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EDITED BY
Keith B. Aubry, William J. Zielinski,
Martin G. Raphael, Gilbert Proulx,
and Steven W. Buskirk
BIOLOGY AND CONSERVATION OF
MARTENS, SABLES,
AND FISHERS
A New Synthesis
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Copyright © 2012 by Cornell University except for chapters 4, 15, and 19 and
portions of chapters 3, 10, 12, 13, 16, and 17, which were written by federal
employees and cannot be copyrighted.
All rights reserved. Except for brief quotations in a review, this book, or parts
thereof, must not be reproduced in any form without permission in writing from the
publisher. For information, address Cornell University Press, Sage House, 512 East
State Street, Ithaca, New York 14850.
First published 2012 by Cornell University Press
Printed in the United States of America
Library of Congress Cataloging-in-Publication Data
Biology and conservation of martens, sables, and fi shers : a new synthesis / edited by
Keith B. Aubry … [et al.].
p. cm.
Includes bibliographical references and index.
ISBN 978-0-8014-5088-4 (cloth : alk. paper)
1. Martes. 2. Martes—Ecology. 3. Wildlife conservation. I. Aubry, Keith Baker.
QL737.C25B516 2012
599.76'65—dc23 2012003137
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209
9
Improved Insights into Use of
Habitat by American Martens
IAN D. THOMPSON, JOHN FRYXELL, AND DANIEL J. HARRISON
ABSTRACT
We reviewed habitat selection by American martens ( Martes americana ) across
North America based on information published from 1993 to 2010. Habitat use
by the species is variable across the continent, with populations occupying a
range of forest ages and types, making generalities regarding habitat preferences
diffi cult to infer at the stand level. Clear differences exist between habitat selec-
tion in boreal and montane forests and selection in midcontinent and eastern
transitional mixed forests. Most models indicate that habitat selection occurs at
3 spatial scales: landscapes (i.e., home range scale), stands within home ranges
(i.e., stand scale), and individual structures at the scale of specifi c sites within
stands (within-stand or site scale). Overall, the strongest patterns of selection oc-
cur at the landscape scale, suggesting a strong connection between home range
composition and individual fi tness. Although American martens are not restricted
to mature and old mixed-wood and conifer forests, most studies have suggested
that these forest types receive the highest relative use. Use of managed landscapes
by American martens is common where suffi cient cover and structures important
to fi tness are present and where the species can exhibit landscape-scale selection
to maintain 70% or more of home ranges in suitable habitat conditions. Habitat
selection may be a function of availability but may also be infl uenced by density
dependence, mortality from commercial trapping and predation, the capacity for
dispersal, and the availability of untrapped reserves as source populations. Few
studies have tested multiple hypotheses of the mechanisms affecting habitat
choice. Further, since most dispersing animals are juveniles, observed patterns of
habitat selection by unmarked American martens could represent use of poor
habitats by subordinate or inexperienced individuals. Hence, we argue that indi-
vidual fi tness, rather than simply the presence of the species, should be used to
evaluate habitat selection and quality.
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210
Ian D. Thompson et al.
Introduction
American martens ( Martes americana ) live primarily in boreal forests, but
their distribution extends south into some temperate and montane forest ar-
eas, including as far south as central California and northern New Mexico.
Like all predominantly boreal forest species, American martens must have
become adapted to the unpredictability resulting from various disturbance
regimes, because, over ecological time, fi re, blowdown, and insect outbreaks
alter forest structure and age composition across the landscapes they occupy
(e.g., Fisher and Wilkinson 2005; Drapeau et al. 2009). At any point in time,
the boreal forest landscape presents animals with a range of habitat types and
forest ages as a matrix at large spatial scales. Selection from among these
habitats is ultimately related to the fi tness of individual animals as mediated
through a variety of choices made and factors acting at those various scales
(Morris 1987; Levin 1992). Choice of habitats affects the capacity of individ-
uals to acquire resources while minimizing energy expended and avoiding
predation to gain those resources (Charnov 1976; Krebs 1978). Habitat selec-
tion represents a suite of choices at several scales, each of which may infl uence
individual fi tness (e.g., Johnson 1980; Morris 2002). Understanding the scale
at which key habitat choices are made, and the mechanisms behind the
choices, improves our capacity to predict how animals may react to manage-
ment alternatives (e.g., Buskirk and Ruggiero 1994; Ciarniello et al. 2007;
Oatway and Morris 2007).
Habitat selection has several key components: fi rst, animals might be ex-
pected to seek out high-quality patches, and, second, among these patches,
variance in the availability of key resources (food, cover) should be reduced
(Mayor et al. 2007); both will affect fi tness, and selection at the larger scale
may constrain selection at smaller scales. Furthermore, habitat selection could
be infl uenced by the density of conspecifi cs (Fretwell and Lucas 1969; Morris
1987; Oatway and Morris 2007). At low densities, animals may readily fi nd
high-quality patches, but as density rises, fewer such patches are available and
mean fi tness declines. Greene and Stamps (2001) have argued that Allee ef-
fects may be observed at low densities such that, up to an infl ection point,
fi tness may increase as density rises before becoming negatively density de-
pendent.
Here we review information on habitat selection by American martens
from studies across North America from 1992 to early 2010, and following
from summaries by Buskirk and Ruggiero (1994), Buskirk and Powell (1994),
and Thompson and Harestad (1994). Since these reviews were published, the
focus of marten research has changed in 4 key respects. First, most of the
work after 1992 has focused on habitat selection across a landscape matrix of
forest types and ages, rather than on comparisons between old forests and
younger regenerating forests. Such heterogeneous landscapes are common in
most jurisdictions, where a matrix of managed and unmanaged forest stands
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211
Habitat Use by American Martens
of various ages and types extends over large spatial scales. Second, research-
ers have increasingly recognized that American martens make habitat choices
at multiple spatial scales. This understanding began with work by John Bis-
sonette and Daniel Harrison and their respective students (e.g., Bissonette
et al. 1989; Bissonette et al. 1997; Chapin et al. 1997a, 1998). Although much
of the earlier research on habitat use by martens (e.g., Soutiere 1979; Bateman
1986; Snyder and Bissonette 1987; Thompson and Colgan 1987) did not nec-
essarily ignore the landscape scale, the main focus was clearly on habitat se-
lection at the stand scale and, specifi cally, on the relative selection of recent
clear-cuts versus residual forest stands within the home range. Third, im-
proved statistical models became widely available after about 1990, at about
the same time as an understanding that pseudoreplication from telemetry
data could infl uence conclusions about habitat use. Hence, studies after about
1990 often employed superior statistical modeling and reduced sampling er-
ror. Finally, many recent studies have focused specifi cally on selection of indi-
vidual within-stand habitat features, such as den sites, resting sites, and cover
attributes. We have structured our review around 3 of Johnson’s (1980) 4 or-
ders of habitat selection, although we prefer to use the term scale. There are
strong linkages across these scales (or orders); thus, we review landscape (sec-
ond order) and stand-scale (third order) selection together, followed by
within-stand or site-level selection (fourth order).
