Spreading free-riding snow sports represent a novel serious threat for wildlife

Abstract

Stress generated by humans on wildlife by continuous development of outdoor recreational activities is of increasing concern for biodiversity conservation. Human disturbance often adds to other negative impact factors affecting the dynamics of vulnerable populations. It is not known to which extent the rapidly spreading free-riding snow sports actually elicit detrimental stress (allostatic overload) upon wildlife, nor what the potential associated fitness and survival costs are. Using a non-invasive technique, we evaluated the physiological stress response induced by free-riding snow sports on a declining bird species of Alpine ecosystems. The results of a field experiment in which radiomonitored black grouse (Tetrao tetrix) were actively flushed from their snow burrows once a day during four consecutive days showed an increase in the concentration of faecal stress hormone (corticosterone) metabolites after disturbance. A large-scale comparative analysis across the southwestern Swiss Alps indicated that birds had higher levels of these metabolites in human-disturbed versus undisturbed habitats. Disturbance by snow sport free-riders appears to elevate stress, which potentially represents a new serious threat for wildlife. The fitness and survival costs of allostatic adjustments have yet to be estimated.

Figure 1

Sampling design during the flushing experiment. The light grey-shaded blocks show the approximate average duration of dawn and dusk foraging bouts (above the x-axis, which depicts time) and of igloo resting (below the same axis) in our three free-ranging, experimentally disturbed birds. The arrows depict approximate time of flushing and faeces collection. The dark grey blocks marked with a circle indicate the period during which faecal material collected for the analysis was deposited inside snow burrows. Black line: temporal pattern of a corticosterone excretion curve (redrawn from Baltic et al. 2005) after injection of ³H-corticosterone, as retrieved from the droppings excreted in a chronological sequence by four black grouse in captivity. After a peak within 1–3 h after administration, the concentrations of corticosterone metabolites progressively decreased to almost baseline levels after 24 h.

Figure 2

Stress level faced by wild free-ranging black grouse, measured as the relative concentration of faecal corticosterone metabolites (mean+s.e., in nmol kg⁻¹ of dry material) in droppings collected from individual snow burrows. Data are presented as percentage, with the mean value of the control reference set at 100% (i.e. day before initial flushing in the experiment, or habitat with null or limited disturbance in the comparative approach). (a) Results of a disturbance experiment with three radiomonitored males that were experimentally flushed in winter, once a day over four successive days. Faecal corticosterone metabolite levels increased significantly from control day 0 throughout to day 3 of experiment (linear mixed model), stemming mainly from a difference between the control and the experimental days (χ₁²=7.9, p=0.005), and not from differences among experimental days (χ₂²=0.3, p=0.86); (b) Faecal corticosterone metabolite concentrations in the droppings collected from 32 sites spread across SW Switzerland (totalling 132 igloo samples) with various levels of disturbance by skiers and snowboarders (sample size at column foot), after correction for a seasonal effect. Stress hormone concentrations differed significantly between sites with no or limited human disturbance versus sites with either moderate or high disturbance; the latter two categories did not differ between each other. ^*p<0.05.