Population Modeling for Eastern Wild Turkey (Meleagris gallopavo silvestris)
Across the Midwestern United States using Sustained Yield Harvesting.
DOUGLAS STRAUB1, The Pennsylvania State University, University Park Campus, University
Park PA, 16802, USA
ABSTRACT Survival and productivity estimates were taken from four midwestern states and
one Canadian province to obtain a simulated population of eastern wild turkeys (Meleagris
gallopavo silvestris). The numbers that were collected from the different states and province
were averaged to obtain a survival and productivity rate for the entire region as well as
coefficients of variation, as a measure of variability. The productivity and survival values were
plugged into a Leslie matrix in the program RAMAS Metapopulations. Exponential growth was
calculated to determine Lambda. The growth rate for the population of eastern wild turkeys in
the Midwest was 57%. A carrying capacity was set to 45,000 individuals to calculate logistic
growth. It took 25 years for the population to reach carrying capacity. A harvest curve was
obtained by using multiple harvest rates and the logistic growth model. The rate at which
individuals in this population was harvested was 0.28 (3,037 individuals). It took 39 years for the
population to reach one half carrying capacity when the harvest rate of 0.28 was implemented.
By knowing the rate of multiplication and the rate at which Midwestern eastern wild turkeys can
be harvested, proper management can be implemented to keep turkeys at healthy population
levels.
INTRODUCTION
Eastern wild turkeys (Meleagris gallopavo silvestris) are large galliform birds that inhibit most
eastern forests, from southern Canada to north Florida and westward to Texas and Iowa.
1
dws5035@psu.edu
Isolated populations of eastern wild turkeys can be found in western states. Of the five
subspecies of turkey, eastern wild turkeys are the largest in the United States. The weight of
adult female eastern wild turkeys range from 3.6 - 5.5 kg and adult male’s weight range from
7.7 - 9.5 kg (Lopez et al. 2000). Eastern wild turkeys utilize a variety of habitat including mature
hardwood and coniferous forests, as well as agricultural settings (Miller 1999). Adult wild
turkeys are mainly herbivorous critters feeding on seeds, tender shoots and some flowers but also
consume insects in their diet, whereas the juvenile’s diet consist primarily large portions of
insects (Lopez et al. 2000). Based on their distribution, eastern wild turkeys begin breeding in
early February and extend through early June (Lopez et al. 2000). Males are polygamous and
readily display their plumage to attract females and to out-compete other males for dominance
(Lewis and Breitenbach 1966). Eastern wild turkey hens clutch size range from 10-12 eggs
(Lopez et al. 2000). Females typically lay one egg per day until the clutch is complete.
Incubation lasts approximately 28 days and females produce no more than one brood per year
(Lopez et al. 2000). Eastern wild turkeys have a short life expectancy, ranging from 2-6 years.
Productivity rates vary year to year due to temperature and precipitation. Annual survival
rates also vary from year to year based on food supply, predators and harsh winters. Due to
relatively high productivity and annual survival rates, eastern wild turkeys populations grow
quickly. Harvesting is an excellent management tool for wildlife managers to utilize for turkeys
in order to keep healthy turkey populations. Multiple productivity and annual survival rates
were observed to model population growth and a sustained yield harvest for eastern wild
turkeys in the Midwestern United States.
METHODS
The Leslie Matrix was used to model growth of midwestern eastern wild turkeys. The turkeys
were broken up into three age classes: juvenile (> 1 year old), sub-adult (1-2 years old), and adult
(< 2 years old). Productivity rates were taken from two midwestern populations of turkeys.
Annual survival rates were taken from three midwestern populations of wild turkeys and one
population in Canada.
Productivity rates were defined as the number of females produced per female. Eastern
wild turkeys have equal sex ratios within a clutch. Other factors that were included to define
productivity were clutch size, nest success, and hatching rates. Juveniles did not have any
productivity since hens become reproductively capable at approximately one year, placing them
into the sub-adult year class. In Wisconsin, the average clutch size for sub adults was 10.3 eggs
(Paisley et al. 1998). With eastern wild turkeys having equal sex ratios, 10.3 was divided by two
to obtain 5.15, representing female poults. Five point five was then divided again by 2 to account
for the number of juvenile females that were able to fledge the nest. The productivity for subadults in the population examined in Wisconsin was 2.575 and adults had a productivity of 2.8
(Paisley et al. 1998). In Minnesota, the productivity for sub-adults was 3.2 and 2.925 for adults
(Kane et al. 2007).
The survival rate for a population for juvenile turkeys in central Ontario was 0.54
(Nguyen et al. 2003). Survival rates for a population of juveniles in south central Iowa was 0.52,
0.713 for sub-adults and 0.676 for adults (Hubbard et.al 1999, Hubbard et al. 1999). In
Minnesota, a population of sub-adults had a survival rate of 0.58 and 0.73 for adults (Kane et al.
