Introduction
Reproduction is vital to population persistence and distribution
dynamics. Reproductive success is tightly linked to the quality and
spatial distribution of available suitable habitat
(Pulliam & Danielson 1991;
Kurki et al. 2000) and so
anthropogenic landscape change can markedly alter a species’ spatial
distribution. These effects are typically negative, through
fragmentation and habitat loss (Fahrig
1997; Fahrig 2002;
Fahrig 2003) but are positive for some
species, facilitating range expansions or invasions
(Ewers & Didham 2006;
Didham et al. 2007). Linking
spatial variability in reproductive success with landscape change (or
disturbance) is key to understanding mechanisms of invasion and range
shifts, an increasingly important endeavor under climate change
(Lawler et al. 2008;
Lawler et al. 2009).
Quantifying spatial variation in reproductive success has been mostly
limited to taxa with stationary offspring such as plants
(Muñoz & Arroyo 2006) and nesting birds
(Rosenberg, Swindle & Anthony 2003;
León-Ortega et al. 2017). Mammals
are much harder to quantify due to their large size, widespread ranges,
and vagile young. Camera trapping (Burtonet al. 2015; Steenweg et
al. 2016) can bridge this data gap, generating data on mammalian
distribution and density. Many mammal species keep young at heel during
early maternal care and this state can can be likewise observed with
camera traps. Applied to camera data for grizzly bears
(Fisher, Wheatley & Mackenzie 2014) and
European brown bears (Burton et al.2018), we showed how spatial variation in reproductive success can be
modelled to identify landscape mechanisms affecting success. Though
further elaborated since (MacKenzieet al. 2017) the diverse opportunities of this approach have yet
to be widely realized. Here, we illustrate how camera trap data can help
infer mechanisms of species invasion and range expansion, using an
example from the Nearctic boreal forest.
Boreal landscapes have been markedly changed by widespread and
economically important resource extraction
(Schindler & Lee 2010;
Venier et al. 2014). The epicenter
of change are Canada’s oil sands, the third largest global oil deposit
and a driver of global economies (Bayoumi
& Mhleisen 2006). Petroleum exploration and extraction create an
altered landscape without analogs
(Schneider, Dyer & Parks 2006;
Pickell, Andison & Coops 2013;
Pickell et al. 2015). Landscape
change affects the entire boreal forest mammal community
(Fisher & Burton 2018), but most notably
manifest in woodland caribou declines (Rangifer tarandus )
(Hervieux et al. 2013;
Hebblewhite 2017). Wolf predation is a
primary cause (Boutin et al. 2012),
with wolf populations bolstered by high-density invading white-tailed
deer (deer; Odocoileus virginianus )
(Latham et al. 2011;
Latham et al. 2013).
White-tailed deer range expansion is a pan-continental phenomenon
(Laliberte & Ripple 2004;
Heffelfinger 2011) impacting entire
ecosystems (Côté et al. 2004).
Research on deer expansion south of the boreal has focused on population
biology (DeYoung 2011), movement
(Beier & McCullough 1990), and predation
(Ballard et al. 2001). Boreal deer
invasion has been linked to landscape and climate change
(Dawe, Bayne & Boutin 2014;
Fisher & Burton 2018;
Fisher et al. 2020) but the
mechanisms remain unidentified. We sought to examine whether
anthropogenic landscape change is linked to spatial patterns of deer
reproductive success, as a possible mechanism of boreal forest
invasion.
Deer balance energy intake from early-seral deciduous forage
(Ditchkoff 2011) with metabolic demands
markedly increased by cold temperatures and deep snow, historically
limiting white-tailed deer range (Parker,
Barboza & Gillingham 2009; Hewitt
2011). In the boreal, climate change has produced warmer winters
(Karl & Trenberth 2003); concurrently,
landscape change has generated more abundant early-successional
vegetation (Finnegan, MacNearney & Pigeon
2018; Finnegan, Pigeon & MacNearney
2019; MacDonald et al. 2020) that
is strongly spatially linked to deer abundance and persistence
(Fisher et al. 2020). Deer
mortality risk is greatest in the first year of life
(Lesage et al. 2001), decreasing
markedly for 1-2 year-olds (Delgiudiceet al. 2006). Fawn growth and survival is largely based on
maternal body condition, governed by food availability
(Therrien et al. 2008), so
examining how spatial resource availability contributes to breeding
success within the first year helps us understand how landscape change
contributes to boreal deer expansion.
We hypothesized that anthropogenic landscape change in the northern
boreal forest is providing resource subsidies that bolster reproductive
success for invading white-tailed deer. If true, we predicted that
anthropogenic features representing conversion of mature forest to early
seral vegetation would explain variability in the spatial distribution
of deer reproductive success. We define reproductive success as a deer
occurrence with at least one attendant fawn in the summer months. This
requires that a female achieve oestrus, breed, produce offspring, and
maintain that offspring into the summer months, thus drawing close to
recruitment–and is a measure
that can be consistently applied to all mammal species with attendant
young at heel.