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OCR for page 227
17
Conserving a Regional
Spotted Owl Population
Human influence on the environment has reduced the populations of
many plants and animals. Small populations are vulnerable to extinction,
because of the difficulty of finding mates and because of random fluc-
tuations, disease, unfavorable weather, and other catastrophic events. When
populations of a species are small and fragmented (a common situation
when habitat destruction is the cause of the reduction in population size),
long-term survival can also be threatened by genetic deterioration the
deleterious effects of inbreeding and loss of genetic variability.
The survival of spotted owls is of increasing concern, because their
required old-growth forest habitat is being reduced and fragmented by
logging. The planning for spotted owl management described here is based
on theoretical population genetics and ecology, as well as on quantitative
natural history observations on reproductive ecology, dispersal, and for-
aging behavior. Although the underlying theory is not yet fully developed,
it is applicable to many attempts to conserve populations of vertebrates
with low reproductive potential.
227
OCR for page 228
Case Smutty
HAL SALWASSER, USDA Forest Service, Wildlife and Fisheries
Staff, Washington, D.C.
INTRODUCTION
Maintaining the full diversity of native vertebrates is a legal mandate
of the federal government (Endangered Species Act of 1973, as amended;
National Forest Management Act of 1976), as well as a policy of many
state resource management agencies. In the late 1970s, the United States
Department of Agriculture (USDA) Forest Service adopted regulations
(36 CFR 219) requiring habitats to be managed to maintain viable pop-
ulations of all native vertebrates in the national forest system over 190
million acres of land and water.
Since the establishment of the national forests in the early 1900s, state
and federal policies and lack of attention have resulted in the loss of
species, such as wolves (Cants lupus) and grizzly bears (Ursus arctos),
from some forests. Land and resource management planning under the
National Forest Management Act of 1976 is intended to prevent further
loss of species.
The spotted owl (Strix occidentalisJ constitutes a major test of the policy
of maintaining species in national forests. The northern subspecies (S. o.
caurina Merriam 1898) inhabits mature and old-growth coniferous forests
in the Cascade, Klamath, and Coast Range mountains of Washington,
Oregon, and California. It appears to need stands of trees more than 24
in. in diameter, a multilayered canopy more than 70% closed, and large
standing and fallen dead trees (Foreman et al., 19841. Such forests have
extremely high commercial timber value (Heinrichs, 1983), and therein
lies the spotted owl dilemma: the kind and amount of habitat required for
survival of each pair of owls is a resource highly valued by an industry
that is the economic backbone of the Pacific Northwest.
Further fragmentation of old-growth forest might impede dispersal of
owls and isolate populations that are too small to survive for long. Thirty
to forty years ago, half the original 15 million acres of old-growth forest
in the Pacific Northwest remained (Franklin, 1984), and the spotted owl
was not an issue in forest management. Timber harvests in the last few
decades have removed nearly all the easily accessible lowland old-growth
forest, and the much-reduced spotted owl population now exists primarily
in rugged, mountainous terrain.
A major purpose of the national forests is to sustain yields of different
228
OCR for page 229
CONSERVING A REGIONAL SPOlTED OWL POPULATION
229
kinds of resources. Timber, wildlife, and wilderness are specifically men-
tioned in the law (Multiple-Use Sustained Yield Act of 1960; Wilderness
Act of 1964~. What is the most efficient way to manage forests to maintain
both a continuous flow of timber products and viable populations of spotted
owls and their coinhabitants of old-growth forests?
By the mid-1970s, the biology of the spotted owl was under study, and
inventories had been initiated. In 1974, biologists began to develop man-
agement guidelines based on the new information, to prevent the owl from
declining so much that formal listing as a threatened species would be
necessary. Research and inventories have continued, and the management
guidelines have evolved to incorporate new findings. In 1981, the guide-
lines were revised, largely under the influence of the work of Soule (1980),
to include new theories and observations concerning the genetic basis of
population viability.
This chapter presents an overview of an evolving population manage-
ment plan for the fragmented spotted owl population over a large area of
the Pacific Northwest. The plan attempts to integrate management of
individual national forests over the whole Pacific Northwest region. Rather
than attempting to review the considerable research on spotted owl biology
in detail, this discussion incorporates demographic estimates based on the
research.
THE BASIC PROBLEM AND APPROACH
One objective of the Forest Service has been to develop plans for the
national forests in the Pacific Northwest that protect resident populations
of the spotted owl while allowing the multiple uses of the forest required
by law. The primary threat to the owl is further reduction and fragmentation
of its old-growth forest habitat through logging. Thus, the plan must deal
with the effects of logging on the suitability of habitat for long-term
maintenance of individual pairs, populations in specific national forests,
and the whole regional population. Hence, the problem is to manage
logging and related activities so that remaining old growth will support
long-term survival of spotted owls in all national forests in the Pacific
Northwest. Four general ecological issues are most important:
· It is necessary to determine the habitat characteristics required for
the survival and successful nesting of individual pairs of owls. This ul-
timately involves detailed studies of owls to determine patterns of habitat
use and factors that influence habitat quality, the relationship between
habitat quality and home range size, and specific requirements for nesting,
foraging, roosting, and dispersal. Several studies are being conducted.
OCR for page 230
230
SELECTED CASE STUDIES
Owls have been censused to establish which habitats they occur in and
with what frequency and to determine the spacing between pairs. Radio-
telemetry has helped to determine patterns of habitat use and the size of
home ranges of individual birds, and cast pellets have been examined to
determine what prey are taken. These studies have provided the input for
habitat-suitability index models that can evaluate habitat quality on the
basis of specific features (Laymon et al., in press).
