POPULATION ECOLOGY OF THE SPOTTED OWL IN THE

POPULATION ECOLOGY OF THE SPOTTED OWL IN THE CENTRAL SIERRA NEVADA, CALIFORNIA by Daryl W. Lutz A Thesis Presented to The Faculty of Humboldt State University In Partial Fulfillment of the Requirements for the Degree Master of Science FEBRUARY, 1992 POPULATION ECOLOGY OF THE SPOTTED OWL
IN THE CENTRAL SIERRA NEVADA, CALIFORNIA
by Daryl W. Lutz Approved by the Master's Thesis Committee
R.J. Gutiérrez
Robert J. Cooper
Charles M. Biles
Director, Natural Resources Graduate Program Date
92/W-236/02/28 Natural Resources Graduate Program Number Approved by the Dean of Graduate Studies Susan H. Bicknell ABSTRACT I studied the demography of California spotted owls (Strix occidentalis occidentalis) in the central Sierra Nevada, California between 1986 and 1989. Fifty-five spotted owls were banded during the study. Estimates of fecundity, nest success, and productivity were 0.34, 42.0%, and 1.70, respectively. Of thirty-nine territories monitored, on average 55% and 21% were occupied by pairs or single owls, respectively, while 24% were unoccupied. Population size was estimated empirically, from Jolly-Seber models A and D, and from Leslie regression analysis. The population estimates were 36, 33, 32, and 40 birds, respectively. All estimates were statistically the same. Goodness of fit tests indicated that the data from this study satisfied the assumptions of the Jolly-Seber models. Crude density was 0.09 owls/km 2 and was significantly less than that reported for the northern spotted owl. Standard Lotka-Leslie models were used to assess the sensitivity of A to variation in the vital rates and to project population trends. Parameters estimated were adult survival rate (S) = 0.76, juvenile survival rate
(S0 ) = 0.16, and fecundity = 0.34. The empirical estimate for adult survival was not
significantly different from the Jolly-Seber model A
estimate. The population's finite rate of growth,
iii
λ, was
iv
most sensitive to adult survival rate and age of senescence. Fecundity, subadult, and juvenile survival rate did not dramatically influence estimates of A.
The value of A for this population was 0.811, which was significantly less than 1.0. At this rate of decline the population's half life is 4 years. ACKNOWLEDGMENTS The spotted owl has opened the eyes of a nation, whose beleaguering greed has resulted in an unacceptable lack of respect for our natural resources. For that, I am grateful to the spotted owl. The success of this study is owed to many. I am indebted to everyone who helped finish this work, and I humbly apologize to those whom I have forgotten. I am especially grateful to Kimberly, my loving wife, for her companionship, sacrifice, support, hard work and tolerance as a field assistant, and the love she has brought into my life. A special thanks to Mom and Dad for their support and encouragement. I kindly thank David Anderson and Jim Heaney for their hard work and comradery as field assistants. Similarly, I kindly thank Michael Bias for the contribution he has made to this study by allowing me to use his data from the 1986 and 1987 field seasons. To Rocky Gutierrez, my major professor, a sincere thanks for the opportunity to be a part of the spotted owl crew. Your professional ethics and lessons will endure a lifetime. I thank my committee members, Charlie Biles and Bob Cooper, for their effort in reviewing this manuscript and offering their advise and expertise. v
vi
I am especially indebted to John and Ellen Thornton for saving my *!@ during a Sierran snow storm while trying to get unstuck. Your friendship has been enlightening. I thank Gordon Gould and the California Department of Fish and Game for their concern for the spotted owl and subsequent funding for this project (contract FG7509). Additional support was received from the Pacific Southwest Forest and Range Experiment Station (PSW 89-0018CA) and Region 5, USDA, Forest Service (PSW 91-0034CA). I appreciate the support and cooperation that personnel from the U.S. Forest Service, Georgetown Ranger District gave to me during the field seasons. I especially thank Karen Hayden for her insight, Art Allen for his "jolly" attitude and smile, and all of the forest service employees with whom I was associated. I thank A. Franklin, P. Ward, D. Call, S. Rinkevich, R. Smith, J. Allen, J. Verner, B. Noon, C. Moen, K. McKelvey, and R. Lamberson for their insights and discussions of owl biology. I thank Dr. Stan Harris for his positive outlook, down home discussions, and friendship. I dedicate this thesis to my Dad and Grandpa Fuller, who through hunting, fishing, and enjoying the country, taught me to love and respect nature and its wildlife. TABLE OF CONTENTS Page ABSTRACT �
iii ACKNOWLEDGMENTS �
LIST OF TABLES �
LIST OF FIGURES �
viii ix 1 �
INTRODUCTION
STUDY AREA �
3
MATERIALS AND METHODS �
5
Surveys and Owl Capture
5 �
Data Analysis
6
�
6 �
Demographic Parameters
8 Sensitivity Analysis and Population Projections . �
RESULTS �
12 Spotted Owl Life History �
12 Population Dynamics
20 �
Sensitivity Analysis �
20 Population Projections �
23 DISCUSSION �
32 LITERATURE CITED �
37 vii
LIST OF TABLES Table
1
Parameter estimates
used in demographic analyses for northern spotted owls and the estimates used in this analysis for the California spotted owl in the EDSA
Page 9 2
California spotted
owl reproduction in the 13 central Sierra Nevada, 1986-1989
3
Spotted owl territory status in the central Sierra Nevada, California, 1986-1989
15 4
Empirical and Jolly Seber model A estimates of annual mortality and survival rates of California spotted owls in the central Sierra Nevada, California, 1986-1989 . .
