Snowshoe hare populations in woodlot habitat

Snowshoe hare populations in woodlot habitat

LAMAR A. WINDBERG AND LLOYD B. KEITH Department of Wildlife Ecology, University of Wisconsin-Madison, Madison, WI , U.S .A. 53706

Received July 25, 1977

WINDBERG, L . A,, and L . B. KEITH. 1978. Snowshoe hare populationsin woodlot habitat. Can. J . Zool. 56: 1071-1080.

Snowshoe hare (Lepus americanus) population dynamics were monitored on six woodlots totaling 33 ha in an agricultural area near Westlock, Alberta, from May 1970 to May 1974. Numerical trends in woodlot and nearby forest hare populations at Rochester were similar, but densities were consistently lower in woodlot habitat until the final stages of a general population decline. Annual reproductive rates were greater in woodlot populations in 1971 (11.2 vs. 9.1 young per adult female) and 1973 (1 1.1 vs. 7.4); and similar to forest populations in 1970and 1972. Consistently low rates of 1st-year survival (4 to 7%) in woodlot hare populations resulted in lower annual population growth rates and hence lower population densities. Browsing-intensity surveys indicated that both forest and woodlot hare populations encountered overwinter food shortages. In addition, woodlot hare populations may have suffered higher rates of predation.

WINDBERG, L . A., et L. B. KEITH. 1978. Snowshoe hare populations in woodlot habitat. Can. J . Zoo]. 56: 1071-1080.

Ladynamique depopulation du lievre Lepus american~ts afait I'objet d'une etude sur six boises totalisant 33 hadans une region agricole pres de Westlock, en Alberta, de mai 1970arnai 1974. Les tendances numtriques de ces populations et de celles d'une forit avoisinante a Rochester sont sernblables, mais les densites sont toujours plus faibles dans les boisds jusqu'aux phases finales d'une diminution generale de I'effectif de la population. Les taux annuels de reproduction se sont averes plus eleves dans les boises en 1971 (1 1.2:9.l jeunes par femelle adulte) et en 1973 (11.1:7.4), et semblablesdanslesdeux typesd'habitatsen 1970eten 1972. Des tauxde survie toujoursfaibles chez les petits de moins d ' l an (4 a 7%) au sein des populations des boises entrainent des taux de croissance annuelle de la population plus faibles et, coniquemment, des densites de population plus faibles. L'etude de I'intensite du broutage indique que les populations des deux types d'habitatsont ifaire face a des pknuriesde nourriture au cours de I'hiver. De plus, les populations des boises ont probablement subi une plus forte predation.

[Traduit par le journal]

Introduction The snowshoe hare (Lepus americanus) is

adapted to a forest environment, its range being historically bounded by prairies on the south and tundra on the north (Hall and Kelson 1959). Leopold (1945) noted that as the forests were cleared in Wisconsin, hare range there had receded slowly northward; and DeVos (1962) noted a similar northward recession with settlement in the Great Lakes region of Ontario. Agricultural development has likewise eliminated much hare habitat along the southern fringe of the boreal forest in Alberta. Yet small remnants (woodlots) of forest in this now primarily agricultural environment are still occupied by hares.

Our snowshoe hare study areas near Rochester in central Alberta were located on the present edge of the boreal forest (Meslow and Keith 1968). Southwest of Rochester a block of more arable land extends northward into surrounding forest (Fig. 1). Beginning in the winter of 1965-1966, coyote (Canis latrans) populations were censused by helicopter in forested habitat near Rochester; and

since 1967-1968 in the agricultural area near Westlock, 20 mi (1 mi = 1.609 km) southwest. Hare observations during these censuses provided a crude annual index to population trends on each area (Fig. 2). Counts from the Rochester area indicated a scarcity of hares during the winters of 1965-1966 to 1967-1968. In 1968-1969 numbers observed began increasing, reaching a peak in 1970- 1971, and then declined steadily through 1973-1974. In contrast, the hare population in the woodlots at Westlock remained low for 1 year longer, increased abruptly in 1969-1970, and remained high through 1971-1972 (1 year later than at Rochester) before declining sharply by winter 1972- 1973.

We thought this apparent eruption of the woodlot hare population might have been the result of im- migration from surrounding forested areas as densities increased there, the primarily agricultural area serving as a region of acceptable but less favorable (marginal) habitat during periods of high hare densities in the forest. Hence we hypothesized that hare population dynamics in the woodlot

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.

1072 CAN. J. ZOOL. VOL. 56, 1978

FIG. 1. Forest cover and location of snowshoe hare study areas in forest and woodlot habitat (1 mi = 1.609 km).

habitat differed from populations in adjacent forest habitat. During May 1970 we initiated a study of hare populations in the small woodlots near Westlock; the study continued until May 1974. The present paper compares demographic parameters of these woodlot and forest hare populations, and some environmental factors affecting each.