· The distribution of habitats is critical to the successful dispersal of
juveniles and movement of adults into suitable vacant habitat. Radio-
telemetry has been used to determine patterns of dispersal of juveniles
and to determine what kinds of habitats they will not cross. Adults with
radio transmitters have also been followed to determine their movements
between areas and throughout the year. The information can be used to
develop criteria for minimal distances between patches of habitat during
dispersal and for resident birds and to determine the best configuration of
habitat that connects populations in adjacent forests.
· The population in each forest must be large enough to withstand
normal environmental fluctuations and random demographic changes with-
out becoming locally extinct. In addition, the regional population must
be sufficiently large and well distributed to withstand severe environmental
fluctuations and reductions and even extinction of local populations. These
requirements are being studied with a general model of regional population
dynamics and with demographic values determined for the spotted owl
from field studies and from the literature on better-studied species. The
model is evolving to incorporate estimates of adult and juvenile mortality,
range of dispersal, reproductive success, and other variables.
· An effective population large enough to minimize the deleterious
genetic effects associated with small and isolated populations must be
maintained. The northwest spotted owl population is a metapopulation
(Levins, 1970), i.e., a large regional population made up of many smaller
populations of varied sizes, densities, and degrees of isolation from one
another. Metapopulations are subject to two genetic problems: a loss of
average individual fitness through an increase in the frequency of breeding
of close relatives (inbreeding depression) and a reduction in potential for
evolutionary adaptation because of a loss of genetic variability. Both these
problems can be reduced by maintaining a number of relatively large local
populations with substantial gene flow among them. A model similar to
that used for population dynamics is being used with values for the spotted
owl from field studies and from the literature on more intensively studied
species.
Planning for protection of the spotted owl began in the early 1970s and
OCR for page 231
CONSERVING A REGIONAL SPOTTED OWL POPULATION
231
continues today. An interagency task force of state and federal scientists
and managers developed interim spotted owl management guidelines in
the middle and late 1970s; later research led to a revision in 1981. The
focus was initially on the kinds and amounts of habitat needed by individual
pairs. The late 1970s saw increased academic interest in long-term pop-
ulation viability as the threat of mass extinction of species became widely
recognized. The result has been more research on factors underlying ex-
tinction (e.g., Frankel end Soule, 1981; Schonewald-Cox, 1983; Soule
and Wilcox, 19801. A 1982 workshop that included management biologists
and academic ecologists studying population viability used the spotted
owl as a basis for developing a general risk-assessment planning process
for long-term population management (Salwasser et al., 19841. The ap-
proach has been revised to incorporate new information on other species
and the results of a second workshop that was held in the fall of 1984.
Debate concerning a plan for managing spotted owls has focused on
several issues. First, what are the specific habitat requirements of indi-
vidual pairs of owls? The management unit for individual pairs or small
groups of pairs with contiguous home ranges is called a spotted owl
management area (SOMA). The type, amount, and distribution of habitat
that should constitute an adequately managed SOMA have been contro-
versial. Underestimation of minimal home range size or quality of sites
needed or too much fragmentation of habitat within a SOMA could reduce
the probability of survival and reproduction of a pair. Second, are the
demographic estimates used in the management models sufficiently ac-
curate? At issue are estimates of adult and juvenile mortality, reproductive
rate, dispersal distance, habitat occupancy, and related population char-
acteristics. Third, are the models themselves adequate and appropriate?
Are demographic or genetic problems more important to long-term via-
bility? The current guidelines are based on the assumptions that genetic
problems are more critical at the scale of the regional population and that
preventing genetic deterioration will prevent demographic collapse. De-
mographics and biogeography are assumed to be most critical at the scale
of a forest population.
The overriding issue concerns the minimal regional population size
necessary for long-term survival and consequently the amount and distri-
bution of old-growth forest to be provided in the future. Points of view
range from the position of some environmental interest groups, that too
much old growth has already been cut and that habitat management criteria
(particularly minimal SOMA characteristics) are inadequate, to the view
of some representatives of the timber industry, that current management
guidelines are too stringent and that the current estimate of minimal ac-
ceptable population size is too large. The current guidelines were under
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232
SELECTED CASE STUDIES
administrative appeal in March 1985, and legal action might be taken by
parties at both extremes.
ECOLOGICAL KNOWLEDGE USED IN DEVELOPING
THE MANAGEMENT GUIDELINES
The management guidelines are based on data, principles, and theory.
In addition to field research on the biology of spotted owls themselves,
the guidelines rely heavily on the findings of recent theoretical and em-
pirical research on the long-term viability of populations of other species,
both in captivity and in the wild. Studies on spotted owls are used to
estimate values for general models of population viability based on this
recent literature.
Studies of the Ecology of Spotted Owis
Research on spotted owls has focused on breeding biology, foraging
ecology, habitat use, and general distribution. Because spotted owls are
nocturnal and now inhabit rugged, mountainous terrain almost exclusively,
field research is very difficult, and much of their biology is still poorly
known.
Individual pairs of owls do not reproduce every year, and clutch size
averages only about two eggs (Foreman, 1980~. In addition, the survival
of juveniles appears to be extremely low; no juveniles with radio trans-
mitter harnesses have survived to breed. Hence, spotted owls appear to
have a very low reproductive potential and thus poor ability to recover
from reductions in population size.