We reviewed 11 published papers that provided suffi cient information to
derive a use/availability index ( w
i ). We also report on >30 published articles
that either examined marten habitat selection or provided specifi c informa-
tion about habitat choices at the stand or site scale (e.g., den sites). We have
attempted to report results only for resident adult martens, given the possibil-
ity that juveniles may be observed often in low-quality habitats (Thompson
and Colgan 1987; Ruggiero et al. 1988; Buskirk and Ruggiero 1994). When
necessary, we have referenced older studies as published support for various
aspects of marten ecology.
Buskirk and Powell (1994) and Thompson and Harestad (1994) proposed
several hypotheses for mechanisms that may infl uence marten habitat selec-
tion, including predator avoidance, presence of key site-scale habitat features
(structural complexity: coarse woody debris, cover) that increase the quality
of the habitat, and abundance of preferred prey. Here, we examine support
from studies conducted during the past 18 years for the proposed hypotheses
for habitat selection, and argue for the need to improve our understanding of
the role that density dependence might play in observed patterns.
Some Forestry Definitions
We refer to forest ages as young regenerating , which, for any forest type, is
the shrubby and young-tree stage that follows a major disturbance; regenerat-
ing , which we consider as forests where the tree component (not shrubs)
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212
Ian D. Thompson et al.
dominates, often at high stem density, and where rapid growth in height is oc-
curring; mature refers to forests where the dominant tree cohort has attained
close to maximum height and stem density is often lower than in regenerating
stands; and old or old-growth as a stage where some of the dominant tree
cohort have died, snags and canopy gaps are common, and young trees have
begun fi lling in gaps. Old forests are often referred to as overmature in forest
planning. The age of the trees at each stage depends on the particular forest
ecosystem. We defi ned mixed woods as forest stands with >25 and <75% co-
nifer (or deciduous) trees, otherwise they are conifer or deciduous . In Great
Lakes-St. Lawrence forests and Acadian forests, a transition zone between the
boreal and southern deciduous zones, managed stands may be maintained in
the mature or old age classes through selection harvesting (see below).
Forests are managed if trees are harvested commercially or pre-commercially.
Unmanaged forests are those originating as a result of natural disturbances
(fi re, wind, insects, ice storms). Clear-cut harvesting refers to the even-aged
silvicultural method of removing most or all trees of the dominant cohort of
trees at the same time; this method is common in boreal and some Acadian
forest types. Selection harvesting refers to an uneven-aged silvicultural tech-
nique whereby certain trees are selected for harvesting from the same stands
over long periods of time. The latter technique is a common silvicultural prac-
tice in most Great Lakes-St. Lawrence forests. Patch-cutting refers to clear-
cut harvesting where the size of the stands removed is restricted to small
patches, for example <50 ha, with buffer areas of mature forest between and
surrounding the patches. Pre-commercial thinning refers to the removal of
stems to a specifi ed basal area during regeneration or at an early mature stage,
to improve stem growth of commercial tree species.
Habitat Selection at Landscape and Stand Scales
Forest landscapes can take various forms, but 2 broad types are reported in
the literature: managed/unmanaged and multi-aged managed mosaic. In most
areas of the boreal forest, progressive clear-cutting leaves large areas, often
several thousand km
2 , with a range of forest ages but containing little mature
or old-growth forest, except in small, remnant patches (e.g., along rivers).
Larger areas of older forest are usually found toward the ends of the roads
(i.e., at the edges of the managed landscape) where harvesting has yet to oc-
cur, or in protected areas (e.g., Thompson and Colgan 1994; Gosse et al.
2005; Johnson et al. 2009). However, in many managed temperate and mon-
tane forests, and in some boreal forests where selection harvesting or patch-
cutting has occurred, forest landscapes contain a matrix of forest types and
ages (including old-growth) (e.g., Hargis et al. 1999; Payer and Harrison
2000; Potvin et al. 2000; Fuller and Harrison 2005). These 2 types of land-
scapes present different habitat-selection problems for dispersing martens. In
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Habitat Use by American Martens 213
the progressive clear-cut landscapes, martens must often make a choice be-
tween living in a regenerating forest, or dispersing to try to fi nd a mature or
old forest landscape. In the mosaic landscapes, martens have the opportunity
to occupy a home range that contains some mature or old forest and some
regenerating stands.
Results presented by Thompson and Harestad (1994: 361), based on stud-
ies conducted prior to 1992, were largely confi rmed by more recent research
(Table 9.1, Figure 9.1) that showed an increase in relative use with forest age.
Nevertheless, there are many important exceptions, including studies in
Maine (Payer and Harrison 2003), British Columbia (Poole et al. 2004),
Alaska (Paragi et al. 1996), Quebec (Potvin et al. 2000), and Newfoundland
(Hearn et al. 2010). In several of these newer studies, marten density was
similar in pole-sized or older regenerating, mature, and old forests. For ex-
ample, Hearn et al. (2010) found no apparent difference in selection between
regenerating conifer and old-conifer habitats with a closed canopy, although
martens did select for old insect-killed stands and open old-conifer forests.
Overall, mature and old forests were selected preferentially by martens in 9 of
11 studies reviewed.
We also summarized results from 4 studies of marten preference among
3 broad forest types: conifer, mixed woods, and deciduous. This required re-
calibration of 1 study (Potvin et al. 2000) that had used a more liberal defi ni-
tion (<50% conifer) than we used for deciduous forest. Martens generally
preferred mixed-wood forests, and conifer forests to a lesser extent, whereas
use of deciduous forests was variable (Figure 9.1). Preference for mixed-wood
forests is not surprising, given the generally higher primary productivity of
mixed-wood forests compared with most conifer forests, which is related to
favorable site conditions (e.g., Sims et al. 1989; Longpré et al. 1994), and to
the greater amounts of structure in mixed-wood forests compared with more
monotypic stands (e.g., Fuller et al. 2004; Payer and Harrison 2004; Fuller
and Harrison 2005). Higher primary productivity can be directly related to
prey populations, which have been shown to have a direct effect on marten
abundance (Thompson and Colgan 1987; Fryxell et al. 1999). Mixed-wood
forests often maintain higher small mammal populations than most conifer-
ous forests (e.g., Orrlock et al. 2000; Fuller et al. 2004). Further, at least in
Maine, partial harvesting did not reduce small mammal prey densities (Fuller
et al. 2004). In abundant seed years, however, populations of small mammals
in conifer forests may also be high (e.g., Fryxell et al. 1998; Elias et al. 2006),
especially in older forests (e.g., Thompson and Colgan 1987; Rosenburg et al.