Methods The woodlot study area wasa 1 18-km2 blocksouthofthe town

of Westlock and approximately 50km southwest of Rochester (Fig. 1). This region, described as historically a forested "Poplar Area" by Moss (1932) was homesteaded between 1905 and 1910 (Jorgenson 1966). Long-term residents (personal commu- nications) reported hare population cycles of huge amplitudes in this region prior to forest clearing. The last notable high population allegedly occurred in some of the larger woodlots during the early 1940's. Hares seem to have persisted in the remaining woodlots since that time, but apparently at inconspicuous densities. The woodlot hare population peak which occurred during this study was generally unnoticed by local residents.

The 1 18-km2 area included 101 woodlots larger than 1 ha; the largest was 87 ha; and the average size was 8.5 ha. Woodlot habitat comprised 13%of the total area. Six woodlots within this area were used for continuous intensive population study. They ranged in size from 3.5 to 6.5 ha and totaled 33 ha. Three of the sites were isolated by open farmland (nonhabitat) and the other three were portions of larger woodlots. These study sites were separated by at least 3 kin, but the small size of each, and hence small samples, necessitated pooling of population data for analysis. The southeast corner of the 180-km2 coyote census area referred to earlier (Wetmore et al. 1970) was located adjacent to the west boundary of our woodlot hare study area.

Topography of the woodlot study area is flat to gently undulat- ing. Soils of the woodlot area are classified as 'dark gray' and 'dark gray wooded' in contrast with the predominantly 'gray wooded' soils near Rochester (Odynsky 1962). Woodlot area soils are considered as relatively more fertile, at least with respect to capability for agricultural production (Kjearsgaard 1972). However, remnant woodlots were usually associated with poorly drained sites such as muskegs, marshes, and small streams. The dominant tree species in the woodlots did not differ appreciably from habitat at Rochester. But understory cover in the many low-lying woodlots was comprised o f a higher proportion of lowbush cranberry (Viburnum edule) and red- osier dogwood (Cornus stolon$era) than in the larger tracts of forest. The vegetation on the six intensive-study woodlots was representative of woodlots in the region.

The individual forest study areas at Rochester used for com- parison were Square-mile (60ha of habitat), Landing Trail (28ha), Moores-pinewoods (34 ha), and Camp (16ha) (see Mes- low and Keith (1968) and Windberg and Keith (19766) for de- tailed descriptions of these areas). The coyote census area at Rochester encompassed these four sites and was about two- thirds forested (Wetmore er a / . 1970).

Specimens required for postmortem examination were collected frorn woodlots within the 1 18-kmZ area near Westlock and from forest habitat within a 24-km radius of Rochester. Ne- cropsy procedures were a s described by Windberg and Keith (1976b), and field techniquesasdescribed by Keitheral. (1968). Estimates of population size and survival rates and the analysis of reproductive parameters followed the format given by Mes- low and Keith (1968).

There are slight differences between population and survival estimates for the Rochester hare study area as reported in the present paper and those presented by Keith and Windberg (1978). Because limited samples frorn the woodlot area often restricted the number of independent methods available for estimating population size and survival, we used only those forest-area estimates which were similarly calculated so that results were directly comparable.

Aerial counts of coyotes, hares, and other species were conducted in winter (17 November to 29 March) using a Bell 5-2 or 206 helicopter. In forest habitat, these surveys were conducted by flying parallel transect lines 0.4km apart at a speed of 80-96 kmlh; whereas on the woodlot study area, all small tracts of forest habitat were thoroughly searched at reduced speed or by hovering with the aircraft.

Qualitative analyses were conducted on hare browse collected frorn the woodlot study area in January 1972. A survey of browsing intensity by haresduringprevious winters was made in both woodlot and forest habitat during summer 1972. Vegetative sampling procedures were described by Pease (1977).

In statistical comparisons, only probability levels less than 5% were accepted as significantly different. Levels of probability greater than 10%are indicated in the tables by the notation 'NS.'

Results Population Trend

We have compared population trend using densities calculated from mean population estimates and study-area size as outlined by trap distribution. While resulting density values are undoubtedly biased high (Keith and Windberg 1978), they should be comparable over time and between areas.

Snowshoe hare populations reached peak densities of 17.6 hares per hectare of habitat on forest study areas in November 1970 (Fig. 3). Consider-

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.

WINDBERG AND KEITH

- FOREST

(3)

FIG. 2. Mean number of snowshoe hares observed on 180-km2 woodlot and forest study areas during aerial censuses of coyotes in winter (17 November to 29 March). Censuses were conducted with Bell 5-2 or 206 helicopters, 50 to 80m above ground level; parallel transects were spaced at 0.4 krn on the forest study area and flown at 80 to 96 krn per hour; remnant blocks of forest were searched on the woodlot area at reduced speed or by hovering. Number of censuses for each area is shown in parentheses.