.18 5 Estimates of the finite rate of growth (A) for a spotted owl population over an array of juvenile survival rates (S o ) in the central Sierra Nevada, California
27 6 Estimates of the finite rate of growth (A) for a spotted owl population over an array of adult survival rates (S) in the central Sierra Nevada, California
30 viii
LIST OF FIGURES Page
Figure
1
2
3
4
5
6
7
Estimate of the California spotted owl
population size in the central Sierra
Nevada using Leslie regression
16
Annual finite rate of increase (λ )
with age at first reproduction at 2 years
(solid isopleths) and at 3 years (dashed
isopleths) for various values of adult (S)
and preadult ( S0S1) survival rates for a
California spotted owl population in the
central Sierra Nevada, California
21
Annual finite rate of increase (λ )
with no senescence (solid isopleths) and
senescence at age 16 (dashed isopleths)
for various values of adult (S) and
preadult (S0S1) survival rates for a
California spotted owl population in the
central Sierra Nevada, California
22
Finite rate of increase (λ ) as a
function of fecundity (b) for 2 values of
adult survival rate (S) for a California
spotted owl population in the central
Sierra Nevada, California
23
Annual finite rate of increase (λ) as
a function of the first year survival
rate (S) for a California spotted owl
population in the central Sierra Nevada,
California
25
Annual finite rate of increase (λ ) as
a function of adult survival rate (S) for a
California spotted owl population in the
central Sierra Nevada, California
26
Population projections as a function of
first year survival rates (S 0 ) for a
California spotted owl population in the
central Sierra Nevada, California
28
ix
x
LIST OF FIGURES (cont.)
Figure
8
Page
Population projections as a function of
adult survival rates (S) for a California
spotted owl population in the central
Sierra Nevada, California
31
INTRODUCTION The northern spotted owl (Strix occidentalis caurina) is closely associated with late seral stage Douglas-fir (Pseudotsuga menziesii) forests in the Pacific Northwest (Forsman et al. 1984). Logging of old-growth forests is responsible for the decline of spotted owl populations (Thomas et al. 1990, USDI 1990, Forsman et al. 1984, Gould 1977). This habitat loss is a primary factor for the northern spotted owls' listing as a threatened species (USDI 1990). The California spotted owl (S. o. occidentalis) also inhabits late seral stage conifer forests (Bias 1989) as well as other habitat types (Grinnell and Miller 1944). However, habitat loss also is occurring following clear cut logging in the Sierra Nevada. Thus, the California spotted owl is considered a priority management species (Remsen 1978, USDA For. Serv. 1988). Laymon (1988) and Bias (1989) described habitats used by spotted owls in the Sierra Nevada. In addition, Call (1990) estimated home range and habitat characteristics of the spotted owl in the Sierra Nevada. However, no demographic analyses have been completed for this subspecies, which may be demographically different from the northern spotted owl (Laymon 1988). Therefore, I studied the demography of a Sierra Nevada population of California 1
2
spotted owls to estimate basic life history characteristics. I tested two hypotheses:
1)
The population is at least stable
2)
The finite rate of growth
(λ ≥ 1.0)
,
(A) is not sensitive to
variation in rates of adult, subadult, and juvenile
survivorship, fecundity, age at first reproduction,
and age of senescence.
STUDY AREA The study area was located in the central Sierra Nevada on the Eldorado National Forest (EDSA). The EDSA was approximately 10 km (6 mi) northeast of Georgetown, El Dorado County, California. The EDSA included the Georgetown and Pacific Ranger Districts, with a small portion in the Foresthill Ranger District, Tahoe National Forest, in El Dorado and Placer Counties. The EDSA was approximately 355 km 2 (136 mi 2 ). Elevations ranged from 366 m (1200 ft) to 2,257 m (7400 ft). Public (USFS) lands (62.7%) and private lands (37.3%) were distributed throughout the EDSA in a "checkerboard" (i.e., alternating ownership) pattern. Vegetation of the EDSA was dominated by the Sierran
Montane Forest vegetation type (K ü chler 1977). White fir
(Abies concolor) and sugar pine (Pinus lambertiana) were
the dominant tree species. Understory species included
Pacific dogwood (Cornus nuttallii), Douglas-fir
(Pseudotsuga menziesii), canyon live oak (Ouercus
chrysolepis), California black oak (Quercus kelloggii), and
incense cedar (Libocedrus decurrens) (K ü chler 1977, Rundel
et al. 1977).