Spotted owls in the Northwest appear to be nonmigratory, and radio-
telemetry has shown that adults move over extremely large home ranges
in the course of a year. A home range can be as large as 8,300 acres for
an individual owl and 10,400 acres for a pair (Foreman et al., 19841. The
old-growth coniferous forests occupied by spotted owls in the Pacific
Northwest are over 200 years old and have several layers, with many
standing dead trees and new trees coming up in gaps where old ones have
fallen. The multilayered structure of these forests is believed to affect
foraging success, partly through an effect on the abundance of the owl's
prey primarily arboreal rodents, such as flying squirrels, voles, and
wood rats. Multilayered forests also provide cool microsites that allow
owls to avoid heat stress, to which they are apparently sensitive. The
broken-off tops of mature trees characteristic of old-growth forest also
provide nest sites for the owls (Foreman, 19801. Spotted owls are not
known to breed in young second-growth forests and are rarely found there.
OCR for page 233
CONSERVING A REGIONAL sporrED OWL POPULATION
233
Although it is generally assumed that home ranges must be larger in
areas with lower habitat quality, and home ranges are indeed larger in
heavily logged areas (Foreman, 1980), there is no good evidence of a
general correlation between habitat quality and home range size within a
study area. Territories might be abandoned as a result of timber harvest
(Foreman et al., 19841. In any year, not all territories are occupied; the
average occupancy rate can be as low as 50% and does not exceed 75%
of potentially suitable sites. The minimal home range size that would
adequately support a breeding pair is still being debated and probably
varies with the individual pair, terrain, forest type, and forest distribution.
Sizes estimated from censuses and radiotelemetry studies range from 740
acres to 8,300 acres in heavily logged areas (Foreman et al., 1984; Marcot,
19781. Logging in a pair's home range reduces habitat quality, not only
by reducing the amount of high-quality habitat, but also by fragmenting
the forest. Fragmentation leads to greater travel time for foraging and
greater exposure to predators, such as great horned owls. The effects of
habitat removal can be minimized by retaining travel corridors of old-
growth forest to connect the larger stands.
Some juvenile spotted owls have been reported to disperse as far as
100 miles from their areas of birth, although most do not travel that far
(Gutierrez, personal communication). It is not known why no radiotagged
juveniles have survived to breed. They might be too inefficient in finding
suitable habitat or suffer high mortality when occupying low-quality hab-
itats during dispersal. It is also possible that the radio transmitters them-
selves increase the chance of death in young and inexperienced owls.
Adults apparently do not like to cross large open areas (Foreman et al.,
1984), and they use corridors of old-growth forest, when available, to
travel between old-growth stands, even if much longer distances must be
covered.
Long-Term Population Viability
Studies of population viability have focused on how and why the risk
of extinction increases as populations become smaller and more isolated
from other populations. Interest has been not only in the fate of local
populations, but also in how long-term viability of a regional population
or an entire species can be affected by changes in the size, makeup, and
distribution of its constituent local populations.
Reviews of the factors that have led to recent extinctions indicate that
natural agents such as predation, competition, parasitism, and disease-
have rarely been the cause of extinction and that the reduction, alteration,
OCR for page 234
234
SELECTED CASE STUDIES
and isolation of habitats through human activity are ultimately more im-
portant (Franker and Soule, 198 1; Hester, 1967; Soule, 1983; Terborgh
and Winter, 1980; Ziswiler, 19671.
Two categories of factors increase the risk of extinction of small and
isolated populations (Shaffer, 198 1; Soule, 1983; Terborgh, 19741: inter-
nal changes and external stresses. Population productivity can decrease
as a result of random changes in fertility rate, litter size, sex ratio, death
rate, immigration rate, and so on. Behavioral dysfunction occurs in some
species below a threshold population size, often disrupting breeding. Ge-
netic drift can lead to loss of genetic variation, with a loss of potential
for adapting to environmental change. Studies of captive populations have
also shown that inbreeding depression occurs when fewer individuals-
all closely related-are available to mate; this situation often decreases
reproduction and survival (Soule, 19801. External factors also have more
severe effects as populations become smaller and more isolated. Habitat
change, intense predation or competition, fire, drought, and floods can
leave too few individuals for recovery to occur. In short, when populations
become very small, the probability increases dramatically that random or
even regular periodic events will reduce population numbers so much that
recovery can occur only if individuals immigrate from other populations.
As populations become more fragmented and isolated, the supply of po-
tential immigrants diminishes.
For a population to survive, it must have demographic resilience, i.e.,
resistance to extinction due to random demographic changes or environ-
mental fluctuations. Theoretical studies suggest that as few as 10 adults
can sustain a population for decades; but, for any given species, this
minimum depends on social system, reproductive potential, generation
length, the nature of random events, and other factors (May, 19731. Some
empirical studies suggest that 50 adults might be necessary for a reasonable
chance of surviving for several decades (Shaffer, 19811. For a population
to survive, it must also maintain adequate average individual fitness in
the face of inbreeding depression (Chambers, 1983) and must have suf-
ficient genetic variability to allow adaptation to environmental change
(Soule, 19801. Populations lose genetic variation through random changes
in gene frequency (Kimpra, 19831. In small populations, inbreeding is
more likely and genetic variation is more likely to be lost through genetic
drift. Genetic variation arises through mutation and immigration, and
rough estimates indicate that, to avoid extinction through genetic deteri-
oration, a population must be about 10 times larger than that necessary
to maintain demographic resilience (Soule, 19801.
The initial guidelines for management described below were based on
the assumption that adequate demographic resilience will be maintained
OCR for page 235
CONSERVING A REGIONAL SPOITED OWL POPULATION
235
if a population is managed to minimize genetic deterioration. Current
planning is assessing the roles of demographic variation and population
fragmentation as well.
