American marten (Martes americana) COSEWIC assessment and status report: chapter 6

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Habitat

Habitat requirements

Historically, marten have been strongly associated with mature and overmature conifer and mixed-wood forests throughout their range in North America (Thompson and Curran 1995, Payer and Harrison 2003). Structural features of these forests important to marten include dense overhead cover, coarse woody debris, low-hanging branches and shrub understory. These features provide protection from predators, subnivean access for hunting, denning and resting sites (also important for thermoregulation), and habitat for prey species.

Recent research has demonstrated that resident adult marten also use partially harvested stands, stands defoliated by insects, early successional forests regenerating after clearcutting (reviewed in Payer and Harrison 2000, 2003; also Potvin et al. 2000, Poole et al. 2004, Hearn et al. 2005) and burns and partial burns (Paragi et al. 1996), if important horizontal and vertical structural features are retained. Marten have evolved in Newfoundland to be habitat generalists, aided in part by a release from predation, which allows them to occupy a naturally fragmented landscape. Hearn et al. (2005) concluded that marten in southwest Newfoundland selected an array of habitats including mid-successional and young regenerating softwoods. Stands logged within 20 years and regenerating stands that had been pre-commercially thinned were also used in proportion to their availability in conjunction with mature and overmature forests.

Previous studies in Newfoundland documented a preference for old growth forests, specifically mature balsam fir (Abies balsamea) (Snyder and Bissonette 1987, Bissonette et al. 1989, Thompson and Curran 1995, Forsey and Baggs 2001). During this period marten were restricted to old growth because trapping and snaring prevented their colonization of other habitat types (Hearn et al. 2005; D. Harrison, pers. comm., 2007). Although snowshoe hares were most abundant in 40-year-old second growth stands (Thompson and Curran 1995), both marten and their primary prey species, meadow voles, occurred in low numbers. The abundance of meadow voles was negatively correlated with tree density and positively correlated with coarse woody debris. Voles were most abundant in overmature stands (Thompson and Curran 1995, Sturtevant 1996, Sturtevant and Bissonette 1997).

Marten habitat associations in Newfoundland were not studied when all historically occupied regions contained marten populations. Rather, research began after the populations had contracted to inaccessible areas of the island, where forest harvesting and trapping were limited or absent. By default these were mature and overmature forests. Hearn et al. (2005) hypothesized that immature forests can support marten due to the availability of snowshoe hare as prey, and the reduced need for cover from predators, which are less common in Newfoundland as compared to the mainland.

Fuller et al. (2006) modelled percent suitable habitat within marten home ranges, and found the probability of occupancy to be 90% when home range-sized habitat units contained at least 60% suitable habitat. The probability of occupancy declined sharply when suitable habitat in the landscape was <60%. The rate of decline in occupancy was steepest between 30 – 40% of suitable habitat within the home range. The mean amount of suitable habitat within home ranges was 47%, and 80% of all marten had ≥38%, while only 6.5% of marten had ≤30% suitable habitat within their home range. Models including fragmentation variables did not perform any better, suggesting that habitat fragmentation is not as important a variable influencing home range occupancy as the amount of suitable habitat alone. Marten in Maine exhibited declines in home range occupancy much sooner as percent suitable habitat declined. Fuller et al. (2006) hypothesized that the Newfoundland population has evolved in a naturally fragmented landscape where a large body size and larger home ranges allow it to include more unsuitable habitat in home ranges. Suitable habitat included insect-killed stands, pre-commercially thinned stands, medium and tall closed canopy softwood stands, tall open-canopy softwood stands, and regenerating clearcuts (≤6.5m tall, ≥75% canopy closure) (Hearn et al. 1995).

Overall, there was equivocal evidence that martens prefer mature forest types. Tall open-canopy softwood had the highest positive selection index at both the landscape and stand scale. Tall-closed softwood and medium-open softwood had intermediate selection values and were used in proportion to their availability. Median availability of mature and overmature forests within the study area ranged between 20% to 40%. The median amount of mature and overmature forest within occupied home ranges was 30% (Hearn et al. 2005). However, marten did not select home ranges dominated by mature and overmature coniferous forests at the landscape scale. Unsuitable habitat types included bog, rock, soil barrens, unmerchantable softwood (≤6.5m tall) and medium height open-canopy softwood. The maximum cumulative amount of unsuitable habitat types within marten home ranges was 35% (Hearn et al. 2005).

Habitat trends

Given the new habitat model for Newfoundland marten (Hearn et al. 2005), past (at the times of previous status assessments) and current habitat trends are unclear. Past predictions of habitat shortages and extinction risks due to logging, natural forest mortality, and trapping/snaring are now debatable (Thompson 1991, Schneider and Yodzis 1994, Schneider 1997). Similarly, past forest management prescriptions are now ambiguous until reviewed against Newfoundland marten habitat requirements (e.g., Sturtevant et al. 1996, Sturtevant et al. 1997, Thompson and Curran 1995, Chapin et al. 1998, Hargis et al. 1999, Potvin et al. 2000, Payer and Harrison 2003).

Continued logging in Newfoundland is reducing the quantity of old growth habitat (Forsey et al. 1995). An estimated 73.5 km²/year of mature and overmature forest was harvested annually between 1985 and 1993 (Lemon 1996). The current forest harvest rate is approximately 200 km²/year (B. English, pers. comm., 2006). While the current average harvest age is 100 years, many natural second growth forests will be harvested in the future as early as 60 to 80 years of age (B. English, pers. comm., 2006).

Habitat protection/ownership

Most of the marten’s range is on public or “Crown” lands. Approximately 14% of the current area of occupancy by American marten is within protected areas (27% of the Terra Nova core area, 6% of the Main River core area, and 20% of the Little Grand Lake-Red Indian Lake core area) (J. Brazil and I. Schmelzer, pers. comm., 2007). Additional protected areas include provincial parks and wilderness reserves. Ten percent of the area of occupancy is protected from wood harvesting alone, 21% is closed to all trapping and snaring, and an additional 18% is within modified trap and snare zones (I. Schmelzer, pers. comm., 2007).

Sixteen percent of all critical habitat is fully protected; 16% is protected from wood harvesting alone, 29% is closed to all trapping and snaring, and an additional 28% occurs in modified trap and snare zones (I. Schmelzer, pers. comm., 2007). Critical habitat encompasses approximately 51% of the area of occupancy.

Protected areas include Gros Morne (1,805 km²) and Terra Nova (392 km²) National Parks. Limited timber harvesting continues within one national park and in the sections of the Pine Marten Study Area which are not within reserves. The Little Grand Lake Provisional Ecological Reserve was established in 2002. Two other protected areas were established in 2002; the Little Grand Lake Wildlife Reserve (569 km²) and the Glover Island Public Reserve (178 km²). A total of 6,160 km² has been proposed as critical habitat for forest habitat management planning purposes, and there is no harvest in critical habitat (J. Brazil, pers. comm., 2007).