1994).
In western montane populations of American martens, the species appears
to prefer mature and old conifer forests (e.g., Buskirk and Ruggiero 1994;
Bull et al. 2005; Mowat 2006; Slauson et al. 2007), whereas in eastern North
America, especially toward the southern limit of the species’ distribution, a
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214
Table 9.1. Studies that reported habitat selection by American martens based on comparisons of use vs. availability (wi) in 3 forest types and 4
forest developmental stages (see text) following disturbances
Author Location ScaleaSeason
Marten
density
Marten
statusbncTechnique
Forest types
preferred
Shrub/
sapling
Immature/
pole Mature Old
Potvin et al.
2000d
Quebec Landscape Winter UnkeAd and J with
HRf
20 Telemetry Mixed woods 0.6 (1.3) 0.9 —
Potvin et al.
2000
Quebec Stand Winter Unk Ad M with
HR
20 Telemetry Mixed woods 0.9 1.2 (1.7) —
Poole et al.
2004
British
Columbia
Stand All Low Ad and J with
HR
31 Telemetry Mature conifer 0.9 1.2 (1.7) —
Gosse et al.
2005
Newfound-
land
Stand All Low Ad with HR 23 Telemetry Mature conifer 0.6 0.3 (1.8) —
Fuller and
Harrison
2005
Maine Stand All High Ad with HR 18 Telemetry None, avoided
young stands
0.25 — (1.0) —
Bull et al.
2005
Oregon Landscape All Unk Ad 20 Telemetry Mature and old
conifer
0.08 0.4 1.7 (1.9)
Proulx 2006b Alberta Stand Winter Unk Unk 44
tracks
Track
transects
Mature conifer
and mixed
wood
0.0 1.0 (1.7) —
Mowat 2006 British
Columbia
Landscape Winter Unk Unk 177
hairs
Hair snags No preference 0.9 0.8 1.1 (1.3)
Dumyahn
et al. 2007
Wisconsin Landscape Winter Low Unk age with
HR
13 Telemetry Deciduous 0.5 0.5 (1.3) —
Slauson et al.
2007d
California Stand Summer/
fall
Low Unk 26
tracks
Track plate Old coniferg0.0 0.0 0.5 (3.2)
Godbout
and Ouellet
2008
Quebec Landscape All Mod. Unk age with
HR
15 Telemetry Mature conifer 0.5 0.5 (1.7) —
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215
Table 9.1—cont.
Author Location Scalea Season
Marten
density
Marten
statusb ncTechnique
Forest types
preferred
Shrub/
sapling
Immature/
pole Mature Old
Hearn et al.
2010
Newfound-
land
Landscape All Low Ad and J with
HR
58 Telemetry Open old
conifer
1.2h1.9i0.8 1.1j
Hearn et al.
2010
Newfound-
land
Stand All Low Ad and J with
HR
58 Telemetry Insect-killed
conifer with
understory
0.75 1.0k0.9 1.0j
Note: Values in parentheses are the maximum wi for each study
a Scale at which use and availability data were provided (stand = within home range scale)
b Ad = adult, J = juvenile; M = male only; with home ranges (HR) only if specifi ed
c n = sample size of martens with home ranges, unless noted, for all forest types
d Estimated wi only
e Unk = unknown
f HR = home range
g Data from “non-serpentine stands”; sample from “serpentine stands” was very low
h Inferred from nonsignifi cant selection for recent cuts (P = 0.7) based on 19 martens whose use exceeded availability and 16 martens whose use was less than
availability for this type
i Inferred from positive selection (P = 0.006) for regenerating cuts with dense conifer regeneration >3.5 m in height as indicated by 55 martens with positive
selection for this type and 29 martens with negative selection for this type
j Mean for tall open conifer (1.3) and tall closed conifer (0.9), for landscape scale; and 1.0 and 1.1, respectively, for stand scale; highest stand selection index was
1.5 for insect-killed old conifer with a regenerating understory
k Combined pole-sized and young regenerating forest; pole-sized was used and <3.5 m was avoided
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216
Ian D. Thompson et al.
broader selection of forest conditions has been observed (e.g., Francis and
Stephenson 1972; Chapin et al. 1997a; Dumyahn et al. 2007; Hearn et al.
2010). Boreal forests constitute >90% of the marten’s geographic distribution,
and in that zone, mixed woods and conifer forests are preferred, whereas de-
ciduous forests are avoided (e.g., Thompson and Harestad 1994; Potvin et al.
Figure 9.1. Habitat selection by American martens for broad forest stage and type classes (wi),
from studies reported in Table 9.1 that also reported selection of forest types.
Forest development stage
shrub pole mature old
Use/availability, wi
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Forest type
conifer mixedwood deciduous
Use/availability, wi
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
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Habitat Use by American Martens 217
2000; Proulx 2006b; Godbout and Ouellet 2008). In Newfoundland, where
almost all forests are coniferous, martens used old open and insect-killed for-
ests and selected for dense regenerating stands >3.5 m tall (Hearn et al. 2010).
In 2 studies where deciduous forests were used extensively by martens (e.g.,
Chapin et al. 1997a; Dumyahn et al. 2007), selection was observed for ma-
ture and old forests and the species composition of deciduous trees was di-
verse. Poole et al. (2004) also observed use of deciduous stands where a single
genus ( Populus spp.) of deciduous trees predominated; however, they also re-
ported selection by American martens for the limited amount of mature
conifer forest found on their landscape, along with use of the regenerating
deciduous stands composing most of that landscape. The overall implication
is that martens may exhibit substantial use of a diverse array of forest types,
if the required habitat structures are present and if suffi cient food is available.
Studies of habitat selection generally assume that occupancy of a particular
habitat refl ects individual fi tness (Buskirk and Powell 1994); nevertheless, as
discussed by Van Horne (1983), Buskirk and Powell (1994), and Powell
(2004), in the absence of data on fi tness, we must interpret patterns of habitat
occupancy cautiously. This is particularly true in cases where indirect meth-
ods, such as hair snags or tracks, were used as an index of use, in circum-
stances where juvenile animals or animals of unknown age and residency
status could confound interpretations, and in extensively managed landscapes
(e.g., boreal) where martens occur at low densities and there is a high proba-
bility of Allee effects (e.g., Morris 1992, 2002).
