Forest and Rangeland Ecosystem Science Center 

Oregon Spotted Frog (Rana pretiosa) Movement 

and Demography at Dilman Meadow: 

Implications for Future Monitoring 

By Nathan D. Chelgren, Christopher A. Pearl, Jay Bowerman, and Michael J. Adams 
Prepared in cooperation with the Sunriver Nature Center 

Open-File Report 2007-1016 


U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

Dirk Kempthorne, Secretary 

U.S. Geological Survey 

Mark D. Meyers, Director 

U.S. Geological Survey, Reston, Virginia 2006 

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ACKNOWLEDGEMENTS 

This analysis was made possible by a grant from the Interagency Special Status/Sensitive Species Program. We thank C. 

Korson and L. Zakrejsek at the US Bureau of Reclamation for their work in all aspects of the translocation. Reviews by L. Bailey 

and two anonymous reviewers improved the manuscript. We thank R. B. Bury for suggestions on field techniques and assistance 

throughout the early years of the study. We thank S. Ackley, R. Diehl, B. McCreary, and C. Rombough for assistance with field 

work. We also thank Deschutes National Forest, US Fish and Wildlife Service, North Unit Irrigation District, and Sunriver 

Nature Center for the logistical and personnel support they dedicated. 
Abstract 

From 2001 to 2005, we studied the demography and seasonal movement of Oregon spotted frogs (Rana pretiosa) 

translocated into created ponds in Dilman Meadow in central Oregon. Our objectives were to inform future monitoring and 

management at the site, and to elucidate poorly known aspects of the species’ population ecology. Movement rates revealed 

complementary use of sites seasonally, with one small spring being preferred during winter that was rarely used during the rest of 

the year. Growth rates were significantly higher in ponds that were not used for breeding, and larger size resulted in significantly 

higher survival. When variation in survival by size was accounted for there was little variation among ponds in survival. 

Seasonal estimates of survival were lowest for males during the breeding/post-breeding redistribution period, suggesting a high 

cost of breeding for males. Overwintering survival for both genders was relatively high. Our study supports others in suggesting 

Oregon spotted frogs are specific in their overwintering habitat requirements, and that predator-free springs may be of particular 

value. We suggest that any future monitoring include measures of the rate of pond succession. Demographic monitoring should 

include metrics of both frog reproduction and survival: counts of egg masses at all ponds during spring, and capture-recapture 

study of survival in mid and late summer when capture rates are highest. Additional study of early life stages would be 

particularly useful to broaden our understanding of the species’ ecology. Specifically, adding intensive capture and marking 

effort after larval transformation in fall would enable a full understanding of the annual life cycle. Complete study of the annual 

life cycle is needed to isolate the life stages and mechanisms through which Oregon spotted frogs are affected by stressors such as 

nonnative predators. Dilman Meadow, which lacks many hypothesized stressors, is an important reference for isolating the life 

stages most responsive to management elsewhere in the species’ range. 
Oregon Spotted Frog (Rana pretiosa) Movement 

and Demography at Dilman Meadow: 

Implications for Future Monitoring 

By Nathan D. Chelgren, Christopher A. Pearl, Jay Bowerman, and Michael J. Adams 

Introduction 

The Oregon spotted frog (Rana pretiosa) is a highly aquatic frog that has been extirpated from a large 

portion of its historic range in the Pacific Northwest, and remaining populations are reduced and isolated (Hayes 

1997, Pearl and Hayes 2005). Loss and alteration of marsh habitat, predation and competition from exotic fish and 

bullfrogs, and degraded water quality from agriculture and livestock grazing are implicated in their decline (Hayes 

1997, Pearl and Hayes 2005). In 2001, an interagency team translocated a population of frogs from a site that was to 

be eliminated by the renovation of the dam impounding Wickiup Reservoir, to newly created ponds at Dilman 

Meadow (121o 39’ 52” W, 43o 41’ 58” N), 2.5 km from the original site in central Oregon, USA. We monitored 

Oregon spotted frog demography and movements at Dilman Meadow for > 4 yr to assess the efficacy of these 

mitigation efforts, determine metrics for long-term monitoring, and inform future management at the site. More 

broadly, many aspects of Oregon spotted frog life history are poorly known, so understanding demography and 

movement patterns is likely to be useful in its conservation. Although wildlife translocations have been attempted 

extensively as conservation means, few such projects have been sufficiently monitored for demographic rates to 

understand the causes for the translocation’s success or failure (Dodd and Seigel 1991). Our objective here is to 

document demographic and movement patterns in the population of Oregon spotted frog at Dilman Meadow so that 

this information will be available to guide management decisions. 

To better evaluate amphibian population responses to management actions it is important to consider the 

contribution of each life history stage and both genders to the balance of reproduction and mortality. Population 

growth or contraction occurs as a complicated function of the probability of breeding, fecundity, and survival during 

multiple life history stages and size classes and the transition between these classes. Body size in amphibians is 

strongly positively linked with the probability of breeding (Semlitsch et al. 1988, Smith 1987), fecundity (Howard 

1980, Berven 1981, Berven and Gill 1983), and survival (Altwegg and Reyer 2003, Chelgren et al. 2006). Thus, 

growth of individuals is an important component of population change. Estimates of demographic rates for one 

gender are often used to infer population growth rates or population viability (Caswell 2001). However, in anurans 

such as Ranid frogs, gender is thought to affect survival rate (Wood et al. 1998, Lyapkov et al. 2004), probability of 

dispersal (Austin et al. 2003, Palo et al. 2004), age at sexual maturation (Lyapkov et al. 2004), and breeding 

probability (Muths et al. 2006). Moreover, males and females differ in energetic costs associated with breeding 

(Feder and Burggren 1992) and in growth rate (Lyapkov et al. 2004). Differences in demographic rates between 

genders will generally affect population growth rate for small populations (Engen et al. 2003, Sæther et al. 2004, 

Husby et al. 2006), so it is important to distinguish these differences during monitoring. For example, it has been 

hypothesized that differences in the frequency at which male and female western toads (Bufo boreas) visit breeding 

sites have led to differential mortality from Chytridiomycosis, resulting in highly skewed sex ratios and diminished 

reproductive output (Muths et al. 2003). We examined sex- and size-specific demography at Dilman Meadow with 

particular focus on a priori hypotheses regarding survival, movement, and growth. 

