Skip to main content Accessibility help

Monitoring an Endangered savannah ungulate, Grevy's zebra Equus grevyi: choosing a method for estimating population densities

  • Victoria H. Zero (a1), Siva R. Sundaresan (a2), Timothy G. O'Brien (a3) and Margaret F. Kinnaird (a1)

Methods that accurately estimate animal abundance or density are crucial for wildlife management. Although numerous techniques are available, there have been few comparisons of the precision and cost-effectiveness of different approaches. We assess the precision and cost of three methods for estimating densities of the Endangered Grevy's zebra Equus grevyi. We compare distance sampling and photographic capture–recapture, and a new technique, the random encounter model (REM) that uses camera-trap encounter rates to estimate density. All three methods provide comparable density estimates for Grevy's zebra and are preferable to the common practice of raw counts. Photographic capture–recapture is the most precise and line-transect distance sampling the least precise. Line transects and photographic capture–recapture surveys are cost-effective in the first year and REM is most cost-effective in the long-term. The methods used here for Grevy's zebra may be applied to other rangeland ungulates. We suggest that for single species monitoring programmes in which individuals can be identified, photographic capture–recapture surveys may be the preferred method for estimating wildlife abundances. When encounter rates are low, distance sampling lacks the precision of the other methods but its cost advantage may make it appropriate for long-term or multi-species monitoring programmes. The REM is an efficient and precise method of estimating densities but has high initial equipment costs. We believe REM has the potential to work well for many species but it requires independent estimates of animal movements and group size.

  • View HTML
    • Send article to Kindle

      To send this article to your Kindle, first ensure is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about sending to your Kindle. Find out more about sending to your Kindle.

      Note you can select to send to either the or variations. ‘’ emails are free but can only be sent to your device when it is connected to wi-fi. ‘’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

      Find out more about the Kindle Personal Document Service.

      Monitoring an Endangered savannah ungulate, Grevy's zebra Equus grevyi: choosing a method for estimating population densities
      Available formats
      Send article to Dropbox

      To send this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Dropbox.

      Monitoring an Endangered savannah ungulate, Grevy's zebra Equus grevyi: choosing a method for estimating population densities
      Available formats
      Send article to Google Drive

      To send this article to your Google Drive account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Google Drive.