Where habitat selection was examined at both landscape and stand scales,
results have generally indicated that models tended to predict occurrence bet-
ter at the landscape scale than at the stand scale (Bissonette et al. 1997; Potvin
et al. 2000; Fuller and Harrison 2005; Mowat 2006; Slauson et al. 2007;
Baldwin and Bender 2008; Godbout and Ouellet 2008). This fi nding may be
partly explained by the fact that territorial animals must regularly visit all of
their territory to exclude other adults of the same sex and may traverse a vari-
ety of forest types, including those that are not preferred. Further, American
martens have large spatial requirements and may exhibit strong selection
when positioning their home ranges on heterogeneous landscapes to maxi-
mize fi tness. The species responds to habitat loss through habitat selection at
the landscape scale by maintaining high densities in high-quality, unmanaged
habitats (e.g., Chapin et al. 1997a), at least until a habitat-loss threshold is
reached (Thompson and Harestad 1994; Bissonette et al. 1997). Occupancy
by American martens across a forested landscape declines when 25–30% of
the landscape is regenerating following timber harvesting (Chapin et al. 1998;
Hargis et al. 1999; Potvin et al. 2000; Fuller and Harrison 2005) (Table 9.2).
Further, a link has been found between landscape pattern, habitat quality,
and home range size. For example, Potvin and Breton (1997) reported that
increased area in openings resulted in larger home ranges, Thompson (1994)
found larger ranges in regenerating than in old forests, and Fuller and
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Table 9.2. Forest variables or metrics measured at landscape or home range (HR) scales that suggested which forest type or condition was
important to resident American martens in managed forests
Study Location Methods naSignifi cant variables (values if given) Fitness
Chapin et al. 1998 Maine Telemetry /
GIS grid
cells
33 Median area of used forest patches 27 ha;
at 250 ha grid, 59% of entire landscape
was residual forest; male and female
residents averaged <22% of regenerating
forest in HRs
Lowb
Hargis et al. 1999 Utah Livetrap /
18 9-km2
plots / GIS
Fragstats
46 Area of clear-cut >25% resulted in reduced use N/A
Potvin et al. 2000 Quebec Telemetry
/ GIS
Fragstats
20 HR size proportional to area of mature forest
(mixed and conifer); HRs had >40% uncut
and <30% regenerating forest
N/A
Poole et al. 2004 British Columbia Telemetry 29 Stands with >25% conifer preferred Population stablec
Fuller and Harrison
2005
Maine Telemetry 18 Maximum proportion of partial harvest in
HR in winter 34% (increases to 73% in
summer)
N/A
Bull et al. 2005 Oregon Telemetry 20 Stands with no harvesting preferred; especially
old subalpine fi r/Engelmann spruce types
N/A
Dumyahn
et al. 2007
Wisconsin Telemetry /
GIS grid
cells
13 At least 70% suitable cover types; high
use of mature upland deciduous forest
N/A
Hearn et al. 2010 Newfoundland Telemetry /
GIS
58 Open old conifer preferred at landscape scale;
insect-killed old forest with regenerating
understory preferred at stand and landscape
scales. High use of regenerating forests but
avoidance of recent cuts.
N/A
a n = number of adult marten with home ranges, except for Hargis et al. (1999) in which n = number of adults live-trapped
b High human-caused mortality with annual replacement; no measure of lambda
c No measure of lambda
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Habitat Use by American Martens 219
Harrison (2005) reported that, in fall and winter, home ranges containing
partially harvested forests were twice the size of those composed primarily of
mature forest. In addition, Fuller and Harrison (2005) reported that martens
exhibited landscape-scale selection during winter for habitats with higher
overstory canopy cover by expanding their home ranges, and suggested that
this may be a predator-avoidance behavior. Chapin et al. (1997a) and Phillips
et al. (1998) reported no habitat shifts in winter at the scale of the home
range, and Payer et al. (2004) observed no differences in seasonal or annual
home-range fi delity among martens inhabiting managed versus unmanaged
landscapes. The resident adult martens studied by Payer et al. (2004) selected
for mature forest within heterogeneous landscapes and exhibited high fi delity
despite large variation in landscape composition. Collectively, these fi ndings
argue strongly in favor of landscape-scale habitat selection as a primary infl u-
ence on the individual fi tness of martens.
Density-dependent effects have rarely been explicitly examined for martens
as a hypothesis to explain habitat selection (Table 9.3) by testing predictions
among competing models. However, several studies (Hodgman et al. 1994;
Thompson 1994; Johnson et al. 2009) have reported density-related behav-
iors, such as dispersal to habitats with a low density of resident martens, and
Payer et al. (2004) reported that the degree of territorial overlap was density
Table 9.3. Support (yes/no) for 4 habitat-selection hypotheses from studies of American
martens published since 1993 that considered stand- or landscape-scale habitat selection
Study
Prey
abundance
Habitat
features
Predator
avoidance
Density
dependence
Thompson and Curran 1995aYes Yes N/A N/A
Paragi et al. 1996 Yes No N/A Yesb
Chapin et al. 1997a N/A Yes N/A N/A
Hargis et al. 1999 No Yes N/A N/A
Potvin et al. 2000 N/A Yes N/A N/A
Payer and Harrison 2003 N/A Yes YescN/A
Poole et al. 2004 No Yes No (Stable
density)
Gosse et al. 2005 N/A Yes N/A N/A
Fuller and Harrison 2005 Yes Yes YescN/A
Bull et al. 2005 N/A Yes N/A N/A
Mowat 2006 N/A Yes N/A N/A
Dumyahn et al. 2007 N/A Yes N/A N/A
Slauson et al. 2007 N/A Yes N/A N/A
Coffi n et al. 1997 Yes Yes N/A N/A
Godbout and Ouellet 2008 N/A Yes N/A N/A
Hearn et al. 2010 YescYes No cN/A
Note: N/A = factor not tested
a Marten density inferred from earlier work
b Selection infl uenced by density although lambda was not reported
c Inferred but not measured directly
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220
Ian D. Thompson et al.
dependent. Fryxell et al. (1999) related marten density directly to prey levels,
and Hearn et al. (2010) suggested that marten habitat choice was related to
snowshoe hare ( Lepus americanus ) abundance (as prey) in Newfoundland. In
Ontario, however, high hare abundance in regenerating forests did not entice
marten to use these habitats, even during years of high densities of martens or
hares (Thompson and Colgan 1987), suggesting that factors other than food
alone were strongly infl uencing habitat selection in that area. Similarly, Fuller
and Harrison (2005) reported high predation on hares by American martens
during winter and an order of magnitude higher hare densities in regenerating
stands compared with mature forest stands. Despite apparent foraging advan-
tages in those regenerating habitats, American martens strongly avoided re-
generating forests at the stand scale, which the authors postulated was a result
of inadequate escape cover for the species in regenerating stands. Thus, recent
work has demonstrated that population densities of American martens and
their prey are related to many aspects of habitat use, but uncertainty remains
regarding possible density-dependent mechanisms of habitat selection and
their effects on the individual fi tness of American martens.