Bias (1989) described the vegetation of the
area in detail.
3
4
The EDSA was characterized by cold, snowy winters and hot, dry summers. Average annual precipitation in the study area was 130 cm. Average annual temperatures ranged from 13 °C at the higher elevations to 15 °C at the lower elevations. Average, winter time, minimum temperatures ranged from -1 °C at higher elevations to 2 °C at lower elevations. Average, summer time, maximum temperatures ranged from 32 °C at the higher elevations to 35 °C at the lower elevations (Elford 1974). MATERIALS AND METHODS Surveys and Owl Capture All habitats within the EDSA were surveyed according to the methods of Forsman (1983) and Franklin et al. (1990b). Night surveys consisted of point and cruise surveys (Franklin et al. 1990b) and were done from dusk to 2400. Birds detected during night surveys were located the following day during walk-in surveys (Franklin et al. 1990b). Reproductive and social status (nesting, number of fledglings, pair, single, etc.) were estimated using Forsman's (1983) and Franklin's et al. (1990) criteria. Birds were captured using a noose pole or a mist net (Forsman 1983); captured birds were banded with a U.S. Fish and Wildlife Service locking band on one leg and a unique color band on the other (Franklin et al. 1990b). Subsequent resightings (recaptures) were made by identifying the color band. Individuals that could not be accurately identified were recaptured. Birds were sexed by voice and aged (as juveniles, subadults, or adults) by plumage characteristics (Forsman 1983). Bird locations were plotted on U.S. Geologic Survey (USGS) topographic maps to the nearest universal transverse mercator (UTM) coordinate. 5 6
Data Analysis Data were analyzed using descriptive and inferential statistics (Zar 1984; Snedecor and Cochran 1980, SPSS Inc. 1988). Differences in proportions were tested using chi-
square tests of homogeneity (Zar 1984:49). Pairwise comparisons were tested using t statistics (Zar 1984:150). Point estimates were compared using z statistics with two-
tailed probabilities (Brownie et al. 1985: 180-182). Null hypotheses were rejected if the probability associated with the test statistic was less than 0.05. Demographic Parameters Nest success was defined as the proportion of nesting pairs that successfully fledged young. Productivity was defined as the number of young fledged per productive female (Franklin et al. 1990a). Fecundity was defined as the number of female owls fledged per paired female per year, assuming a 1:1 sex ratio. Territory occupancy was defined as the proportion of territories occupied in a single year relative to the total number of territories checked between 1986 and 1989. Territory turnover was defined as the proportion of marked adult and subadult owls replaced or missing from their territories relative to the total number of individuals checked (Franklin et al. 1990a). 7
Crude density estimates were computed from the number of individual owls detected divided by the study area size. Population size was estimated from empirical estimates (i.e. direct counts), Leslie regression analysis, and Jolly-Seber (J-S) capture-recapture models A and D. Leslie regression analysis of catch per unit effort against total cumulative catch (Leslie and Davis 1939; Caughley 1977) has been used to estimate numbers of northern spotted owls (Ward et al. 1991). J-S estimates were generated using program JOLLY (Brownie et al. 1986). Goodness-of-fit tests were used to evaluate violations of model assumptions (Pollock et al. 1985, Brownie et al. 1986). Annual mortality rates were computed empirically as
the proportion of banded territorial owls that were missing
from their territory for
≥
2 years in year t + 1 relative
to the total number of banded, territorial owls identified
at or before year t that were checked in year t + 1.
Annual survival rates were computed as: [1-mortality
rate]. Adult survivorship also was estimated using the J-S
capture-recapture model A (Franklin et al. 1990b). An
estimate of subadult survival was not calculated due to
inadequate sample size, and therefore was assumed to be
equal to the adult survival rate. Juvenile survival rates
were estimated using program MICROMORT (Heisey and Fuller
1985) and radio-telemetry data from Laymon (1988:165).
8
Sensitivity analysis and population model A three-stage Leslie projection model (Table 1) was
used to estimate A (finite rate of growth) and its
sensitivity to variation in: adult, subadult and juvenile
survival rates, fecundity, senescence, and age of first
reproduction. The Leslie model was also used to project
population trends. The Lotka equation,
1= Σ
λ-x1xbx
(1)
given constant estimates for juvenile survival (S 0 ), subadult survival (S 1 ), adult survival (S), and fecundity (b), can be simplified to 1 = bS0S1/ (λ 2 - λ (S)),
(2)
where 0 < S < 1 and A > S. Equation (2) can be rewritten,
λ2 - λ (S) - bS0S1 = 0,
(3)
and for the spotted owl, A can be computed as the single,
positive, real root of the characteristic equation:
λ = [S + (S2 + 4S0S1b) ½ ]/ 2.
