Introduction
Over the past century the range of many species has changed, often
attributed to climate change and land cover modification (Thomas 2010,
Laliberte and Ripple 2004, Walther et al 2002). A species can
respond to environmental changes by exploiting resources at the
extremities of its niche breadth (Sexton et al. 2017), by
phenotypic plasticity (Valladares et al. 2014, Nicotra et
al. 2010), or by adaptation (Williams et al. 2008). However, the
rate at which current conditions are changing might make adaptation
impossible for many species because the process of natural selection is
too slow (Davis and Shaw 2001). Consequently, species will have to track
their bioclimatic niche (Visser 2008) the ability to do so will be an
essential characteristic influencing future biodiversity (Bell and
Gonzalez 2011, Schloss et al. 2012, Travis et al. 2013).
Terrestrial species that have ranges near the poles will be limited in
their ability to track climate because they are limited by the
availability of space to move to higher latitudes (Kerr and Packer
1998). Therefore, many unique cold adapted species will eventually
perish unless they somehow adapt to much warmer conditions and to new
biotic interactions. Understanding how and why the warmer range edge of
cold adapted species has been changing would help us in making better
informed conservation decisions, since anthropogenic change is not
slowing down.
The Canada lynx (Lynx canadensis ) is an iconic carnivore that
largely resides in the boreal forest of North America and its northern
range has some expansion potential into northern taiga landscapes but is
generally bounded by tundra and the Arctic Ocean (Poole 2003). The lynx
is a habitat specialist because it almost exclusively preys on snowshoe
hares in the boreal forest (O’Donoghue et al. 1998).
Consequently, its population dynamics are highly coupled to the 8-11
year population cycle of the snowshoe hare, mirroring it with a 1-2 year
delay (Poole 2003). Since presettlement times the Canada lynx range has
contracted by 40% (Laliberte and Ripple 2004). However, most of this
range reduction took place prior to the 20th century
and was attributed to unregulated harvest and habitat loss due to land
clearing during European colonization (de Vos 1952, de Vos 1964, Hovinget al. 2003, McKelvey 2000, Poole 2003).
Canada lynx are predominantly found in areas where snowshoe hare density
is above 0.5 per hectare (Zahratka and Shenk 2008, Hodges et al.2009, Berg et al. 2012, Ivan et al. 2014). In the southern
periphery of the lynx range, hare population densities have declined
compared to historic levels (Aubry et al. 2000, Hodges 2000,
Murray 2000) and this most likely accounts for the contraction of the
lynx from its historic range (Poole 2003). Following years with high
peak hare abundance, Canada lynx appear to migrate from range core to
range periphery as a result of density dependent dispersal (McKelvey et
al. 2000). Such dispersal pulses might lead to higher occupancy of the
southern range periphery (McKelvey et al 2000, Murray et al.2008). Consequently, lower peaks in hare abundance might decrease the
likelihood of dispersal of lynx into the southern periphery (Poole
2003). Southern dispersal might also be limited in some locations by
connectivity with the range core (Ruggiero et al. 2000, Buskirk
2000, Walpole et al. 2012).
The warming climate might
indirectly impact the lynx through its main food source the snowshoe
hare. The timing between molt and season change for the snowshoe hare is
important in decreasing predation rates (Zimova et al. 2016). A
changing snow regime could increase snowshoe hare predation rates by
increasing the rate of mismatch between snowshoe hare molt and season
change. Increased predation rates might also reduce the amplitude of the
hare cycle (Krebs 2010).
Climate change will also open formerly inhospitable habitat to new
species in the lynx range. Bobcats (Lynx rufus ) and coyotes
(Canis latrans ) have smaller feet than Canada lynx, consequently
they cannot support as much weight as lynx in deep snow without sinking
(Parker et al. 1984). This might be one factor that has hindered
the bobcat from invading Canada lynx territory in the past (Marston
1942, McCord 1974, Murray et al. 2008, Parker et al.1984). However, since the climate is warming, and snow depths across the
southern periphery of the lynx range are shallower, southern competitors
might be less hindered by snow, increasing their competitive potential
(Buskirk et al. 2000, Ruediger et al. 2000, Scullyet al. 2018). In fact, Parker et al. (1984) found that
after several years of low snow the bobcat invaded the lowlands of Cape
Breton while the Canada lynx left the area. Marrotte et al.(2020a) found that deep winter snow in the Great Lakes region limited
bobcat expansion northward, suggesting that greater expansion will
result from additional climate warming.
The lynx once occurred in 24 of the United States (McKelvey 2000), but
currently only occurs in 7 (US Fish and Wildlife Service 2017). As a
result, the lynx is designated as ‘threatened’ in the contiguous United
States (US Fish and Wildlife Service 2000). Despite the range
contraction, its protection status is being debated and it might be
removed from the list of endangered species in the United States. In
Canada, the lynx occupies 95% of its historic range (Poole 2003).
However, it is designated as provincially endangered in Nova Scotia
(Parker 2001), and New Brunswick (New Brunswick Endangered Species
Regulation 2013) and was extirpated from Prince Edward Island (Poole
2003). Further analysis has demonstrated that the range of lynx in
British Columbia has been stable since the 1930s (Gooliaff and Hodges
2018). In contrast, the lynx range in Ontario appears to have contracted
northwards by 175 km from 1972 to 2010 (Koen et al. 2014).
We estimated the past extent of the Canada lynx southern range in
Ontario, Canada using harvest records and then determined whether the
spatial-temporal patterns could be attributed to snowshoe hare and
boreal lynx population dynamics, connectivity, climate, land use and
competition. We predicted that years with low Canada lynx abundance in
the boreal forest were associated with a reduction in the extent of the
southern lynx range. We also predicted that areas with high human
disturbance, shallow snow, presence of competitors, and with low
connectivity to boreal lynx populations were less likely to be part of
the southern range.