Download RISKMAN Software Manual (Version 1.9)

RISKMAN
Concepts and Common Functions
The term Carrying Capacity (also sometimes referred to as K) is the population
number that causes a density dependent reduction in the population growth rate to 0.
However, density does not affect the population growth rate directly. Rather density
affects birth and death rate, and those cause the change in population growth rate.
Density may only affect the demographic rate of same sex and age strata. It may be
that only the density of same sex and age strata is causing the density effect.
RISKMAN provides a flexible empirical equation (inverted threshold-corrected
Michaelis-Menton equation) to model the mechanism of density effect.
The rate value entered (e.g. survival rate) is considered to be the maximum rate value
for a given iteration. The maximum rate is reduced based on the values of the X-Axis
intercept (CC) which is the value where the rate goes to zero. The shape parameter
(KS) determines whether the decline is linear (classical density effect) or non-linear.
Many independent density effects can be specified simultaneously. If the sex and age
distribution of the population are changing the impact of these demographic rate
effects on overall population growth rate can be dynamic.
The values of the parameters used to model density effects can be determined by nonlinear least squares regression. The density effects equation has been based on a
threshold-corrected Michaelis-Menton curve. The equation is:
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