1999). Juvenile turkeys had a survival rate of 0.47, 0.66 for sub-adults and 0.52 for adults for a
population in Missouri (Kurzejeski et al.1987). After all data was recorded for productivity and
survival, the numbers were then averaged to obtain overall productivity and survival for the
Midwest. The standard deviation from the productivity and survival values was converted to
coefficients of variation to express variability in the data.
Using RAMUS Metapopulations (Akcakaya and Root 2002), the duration was set to 100
years with 20 replications. The numbers obtained from the averaged productivity and survival
was plugged into the stage matrix. The coefficients of variation were plugged into the standard
deviation matrix. The initial abundance of individuals was set 30 and was evenly distributed
among the three age classes. After all data was recorded into the program and correct settings
were activated, exponential growth was calculated. After exponential growth was calculated, I
went under the populations tab and went to the display tab to obtain the annual rate of increase
(Lambda). The annual rate of increase was then plugged into the maximum growth rate under the
populations tab. The carrying capacity was set to 45,000 individuals and logistic growth was
modeled to see how the population of turkeys would react to constraints set by RAMUS.
Elasticity and sensitivity were also calculated to see which age classes would be affected by a
harvest. After logistic growth and elasticity/sensitivity values were examined, I initiated a
harvest for each year. The harvest rate was examined during the fiftieth year of the logistic
growth model. A sustained yield curve was obtained by using multiple harvest rates and the
logistic growth model.
RESULTS
When exponential growth was calculated, it took a short amount of time for the population to
reach 2.1 billion individuals (Fig. 1). The average rate of increase for the population of wild
turkeys in the Midwest was 1.5756. For logistic growth and a carrying capacity set to 45,000
individuals, it again took a short amount of time for the population to reach carrying capacity
(Fig. 2). Sub-adults had the highest survival rate (Table 1). Sub-adults had the highest
productivity (Table 2). Elasticity and sensitivity values indicated that juvenile’s survival was
crucial in sustaining a population of turkeys. Only sub-adults and adults were harvested. For the
50th year, the maximum number of individuals harvested was 3,037 with a harvest rate was 0.28
(Fig. 3). When the logistic growth and harvest were modeled together, it took a little longer for
the population to reach one half carrying capacity when compared to logistic growth reaching
carrying capacity (Fig.4).
Fig. 1. Maximum, average, and minimum exponential growth for eastern wild turkey populations
across the Midwestern United States to determine the growth rate of the population.
Fig. 2. Maximum, average, and minimum logistic growth for eastern wild turkey populations
across the Midwestern United States with a carrying capacity set at 45,000 individuals.
Table 1. Mean survival and Coefficients of variation for eastern wild turkeys
in the Midwestern United States. Survival rates were taken from central Ontario,
southcentral Iowa, Minnesota and Missouri from 1987-2007.
Age Class
Juvenile
Sub-adult
Adult
Mean Survival (%)
0.51
0.651
0.642
Coefficient of
Variation
0.08
0.11
0.17
Table 2. Mean productivity and Coefficients of Variation for eastern wild
turkeys in the Midwestern United States. Productivity rates were taken from
Wisconsin and Minnesota from 1998-2007.
Age Class
Juvenile
Sub-adult
Adult
Mean Productivity
(females per female)
0
2.88
2.85
Coefficient of
Variation
0
0.147
0.0245
Fig. 3. Maximum sustained yield harvest with respect to harvest rates during the 50th year of the
logistic growth model for eastern wild turkeys in the Midwestern United States.
Fig. 4. Maximum, average, and minimum population size for eastern wild turkeys in the
Midwestern United States with a sustained harvest of 28% and a carrying capacity set at
45,000 individuals.
DISCUSSION
After exponential growth was calculated, the growth rate was 1.5756, meaning the population
was growing at a rate of 57 %. This is a high growth rate but is reasonable since turkeys have
high survival and productivity rates. When the variation and the carrying capacity were added to
the logistic model, the population still grew fast. In a real world situation, the time it would take
to reach carrying capacity would take longer since the survival of juveniles is not going to be that
high and consistent over 100 years due to environmental constraints. Temperature and
precipitation play a critical role in the ability for juveniles to successfully survive (Roberts and
Porter et al. 1998).
With growth rates being high, eastern wild turkeys can reach their carrying capacity fast.
This may hurt the population because food sources can become depleted which may allow little
growth among the population. Since the growth rates of eastern wild turkeys are high, harvesting
is a management tool that works well to keep turkey populations from reaching their carrying
capacity. By obtaining a harvest yield curve from the logistic growth model and using different
multiple harvest rates, the maximum number of turkeys that can be taken off with a carrying
capacity set at 45,000 individuals can be determined. Once the highest number of individuals that
can be harvested is determined, harvesting regulations can be implemented for a certain county,
state etc. The rate at which the population in the Midwest should be harvested was 0.28 since
productivity and survival stayed constant throughout the 100 years and no environmental
constraints and catastrophes occurred that could have depleted an entire year class. Although the
harvest rate in this model is for the entire midwestern region, certain states may have higher or
lower growth rates and may need to implement a higher or lower harvest rate in order to
maximize growth within the population.
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