It is generally no longer defensible to use captures or sightings of individuals to estimate demographic rates 

when numbers are uncorrected for differences in the probability of capture. Instead, capture, survival, and 

movement probabilities are modeled simultaneously to reduce bias in estimates of demographic rates (Williams et 

6
al. 2001). The missing-data factorization of the multi-state model (Dupuis 1995) and Bayesian estimation allowed 

us to incorporate important features of Oregon spotted frog ecology, central to our hypotheses, into our statistical 

modeling. 

Specific hypotheses 

We examined several a priori hypotheses regarding variation in survival at Dilman Meadow. Potential 

losses to emigration, frogs’ unfamiliarity with the new site, and handling effects led us to hypothesize that survival 

would be initially lower for individuals translocated to Dilman Meadow than for individuals produced in the 

meadow in subsequent years, hereafter ‘native’. We also hypothesized that density effects associated with a 

growing frog population in the meadow should lead to decreased survival with increasing time since ponds were 

established. As has been seen in other amphibians, we hypothesized that survival should be higher for larger frogs, 

and should be higher for females than males. In addition to hypotheses about survival, we examined two hypotheses 

regarding movement and growth to illuminate the value of specific ponds for growth versus reproduction. We 

hypothesized that 1) after breeding and prior to the dry summer when most growth is expected to occur, frogs 

disproportionally moved away from breeding sites to non-breeding sites, and 2) that the sites not used for breeding 

had higher growth rates than sites which were used for breeding. Other objectives were to compare the magnitude 

of the hypothesized effects with the magnitude of variation among ponds and seasons, and to estimate movement 

probabilities among ponds to determine how specific ponds were used seasonally. 

Methods 

Study Area Description 

Dilman Meadow is an emergent wetland located at approximately 1300 m elevation in the Deschutes 

National Forest, Oregon. In the 2 years prior to pond construction, open water in the meadow was limited to a 

narrow channel that conveyed steady ground-water flow. Our observation wells indicated that most of the meadow 

was saturated to near the surface throughout the year. The meadow is largely isolated from other surface water 

during summer, with the exception of the small outflow stream that drains to the Deschutes River located 0.5 km 

distant. The nearest permanent lentic water is Wickiup Reservoir, 2.5 km distant. Dilman Meadow did not support 

breeding frogs at the beginning of the translocation effort in 2001. From 2001 to 2005, 5 adult western toads and 

many long-toed salamanders (Ambystoma macrodactylum) were observed at the meadow. The nearest known 

Oregon spotted frogs are a small number that breed sporadically in upper Wickiup Reservoir, circa 5 km distant, and 

separated from Dilman Meadow by the dam, a road, and arid upland forest. The landscape surrounding Dilman 

Meadow is ponderosa (Pinus ponderosa) and lodgepole pine (Pinus contorta) forest, with the exception of a thin 

willow (Salix sp.) – sedge (Carex sp.) riparian strip downstream of Dilman Meadow and along the Deschutes River. 

Dilman Meadow itself is relatively undisturbed: livestock grazing does not occur, and there is no agriculture in the 

wetland’s catchment other than timber harvest. Neither bullfrogs nor fish are present at the meadow. As a result of 

the absence of these stressors, Oregon spotted frog demography at Dilman Meadow is invaluable as a reference to 

which other populations may be compared. 

Field Methods 

In fall and winter of 2000-2001, six ponds were created at Dilman Meadow as mitigation for the loss of 

Oregon spotted frog habitat associated with the renovation of the Wickiup Reservoir dam. Ponds were constructed 

by blasting and ranged from circa 100 to 800 square meters in surface area (0.01 - 0.08 ha.) and the deepest water in 

all ponds was circa 2 m. Pond succession prompted the creation of three additional ponds around the margin of the 

meadow in 2004 by mechanical excavation using heavy equipment. In spring 2001, we moved 9 Oregon spotted 

frog egg masses from the ditch below Wickiup Reservoir to ponds in Dilman Meadow. During intensive search and 

trapping efforts in June through September 2001, 48 frogs (9 females, 11 males, and 28 post-metamorphic subadults) were captured at the Wickiup ditch and were translocated to Dilman Meadow. All adult frogs captured had 

unique passive integrated transponder (PIT) tags from an extensive survey of the site the prior year, and presumably 

represented the entire adult population. 

7
We counted egg masses during complete searches of Dilman Meadow in April-May each year from 2002 to 

2005. In addition, we captured frogs by hand and with funnel traps (circa 3-cm gape) during bi-monthly sampling 

occasions. In winter months, when water temperatures were cold (< 12 oC), traps were submerged. Otherwise traps 

were placed at the water’s surface to enable captured animals to exchange air. Captured frogs were measured for 

snout-urostyle length (sul) and snout-vent length (svl), and sex was determined. Based on limb morphology, we 

attempted to independently determine the sex of frogs each time the individual was captured. If gender could not be 

determined, or if our field determination at one capture conflicted with the individual’s final gender determination, 

the gender data were treated as missing. We marked frogs >40 mm svl with PIT tags inserted subcutaneously 

through incisions made on the anterior dorsum. To avoid additional tissue damage, we did not close incisions with 

medical glue, but rather massaged tags to the posterior dorsal side to prevent tag loss back through the incision. 

McAllister et al. (2004) reported “negligible or no side effects” of PIT tagging on Oregon spotted frogs in 

Washington. At each capture, frogs were checked for PIT tags using a Destron Fearing Mini Portable ReaderTM. 

