Spatial patterns of reproductive success: GLMs
Months of occurrence of reproductively successful deer was positively
related to anthropogenic landscape features, as well as natural
landscape features. Models with petroleum features best explained deer
with fawn occurrence, thus corroborating the multi-state occupancy
models. Occurrence of deer with fawns increased with increasing seismic
line density, 3D seismic line density, pipeline density, and deciduous
forest cover; model 31 (AICw = 0.88) and 29
(AICw = 0.11) together carried 99% of the weight of
evidence (Table 2; Table S3). The effect size (model β coefficients) of
seismic lines on the occurrence of fawns was 100 times greater than the
effect size of the best natural landcover feature: upland deciduous
forests (Fig. 3). Projected across the northeast boreal forest
surrounding the study area, areas of higher probability of deer
reproduction correspond to intensive development (Fig. 4).
Discussion
As mammal distributions shift with climate change, decrease with habitat
loss or capitalize upon change to invade, understanding the features
facilitating reproduction in once barrens landscapes allow us to
elucidate, and address, those mechanisms of change. Global camera-trap
networks coupled with spatial distribution models can yield these
insights.
In our example widespread landscape change from energy extraction is
strongly linked to white-tailed deer reproduction where they have
invaded the western Nearctic boreal forest. Deer invasion of the cold
northern latitudes is consequent to expansion from southern agricultural
areas, a continental phenomenon borne from widespread conversion of
mature forest into early seral vegetation
(Côté et al. 2004;
Heffelfinger 2011). In the last few
decades, new advances in forest harvesting and the dramatic growth of
energy exploration and extraction have radically altered the Alberta
boreal landscape (Pickell, Andison &
Coops 2013; Pickell et al. 2015).
The density of petroleum exploration “seismic” lines in the landscape
had a 100 times greater effect on deer breeding success than did natural
deciduous forest, an important predictor of adult white-tailed deer
individual habitat selection and distribution
(Darlington 2018;
Fisher & Burton 2018;
Fisher et al. 2020). Although we
hypothesized that forest harvesting might play a substantial role
(Fisher & Wilkinson 2005), we found no
evidence to support this; and petroleum extraction features are much
more widespread than forest harvest blocks in this region
(Pickell, Andison & Coops 2013;
Pickell et al. 2015). We conclude
that the 1000s of kilometres of seismic lines, as well as pipelines and
3D seismic lines, spread across the western Nearctic boreal forest play
a significant role in facilitating the northward expansion of
white-tailed deer.
The mechanism for the relationship between linear features and deer
reproductive success is centred on available forage. Nutrition affects
ungulates’ probability of pregnancy, over-winter survival, parturition,
and neonatal survival (Parker, Barboza &
Gillingham 2009; Hewitt 2011). Greater
nutrition from abundant available forage prevents metabolic stress,
increasing deer survivorship and reproductive success
(Hewitt 2011). However forage biomass is
in itself not a good predictor of deer nutrition, as forage distribution
relative to inedible vegetation plays a significant role
(Spalinger & Hobbs 1992). In this
landscape, abundant edible forage is available in linear features
(Finnegan, MacNearney & Pigeon 2018;
Finnegan, Pigeon & MacNearney 2019;
MacDonald et al. 2020), and may be
especially important in spring during green-up, when energetic demands
of gestation are great (Pekins, Smith &
Mautz 1998).
Research on deer pregnancy rates and recruitment suggests that female
age and body condition affect breeding success
(Ozoga, Verme & Bienz 1982;
Ozoga & Verme 1986;
Verme 1989;
DelGiudice, Lenarz & Powell 2007); body
condition, in turn, is primarily a function of nutrition afforded by
available browse (Hewitt 2011). Winter
induces substantial metabolic costs on white-tailed deer, but pregnancy
and lactation induces markedly greater metabolic costs on females
(Pekins, Smith & Mautz 1998;
Therrien et al. 2008;
Ditchkoff 2011). If female deer in this
landscape were metabolically stressed after severe winters, female
mortality, small fawns with low survival
(Ditchkoff 2011), and starvation-induced
abortions (Worden 1992, in Pekins et al. 1998) might be expected to
reduce reproductive success. If the early seral vegetation abundant in
anthropogenic landscape features provides forage subsidies, then
metabolic costs would be offset and reproductive success enhanced. We
contend our evidence here, as well as corroborating past research on
adult deer showing positive links to anthropogenic features
(Darlington 2018;
Fisher & Burton 2018), strongly infers
that landscape change is enhancing breeding success and hence,
facilitating and maintaining boreal deer invasions.