(4)
The effects of age at first reproduction, a, on the
value of A were incorporated into the characteristic
equation (3):
λa - λ (S)a-1 - bS0S1Sa-2 = 0,
given that a
≥
(5)
1 and 0 < S < 1 (Nichols et al. 1980, Noon
Table 1. Parameter estimates used in demographic analyses for northern spotted owls and the estimates used in this analysis for the California spotted owl in the EDSA. Barrowclough
and Coats
(1985)
Parameter Marcot and
Holthausen
(1987)
Lande
(1988)
Noon and
Biles
(1990)
This
Study
Juvenile Survival (S0 )
0.19
0.11
0.11
0.11
0.16 a
Subadult Survival (S i )
0.85
0.85
0 .34
0.96
0.96
0.71
0.71
0.76
0.94
0.94
0.76 c
0.28
0.24
0.24
0.34 d —
3
2
3
2
2
10
15
∞
∞ (16,21,26) ∞ (16)
Adult Survival (S ) Fecundity (b) Age at first Repro.
(a)
Age of Senescence (d )
(3)
b
(3)
Estimates
A
re -0.237 -0.170 -0.040 -0.040 -0.211
0.789
0.840
0.961
0.961
a Upper 95% confidence limit of the estimate computed from MICROMORT.
b
Assumed equal to the estimate for S.
c Estimate computed from J-S model A.
d
Computed as the average female young/female between 1986-1989.
e Intrinsic rate of growth.
0.810
10
and Biles 1990). Thus, the probability of surviving to age x was indepe ndent of the age at first reproduction (Noon and Biles 1990). Senescence, d, was incorporated into equation (5): λa - λ a-1 (S)- bS0S1Sa-2[1-(S/ λ)d-a] = 0,
(6)
where a ≥ 1, d ≥ 2, 0 < S ≥ 1.
This analysis followed Noon and Biles' (1990) model
for the northern spotted owl. Fo rmula derivations are
outlined in Noon and Biles (1990). Assumptions of the
stage-projection model were: repr oduction was
characteristic of a birth pulse population (Caughley 1977),
no density dependence, and a 1:1 sex ratio. The model was
based on the survivorship and fecundity estimates from the
banded study birds. To assess the sensitivity of A,
variation was introduced into one parameter at a time while
holding all other parameters constant. The sampling
variance of λ (σ2λ ) was computed as:
σ2λ = Σ (α λ / α π)2 σπ2 (7)
where π represents each of the vital rates (i.e., S o , S l ,
S, and b) and σπ2 their sampling variance.
represents the sensitivities of
α λ /απ
λ to each of the vital
rates and were computed by implicit differentiation as
follows:
S0 : α λ / αS0 = S1b / (2 λ - S);
(8)
S1 : α λ / αS1 = S0b / (2 λ - S); (9)
11
S : α λ / αS = λ / (2 λ - S); and
(10)
b : α λ / αb = SoS1 / (2 λ - S).
(11)
An estimate of A and its standard error (square root of the
variance) allowed one-tailed tests of the hypothesis
Ho : λ ≥
1, versus the alternative hypothesis: H a : λ < 1.
The test statistic followed a Z-distribution and was
computed as: |( λ -1) / α λ | ( Thomas et al. 1990:232).
Population parameter estimates for the population
projection model for adult survivorship (S) were taken from
J-S model A, subadult survivorship (SO was assumed equal to S, juvenile survivorship (S 0 ) was equal to the upper 95% C.L. of the MICROMORT estimate (see Results and Franklin et al. 1990a), and fecundity (b) was equal to the mean number of female owls produced per paired female per year from 1986 through 1989 (Table 1). RESULTS During four field seasons, 1 April to 30 August of 1986, 1987, 1988, and 1989, approximately 1565 person hours were used to survey, locate, estimate reproductive status, and band California spotted owls in the EDSA (Bias and Gutierrez 1987, 1988; Lutz and Gutierrez 1989a, 1989b). Thirty-nine spotted owl territories were identified within the EDSA. Fifty-five spotted owls were banded resulting in fifty-five resightings and/or recaptures. Spotted Owl Life History Nest success estimates from 1986 and 1988 were pooled (x2 = 0.91, df = 1, P > 0.25) as were estimates from 1987 and 1989 (x 2 = 0.53, df = 1, P > 0.25). The pooled estimates were significantly different (x 2 = 10.65, df = 1, P < 0.01) (Table 2). Pooled productivity estimates for 1986/1988 and 1987/1989 were not significantly different (t = 0.77, df = 3, P > 0.25) (Table 2). Fecundity ranged from 0.12 females produced/female in 1987 to 0.63 in 1986 (Table 2) and was variable over the study period. The fecundity rates in 1986 and 1988 were pooled (t = 0.98, df = 23, P = 0.34) as were the rates observed in 1987 and 1989 (t = -0.67, df = 25, P = 0.51). The pooled estimates 12 13
Table 2. California spotted owl reproduction in the central
Sierra Nevada, 1986-1989. The standard error of the estimate
is in Q.