Habitat Selection and Dispersal
at the Landscape Scale
It is adaptive for animals to select habitats that will maximize their indi-
vidual fi tness (Stephens and Krebs 1986; Powell 2004). During periods of
high population density, however, juvenile American martens born in supe-
rior habitat may be forced to disperse to inferior habitats (Johnson et al.
2009). Thus, managing marten habitat at very large spatial scales may be
particularly important if selection at the landscape scale affects home range-
and site-scale habitat selection in a density-dependent manner; however, the
process of habitat selection may be limited in scale (Morris 2002). For exam-
ple, such a threshold could be determined by the capacity of a species to dis-
perse. Johnson et al. (2009) found that 80% of dispersing juvenile martens
settled <20 km from where they were born to where they established a home
range, although in some cases the animals moved much farther than 20 km
before returning (Broquet et al. 2006a). The dispersal process is likely depen-
dent to some extent on the availability of suitable habitat, but it provides some
guidance about the appropriate scale for studying habitat selection.
Johnson et al. (2009) showed that dispersal distance and success (survival
and settling) were highly dependent on the quality of the landscape through
which the martens were moving. Marten dispersal success and distance, and
hence habitat selection, were probably related primarily to the population
density of adult animals, and hence territorial vacancies, with lower popula-
tion density on a regenerating managed landscape than on an unmanaged
mature–old forest landscape. If surrounding preferred habitat is saturated,
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Habitat Use by American Martens 221
animals might be compelled to move farther than in a landscape where space
is available, including moving through or into poor habitat to fi nd a vacant
home range. In very large progressively logged landscapes, most martens do
not move far enough to leave the landscape in which they were born and settle
in an unmanaged landscape (Johnson et al. 2009). Thus, it may be that use of
managed forests refl ects a long-term (e.g., decadal-scale) accumulation of dis-
persal events. In years when American martens are abundant, some animals
must leave the unmanaged forests and settle in managed landscapes, suggest-
ing either a source-sink or an ideal despotic (Fretwell and Lucas 1969) density
model. Certainly, some American martens are capable of long-distance dis-
persal (Thompson and Colgan 1987; Broquet et al. 2006a; Johnson et al.
2009), although survival and fi tness are generally lower in managed than in
unmanaged forests as a result of commercial trapping and increased natural
(mostly predation-caused) mortality (Hodgman et al. 1994; Thompson 1994;
Johnson et al. 2009; but see Hodgman et al. 1997). A population in poor
habitat (sink) could persist with periodic emigration from preferred habitats
(source), as was suggested by Paragi et al. (1996) for young regenerating
burned forests adjacent to unmanaged old forests in Alaska, and by Hodg-
man et al. (1994) for managed forests next to a preserve in Maine. Other
population models, such as ideal despotic for example, might apply for mar-
ten populations living in a more heterogeneous matrix of forest ages and
types.
Habitat Selection Within Stands
Stand-scale habitat selection has been observed in most studies, and forest
structures are important features that infl uence marten selection of individual
forest stands, often regardless of stand age or type (Chapin et al. 1997b; Payer
and Harrison 2003). Linkages have rarely been made between stand-scale
variables and the individual fi tness of American martens. Various studies
have described the stand conditions martens prefer, however, with some (e.g.,
Payer and Harrison 2000, 2003) providing considerable detail on complex
forest structures (Table 9.4). For example, Payer and Harrison (2003) indi-
cated that, in Maine managed forests, martens used stands with a basal area
>18 m
2 /ha, mean tree (dbh 7.6 cm) height 9 m, and snag (dbh 7.6 cm and
>2 m tall) volume 10 m
3 /ha. These variables are believed to have functional
importance to martens by providing cover from predators, facilitating forag-
ing for prey, enabling resting or denning, and providing other benefi ts. Coffi n
et al. (1997) and Bowman et al. (2000) noted a relationship between southern
red-backed voles ( Myodes gapperi ), a main prey of martens in many regions,
and forest structures. Similarly, Andruskiw et al. (2008) illustrated the func-
tional importance of coarse woody debris to hunting success. A nutritional
link to breeding success has previously been well established for American
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222
Table 9.4. Stand- or site-scale variables that were important in models or univariate tests
of habitat use by American martens within home ranges
Study
Location /
method Forest stage na
Signifi cant
variables (and
values if given)
Thompson
1994b
Ontario /
telemetry
Old and
regenerating
37 Mean tree density
(>10 cm) 680/ha;
conifer trees 80%;
canopy 60%; mean
tree (>10 cm dbh)
height 18 m
Bowman and
Robitaille
1997c
Ontario / winter
tracks
Regenerating 151 used sites in
regenerating
forest
Conifer trees 76%;
mean tree height
13 m; canopy 80%;
logs/100-m2 quadrat
3.5
Chapin et al.
1997a
Maine /
telemetry
Mature and
insect-killed
38 No selection among
stand types
Smith and
Schaefer
2002
Labrador/
telemetry
Old and
mature
26 (includes
some
juveniles)
Canopy >20%
Payer and
Harrison
2000, 2003
Maine /
telemetry
Mature and
regenerating
24 Mean tree basal
area >18 m2/ha;
tree height (>8 cm
dbh) >9 m; snag
volume >10 m3/
ha; snag and cull
tree BA >18 m2/
ha; CWD 55 m2/
ha; mean diameter
CWD >22 cm;
winter canopy
>30%d
Poole et al.
2004
British
Columbia /
telemetry
Mature and
regenerating
29 Shrub cover >15%;
snag density 4.5/
ha; conifer >25%
Payer and
Harrison
2004
Maine /
telemetry
Mature 57 Volume of exposed root
masses 25.2 m3/ha
Bull et al.
