Species data collection

Observational records of amphibian activity were obtained through the Natural Heritage Information Centre of Ontario and the Ontario Herpetofaunal Summary (OHS) Atlas (http://nhic.mnr.gov.on.ca). OHS began its active volunteer‐based data collation in 1984 and currently collects observational data through an online submission form. They request that the observer report any reptile or amphibian sighting in Ontario and provide information on the number of individuals observed, the time and location of the observation, as well as descriptions of the species and site. The majority of the data from both the center and the atlas are from primary sources and historical records from museum and university collections that have been verified by experts.

Many phenological studies to date have used historical volunteer‐based observations, or Citizen Science, to determine correlates of seasonal breeding activity (e.g., Dunn and Winkler 1999; Gibbs and Breisch 2001; Gordo and Sanz 2006). Frogs are good candidates for Citizen Science as each species typically has a unique and easily recognizable advertisement call, meaning that acoustic surveys offer a fairly accurate indicator of male reproductive activity (Crouch and Paton 2002; Steelman and Dorcas 2010). In Ontario, the focus of our study, frog species are few and are distinguishable in their call, coloration, size, and preferred habitat types, facilitating both visual and acoustic species identification.

Data were requested for an approximately 200 × 200 km area from southeastern Ontario as this allowed for a reasonably large dataset of observations that included the Queen's University Biological Station (a focus for our phenological studies), while also potentially minimizing confounding spatial effects (de Solla et al. 2006; Fig. 1). Data were available from 1930 to 2010 for nearly all years and for 10 species of anurans, (nine frogs and one toad species). The earliest record in which a species was observed emerging or calling for each year is considered the onset of emergence/calling for that year.

Figure 1 Open in figure viewer PowerPoint Sampling area (transparent shaded area) from which historical frog observations were obtained for this study. The solid circles within the sampling area indicate the locations of the weather stations used to test long‐term trends in temperature and precipitation. IHD: International Hydrological Decade station, PCC: Pollution Control Centre. Map obtained and modified from the Brock University Map Library.

Given that we were interested in breeding phenology, any observations of juveniles or tadpoles were discarded. Observations that did not include a complete date, with day, month and year, or did not include geographic coordinates were also excluded. Years with fewer than five records were eliminated (Dunn and Winkler 1999). Records from the earliest years of data collection were sparse and to minimize data biases, any records that were not within 10 years of another observation were discarded. For our analysis, we only used records of the absolute first day of calling activity or the first day of emergence as indicated by either calling activity or observation for each year and species.

Ultimately, eight anuran species had sufficient data for analysis; American toad (Bufo americanus), American bullfrog (Rana catesbeiana), chorus frog (Pseudacris sp. – either maculatus or triseriata; see below), gray tree frog (Hyla versicolor), green frog (Rana clamitans), northern leopard frog (Rana pipiens), spring peeper (Pseudacris crucifer), and wood frog (Rana sylvatica). American toads have been placed within the genus Anaxyrus and North American members of the genus Rana within the genus Lithobates (Frost et al. 2006). As Rana and Bufo are considered scientifically valid by some (Hillis 2007; Pauly et al. 2009) and are the most common names used for these species in previous literature (e.g., Oseen and Wassersug 2002; de Solla et al. 2006; Brodman 2009), we use these naming conventions throughout the article.

Bufo americanus, Pseudacris sp., P. crucifer, and R. sylvatica are all early spring breeders that breed in shallow waters. Hyla versicolor, R. clamitans, and northern R. pipiens begin breeding in late spring, whereas R. catesbeiana are often the last to begin their breeding season in early summer. Hyla versicolor prefer to breed in shallow woodland marshes, whereas R. clamitans, R. pipiens, and R. catesbeiana tend to breed in deeper, permanent bodies of water.

Western chorus frogs (Pseudacris triseriata) and boreal chorus frogs (Pseudacris maculata) were only relatively recently determined to be genetically distinct species (Platz 1989). The two species have very similar breeding calls and coloration and the extent to which their ranges overlap in its southern Ontario range is currently unknown. Consequently, we cannot be certain of which species of chorus frog is present in the dataset and hereafter refer to chorus frogs as Pseudacris sp. None of the aforementioned species are at their northern range limit within the study area.

In this dataset, both R. pipiens and R. catesbeiana had been documented emerging early in the year when frogs are expected to still be overwintering. Indeed, these species occasionally become active during the core winter months; however, these behaviors are not related to breeding, but instead may be a method of avoiding freezing temperatures and anoxia (Stinner et al. 1994). We thus discard any observations of anurans before February of each year, as well as observations of anurans that were active under the ice surface of water bodies. The resulting pruned dataset comprised approximately 40 years of data between 1970 and 2010 on emergence and the onset of calling activity for all eight focal species.