In a genetically "ideal" population, individuals have an equal proba-
bility of mating with each other, fecundity is constant, population size is
constant, and generations are discrete. Few real populations meet these
criteria (Hartl, 1980; Kimura, 1983; Kimura and Crow, 1963; Wright,
19381. To predict random genetic change in a population of average size
N. when the population characteristics depart from the ideal, an effective
population size, Ne' is often used in lieu of the actual census size. The
calculation of Ne typically incorporates adjustments for deviation from the
ideal in sex ratio, variation in litter size or survival of offspring to repro-
ductive age, overlapping of generations, random mating, and fluctuation
of population number (Kimura, 19831. Ne is used to calculate inbreeding
coefficients, and it is usually less than the census N sometimes less than
20~o of it. Formulas for estimating the effects of population structure on
Ne have been given by Franklin (1980), Hartl (1980), Frankel and Soule
( 1981), Kimura ( 1983), and Thomas and Ballou ( 19831.
Corrections for three factors are often incorporated into the calculation
of Ne: sex ratio different from 1:1, variation in offspring survival, and
population fluctuations.
If not all adults breed, some genes are less likely than others to be
passed on. A biased sex ratio can create such an effect, and Ne should be
adjusted as follows:
Ne = 1/t11~4Nm) + 11~4Nf)l,
(1)
where Nm and Nf are the numbers of adult males and females.
The reproduction of genes is biased when the production of offspring
varies between parents. The effect of this variation on Ne is given by:
Ne = 4NI(2 + A,
(2)
where V is the variance in survival of offspring per parent and N is the
actual population size.
When a fluctuating population is well below the average size, genes
can be reproduced in a biased fashion as a result of genetic drift. The
longer a population remains small (i.e., fails to recover from decrease),
the greater the effect. It will be more pronounced, therefore, in species
with low powers of increase. The effect on Ne is as follows:
Ne = tl~llNe~ + 1lNe2 + ... + 11Net).
(3)
where t is the number of generations stipulated and Newt is the effective
number (N adjusted for sex ratio and offspring variance) in generation t.
OCR for page 236
236
SELECTED CASE STUDIES
The total effect of these three factors can be approximated by:
Ne = NSOP,
(4)
where S. O. and P are the ratios of Ne to N calculated for sex ratio,
offspring variance, and population fluctuation, respectively. This approx-
imation should be sufficiently precise for general wildlife habitat man-
agement, in which case other information is likely to be much less precise
than the estimate of Ne
The inbreeding coefficient, F. is calculated with the overall value of
Ne as derived above. Empirical data on the actual effects of inbreeding
in small populations of normally free-ranging and outbred species of mam-
mals have shown that even a small amount of inbreeding is correlated
with reduced fecundity and reduced survival of offspring (Rails and Ballou,
1982, 1983; Rails et al., 1979~. Free-ranging populations normally have
behavioral and ecological patterns that keep natural inbreeding low. The
formula for F is as follows:
Ft = 1 - (1 - [11~2Ne) + O.5]t,
(5)
where Ft is the inbreeding coefficient in generation t, t is the number of
generations from time zero, and Ne is the effective population number
during the period of interest.
Some of the variables in these formulas cannot be measured in wild
populations, and Soule (1980) cautioned that basing management on es-
timates of Ne can yield only rules of thumb, rather than reliable quantitative
results. As a rule of thumb, Franklin (1980) and Soule (1980) suggested
that an Ne of 500 or more might approach the balance point between
random loss and addition of genetic variation for many species, and Lande
(1980) suggested that populations of several hundred are at little more
risk than very large ones. The life history and population structure of a
species must be considered before such a rule of thumb is applied, but
there is now little basis for determining a more specific threshold value
for any species.
Soule ~ 1980) has proposed that wild populations are at risk of extinction
because of genetic factors when F' reaches 0.5 (Figure 11. Because For-
mula 5 does not consider the mitigating effects of migration between
populations or mutations, it overestimates the inbreeding coefficient,
sometimes substantially (Hartl, 1980; Kimura, 1983; Figure 21. Figure 1
shows that, assuming a generation time of 2 years for the spotted owl, a
regional population of 500 or more will be sufficient to provide protection
against genetic deterioration for many centuries. It is assumed that the
inbreeding coefficient calculated for the regional population also applies
OCR for page 237
CONSERVING A REGIONAL SPOOLED OWL POPULATION
~ 1.0
_'
·_.
V
·_l
o
·_.
Q .1
.8
.7
.6
.5
.4
.3
.2
50 100 150
Generations (t)
Ne=2~
- 1/ ~
- / /
/e = 40
~ Ne = 100
~A= 200
. .
200 250 300
237
FIGURE 1 Inbreeding coefficient, F. increases as function of effective population
number, Ne, and number of generations. At low Ne, F approaches dangerous extent
of inbreeding in fewer generations.
1.0
at, .8
Fin
d
a
~ .6
·_.
·_.
~ .4
Fly
.2
o
0 .5 1.0
A
1 1 1 1 1
1.5 2.0 2.5
Migrants per Generation (Nm)
FIGURE 2 Higher migration rates of reproductively successful individuals into small
population from larger population offset effects of inbreeding.
OCR for page 238
238
SELECTED CASE STUDIES
to all local forest populations, because they are not totally isolated from
each other.
THE PLANNING PROCESS FOR SPOTTED OWLS
The planning process for spotted owls is derived from a general protocol
(Salwasser et al., 1984) and now consists of eight steps (Figure 31.
