Arctic Refuge Coastal Plain Terrestrial Wildlife Research Summaries

Section 3: The Porcupine Caribou Herd - Part 2

Subsections:
Herd Dynamics and Demography
Seasonal Distribution and Movements
Foraging on the Calving Ground

Herd Dynamics and Demography

The growth curve of the Porcupine caribou herd suggested an approximate 30- to 40-year cycle of increase and decrease in abundance (Fig. 3.8). The herd numbered ~100,000 in 1972, increased at about 4.9% per year from 1979 through 1989 when it reached ~178,000 animals, then declined at about 3.6% per year from 1989 to 1998 (Fig. 3.8). The decline from 1998 to 2001 was only about 1.5% per year, and the herd now totals ~123,00 animals. If the current decline continues, the herd would be expected to again reach the lowest levels ever recorded during 2005-2010. If the herd continues to decline below ~100,000 animals, then the length of a complete herd cycle may exceed 30 years.

Figure 3.8 plots the population size of the Porcupine caribou herd, 1972-2001, as estimated from aerial photo-censuses by Alaska Department of Fish and Game.

Figure 3.8. Population size of the Porcupine caribou herd, 1972-2001, estimated from aerial photo-censuses by the Alaska Department of Fish and Game.

Porcupine caribou herd size appeared correlated with Arctic Oscillation although there were too few data to conduct a proper time series analysis (Fig. 3.5). In contrast to the Porcupine caribou herd, other Alaska barren-ground caribou herds (Western Arctic, Teshekpuk Lake, Central Arctic), generally continued to increase during the downward trend in the Arctic Oscillation that was evident during the 1990s (Fig. 3.5).

Capacity for growth (defined as the maximum realized long-term growth rate) of the Porcupine caribou herd appeared substantially less than for other Alaska herds. Capacity for growth among herds of dramatically different sizes is best visualized by plotting relative herd sizes (Fig. 3.9). Maximum long-term growth rate (~4.9%, assumed linear, 1979-1989) (Fig. 3.8) of the Porcupine caribou herd was never more than about half the rate observed for other Alaska barren-ground caribou herds [Western Arctic herd (1976-1996, ~9.5%), Teshekpuk Lake herd (1978-1993, ~13%), Central Arctic herd (1978-1992, ~10.3%)] (Fig. 3.9).

Figure 3.9 plots the relative population sizes of the four Alaskan arctic-calving caribou herds from 1976 to the present. On the plot, each herd begins with a value of 1.0. Plotting relative population size illustrates the relative growth rates among the four herds. The curve for the Porcupine herd is relatively flat, rising very slightly during the herd's increasing phase and then falling back during the herd's decreasing phase. In contrast, sizes of the other 3 herds (Central Arctic, Western Arctic, and Teshekpuk Lake) increase 5 to 7 fold. Prior to the mid-1980s, the Central Arctic, Western Arctic, and Teshekpuk Lake herds had almost identical rates of population size increase. Beginning in the mid-1980s, growth rate of the Central Arctic departed slightly (reduced) from that of the Western Arctic and Teshekpuk Lake.

Figure 3.9. Relative post-calving herd sizes (minimum observed = 1.0) of the 4 Alaska barren-ground caribou herds (PCH = Porcupine caribou herd; WAH = Western Arctic herd; CAH = Central Arctic herd; TLH = Teshekpuk Lake herd), 1976-2001. Maximum observed population size for each herd is noted in the legend.

The Porcupine caribou herd was the first Alaska barren-ground caribou herd to begin and maintain a prolonged decline in the last 2 decades (Fig. 3.9). Annual survival of Porcupine caribou herd adult females was only about 84% (Fancy et al. 1994, Walsh et al. 1995), which was lower than that generally observed in other caribou herds (Bergerud 1980); and adult female survival may have been responsible for the relatively low growth rate of the Porcupine caribou herd.

Annual calf survival averaged about 48% with about half (56%) of the annual mortality occurring on the calving ground (Whitten et al. 1992, Fancy et al. 1994, Walsh et al. 1995).

