Microrefuges and the occurrence of thermal specialists: implications for wildlife persistence amidst changing temperatures

Abstract

Background: Contemporary climate change is affecting nearly all biomes, causing shifts in animal distributions, phenology, and persistence. Favorable microclimates may buffer organisms against rapid changes in climate, thereby allowing time for populations to adapt. The degree to which microclimates facilitate the local persistence of climate-sensitive species, however, is largely an open question. We addressed the importance of microrefuges in mammalian thermal specialists, using the American pika (Ochotona princeps) as a model organism. Pikas are sensitive to ambient temperatures, and are active year-round in the alpine where conditions are highly variable. We tested four hypotheses about the relationship between microrefuges and pika occurrence: 1) Local-habitat Hypothesis (local-habitat conditions are paramount, regardless of microrefuge); 2) Surface-temperature Hypothesis (surrounding temperatures, unmoderated by microrefuge, best predict occurrence); 3) Interstitial-temperature Hypothesis (temperatures within microrefuges best predict occurrence), and 4) Microrefuge Hypothesis (the degree to which microrefuges moderate the surrounding temperature facilitates occurrence, regardless of other habitat characteristics). We examined pika occurrence at 146 sites across an elevational gradient. We quantified pika presence, physiographic habitat characteristics and forage availability at each site, and deployed paired temperature loggers at a subset of sites to measure surface and subterranean temperatures. Results: We found strong support for the Microrefuge Hypothesis. Pikas were more likely to occur at sites where the subsurface environment substantially moderated surface temperatures, especially during the warm season. Microrefugium was the strongest predictor of pika occurrence, independent of other critical habitat characteristics, such as forage availability. Conclusions: By modulating surface temperatures, microrefuges may strongly influence where temperature-limited animals persist in rapidly warming environments. As climate change continues to manifest, efforts to understand the changing dynamics of animal-habitat relationships will be enhanced by considering the quality of microrefuges.

Variation in subsurface thermal characteristics of microrefuges used by range core and peripheral populations of the American pika (Ochotona princeps)

“…Even after accounting for within‐site variation and variation in habitat conditions, the occupied sites and sites in preferentially used substrates (i.e., pahoehoe and talus; Rodhouse et al., ; Jeffress et al., ; Rodhouse & Hovland, ) exhibited considerably more thermal buffering. These results add to the growing evidence (Hall et al., ; Henry et al., ; Millar & Westfall, ; Millar et al., , ; Rodhouse et al., ; Varner & Dearing, ; Wilkening et al., ) that thermally stable subsurface microrefuges decoupled from regional climates are the mechanism by which American pikas persist in otherwise inhospitable conditions along the range periphery. Our study advances this line of inquiry beyond previous studies that have shown that subsurface microclimates are decoupled from aboveground climates (Hall et al., ; Henry et al., ; Millar et al., , ; Shi et al., ; Varner & Dearing, ) and dispersal habitats (Millar et al., ) by demonstrating important differences in microclimates among used and unused sites and substrate types.…”

Behavioural plasticity modulates temperature‐related constraints on foraging time for a montane mammal

“…Contrary to our predictions, pikas exposed to higher daytime temperatures did not increase food-collecting activity at night. At night, interstitial spaces in the talus are warmer than ambient temperatures (Hall et al, 2016;Millar, Westfall, & Delany, 2016). Nocturnally active pikas may also be exposed to increased thermoregulatory costs.…”

“…We selected 10 sites from a candidate set of 25 where both pikas were present (determined from occurrence surveys) and we had year-round, in situ thermal profiles -2012Hall, Chalfoun, Beever, & Loosen, 2016). We evaluated temperature data from loggers deployed 50 cm below the surface of the talus at each candidate location and selected sites that encompassed a range of temperatures during the snow-free period (1 June-30 September; range of mean temperatures at selected sites = 8.5-16.…”

“…We wired each logger to the underside of a rock, such that it was not exposed to direct sunlight, and sealed it in a water-tight 5-g jar made of clear plastic (Beever, Ray, Mote, & Wilkening, 2010;Hall et al, 2016). The sensors were intended to capture the temperatures that pikas were experiencing on the talus surface, rather than ambient conditions.…”

Beyond exposure, sensitivity and adaptive capacity: a response based ecological framework to assess species climate change vulnerability

“…Past work has described migration, tolerance, evolutionary adaptation, and extinction as the range of species responses to climate change [16,26]. We expand on these species responses by considering an increasingly clear and distinct microrefugia response [27][28][29], and by dropping extinction as a response, as it is effectively the consequence of a lack of response. In this ecologicallybased formulation, species vulnerability is the degree to which a species is unable to exhibit any of the four responses necessary for persistence: 1) the migration response, in which a species follows (in a biogeographical sense) its moving climatic limits through dispersal and establishment in new areas where its niche becomes available [30][31][32]; 2) the microrefugia response, when the niche of a species persists in locations that retain suitable climate characteristics [27,[33][34][35] within a greater area that becomes unsuitable at the macro-climatic scale; 3) the toleration response, a situation when climate shifts fit within a species niche breadth [36][37][38][39]; 4) and the evolutionary adaptation response, when a species alters its niche to withstand changes in climate through natural selection [40,41].…”