Factors Regulating A Declining Snowshoe Hare Population

Margaret Klepack

The Hare Area stand.

Background
The Adirondack Ecological Center, located in the central Adirondack region of New York State, has been monitoring the abundance of snowshoe hare (Lepus americanus) and several predator species for 15 years. Data collected from 1986-2001 suggests that snowshoe hare abundance in the region has declined in recent years. Previous studies conducted elsewhere have shown habitat suitability and predation to be 2 determining factors in the regulation of snowshoe hare abundance. This study was an effort to evaluate how these factors may have affected a declining snowshoe hare population in the more southern limits of the species' range.

Habitat factors found to regulate hare populations are the availability of suitable browse and cover. Snowshoe hare feed on a wide variety of plant species, and few of the common woody species in the Adirondacks are considered inedible. Cover is an important element of snowshoe hare habitat as it provides protection from predators and shelter from inclement weather.

Many studies have found snowshoe hare populations to be affected by predators. Previous research has concentrated on strongly cyclic populations found in areas north of the Adirondack Park. Few studies have investigated predation as a regulating factor in the non-cyclic hare populations found in the northeastern US.

This study investigated long-term snowshoe hare and predator abundance in a softwood stand (the "Hare Area") at Huntington Wildlife Forest (HWF) in the central Adirondack region of New York State. The purpose of my study was to investigate how cover, browse availability and predator abundance contribute to a declining snowshoe hare population in the study area. The objectives of the study were to 1) Calculate population growth rates for hare and predators in the study area, 2) evaluate the suitability of snowshoe hare habitat in the study area, and 3) evaluate impacts of habitat suitability and predator abundance on the HWF hare population using results obtained from this study and literature review.

Methods

Hare and Predator Populations
Hare and predator population growth rates were determined through track counts conducted in the winters of 1986-2001 on a network of transects established throughout the Hare Area. Hare, fisher (Martes pennanti), marten (Martes americana), weasel (Mustela spp.) and coyote (Canis latrans) tracks were counted. From these data, the population growth rates of hare and predator species were determined. Trends in these populations were compared for correlations.

Snowshoe Hare Habitat
Winter habitat was evaluated at 100 sites in the Hare Area using a snowshoe hare habitat model created by the U.S. Fish and Wildlife Service. Habitat factors were estimated as to their winter condition because winter has been found to be the most critical season for snowshoe hare survival.

The visual obscurity board.

Cover was evaluated using a "visual obscurity board". The board, with alternating black and white blocks, was observed at a distance of 15 m, and the percentage of each of 6 blocks obscured by vegetation was estimated. Deciduous leaves and other seasonal vegetation were excluded from the estimate to approximate winter conditions. Browse was estimated at each site by clipping all available winter browse within a 1-m2 quadrat. Available winter browse was defined as living woody vegetation, rooted within the quadrat, with a diameter <1.5 cm within 98 cm of the ground. Species considered inedible to hare, such as balsam fir, were not clipped. Samples were weighed in the lab using an electronic balance.

Samples of available browse found within the 1-m2
quadrat were taken to the lab for weighing.

Results

Population
The snowshoe hare population and most predator populations in the Hare Area were found to be declining between 1986-2001. Significant correlations were found between the abundance of snowshoe hare and weasel (rs=0.567, P=0.028) and fisher (rs=0.477 P=0.07) in the Hare Area. No correlation was found between the snowshoe hare population and the coyote or pine marten populations.

Snowshoe Hare Habitat
Cover and browse availability were evaluated at 100 sites distributed evenly along the transects in June 2002. Average cover was between 20-40%. There was found to be 437.9 kg/ha of available winter browse. Through the hare habitat model, I found that cover condition was not suitable but browse availability was suitable for snowshoe hare use.

The 1-m2 quadrat in which all available browse was clipped.

Discussion

I found that snowshoe hare habitat in the Hare Area was unsuitable, since cover appeared to be limiting. Other studies have found that greater than 40% cover is required to support a population of hares. Thus, the average visual obscurity of 20-40% found in the Hare Area was not high enough to meet habitat requirements of snowshoe hare. The maturing forest conditions in the Hare Area may have degraded cover conditions during the period of the track count surveys. The Hare Area, composed of trees >50 years old, may have matured beyond the point of providing adequate cover for snowshoe hares. Larger conifers (>10 m in height) lose lower branches thereby reducing visual obscurity and consequently providing less cover for snowshoe hares. A similar maturing process may have been taking place in the predominantly softwood Hare Area. Trees in the Hare Area are generally 14-17 m tall, therefore it is possible the predominance of dead lower branches provided less suitable cover for snowshoe hare.

As cover in the Hare Area was limiting, predators may have been negatively affecting the hare population. Limited cover provides less protection from predators. I found a significant positive correlation between the weasel population and the hare population in the Hare Area. Similarly, other studies have found that hare cycles in the North are synchronous with weasel cycles. A significant correlation was also found between the fisher population and the hare population in the Hare Area. However, a previous study found that fisher numbers did not track hare cycles. Though no correlation was found between the coyote population and the hare population in the Hare Area, studies conducted elsewhere found relationships to exist. However, it should be noted that most other hare and predator population studies have been conducted in areas exhibiting strong cyclic hare populations. Hare populations in the northeastern United States do not exhibit such extreme fluctuations as in cyclic populations. Thus, the population dynamics of hare and predators in the Adirondacks could be quite different from the patterns observed in other parts of North America where cycles exist.

The Hare Area stand.

In conclusion cover conditions in the Hare Area were degraded as the stand matured over the course of the ALTEMP study. These degraded habitat conditions may have caused the observed snowshoe hare population decline. Reduced cover may have led to increased predation on snowshoe hares, or may have left hares more vulnerable to harsh winter weather.

Management Recommendations

Snowshoe hare populations require suitable cover conditions that are best provided by young, regenerating forests. Thus, for the management of a large snowshoe hare population, forests must be periodically thinned or cut to encourage the regenerative growth that provides the most suitable habitat cover conditions for snowshoe hares.

Acknowledgements
I am grateful to the Roosevelt Wildlife Station for funding this study and to my supervisor, Scott Haulton, for his immeasurable help with this study. I thank Madeleine Fairbairn, Tim Marvin and Patricia Baretsky for helping me with the fieldwork.

Contact info:
For further information on any of these projects, you can contact:
Adirondack Ecological Center: aechwf@esf.edu
Margaret Klepack: mfk7@cornell.edu