· Step 1: The northern spotted owl in the Pacific Northwest was
identified as a species of concern, for several reasons. The owl is obligately
dependent on a habitat that is now rare (old-growth coniferous forest) and
that is being reduced rapidly, and individuals require large amounts of the
habitat. The population is becoming increasingly fragmented, as a result
of continued timber operations, and some individual forest populations
might be close to the lower limit of adequate short-term demographic
resilience-about 50-100 individuals. Planning is required now, so that
the regional population does not decrease below the size necessary for
long-term survival. Because the spotted owl has the most stringent re-
quirements for old-growth habitat, managing old growth to protect spotted
VIII. Monitoring &=\
Adaptive Management thru Timed /
VII. Decision & Actions ~
_
-
.~
/ \r-- ~
' /1\\ "
/1 \\
/1 \\
\ \
/ 11 \ \
/' 1 \\
\ \
VI. Viability Risk Evaluation ~/
~ ~
\ ~- I \
7
V. Estimation of Effects
on Populations
I. Species Selection
Ax\\
\ \
1
I
II. Coordination of Responsibilities
.~
- III. Species Biology & Habitat
Relationships Models
~
~
'N 1
IV. Alternative Management
Strategies
FIGURE 3 Eight-step process for planning viable population and analyzing risk.
OCR for page 239
CONSERVING A REGIONAL SPORED OWL POPULATION
239
owls will probably also protect most, if not all, other species that depend
on old-growth forest (Raphael and Barrett, 19841.
· Step 2: Planning responsibility is assigned on the basis of range and
distribution of the species. Censuses have been performed on all national
forest land and evaluated in conjunction with information from adjacent
lands to identify areas that might become effective biological reserves for
the species, regardless of ownership and prevailing land use.
· Step 3: Habitat requirements for spotted owls and the best distri-
bution of habitat within and between individual forest populations are
determined, and the results lead to development of habitat capability mod-
els that describe the full range of habitats over which the species occurs
(Nelson and Salwasser, 19821. The models can be used to guide habitat
planning and to determine the effects of different land-use patterns on the
species (Laymon et al., in press). On the basis primarily of radiotelemetry
studies of spotted owl habitat use in Oregon, a minimally suitable year-
round home range for a pair is presumed to include 300 contiguous acres
of mature to old-growth forest and 700 acres within 1.5 miles of the nest
site (Foreman et al., 19841.
To facilitate interchange among local populations and to make the oc-
cupancy of suitable habitat more likely, each managed habitat area must
be within the normal dispersal distance of the species-6-12 miles, ac-
cording to the results of radiotelemetry studies. The key concern is that
loss of a piece of habitat not lead to permanent isolation of a local pop-
ulation. The best pattern of habitat distribution would entail several con-
nections among suitable pieces of habitat, so that the loss of one connection
would not isolate any piece. Linear patterns should be avoided.
· Step 4: Population and habitat requirements are translated into land-
use planning variables. This is accomplished by assigning each individual
forest (or other planning unit) a quota for the number of pairs to be
maintained (as determined from the overall risk analysis, discussed below)
and by specifying how habitat in each forest is to be managed. In the
national forests, this plan must be flexible to accommodate multiple land
uses. There will inevitably be alternative plans for meeting these multiple
objectives, each of which will involve trade-offs among individual goals,
such as protecting spotted owls.
· Step 5: The alternative management strategies are projected to es-
timate their effects on key population dimensions, such as N. Ne' and the
structure of local domes. The projections use habitat and population sim-
ulation models with explicit assumptions about rates of systematic changes
and the importance of random variation.
For each planning alternative, two substeps are performed: estimation
of the population size in each forest that the planned habitat distribution
OCR for page 240
240
SELECTED CASE STUDIES
can support (habitat capability) and estimation of the effective population
size (Ne)
Habitat capability for each forest can be estimated with the habitat
management criteria for owl pairs described above and with censuses of
owls and maps of projected habitat distribution. The formulas presented
earlier can be used with population data on spotted owls to estimate Ne
both for individual forests and for the regional population (Formulas 4
and 5). Spotted owls breed monogamously, and, because there is no
indication of a biased sex ratio, the coefficient S is assumed to be 1.0.
Data on variance in offspring production are few, but studies have shown
that complete reproductive failure occurs in some years (Gutierrez, per-
sonal communication). Assuming that variance in offspring production
would be at the high end of the range for vertebrates, the coefficient O
is assigned a value of 0.66 (inferred from Crow and Morton, 1955).
Accurate counting of the nocturnal spotted owl is difficult, because the
failure of individuals to respond to imitated or recorded calls might indicate
only unresponsiveness, rather than absence. Population fluctuation in spot-
ted owls is probably small, because populations of other species with low
reproductive rate and high adult survivorship generally fluctuate little. It
is assumed that populations do not decrease to less than 50% of the average
more frequently than 1 year in 5 and that it takes about 3 years to recover
from a decrease to 50% of average N. Use of these assumptions in Wright's
(1938) harmonic-mean formula gives a value of 0.76 for the coefficient
P. Combining these coefficients (Formula 4) yields an estimated Ne of
half the census N (1.0 x 0.66 x 0.761. Thus, Ne for the northwest
regional spotted owl population, currently estimated at more than 2,000
adults, would be about 1,000, or 500 pairs. The Ne for any isolated
population can be calculated similarly by multiplying the census number
by 0.5.
· Step 6: An effective degree of protection (Table 1) is determined
for a species through a risk analysis of the estimated demographic, generic,
and geographic results of the management alternatives. The eventual man-
agement goals will depend explicitly on the degree of protection desired
for the species and will be in essence a value judgment involving a cost-
benefit analysis of possible protective measures. Obviously, the greatest
protection would be obtained if no more habitat were altered or removed,
but that is not always practical.