      Monitoring an Endangered savannah ungulate, Grevy's zebra Equus grevyi: choosing a method for estimating population densities
      Available formats
Corresponding author
(Corresponding author) E-mail
Hide All
Amstrup, S.C., McDonald, T.L. & Manly, B.F.J. (eds) (2005) Handbook of Capture–Recapture Analysis. Princeton University Press, Princeton, USA.
Anderson, D.R. (2001) The need to get the basics right in wildlife field studies. Wildlife Society Bulletin, 29, 12941297.
Buckland, S., Anderson, D.R. & Burnham, K.P. (2001) Introduction to Distance Sampling: Estimating Abundance of Biological Populations. Oxford University Press, Oxford, UK.
Buckland, S.T., Plumptre, A.J., Thomas, L. & Rexstad, E.A. (2010) Design and analysis of line transect surveys for primates. International Journal of Primatology, 31, 833847.
Burnham, K.P. & Anderson, D.R. (2002) Model Selection and Inference: A Practical Information-Theoretic Approach. Springer-Verlag, New York, USA.
Calambokidis, J. & Barlow, J. (2004) Abundance of blue and humpback whales in the eastern North Pacific estimated by capture–recapture and line-transect methods. Marine Mammal Science, 20, 6385.
Chao, A. & Huggins, R.M. (2005) Classical closed-population capture–recapture models. In Handbook of Capture–Recapture Analysis (eds Amstrup, S.C., McDonald, T.L. & Manly, B.F.), pp. 2235. Princeton University Press, Princeton, USA.
Cochran, W.G. (1977) Sampling Techniques. Wiley, New York, USA.
Forcada, J. & Aguilar, A. (2000) Use of photographic identification in capture–recapture studies of Mediterranean monk seals. Marine Mammal Science, 16, 767793.
Franz, T.E. (2007) Ecohydrology of the Upper Ewaso River Basin, Kenya. MSc thesis, Princeton University, Princeton, USA.
Ginsberg, J.R. (1987) Social organization and mating strategies of an arid-adapted equid, Grevy's zebra. PhD thesis, Princeton University, Princeton, USA.
Hiby, L. (2010) Downloadable software for automated photo-id of Grevy's zebra. Http:// [accessed 18 March 2013].
Hiby, L., Lovell, P., Patil, N., Kumar, N.S., Gopalaswamy, A.M. & Karanth, K.U. (2009) A tiger cannot change its stripes: using a three-dimensional model to match images of living tigers and tiger skins. Biology Letters, 5, 383386.
Huggins, RM. (1991) Some practical aspects of a conditional likelihood approach to capture experiments. Biometrics, 47, 725732.
Hutchinson, J.M.C. & Waser, P.M. (2007) Use, misuse and extensions of ‘ideal gas’ models of animal encounter. Biological Reviews of the Cambridge Philosophical Society, 82, 335359.
Jennelle, C.S., Runge, M.C. & MacKenzie, D.I. (2002) The use of photographic rates to estimate densities of tigers and other cryptic mammals: a comment on misleading conclusions. Animal Conservation, 5, 119120.
Jett, D.A. & Nichols, J.D. (1987) A field comparison of nested grid and trapping web density estimators. Journal of Mammalogy, 68, 888892.
Karanth, K.U. (1995) Estimating tiger Panthera tigris populations from camera-trap data using capture–recapture models. Biological Conservation, 71, 333338.
Karanth, K.U. & Nichols, J.D. (1998) Estimation of tiger densities in India using photographic captures and recaptures. Ecology, 79, 28522862.
Karanth, K.U., Nichols, J.D., Kumar, N.S. & Hines, J.E. (2006) Assessing tiger population dynamics using photographic capture–recapture sampling. Ecology, 87, 29252937.
Lancia, R.A., Nichols, J.D. & Pollock, K.H. (1994) Estimating the number of animals in wildlife populations. In Research and Management Techniques for Wildlife and Habitats, 5th edition (ed. Bookhout, T.A.), pp. 215253. The Wildlife Society, Bethesda, USA.
Lyra-Jorge, M.C., Ciocheti, G., Pivello, V.R. & Meirelles, S.T. (2008) Comparing methods for sampling large- and medium-sized mammals: camera traps and track plots. European Journal of Wildlife Research, 54, 739744.
MacKenzie, D.I., Nichols, J.D., Lachman, G.B., Droege, S., Andrew Royle, J. & Langtimm, C.A. (2002) Estimating site occupancy rates when detection probabilities are less than one. Ecology, 83, 22482255.
Manzo, E., Bartolommei, P., Rowcliffe, J.M. & Cozzolino, R. (2011) Estimation of population density of European pine marten in central Italy using camera trapping. Acta Theriologica, 57, 165172.
Moehlman, P.D., Rubenstein, D.I. & Kebede, F. (2008) Equus grevyi. In IUCN Red List of Threatened Species v. 2010.4. Http:// [accessed 29 January 2011].
