Report – ZL3204 – Population Analysis of Wombat

ZL3204, 2001-04-08

Evaluation of Population Viability Analysis and its use in

Management of the Northern Hairy-nosed Wombat


The disappearance of the northern hairy nosed wombat (Lasiorhinus krefftii) from two of its three historic habitats in the last 100 years trouble scientists. The present range has been much reduced, apparently as result of competition with cattle (Johnson, 1991). In the remaining habitat, Epping Forest National Park, cattle have been excluded and the forest is now protected.

Due to the very small population, about 100 animals, management is a serious problem. Any ill-considered intervention with the animals can have dramatic effects on the entire population and can potentially drive it to extinction (Johnson, 1991).

This paper will discuss the potential management options for the northern hairy nosed wombat. Our discussion is based on analysis with a population viability analysis package (PVA) called VORTEX and how PVA packages are used and how reliable they are.


Evaluating the likely effect on the risk of extinction of the Epping Forest wombat population of:

  1. Increasing the size of the reserve (varying K, the carrying capacity of wombat’s habitat)
  2. Removing some animals to start a captive breeding program (varying initial population size).


VORTEX one of the most widely used Population Viability Analysis packages, was used in our study to model if the change of area (K) of the reserve would influence the population size in Epping Forest National Park. We also looked at the option of removing some animals for a captive breeding program (changing initial population) and how that would influence the population’s risk of extinction.

By testing the effects of changing the carrying capacity and the initial population size (removal of some animals for a captive breeding program) on experimental risk, one can evaluate the benefit of the option of increasing the area and the cost of the option of the breeding program.

Fig. 1


Figure 1 shows the projected extinction probability with increasing carrying capacity (area) in three different population sizes.

The purple line (squares) indicates the projected outcome for the control (100 animals) that is without any manipulation of population size nor change of available area.

The green line (triangles) is the projected outcome for a population twice as large as the control.

The blue line (diamonds) indicates the projected outcome if half the population would be taken into a captive breeding program.

The projections indicate that the population is more stable and less prone to extinction when in larger numbers. The projected outcome between the 200 individual population and the control is clearly showing differences reaching as high as 15%.

However, the larger the area the more ‘unstable’ the data tended to be, this can be seen in fluctuations from K=450 and upward. This can perhaps be remodelled by running more simulations.


Management Issues

Our analysis indicated quite clearly that the population and area did not effect the population significantly when increasing the area. This is a quite common conclusion.

We also found (Figure 1) that the larger population the less prone to extinction the population was, therefore ex-situ conservation would indeed increase the probability of extinction for the species. If this however, is the only way to preserve the northern hairy-nosed wombat perhaps that chance is worth taking.

No specific reasons has been found to why the northern hairy nosed wombat died out from the two former ranges. However clearing of land is probably the main reason. Without knowing the problem about the animal it is hard to intervene with the animal. Caughley & Sinclair (1994) stated, however, that once a reason for a decline has been recognised, it can be treated. Ex-situ conservation, i.e. taking some of the animals into a captive breeding program and later re-introduce them, has been tested with a couple of species of kangaroo and wallaby around Australia. Caughley & Sinclair (1994) continued, that reintroduction of an animal to its former range should not be attempted without understanding why it died in the first place.

VORTEX Evaluation

Population viability analysis is widely used in conservation biology to predict extinction probabilities and compare management strategies for endangered species. However, it is unclear if different PVA packages produce similar predictions, and whether the predictive models realistically describe the behaviour of wildlife populations (Brook et al., 1999; Brook et al., 2000; Lacy, 1993). Brook et al.(1999) tested six commonly used PVA packages, GAPPS, INMAT, RAMAS (Age, Stage and Metapop) and VORTEX, to compared for a range of life-history types.

The major findings were:

  1. when complex processes were included in the models, large differences were found between some packages, and even versions of the same package. The pattern of similarities and differences altered depending on the species examined.
  2. When completely standardised, a consistent difference was revealed between the predicted extinction probabilities of the matrix-based packages (INMAT and RAMAS) compared to those that were individual-based (GAPPS and VORTEX).
  3. A set of tests using the historical data of over 20 long-term population studies was conducted, to test the predictive reliability of PVA. It was found that the quasi- extinction and -recovery probabilities predicted by the PVA packages did not realistically reflect the actual population fluctuations. The models also failed to accurately predict future population abundance.
  4. A validation and refinement process for PVA was developed, which helped to improve the projections. Current PVA packages cannot be relied upon to produce accurate quantitative predictions.

However, the same author, Brook et al. (2000) found a year later quite the opposite when they conducted a retrospective test on 21 long-term ecological studies. They estimated the parameters from the first half of each data set and the second half was used to evaluate the performance of the model. The risk of population decline closely matched the observed outcome and there was no significant different in population size projections from the estimated and reality. They also found that the outcomes from the five PVA software packages were highly similar. They concluded that PVA was a valid and sufficiently accurate to for managing endangered species.

This tend to indicate that the PVA packages has been updated and has become more accurate and realistic from the previous studies.

However, the literature (Beissinger & Westphal, 1998; Norton, 1995; Taylor, 1995; Brook et al. 1997) has still not agreed on the accuracy of different PVA packages and if they predict the behaviour of real populations. Scientists and managers are therefore still trying to develop better analysis tools instead of ‘trail and error’ management is carried out on endangered species.


Beissinger, S.R. & Westphal, M.L. (1998) On the use of demographic models of population viability in endangered species management. Journal of Wildlife Management. 62: 821-841.

Brook, B.W., Lim, L., Harden, R and Frankham, R. (1997) Does population viability analysis software predict the behaviour of real populaitons? A retrospective study on Lord Howe Island woodhen Tricholimnas sylvestris (Sclater). Biological Conservation 82: 119-128.

Brook, B.W., Frankham, R and Burgman, M.A. (1999) Evaluating the Predictions of Population Viability Analysis (PVA). In Population Viability Analysis Conference: Assessing Models for Recovering Endangered Species; March 15-16, 1999, San Diego, California, URL:

Brook, B.W., Grady, J.J.O., Chapman, A.P.Burgman, M.A., Akcakaya and Frankham, R. (2000) Predictive accurace of population viability analysis in conservation biology. Nature 404: 23 March.

Caughley, G. & Sinclair, A.R.E. (1994) Wildlife Ecology and Management. Blackwell Science, Cambridge, USA.

Johnson, C.N. (1991) Utilization of Habitat by the Northern Hairy-Nosed Wombat Lasiorhinus krefftii. J. Zool., Lond. 225: 495-507.

Lacy, R.C. (1993) VORTEX: A Computer Simulation Model for Population Viability Analysis. Wildlife Research. 20: 45-65.

Norton, T.W. (1995) Special issue applications of populationviabillity analysis to biodiversity conservation. Biological Conservation. 73: 91-176.

Taylor, B.L. (1995) The reliability of using populaiton viability analysis for risk classification of species. Conservation Biology. 9: 551-558.

Leave a Reply