Manuscript / Can ground counts reliably monitor ibex populations?
Can ground counts reliably monitor ibex populations?
Emilie Largo, Jean-Michel Gaillard, Marco Festa-Bianchet, Carole Toïgo, Bruno Bassano, Hervé Cortot, Gilles Farny, Benoît Lequette, Dominique Gauthier & Jean-Pierre Martinot
Although ground counts are often used to monitor ungulate populations, several studies show that counts of ungulates have low precision and often under-estimate population size. We assessed the reliability of ibex counts as performed in French National Parks, by analyzing up to 23 years of annual censuses of six ibex populations for which a subset of animals were individually marked. We compared the population growth rate obtained from census data (estimated from four different methods) with the growth rate calculated from a demographic model including parameters estimated from Capture-Mark-Recapture methods. The correlations between count-based estimates and growth rate obtained from demographic models were adequate to suggest that ground counts can monitor trends in population size of ibex, provided that the occasional under-counts are identified. Substantial undercounts in some years led to biologically impossible values of yearly population growth (l>1.35) and, in the longest time series available, to marked autocorrelations in counts. Managers should replicate counts within the same year to check for underestimated counts. To reduce errors, population biologists analysing time series of ungulate counts should check the plausibility of annual growth rates estimated from two consecutive counts.
Key words: capture-mark-recapture, census, demography, density estimate, monitoring, population growth, ungulate