Ecological Archives A025-141-A2
Elizabeth B. Harper, David A. Patrick, and James P. Gibbs. 2015. Impact of forestry practices at a landscape scale on the dynamics of amphibian populations. Ecological Applications 25:2271–2284. http://dx.doi.org/10.1890/14-0962.1
Appendix B. Description of parameter estimates and model structure for dispersal rates and temporal and spatial variation in hydroperiod.
Dispersal Rates
In the simulations, we assumed that all dispersers are first time breeders. Gamble et al. (2007) found that approximately 9% of A. opacum first time breeders disperse. Trenham et al. (2001) found higher dispersal rates in A. californiense, with overall dispersal of approximately 22%. In all of our Ambystoma models we used the average of these estimates and assumed that 12% of individuals in a population attempt to disperse each year. Of these dispersers, approximately 50% move to pools within 300 m of their natal pool, around 80% disperse to pools within 600 m, 95% remain within 900 m, and roughly 5% disperse to pools at distances greater than 900 m (Trenham et al. 2001; Gamble et al. 2007). In our Ambystoma simulations we used these percentages and set an absolute maximum dispersal distance of 2600 m (roughly twice the maximum dispersal distance recorded by Gamble et al. (2007) for A. opacum), to assign relative probabilities of dispersal between pairs of pools based on interpool distance and the number of potential breeding sites within each distance category. For example, if there was only one pool within 300 m, it would attract 50% of dispersers leaving a pool, but if there were two pools within this distance, the dispersers would be split equally between them. When there were no pools within a given distance category (e.g., no pools between 300–600 m), those potential dispersers were returned to their natal pool. We used the same method for determining dispersal in the Rana model, but used estimates from Berven and Grudzien’s (1990) study of wood frog dispersal in which 20% of first time breeders were dispersers and approximately 50% of dispersers move to pools within 600 m, 80% within 1200 m, 95% within 2400 m and 5% beyond 2400. In the Rana models we limited dispersal to within 5000 m of the natal pool, approximately twice the maximum dispersal distance recorded by Berven and Grudzien (1990).
Temporal and spatial variation in hydroperiod
The timing and frequency of pool drying are important factors affecting the density and diversity of amphibians in a landscape (Babbitt et al. 2003); pools that dry frequently have decreased predator loads (Woodward 1983, Skelly 1996), but in some years may dry before larval amphibians have reached metamorphosis, resulting in years without successful recruitment (Semlitsch et al. 1996). Our estimates of average hydroperiod (134 days) and spatial variation (CV = 0.33) were based on five studies, each conducted in a different U.S. state, which calculated the mean hydroperiod and standard deviation for at least 15 pools in a single year (Palik et al. 2001; O’Driscoll and Parizek 2003; Weyrauch and Grubb 2004; Skidds and Golet 2005; Baldwin et al. 2006). These studies each defined hydroperiod as the number of consecutive days that a pool held water beginning in the spring either with the initiation of amphibian breeding or when pool ice began to melt. These dates varied among studies (range 1 Feb. – 1 April), but average hydroperiod varied independently of the initiation date. This allowed us to create a geographically generic model in which hydroperiod could be quantified in days beginning with the start of the spring breeding season, making it unnecessary to define a geographically specific calendar date on which breeding was initiated. The larval period for each species was calculated relative to this spring breeding season, and we assumed that pools filled in the fall and held water over winter. Our estimate of temporal variation in hydroperiod (CV = 0.32) was based on data from five individual pools that were studied for a minimum of eight consecutive years (Semlitsch et al. 1993; Brooks 2004).
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