DISCUSSION
The luxury effect – the relationship between urban biodiversity and
human affluence – observed globally (Leong, Dunn and Trautwein 2018,
Magle, Fidino, Sander, Rohnke, Larson, Gallo, Kay, Lehrer, Murray and
Adalsteinsson 2021) manifests here in the behavioral resource selection
by urban black-tailed deer. The greatest drivers of deer selection in
this former savannah urban landscape are residential lot size and
vegetation greenness (i.e., NDVI). Both are products of wealth. Oak Bay
is a highly affluent neighborhood with 2022 house prices ranging from
$1-12 million CAD (mls.ca). Large lots (mean = 3601
m2, s.d. = 1609, range 2530-14766) are highly
manicured, with gardens offering abundant resource subsidies. Although
the smaller “high density” lots in Oak Bay are not very small (mean =
679 m2, s.d. = 176, range 202-1052) and still heavily
gardened and watered they are still not as strongly selected as the
large lots.
Deer selection for areas with high-productivity vegetation highlights
the importance of high-quality forage availability in urban deer
resource selection. Higher vegetation greenness is represented in areas
with healthy and dense vegetation, and is inversely linked to dry or
drought conditions (Drisya and Roshni 2018). We observed a tight
correlation with vegetation greenness (measured as NDVI) and the
normalized difference moisture index (Supplementary Information, Fig.
1), with higher soil moisture associated with higher vegetation
greenness. The unirrigated regions of this landscape experience extended
summer drought; Garry Oak meadows provide dry, nutrient-poor vegetation
(Barlow 2017, Fuchs 2001, GOERT 2021, Pellatt and Gedalof 2014, Pellatt,
McCoy and Mathewes 2015). Parsing apart the effects of urban development
is difficult in any system, as pre-development records are typically
scant; but historically Cowan (1945) observed “from the standpoint of
deer the food potential of a west coast climax forest is so low that
over vast areas deer are almost non-existent”. At that time dense deer
were only observed in regenerating forestry clearcuts. In the dry summer
months, water was noted as particularly in demand, with a strong
selection for plants in hygric areas (Cowan 1945). Thus, following
European colonization the conversion of historically well-drained,
drought-resistant Garry oak savannah ecosystems maintained by Indigenous
peoples (Barlow, Pellatt and Kohfeld 2021, McCune, Pellatt and Vellend
2013, Pellatt and Gedalof 2014), to modern watered lawns and
high-productivity vegetation is a key contributor to urban black-tailed
deer abundance. This supports other research that shows the luxury
effect is most commonly observed in arid and semi-arid landscapes,
linked to irrigation and diverse plant communities (Leong, Dunn and
Trautwein 2018).
Beyond the effects of natural vegetation, large residential lots are
also a highly significant predictor of urban deer habitat-use in our
study area. Large-sized residential lots are embedded in neighborhoods
of similar-sized lots, generating low housing density. Deer are
therefore likely responding to the decreased human disturbance
associated with these neighborhoods, as well as higher densities of
high-productivity vegetation associated with larger residential lots,
and vegetative cover on lots. Neighborhoods with larger lot sizes and
high investment into landscaping – features generated by financial
affluence – are therefore more likely to experience higher deer use,
and to perceive these interactions negatively (Wine et al. 2015).
Concentrated, high-quality resources mean deer can maintain smaller home
ranges, which we observed here. Female urban deer home ranges were a
quarter of the size of wild females in nearby Washington State (Bender,
Anderson and Lewis 2004). Small home ranges in suburban environments
have been noted by Happe (1982) and Bender, Anderson and Lewis (2004).
Ideal free distribution theory suggests animals occupy the smallest
areas that provide the resources they require (Fretwell 1969, Harestad
and Bunnel 1979), and this phenomenon has been noted for other deer
species (Said and Servanty 2005). In natural landscapes black-tailed
deer strongly select shrubs (as opposed to graminoids or forbs) in early
successional conifer stands (14-20 years) (Hanley 1984), and the
abundance of hygric shrubs throughout large residential lots offers
substantial subsidies that keep home-ranges small. People living in
large residential lots tend to have the highest per capita income yet
react most negatively to human-wildlife conflict (Wine et al. 2015.
Thus, citizen reports of hyperabundant deer stem from peoples’
negativity bias (Jacobs & Vaske 2019; Buijs & Jacobs 2021) and
repeated sightings of deer.
Citizen sightings suggested urban deer were strongly associated with
golf courses and green spaces and our analysis corroborates this
observation. Natural green spaces remain semi-arid oak savannahs but
provide abundant escape cover; golf courses offer abundant well-watered
grazing opportunities. Both parks and golf courses are elements of
wealthy landscapes (Chamberlain, Henry, Reynolds, Caprio and Amar 2019,
Schell, Dyson, Fuentes, Des Roches, Harris, Miller, Woelfle-Erskine and
Lambert 2020), and the relationship between affluence and negativity
bias towards urban wildlife (Wine et al. 2015) are bound to make these
citizen sightings noteworthy.
Habitat selection by urban deer occurs on a predator-free backdrop. No
wolves (Canis lupus ), cougars (Puma concolor ), or black
bears (Ursus americanus ) – primary prey of Columbia black-tailed
deer – live in these urban landscapes, as none were ever detected on
cameras. Wolves in particular regulate black-tailed deer; in wild
landscapes on northern Vancouver Island wolf control was associated with
increased deer numbers; modelling (and mortality observations, (McNay
and Voller 1995)) suggested that increased recruitment was the primary
mechanism (Hatter and Janz 1994). Therefore, a predator-free urban
environment is expected to markedly increase recruitment (and hence
abundance) over natural landscapes. However, predator regulation of
black-tailed deer populations is mediated by forage availability and
proximity to carrying capacity; dense deer populations are less affected
by predators (Ballard, Lutz, Keegan, Carpenter and deVos Jr 2001). Even
if urban environments did allow predators, the abundant resource
subsidies are likely to sustain abundant deer populations.