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.