2005
Oregon /
telemetry
Old and
regenerating
20 Canopy >50%; large
snags (25 cm) 22/
ha; trees (>25 cm
dbh)100/ha
Fuller and
Harrison
2005
Maine /
telemetry
Partial-harvest 18 Tree basal area
>18 m2/ha; winter
canopy >30%
Slauson et al.
2007c
California /
track plates
Old 26 detections Shrub cover; %
conifer
a n = number of adult home ranges, unless specifi ed differently
b Study did not explicitly test used vs. random or unused sites
c Home ranges were not determined
d BA = basal area, CWD = coarse woody debris
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Habitat Use by American Martens 223
martens (e.g., Thompson and Colgan 1987). Substantial variation occurs
among published values for some key variables, however, such as canopy clo-
sure, percent conifer in a stand, woody debris volume, and tree and large-snag
density (Table 9.4). These differences likely refl ect variation among types of
forest ecosystems (e.g., montane vs. boreal vs. transitional) resulting from dif-
ferences among endemic tree species and the ecology of the systems, as well as
to regional differences in prey use by American martens.
Given that habitat selection by martens has been documented to occur at
the within-stand (site) scale, analyses conducted at broader scales may not
capture some important density-dependent variation. Few models have been
developed for marten habitat selection at site scales. Models described in
Chapin et al. (1997b), Ruggiero et al. (1998), and Porter et al. (2005) found
predictability for resting- and den-site selection. These models suggested that,
within home ranges, martens may make important choices about fi ne-scaled
features such as large snags, overhead cover, and conifer-tree density.
Natal Dens, Maternal Dens,
and Resting Structures
American martens appear to be more selective of habitat conditions at den
sites than at resting sites. Ruggiero et al. (1998) noted the importance of
structures associated with late-successional forests to den selection in Wyo-
ming; both the characteristics of denning structures and the features of the
stands where the structures occurred seemed to infl uence den-site selection.
Ruggiero et al. (1998) cautioned, however, that selection among natal dens,
maternal dens, and resting structures needed to be evaluated separately, given
the possibility of greater selectivity for natal dens than for other structures or
sites. The most common feature reported in studies of natal and maternal
dens was a large dead or live tree that provided a hollow space that could con-
tain a mother and her kits (Table 9.5). Such hollow spaces could result from a
split in the trunk, a woodpecker cavity, or interior rot accessible from the base
of the tree. Other common den structures included rock crevices and stumps.
Resting structures were most commonly associated with live trees, especially
those where various fungi had caused excessive growth on branches (Bull and
Heater 2000), tip-ups, and large coarse woody debris (Table 9.5).
None of the studies on fi ne-scale habitat selection (Table 9.5) provided any
indication that den structures were limiting at the population level. Several
researchers have found little difference in the abundance of important struc-
tures, such as coarse woody debris, between used and unused habitats (e.g.,
Thompson and Colgan 1987; Payer and Harrison 2004). This suggests that,
at least in some ecosystems, certain fi ne-scale habitat elements were either not
limiting or were not quantifi ed at a suffi ciently refi ned level (e.g., volume or
number of coarse woody debris >25 cm in class 2 as opposed to all coarse
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224
Ian D. Thompson et al.
Table 9.5. Selection of resting and denning structures by American martens
Study
Location /
method naStructures used (%)
Mean
characteristics
(if values given)
Gilbert et al.
1997
Wisconsin /
telemetry
32 rw
19 rs
7 d
rw: tip-up (44),
CWDb (28)
rs: live tree (68)
d: live tree (71)
d: >50 cm tree
Chapin et al.
1997b
Maine /
telemetry
73 rw
67 rs
rw: subnivean (40),
tip-up (18),
stump (18)
rs: live tree (53),
snag (15)
—
Raphael and
Jones 1997
Oregon /
telemetry
95 rw
163 rs
31 d
rw: subnivean (41),
CWD (31)
rs: slash (60),
CWD (16)
d: CWD (32),
slash (29)
—
Raphael and
Jones 1997
Washington /
telemetry
140 rw
240 rs
26 d
rw: live tree (53),
snag (20)
rs: live tree (46),
snag (22)
d: live tree (54),
snag (31)
r: live tree 88 cm
d: live tree or snag
98 cm
Ruggiero et al.
1998
Wyoming /
telemetry
105 d d: rock crevice (28),
snag (25), squirrel
midden (19)
d: snag 55 cm
Bull and Heater
2000
Oregon /
telemetry
1184 r
w and s
30 d
r: live tree (58),
log pile or
hollow log
(w 23, s 9),
snag (17)
d: cavity (40),
hollow log (11)
r and d: live tree
52 cm,
hollow log
(CWD) 66 cm,
snag 79 cm
Wilbert et al.
2000
Wyoming /
telemetry
190 rw rw: subnivean
CWD (40),
live tree (35),
snag (15)
rw: snags and live
trees 52 cm;
CWD 39 cm
Porter et al.
2005
British
Columbia /
backtracking
52 rw snag (unk) rw: snags Class 3+
a n = number of resting structures (r) in summer (s) or winter (w), and natal or maternal den
structures (d); only the 2–3 most abundant structures are provided from each study
b CWD = coarse woody debris
woody debris). However, high-quality natal den structures could be a limiting
feature to a marten population, especially in managed forests with small-
diameter trees, few snags, and little woody debris (Ruggiero et al. 1998). Fu-
ture work should be directed toward determining which fi ne-scale features
may limit the population persistence of American martens in various forest
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Habitat Use by American Martens 225
types. For example, Raphael and Jones (1997) observed that, where natural
structures were scarce in managed areas, American martens used slash piles
for denning, but the implications for relative fi tness were not explored.
Energetic Considerations
Martens are believed to have high energy demands because of their body
form and exposure to winter weather (Buskirk and Harlow 1989; Harlow
1994). Thompson and Colgan (1994) showed a linear decline in marten activ-
ity at ambient temperatures below –5 °C, and Wilbert et al. (2000) found that
accumulating snow reduced activity levels. An important component of habi-
tat for American martens is access to structures that enable them to conserve
body heat, especially in winter. For example, resting sites in winter are often
subnivean (Table 9.5), and Buskirk and Powell (1994) and Taylor and Buskirk
(1994) found that the microenvironments of resting sites enabled behavioral
thermoregulation in all seasons. Use of subnivean sites for resting in Wyo-
ming was more likely when snow cover was deep, when snowfall the previous
day had been heavy, and when low temperatures prevailed (Wilbert et al.