Year
Nest Success
Fecundity
Productivity
1986
75.0% (0.14)
n = 12
0.63 (0.13)
n = 12
1.67 (0.18)
n = 9
1987
15.4% (0.11)
n = 13
0.12 (0.09)
n = 13
1.50 (0.71)
n = 2
1988
35.7% (0.14)
n = 14
0.54 (0.14)
n = 14
1.88 (0.13)
n = 8
1989
26.7% (0.12)
n = 15
0.20 (0.10)
n = 15
1.50 (0.33)
n = 4
Mean
42.0% (0.07)
n = 54
0.34 (0.06)
n = 54
1.70 (0.10)
n = 23
14
from the first and third years were significantly different from the pooled estimates from the second and fourth year (t = 4.16, df = 45, P = 0.0001) (Table 2). Of the thirty-nine territories observed, an average 55% were occupied by pairs of spotted owls, 21% by single owls, and 24% were not occupied at any time between 1986 and 1989 (Table 3). Territory turnover averaged 0.16 between 1986 and 1989 (Table 3). An increase in the number of territories surveyed and, subsequently, the number of unoccupied territories may be a reflection of increased survey effort within the study area. In addition, historic sites (i.e. sites located prior to this study) which were unoccupied during the study lowered the occupancy rate (Table 3). The empirical estimate of the population size (N 0 ), based on owls located during walk-in surveys, was 36 owls in 1989. The Leslie regression estimate was 40 owls (± 5) (Figure 1). The empirical estimate was not significantly different from the Leslie regression estimate (t = 1.52, df = 26, 0.10 < P < 0.20). The capture-recapture data fit J-S models A (x2 = 0.59, df = 1, P = 0.74) and D (x2 = 2.16, df = 1, P = 0.71), indicating that the assumptions of both models were met. Estimates from model A were not significantly different from those for model D (x 2 = 1.57, df = 2, P = 0.46). Model A and D population size estimates were 15
Table 3.
Spotted owl territory status and turnover rates
in the central Sierra Nevada, California, 1986-1989.
Year
Territories
Surveyed
Survey
Effort
(hrs.)
Pair
Single
Unocc.
Territory
Turnover
1986
34
300
0.53
0.35
0.12
----
1987
33
324
0.57
0.18
0.24
0.08
1988
39
498
0.59
0.18
0.23
0.18
1989
39
443
0.49
0.13
0.38
0.17
Mean
(SE)
--
---
0.55
(0.03)
0.21
(0.10)
0.24
(0.11)
0.16
(0.02)
16
Figure 1. Estimate of the California spotted owl population size in the central Sierra Nevada using Leslie regression. 17
32.65 (±7) and 32.20 (±5), respectively. In addition, the empirical estimate, 36 owls, was not significantly different from either model A (Z = 0.95, P = 0.17) or model D estimates (Z = 1.43, P = 0.08). Estimated crude densities (owls/km 2 ) ranged from 0.09 to 0.11. The empirical estimate (0.10) was between the statistical estimates (Jolly-Seber A = 0.09; Jolly-Seber D = 0.09; Leslie = 0.11). Crude densities estimated using J-S model D for the EDSA and northern California (0.235 owls/km2 ; Franklin et al. 1990b) were significantly different (Z = 2.03, P = 0.02). Empirical estimates of owl mean survival rates for males and females were not significantly different (t = 1.70, df = 55, 0.10 < P < 0.05) (Table 4). J-S model A did not fit the data for males, but it did fit the data for females (x 2 = 0.12, df = 1, P = 0.72). The model A survival estimate for females, 0.70, was not significantly different from the empirical estimate (Z = 0.46, P = 0.46) (Table 4). Further, the model A survival estimate for females was not significantly different from the model A estimate for the total sample (i.e., males and females combined; Z = 0.49, P = 0.35). Survival estimates from the total sample were more precise and were therefore used in the model. J-S model A survival estimates for the northern spotted owl (0.92; Franklin et al. 1990b) were significantly different from the J-S model A estimates for Empirical and Jolly-Seber model A estimates of annual mortality and survival Table 4.
rates of California spotted owls in the central Sierra Nevada, California, 1986-1989. Year
Number of Banded
Owls
missing
checked
Time t+1,
Time t
≥2 yrs.