Assessing the degree of protection with each alternative involves eval-
uating the expected size of the whole regional population and the size and
degree of isolation of each forest unit. If isolation is not expected, the
potential demographic resilience of a forest population can be evaluated
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CONSERVING A REGIONAL SPOTTED OWL POPULATION
TABLE 1 Degrees of Protection for Species, Expected Population
Viability, and Habitat Required for Use in National Forest Planning
Degree of
Protection
241
Viability
Population Pattern Supported
1
4
5
6
7
8
9
Individual: Survival likely for
only a few years to a few de-
cades
Individual: Survival likely up to
several decades, depending on
N and distribution
Short-term local population resili-
ence: Survival likely for 1 to a
few decades
Mid-term local population resili-
ence: Survival likely for several
decades
Long-term local population resili-
ence: Survival highly likely for
several decades to a century
Short-term adaptability: Survival
of populations likely for a cen-
tury
Mid-term adaptability: Survival of
populations highly likely for a
century
Long-term adaptability: Survival
of populations likely for millen
n~a
Evolutionary fitness: Survival of
populations highly likely for
millennia
Several individuals, isolated in
forest; no interchange with spe-
cies out of forest
Family, social group, or small
population isolated in forest;
deme of 10-30 adults
Several reproductive or social
groups isolated in forest; deme
of 30-60 adults
Well-distr~buted forest population,
isolated from rest of species;
deme of 60-100 adults
Well-distr~buted forest population
with at least degree 4 protec-
tion; part of population with Ne
in mid-lOOs
Well-distributed forest population,
with at least degree 4 protec-
tion; part of population with Ne
in mid-lOOs
Well-distributed forest popula-
tion(s), with at least degree 4
protection; part of population
with Ne of 500-1,000
Distinct, well-distr~buted forest
populations each with at least
degree 4 protection; part of
population with Ne greater than
1,000, whose demes could di-
verge genetically
Distinct, well-distributed forest
populations, each with at least
degree 4 protection; part of
population whose demes with
Ne greater than 1,000 could di-
verge genetically
NOTE: Adapted from Schonewald-Cox (1983).
with the habitat capability model, because some immigration can be ex-
pected to offset the risk of temporary population declines. Local popu-
lations that are not isolated from others are assumed to experience the
same inbreeding coefficient as the regional population as a whole (Figure
OCR for page 242
242
SELECTED CASE STUDIES
11. When a proposed alternative is expected to result in isolation of a local
population, Steps 5 and 6 should be performed for that population. Land-
use planning should evaluate the likelihood that planned or unplanned
events will eliminate adjacent habitats at the weakest points in the habitat
distribution.
· Step 7: This step, the decision process, involves consideration of
other resource concerns and values for the areas of land in question.
Biological assessments are blended with social, political, and economic
issues. The decisions often entail a social preference among competing
uses of the land.
· Step 8: A monitoring and research program is developed. It should
reflect the degree of protection and the potential environmental costs of
management and should allow for evaluation of critical assumptions used
in the risk assessment protocol (Salwasser et al., 19831. It should also
stipulate the variables to be measured and the frequency of measurement
and should address the issue of measurement reliability.
A monitoring program has three major goals. First, compliance with
the plan must be monitored. For example, if 100 SOMAs are allocated
to one national forest, it must be determined whether all SOMAs are being
maintained properly and whether the actual spatial relationships of the
various SOMAs are acceptable. Second, monitoring must show whether
management is achieving its resource goals. For example, are the SOMAs
supporting as many pairs of owls as expected, is reproductive rate adequate
for long-term population stability, and is genetic variation being main-
tained? Third, information must continually be gathered and used to update
and revise the plan. Some of this information must come from experimental
research.
Key assumptions in current planning involve dispersal behavior of ju-
veniles, the nature of the owls' dependence on stands of old-growth forest
for reproduction and survival, and the adequacy of population sizes for
maintaining demographic resilience. These assumptions are being studied
by a number of agencies (Ruggiero and Carey, 19841. There are no plans
to conduct intensive research on genetic factors, as long as at least 1,000
adults are maintained in the regional population. A demographic model
for the spotted owl is being modified to incorporate genetic considerations
and the effects that demographic variation might have on viability.
CONTRIBUTION OF ECOLOGICAL KNOWLEDGE TO THE
CASE STUDY RESULTS
Until recently, efforts to protect populations were based primarily on
site-specific habitat management, to ensure the survival and reproductive
OCR for page 243
CONSERVING A REGIONAL SPOlTED OWL POPULATION
243
success of individuals and breeding pairs. Although such efforts are still
critical to species management, it is now recognized that local populations
cannot be protected for long without studies of interactions among pop-
ulations that might exchange individuals studies that have been central
to developing a regional management plan for the spotted owl.
But metapopulation and other genetic models are only as good as the
values that go into them. In spite of recent research on spotted owls, there
is little assurance that the values of variables used in the models are
accurate. Many must be derived from studies on unrelated species. In-
formation on distribution and abundance is generally accurate, but knowl-
edge of survivorship, reproductive rate, dispersal, and other demographic
characteristics is sparse. Thus, it is essential to view current management
plans and their assumptions as hypotheses to be tested. Both monitoring
and research must be involved in the tests. The current plan is flexible
and adaptive, and it takes the uncertainty of the effects of our proposed
management actions into account.
ACKNOWLEDGMENTS
Contributions to this chapter have come from many people. Michael
Soule's lucid writing on conservation biology prompted the planning pro-
cess described. Eric Forsman, Rod Canutt, and Dean Carrier worked out
the basic method for specifying suitable habitats and distribution (Step 31.
