Introduced ferrets (Mustela furo) in New Zealand are
subject to population control to reduce their threat to native fauna and the
incidence of bovine tuberculosis (Tb) in livestock. To help in evaluating
control options and to contribute to a multi-species model for Tb dynamics, a
simple Ricker model was developed for ferret population dynamics in a
semi-arid environment. The model was based on two data sets and suggested an
intrinsic rate of increase for ferrets of 1.0–1.3
year–1 and a carrying capacity of 0.5–2.9
km–2. There was evidence for direct
density-dependence in both data sets and the effect appeared to act mainly on
recruitment. Dependence of the rate of increase of predators on the density of
wild rabbits (Oryctolagus cuniculus) was exhibited in
one of the two data sets, together with a numerical response relating current
density of predators asymptotically to current density of rabbits, their
primary prey. Predators in this data set included both cats and ferrets,
estimated from spotlight counts, but the other data set demonstrated a direct
proportionality between predator (cat and ferret) spotlight counts and minimum
ferrets known to be alive by trapping. The model suggested, firstly, that
populations are hard to suppress by continuous culling, with at least a
50% removal per year necessary to effect a suppression of 50% in
long-term average density. Secondly, if control is episodic rather than
continuous, culling in autumn gives a greater degree of suppression over time
(280%, accumulated over time) than culling in spring (180%). A
differential equation version of the model provides a component for a general
Anderson/May bovine Tb/wildlife (possum/deer/ferret) model.
Explaining Spatial Heterogeneity in Population Dynamics and Genetics from Spatial Variation in Resources for a Large Herbivore