Mortality
Rate (SE)
Survival
Rate (SE)
Jolly-Seber
A
1986
males
females
18a
12 b�
6
----
----
----
1987
males
females
12
9
3
1
1
0
0.08 (0.09)
0.11 (0.12)
0.00
0.92 (0.09)
0.89 (0.12)
1.00
1988
males
females
17
11
6
2
0
2
0.12 (0.08)
0.00
0.33 (0.12)
0.88 (0.08)
1.00
0.67 (0.12)
--
--
--
1989
males
females
28
15
13
4
1
3
0.14 (0.07)
0.07 (0.07)
0.23 (0.13)
0.86 (0.07)
0.93 (0.07)
0.77 (0.13)
--
--
--
0.12 (0.05)
0.06 (0.11)
0.23 (0.09)
0.88 (0.05)
0.94 (0.11)
.77 (0.09)
Mean (SE)
males
females
a
Number of spotted owls banded at the beginning of the study.
b
Insufficient data for male owls to satisfy the assumptions of J-S model A. 0.80 (0.10)
-0.70 (0.10)
0.73 (0.10) --
0.57 (0.15) 0.76 (0.07) --
0.70 (0.10) 19
the California spotted owl (Z = 2.26, P = 0.01). Three subadults (5.5%) were banded in the EDSA between 1986 and 1989. Only one was banded prior to the 1989 field season, yielding one subadult recapture. Therefore, estimates for subadult survival rates were not computed and were assumed to be equal to the estimated adult survival rate. This assumption was a conservative (optimistic) estimate of survival among subadults (Thomas et al. 1990). Nine (16.4%) juveniles were banded during the study, of which eight were banded prior to the 1989 field season. However, banded juveniles were not recovered during the study. This does not mean that all the juveniles died, only that they were not detected or resighted. Because of this low recapture rate, juvenile survival was estimated from daily survival of radio-tagged juveniles observed near the study area (Laymon 1988). Juvenile survival rate was estimated to be 0.005 (95% confidence interval = 0.0002 to 0.16). Since the point estimate was probably an unrealistic survival rate, I used the upper 95% confidence limit (0.16), which was similar to the juvenile survival rate reported by Franklin et al. (1990a). 20
Population Dynamics Sensitivity Analysis Isopleths illustrate values of preadult and adult
survivorship that give constant values of
λ assuming a
stable age distribution (Figure 2). Conclusions drawn from
this analysis apply only to long-term trends.
λ
was relatively insensitive to age at first
reproduction at high values of adult survival (Figure 2).
For example, to maintain λ = 1.00 when a = 2 or 3 and
preadult survival = 0.12 (S oS i ), adult survival must
be ≥ 0.97.
However, when adult survival decreases, the
departure of the isopleths increases.
Spotted owls are thought to be relatively long-lived
birds given the high estimates of adult survival rates
reported previously (Noon and Biles 1990). The effects of
a senescent decline become more pronounced as the age of
senescence decreases and is most pronounced at values of
high adult survival (Figure 3). For example, to maintain
λ = 0.80 when d = ∞ and 16 years, at a preadult survival
rate of 0.12, adult survival must equal 0.77 and 0.83,
respectively. The effects are obvious for all values of
λ (Figure 3), and they become dramatic when λ approaches 1.0.
Variation in fecundity (b) did not affect the value
of λ dramatically (Figure 4). For example, for two values
21
Figure 2. Annual finite rate of increase (A) with age at
first reproduction at 2 years (solid isopleths) and at 3
years (dashed isopleths) for various values of adult (S) and
preadult (S0S1)survival rates for a California spotted owl
population in the central Sierra Nevada, California. 22
Figure 3.
Annual finite rate of increase ( λ ) with no
senescence (solid isopleths) and senescence at age 16 (dashed
isopleths) for various values of adult (S) and preadult
(SoS i ) survival rates for a California spotted owl population in the central Sierra Nevada, California. 23
Figure 4. Finite rate of increase ( λ ) as a function of
fecundity (b) for 2 values of adult survival rate (S) for a
California spotted owl population in the central Sierra
Nevada, California.
24
of adult survival (S = 0.90, S = 0.76) the function stayed relatively parallel to the line where A = 1.0. For S = 0.90 the function was greater than 1.0 at b = 0.75. Conversely, when S equals the estimated 0.76, the function stayed below the line where A equals 1.0, for all values of b (Figure 4). Variation in juvenile survival rate resulted in
relatively constant estimates of λ (Figure 5). The
function for S = 0.90 stayed close to the line where
λ = 1.0. The function for S = 0.76 was also close to this
line, but never exceeded λ = 1.0.
Adult survivorship was the key parameter influencing
the value of λ (Figure 6). Given the estimated parameters
of So , S 1 , S, and b, the value of λ exceeded 1.0 only at
very high values of S, and it obtained a maximum value of
λ = 1.04 (assuming S = 1.0).
