Fitness is influenced by spatial and temporal heterogeneity in the distribution of multiple, often complementary, resources (Gaillard et al. 2010). A given habitat may not fulfil all the needs of an animal simultaneously, while the functional role of a habitat may fluctuate over time in relation to modifications in an individual’s requirements, or in the value of the habitat per se (Peters et al. 2017; Couriot et al. 2018), for example, driven by vegetation phenology. Mandelik et al. (2012) defined ‘complementary habitat use’ as the use of different habitats at different times by individuals during the course of their daily and seasonal activity (Complementary Habitat Hypothesis, CHH). For many ungulates, for example, accessibility of forage acquisition and protection from predation risk are two key resources that are selected (Hebblewhite and Merril 2009). Indeed, these two types of resource are both positively related to fitness, but often spatially distinct (Benhaiem et al. 2008) and temporally variant, driving complementary use (sensu Mandelik et al. 2012) of ‘open’ vs ‘cover’ habitats (Mysterud and Ostbye 1999, Berryman and Hawkins 2006). For example, forest canopy can provide cover from adverse conditions (e.g., deep snow: Mysterud and Ostbye 1995, Ewald et al. 2014, Ossi et al. 2015) and protection from predators or human disturbance (Gehr et al. 2020), while the understorey provides seasonally rich foraging (i.e., during the vegetation green up and re-growth, Mancinelli et al. 2015). In contrast, open areas may be used by ungulates as a seasonal source of forage, but mainly at night to avoid human disturbance (Godvik et al. 2009, Bonnot et al. 2013, Dupke et al. 2017, Bonnot et al. 2020), or to diminish the risk of ambush predation (Lone et al. 2014; Gehr et al. 2020). Hiding cover may also be seasonally available in open habitats, for example, in summer, when crops are high (Mysterud et al. 1997, Bjørneraas et al. 2011, Bonnot et al. 2013, Dupke et al. 2017).
In temperate ecosystems, open and closed habitats, hence, may represent composites of different resource types, and animals have to alternate between them to satisfy their requirements (see Dunning et al. 1992). Large herbivores use these composites at different spatio-temporal scales, so that the open and closed habitats used along the movement trajectories generate specific patterns of sequential habitat use (De Groeve et al. 2016). For example, at the daily scale, the day-night alternation between open and closed may be linked to activity cycles e.g., foraging vs. resting and ruminating, or forage acquisition-predation risk avoidance trade-off (Hebblewhite and Merril 2009). In turn, daily alternating use of open and closed habitats may vary in relation to seasonal changes in perceived risk (e.g., anthropogenic disturbance, hunting activity; Gehr et al. 2017, Bonnot et al. 2020), vegetation productivity (i.e., green up and senescence, or cultivation / harvesting; Peters et al. 2019), and physiological cycles (e.g., growth, reproduction, dispersal; Bonnot et al. 2018). The understanding of the complementary use of resource composites across spatio-temporal scales by ungulates could inform managers regarding the functional role of different habitat types in human-modified environments. For example, the spatial association between open agricultural habitats and forest patches could support thriving populations of wild ungulates in matrix landscapes (Hewison et al. 2009, Linnell et al. 2020). These concepts are well-addressed within the Complementary Habitat Hypothesis framework and well established in ecological theory, but rarely tested for wild populations.
Here, we considered the patterns of alternation between open and closed habitats in European roe deer (Capreolus capreolus) to test the Complementary Habitat Hypothesis in a highly managed large herbivore (Apollonio et al. 2010) that has successfully adapted to human-modified landscapes (Hewison et al. 2009), including open agricultural areas (Andersen et al. 1998). Roe deer is an ideal model species to test the Complementary Habitat Hypothesis because it occupies a wide range of landscapes (i.e., different arrangements of complementary habitats, sensu Dunnings et al. 1992), from completely forested areas to wide open agroecosystems, exhibiting marked behavioural and ecological plasticity (Morellet et al. 2013). Moreover, roe deer are known to prefer ecotonal and forest habitats, complemented by the use of open habitats (meadows and crops), typically at night (Bonnot et al. 2013, Dupke et al. 2017), in relation to their bimodal crepuscular activity pattern (Pagon et al. 2013; Krop-Benesch et al. 2013; Bonnot et al. 2020). Finally, roe deer are characterized by sex-dependent space use patterns (Malagnino et al. 2021), especially in spring and summer, when adult males display territorial behaviour (Hewison et al. 1998), and females give birth and care for their young.
