9th International Deer Biology Congress

AUTHORS: Perry S. Barboza, Department of Wildlife & Fisheries Sciences, Texas A&M University; Lindsay van Someren, Department of Biology & Wildlife, University of Alaska Fairbanks; Keith W. Oster, Department of Wildlife & Fisheries Sciences, Texas A&M University; David D. Gustine, National Park Service, Grand Teton National Park and U.S. Geological Survey, Alaska Science Center; Heather E. Johnson, U.S. Geological Survey, Alaska Science Center

ABSTRACT: Reindeer and caribou (Rangifer tarandus) are exposed to a wide range of anthropogenic (e.g. mineral & energy industries) and natural perturbations (e.g., extreme cold, rain-on-ice events, shrub encroachment) that affect the relationship between population size and food supply (i.e. carrying capacity). Rangifer populations can change dramatically through a combination of factors that are dependent on animal density (e.g., food supply) and independent of animal density (e.g., severe winter weather) [1]. The negative effect of high population density on diversity and abundance of forages is most evident for island populations of Rangifer [2] but the relationship between food abundance and population size is more difficult to predict in mainland populations especially for large migratory herds. The feedback between forage supply and abundance of Rangifer is weakened by migration and by storage and conservation of energy and nutrients in body tissue [3] that could predispose these populations to irruptions and crashes [4]. Conversely, highly seasonal patterns of forage growth strengthen the relationship between food supply and animal abundance by influencing life history patterns and movement: male caribou migrate shorter distances than female caribou, which aggregate in calving areas that vary with population size, weather conditions, and plant biomass [5]. The summer window of forage growth supplies the energy and nutrients to restore and grow body tissues. It is well known that summer storage of energy in body fat positively affects both winter survival and spring reproduction in arctic Rangifer but summer supplies of nutrients may have an even greater positive effect on reproduction. Body protein stored from the previous year in maternal muscle supplies more that 85% of the nitrogen (N) transferred to calves in pregnancy and lactation [6].

We recently tested the hypothesis that the supply of N is more limiting than energy for arctic caribou by collecting key forages (three species of graminoids, three species of woody browse and one genus of forb) over three summers in the migratory range of the Central Arctic Herd (CAH) in Alaska from the Brooks Range to the Coastal Plain on the Arctic Ocean[7]. We combined in vitro digestion and detergent extraction to measure fiber, digestible energy and usable fractions of N in forages (n = 771). Digestible energy content fell below the minimum threshold value of 9 kJ/g for one single forage group: graminoids, and only beyond 64-75 days from parturition (6 June), whereas all forages fell below the minimum threshold value for digestible N (1% of dry matter) before female caribou would have weaned their calves at 100 days from parturition. The window for digestible N was shortest for browse, which fell below 1% at 30–41 days from parturition, whereas digestible N contents of graminoids were adequate until 46–57 days from parturition. The low quality of browse as a source of N was also apparent from concentrations of available N (i.e. the N not bound to fiber) that were <1% at 72–80 days from parturition. We also examined variation of minerals in summer forages for arctic caribou in the migratory ranges of the CAH and the Western Arctic (WAH) caribou herds [8]. Spatial distribution of forage mineral content was associated with variation in soil pH, while temporal variation was related to plant maturity, and thus N and fiber content of forages. Concentrations of sodium were below minimum requirements in all forage species for most of the summer and adequate only on the Coastal Plain during the second half of summer. Phosphorus declined in plants from emergence to senescence and were below requirements in all forages by mid-summer, while concentrations of copper declined to marginal concentrations at plant senescence. Interactions of sodium with potassium, phosphorus with calcium, and copper with zinc likely exacerbate the constraints of low concentrations of sodium, phosphorus and copper in forages. However, these effects may be weaker for forages on the WAH where significantly higher concentrations of phosphorus and copper coincided with lower concentrations of calcium, than those collected on the CAH transect.

The Coastal Plain may be favored by female caribou because available and digestible concentrations of N are not only greater than those on the Brooks Range, the window of usable N on the Coastal Plain extends the period of protein gain for females and their calves by 17 days. Migration to the coast may also allow parturient females to replenish sodium stores depleted by foraging inland through the long arctic winters, while also extending the availability of adequate phosphorus by 43 days. Our studies provide evidence that phenological windows for N and mineral gain in female caribou are both spatially and temporally dynamic and likely to affect the distribution and growth of the populations. We are continuing research on carrying capacity of these populations by combining models of forage growth with those of functional response to predict the relative value of regions for population growth under scenarios of natural and anthropogenic change.