Population Projections The solution to equation (2), 1 = bSoS i / ( λ 2 - λ (S)),
given the parameter estimates (Table 1, Table 5), was
A = 0.81, which equates to a 19% annual decline in the
population size and a population half-life equal to 4
years, given No = 40 owls (Figures 1 and 7).
This
estimated value of λ was significantly less than 1.0
25
Figure 5. Annual finite rate of increase ( λ ) as a function
of the first year survival rate (S 0 ) for a California spotted
owl population in the central Sierra Nevada California.
26
Figure 6. Annual finite rate increase ( λ ) as a function of
adult survival rate (S) for a California spotted owl
population in the central Sierra Nevada, California.
Table 5. Estimates of the finite rate of growth (A) for a spotted owl population over an
array of juvenile survival rates (S 0 ) in the central Sierra Nevada, California.
Parameter
SE
Estimate Estimate
A
SE
A
Z
—
P
—
Ho:A > 1
Conclusion
So
0.100
0.100 8
0.793
0.083
2.49
0.006
Reject Ho
So
0.160
0.130
0.811
0.098
1.93
0.027
Reject Ho
So
0.200
0.141
0.823
0.108
1.65
0.050
Reject Ho
So
0.250
0.153
0.837
0.120
1.36
0.087
S1
0.760
0.068
S
0.760
0.068
b
0.340
0.443
Do not reject Ho
Estimate of the Standard Error of S o computed by: [S 0 (1-50 )]/8; where the denominator
(8) equalled the number of banded juveniles available for recapture between 1986 and
1989 within the EDSA.
a
28
Figure 7. Population projections as a function of first year survival rate (S 0 ) for a California spotted owl population in the central Sierra Nevada, California. 29
(Z = 1.95, P = 0.026) (Table 5). When age at first reproduction was set to the third year (i.e. shifting the life table up), the value of A was equal to that computed
at a = 2, 0.81, within two significant digits. When
senescence was introduced into the model at year 16 (i.e.,
truncating the life table at year 15) the value of λ
changed from 0.81 to 0.75, equating to a 25% annual
reduction in population size.
Because juvenile survival was probably underestimated
using MICROMORT, I bracketed this parameter to evaluate an
array of values for S 0 . Holding all other parameters
constant, λ was significantly less than 1.0 when S o < 0.250
(Table 5). The influence of S o on the population projection was minimal (Figure 7). The standard error of the estimate of adult survival
rate from the J-S model A was 0.068 (Table 6), resulting in
a 95% confidence interval of 0.63 to 0.90. Because of the
poor precision of this estimate, I evaluated λ at the full
range of values within the estimates' 95% confidence limit
(Table 6).
λ was significantly less than 1.0 when S < 0.78
(Z = 1.63, P = 0.052) (Table 6). Population projections
showed the relative influence of S on the finite rate of
growth for spotted owls in the EDSA (Figure 8). The
population half-life equalled eleven years when S = 0.90
and four years when S = 0.76.
Estimates of the finite rate of growth ( λ ) for a spotted owl population over an
Table 6.
(S 0 ) in the central Sierra Nevada, California.
array of adult survival rates
Parameter
Estimate
SE
Estimate
So
0.160
0.130
Si
=
=
Sa
0.630
0.760
S
S
Z
Ho:λ
1
Conclusion
P
b
0.068
0.680
0.096
3.32�
0.000
0.811
0.098
0.098
1.93
0.027
1.72
0.49
0.043
S
S
0.780
0.900
0.068
b
0.340
0.443
b
A
S
0.068
0.068
a
≥
SE
A
0.831
0.951
0.099
0.312
Reject Ho
Reject Ho
Reject Ho
Do not reject Ho
Parameter estimates and confidence limits computed using Jolly-Seber model A.
Estimate of the Standard Error of S o computed by: (S0 (1-S0 )]/8; where the denominator
(8) equalled the number of banded juveniles available for recapture between 1986 and 1989 within the EDSA. 31
Figure 8. Population projections as a function of adult survival rates (S) for a California spotted owl population in the central Sierra Nevada, California. DISCUSSION California spotted owls in the central Sierra Nevada exhibited declining population trends similar to both northern spotted owls and insular populations of California spotted owls (Franklin et al. 1990a, Franklin et al. 1990b, Gutierrez and Pritchard 1990, LaHaye et al. In press). Reproduction was quite variable and annual adult survival was relatively high. However, unlike a small insular population in southern California (Gutierrez and Pritchard 1990), the proportion of subadult owls in the population was small. Further, population density was significantly lower than that reported by Franklin et al. (1990b), but similar to densities reported by LaHaye et al. (In press). Since habitat distribution was more continuous historically than in LaHaye's et al. (In press) study area, the density of owls in my study area may reflect habitat loss, short-
term environmental responses (i.e. a series of bad weather years which resulted in reduced numbers of birds), different evolutionary histories that have influenced the population's demographics (Gutierrez and Pritchard 1990), or logging activity that has negatively affected the distribution of spotted owls (Bias and Gutierrez In Press). Like northern spotted owls (Noon and Biles 1990, Thomas et al. 1990), everglade kites
32 (Rostrhamus 33
sociabilis) (Nichols et al. 1980), and California condors (Gymnogvps californianus) (Mertz 1971), the value of A for
my study population of California spotted owls was most
sensitive to variation in adult survival rates relative to
preadult survival rates. Further, fecundity exhibited
relatively little influence on λ .
