Investigating the Dynamics of Elk Population Size and Body Mass in a Seasonal Environment Using a Mechanistic Integral Projection Model

Environmentally mediated changes in body size often underlie population responses to environmental change, yet this is not a universal phenomenon. Understanding when phenotypic change underlies population responses to environmental change is important for obtaining insights and robust predictions of population dynamics in a changing world. We develop a dynamic integral projection model that mechanistically links environmental conditions to demographic rates and phenotypic traits (body size) via changes in resource availability and individual energetics. We apply the model to the northern Yellowstone elk population and explore population responses to changing patterns of seasonality, incorporating the interdependence of growth, demography, and density-dependent processes operating through population feedback on available resources. We found that small changes in body size distributions can have large impacts on population dynamics but need not cause population responses to environmental change. Environmental changes that altered demographic rates directly, via increasing or decreasing resource availability, led to large population impacts in the absence of substantial changes to body size distributions. In contrast, environmentally driven shifts in body size distributions could occur with little consequence for population dynamics when the effect of environmental change on resource availability was small and seasonally restricted and when strong density-dependent processes counteracted expected population responses. These findings highlight that a robust understanding of how associations between body size and demography influence population responses to environmental change will require knowledge of the shape of the relationship between phenotypic distributions and vital rates, the population status with regard to its carrying capacity, and importantly the nature of the environmentally driven change in body size and carrying capacity.

Animal-unit equivalence of bison, wapiti, and mule deer in the aspen parkland of Alberta

Animal-unit equivalences needed for joint-stocking calculations of ungulates are rarely studied because of difficulties deriving accurate estimates of forage intake. We used indigestible internal and external markers (double n-alkane ratio) to determine animal-unit equivalence of bison ( Bison bison (L., 1758)), wapiti ( Cervus elaphus canadensis L., 1758), and mule deer ( Odocoileus hemionus (Rafinesque, 1817)) in the aspen parkland of east-central Alberta. We hypothesized that dry matter intake would differ among species in absolute terms (kg·day–1) but not when expressed as percent body mass (%BM) or when related to metabolic mass (g·kg–1·day–1 BM0.75). We administered slow-release n-alkane capsules to 12 bison, 3 wapiti, and 11 mule deer and released them into separate 3–7 ha paddocks during summer and fall. Intake of dry matter by bison, wapiti, and mule deer differed significantly among species when expressed as total intake (p = 0.009) and %BM (p = 0.014) but not when related to metabolic mass (p = 0.237). Dry matter intake related to metabolic mass did not differ between bison and mule deer in any comparison despite a 6.8-fold difference in body size, suggesting that intake scales to metabolic mass, and is the preferred expression of animal-unit equivalents.

Using n-alkane markers to estimate forage intake of mule deer

Dry matter intake (DMI) of free-ranging mule deer ( Odocoileus hemionus (Rafinesque, 1817)) in the aspen parkland of east-central Alberta was estimated using the double n-alkane ratio and bite count methods. Eleven female mule deer were given an intraruminal capsule containing synthetic n-alkanes and released into 4–8 ha paddocks. Known concentration of even-chained carbon (C32) was released from the capsules and recovered 7–10 days later from freshly voided faeces. Odd carbon chains of adjacent length (C31:C33) were extracted from vegetation samples gathered during behavioural observations of four tame deer. Calculations from the pairings (C31:C32; C33:C32) provided estimates of DMI that ranged from 1.29 to 2.73 kg/day. DMI was highest during autumn, possibly to prepare for increased energy demands for the upcoming winter. No differences were found in seasonal or annual comparisons of bite rates, but bite sizes differed in all comparisons, suggesting bite size was a critical factor affecting consumption rates for mule deer on northern ranges. Maximum bite sizes rose from 49 ± 5 mg/bite in July to 213 ± 58 mg/bite in October. Maximum consumption rates were in October of both years (3.6–4.4 g/min) and occasionally approached theoretical maxima (14.3–15.6 g/min).

Prolonged winter undernutrition and the interpretation of urinary allantoin:creatinine ratios in white-tailed deer

The urinary allantoin:creatinine (A:C) ratio (expressed in micromoles of allantoin to micromoles of creatinine) has shown potential as an index of recent winter energy intake in preliminary controlled studies of elk (Cervus elaphus) involving mild condition deterioration (up to 11% loss of body mass). To ensure reliable nutritional assessments of free-ranging cervids by measuring A:C ratios of urine in snow, it is essential to extend this work. We assessed the effect of moderate and severe winter nutritional restriction on urinary A:C ratios of captive white-tailed deer (Odocoileus virginianus) that lost up to 32% body mass and related these ratios to metabolizable energy intake (MEI), body-mass loss, and other reported nutritional indicators. Deer in the control group were fed a low-protein, low-energy diet ad libitum, whereas deer in the treatment group were fed restricted amounts of the same diet. MEI was below the winter maintenance requirement for all deer, but was lower (P = 0.029) in treatment deer than in control deer. Percent body-mass loss differed between the two groups as the study progressed, and represented the full range of physiological tolerance (0-32% loss). Mean A:C ratios of control deer, which lost up to 17.4% body mass, showed a slight increasing (P = 0.086) trend, whereas initially similar A:C ratios of severely restricted deer increased (P = 0.0002) markedly by the eighth week (0.52 vs. 0.09 µmol:µmol). The urinary A:C ratio was not related (P = 0.839) to recent (2 days prior to urine sampling) MEI, but there was a marginally significant relation (r2 = 0.42, P = 0.110) between the A:C ratio and cumulative percent mass loss. The urinary A:C ratio was directly related to urinary urea nitrogen:creatinine (r2 = 0.59, P < 0.0001) and 3-methylhistidine:creatinine (r2 = 0.43, P < 0.0001) ratios. This study confirms that elevated and increasing A:C ratios may be due either to increasing energy intake or to accelerated tissue catabolism and increased endogenous contributions to urinary allantoin excretion.

Seasonal foraging behavioural compensation in reproductive wapiti hinds (Cervus elaphus canadensis)

Captive wapiti hinds were observed during seven periods between June 1996 and November 1997. We investigated their activity and foraging behaviour on two pastures, heavily and lightly grazed, during four phases of the reproductive cycle (early gestation, late gestation, peak lactation, and late lactation). Within season, differences in forage availability between pastures had little apparent effect on wapiti feeding behaviour (p0.05). However, within each pasture, hinds tended to select locations of higher phytomass than the pasture average. Among seasons, effects of forage availability on feeding behaviour were pronounced. Hinds grazed longest in late lactation (12.8 h/day), when they spent 94% of their active time foraging, whereas during early gestation they foraged fewer hours (8.2 h/day) and less intensively (66% of active time). The longest foraging bouts also occurred in late lactation (100 min) and decreased linearly as the number of bouts per day increased (R(2)=0.88). The annual peak bite rate (BR) was in late lactation (62 bites/min), whereas the annual nadir occurred in early gestation (37 bites/min). Smallest bite sizes (BS) (mg) were recorded in late gestation (127 mg), and increased linearly with forage availability (R(2)=0.46), with largest BS occurring during peak lactation (280 mg). This study demonstrated how seasonal modifications in activity and foraging behaviour enabled gestating and lactating wapiti hinds to satisfy their changing nutritional requirements on seasonal pastures. Knowledge of behavioural compensation in response to nutritional demand and pasture conditions will be useful in designing supplementation program for farmed wapiti.

Evaluation of the urinary allantoin:creatinine ratio as a nutritional index for elk

We conducted experiments with captive female elk (Cervus elaphus) to explore the utility of allantoin:creatinine (A:C) ratios as a noninvasive index of nutrition in free-ranging ungulates. Pens of animals were assigned to one of four dietary treatments, each treatment being given to 6 animals. One treatment involved gradually increasing the daily metabolizable energy intake (MEI) from approximately 650 to 1200 kJ metabolizable energy (ME)/kg body mass (BM)0.75in weekly increments over 8 weeks. In a second treatment, daily MEI was decreased in a similar manner from approximately 900 to 250 kJ ME/kg BM0.75. Urine samples (96 in total) were collected from each animal in these treatments 5 days after each change in diet. Two additional treatments involved relatively large and abrupt changes in daily diet ranging from 120 to 500 kJ ME/kg BM0.75, with successive diets being alternately increased and decreased at 2-week intervals for 10 weeks. Urine samples (179 in total) were collected from each animal 3, 5, and 13 days after a dietary change was initiated. The underlying relation between A:C ratio and MEI was consistent among the four treatments and indicated that the urinary A:C ratio is a reliable index of dietary intake in elk across a variety of dietary regimes. A:C ratios responded relatively quickly to changes in diet, indexing the dietary intake of the animals during a several-day period immediately prior to the time the urine was voided. The results of this and previous studies indicate that the urinary A:C ratio has significant potential as a nutritional index for wild ruminants; however, we recommend additional research before the use of A:C ratios is integrated into ecological or management applications.

Forage intake by black-tailed deer in a natural environment: bout dynamics

We examined the within-bout and seasonal dynamics of intake rate for free-ranging tractable black-tailed deer (Odocoileus hemionus sitkensis) throughout a 2-year period. Based on log frequency analyses of 487 complete active bouts, we concluded that active bouts consisted of a series of foraging bouts separated by ~2 min of nonforaging activity. Black-tailed deer spent ~92% of their active time foraging, independent of sex or season. Dry matter intake rates varied significantly with season, from ~0.5 g/min active in winter up to 1.5 g/min in summer, but few differences were attributable to sex. The large leaf size of skunk-cabbage (Lysichiton americanum) enabled deer to attain and sustain intake rates that approached predicted theoretical Rmax values of 10–12 g dry matter/min (L.A. Shipley and D.E. Spalinger. 1992. Can. J. Zool. 70: 1743–1752). Daily dry matter intake varied from 300 g/day in winter to > 1300 g/day in summer. Deer maintained a constant intake rate throughout only one-third of all active periods; intake was reduced at the end of ~40% of-all bouts, while intake rates increased towards the end of 30% of bouts. We discuss the implications of these dynamics for sampling of incomplete bouts, which is frequently employed by observers. Finally, we discuss the importance of selected, not available, biomass in understanding the foraging intake of ungulates.