Nelson, A. (2003) Status, distribution and structure of Grevy's zebra populations in northern Kenya. MSc thesis, University of Oxford, Oxford, UK.
Nelson, A.P.W. & Williams, S.D. (2000) Grevy's Zebra Survey: Kenya 2000 Final Report. Kenya Wildlife Service, Nairobi, Kenya.
O'Brien, T.G. (2011) Abundance, density and relative abundance: a conceptual framework. In Camera Traps in Animal Ecology: Methods and Analyses (eds O'Connell, A.F., Nichols, J.D. & Karanth, K.U.), pp. 7196. Springer-Verlag, Tokyo, Japan.
O'Brien, T.G., Kinnaird, M.F. & Wibisono, H.T. (2003) Crouching tigers, hidden prey: Sumatran tiger and prey populations in a tropical forest landscape. Animal Conservation, 6, 131139.
Ogutu, J.O., Bhola, N., Piepho, H.P. & Reid, R. (2006) Efficiency of strip- and line-transect surveys of African savanna mammals. Journal of Zoology, 269, 149160.
Otis, D.L., Burnham, K.P., White, G.C. & Anderson, D.R. (1978) Statistical inference from capture data on closed animal populations. Wildlife Monographs, 62, 3135.
Otto, M.C. & Pollock, K.H. (1990) Size bias in line transect sampling: a field test. Biometrics, 46, 239.
Parker, G., Sundaresan, S., Chege, G. & Brien, T.O. (2011) Using sample aerial surveys to estimate the abundance of the endangered Grevy's zebra in northern Kenya. African Journal of Ecology, 49, 5661.
Parmenter, R.R., Yates, T.L., Anderson, D.R., Burnham, K.P., Dunnum, J.L., Franklin, A.B. et al. (2003) Small-mammal density estimation: a field comparison of grid-based vs. web-based density estimators. Ecological Monographs, 73, 126.
Pollock, K.H. (1982) A capture–recapture design robust to unequal probability of capture. The Journal of Wildlife Management, 46, 752757.
Rovero, F. & Marshall, A.R. (2009) Camera trapping photographic rate as an index of density in forest ungulates. Journal of Applied Ecology, 46, 10111017.
Rowcliffe, J.M., Field, J., Turvey, S.T. & Carbone, C. (2008) Estimating animal density using camera traps without the need for individual recognition. Journal of Applied Ecology, 45, 12281236.
Royle, J.A. & Nichols, J.D. (2003) Estimating abundance from repeated presence–absence data or point counts. Ecology, 84, 777790.
Seber, G.A.F. (1982) The Estimation of Animal Abundance and Related Parameters. Macmillan, New York, USA.
Seddon, P.J., Ismail, K., Shobrak, M., Ostrowski, S. & Magin, C. (2003) A comparison of derived population estimate, mark–resighting and distance sampling methods to determine the population size of a desert ungulate, the Arabian oryx. Oryx, 37, 286294.
Silveira, L., Jacomo, A.T.A. & Diniz-Filho, J.A.F. (2003) Camera trap, line transect census and track surveys: a comparative evaluation. Biological Conservation, 114, 351355.
Smith, T.D., Allen, J., Clapham, P.J., Hammond, P.S., Katona, S., Larsen, F. et al. (1999) An ocean-basin-wide mark-recapture study of the North Atlantic humpback whale (Megaptera novaeangliae). Marine Mammal Science, 15, 132.
Sundaresan, S.R., Fischhoff, I.R., Dushoff, J. & Rubenstein, D.I. (2007) Network metrics reveal differences in social organization between two fission-fusion species, Grevy's zebra and onager. Oecologia, 151, 140149.
Thomas, L., Buckland, S.T., Burnham, K.P., Anderson, D.R., Laake, J.L., Borchers, D.L. & Strindberg, S. (2002) Distance sampling. In Encyclopedia of Environmetrics (eds El Shaarawi, A.H. & Piegorsch, W.W.), pp. 544552. John Wiley, Chichester, UK.
Thomas, L., Buckland, S.T., Rexstad, E.A., Laake, J.L., Strindberg, S., Hedley, S.L. et al. (2010) Distance software: design and analysis of distance sampling surveys for estimating population size. Journal of Applied Ecology, 47, 514.
White, G.C. & Burnham, K.P. (1999) Program MARK: survival estimation from populations of marked animals. Bird Study, 47, 120138.
Williams, B.K., Nichols, J.D. & Conroy, M.J. (2002) Analysis and Management of Animal Populations. Academic Press, San Diego, USA.
Wilson, K.R. & Anderson, D.R. (1985) Evaluation of two density estimators of small mammal population size. Journal of Mammalogy, 66, 1321.
Recommend this journal

Email your librarian or administrator to recommend adding this journal to your organisation's collection.

  • ISSN: 0030-6053
  • EISSN: 1365-3008
  • URL: /core/journals/oryx
Please enter your name
Please enter a valid email address
Who would you like to send this to? *



Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed