An assessment of the reliability of ground counts of ungulate populations

 Ground counts are often used to monitor ungulate populations. The minus point is that ground counts have low precision and often underestimate population size.

The authors of this paper assessed the reliability of ibex Capra ibex counts as performed in French national parks. They analysed up to 23 years of annual censuses of six ibex populations. The population growth rate obtained from census data (estimated by use of four different methods) was compared with the growth rate calculated from a demographic model including parameters estimated from capture-mark-recapture methods.

The results suggested that ground counts can monitor trends in population size of ibex, provided the occasional undercounts are identified. The authors say substantial undercounts in some years led to biologically impossible values of yearly population growth (λ > 1.35) and, in the longest time series available, to marked autocorrelations in counts.

The researchers come up with the recommendation that managers should replicate counts within the same year to check for underestimated counts. To reduce errors, population biologists analyzing time series of ungulate counts should check the plausibility of annual growth rates estimated from two consecutive counts.

Can ground counts reliably monitor ibex Capra ibexpopulations?

Emilie Largo, Jean-Michel Gaillard, Marco Festa-Bianchet, Carole Toïgo, Bruno Bassano, Hervé Cortot, Gilles Farny, Benoît Lequette, Dominique Gauthier & Jean-Pierre Martinot

Wildl. Biol. 14: 489-499.