gauge is installed)

RASY.Count.1 = independent whole pond count of Rana sylvatica egg masses by observer 1

RASY.Count.2 = independent whole pond count of Rana sylvatica egg masses by observer 2

Avg.RASY.Count = average of the two independent counts

RASYdens = density of RASY egg masses (Avg.RASY.Count/pond area) in egg masses/m²

GrowthRate = population growth rate as estimated by egg masses (see explanation above)

RASYdens_t2 = RASY density two years prior to the current year (egg masses/m²)

days_thawed = number of days between 1 October and 30 March above freezing in the winter prior to breeding

spfaPDSI = average monthly drought severity value from 1 March to 30 September of the breeding year

spfaPDSIlag = average monthly drought severity value from 1 March to 30 September two years prior to the breeding year

wEggs = number of egg masses in neighboring ponds within 500 m of focal pond weighted by distance from pond (see data description)

distance.mat.csv

Distance matrix for distance (in meters) between each pond. Dataset is 65 columns x 65 rows.

pondinfo.csv

Area (m²) and canopy data (as Global Site Factor, see data description) for each pond. Dataset is 3 columns x 65 rows.