TABLE 1. Estimated mean conception date of first litter, and mean date of testis regression among snowshoe hares on woodlot and forest study areas (sample sizes in parentheses)

Mean conception date Mean date of testis of first litter regression to 4 g

Woodlot Forest Woodlot Forest Year study area study area Pa study area study area

1970 3 April (12) 8 April (136) <0.01 - D 24 July (39) 1971 9 April (25) 15 April (1 18) <0.01 24 July (14) 9 July (34) 1972 7 April (19) 13 April (54) <0.01 24 July (18) 10 July (23) 1973 1 April (16) 12 April (63) <0.01 - 16 July (20) 1974 8 April (12) 14 April (49) <0.01 - -

'Level of probability determined by r-test (unpaired observations; common variance). blnsufficient data for estimation of testis regression.

ably lower peak densities of 5.6 per hectare were recorded in the woodlots at that time. On both areas in 1970, densities increased from May to November; but in 1971, 1972, and 1973, November densities were either lower or about the same as in May of the same years.

During the peak year (1970-1971) and the first 2 years of population decline, woodlot and forest hares exhibited generally similar seasonal fluctua- tions, but densities were always lower in woodlots. During the 3rd year (1973-1974), densities in woodlots remained about the same but forest populations continued to decrease. By May 1974 woodlot hare densities (0.9 per hectare) were double those on forest study areas (0.5 per hectare). The ratio of decrease from November 1970 to May 1974 was 35: 1 in forest habitat, but only 6.1 in the woodlots.

Reproduction Closely synchronized initiation of breeding and

immediate postpartum mating results in distin-

guishable litter groups among snowshoe hares (Meslow and Keith 1968). Either three or four litters are born yearly dependent upon the phase of the population cycle (Cary and Keith, in preparation). The annual onset of breeding was indexed by mean first-litter conception date, and its termination by regression of mean testis weights to 4g (Meslow and Keith 1968). Breeding season length, as indicated from the interval between these two dates, tended to be longer among woodlot hares (Table 1). In all years of study, woodlot hares conceived their first litter earlier than forest populations. In both 1971 and 1972, the date of testis regression was 2 weeks later on the woodlot area. Nevertheless, there were no fourth-litter pregnan- cies in a sample of six hares from the woodlot population during those years (Table 2). Testis regression dates on the woodlot area could not be established because of insufficient data during the other years.

During the 5 years of study, pregnancy rates

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.

1074 CAN. J . ZOOL. VOL. 56, 1978

TABLE 2. Pregnancy rates of snowshoe hares from woodlot and forest study areas (sample sizes in parentheses)

Year and study area Litter 1 Litter 2 Litter 3 Litter 4

1970 Woodlot Forest

1971 Woodlot

Forest 1972

Woodlot

Forest 1973

Woodlot Forest

1974 Woodlot Forest

'No data 'Denotes

collected from woodlot study area. statistically significant difference (P determined chi-square contingency table.

TABLE 3. Mean number of corpora lutea and viable embryos among snowshoe hares (yearling and older) from woodlot and forest study areas (sample sizes in parentheses)

Corpora lutea Viable embryos P

Litter group Woodlot Forest Woodlot Forest and years area area P area area P

Litter 1 1971 1972 1973 1974 Total

Litters 2, 1970 1971 1972 1973 1974" Total

3.38 (16) 3.17 (6) 3.38 (8) 3.00 (3) 3.30 (33)

3, and 4 6.17 (12) 5.36 (11) 5.00 (11) 4.63 (8) 6.86 (7) 5.57 (49)

'Litter 2 only.

among woodlot females tended to be higher than among forest hares, significantly so for litters 2 and 3 in 1971 and litter 1 in 1972 (Table 2).

Ovulation rates and counts of viable embryos were closely related in snowshoe hares (Meslow and Keith 1968; Cary and Keith, in preparation) because preimplantation and intrauterine losses were consistently low. First litters were consistently smaller than later litters (Cary and Keith, in preparation), and hence were segregated in our analysis. During this study, meanin utero litter size (viable embryos) for the first-litter group was statis-

tically similar among years (1971 to 1974) in both the woodlot and forest populations (Table 3). Woodlot hares had larger first litters than forest populations in 1971 (3.20 vs. 2.55) and 1973 (3.13 vs. 2.53). The mean litter size of the litter 1 group for all years of study (1971-1974) was also significantly larger (3.09 vs. 2.68) in the woodlot population.

Data for the later litter groups (2, 3, and 4) similarly showed a trend for larger litters among woodlot hares (Table 3). Mean litter sizes were high in 1970 and 1974 and relatively low during intervening

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.

WINDBERG AND KEITH 1075

TABLE 4. Estimated interval and annual survival rates of adult and juvenile snowshoe hares on woodlot and forest study areas

Adult survival ratesa Juvenile survival rates

Year and May to November May to Birth to November Birth study area November to May Mayb Novemberc to Mayd to MayC

1970-1971 Woodlot Forest




- -- -

"Survival rate estimates based on average of (1) census data estimate (calculated from population estimate and age ratios); and (2) triple catch estimate (termed "survival series estimate" in Meslow and Keith 1968).

bEstimate of annual survival is independent of preceding interval surv~val estimates. CDerived from recruitment rate and November and May census data (population estimates and age ratios). 'Derived from census data (population estimates and age ratios). 'Only one survival rate estimate (census data) was available for adults during this year.

years (197 1- 1973) among forest-area hares. On the woodlot area, litters were high in both 1970 and 1971, relatively low in 1972 and 1973, and high again in 1974. In 1971, the mean size of later litters was significantly higher in the woodlots than in forest habitat (5.36 vs. 4.17). Woodlot-area hares averaged significantly larger later litters than forest populations (5.20 vs. 4.50) over all years of study (1970-1974).

Survival Estimated annual survival of adult snowshoe

hares on the woodlot study area (0.31) was lower than in forest habitat (0.53) from May 1970 to May 1971 (Table 4). During 1971- 1972, woodlot adults had a higher survival rate (0.45 vs. 0.26). In 1972- 1973, adult survival was equally low in woodlot (0.19) and forest habitat (0.18). During 1973-1974, the survival rate of adult hares in the woodlots was more than double that of the forest population (0.58 vs. 0.25). In each year of the study, relative survival of adults in both populations during the May-to-November and November-to-May intervals (Table 4) generally reflected the annual survival rates.

Juvenile survival from birth to the following spring (May) ranged from 0.04 to 0.07 in the woodlot population during the 4 years of study (Table 4). In 1970-1971, the survival rate of woodlot juveniles during their 1st year of life was only 0.05 in com- parison with 0.10 in the forest population. Over the next 3 years of population decline, 1st-yearjuvenile survival was very low on both study areas but was numerically higher in the woodlot population in

each annual period. During each year of study, the major between-area difference in juvenile survival occurred during the birth-to-November interval (Table 4).

Age Structure Age structure in the adult cohort (8 months and

older) was sampled during 16 March through 15 September each year. The proportion of yearlings in forest populations declined from a high of 77% in 1970 to 53% in 1971, and to even lower levels (26 to 35%) during the following 3 years (Table 5). In contrast, the proportion of yearlings in the woodlot population did not differ significantly (P > 0.05, chi-square tests) from 1970 to 1974. The average age of forest hares increased from 1.3 years in 1970 to 2.3 years in 1974, but the average age of the woodlot population was approximately the same (1.7 to 2.0) from 1970 to 1974. The annual age distributions of woodlot and forest populations differed statistically only in 1970 when the forest population contained more yearlings (45 vs. 77%).

Discussion A conceptual model of the snowshoe hare popu-

lation cycle in the boreal forest ecosystem was outlined by Keith (1974). This model postulates that a critical interaction between increasing hare populations and their overwinter food supply ini- tiates a major decline from peak densities, thereby reducing the hare-predator ratio and triggering a predator-hare interaction which extends the period of decline. Because population trend was similar on woodlot and forest areas during the

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.

1076 CAN, J. ZOOL. VOL. 56, 1978

TABLE 5. Age distribution among adult snowshoe hares collected from 16 March to 15 September (age determined from eye-lens weights)

Average % in each age class, years Year and Sample age,

study area size years 1 2 5 3 Pa

1970 Woodlot 27 2.0 45 22 33 Forest 150 1.3 77 17 7 <0.01

1971 Woodlot 67 Forest 247



1974b Woodlot 38 Forest 42

'Level of probability determined by chi-square contingency table. %ample from 16 March to 31 May only.

- FOREST ---+--- WOODLOTS

F I G . 3. Mean snowshoe hare population densities on woodlot and forest study areas from May 1970 to May 1974. Population estimates for both areas during each period determined from independent estimators shown in Appendix I . Densities were calculated by division of mean population estimates by amount of habitat on study areas.

4-year study (Fig. 3), it appeared likely that similar factors were affecting the major demographic changes in both types of habitat. However, the marked difference in amplitude of fluctuation between woodlot (6: 1) and forest (35: 1) populations indicated that such factors were operating with different intensity. Leopold (1933, p. 64) suggested that the amplitude of hare population cycles was higher on large continuous blocks of range than on small, dispersed or discontinuous blocks.

Effect of Environmental and Demographic Vari- ables on Reproductive Rates

A direct relationship between photoperiod and

onset of breeding in snowshoe hares was suggested from laboratory experiments (Davis and Meyer 1972). Correlation analyses between reproductive parameters and weather factors revealed that annual initiation of breeding in Rochester hare populations was associated with the "earliness of spring" (Cary and Keith, in preparation) as reflected in several corelated weather variables. Because the woodlot and Rochester study areas were only 50 km apart, it seemed unlikely that weather could have varied enough to have caused first-litter conception dates to be 5 to 11 days earlier in the woodlots (Table 1).

There is now strong evidence that early regres-

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.


sion of hare testis weights at Rochester was closely linked to food shortage during the previous winter (Keith and Windberg 1978). In all 4 years of this study, the latest mean date of testis regression in either population was 24 July (Table 1). This date was 1 to 2 weeks earlier than testis regression during years of population increase (Cary and Keith, in preparation) and indicative of termination of the breeding season before a fourth litter could have been conceived (Table 2). Our testis regression data suggests then that both forest and woodlot hares were affected by poor overwinter nutrition during the breeding seasons of 1971 and 1972. Such nutritional problems were probably more severe among forest hares because testis regression occurred 2 weeks earlier at Rochester.

The combination of higher rates of pregnancy (Table 2) and ovulation (Table 3) resulted in greater reproductive rates on the woodlot study area in 2 of the 4 years of study. Mean annual reproductive outputfavored woodlot populations by i1.2 vs. 9.1 young per adult female in 1971 and 11.1 vs. 7.4 in 1973. Reproductive rates were similar on each area in 1970 (12.9 vs. 12.2) and 1972 (7.4 vs. 8.3).

Estimated densities were higher in forest populations during the first 3 years, but other recent studies have been unable to demonstrate any adverse effects of high population densities per se on reproductive output in wild rabbits (Oryctolagus cuniculus) (Myers 1966), cottontails (Sylvilagus Joridanus) (Kirkpatrick and Baldwin 1974), or snowshoe hares (Windberg and Keith 19766; Cary and Keith, in preparation). Thus, it was unlikely that differences in population density contributed to the observed differences in reproductive rates in woodlot and forest habitat.

Cary and Keith (in preparation) found no significant relationship between ovulation rates and age at Rochester. The overall trend for larger litters among woodlot hares during the 1970- 1974 period (Table 3) was not likely influenced by female age because the average age distributions of the two populations (Table 5) were not significantly different (P > 0.05; chi-square test).

A direct relationship between insufficient food and lower reproduction has been noted among lagomorphs, including wild rabbits (Shipp et al. 1963: Stodart and Mvers 1966). cottontails (Russell 1966; Kirkpatrick a id Kibbe 1971), and sndwshoes (Keith and Windberg 1978). Further, a direct link between plant nutrition and soil fertility has been hypothesized as responsible for higher ovulation rates in cottontails (Williams and Caskey 1965; Russell 1966; Hill 1972). Because soils of the Westlock region are more fertile than those near Rochester (Odynsky 1962; Kjearsgaard 1972), we

hypothesized that nutrient levels in woodlot vegetation would reflect the difference in soil fertility and hence possibly account for the larger litters found in hare populations there. In January 1972, samples of four woody-browse species were collected from four of the woodlot study sites. Results of proximate and elemental analyses of these samples (Table 6) were compared with samples taken on the forest study area by Pease (1977). There was a trend for more fat and carbohydrate (two of the four species) and less fiber (three of four species) in forest-area browse. But, perhaps most significant, there were no differences in protein content, and few differences in five important elements (phos- phorus, potassium, calcium, magnesium, and so- dium) analyzed. We thus concluded that the nutrient content of woody browse on woodlot and forest areas was similar during winter 1971-1972 and that differences in ovulation rates were related to some other factor.

Relationship Between Overwinter Food Supply and Numerical Trend

Pease (1977) concluded that available browse was insufficient to support existing hare populations at Rochester over the winters of 1970-1971 and 1971-1972. We have no similar quantitative measurement of hare browse in woodlot habitat, but during summer 1972 evidence of browsing in previous winters was surveyed on both forest and woodlot areas (J. L. Pease, unpublished data). In forested habitat, 70% of 3490 woody plants had been clipped and (or) girdled; while in woodlots, 64% of 1263 plants were similarly damaged by hares. These data suggest that browsing intensity had been comparable. Available browse in the woodlots was, however, comprised of a lower proportion of such palatable species as willow (Salix spp.) (6 vs. 17%), aspen (Populus tremuloides) (5 vs. 1 I%), and hazel (Corylus cornuta) (13 vs. 22%). The percent occurrence of several unpalatable species was notably higher in the woodlots: snow- berry (Symphoricarpos occidentalis) (34 vs. 25%); lowbush cranberry (24 vs. 15%); red-osier dogwood (22 vs. 5%); and honeysuckle (Lonicera spp.) (21 vs. 13%). Thus, even though hare densities were lower in the woodlots (Fig. 3), suitable overwinter food may still have been in short supply.

Impact of Predation and Other Mortality factors on Numerical Trend

Data on predator populations and food habits within the woodlot area are limited. Lynx (Lynx canadensis) apparently reached peak densities at Rochester during winter 1971-1972 (Brand et al. 1976). We observed lynx tracks in several woodlots near Pickardville (Fig. 1) during the winters of

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.

1078 CAN. J . ZOOL. VOL. 56. 1978

TABLE 6. Mean percentages of selected proximate and elemental components of woody browse species collected in January 1972 from woodlot and forest study areasa

Nutrient content of browse, % dry weight No. of 3

Study pooled Carbo- Species areab samples Ash Fat Protein Fiber hydrate P K Ca Mg Na

Aspen (Populus tremuloides)

Willow (Salix spp.)

Hazel (Corylus cornuta)

Rose (Rosa spp.)