2000). Although snow as well as features enabling subnivean access (Corn
and Raphael 1992) and those providing thermal cover (Chapin et al. 1997b)
likely infl uence resting-site selection by martens, several authors (Corn and
Raphael 1992; Thompson and Colgan 1994; Chapin et al. 1997b) have con-
cluded that these features are not necessarily limiting to populations. Again,
additional work is needed on the relationships between the availability of den-
ning and resting structures (including those that are subnivean) and the fi tness
of American martens. Finally, ambient temperature and snow are key envi-
ronmental infl uences on microhabitat selection by American martens, but
unless they are studied at fi ne spatial scales, subtle behavioral and fi tness im-
plications may not be apparent.
Effects of Forest Management
Over the long term, an important question in our understanding of the ef-
fects of forest management is whether or not second-growth forests will
converge with primary forests in terms of ecosystem processes, patterns,
and structures. Findings from several areas (Maine, Quebec, Ontario, and
British Columbia) suggest that, in some forest types and with management
guidance, martens may be able to persist in managed forests, although long-
term data are lacking. Further, densities in managed landscapes may be
similar to, or lower than, densities in unmanaged forests, depending on for-
est type and prevailing forest management practices (Thompson 1994;
Poole et al. 2004; Fuller and Harrison 2005). Mechanisms that are impor-
tant to the relationships among marten density, forest management, and
available forest types require consideration of alternative explanations
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226
Ian D. Thompson et al.
tested against multiple hypotheses over time to fully elucidate these complex
relationships.
Effects of Clear-cut Harvesting
As noted above, where clear-cut harvesting is practiced, marten popula-
tion declines can be expected for 40 years if >25–30% of the forest is com-
posed of regenerating stands (Thompson and Harestad 1994; Chapin et al.
1998; Hargis et al. 1999; Potvin et al. 2000; Poole et al. 2004). An exception
was reported from Newfoundland, where martens fully exploited landscapes
in which much of the forest comprised regenerating harvested areas or insect-
killed stands, but where most predators and competitors of American martens
were uncommon or absent (Hearn et al. 2010). Few data are available to de-
termine whether American martens will persist in regenerated clear-cut for-
ests once they reach maturity, because none of these stands have reached old
age. During a 4-year study in British Columbia, Poole et al. (2004) reported
adult resident martens living at moderate densities in second-growth forests
about 40 years after clear-cutting and previous agricultural use. In Maine
(Chapin et al. 1998; Payer and Harrison 2003; Fuller and Harrison 2005) and
Quebec (Potvin et al. 2000), martens were studied in matrix landscapes com-
prising a mixture of stands regenerating after modifi ed clear-cut and patch
harvesting, interspersed with large stands of unharvested mature forest.
These residual stands infl uenced home range placement and movements across
the landscape by American martens, but the animals made some use of regen-
erating stands and avoided the youngest stands. Median size of used residual
stands in Maine was 150 ha, whereas in Quebec, the size of home ranges was
inversely proportional to the amount of residual uncut forest they encom-
passed, which on average was 60–70% of a home range. Landscapes with
progressive and large-scale clear-cutting with few remaining old or mature
stands, however, supported substantially fewer animals during the regenerat-
ing period than matrix landscapes (e.g., Thompson 1994; Potvin et al. 2000;
Payer and Harrison 2003; Godbout and Ouellet 2008).
The length of time in which regenerating habitats are unsuitable to Ameri-
can martens seems to depend strongly on landscape context, forest type, and
silvicultural practices. Certain forest types lack suitable structures for long
periods of time, whereas in others, the structures needed by the species are in
more continuous supply (e.g., Chapin et al. 1997a; Payer and Harrison 2004).
Although understory conditions may not be limiting in some areas, required
overstory conditions (e.g., overstory canopy closure and the basal area of re-
sidual trees) may be limiting for longer periods in managed landscapes (Payer
and Harrison 2003). For example, low residency of American martens has
been observed in extensively clear-cut boreal forests up to 50 years after har-
vest (Thompson 1994; Godbout and Ouellet 2008; Thompson et al. 2008). In
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Habitat Use by American Martens 227
contrast, Newfoundland martens used stands that were considerably less than
50 years old (including dense regenerating stands with open overstories and
pre-commercially thinned stands) but where ecological release from predation
may have infl uenced habitat selection (Hearn et al. 2010). In many forest
types, even young regenerating stands may be used to obtain berries that are
seasonally abundant in these areas, but these areas are usually avoided at all
other times (Steventon and Major 1982; Thompson and Colgan 1987; Poole et
al. 2004). Paragi et al. (1996) found American martens occupying 30-year-old
regenerating burned habitats surrounded by older forests, but suggested that
the animals were mostly juveniles and could not persist. Ecosystem type and
regeneration following disturbances are related to structural features and prey
densities, which may infl uence occupancy in both time and space (e.g., Chapin
et al. 1997a; Payer and Harrison 2003; Poole et al. 2004; Johnson et al. 2009).
Effects of Partial Harvesting
Consistent with the concept that the American marten can persist in man-
aged landscapes if certain thresholds are not surpassed, Fuller and Harrison
(2005) found that the species tolerated partial harvesting to a basal area of 18
m
2 /ha with 30% canopy closure in transitional forests in Maine; however,
martens expanded their home ranges in winter to include more residual for-
est. In Quebec, Godbout and Ouellet (2008) found that mature conifer forest
was the main forest type (49%) in American marten home ranges and that the
animals tolerated partial harvesting that left at least 40% canopy closure,
while avoiding stands that were pre-commercially thinned. Their results were
similar to the fi ndings of Potvin et al. (2000) in an area of mixed modifi ed
and clear-cut harvesting in mixed-wood forest. American martens sometimes
tolerate more partial harvesting than clear-cut harvesting within the forest
matrix; however, they appear very sensitive to the residual basal area of ma-
ture overstory trees and the extent of winter canopy cover remaining after
partial harvests (Fuller and Harrison 2005). This suggests that with careful
consideration and additional studies, some opportunities may exist to manage
for timber production and still maintain viable populations of American mar-
tens. For any forest system in which selection harvesting is conducted, thresh-
old values for canopy closure and species composition need to be developed if
the species is to be conserved. It should also be recognized that partial har-
vesting may reduce the total available uncut forest by requiring a larger har-
vesting footprint to meet timber-harvest goals.