Statistical Methods 

Survival and movements 

We used a modified version of the Arnason-Schwarz multi-state capture-recapture model (Arnason 1972, 

1973; Brownie et al. 1993; Schwarz et al. 1993) to relate characteristics of individual frogs to survival, movement, 

and capture probabilities. The Arnason-Schwarz model differs from other models in that it allows the estimation of 

survival and movement rates among multiple sites when capture probabilities are less than 1.0. Because sex is 

thought to strongly affect survival in Ranid frogs, and because improper treatment of missing gender data can lead to 

substantial bias in sex-specific survival estimates (Nichols et al. 2004), we modified the Arnason-Schwarz model to 

account for missing sex data. We augmented the Arnason-Schwarz model to simultaneously model the probability 

of being female and the mechanism, δ, of the sex data being observed in a manner analogous to sampling situation A 

of Nichols et al. (2004). Model A of Nichols et al. (2004) may be written in a missing data factorization as a joint 

model with 2 factors: a model for sex with informative missing (Little and Rubin 2002) data and a model for 

survival and capture equivalent to the Cormack-Jolly-Seber model (Lebreton et al. 1992). Our new model amounts 

to simply substituting, for the Cormack-Jolly-Seber component its multi-state generalization, the Arnason-Schwarz 

model. We modeled logit(δ), as a linear function of sex; large, an indicator for animals greater than 57 mm snoutvent length; and the interaction sex*large. To make this augmentation, we monopolized the tractability of the 

missing data likelihood (Dupuis 1995) for the Arnason-Schwarz model, which can be concisely coded in program 

WinBugs 1.4. In summary, the estimation procedure used the pattern of whether or not gender was determined 

among all captures to adjust survival, movement, and growth rates for the sampling bias that large individuals and 

males were more likely to have their gender determined than small and female frogs. 

To accommodate the large number of potential movement patterns among sites and seasons, we also 

factored site transition probabilities into probabilities of emigration and immigration conditional on the frog having 

emigrated (Grosbois and Tavecchia 2003), then simplified the model for immigration. Transition probabilities, ψ, 

of the Arnason-Schwarz multi-state model were decomposed in the πμ formulation of Grosbois and Tavecchia 

(2003) into probabilities of emigrating sites and probabilities of immigrating sites conditional on emigration having 

occurred. Adopting the notation of Grosbois and Tavecchia (2003), let ψi

RT be the probability that an individual 

which originates in site R in period i and survives the interval, moves to site T at the end of the interval. Grosbois 

∑

T≠R 

from site R; and μi

RT , as the probability of immigrating to site T given emigration from R occurred, with the 

ψi

RT R RT and Tavecchia (2003) factored into two processes, defining π = ψ , as the probability of emigrating i i 

constraint that μ RT ∑ i = 1. Although the πμ formulation provides opportunity to separately model leaving and 

T≠R 

settlement decisions (Grosbois and Tavecchia 2003), with even a moderate number of states (here sites), a large 

number of movement parameters exist. In our case, with 8 sites and 5 seasons, there were 8*7*5 = 280 movement 

probabilities to be accounted for. To stabilize parameter estimates, and to summarize frog movements seasonally in 

8 
a manner that could be easily interpreted, we considered the immigration probabilities to be non-Markovian, or 

independent of the site of origin. To accomplish this under the constraint that transition probabilities sum to 1, we 

define the relative 

probability of preference, αT

i , as the probability site T is preferred as a destination. Here we assume that animals 

do not emigrate and immediately immigrate back to the same site within the same interval. The probability that site

K 

j T is preferred as a destination, relates to immigration probability by μi

RT = αiT (1− αiR ), where ∑αi = 1 and K 

j=1 

is the total number of sites. In this way the μi parameters drop from the likelihood, and the number of immigration 

parameters for an interval reduces from K(K-1) to K. To reduce the parameter space further, and because seasonally 

we expected that environmental conditions such as desiccating conditions in summer would largely determine the 

probability that individuals redistribute among sites, we used random effects of site nested within 5 discrete seasons 

as fixed effects to model π . The total number of transition parameters was thus reduced from 280 to between 40 

and 75 depending on the amount of shrinkage associated with the random effects. 

Because body size of amphibians is an important factor affecting survival, sexual maturation, and 

fecundity, feeding areas favorable for growth are important to population change. In addition to spatial movements 

among 8 sites, our multi-state model also accounted for size-transition probabilistically between 2 discrete sizeclasses (Wood et al. 1998). As described above, we used the indicator large = 1 for size greater than 57 mm snoutvent length, otherwise large = 0. The cutoff between size classes, 57 mm, was the average minimum snout-vent 

length for individuals in the study. Because of sexual size dimorphism, we considered size-class transition 

probabilities to be sex-specific. See below for a more detailed analysis of logistic growth separate from the capturerecapture modeling effort. 

We tabulated the capture-recapture data for bi-monthly survival and movement rates at the network of 8 

sites which comprise Dilman Meadow. Of the 8 sites, 6 were created in 2001 for mitigation and one (site 3) was a 

natural spring. Three ponds were newly excavated in 2004 as mitigation. The 3 new ponds, which were excavated 

in 2004, were grouped as a single site (site 8) for the purpose of this analysis because of the small numbers of 

captures and identical search schedule at these ponds. Relative preference for site 8 was set to 0.0 in the model 

during intervals that occurred prior to the ponds’ excavation. We used a bi-monthly survival-movement interval as a 

compromise between the time-scale of frog movements, our search efforts, and the dimension of the problem 

computationally. We constrained bi-monthly parameter estimates to be equal within 5 discrete seasons: spring 

redistribution (March), breeding/post-breeding redistribution (April-May), summer growth (June-September), fall 

redistribution (October), and over-winter (November-February). Seasons were chosen based on a priori knowledge 

of spotted frog ecology and to address our hypotheses about seasonal movements. 

We used a generalized linear mixed model framework to relate the survival and capture probabilities to 

explanatory variables. With the logit link function, we used normally distributed random effects to stabilize 

parameter estimates while still allowing generality (Barry et al. 2003). We modeled survival with the fixed effects: 

sex; trans, an indicator for the original translocated individuals in the first year of the study; large; and year, a 

continuous pond-specific variable for years since pond construction. We included random effects on survival for 

season nested within sex, and for the site in use at the start of the survival interval. We accounted for variation in 

capture probabilities with the fixed effects: sex; year; and days, the number of days the site was searched in the 14 

day interval; and included random effects of site and season for capture probability. 

