Evaluating control strategies for African multimammate mice: a modelling approach including economy

1
Evaluating control strategies for African
multimammate mice a modelling approach including
economy
Brevig, T. (1), Stenseth, N.C. (1), Leirs, H.
(2,3) 1 University of Oslo, Norway 2 University
of Antwerp, Belgium 3 Danish Pest Infestation
Laboratory, Denmark
A stage structured population model Leirs et al.
(1997) developed a discrete demographic model
that simulates fluctuations in the density of
Mastomys natalensis populations. The model
differentiates between three classes of
individuals. Monthly survival, maturation and
birth depend on both density and recent rainfall.
An extended version of this model is used to
evaluate control strategies in an economic
perspective.

• Links to economic aspects
• Simple equations link population dynamics to the
  economic aspects of mortality based rodent
  control and rodent damage to maize
• Maize yield depends on rodent damage to seeds and
  maize, which depends on rodent density during
  planting and harvesting.
• The effect of control on mortality depends on the
  control effort and rodent density.

Systematic testing Control strategies are varied
systematically. Each strategy is tested over a
ten year period, and its achieved profit is
recorded.
Figure 1. The demographic processes.
A quantitative definition of control
strategies A control strategy is defined by the
array of density and rainfall conditions under
which control is applied, together with the
effort that is put into the control action.
Figure 3. Simulation example of a single ten year
period (120 months).
a. The population that is subject to control
fluctuates in time. An unaffected population is
simulated for comparison.
b. Values for rainfall are picked randomly from
collected rainfall data.
Figure 2. Illustration of the decision process.
Control is applied during a particular month if
and only if density is between LLD and ULD and
precipitation is between LLR and ULR. In this
hypothetical case, the conditions for control are
satisfied, and control would be applied.
d. profit benefit - cost. Each time control is
applied, the accumulated cost increases. At
planting and harvesting, the accumulated benefit
changes. The end value for profit is used as the
profitability of the current control strategy.
c. Differences in population density affect
benefit only during planting and harvesting.

• Results are clear
• Average profit increases with control effort up
  to a certain level. Increasing effort beyond this
  point leads to decreased profit.
• Restricting control to months of high population
  densities result in reduced profit. This
  contradicts the traditional view that poison
  should be applied when rodents are abundant.
• Rainfall is not helpful for determining whether
  to apply control in particular months.
• Simulated application of the most successful
  strategies suggests that long term profitability
  can be achieved by specific periods of sustained
  control.

• but more empirical data are needed.
• The profitability of control is strongly affected
  both by rates of immigration and the nature of
  density dependent survival. Therefore, accurate
  data on immigration and demographic rates are
  crucial.
• A better empirical basis for damage by rodents
  and mortality due to control is needed.

• Ideas for further research
• The strategy that is to be optimized could also
  include decisions on planting, harvesting and use
  of fertilizer, turning the model into an
  integrated tool for crop management.
• Evaluation of strategies could be improved by
  looking at probabilities of different economic
  outcomes instead of average profit.
• Weather forecasts from parallel climatological
  simulations could be used to improve the basis
  for management decisions.

• References
• Leirs, H., N. C. Stenseth, J. D. Nichols, J. E.
  Hines, R. Verhagen, and W. Verheyen. 1997a.
  Stochastic seasonality and nonlinear
  density-dependent factors regulate population
  size in an African rodent. Nature 389 176-180.
• Brevig, Thomas. 2003. Evaluating control
  strategies for African multimammate rats. Cand
  Scient thesis, Department of Biology, University
  of Oslo. The thesis can be found at
  http//folk.uio.no/brevig/

brevig_at_bio.uio.no