Testing Hypotheses for Habitat Selection
Few explicit tests of the hypotheses proposed by Buskirk and Powell (1994)
and Thompson and Harestad (1994) have been conducted for habitat selection
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228
Ian D. Thompson et al.
by American martens at the stand or landscape scale. Among the 16 studies
reviewed (Table 9.3), only 6 attempted to measure variables other than vege-
tative characteristics to explain habitat use by American martens. Support for
the site-scale habitat features hypothesis was suggested in 15 studies, whereas
4 of 6 studies provided evidence that prey abundance was important (Ta -
ble 9.3). Density-dependent habitat selection was suggested by Paragi et al.
(1996) in burned and unburned forests in Alaska. The possibilities of preda-
tor avoidance and food limitation were invoked in several studies but not as-
sessed with data on predator abundance or mortality rates. Although many
habitat-selection studies have been conducted during the past 18 years, few
have controlled for potentially mechanistic variables, including relative food
abundance, predators, or density dependence.
Conclusions
American martens generally prefer mature or old forests over regenerating
stands in areas where both are available in the landscape. This fi nding is espe-
cially true in conifer and mixed-wood forests, but less so in deciduous forests,
particularly those dominated by only 1 or 2 species. Martens seem to have
greater affi nity for mature deciduous stands where transitional northern
hardwood forest stands intersperse with mixed-wood and coniferous stands
across the landscape. Where they still occur, the importance of mature and
old forests to martens is clearly evident. When the availability of these habi-
tats in a landscape is reduced to <70% from wildfi re or forest management,
the population density of American martens declines. In matrix landscapes
that include a mixture of high- and low-quality habitats, American martens
appear to select home ranges dominated by high-quality habitat types, but
also include some recent clear-cuts and regenerating forests (e.g., Chapin et al.
1997a; Potvin et al. 2000; Fuller and Harrison 2005). In progressively clear-
cut boreal habitats, declines in the population density of American martens
result from reduced habitat quality, and there is limited evidence of recovery
to pre-harvest levels in regenerating forests after 40–50 years (e.g., Thompson
1994; Godbout and Ouellet 2008), which represent the oldest second-growth
boreal forests currently available. A notable exception occurs in Newfound-
land, where martens use regenerating forests and pre-commercially thinned
stands, but where there are few predators and competitors of martens (Hearn
et al. 2010). In contrast to fi ndings from most boreal and montane forests,
American martens in the transitional forests of the northeastern United States
and southeastern Canada can persist in managed forest landscapes where re-
sidual forests remain the dominant background matrix within occupied home
ranges (Taylor and Abrey 1982; Chapin et al. 1998; Fuller and Harrison
2005; Dumyahn et al. 2007). Although there is a preference for mature stands
in these forests, American martens persist in stands with basal areas >18 m
2 /ha
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Habitat Use by American Martens 229
and trees >9 m tall, but avoid young regenerating forests (Chapin et al. 1997a;
Payer and Harrison 2004; Fuller and Harrison 2005).
The apparent disparity among studies suggesting a requirement by Ameri-
can martens for old forest stages and those fi nding the species persisting either
in mixtures of young and old forest on a landscape or in regenerating forests
appears to be linked to key aspects of marten ecology in different forest types.
Other factors include differences in prey abundances and diet preferences,
variation in the local predator community, or perhaps differing climatic con-
ditions. Martens need certain structures to improve hunting success (An-
druskiw et al. 2008) and for denning (Ruggiero et al. 1998). Studies have
shown that in some forest types, these structures are available in stands well
before the mature or old stages, such as in insect-killed stands (Payer and
Harrison 2000; Hearn et al. 2010), or they can potentially be maintained
during some forms of partial harvesting if suffi cient residual tree basal area
and overstory canopy closure in winter are maintained (e.g., Fuller and Har-
rison 2005). In most forest types, however, the structures required by Ameri-
can martens are absent for >50 years after clear-cutting (e.g., Godbout and
Ouellet 2008; Thompson et al. 2008). Further, martens seem to require over-
head cover, possibly in multiple layers, presumably to avoid or escape preda-
tors (e.g., Buskirk and Powell 1994; Thompson and Harestad 1994). Such
cover can occur in regenerating conifer forests at very early stages, especially
where stem densities are high (Hearn et al. 2010), whereas in deciduous-
dominated forests (especially during winter), these cover requirements may be
met only in mature deciduous stands with high species richness (Chapin et al.
1997a). Where adequate overhead cover is not maintained during winter,
American martens may seasonally expand their home ranges to include more
cover (e.g., Fuller and Harrison 2005). Poole et al. (2004) also found that the
species used open, deciduous-dominated forests that maintained high struc-
ture and food levels, but they did not provide data on potential predators of
American martens. Hearn et al. (2010) suggested that, under reduced risk of
predation, American martens may make greater use of open stands, especially
where there are higher prey densities than in older stands. Marten habitat use
varies among regions; consequently, if maintaining marten populations is a
management objective, assumptions about stand-scale habitat relationships
should be ecosystem-specifi c and not based on general understandings.
Since the 1994 reviews, our understanding of use and selection of habitats
by martens has improved considerably. This is especially true regarding the
importance of spatial scale from sites to landscapes. At the same time, our
understanding of the mechanisms affecting habitat selection has not advanced
to the same degree. Most authors have implicitly assumed that habitat choices
made by martens maximize or improve individual fi tness. In general, this is a
reasonable assumption for American martens (Buskirk and Powell 1994; Bus-
kirk and Ruggiero 1994), although it is widely understood that density and
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230
Ian D. Thompson et al.
fi tness may not be correlated (Fretwell 1972; Van Horne 1983; Hobbs and
Hanley 1990). For American martens and other Martes species, this discon-
nect may be related to nonoverlapping home ranges and, possibly, an ideal
despotic distribution. There are limitations to habitat-selection models, espe-
cially in the absence of fi tness correlates to improve our understanding of the
ways in which selection might change under varying conditions. For example,
habitat-selection theory suggests that individual fi tness declines as a function
of population density (e.g., Fretwell and Lucas 1969). For American martens,
however, we might not expect such a relationship if females always occupy
and protect the best habitats to buffer any negative effect (despotic habitat
selection). At a minimum, we believe that habitat choices can be understood
only in the light of some measure of both population density and survivor-
ship. For martens, few data are available on the relationships among demog-
raphy, dispersal, and habitat selection. Predicting the likelihood of long-term
population persistence is not possible without this information. Consequently,
the challenge for future research is to provide a more detailed demographic
approach to understanding the ways in which martens will respond to altered
forest conditions and climate change (Carroll 2007) by employing testable
predictions based on the knowledge we have gained over the past 18 years.
Acknowledgments
This chapter benefi ted immensely from reviews by an anonymous reviewer
and by Steve Buskirk, and from editing by Keith Aubry.
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