Polar Biology (2019) 42:1005–1023 https://doi.org/10.1007/s00300-019-02492-6 ORIGINAL PAPER Long‑term changes in the primary productivity of migratory caribou (Rangifer tarandus) calving grounds and summer pasture on the Quebec‑Labrador Peninsula (Northeastern Canada): the mixed infuences of climate change and caribou herbivory Allen Brett Campeau1 · Gregory J. M. Rickbeil2 · Nicholas C. Coops2 · Steeve D. Côté1 Received: 7 May 2018 / Revised: 27 January 2019 / Accepted: 18 March 2019 / Published online: 30 March 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract At high herbivore density, herbivory can reduce forage abundance, potentially contributing to habitat degradation and driving changes in herbivore population size or range use, in accordance with the exploitation ecosystem hypothesis. The migratory Rivière-George caribou herd (RGH, Rangifer tarandus) of the Quebec-Labrador Peninsula (Canada) has experienced a large decline in population size since the population peaked in the early 1990s, with similarly large changes in seasonal range use. Demographic changes are suspected to have infuenced forage abundance and caribou range use through density-dependent interactions between caribou and their habitat. We used the Normalized Diference Vegetation Index (NDVI) to examine relationships between RGH caribou density and range productivity from 1991 to 2011. A modelling approach was used to control for the response of climate and to isolate the infuence of caribou herbivory on primary productivity. Signifcant negative relationships were identifed between caribou density and primary productivity, after controlling for climatic varia- tion, for the global RGH calving grounds (r2 = 0.54–0.55) and summer range (r2 = 0.42–0.51), but not for the “core” ranges, where caribou density was highest. Positive temporal trends in primary productivity appeared to refect the decline in RGH population size, suggesting vegetation recovery following reductions in caribou abundance. Climate warming (of up to + 1.5 °C per decade) was most responsible for the strong positive trends in primary productivity observed over the 1991–2011 period, but decreases in RGH herbivory likely also contributed to the increases in range productivity. Forage access likely improved over the study period, which may have infuenced RGH range use and habitat selection. Keywords Caribou · Herbivory · Exploitation ecosystem hypothesis · Remote sensing · Primary productivity · Climate change Introduction function according to the exploitation ecosystem hypoth- esis (EEH), which postulates that vegetation is regulated by Arctic terrestrial ecosystems exhibit slow vegetation dynam- top-down herbivory in the absence of signifcant predation ics because of harsh climatic and soil conditions, giving Arc- pressure (Fretwell 1977; Oksanen et al. 1981). The extent to tic herbivores like caribou (or reindeer, Rangifer tarandus) which low-productivity terrestrial ecosystems are regulated an important role in governing ecosystem functioning (Post by herbivory is debatable, however, because of the varying and Pedersen 2008). These low-productivity ecosystems may degrees of predation intensity and the confounding efects of climate on vegetation productivity (Legagneux et al. * 2012). It is therefore necessary to consider the infuences Allen Brett Campeau of climatic and environmental variation when assessing [email protected] plant–herbivore relationships in low-productivity environ- 1 Caribou Ungava, Département de Biologie, Université Laval, ments, especially under a changing climate. For migratory Québec, Québec, Canada caribou herds, migration to relatively predator-free calving 2 Department of Forest Resources Management, Integrated grounds and summer areas helps to reduce predation risk Remote Sensing Studio, University of British Columbia, (Fryxell and Sinclair 1988; Heard et al. 1996; Bergerud and Vancouver, BC, Canada Vol.:(0123456789)1 3 1006 Polar Biology (2019) 42:1005–1023 Luttich 2003); however, the low primary productivity and harassment (Toupin et al. 1996). These behaviours can result slow vegetation dynamics of these areas make them suscep- in caribou occurring at very high densities (several hundred/ tible to overgrazing, especially when caribou are abundant km2), potentially reducing local forage abundance through (Messier et al. 1988; Allard 2003). These environments thus acute foraging and trampling pressure (Manseau et al. 1996). represent appropriate test areas for the EEH prediction of a Intense herbivory may ultimately degrade caribou habitat top-down regulatory role for herbivory in low-productivity and force caribou to move to areas where forage is more areas. A large-scale and long-term examination of herbivory readily