Click for next page ( 228


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

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 227
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 227
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 227
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 227
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

OCR for page 227
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 227
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 227
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 227
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 227
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 227
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 227
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 227
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 227
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

OCR for page 227
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 227
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 227
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 227
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 227
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 227
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

OCR for page 227
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.