It is likely that for
the California spotted owl, natural selection has favored
the evolution of longevity in this environment. Further,
the low and variable fecundity rates observed may indicate
that recruitment has always been variable, but given high
adult survival rates the owl has been able to persist
through periods of low reproductive activity (Noon and
Biles 1990, Nichols et al. 1980). This is a critical
observation when considering land management activities
that may reduce adult survivorship, because given sporadic
reproductive output, this spotted owl population would not
be able to recover quickly from population decline, and
therefore may locally go extinct. The effects of
environmental perturbations (i.e., habitat loss) on the
population dynamics of other rare species has been
demonstrated by Nichols et al. (1980), Binkley and Miller
(1980), and Mertz (1971). Population projections indicate that this California spotted owl population is declining. Several factors, including the precision and accuracy of parameter estimates, habitat loss, and population response to 34
relatively short-term environmental conditions (i.e., drought) may have influenced these projections. Boyce (1987) points out that variance associated with parameter
estimates that is due to sampling error (i.e. small sample
size) will overestimate the probability of extinction. For
example, in this analysis the estimate of juvenile survival
was not reliable, and therefore, it was assumed to be equal
to that observed in other spotted owl populations (Franklin
et al. 1990a, Thomas et al. 1990). It may be reasonable to
observe a juvenile survival rate exceeding 0.25 (Thomas et
al. 1990, Ricklefs 1983, Southern 1970), and an associated
estimate of A that would not be statistically less than 1.0
(Table 5). Further, Lahaye et al. (In press) pointed out
that when an adult owl leaves a territory it may not
reenter the breeding population for several years. Thus,
it may require many years (i.e., > 4) to accurately
estimate survival rates. Sources of variation were not
considered in these analyses. However, I evaluated
potential variation and its effects on A by bracketing the
point estimate of the parameter with extreme values (Tables
5 and 6). Further, clearcutting late seral forests has
resulted in a fragmented landscape and reduced spotted owl
habitat in the central Sierra Nevada (Bias 1989). Habitat
fragmentation may increase owl susceptibility to starvation
and predation, thus negatively affecting estimates of the
vital rates. Finally, estimates of the vital rates were
35
measured during a drought period which may have caused abnormally low and/or variable survival and fecundity estimates. It is apparent that the population sampled in the
EDSA was declining between 1986 and 1989 given the
parameter estimates and computed value of λ .
Further, the
number of unoccupied territories increased by 15% from 1989
to 1990 (Table 3). Wilcove and Terborgh (1984) suggest
that a small observed decrease in the number of occupied
territories may signal a decrease in the population as a
whole. A continuation of this trend would indicate an
overall population decline. Conversely, such observations
may reverse in subsequent years.
If the parameter estimates were to remain constant, this population would continue to decline at an annual rate of 19% and the estimated population of 40 owls would be extinct in 8 years. However, this rate of decline was not apparent from field observations of breeding bird replacement. A possible explanation for this contradiction may be the existence of a non-territorial or floater population (Smith 1978, Wilcove and Terborgh 1984, Arcese 1987, Franklin in press). All territorial birds that died or left a territory were replaced the next year by an unbanded owl, usually an adult, indicating that a surplus population of owls existed. Logging practices have created a fragmented landscape 36
with isolated pockets of suitable habitat in the Sierra Nevada (Bias 1989). Given this fragmented landscape, dispersing and floater owls may frequently encounter large patches of unsuitable habitat, thus increasing their susceptibility to starvation and predation. Further, recruitment and juvenile survival rates in the central Sierra Nevada were low compared to insular populations in southern California (Gutierrez and Pritchard 1990). Lower recruitment and juvenile survival is most likely due to a higher degree of habitat fragmentation in the Sierra Nevada, which unlike the southern California study area (Gutierrez and Pritchard 1990), reduces the owl's capability to search available habitat for vacant territories. If the population in the central Sierra Nevada continues to decline, the potential for recruitment, either from reproductive output or from the floater population, will decrease. Therefore, territorial birds will not be replaced and an overall population decline will escalate. LITERATURE CITED Arcese, P. 1987. Age, intrusion pressure and defence against floaters by territorial male song sparrows. Anim. Behay. 35:773-784. Barrowclough, G.F. and S.L. Coats. 1985. The demography and population genetics of owls, with special reference to the conservation of the spotted owl R.J. Gutierrez and A.B. (Strix occidentalis).
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