Woodlot

Forest Woodlot

Forest Woodlot

Forest Woodlot

Forest

- .-

0.14 0.03 '*

0 . 1 1 0.02 0.07 0.03

0.06 0.02 0.09 Trace

0.09 0 0.13 0

0.13 0

'Each pooled sample includes terminal twigs collected from 10-20 plants at random. Maximum twig diameter was about 4 mm. bForest-area data from Pease (1977). *Denotes significant difference (P < 0.05) between areas as determined by one-way analysis of variance.

TABLE 7 . Estimated mean number of snowshoe hares and coyotes per 180 kmz on woodlot and forest study areas during winter and percent biomass of snowshoe hares in diet of coyotes on each area

Woodlot study area Forest study area

lomass % b' % biomass No. of No. of of hares in No. of No. of of hares in

Year haresa coyotesb coyote dietc haresc coyotesb coyote dietc

1970-1971 8600 32 (2) - 109 000 79 (3) _d d

1971-1972 5400 26 (1) - 42 800 76 (3) - 1972-1973 3400 32 (3) 27 ( 1 13) 18 100 62 (5) 67 (261) 1973-1 974 2200 29 (3) 8 (74) 5 700 52 (5) 1 1 (55)

-- -

'Based on estimated population densities on 1 December and proportion of hare habitat on each area. bMean number of coyotes observed on aerial censuses conducted between 17 November and 29 March extrapolated with assumption that 100% of coyotes

on woodlot area and 33% of those in forest habitat wereseen (Keith rt a/. 1977; A. W. Todd, unpublished). Number of censuses per winter is given in parentheses.

<Determined by analysis of coyote scats and intestines collected on each study area from November to March (A. W. Todd, unpublished). Percent biomas s was calculated by multiplying percent occurrence of prey times average weight of prey items. Sample sizes in parentheses.

'No data collected.

1971- 1972 and 1972- 1973. We suspect these individuals were transients because tracks were observed only sporadically. Coyote densities were about two to three times higher in forest than in woodlot habitat during winters 1970-1971 to 1973-1974 (Table 7). Great horned owls (Bubo vir- ginianus) and goshawks (Accipiter gentilis) were resident hare predators on both study areas. Other less important or seasonal hare predators which occurred on each study area were as follows: long- tailed and short-tailed weasels (Mustela frenata and M. erminea), red-tailed hawks (Buteo jamai- censis), snowy owls (Nyctea scandiaca), and domestic and feral dogs and cats.

Our assessment of relative predation rates on hares in woodlot vs. forest habitat is restricted to coyotes and great horned owls. Unless otherwise

indicated, the information on those species cited below is from A. W. Todd (unpublished data). Dur- ing winter 1972-1973, coyote numbers were 1.9 times higher on the forested than on the woodlot study area, while hare numbers were 5.3 times higher (Table 7). Percent biomass of hares in the diet of forest-dwelling coyotes was 2.5 times that of coyotes in woodlot habitat. The relative predation rate on woodlot hares was thus probably comparai ble with that in forest habitat (5.3 + [1.9 x 2.51 = 1.1). The same type of calculation for winter 1973- 1974 also yielded an estimated relative predation rate which was similar (1.0).

During the breeding season of 1972, there were 1.7 times as many resident territorial pairs of great horned owls in forest habitat (16 pairs on 162 km2) as in the woodlots (we observed 7 pairs on

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.


118km2), and total hare numbers were 5.5 times higher (21 400 vs. 3900). The estimated biomass of hares in the diet of woodlot owls during spring and early summer 1972 was 50%, and not significantly less (P > 0.05; chi-square test) than the 64% found in forest owls. In 1973, territorial pairs of great horned owls were 1.6 times higher on the forest area; hare numbers were 6.0 times higher (7800 vs. 1300); and again percent biomass of hares in the owls' diet was similar (64% in woodlots; 74% in forest). Thus the relative proportion of hares con- sumed by owls was markedly higher (2.5 times in 1972 and 3.1 times in 1973) in woodlot habitat during the two breeding seasons in which we were able to make comparisons.

Other possible factors of hare mortality include adverse weather conditions and infectious and parasitic disease. We have discounted the possibil- ity of any significant differences in weather between our woodlot and forest study areas because they were only 50 km apart, and our data on tem- perature, precipitation, and snow depth from weather stations near Rochester and Pickardville (Fig. I) were not notably different.

Comparisons of percent occurrence (presence or absence) of the most common helminth parasites of hares (Obeliscoides cuniculi, Dirofilaria scapiceps, Protostrongylus boughtoni, and two Taenia spp. (cystercerci)) yielded no significant differences (chi-square tests) between woodlot and forest populations. Likewise, the occurrence of three common arbovirus antibodies (eastern equine encephalitis, western equine encephalitis, and Cali- fornia encephalitis) was also found to be comparable (chi-square tests).

