Age structure: an indicator to monitor populations of large herbivores

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Most populations of large herbivores are hunted over much of their distribution. The size of these populations over time must be monitored to detect responses to management actions and environmental changes. Here, I first review the limits in precision and accuracy of most methods used to estimate population size in wild herbivores. I then show how density-dependence of age structure can be used to monitor harvested populations, using simulated and empirical data. Accurate estimates of population size are difficult, costly in time and effort, and mostly impracticable at a large scale. Indices are widely used to monitor population size at a large scale, but are rarely tested, and assumptions of constant detection rates over years are questionable. In a recent approach, the effects of population density on habitat quality and individual performance were used as ecological indicators to infer changes in population size over time. Populations of large mammals are strongly age structured. In harvested populations, density-dependence and harvest affect age structure through changes in age-specific vital rates. Because age- and sex-specific vital rates are density-dependent, changes over time in age structure should reflect the response of population to changes in environmental conditions such as density and harvest rate. Time series of age-structure may be used as an ecological indicator to monitor harvested populations, and to infer their demographic trajectory, especially when used within a set of ecological indicators. However, attention needs to be paid in the use of ecological indicators because of the potential bias introduced in data collection by hunter selectivity.