accessible, but where predation risk might also be efects can help improve our understanding of complex higher (Bergerud and Luttich 2003). The relative importance plant–herbivore relationships, potentially linking herbivore of predation avoidance and forage access in driving habitat population fuctuations with vegetation degradation and selection may therefore vary in response to caribou popula- recovery (Hansen et al. 2007; Newton et al. 2014). tion dynamics and associated changes in forage availability. Most migratory caribou populations in North America Forage availability has been shown to limit caribou pop- are currently in a state of decline, suggesting a general dete- ulations, with diferent authors attributing primary impor- rioration of habitat conditions and subsequent reductions tance to either the availability of lichens on the winter range in caribou survival and fecundity (Vors and Boyce 2009; (Skogland 1986; Ferguson et al. 2001) or the availability of Festa-Bianchet et al. 2011). The decline phase is arguably green forage (i.e. shrubs, herbs, and grasses) on the sum- part of natural population fuctuations, which for many herds mer range and calving grounds (Crête and Huot 1993; Crête appear to follow the same, predictable pattern: a low-abun- and Doucet 1998). Lichen degradation has been observed dance phase followed by rapid growth, a high-abundance on reindeer winter pastures in Fennoscandia (Colpaert and plateau, and, when habitat conditions deteriorate, a subse- Kumpula 2012; Kumpula et al. 2014) and on the winter quent decline in population size (Messier et al. 1988; Coutu- ranges of wild insular Rangifer herds (Klein 1987; Hansen rier et al. 1990). These fuctuations are thought to be driven et al. 2009), suggesting a strong relationship between cari- by several interacting factors, including predation by grey bou density and winter forage availability. Lichen degrada- wolf (Canis lupus) (Heard et al. 1996; Bergerud and Luttich tion can also be observed on the summer ranges of migra- 2003), climatic variability and extreme weather (Klein and tory caribou herds (Boudreau and Payette 2004; Théau and Shulski 2009; Tyler 2010), and perhaps most importantly, Duguay 2004), but lichen is considered less important than changes in caribou forage availability (Couturier et al. 2009). green forage for parturient and lactating females during the Caribou herbivory can contribute to habitat degradation critical calving and summer foraging periods (Crête et al. throughout caribou annual ranges and thus lead to declines 1990; Crête and Huot 1993). Indeed, observational and in caribou abundance, which could in turn promote vegeta- faecal analysis studies from Alaska (USA) and the Yukon tion recovery and an eventual return to high caribou den- (Canada) have shown that deciduous shrubs, such as wil- sity. The length of time necessary for vegetation recovery low (Salix spp.), amount to nearly 50% of caribou diets in will vary depending on the intensity and duration of cari- the spring and summer months (Trudell and White 1981; bou herbivory and the resilience of the afected vegetation Boertje 1984; Russell et al. 1993). Additionally, there is community, potentially introducing temporal fuctuations in evidence that Arctic Rangifer will primarily select for high forage availability (Newton et al. 2014). Density-dependent vegetation biomass (as opposed to high vegetation qual- feedback between caribou and their preferred forage spe- ity, indexed by nitrogen content and C:N ratio) during the cies may therefore have important consequences for caribou growing season (Van der Wal et al. 2000). The hypothesis population dynamics. of green forage limitation is thus highly tenable for conti- Changes in the size of a caribou population can lead to nental migratory caribou herds with relatively small calving changes in caribou range use through density-dependent grounds or low-productivity summer areas. interactions between caribou, predators, and habitat. Migra- Other work has demonstrated calving ground and summer tory caribou move between diferent seasonal pastures to range degradation during periods of high caribou population avoid predation and to access forage, often congregating in size, with localised feld measurements showing decreases large numbers and demonstrating high fdelity to specifc in green forage biomass (Manseau et al. 1996) in associa- areas or movement corridors (Fryxell and Sinclair 1988; tion with observed changes in caribou population dynamics Bergerud et al. 2008). Parturient females migrate from (Messier et al. 1988; Couturier et al. 1990). The negative boreal forest winter pastures to small, well-defned calv- efects of caribou herbivory on forage species have also been ing grounds on