Spatial extent of human-intrusion effects on subalpine bird distributions

Abstract

Low levels of human intrusion have become ubiquitous, yet the distance at which they affect bird distributions remains unclear. By testing for changes in bird abundance, we assessed whether low levels of intrusion altered bird distributions within and beyond intruded sites. In Wyoming subalpine forests, we experimentally implemented intrusions within circular 1.0-ha (113-m diameter) sites for 1-2 hr (Snowy Mountains, 1989-1993) or for 5 hr (Pole Mountain, 1991-1993) each week during 10 consecutive weeks of the breeding season. The intrusions did not displace birds during most years, with the following exceptions. Mean abundances for Mountain Chickadee (Parus gambeli) in the Snowy Mountains (1992) and at Pole Mountain (1993), and mean abundances for American Robin (Turdus migratorius) and Hermit Thrush (Catharus guttatus) in the Snowy Mountains (1989), were 46-57% lower within intruded sites than they were within control sites. Intrusion did not influence abundances outside of the 1.0-ha intruded sites. Minimum detectable effect sizes (R2s for the intrusion effect) were 18-32%; effects of this magnitude and larger were detectable with a probability of 0.80. We would therefore have readily detected moderate and large abundance changes had they occurred. The spatial extent of intrusion effects on distributions was thus limited to the actual sites of intrusion, and the effects occurred infrequently. Knowledge about the distance at which low levels of intrusion do and do not alter bird distributions is essential for protecting intrusion-sensitive species and avoiding unnecessary restrictions on landscape use by the public.