Given these movement patterns throughout the life of salamanders, we explore
the time it would take for salamanders to colonize habitat that becomes available to
them, such as ponds that are created nearby their current location. To do so, we
begin with a population of spotted salamanders inhabiting a single woodland pond,
but with access to a new pond—Pond 2—in the same woodland area as Pond 1. We
also assume a third pond—Pond 3—in a nearby woodlot, accessible only through
an open grassy field.
We assumed that dispersal only occurs during the juvenile stage since the eggs/
larvae are aquatic and adults are known to have a high level of site fidelity. Initially,
we assumed that each life stage has its own death rate, but that these death rates
would be the same across all three ponds.
Following work by Rothermel and Semlitsch [1] we assume that only 14 % of
juveniles disperse farther than 100 m, so we used this number to represent the total
percentage of juveniles moving far enough from their natal pond to colonize
adjacent ponds. Several studies show that juveniles show a preference to disperse
through forest versus grass, and so we assume that 10 % disperse through the forest
and 4 % disperse through grass. Recapture rates from different habitats are used to
represent the survival rates of the juvenile salamanders as they moved through grass
(0.167) and through forest (0.333) Rothermel and Semlitsch [1]. And because it
takes 7 years for the females to mature and reach reproductive age, we simply
account for this aging delay by dividing the juvenile population of each pond by
seven, so that 1/7 of each juvenile cohort would enter the adult population.
Dispersal rates, death rates, hatching success, and average clutch size all are
highly dependent on environmental factors, such as predation, water characteristics,
and weather. The parameters used here have been chosen to reproduce an observational 4-year study monitoring the adult population size of a single pond Blackwell et al. [2]. There are 236 adults and 365 juveniles in Pond 1 at a carrying
capacity of 274.
Figure 38.1 shows the STELLA model for the three ponds. Here, we use a
converter to capture the field observations (Observed Pop) on population in a
pond—data against which we will compare our model results. For simplicity, we
assume that the final population size in that pond corresponds to the steady-state
population. The hatching success rate has been calibrated (through sensitivity
analysis) to yield population sizes that are, in the long run, close to that steady
state (Fig. 38.2). The particular value we chose is 0.27, and the larval, juvenile, and
adult death rates that correspond with that hatching success rate to generate
acceptable results are, respectively, 0.5, 0.43, and 0.17.
The adults in Pond 2 and 3 rise to an initial surge after the juveniles disperse
from Pond 1 and colonize the other two ponds (Fig. 38.2). Adults reach a peak in
Pond 2 before they do in Pond 3 because juvenile dispersal through forest to Pond
2 is easier than dispersal through grass to Pond 3. Initial adult populations peak after
juveniles because the juveniles are the ones dispersing from the natal pond (Pond 1)
to the other two ponds (Fig. 38.3). Upon maturation, they grow into adults and
begin reproducing in each of the two new ponds.
350
38 Salamander Dispersal
the time it would take for salamanders to colonize habitat that becomes available to
them, such as ponds that are created nearby their current location. To do so, we
begin with a population of spotted salamanders inhabiting a single woodland pond,
but with access to a new pond—Pond 2—in the same woodland area as Pond 1. We
also assume a third pond—Pond 3—in a nearby woodlot, accessible only through
an open grassy field.
We assumed that dispersal only occurs during the juvenile stage since the eggs/
larvae are aquatic and adults are known to have a high level of site fidelity. Initially,
we assumed that each life stage has its own death rate, but that these death rates
would be the same across all three ponds.
Following work by Rothermel and Semlitsch [1] we assume that only 14 % of
juveniles disperse farther than 100 m, so we used this number to represent the total
percentage of juveniles moving far enough from their natal pond to colonize
adjacent ponds. Several studies show that juveniles show a preference to disperse
through forest versus grass, and so we assume that 10 % disperse through the forest
and 4 % disperse through grass. Recapture rates from different habitats are used to
represent the survival rates of the juvenile salamanders as they moved through grass
(0.167) and through forest (0.333) Rothermel and Semlitsch [1]. And because it
takes 7 years for the females to mature and reach reproductive age, we simply
account for this aging delay by dividing the juvenile population of each pond by
seven, so that 1/7 of each juvenile cohort would enter the adult population.
Dispersal rates, death rates, hatching success, and average clutch size all are
highly dependent on environmental factors, such as predation, water characteristics,
and weather. The parameters used here have been chosen to reproduce an observational 4-year study monitoring the adult population size of a single pond Blackwell et al. [2]. There are 236 adults and 365 juveniles in Pond 1 at a carrying
capacity of 274.
Figure 38.1 shows the STELLA model for the three ponds. Here, we use a
converter to capture the field observations (Observed Pop) on population in a
pond—data against which we will compare our model results. For simplicity, we
assume that the final population size in that pond corresponds to the steady-state
population. The hatching success rate has been calibrated (through sensitivity
analysis) to yield population sizes that are, in the long run, close to that steady
state (Fig. 38.2). The particular value we chose is 0.27, and the larval, juvenile, and
adult death rates that correspond with that hatching success rate to generate
acceptable results are, respectively, 0.5, 0.43, and 0.17.
The adults in Pond 2 and 3 rise to an initial surge after the juveniles disperse
from Pond 1 and colonize the other two ponds (Fig. 38.2). Adults reach a peak in
Pond 2 before they do in Pond 3 because juvenile dispersal through forest to Pond
2 is easier than dispersal through grass to Pond 3. Initial adult populations peak after
juveniles because the juveniles are the ones dispersing from the natal pond (Pond 1)
to the other two ponds (Fig. 38.3). Upon maturation, they grow into adults and
begin reproducing in each of the two new ponds.
350
38 Salamander Dispersal