Jack Ward Thomas showed how to use the species habitat niche to rep-
resent resource needs in a practical way (Steps 3 and 41. Karl Sider~ts and
Bob Radtke pioneered the use of diversity standards to provide for the
habitat needs of all wildlife (Step 41. John Lehkuhl, Ed Harshman, and
Daniell Jerry tested the application of population theories to determining
the number of individuals needed (Step 51. Michael Soule, Daniel Good-
man, Michael Gilpin, James Brown, Linda Joyce, Tom Hoekstra, Mark
Shaffer, Curt Flather, Dick Holthausen, Bill Burbndge, Brad Gilbert,
Charlie Phillips, Maureen Beckstead, Tom Burke, and Paul Brouha as-
sisted in workshops, brainstorming, and review of manuscripts to develop
the analytical strategy and process.
REFERENCES
Chambers, S. M. 1983. Genetic principles for managers. Pp. 15-46 in C. M. Schonewald-
Cox, S. M. Chambers, B. MacBryde, and W. L. Thomas, eds. Genetics and Conser-
vation: A Reference for Managing Wild Animal and Plant Populations. Benjamin/Cum-
mings, Menlo Park, Calif.
Crow, J. F., and N. E. Morton. 1955. Measurement of gene frequency drift in small
populations. Evolution 9:202-214.
OCR for page 244
244
SELECTED CASE STUDIES
Forsman, E. D. 1980. Habitat Utilization by Spotted Owls in the West-Central Cascades
of Oregon. Ph.D. thesis, The Oregon State University, Corvallis.
Forsman, E. D., E. C. Meslow, and H. M. Wight. 1984. Distribution and biology of the
spotted owl in Oregon. Wildl. Monogr. 87:5-68.
Frankel, D. H., and M. E. Soule. 1981. Conservation and Evolution. Cambridge University
Press, New York.
Franklin, I. R. 1980. Evolutionary change in small populations. Pp. 135-149 in M. E.
Soule and B. A. Wilcox, eds. Conservation Biology: An Evolutionary-Ecological Per-
spective. Sinauer, Sunderland, Mass.
Franklin, J. F. 1984. Characteristics of old-growth Douglas-fir forest. Pp. 328-334 in
Society of American Foresters. New Forests for a Changing World. Society of American
Foresters, Bethesda, Md.
Hartl, D. L. 1980. Principles of Population Genetics. Sinauer, Sunderland, Mass.
Heinrichs, J. 1983. The winged snail darter. J. For. 81:212-262.
Hester, J. 1967. The agency of man in animal extinctions. Pp. 169-192 in P. S. Martin
and H. E. Wright, eds. Pleistocene Extinctions: The Search for a Cause. Yale University
Press, New Haven, Conn.
Kimura, M. 1983. The Molecular Theory of Evolution. Cambridge University Press, New
York.
Kimura, M., and J. F. Crow. 1963. The measurement of effective population number.
Evolution 17:279-288.
Lande, R. 1980. Genetic variation and phenotypic evolution during allopatric speciation.
Am. Nat. 116:463-479.
Laymon, S. A., H. Salwasser, and R. H. Barrett. In press. Habitat suitability index models:
Spotted owl. U.S. Fish Wildl. Serv. Biol. Rep.
Levins, R. 1970. Extinction. Pp. 77-107 in M. Gerstenhaber, ed. Some Mathematical
Questions in Biology. Vol. II. American Mathematical Society, Providence, R.I.
Marcot, B. G. 1978. Prolegomena of the Spotted Owl (Strip occidentalis) in Six Rivers
National Forest. Tech. Rept. USDA For. Serv. Six Rivers National Forest, Eureka, Calif.
May, R. M. 1973. Stability and Complexity in Model Ecosystems. Princeton University
Press, Princeton, N.J.
Nelson, R. D., and H. Salwasser. 1982. The Forest Service wildlife and fish habitat
relationship program. Trans. N. Am. Wildl. Nat. Resour. Conf. 47:174-183.
Ralls, K., and J. Ballou. 1982. Effect of inbreeding on juvenile mortality in some small
mammal species. Lab. Anim. 16: 159- 166.
Ralls, K., and J. Ballou. 1983. Extinction: Lessons from zoos. Pp. 164-184 in C. M.
Schonewald-Cox, S. M. Chambers, B. MacBryde, and W. L. Thomas, eds. Genetics
and Conservation: A Reference for Managing Wild Animal and Plant Populations. Ben-
jamin/Cummings, Menlo Park, Calif.
Ralls, K., K. Brugger, and J. Ballou. 1979. Inbreeding and juvenile mortality in small
populations of ungulates. Science 206: 1101 - 1103.
Raphael, M. G., and R. H. Barrett. 1984. Diversity and abundance of wildlife in late
successional Douglas-fir forest. Pp. 352-360 in Society of American Foresters. New
Forests for a Changing World. Society of American Foresters, Bethesda, Md.
Ruggiero, L. F., and A. B. Carey. 1984. A programmatic approach to the study of old-
growth forest-wildlife habitat relationships. Pp. 328-334 in Society of American For-
esters. New Forests for a Changing World. Society of American Foresters, Bethesda,
Md.
Salwasser, H., C. K. Hamilton, W. B. Krohn, J. F. Lipscomb, and C. H. Thomas. 1983.
OCR for page 245
CONSERVING A REGIONAL SP07JED OWL POPULATION
245
Monitoring wildlife and fish: Mandates and their implications. Trans. N. Am. Wildl.
Nat. Resour. Conf. 48:297-307
Salwasser, id., S. P. Mealey, and K. Johnson. 1984. Wildlife population viability A
question of nsk. Trans. N. Am. Wildl. Nat. Resour. Conf. 49:421-439.