Our initial hypothesis that the sudden increase of woodlot hare populations in the winter of 1969-1970 (Fig. 2) was a result of ingress from surrounding forest remains untested. Dispersal from forest to woodlot habitat is certainly not im- possible because we had recorded movements of up to 10 km by marked hares on Rochester study areas (Windberg and Keith 1976~). A field experi- ment conducted in forest habitat simultaneous with this study (Windberg and Keith 1976a) dem- onstrated conclusively that dispersing hares were nearly 90% juveniles and yearlings. Therefore, if woodlot hares were largely immigrants during winter 1969-1970, we would have expected a very high proportion of yearlings on that area during spring-summer 1970. Instead the age ratio was comprised of only 45% yearlings (Table 5). Thus, it is most likely that the rapid population increase in the woodlots was a function of higher than average juvenile survival from birth to winter in 1969. Dur-

ing that year, 1st-year survival of young inforests at Rochester was the highest (0.36) ever recorded there (Keith and Windberg 1978).

In summary, snowshoe hare populations in woodlot habitat fluctuated in synchrony with nearby forest populations but densities and the amplitude of decline from peak to low population levels were less (Fig. 3). The significant demographic parameter which prevented woodlot populations from reaching the high peak densities which occurred in forest habitat was higher juvenile mortality from birth to November in 1970 (Table 4). Although reproductive rates were consistently higher among woodlot hares, the combined survival of adults and their progeny was insufficient to prevent population decline. Low survival of juvenile hares was characteristic of the woodlot population (Table 4). Likewise, Dolbeer and Clark (1975) found that juvenile survival averaged much lower on their sparsely forested study area (0.04) than on a densely forested area (0.25) during 2 years of hare population study in Colorado. The factors of hare mortality in woodlot and forest habitats appeared to have been similar but the relative intensity probably differed. Evidence indicated that both populations encountered food shortage during the winter of peak densities and the first winters of population decline. But higher rates of predation on woodlot hares may have contributed to the overall lower population densities in the woodlot habitat.

Acknowledgments We gratefully acknowledge the laboratory assis-

tance of Diane Mathison, Diane Keith, and Bette Kuhs, and the field assistance of Jay Gorham, Terry Vowel, and Bryan Keith. We thank Allen Arndt, Roy Edgar, Cameron Klassen, John McLaughlin, Roland Nelson, Alfred Victor, and George Watt for allowing us to use their private lands for this study. The advice and assistance in computer programming provided by John R. Cary, Department of Wildlife Ecology, and Arden R. Hardie, Department of Dairy Science, were most helpful. We thank Arlen W. Todd and James L. Pease for providing the unpublished data used in this paper. Financial support for the study was provided by the following: The University of Wis- consin, College of Agricultural and Life Sciences, and the Research Committee of the Graduate School; the Research Council of Alberta, and the Alberta Departments of Lands and Forests (Fish and Wildlife Division) and Agriculture (Veterinary Services Branch); the National Science Founda- tion (grant GB-33320x1); and the Canadian Wildlife Service.

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.

1080 CAN. J . ZOOL. VOL. 56, 1978

A ~ ~ e n d i x 1 HILL, E. P., 111. 1972. Litter size in Alabama cottontails as . . Independent estimates of snowshoe hare populations on woodlot and forest study areas (confidence limits (P = 0.95) in

parentheses)

Estimated no. of hares Month Population

and year estimatora Woodlot area Forest area

Apr.1970 R T 143 (101-211) 772 (610-958) NOV. 1970 RO 155 (86-255) 2658 (2097-341 1)

R T 214 (109-375) 2624 (1979-3434) T E 179 (97-302) 1932 (1502-2362)

Jan. 1971 R O 79 (43-132) 1053 (943-1352) R T 179 (76-368) 1417 (1029-1888) TE 112 (56-210) 1273 (989-1557)

Apr. 1971 R O 11 3 (50-220) 942 (800-1200) R T 98 (55-161) 1009 (783-1292) T E 89 (43-162) 823 (654-992) I 88 (52-137) 805 (532-1 154)

NOV. 1971 R O 136 (49-317) 881 (695-1118) R T 74 (40-1 23) 654 (470-874) T E 113 (55-206) 657 (541-773)

Jan. 1972 RO 32 (14-67) 694 (5 1 8-9 14) R T 97 (33-250) 684 (456-973) T E 84 (30-196) 596 (431-761)

Apr. 1972 R T 87 (28-224) 439 (318-585) T E 74 (24-1 89) 358 (291-425) I 62 (20-158) 390 (256-562)

Nov. 1972 R T 65 (26-141) 322 (202-484) T E 67 (27-148) 389 (253-525)

Jan. 1973 R T 50 (21-102) 245 (138-399) T E 46 (15-1 17) 222 (14@-304)

Apr. 1973 R O 24 (9-55) 138 (91-198) R T 37 (1 3-93) 166 (89-281) T E 26 (9-62) 127 (84-170)

NOV. 1973 R T 27 (4-152) 90 (25-266) T E 54 (4-1055) 110 (23-197)

Apr. 1974 R T 31 (8-89) 60 (9-339)

'Population estimators are coded as follows: RO, Lincoln-index estimator from reobservation of color-marked individuals; RT, Lincoln-index estjmator from retrapping of marked individuals,; TE. trapping efficiency estimator (Meslow and Keith 1968); 1, Lincoln-~ndex estimator from retrapping of radioactive young from spring-implanted females. All confidence limits are based on Chapman's (1948) Tables 1 and 2.