Logistic growth 

In a separate analysis from the multi-state capture recapture modeling just described, we used a 2­

parameter discrete-time logistic growth model for 251 PIT tagged frogs that had two or more measurements and for 

which gender was known. We modeled sul at time t as the response, conditioning on initial size at marking 

⎛ sul ⎞ sult+1 ~ sult + ρ*sult *⎜

⎜1− t

⎟

⎟. 

⎝ γ ⎠

9
We modeled the growth rate parameter, ρ, by sex, trans, site, and season, with a continuous effect of year, and 

modeled asymptotic size, γ, by sex and trans. Site was treated as a normal random effect. We handled missing site 

data for individual i by imputing missing values based on the proportion of observed data at each site. Specifically, 

we treated the site values for each individual at each time as single multinomial trials where the multinomial 

probabilities, θ, were common to all individuals and time intervals sitei,t ~ Mult(1;θ1,θ2 ,θ3 ,L,θ8 ). Bivariate 

normal random effects of individuals for ρ and γ accounted for repeated measures in the precision of estimates. In 

addition to the random site model, we ran a similar model without the random site effect, but where we estimated an 

offset, breeding, for growth rate in the ponds primarily used for breeding (i.e. breeding = 1 for ponds 1, 5, 6, and 7; 

breeding = 0 for ponds 2, 3, 4, and 8). 

Abundance 

We estimated abundance of juvenile, small adult, and large adult frogs at each site during September each 

year prior to the majority of fall movement. We used the Bayesian estimates of capture probability from the 

capture-recapture analysis of PIT tagged adults at each pond and bi-monthly interval, to adjust the number of 

captures of frogs in each size class for differences in capture probability. Because juvenile frogs, svl < 40 mm, were 

not individually marked and we could not know the total numbers of individuals captured in the bimonthly intervals, 

we computed the Bayesian estimators of capture probabilities for days = 1, then modeled the numbers of juveniles 

captured each day, nday,site, as binomially distributed, nday,site

 ~ Binomial(Nyear,site, Pday,site), to estimate year and site-specific 

population size Nyear,site. To estimate numbers of small (40 - 57 mm) and large (> 57 mm) adults, we took the same 

approach, but used bi-monthly capture rates and numbers of unique frogs captured in bi-monthly intervals to 

estimate site- and year- specific numbers. 

Notation 

We report 95% credibility intervals for Bayesian parameter estimates. Credibility intervals are interpreted 

as the values between which we are 95% certain the true parameter lies. Statistical significance is evaluated at the 

level of α = 0.05. We do not report P-values, but report the probability (Pr) a parameter is greater than or less than a 

specified value [e.g. Pr(Y > 0) = 0.93 states that we are 93% certain the value of Y is greater than 0]. 

RESULTS 

Survival 

Survival rate had a strong, positive relationship to size, was lower for translocated individuals than frogs 

native to Dilman Meadow, and differed seasonally by sex (Table 1). Estimates of sex-specific seasonal survival 

rates reveal different seasonal patterns for the genders that suggest an acute cost of breeding for males. Mortality 

was highest for males in the breeding/post-breeding redistribution period (April-May), and during summer (JuneSeptember), whereas females tended to die later in the year, during June-September and October (Figure 1). The 

lowest mortality rate occurred over winter, November through February (Figure 1). The probability that the survival 

odds for females exceeded that for males was Pr(βFemale > 0) = 1.00 during the breeding/post-breeding redistribution 

period but was not supportive of gender differences in the other seasons: spring redistribution, Pr = 0.37; summer, Pr 

= 0.13; fall redistribution, Pr = 0.27; and over-winter, Pr = 0.23. There was strong support (Pr = 0.99) that survival 

was higher for large frogs than small. Female frogs survived at an annual rate of 0.19 (0.06, 0.37) and 0.69 (0.57, 

0.82) for small and large individuals, respectively. Males survived at an annual rate of 0.08 (0.03, 0.16) and 0.57 

(0.49, 0.66) for small and large individuals, respectively, these estimates being computed for individuals native to 

Dilman Meadow at the median site and in 2003. There was strong support (Pr = 1.00) that translocated frogs had 

lower survival during the first year following relocation, the odds of survival being lower by a factor of 0.39 (0.20, 

0.78; Figure 2). However, there was no support (Pr = 0.41) for a trend by year and there was little variation between 

sites in survival (Figure 1). There was little probabilistic support for site differences based on pair-wise 

10
π α

comparisons of the survival odds between sites: the largest measure of support being Pr = 0.87 comparing sites 5 

and 6. The pattern of variation in survival rate at Dilman Meadow was dominated by the translocation effect, size, 

and differences between the sexes seasonally. 

Movements 

Movements between sites occurred primarily during March prior to breeding, and during October, 

following the dry summer (Figure 3). Frogs did not redistribute appreciably during the breeding/post-breeding 

period prior to the summer dry season when growth conditions are most favorable. In addition, the pattern of 

movements post-breeding did not support preference for non-breeding sites. On the contrary, the ratio of emigration 

to immigration during the breeding/post-breeding redistribution period was actually lower at the sites most 

important for breeding: site 1, 0.03; site 5, 1.12; site 6, 2.43; and site 7, 0.73, compared with sites with little or no 

breeding: site 2, 5.08; site 3, 8.81; site 4, 0.80; and site 8, 13.85. The pattern of movements for site 3, the natural 

spring, was distinctly different than for the other ponds. Animals used site 3 during the over-winter period, 

emigrating site 3 during the March redistribution period, and immigrating site 3 during the fall redistribution period 

(Figures 3, 4). The pattern of movements for site 5, the closest pond to site 3, was consistent with steppingstone 

movement. The group of 3 ponds excavated in 2004 (Site 8) were not used preferentially within the first year 

following construction; relative probability of preference for the site was intermediate to preference for other sites 

(Figure 4). 

