10. Using Matrix Models to Focus Research and Management Efforts
159
model results indicated that widespread use and enforcement of TEDs should lead
to population increases of 3–6% per year.
The Loggerhead Turtle’s extremely long time to maturity suggests that the
population will take many years to reach a stable age distribution. The stage-based
model does not tell us what the population size or stage distribution will look like
in the next two or three decades as the population responds to the changes in vital
rates associated with TED regulations. Crowder and co-workers (1994) expanded
their five-stage model into a modified Leslie matrix to examine transient effects
on the expected numbers of nesting females as the model population responded to
new vital rates associated with the TED requirements. This 22 × 22 age-based
matrix had all adults collapsed into a single stage, primarily because neither life
span nor age-specific fecundity of sea turtles is known; thus, the model looked
like a Leslie matrix (Fig. 10.1a) except for an adult survival entry in the lower
right-hand corner of the matrix.
Because the population has been declining for several years and large juvenile
turtles have been vulnerable to shrimp trawls, there is likely to be a “hole” in the
population size distribution that will mature in the next decade or two. The agebased model revealed a transient dip in the nesting population in that time frame,
completely independent of stochastic fluctuations (Fig. 10.2). To the inexperienced, this dip could be interpreted as a failure of the TEDs. By anticipating this
transient decline in nesting females, such misinterpretations can be avoided.
Today, most monitored nesting beaches report that Loggerhead populations are
stable or increasing (Turtle Expert Working Group 1998). A time-series analysis
of strandings indicate that periods of TED use in South Carolina reduced stranding numbers by 40% (Crowder et al. 1995). Research continues to determine the
population-level effects of TEDs and other management efforts for Loggerheads
and other sea turtle species.
Case Study 2: Red-Cockaded Woodpeckers in
North Carolina
Red-cockaded Woodpeckers (RCWs) are cooperatively breeding birds that inhabit long-leaf pine forests in the southeastern United States (Thompson 1971;
Wood 1983; Walters 1990). RCW population dynamics are driven by territory
availability, and males may spend several years as nest helpers before reproducing
themselves. Because territories are only held by males and larger populations do
not appear to be female limited (Walters 1990), Heppell and colleagues (1994)
produced a male-only stage-based model for RCWs in the North Carolina sandhills. The object of this modeling exercise was to determine which life history
stages had the greatest impact on population growth and to evaluate qualitatively
several management proposals.
Unlike the sea turtle model, the woodpecker model was based on life history
stages that are not dependent on age: fledglings, helpers, floaters (without a
territory), solitary males (with a territory but no mate), 1-year-old breeders, and
2-year-old or older breeders (Fig. 10.1c). Multiple transitions between stages
159
model results indicated that widespread use and enforcement of TEDs should lead
to population increases of 3–6% per year.
The Loggerhead Turtle’s extremely long time to maturity suggests that the
population will take many years to reach a stable age distribution. The stage-based
model does not tell us what the population size or stage distribution will look like
in the next two or three decades as the population responds to the changes in vital
rates associated with TED regulations. Crowder and co-workers (1994) expanded
their five-stage model into a modified Leslie matrix to examine transient effects
on the expected numbers of nesting females as the model population responded to
new vital rates associated with the TED requirements. This 22 × 22 age-based
matrix had all adults collapsed into a single stage, primarily because neither life
span nor age-specific fecundity of sea turtles is known; thus, the model looked
like a Leslie matrix (Fig. 10.1a) except for an adult survival entry in the lower
right-hand corner of the matrix.
Because the population has been declining for several years and large juvenile
turtles have been vulnerable to shrimp trawls, there is likely to be a “hole” in the
population size distribution that will mature in the next decade or two. The agebased model revealed a transient dip in the nesting population in that time frame,
completely independent of stochastic fluctuations (Fig. 10.2). To the inexperienced, this dip could be interpreted as a failure of the TEDs. By anticipating this
transient decline in nesting females, such misinterpretations can be avoided.
Today, most monitored nesting beaches report that Loggerhead populations are
stable or increasing (Turtle Expert Working Group 1998). A time-series analysis
of strandings indicate that periods of TED use in South Carolina reduced stranding numbers by 40% (Crowder et al. 1995). Research continues to determine the
population-level effects of TEDs and other management efforts for Loggerheads
and other sea turtle species.
Case Study 2: Red-Cockaded Woodpeckers in
North Carolina
Red-cockaded Woodpeckers (RCWs) are cooperatively breeding birds that inhabit long-leaf pine forests in the southeastern United States (Thompson 1971;
Wood 1983; Walters 1990). RCW population dynamics are driven by territory
availability, and males may spend several years as nest helpers before reproducing
themselves. Because territories are only held by males and larger populations do
not appear to be female limited (Walters 1990), Heppell and colleagues (1994)
produced a male-only stage-based model for RCWs in the North Carolina sandhills. The object of this modeling exercise was to determine which life history
stages had the greatest impact on population growth and to evaluate qualitatively
several management proposals.
Unlike the sea turtle model, the woodpecker model was based on life history
stages that are not dependent on age: fledglings, helpers, floaters (without a
territory), solitary males (with a territory but no mate), 1-year-old breeders, and
2-year-old or older breeders (Fig. 10.1c). Multiple transitions between stages