(
2010-07)
Cooch, Evan G.; Cam, Emmanuelle; Caswell, Hal
Advances in the estimation of population parameters
using encounter data from marked individuals have
made it possible to include estimates of the probability of
recruitment in population projection models. However, the
projected growth rate of the population, and the sensitivity
of projected growth to changes in recruitment, can vary
significantly depending upon both the structural form of the
model and how recruitment is parameterized. We show that
the common practices of (1) collapsing some age classes
into a single, terminal ‘aggregated’ age-class, and (2) parameterizing
recruitment using the proportion of recruited
individuals (breeders) in a given age-class may confound
analysis of age-based (Leslie) matrix projection models in
some instances, relative to state-based projection models
where recruited and pre-recruited individuals are treated as
separate states. Failing to account for these differences can
lead to misinterpretation of the relative role of recruitment in
the dynamics of an age-structured population.We show that
such problems can be avoided, either by structural changes
to the terminal aggregated age-class in age-based models,
or by using using a state-based model instead. Since all
the metrics of general interest from a classical age-based
matrix models are readily derived from a state-based model
equivalent, this suggests there may be little reason to use the
classical age-based approach in situations where recruitment
is a parameter of interest.
