Using Egg Mass Surveys to Monitor Productivity and Estimate Population Sizes of Three Pool-Breeding Amphibians at Marsh-Billings-Rockefeller National Historical Park

Faccio, Steven

The Marsh-Billings-Rockefeller National Historical Park was established to interpret conservation history, the evolving nature of land stewardship, and to continue careful management of the park’s historic 555-acre Mount Tom forest, the oldest professionally-managed forest in the United States. To improve upland habitat for pool-breeding amphibians, the park’s forest management plan called for selective thinning of two conifer plantations in order to slowly convert them to native hardwood cover. A three-year study to monitor amphibian productivity via egg mass counts, estimate population sizes of Wood Frog (Rana sylvatica), Spotted Salamander (Ambystoma maculatum), and Jefferson Salamander (Ambystoma jeffersonianum), and provide baseline data with which to assess the effects of forestry operations on these species, was conducted at MABI from 2009–2011. Dependent double-observer egg mass counts were conducted twice annually at seven vernal pools. This technique allowed for the estimation of detection probabilities and removal of observer bias, resulting in more robust estimates of the total number of eggs present. For each species, detection probabilities were modeled as a function of four covariates hypothesized to affect the probability of detecting egg masses of each species. The population size of each species was estimated for three distinct breeding populations deemed independent based on their distance from other pools in the study area. Hydrology (pool depth and size) and water chemistry data (pH, dissolved O2, conductivity, tannic acid, and water temperature) were also collected at each pool during egg mass surveys. Maximum annual egg mass surveys resulted in a total of 1,498 Jefferson Salamander egg masses, 971 Wood Frog egg masses, and 846 Spotted Salamander egg masses over three years. The largest breeding pool in the study area consistently supported the highest counts of egg masses for all three species, with an average of 280.3 Jefferson Salamander egg masses, 144.7 Spotted Salamander egg masses, and 136.3 Wood Frog egg masses per year. Although egg masses of all three species were detected in all seven pools, only Spotted Salamander eggs were detected in all pools every year. Estimated detection probabilities were highest for Wood Frog (91%), followed by Jefferson Salamander (83%), and Spotted Salamander (80%). There was considerable variation in detection probabilities among different observers, underscoring the need to account for observer bias when estimating population trends over time. Annual egg mass counts, and therefore population estimates showed considerable variation between years. The cluster of four pools just north of the Pogue supported 90% of the study area’s Wood Frog population, 81% of the Spotted Salamander population, and 58% of the Jefferson Salamander population. Water chemistry and hydrology measurements were similar between pools. The number of eggs present at MABI pools was positively correlated to pool size for all three species. Currently, acidity does not appear to be a limiting factor for amphibian populations at MABI. However, the Saddle Pool, near the Mt. Tom overlook, had the lowest ph (mean = 6.24), and further acidification due to acid precipitation could drop its pH below 5.5, which can reduce larval survival of Jefferson Salamanders. Although these data were insufficient to evaluate the effects of forestry operations on pool-breeding amphibians at MABI, they established valuable baseline data on estimated population sizes for all three pool-breeding species, and demonstrated that dramatic annual variations in reproductive effort occur. Recommendations for future monitoring to assess the effectiveness of forest management activities on pool-breeding amphibians are discussed.

Using Egg Mass Surveys to Monitor Productivity and Estimate Population Sizes of Three Pool-Breeding Amphibians at Marsh-Billings-Rockefeller National Historical Park

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