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environmental education and land stewardship at Wachusett Mountain.
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Spotted Salamanders
General Project Background and Summary of Interim Results
Beginning in 2001, Scott Smyers, Doug Williams and many other volunteer collaborators collected body size data on male spotted salamanders from five breeding populations at Wachusett Mountain State Reservation, along with 15 sites across Massachusetts and Rhode Island. These populations encompass a broad geographic area and include two island populations (Martha's Vineyard and Prudence Island, RI). Salamander body length (snout-vent length (SVL)) and mass were measured, and non-lethal tissue samples from regenerative tissues such as toes or tails were obtained for genetic and skeletochronological analysis. Body size varies significantly among the populations, with most populations averaging 12-15 grams/individual male. We used measurements from male salamanders for all our analyses to standardize comparisons. Strikingly, salamanders from the summit of Wachusett Mountain are the largest, averaging 18.3 grams. In contrast, a population in Milford averages 10.5 grams/individual.
Why are the Wachusett salamanders getting smaller?
After reviewing new data from 2007, it seems that the size of the breeding male salamanders has varied between 2002, 2004 and 2007. Although our data are not from every year, the Mass vs. Year graph illustrates the variability in body size from the two ponds at the summit of Wachusett. Interestingly, the salamanders from Machias Pond, located at the base of the Mountain, did not vary as greatly as the summit ponds.
It seems that the goldfish made an impact on the spotted salamanders that are breeding in summit pond. Both the number of breeding adults was low and the total eggs deposited were down as well. For the first time, we observed breeding salamanders infested with leeches, including two confirmed deaths due to leeches. Therefore, not only do the fish eat nearly everything trying to survive in the pond (plants, animals, and bread people feed them), but now the adults are threatened by leeches. GOLDFISH ARE BAD FOR THIS POND!
It remains unclear weather the same individual animals are gaining or losing a couple grams depending on the feeding the previous fall and/or winter stress conditions, or if breeding effort per year by larger or smaller individuals is bias to some environmental conditions yet to be understood.
Life History and Genetics
Candidate non-mutually exclusive explanations for variation in body sizes include the effects the local environment, the input of genetics INSERT LINK, or both. To begin our life history evaluation, we analyzed salamander bone growth rings to estimate ages of individuals from a subset of our study sites, including extremes of the body size spectrum and two "outpost" populations in southern New England: Martha's Vineyard, MA and Prudence Island, RI. The results indicate that skeletochronology can be used as a tool to approximate age classes of spotted salamanders. In the populations reviewed in this study, it is not likely to be very accurate for any animals greater than 12 years, therefore those animals are assigned to the 12+ age class. Furthermore, by analyzing both the correlation (R2 = x) and slope (y = slope x ...), we can possibly determine how the age:mass ratio varies between populations and what that could explain about life history variability.
These data may indicate that salamanders from the Summit Pond of Wachusett Mountain delay maturity, live longer (in comparison to three other populations) and increase in mass after reaching sexual maturity, but do not increase in overall body length. The fish eating nearly all the baby salamanders over the past 20+ years of fish introduction could be the cause this "delay" in maturation and we may be witnessing a population about to die out due to minimal recruitment of young mature salamanders.
In 2004 (the year after the successful fish removal), we investigated differences in the aquatic environment, where eggs and larvae develop, and the terrestrial habitat where juveniles and adults spend approximately 95% of their lives (underground). Sizes of late stage larvae at two ponds located at the summit of Wachusett were significantly different, likely due to the fact that one pond is shallow and dries every year, while the other is deep and never dries. The deep pond contained significantly larger larvae than did the shallow pond; however, the adults that breed in each pond are equivalent in body size. Therefore, it is unlikely that larval development and subsequent size at metamorphosis influences adult body size.
INSERT LARVAE GRAPHBeyond Wachusett
There are plenty of other interesting populations of spotted salamanders beyond the WMSR. By plotting mass vs. body length, or SVL, you can see differences in the correlation and slope from population to population and even from year to year (where data are available, i.e., Wachusett) or between populations at different locations. In comparison to the spotted salamanders at the summit of Wachusett Mountain, salamanders from a population in Milford mature early and continue to grow in both mass and length after maturity, but their maximum body size is much smaller than the population at the summit of Wachusett Mountain.
What about body shape?
Also notice how the population from Edgartown, Martha's Vineyard has a very tightly correlated mass vs. length plot, a relatively flat slope, and how the slope is steeper for the length vs. age (Lines of Arrested Growth (LAGs))compared to the mass vs. age. The steeper the slope of the mass vs. length plot, the thicker the body. Conversely, a flatter slope indicates a more slender body. Combining these two data sets, it appears that spotted salamanders from Edgartown on that given year represented an even distribution of age and size classes. On the other spectrum, notice how the population from Prudence Island has a relatively clustered mass-to-length ratio and a relatively weak correlation of mass-to-age, indicating that most of the breeding population is of approximately the same size and since age is weakly correlated with size, growth may slow after a given size is achieved.
This body shape characteristic that may vary year-to-year depending on environmental variables such as local climatic conditions. Note that for both the Summit and Lower Summit populations that the average mass increased from 2002 to 2004, but so did the slope of the mass vs. length plot. This indicates that the salamanders that showed up to breed that year were more robust that two year's previously. The variation of 1-3 grams is substantial, but could be a measure of body condition, e.g., heavy salamanders are healthier.
Could it be the soil conditions?
In the terrestrial habitat, we investigated soil conditions during a normal dry period of summer 2005. Soil in the summit forest of Wachusett Mountain was significantly more saturated and contained more topsoil compared to soils around the pond in Milford. Therefore, we hypothesize the salamander populations have developed alternative life history strategies to adapt to their corresponding habitat conditions. Milford populations maximize fitness by maturing early, to respond to summer stress imposed by limited organic topsoil and moisture. Conversely, the moist, rich soil at Wachusett Mountain provides a more predictable and less stressful habitat; accordingly, individuals allocate more energy to growth than reproduction.
Continued Research
In our ongoing research, we will collect additional field data and design a series of laboratory and field experiments to test these environmental hypotheses. Furthermore, we propose to build upon our preliminary molecular genetic analyses to determine if any of the salamanders' varying life history and body-size traits are associated with population or lineage-specific genetic structure.
Acknowledgements
To date this research has been supported by Oxbow Associates, Inc., The University of Massachusetts at Lowell, The Sheriff's Meadow Foundation ADD LINK, Narragansett Bay National Estuarine Research Reserve ADD LINK, and partially supported by the Department of Conservation and Recreation. Also, many thanks to the volunteers and students associated with this project.
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