Taylor’s law and quasi-experimental hunting of invasive wild boar and axis deer in a protected area of north-eastern Argentina

Abstract

Context Overabundant ungulate populations frequently cause ecological and economic damage and are difficult to control. Sustained culling efforts at El Palmar National Park have largely reduced wild boar (Sus scrofa) numbers and damage but have failed to halt the ever-increasing abundance of axis deer (Axis axis) for undefined reasons. Multi-year camera-trap surveys indicated a mismatch between deer core activity and the usual timing of hunting sessions. Aim We used the spatial and temporal forms of Taylor’s law (TL, a power relationship between the sample mean and sample variance) to test whether overnight hunting sessions are more effective for culling axis deer and wild boar than are daytime sessions, and assessed whether they modify ungulate harvest composition. Methods We implemented a quasi-experimental trial (i.e. ‘experiments in which units are not assigned to conditions randomly’), including nine alternating blocks of four to seven hunting sessions each (overnight or daytime) in which an average of 42 hunter groups performed controlled shooting over bait from fixed elevated blinds, retrieved, sexed, weighed and measured all quarry. Key results The relationship between the mean and variance of ungulate crude catch per hunting-party session and crude catch per unit effort (CPUE, where effort is measured in hunting-party hours) by hunting shift satisfied the spatial and temporal TL. On average, axis deer catch was 2.39–2.61 times greater in overnight than in daytime sessions, whereas CPUE indices were 1.54–1.73 times greater. For wild boar, overnight sessions returned catches similar to (0.94–1.03×), and a significantly lower CPUE (0.63–0.67×) than for daytime sessions. The harvest indices were substantially aggregated in daytime sessions only, and were consistently skewed towards yearlings or subadults and males, especially among adult deer. Overnight sessions culled proportionally more adults and more pregnant hinds and sows than did daytime sessions, and significantly heavier deer and wild boar males. Conclusions Overnight sessions were substantially more effective for culling deer than were daytime sessions, and selected for individuals with greater fitness. Stage- and male-skewed harvest most likely explains the steady population growth of axis deer despite increasing hunting pressure over a decade. Implications Adjusting the schedule of hunting sessions to time intervals of peak ungulate activity substantially increased the efficiency of management efforts in terms of removal rates and reproductive value of the culled specimens.