We applied the Individual Movement-based Sequence Analysis Method (IM-SAM, De Groeve et al. 2020a) to six populations of European roe deer living in contrasting landscapes to evaluate how day-night alternation between closed and open habitats varied across the seasons and environmental contexts. We first hypothesized that day-night alternation between closed and open habitats would mirror the landscape composition (sensu Dunnings et al. 1992; Table 1: Landscape Composition and Structure Hypothesis, LCSH, H1). In particular, we expected that day-night alternation would occur mainly in heterogeneous landscapes (Table 1: H1, P1). Second, we hypothesized that day-night alternation would also vary seasonally, in relation to the phenology of vegetation (Pettorelli et al. 2006), according to the Complementary Habitat Hypothesis (Mandelik et al. 2012). Specifically, we predicted frequent alternation during vegetation green-up to maximize access to high quality food in rich-open habitats, but less alternation in winter, when meadows and crops provide less food resources and are more exposed to extreme weather conditions (e.g., snow cover, wind exposure; Table 1: CHH, H2, P2.1). Also, we hypothesized that day-night alternation between open and closed habitats would vary over seasons according to key life history events, such as reproduction, and other physiological constraints that are linked to sex and age (Table 1: H2, P2.2; Andersen et al. 2000, Bongi et al. 2008). In particular, we predicted that the day-night alternation of females and males should differ the most during the reproductive season, when adult females should alternate less due to the constraints of maternal care (Andersen et al. 2000), while adult males should alternate more to patrol and defend their mating territory (Johannson et al. 1996). Therefore, we expected these seasonal patterns to be more evident in adult males than juveniles linked to reproductive status (Sempéré et al. 1998).
Table 1
Summary of hypotheses and analyses (see footnote for abbreviations).
Hypotheses | Predictions | Analysis | Input data |
H1: Landscape Composition and Structure Hypothesis (LCSH) “Proportion of alternation between closed and open habitats in roe deer populations depends on the proportion and spatial structure of these habitats in the landscape” | P1 Day-night alternation mainly occurs in heterogeneous landscapes | Exploration with mosaic plot and non-parametric statistics based on overall frequencies of habitat use sequences in each population, related to the open/closed composition and structure. | Habitat use sequences, classified according to the following patterns: Homogeneous open (o) Homogeneous closed (c) Random (u) Alternation (a) |
H2: Complementary Habitats Hypothesis (CHH) “Roe deer alternation between habitats varies seasonally, in relation to environmental and roe deer intrinsic cycles” | P2.1. Within a given landscape, alternation between habitats varies seasonally following the phenology of the vegetation, with more day-night alternation during the vegetation green-up in spring. | 1. GAMMs – Alternation (0/1) M1: a ~ s(bw * pop) + pop + 1|ind 2. ANODEV NDVI \(ANODEV=1-\frac{L\left({M}_{NDVI}\right)-L\left({M}_{FULL}\right)}{L\left({M}_{NULL}\right)-L\left({M}_{FULL}\right)}\) Where: MFULL: a ~ bw * pop + 1|ind MNULL: a ~ 1|ind + pop MNDVI: a ~ s(NDVI * pop) + pop + 1|ind | Binomial variable based on the habitat use sequences, defined as follows: c, o, u = 0 a = 1 |
P2.2 Day-night alternation between habitats varies seasonally in relation to intrinsic cycles linked to life-history. | GAMMs – Alternation (0/1) M2: a ~ s( bw * age:sex) + age*sex + 1|ind |
1 a = daily habitat alternation (0/1); pop = population; 1|ind = individual identity as a random effect on intercept; s(bw) = cyclic spline smooth of the biweek; s(NDVI) = cubic spline smooth of the Normalized Difference Vegetation Index; age = fawns & yearlings (< 2 yrs); adults (> 2 yrs); sex = male or female; L = log Likelihood; GAMM; ANODEV