Capture probability 

Capture probability varied considerably and by all sources examined (Table 2). There was strong support 

(Pr = 1.00) that capture probability was higher for females than males, the odds of capture given presence in the 

pond were 1.86 (1.55, 2.22) times higher for females than males. There was substantial variation among ponds, 

capture probability being generally negatively related to pond size. Capture probability was highest at site 3, the 

natural spring, and lowest at site 6. The odds of capture increased (Pr = 1.00) with increasing year at a rate of 1.29 

(1.18, 1.41) times per year, a pattern consistent with the negative correlation of capture probability with pond size 

considering that pond succession led to reduced pond area over the course of the study. The odds of capture also 

increased with the number of days the site was visited (Pr = 1.00), by 1.77 (1.63, 1.92) times per day. 

Missing gender 

Gender was significantly less likely to be determined for females than for males, and for smaller than larger 

individuals (Table 3). Our estimates of survival and movement appropriately account for this sampling bias. 

Logistic growth 

Growth rate and asymptotic size were variable; breeding and trans were important factors affecting growth 

rate, and animals that were translocated from the Wickiup site appeared able to attain larger maximum size than 

frogs native to Dilman. Males grew at a significantly faster rate, relative to their asymptotic size, than females (Pr = 

1.00). Growth rate was significantly higher for translocated frogs than those native to Dilman Meadow (Pr = 1.00), 

again this was relative to asymptotic size. An increasing trend in growth rate with increasing year was also 

significant (Pr = 1.00), which may be explained by increased productivity of ponds as they acquired natural 

communities subsequent to construction. Growth rate did not vary importantly among sites according to the 

estimated standard deviation, but seasonal differences were substantial (Table 4). Growth rate was highest in 

summer and was negligible during the fall redistribution and over-winter periods. The growth model that estimated 

separate growth rates for breeding sites revealed significantly lower growth rates at breeding sites than at sites where 

little or no breeding occurred (Pr = 0.99). Asymptotic size was significantly higher for translocated animals (Pr = 

1.00), and higher for females than males (Pr = 1.00; Figure 5). The significant trans effect for asymptotic size is 

11
consistent with inter-population variation in maximum size among other populations (C. Pearl unpublished data) and 

suggests the differences may have important environmental causes over strictly genetic ones. 

Egg masses, adult abundance and recruitment 

Numbers of egg masses laid at Dilman Meadow increased steadily from a minimum of 2 masses in the first 

year following translocation to 49 masses in 2005 (Table 5). Numbers of large adult frogs also increased steadily 

from 2001 to 2005, with total numbers in this group reaching 179 (154, 205) individuals by 2005 (Figure 6). 

Numbers of small adults reached a total of 93 (76, 112) animals by 2005. The ratio of numbers of egg masses to 

numbers of large adults in the previous fall varied significantly among years and was lowest in 2002, the year 

following translocation: 2002, 0.06 (0.04, 0.09); 2003, 0.32 (0.25, 0.40); 2004, 0.22 (0.17, 0.29); and 2005, 0.38 

(0.30, 0.48). There was no evidence that density dependence acted on numbers of egg masses laid. However, 

density dependence, or a decreasing temporal trend in embryo or larval survival is apparent. Numbers of egg 

masses increased five fold from 2001 to 2005 (Table 5), though numbers of juveniles during September did not 

increase over this same period (Figure 6A). 

DISCUSSION 

Numbers of adult Oregon spotted frogs at Dilman Meadow reached greater than 5 times their original 

number in 5 years. Both the numbers of egg masses and the number of egg masses per large adult increased since 

the translocation was completed, while adult survival stabilized. As a result of intensive seasonal capture efforts 

annually, a large proportion of the frog population has been marked, enabling unique insight to gender and seasonal 

variation in survival and growth. The detailed demography we present is invaluable for understanding the potential 

for population growth of the species because stressors thought responsible for its demise elsewhere were not acting 

at Dilman Meadow. 

Low survival rates of small frogs in the first year of the study emphasize the importance of translocating 

mature individuals for rapid population growth provided these individuals do not attempt to home. Attempts by 

translocated individuals to home may explain the lower apparent survival of translocated frogs at Dilman than frogs 

native to the meadow. Homing has been observed in frogs (e.g. Sinsch 1992, Matthews 2003). However, this was 

not supported by a telemetry study conducted at Dilman Meadow which revealed no inclination to home for radio 

tagged adults (C. Pearl, J. Bowerman and R. B. Bury, unpublished data). Handling effects, unfamiliarity with the 

new site or unexplained annual variation may also be responsible for significantly lower survival in the first year of 

this study. Translocated mountain yellow-legged frogs (Rana muscosa) lost more weight than a random sample of 

individuals that were not translocated (Matthews 2003), but annual variation in frog demography is also commonly 

observed (e.g. Richter and Seigel 2002). 

Annual survival rates for adult frogs at Dilman Meadow differed substantially between size classes, but 

large frogs native to the meadow survived at rates comparable to other species. Large frogs that were native to 

Dilman Meadow survived at rates (55 – 70%) comparable with other anurans: 72-84%, R. lessonae (Peter 2001); 53­

70%, R. esculenta (Peter 2001); 65-92%, R. sevosa (Richter and Seigel 2002); 52-71% Bufo bufo (Frétey et al. 

2004). Size strongly affected survival at Dilman Meadow consistent with studies of European toads (Bufo bufo; 

Goater 1994), water frogs (R. lessonae, R. esculenta; Altwegg and Reyer 2003), and Northern red-legged frogs 

(Rana aurora; Chelgren et al. 2006). No size effect was detected on survival of chorus frogs (Pseudacris triseriata; 

Smith 1987). Estimates of survival for small adults (7 – 20%) were intermediate to rates for adults and juveniles in 

other studies. Monthly survival rate estimates for juvenile pig frogs (R. grylio; Wood et al. 1998) extrapolate to 3% 

survival annually. Any effort to promote growth of individuals at Dilman Meadow could benefit the population 

growth rate by increasing survival probability, but we do not know how current survival rates affect population 

viability. Unfortunately, estimates of annual survival of Oregon spotted frogs from other sites are not currently 

available for comparison. Obtaining annual demographic rates from other Oregon spotted frog populations, where 

suspected stressors are acting, would be most valuable in understanding the mechanisms underlying the species 

declines and in prescribing management actions focused on appropriate life history stages. 