There were no significant differences in mean parturition, calf survival during June, or net calf production (defined as the product of parturition rate and June calf survival) (Fig. 3.10a-c) between the increase and decrease phases of the herd (Fig. 3.8). Parturition rate averaged 0.81 (range 0.71-0.92) during 1983-2001 (Fig. 3.10a) and did not differ between the increase phase (0.80, SE = 0.04, 1983-1989) and the decrease phase (0.82, SE = 0.08, 1990-2001).

Figure 3.10 plots estimates of 3 reproductive parameters for the Porcupine caribou herd, 1983-2001; a) parturition rate of adult females, b) calf survival from birth through the last week of June, and c) net calf production, the product of parturition rate and calf survival. No trends through time, or differences between the increasing and decreasing population phases were evident for any parameter.

Figure 3.10. Reproductive estimates for the Porcupine caribou herd, 1983-2001: a) parturition rate of adult females, b) calf survival from birth through the last week of June, and c) net calf production [the product of parturition rate and calf survival].

Calf survival during June was quite high and averaged 0.75 (range 0.57-0.94) during 1983-2001 (Fig. 3.10b) but did not differ between the increase phase (0.71, SE = 0.07, 1983-1989) and the decrease phase (0.79, SE = 0.13, 1990-2001). Net calf production averaged 0.62 during 1983-2001 (range 0.50-0.82) (Fig. 3.10c) and did not differ between the increase phase (0.58, SE = 0.06, 1983-1989) and the decrease phase (0.63, SE = 0.13, 1990-2001). For all these demographic characteristics, variance tended to be greater during the decrease than during the increase phase of the herd.

Because average parturition, calf survival during June, and net calf production did not differ between the increase and decrease phases of the Porcupine caribou herd, 1983-2001, a reduction in adult, sub-adult, and/or calf survival while animals were off the calving ground in late-summer through winter must have accompanied the herd decline. Emigration to the adjacent Central Arctic herd was an unlikely cause of the Porcupine caribou herd decline because satellite-collared animals that occasionally (4 out of 167 collar-years) wintered with the Central Arctic herd, returned to the Porcupine caribou herd the following summer.

Periodic lows in net calf production and calf survival during June (1992, 1993, 1997; Figs. 3.10b, c) were not sufficient to maintain the herd decline (S. A. Arthur, Alaska Department of Fish and Game, personal communication). Unfortunately, a complete record of adult, sub-adult, and calf survival estimates was not available for late-summer through winter during the decrease phase of the herd, 1989-2001.

Seasonal Distribution and Movements

The Porcupine caribou herd caribou wintered (15 November – 14 April) in Alaska south of the Brooks Range and in Canada in the Richardson and Ogilvie Mountains in the Yukon Territory (Fig. 3.11). Their annual range encompassed ~290,000 km2 (Fig. 3.2). The extent of calving encompassed ~36,000 km2. Spring migration to the annual calving grounds began in mid-April and continued through April and May (Fig. 3.11). Return to fall/winter ranges began with departure from the annual calving grounds in late-June and early-July (Fig. 3.11). In fall (15 September – 14 November), the Porcupine caribou herd was distributed widely.

Figure 3.11 maps the locations of satellite-collared female caribou of the Porcupine caribou herd during seven seasonal time periods, 1985-1995. An average of 10 animals (range 4-17) were collared each year yielding 14,447 observations; 87 percent of these observations were obtained 1985-1990. Not included were locations of 3 females that each spent one winter with the adjacent Central Arctic herd. During 15 November through 14 April, caribou locations were primarily in the southern Brooks Range of the Arctic Refuge and the Richardson Mountains of Canada. Onset of spring migration was evident during 15 April through 26 May when the females moved northward to the coastal plain. Regardless of winter location, entry onto the coastal plain occurred at the eastern edge of the Brooks Range in the northern Yukon Territory of Canada. During 27 May through 23 June, females were located almost exclusively on the coastal plain and foothills, predominantly in the Arctic Refuge from the base of the Brooks Range northward onto the coastal plain and 1002 Area. During 24 June through 6 July, females occupied the coastal plain, almost exclusively in the Arctic Refuge, and predominantly throughout the 1002 Area. Regardless of calving location, females typically used the 1002 Area in late June. During 7 July through 14 August, some females remained on the coastal plain with a westward emphasis in their distribution, while other females moved southward through various mountain passes of the Brooks Range. During 15 August through 14 September, few females remained on the coastal plain while most were dispersed in the southern Brooks Range and northern Richardson Mountains. During 15 September through 14 November, females were absent from the Alaska coastal plain and were located throughout their winter ranges in both Alaska and Canada.

Figure 3.11. Distribution of satellite-collared female caribou of the Porcupine caribou herd during 7 time periods, 1985-1995. An average of 10 animals (range 4-17) were collared each year yielding 14,447 observations; 87% of these observations were obtained 1985-1990. Not included were the locations of 3 females that each spent one winter with the adjacent Central Arctic herd.

Minimum daily travel rates of parturient females were variable throughout the year (Fig. 3.12). Non-parturient females had similar movement rates. Minimum movement occurred during winter. Movement began increasing in mid-April with initiation of migration to the annual calving ground and was directional toward the annual calving ground.

Figure 3.12 plots minimum median daily movement rate of parturient satellite-collared females of the Porcupine caribou herd, 1985-1995. Values were calculated from no more than one location per day. An average of 10 animals (range 4-17) were collared each year yielding 14,447 observations; 87 percent of these observations were obtained 1985-1990. Not included were data for 3 females that each spent one winter with the adjacent Central Arctic herd. During November through April, median movement averaged about 2.5 kilometers per day. This rate more than tripled during the spring migration in May, and then abruptly dropped during the early June calving period. During post-calving, daily movements rates increased, attaining maximums during July that averaged as high as 18 kilometers per day. Rates dropped to about 5 kilometers per day in August, and then rose again during September.

Figure 3.12. Minimum median daily movement rate of parturient satellite-collared females of the Porcupine caribou herd, 1985-1995. Values calculated from no more than one location per day. An average of 10 animals (range 4-17) were collared each year yielding 14,447 observations; 87% of these observations were obtained 1985-1990. Not included are the data for 3 females that each spent one winter with the adjacent Central Arctic herd.

After their calves were born, the direction of movement of satellite-collared parturient females was random for 20 days (Fancy and Whitten 1991). Calf movement rate (minimum, straight line, estimated from conventional radio-collars) in the years 1992-1994 was about 2.5 km/day during the first week after birth. The rate increased gradually during the next week to about 5 km/day and then increased through the end of June to approximately 15-20 km/day. As females and calves departed the calving ground in late June and early July, some individual calves traveled as much as 90 km/day. Relatively high rate of movement continued throughout July. Because movement rates were low during the calving season and direction of movement was random for 20 days after birth (Fancy and Whitten 1991), the distribution of calving sites was assumed to be representative of habitat use by caribou through 21 June.

Movement declined during August perhaps in response to harassment by Oestrid flies or to localized forage abundance. Movement increased during the pre-rut period in late-September and October and then reached a minimum again by mid-November. The average female of the Porcupine caribou herd traveled approximately 4,355 km annually (Fancy et al. 1989).

During 1985-1992, median arrival of satellite-collared parturient females on the annual calving ground ranged from 17 May-4 June and median date of departure ranged from 3-26 July. Non-parturient females tended to lag slightly behind and south of the parturient females from early-May through calving (Whitten et al. 1992), but within 1 week after calving, parturient and non-parturient female distributions were essentially coincident.

Length of stay on the annual calving ground ranged from 34-67 days. Caribou have tended to depart the annual calving grounds earlier since 1995 (F. J. Mauer, U.S. Fish and Wildlife Service, personal communication). This trend may have been related to more advanced plant phenology within the extent of calving in late June during the late 1990s (Fig. 3.4).

Median calving date, 1983-1996, was 1 June (range 30 May-6 June) with 50% of annual calving occurring within 2 days of the annual median calving date. No temporal trends were evident in median calving date, and annual calf survival was not related to median calving date (P > 0.05).

Sizes and locations of annual calving distributions were quite variable. Annual calving grounds encompassed 3,672-16,667 km2 during 1983-2001 (Fig. 3.13, Table 3.1). Similar distributions were observed during aerial surveys, 1972-1982 (Figs. II-5 in Clough et al. 1987). On average, concentrated calving areas occupied 12.3% (range 0.7-25%) of the annual calving grounds (255-2,548 km2) and contained 47% (range 29-61%) of calving locations.

Figure 3.13 shows individual maps of the annual calving distributions of the Porcupine caribou herd, 1983-2001, as estimated from fixed kernel analyses of the sites where radio-collared females were first observed with calves during repeated aerial surveys in May and June. Three zones are shown: 1) the concentrated calving area which is the contour enclosing calving sites with greater than average fixed kernel density, 2) the annual calving ground which is the 99% fixed kernel utilization distribution for a year, and 3) the extent of calving which is the same in all maps and represents the outer perimeter of all annual calving grounds. This figure illustrates the high spatial variability in location of the concentrated calving area among years.

Figure 3.13. Calving distributions of the Porcupine caribou herd, 1983-2001, as estimated from fixed kernel analyses of the sites where radio-collared females were first observed with calves during repeated aerial surveys in May and June. There are 3 zones: 1) concentrated calving area (shown in dark gray), the contour enclosing calving sites with greater than average fixed kernel density, 2) annual calving ground (medium gray), the 99% fixed kernel utilization distribution for a year, and 3) aggregate extent of calving (light gray), the outer perimeter of all annual calving grounds. No concentrated calving was detected in 2001.

There was no concentrated calving area in 2001 when the spring was very late and the extent of calving was almost completely snow covered. Density of parturient females in the concentrated calving area ranged approximately 13-106/km2 over the years and averaged 7 times (range 3.7-10.8) higher than outside the concentrated calving area each year (Table 3.1). None of these estimates differed between the increase and decrease phases of the herd (P > 0.05). Since 1972, there have been only 2 years (2000, 2001) when all calving occurred in Canada and 1 additional year (1982) when all concentrated calving occurred in Canada.

Table 3.1. Number of calving sites, number of calving sites in the concentrated calving area (CCA), area (km2) of CCA, area (km2) of annual calving ground (ACG), ratio of sizes of CCA to ACG, population size of the Porcupine caribou herd, percent of radio-collared female caribou that calved in the CCA, percent of radio-collared female caribou that calved in the 1002 Area, percent of the CCA within the 1002 Area, and percent of the ACG within the 1002 Area, 1983-2001, Alaska, USA, and Yukon Territory, Canada.

Table 3.1 presents the annual (1983-2001) number of calving sites, number of calving sites in the annual concentrated calving area, the area of the concentrated calving area in square-kilometers, the area of the annual calving ground, and the ratio of sizes of the concentrated calving area relative to the annual calving ground. Table 2.1 also presents the population size of Porcupine caribou herd as estimated by censuses in 1983, 1987, 1989, 1992, 1994, 1998 and 2001. The table also presents the percentages of calving sites within the 1002 Area proportions of the calving areas occurring within the 1002 Area.

Neither the areas of annual calving grounds nor areas of concentrated calving areas were correlated (P > 0.05) with the number of calving sites, with the estimated number of parturient females in the herd, with the percent of the extent of calving that was snow free, or with any greenness (NDVI) estimate in either the extent of calving or the annual calving grounds. Thus, neither herd size nor habitat characteristics were clearly related to calving ground size. Factors affecting calving ground size remain unclear.

Distribution of calving sites differed (MRPP, P < 0.05) among all successive years, 1983-2001, except 1983-1984 when the number of calving sites obtained from radio-collared females was lowest and 2000-2001 when late springs restricted calving to Canada (Table 3.1). There was no uni-directional trend to shifts in location of annual calving grounds or concentrated calving areas (Rayleigh’s Test, P = 0.870 and 0.740, respectively). During 1983-1994, parturient females displayed no among-year fidelity to the concentrated calving area (P = 0.951) nor any habitat attribute for calving (P > 0.135), but females that calved in the 1002 Area returned there for calving in the following year more often than expected (P = 0.024).

The percent of females calving in the 1002 Area in the years 1983-2001 was quite variable, averaging 43% (range 0-92%) but not differing (P = 0.128) between the decrease (50%, SE = 32%) and the increase phase (30%, SE = 23%) of the herd (Fig. 3.14). The proportion of the concentrated calving area that was in the 1002 Area followed a similar trend. As the relative amount of green biomass at calving within the extent of calving (NDVI_calving) increased because of earlier springs, the percent of females calving in the 1002 Area increased (r2 = 0.68, P < 0.001) (Fig. 3.15). Thus, the average proportion of Porcupine caribou herd females that calve in the 1002 Area may increase if the climate continues to warm.

Figure 3.14 plots the percent of radio-collared female caribou of the Porcupine herd that calved in the 1002 Area of the Arctic National Wildlife Refuge, 1983-2001. Values range widely, from zero percent in 2000 and 2001 to over eighty five percent in 1991, 1995 and 1998.

Figure 3.14. Percent of radio-collared Porcupine caribou herd females that calved in the 1002 Area of the Arctic National Wildlife Refuge, Alaska, 1983-2001.

Figure 3.15 plots the percent of radio-collared Porcupine caribou herd females that calved within the 1002 area of the Arctic National Wildlife Refuge in relation to the median Normalized Difference Vegetation Index at calving within the extent of calving, 1985-2001. The figure shows that during years with earlier spring conditions, and hence a more advanced vegetation phenology (higher NDVI), more caribou calve within the 1002 Area.

Figure 3.15. Percent of radio-collared Porcupine caribou herd females that calved within the 1002 Area of the Arctic National Wildlife Refuge, Alaska, in relation to the median Normalized Difference Vegetation Index at calving (NDVI_calving) within the aggregate extent of calving, 1985-2001. Point legends indicate the year of the estimates.

The general location of calving in the years 1983-2001 was related to the winter Arctic Oscillation (January, February, March) during previous calendar year, approximately 15 months before calving. In years when the Arctic Oscillation was positive, more than half of the concentrated calving area was likely to be located on the Alaska portion of the coastal plain (83.3% of the years, Fisher’s Exact Test, P = 0.045). Similarly, there was a tendency (66.7% of years, Fisher’s Exact Test, P = 0.057) for more than half the females to calve in the 1002 Area when the Arctic Oscillation in the previous calendar winter was positive.

The time delay in correlation between the Arctic Oscillation and calving location and between the Arctic Oscillation and NDVI_calving (Fig. 3.6) may have been related to a 1-year delay between tiller formation and flower production for Eriophorum vaginatum (cottongrass) (Billings and Mooney 1968, Bliss 1971). Immature cottongrass flowers have been a dominant food item for Porcupine caribou herd when they have calved on the Arctic Refuge coastal plain. Cottongrass tiller formation is probably related to the availability of resources (moisture and soil nutrients).

Positive phases of the Arctic Oscillation may have enhanced resource availability, increased tiller production in the previous year, and resulted in increased flower production during the current spring. We would expect that the increased greenness at calving (NDVI_calving) might reflect leaf area of cottongrass tillers, rather than the pale green immature flowers.

During post-calving (>3 weeks after calf birth), Porcupine herd caribou (regardless of calving location) tended to move westward (Fig. 3.11). Even in exceptional years when calving occurred far to the east in Canada (e.g., 2000, 2001) (Fig. 3.13) caribou reached the Arctic Refuge coastal plain and portions of the 1002 Area by late-June or July (S. A. Arthur, Alaska Department of Fish

and Game, personal communication). As a result of these westward movements, essentially the entire 1002 Area was eventually used by late June or early July. Most of the use of the westernmost portion of the 1002 Area by satellite-collared females of the Porcupine caribou herd occurred during 24 June-14 August (Fig. 3.11).

Foraging on the Calving Ground

The calving season diet of Porcupine herd caribou during 1993-1994, when concentrated calving was primarily in the 1002 Area (Fig. 3.13), was dominated (76-82%) by immature flowers of cottongrass from the time the caribou arrived on the calving ground until about 16-18 June (Figs. 3.16a, 3.17a). Similar diets were observed in 1973 (Thompson and McCourt 1981), but the location of concentrated calving in that year was not documented (Clough et al. 1987).