BRAND, C. J . , C. A. FISCHER, and L . B. KEITH. 1976. Lynx responses to changing snowshoe hare densities in central Alberta. J. Wildl. Manage. 40: 416-428.

CHAPMAN, D. G. 1948. A mathematical study of confidence limits of salmon populations calculated from sample tag ratios. Int. Pac. Salmon Fish. Commission Bull. 2: 69-85.

DAVIS, G. J . , and R. K. MEYER. 1972. The effect ofdaylength on pituitary FSH and LH and gonadal development of snowshoe hares. Biol. Reprod. 6: 264-269.

DEVOS, A. 1962. Changes in the distribution of the snowshoe hare in southern Ontario. Can. Field-Nat. 76: 183-189.

DOLBEER, R. A., and W. R. CLARK. 1975. Population ecology of snowshoe hares in the Central Rocky Mountains. J . Wildl. Manage. 39: 535-549.

HALL, E. R., and K. R. KELSON. 1959. The mammals of North America. Vol. 1. The Ronald Press, New York.

influenced by soil fertility. J . Wildl. Manage. 36: 1199-1209. JORGENSON, R. D. 1966. The squatters. Greig Printing and

Stationery, Westlock. KEITH, L . B. 1974. Some features of population dynamics in

mammals. Trans. 11th Int. Congr. Game Biol., Stockholm, Sweden. pp. 17-58.

KEITH, L. B., E . C. MESLOW, and 0. J . RONGSTAD. 1968. Techniques for snowshoe hare population studies. J . Wildl. Manage. 32: 801-812.

KEITH, L. B., A. W. TODD, C. J . BRAND, R. S . ADAMCIK, and D. H. R u s c ~ . 1977. An analysis of predation during a cyclic fluctuation of snowshoe hares. Trans. 13th Int. Congr. Game Biol., Atlanta. In press.

KEITH, L. B., and L . A. WINDBERG. 1978. A demographic analysis of the snowshoe hare cycle. Wildl. Monogr. No. 58. In press.

KIRKPATRICK, R. L., and D. M. BALDWIN. 1974. Population density and reproduction in penned cottontail rabbits. J . Wildl. Manage. 38: 482-487.

KIRKPATRICK, R. L., and D. P. KIBBE. 1971. Nutritive restric- tion and reproductive characteristics of captive cottontail rabbits. J . Wildl. Manage. 35: 332-337.

KJEARSGAARD, A. A. 1972. Soil survey of the Tawatinaw map sheet (83-1). Alberta Inst. Pedol. Rep. No. S-72-29.

LEOPOLD, A. 1933. Game management. Charles Scribner's Sons, New York.

1945. The distribution of Wisconsin hares. Trans. Wis. Acad. Sci. Arts Lett. 37: 1-14.

MESLOW, E. C., and L. B. KEITH. 1968. Demographic parameters of a snowshoe hare population. J . Wildl. Manage. 32: 812-834.

Moss, E . H. 1932. The vegetation of Alberta. IV. The poplar association and related vegetation of central Alberta. J. Ecol. 20: 380-415.

MYERS, K. 1966. The effectsofdensity on sociality and health in mammals. Proc. Ecol. Soc. Aust. 1: 40-64.

ODYNSKY, W. 1962. Soil zones of Alberta. (Map.) Dept. Ext. University of Alberta, Edmonton.

PEASE, J . L . 1977. Interaction of snowshoe hares and woody vegetation. M.Sc. Thesis, University of Wisconsin, Madison.

RUSSELL, K. R. 1966. Effects of a common environment on cottontail ovulation rates. J. Wildl. Manage. 30: 819-827.

SHIPP, E., K. KEITH, R. L. HUGHES, and K. MYERS. 1963. Reproduction in a free-living population of domestic rabbits, Oryctolagus cuniculus (L.), on a sub-antarctic island. Nature (London), 200: 858-860.

STODART, E., and K. MYERS. 1966. The effects of different foods on confined populations of wild rabbits, Oryctolagus cuniculus (L.). CSIRO Wildl. Res. 11: 11 1- 124.

WETMORE, S. P., C. H. NELLIS, and L . B. KEITH. 1970. A study of winter coyote hunting in Alberta with emphasis on use of snowmobiles. Alberta Dep. Lands For. Fish Wildl. Div. Tech. Bull. No. 2.

WILLIAMS, C. E., and A. L. CASKEY. 1965. Soil fertility and cottontail fecundity in southeastern Missouri. Am. Midl. Nat. 74: 21 1-224.

WINDBERG, L. A., and L. B. KEITH. 1976a. Experimental analyses of dispersal in snowshoe hare populations. Can. J. Zool. 54: 2061-2081.

1976b. Snowshoe hare population response to artificial high densities. J. Mammal. 57: 523-553.

Can

. J. Z

ool.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV C

HIC

AG

O o

n 11

/21/

14Fo

r pe

rson

al u

se o

nly.

Documents

Snowshoe hare populations in woodlot habitat