When Winners Become Losers: Predicted Nonlinear Responses of Arctic Birds to Increasing Woody Vegetation

Abstract

Climate change is facilitating rapid changes in the composition and distribution of vegetation at northern latitudes, raising questions about the responses of wildlife that rely on arctic ecosystems. One widely observed change occurring in arctic tundra ecosystems is an increasing dominance of deciduous shrub vegetation. Our goals were to examine the tolerance of arctic-nesting bird species to existing gradients of vegetation along the boreal forest-tundra ecotone, to predict the abundance of species across different heights and densities of shrubs, and to identify species that will be most or least responsive to ongoing expansion of shrubs in tundra ecosystems. We conducted 1,208 point counts on 12 study blocks from 2012–2014 in northwestern Alaska, using repeated surveys to account for imperfect detection of birds. We considered the importance of shrub height, density of low and tall shrubs (i.e. shrubs >0.5 m tall), percent of ground cover attributed to shrubs (including dwarf shrubs <0.5 m tall), and percent of herbaceous plant cover in predicting bird abundance. Among 17 species considered, only gray-cheeked thrush (Catharus minimus) abundance was associated with the highest values of all shrub metrics in its top predictive model. All other species either declined in abundance in response to one or more shrub metrics or reached a threshold where further increases in shrubs did not contribute to greater abundance. In many instances the relationship between avian abundance and shrubs was nonlinear, with predicted abundance peaking at moderate values of the covariate, then declining at high values. In particular, a large number of species were responsive to increasing values of average shrub height with six species having highest abundance at near-zero values of shrub height and abundance of four other species decreasing once heights reached moderate values (≤ 33 cm). Our findings suggest that increases in shrub cover and density will negatively affect abundance of only a few bird species and may potentially be beneficial for many others. As shrub height increases further, however, a considerable number of tundra bird species will likely find habitat increasingly unsuitable.

Comparison of acoustic recorders and field observers for monitoring tundra bird communities

“…Vegetation density and structure affect the abundance and probability of detection for many species of birds (McShea and Rappole , Pacifici et al ). As expected, bird counts at our study sites were influenced by shrub conditions for all bird groups we evaluated, a pattern we recently confirmed more broadly across the Seward Peninsula (Thompson et al ). Importantly, however, statistically significant interactions between survey method and shrub characteristics were not supported for our models of individual counts.…”

“…We sampled birds and vegetation at 59 sites separated by ≥500 m along 5 transects, which were selected in accessible areas from a larger collection of transects used for long‐term avian monitoring on the Seward Peninsula (McNew et al ; Thompson et al , ). Transects were located near Grant Creek (65.54°N, 165.05°W), Neva Creek (65.48°N, 164.76°W), Bunker Hill (65.15°N, 164.75°W), Horton Creek (65.69°N, 164.03°W), and Fox River (64.84°N, 163.76°W).…”

Plant diversity increases predation by ground‐dwelling invertebrate predators

“…Higher predation rates at higher plant species richness could be caused by reduced intraguild predation (Finke and Denno ), changes in temperature and humidity conditions affecting predator activity (Heck and Crowder ), or reduced foraging efficiencies of top predator, such as birds, reducing predation pressure on intermediate predators (Thompson et al. ).…”

“…Finally, plant diversity effects on predation may also be mediated by the increase in vegetation structural complexity which can cause changes in predation directly and indirectly by affecting prey and predator behavior, movement, and hunting efficiency (Brose 2003, Diehl et al 2013). Higher predation rates at higher plant species richness could be caused by reduced intraguild predation (Finke and Denno 2002), changes in temperature and humidity conditions affecting predator activity (Heck and Crowder 1991), or reduced foraging efficiencies of top predator, such as birds, reducing predation pressure on intermediate predators (Thompson et al 2016).…”

Projecting the future of an alpine ungulate under climate change scenarios

“…Assessing the "winners" and "losers" of climate change is a profound and critical conservation objective that has been addressed in a variety of different ecosystems (Bateman et al, 2015;Brodie et al, 2017;LaRue et al, 2013;Thompson, Handel, Richardson, & McNew, 2016). Yet, the effect of projected future changes in climate on most alpine vertebrates is unclear.…”