12
 Few studies have properly dealt with bias due to missing sex data (Wood et al. 1998, Nichols et al. 2004) 

and ours is the first that we know of to do so in a multi-state model. Here we show that sex-specific survival rates 

varied by season. The highest mortality of frogs at Dilman Meadow occurred for males after breeding. This is 

consistent with recent work on boreal toads (Bufo boreas) that suggests breeding is costly for males (Muths et al. 

2006). Also, monthly survival during autumn through winter was higher than during spring through summer in 

water frogs (R. lessonae, R. esculenta; Peter 2001), consistent with our study. Oregon spotted frogs migrate 

seasonally to special habitats for over-wintering (Watson et al. 2003, our study) where they are relatively 

concentrated in space. We hypothesize that the high over-winter survival we observed at Dilman Meadow related to 

the absence of predatory fish from over-winter sites. 

Size is rarely controlled for in studies of sex-specific survival in frogs. In our analysis, which controlled 

for size, survival rate still tended to be higher for females, as in other studies (Wood et al. 1998), but this difference 

was not statistically significant. We suggest that differences in survival rates between genders in other studies are 

confounded by size variation between genders. Body size tends to be sexually dimorphic in frogs (Shine 1979) and 

is highly plastic (e.g. Peacor and Pfister 2006). Because size may be affected by management, whereas sex 

generally cannot, we suggest that future studies attempt to isolate gender and size effects on demographic rates. 

The results of our study were consistent with other studies of Oregon and Columbia spotted frogs (R. 

luteiventris) that demonstrate seasonal migrations and the importance of specialized wintering habitats. Watson et 

al. (2003) showed that Oregon spotted frogs reduced movements in summer, re-occupied the breeding range in 

September through January, and used specialized habitats as refuge during the coldest periods. Columbia spotted 

frogs in the mountains of central Idaho used multiple wetland sites seasonally and were able to traverse uplands 

(Pilliod et al. 2002). We do not know whether Oregon spotted frogs from Dilman Meadow are able to travel to the 

nearest lentic site outside of Dilman Meadow, Wickiup Reservoir (ca. 2.5 km). We would expect such movements, 

be they seasonal or permanent (i.e., emigration), to show up in our capture-recapture data as greater mortality during 

the fall and spring (wet, warm months when longer movements are most likely). While Oregon spotted frogs appear 

capable of substantial movements (e.g., Watson et al. 2003), there is little indication in literature or in our 

observations that they make such moves outside of riparian areas. Clearly, the differential use of site 3, the natural 

spring, emphasizes the importance of movement within the meadow. We conclude that Oregon spotted frogs at 

Dilman Meadow behave as a single population with exchange of individuals among ponds, which were 

complementary in their value to the population. 

Striking and significant differences in asymptotic size were observed between the animals originally 

translocated to Dilman Meadow and those native to the meadow. Studies of body size at metamorphosis and 

subsequent growth have shown variation in the capacity of frogs to make up for size differences at metamorphosis 

after their transformation. Such catch-up growth has been shown to occur in 4 species of anuran (Boone 2005), 

though it was not detected in European toads (Bufo bufo; Goater 1994), wood frogs (R. sylvatica; Goater and 

Vandenbos, 1997), or spadefoot toads (Spea hammondii; Morey and Reznick 2001). Currently, we cannot 

determine if the variation in asymptotic size we observed resulted from difference in conditions faced by larvae, by 

frogs, or both. There is some evidence, provided by the lower growth trajectories of 3 individuals originally 

translocated to Dilman as sub-adults (Figure 5), that the environment of adults plays a role in the differences in 

asymptotic size. Because of the importance of size in the demography of amphibians, this difference is likely to 

affect the performance of the Oregon spotted frog population at Dilman Meadow. 

We observed a decreasing trend in embryo or larval survival over the course of the study. This pattern is 

consistent with density dependent effects on survival, but could be explained by other factors which were not 

studied, such as the growth of predator populations or other changes in pond communities. Density dependence has 

been regularly observed in mesocosms (Loman 2001) and in some whole-pond experiments (Van Buskirk and Smith 

1991, Loman 2004; but see Beebee et al. 1996). Perhaps most importantly for the future of Oregon spotted frogs at 

Dilman Meadow, open water in the mitigated ponds is rapidly decreasing due to pond succession. Our study 

suggests that the Dilman population may be impacted by succession initially via the reduced survival of larvae. 

There is some indication that density dependence affected the growth of adults as evidenced by differences in 

growth rate between breeding and non-breeding ponds. Density dependence could have occurred if a higher density 

of young frogs at breeding sites reduced growth relative to non-breeding sites. 

13
At Dilman Meadow, increasing numbers of egg masses were laid at sites which were used previously for 

breeding, whereas there was little evidence of true dispersal (resulting in breeding) to sites which were not 

previously used for breeding. Breeding has yet to occur at the 3 ponds newly excavated in 2004 (Site 8), but neither 

did breeding occur immediately at sites 2, 3, or 4. Site 8 has been used increasingly by frogs since 2005 (C. Pearl 

unpublished data) though not for breeding. In other frog species, dispersal is most prevalent in young animals. If 

this is true of Oregon spotted frogs, then spring of 2005 may have been too early to anticipate the deposition of eggs 

at site 8. Sites 2 and 4, the first to be naturally colonized, did not produce an egg mass until the 4th

 year of natural 

egg deposition. Age-related dispersal in Oregon spotted frogs is an important information gap. To confirm agerelated dispersal, it will be necessary to mark young animals, and use genetic techniques or intensive breeding 

season observation to link egg masses to the individual frogs that laid them. Apparent density dependent effects on 

survival during the larval stage coupled with the pattern of true dispersal at Dilman Meadow suggest an advantage 

of careful distribution of egg masses during translocation to minimize tadpole density. Our survival study, and 

others (Smith 1987, Semlitsch et al. 1988, Chelgren et al. 2006) demonstrate that conditions which positively affect 

growth rates of tadpoles should increase the probability of success of translocation efforts by improving multiple 

components of fitness. 