Schonewald-Cox, C. M. 1983. Conclusions: Guidelines for management: A beginning
attempt. Pp. 414-446 in C. M. Schonewald-Cox, S. M. Chambers, B. MacBryde, and
W. L. Thomas, eds. Genetics and Conservation: A Reference for Managing Wild Animal
and Plant Populations. Renjamin/Cummings, Menlo Park, Calif.
Shaffer, M. 1981. Minimum population sizes for species conservation. BioScience 31:131-
134.
Soule, M. E. 1980. Thresholds for survival: Maintaining fitness and evolutionary potential.
Pp. 151-169 in M. E. Soule and B. A. Wilcox, eds. Conservation Biology: An Evo-
lutionary-Ecological Perspective. Sinauer, Sunderland, Mass.
Soule, M. E. 1983. What do we really know about extinction? Pp. 414-446 in C. M.
Schonewald-Cox, S. M. Chambers, B. MacBryde, and W. L. Thomas, eds. Genetics
and Conservation: A Reference for Managing Wild Animal and Plant Populations. Ben-
jamin/Cummings, Menlo Park, Calif.
Soule, M. E., and B. A. Wilcox. 1980. Conservation Biology: An Evolutionary-Ecological
Perspective. Sinauer, Sunderland, Mass.
Terborgh, J. 1974. Preservation of natural diversity: The problem of extinction-prone
species. BioScience 24:715-722.
Terborgh, J., and B. Winter. 1980. Some causes of extinction. Pp. 119-134 in M. E. Soule
and B. A. Wilcox, eds. Conservation Biology: An Evolutionary-Ecological Perspective.
Sinauer, Sunderland, Mass.
Thomas, W. L., and J. Ballou. 1983. Equations and population management. Pp. 414-
446 in C. M. Schonewald-Cox, S. M. Chambers, B. MacBryde, and W. L. Thomas,
eds. Genetics and Conservation: A Reference for Managing Wild Animal and Plant
Populations. Benjamin/Cummings, Menlo Park, Calif.
Wright, S. 1938. Size of population and breeding structure in relation to evolution. Science
87:430-431.
Ziswiler, Z. 1967. Extinct and Vanishing Animals. Spnnger, New York.
Committee Comment
As we continue to alter and fragment the habitat of more and more
species of plants and animals, we add to the list of species that are
composed of metapopulations at an alarming rate. Only recently has it
been widely recognized that such species cannot be protected against the
threat of extinction by managing habitat for their local constituent pop-
ulations. This recognition is the crucial first step toward maintaining the
long-term viability of threatened species, but the scientific issues raised
will not be easy to deal with, and the social and political steps required
will not be easy to accomplish. Habitats of many species are distributed
without regard for national boundaries and are often in countries where
pressure for modifying those habitats is intense.
Even if we had control over habitat destruction, we would be far from
OCR for page 246
246
SELECTED CASE STUDIES
understanding how to protect these species. Theoretical research in meta-
population management is in its infancy, and the data needed for emerging
models are difficult to acquire and are lacking for most species. The spotted
owl management plan constitutes one of the first attempts to incorporate
metapopulation modeling into usable management guidelines. But, as
pointed out by Salwasser, even the basic issue of whether demographic
or genetic constraints are more critical is being hotly debated, and only
very general rules of thumb for determining minimal population sizes are
available. In Salwasser's spotted owl case study, genetic constraints are
assumed to be more stringent, and the model used is based primarily on
limiting genetic deterioration. Until more research is done on demographic
models, we cannot be confident that this assumption is valid.
In a recent study, Lande (1985) suggested that demographic factors
might be more critical than genetic factors to the survival of spotted owls.
Lande used two independent analytical methods a basic life-table anal-
ysis and an analysis of habitat occupancy and concluded that current
management plans for the spotted owl will eventually result in demo-
graphic collapse. Lande's study demonstrates the importance of exploring
every available analytical approach before making irreversible habitat
management decisions.
In addition to the deterministic approaches used by Lande, Shaffer
(1981) and others have been developing a Monte Carlo simulation ap-
proach for determining minimal viable population size. Various demo-
graphic values can be used in an iterative stochastic simulation model that
projects population size and makeup for many generations. By simulating
both environmental and demographic variability, the models clearly dem-
onstrate that population viability is a matter of probability. How long do
we wish the population to avoid extinction? What risk (probability of
extinction) are we willing to accept? Management decisions ultimately
depend on what are essentially value judgments.
On a metapopulation level, we must also address the issue of whether
near-term (centuries) protection against demographic collapse is more
important than providing the potential for long-term evolutionary change.
Overall management plans aimed at these two different goals will often
be very different. Is one very large population less likely to become
extinct more desirable than a system of smaller, connected populations
that are more likely to facilitate evolutionary change in response to locally
changing environments? Clearly, protecting a species against demographic
collapse is a first priority, for, unless we maintain a viable population,
we will not even have the chance to tackle the longer-term problems of
loss of genetic variation.
The management plan presented in this chapter will undergo many
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CONSERVING A REGIONAL SPOlTED OWL POPULATION
247
changes with a better understanding of metapopulation dynamics and ge-
netics and with more demographic and ecological data on spotted owls.
The clear and accessible presentation of the plan, however, is of great
value, because it focuses the debate and research needed for the achieve-
ment of its goals.
References
Lande, R. 1985. Report on the Demography and Survival of the Spotted Owl. Paper prepared
for the National Wildlife Federation, Portland, Oreg.
Shaffer, M. 1981. Minimum population sizes for species conservation. BioScience 31:131-
134.
Representative terms from entire chapter:
spotted owls