Figure 3.16 illustrates the temporal relationship between forage phenology and diet composition of caribou in 1993. During early June, diet was mainly cottongrass with some lichen. In mid-June, corresponding to the period when willow leaves emerged and cottongrass flowers matured, caribou diet shifted to predominantly willow and herbs.

Figure 3.16. Porcupine caribou herd a) diet composition and b) median phenology of major forage items, 1993. Diet composition estimated from microhistological analysis of fecal pellets, corrected for digestibility. Phenology scores for cottongrass: 1 = leaves only, 2 = flowers in boot, 3 = early flower, 4 = full flower; and for willow: 1 = dormant, 2 = bud swelling, 3 = leaf unfolding, 4 = full leaf.

Diet was relatively consistent between years, but somewhat more variable in 1994, and not related to average daily weight-gain of calves in 1993 and 1994. Both cottongrass flowers and young willow (Salix spp.) leaves are easily digestible and are common forage of upland calving caribou when they are available (e.g., Thompson and McCourt 1981, Kuropat 1984, Russell et al.1993). Cottongrass flowers were most common in the vegetation type herbaceous tussock tundra, and willow was most common in shrub tussock tundra and riparian shrub vegetation types (Jorgensen et al. 1994). Herbaceous plants were ubiquitous.

Dietary shifts within the 1993 and 1994 calving seasons apparently allowed caribou to increase nutrient concentration in their diet as the season progressed. By mid-June, 1993-1994, as cottongrass flowers matured, the leaves of willows unfolded (Figs. 3.16b, 3.17b). Then, within about 4 days (Figs. 3.16a, 3.17a), caribou diet shifted to an approximate 50:50 mix of willow and herbaceous plants.

Figure 3.17 illustrates the temporal relationship between forage phenology and diet composition of caribou in 1994. The results of 1994 were similar to those of 1993 (Figure 3.16). However, forage phenology was more advanced in 1994, and the same shift in diet was observed about one week earlier in 1994 compared to 1993.

Figure 3.17. Porcupine caribou herd a) diet composition and b) median phenology of major forage items, 1994. Diet composition estimated from microhistological analysis of fecal pellets, corrected for digestibility. Phenology scores for cottongrass: 1 = leaves only, 2 = flowers in boot, 3 = early flower, 4 = full flower; and, for willow: 1 = dormant, 2 = bud swelling, 3 = leaf unfolding, 4 = full leaf.

The diet shift resulted in an increase of dietary nitrogen concentration (from 3% to 4%) and a decrease in Neutral Detergent Fiber (NDF) concentration (from 57% to 27%) based on nutritional analyses of cottongrass and willow of appropriate phenological stages from the calving ground. Available biomass of willow likely exceeded the biomass of cottongrass flowers during the diet shift and thereafter.

Caribou maintained the willow and herbaceous diet until they departed the calving ground near the end of June. Because climate warming and earlier greening may increase the carbon:nitrogen ratios of individual forage species and reduce their quality on fixed dates (Walsh et al. 1997), rapid shifting among forage species may allow caribou to accommodate time-specific reduction in nutritional quality of individual plant species that accompanies climate warming.

Diet of Porcupine herd caribou was substantially different when they used the Canadian portion of the extent of calving than when they used the Arctic Refuge coastal plain and the 1002 Area. Regardless of timing of snowmelt in Canada, calving diet there was dominated by mosses and evergreen shrubs (58.4-73.5%, Russell et al. 1993). These forage groups were much less digestible than the immature cottongrass flowers and willows (Russell et al. 1993) that dominated the calving diet of the Porcupine caribou herd in 1993 and 1994. This implied that diet quality during calving was reduced when the Porcupine caribou herd used the Canadian portion of the extent of calving rather than the Arctic Refuge coastal plain and the 1002 Area.

(continued to Part 3)

| Home | Section 1 - Introduction | Section 2 - Land Cover | Section 3 - Porcupine Caribou Herd |
| Section 4 - Central Arctic Caribou Herd | Section 5 - Forage Quantity and Quality | Section 6 - Predators |
| Section 7 - Muskoxen | Section 8 - Polar Bears | Section 9 - Snow Geese | Acknowledgements |