Recommendations for future monitoring 

To continue monitoring frog dynamics at Dilman Meadow, we recommend the following components in 

order of priority: 

1) Obtain a complete count of egg masses at all ponds during spring. 

2) Monitor pond succession or the extent of surface water at each pond during the dry period of summer. 

3) Continue capture-recapture sampling of adult and sub-adult frogs during summer when frogs are most easily 

captured. 

4) Conduct capture-recapture sampling during late summer following metamorphosis, applying year- and pondspecific batch marks to newly metamorphosed juveniles. 

Components 1 and 2 are minimum metrics for documenting short-term success of the translocation effort. 

Component 1 is the most effective single measure of population performance but at a minimum should be combined 

with component 2 because without management, pond succession appears inevitable and will ultimately negate the 

population dynamics of frogs. Components 1 and 2 together would relate frog population size to area of open water, 

clarifying the urgency of management. 

The combination of components 1, 2, and 3 would provide information necessary to assess recruitment of 

adult breeders, and survival as well as breeding probability. Including component 3 in monitoring efforts would 

provide a more complete picture of the life history, but it would not permit analysis of the entire annual life cycle. 

That is, without component 4, demography during the two most variable and dynamic periods, the larval period 

from egg to metamorphosis and the juvenile period from metamorphosis to the small adult stage, would be missed. 

Without the fourth component it would not be possible to link recruitment of adults to the numbers of egg masses 

laid, because of variation in both size at metamorphosis and post-metamorphic growth rates. With the fourth 

component, it would be possible to estimate the complete suite of demographic rates annually, which has never been 

done for a single amphibian population. Intensified efforts to sample juveniles during fall would benefit our 

understanding of the ecology of larval and juvenile life stages including the importance and form of density 

dependence during the larval period and the impacts of size at metamorphosis on juvenile survival and proportional 

age of recruitment into the breeding population. 

This work provides context for studies of other Oregon spotted frog populations, particularly populations 

confronted with exotic predators. For example, Pilliod et al. (2002) reported that the presence of fish in 

overwintering sites reduced Columbia spotted frog survival. Our finding of high overwinter survival in a system 

lacking fish is consistent with focused winter effects of fish. Comparison of demographic rates between sites with 

introduced fish or bullfrogs and sites lacking predators (such as Dilman Meadow) is a necessary component for 

proceeding with informed, effective management elsewhere in the species range. 

14
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16
Table 1. Summary statistics of the posterior distributions of capture-recapture model parameters pertaining to survival: model 

coefficients (1-5), random sex*season effects (6-15), random site effects (16-23), and random effect standard deviations (24-25). 

Random effects were zero-centered. Lower and Upper refer to the 0.025th and 0.975th percentiles of posterior distributions, 

respectively. The logit link function relates linear combinations of parameters to bi-monthly survival rates. 

Index Parameter Mean St. dev. Lower Median Upper 

1. Intercept 3.585 0.927 2.092 3.473 5.684 

2. Female -0.083 1.054 -2.232 -0.022 1.878 

3. Year -0.020 0.101 -0.209 -0.024 0.184 

4. Trans -0.941 0.349 -1.617 -0.945 -0.247 

5. Large 1.700 0.274 1.175 1.696 2.245 

6. male, March 0.997 1.648 -1.776 0.807 4.786 

7. male, April-May -2.691 0.981 -5.012 -2.568 -1.136 

8. male, June-Sept. -1.021 0.938 -3.114 -0.906 0.527 

9. male, Oct. 1.308 1.549 -1.118 1.084 5.018 

10. male, Nov.-Feb. 1.735 1.502 -0.608 1.529 5.275 

11. female, March 0.406 1.721 -2.615 0.274 4.255 

12. female, April-May 0.970 1.679 -2.119 0.809 4.785 

13. female, June-Sept. -1.413 1.054 -3.762 -1.355 0.530 

14. female, Oct. 0.148 1.730 -3.007 0.036 3.927 

15. female, Nov.-Feb. 0.585 1.500 -2.162 0.452 4.028 

16. Site 1 -0.034 0.189 -0.418 -0.030 0.334 

17. Site 2 -0.058 0.262 -0.602 -0.051 0.448 

18. Site 3 0.007 0.286 -0.548 0.003 0.586 

19. Site 4 0.006 0.224 -0.436 0.005 0.453 

20. Site 5 -0.119 0.186 -0.505 -0.113 0.229 

21. Site 6 0.173 0.238 -0.238 0.154 0.708 

22. Site 7 -0.014 0.186 -0.386 -0.015 0.355 

23. Site 8 0.057 0.293 -0.489 0.042 0.694 

24. σ 1.791 0.833 0.758 1.604 3.905 sex*season 

25. σ 0.275 0.109 0.141 0.251 0.555 site 

17
Table 2. Summary statistics of the posterior distributions of capture-recapture model parameters pertaining to capture 

probability: model coefficients (1-4), random site effects (5-12), random season effects (13-17), and the random effect standard 

deviation (18). Random effects were zero-centered. Lower and Upper refer to the 0.025th and 0.975th percentiles of posterior 

distributions, respectively. The logit link function relates linear combinations of parameters to capture probability. 

Index Parameter Mean St. dev. Lower Median Upper 

1. Intercept -3.962 0.504 -4.894 -3.965 -2.945 

2. Female 0.620 0.092 0.435 0.621 0.799 

3. Year 0.255 0.046 0.164 0.254 0.347 

4. Days 0.569 0.042 0.487 0.569 0.651 

5. Site 1 -0.409 0.306 -1.033 -0.405 0.175 

6. Site 2 -0.246 0.378 -1.041 -0.234 0.462 

7. Site 3 1.888 0.410 1.104 1.879 2.705 

8. Site 4 0.157 0.327 -0.492 0.160 0.784 

9. Site 5 -0.166 0.303 -0.784 -0.158 0.414 

10. Site 6 -1.030 0.329 -1.714 -1.018 -0.416 

11. Site 7 -0.827 0.306 -1.444 -0.824 -0.241 

12. Site 8 0.404 0.552 -0.675 0.399 1.511 

13. March 0.041 0.363 -0.738 0.048 0.748 

14. April-May -0.262 0.368 -1.052 -0.252 0.444 

15. June-Sept. 0.808 0.355 0.023 0.815 1.500 

16. Oct. -0.292 0.369 -1.087 -0.283 0.409 

17. Nov.-Feb. -0.770 0.389 -1.611 -0.758 -0.033 

18. σ 0.818 0.193 0.532 0.790 1.280 

18
Table 3. Summary statistics of the posterior distributions of coefficients for the mechanism of the missing gender data. 

Parameters relate to the probability that gender was observed using the logit link function. Lower and Upper refer to the 0.025

th 

and 0.975th percentiles of posterior distributions, respectively. 

Index Parameter Mean St. dev. Lower Median Upper 

1. Intercept -0.414 0.169 -0.748 -0.412 0.091 

2. Female -1.651 0.292 -2.235 -1.651 -1.075 

3. Large 3.467 0.301 2.892 3.458 4.089 

4. Female*Large 0.893 0.410 0.087 0.898 1.673 

19
Table 4. Summary statistics of the posterior distributions of logistic growth model parameters: coefficients for growth rate (1-9), 

random site effects on growth rate (10-17), coefficients for asymptotic size (18-20), and the random effect standard deviation (21). 

Random effects were zero-centered. Lower and Upper refer to the 0.025th and 0.975th percentiles of posterior distributions, 

respectively. The log link function relates linear combinations of parameters to growth rate and asymptotic size. 

Index Parameter Mean St. dev. Lower Median Upper 

1. March -3.137 0.486 -4.176 -3.132 -2.275 

2. April-May -2.994 0.456 -3.960 -2.951 -2.200 

3. June-Sept. -2.444 0.283 -3.005 -2.421 -1.946 

4. Oct. -774.525 594.123 -2186 -633.85 -33.284 

5. Nov.-Feb. -807.054 598.731 -2233 -690.90 -36.290 

6. Female -0.455 0.190 -0.810 -0.464 -0.092 

7. Year 0.210 0.067 0.074 0.216 0.330 

8. Translocation 1.520 0.328 0.825 1.548 2.132 

9. Breeding -0.563 0.207 -0.949 -0.572 -0.121 

10. Site 1 -0.295 0.354 -0.977 -0.316 0.463 

11. Site 2 0.039 0.465 -0.932 0.071 0.830 

12. Site 3 -0.518 0.602 -1.926 -0.447 0.457 

13. Site 4 0.609 0.323 -0.031 0.627 1.203 

14. Site 5 -0.109 0.318 -0.75 -0.094 0.499 

15. Site 6 -0.070 0.408 -0.934 -0.048 0.710 

16. Site 7 0.003 0.331 -0.615 -0.005 0.663 

17. Site 8 -0.180 0.666 -1.635 -0.129 0.969 

18. Intercept 4.067 0.012 4.046 4.067 4.091 

19. Female 0.181 0.019 0.143 0.181 0.217 

20. Translocation 0.089 0.023 0.044 0.088 0.134 

21. σ 0.707 0.222 0.414 0.664 1.264 

20
Table 5. Numbers of egg masses laid in 2000 at the original ditch near Wickiup Reservoir, numbers translocated to Dilman 

Meadow in 2001, and numbers laid at Dilman Meadow subsequently. Site 8 was constructed after the completion of oviposition in 

2004. 

Site Number 


Year Count 1 2 3 4 5 6 7 8 


2000 11 - - - - - - - -


2001 9 2.25 0 0 0 2.25 2.25 2.25 -


2002 2 0 0 0 0 2 0 0 -


2003 25 12 0 0 0 4 2 7 -


2004 30 10 0 0 0 6 6 8 -


2005 49 13 1 0 1 14 8 12 0 


21
Figure 1. Variation in bimonthly survival rate by season, gender, and pond. Bimonthly survival rates were constrained to be equal 

within the 5 seasons (x-axis): 1) Spring redistribution (March), 2) Breeding/post-breeding redistribution (April-May), 3) Summer 

(June-September), 4) Fall redistribution (October), 5) Over-winter (November-February). Female frogs are depicted in gray, males 

in black, and plotting symbols represent sites 1-8. Estimates are computed for frogs native to Dilman Meadow at the median year. 

22
Figure 2. Variation in annual survival rate for small (snout-vent length < 57 mm, triangles) and large frogs (snout-vent length > 57 

mm, circles) by year and sex. Males are depicted in black, females in gray. Bars are 95% credibility intervals. The linear 

combination for 2001 incorporates a translocation effect additive to the trend across years. 

23
Figure 3. Probability of emigrating a site given the animal was present in the pond at the start of a bimonthly interval (π). X-axis 

represents season: 1) Spring redistribution (March), 2) Breeding/post-breeding redistribution (April-May), 3) Summer (JuneSeptember), 4) Fall redistribution (October), 5) Over-winter (November-February). Parameter estimates were constrained equal 

within seasons. Plotting symbols represent the site of origin. 

24
Figure 4. Season-specific relative probability of preference of sites conditional on frogs having emigrated. X-axis represents 

season: 1) Spring redistribution (March), 2) Breeding/post-breeding redistribution (April-May), 3) Summer (June-September), 4) 

Fall redistribution (October), 5) Over-winter (November-February). Within seasons, relative probabilities of preference sum to 1.0. 

Plotting symbols represent the site of destination. 

25
Figure 5. Growth of female (upper panel) and male (lower panel) Oregon spotted frogs. Original translocated individuals are 

represented with solid lines. X-axis represents days since first marked. 

26
Figure 6. Estimated abundance of A) juvenile (< 40 mm snout-vent length), B) small adult (40-57 mm) and C) large adult frogs (> 57 

mm). Plotting symbols represent sites 1-8. In A), No juvenile frogs were captured at sites 3, 4, and 8. 

27