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The whooping crane (Grus americana) population of North America
The Whooping Crane (Grus americana) Population of North America RICHARD S. MILLER Oastler Professor o f Wildlife Ecology, School o f Forestry and Environmental Studies', Yale University, New Haven, Connecticut, 06511, USA
DANIEL B. BOTKIN Assistant Professor o f Systems Ecology, School o f Forestry and Env&onmental Studies, Yale University, New Haven, Connecticut, 06511, USA &
ROY MENDELSSOHN School o f Forestry and Environmental Studies, Yale University, New Haven, Connecticut, 06511, USA
ABSTRACT A model o f the whooping crane population o f North America shows that its rate o f increase has been the result o f a stabilized death rate, in spite o f an overall decrease in the birth rate and no significant increase in the breeding population. Projections--based on current trends--predict a total population o f about 76 individuals in 10 years and 114 in 20 years, with a doubling time o f approximately 18 years, but these projections will have to be revised downward i f the birth rate continues to decline, or i f the carrying capacity o f the wintering grounds becomes limiting.
Obviously, conservationists and wildlife biologists would prefer to see a more dramatic increase to a safer population level, but the fact that the recovery of the whooping crane has been so slow, regardless of the protection it has received, suggests that its ecology and demographic properties are strong contributors to its endangered status. The purpose o f this paper is to analyse the population data that are available and to model the past performance and possible future of the species.
METHODS
The endangered status of the whooping crane (Grus Aerial and ground surveys have been conducted americana) is widely known and its past history has been well documented by Allen (1952, 1957). The every year since 1938 of the number of adult and young species was placed under the protection of the whooping cranes that arrive at the wintering grounds Migratory Bird Treaty in 1916, but the population of the Aransas National Wildlife Refuge at the terminacontinued to decrease from 47 individuals in 1918 to tion of fall migration. Juveniles can be distinguished a low of 15 in 1941. When the wintering ground of the from older birds by their size and plumage, but all whooping crane was discovered on the Blackjack whooping cranes have identical 'adult' plumage after Peninsula of Texas, 47,261 acres of this area were their first complete moult. It is, therefore, impossible purchased by the federal government and the Aransas to distinguish between subadults and reproductivelyNational Wildlife Refuge was established in 1937 mature individuals, and all birds older than juveniles (Howard, 1954). Following World War II, intensive will usually be referred to as 'adults' in this analysis. conservation efforts by private and government Annual surveys are also conducted by Canadian agencies in the United States and Canada resulted in Wildlife Service personnel of the number of nesting better protection during spring and fall migration pairs and production of young on the breeding (Novakowski, 1966), and in 1954 a team of Canadian grounds in Wood Buffalo Park. These surveys began Wildlife Service biologists discovered the last remain- in 1954, when the breeding population was discovered, ing breeding area of the species in Wood Buffalo Park, and have continued to the present time. Northwest Territories (Allen, 1957). This area is In spite of our lack of knowledge of many critical remote and relatively inaccessible (Novakowski, 1966), parameters, it is unusual to have an accurate census and nesting pairs are well isolated from human of the total numbers, annual production, and annual disturbance. Nevertheless, in spite of unprecedented mortality of a species, and we have used these data conservation efforts and almost total protection, the to develop a model of the growth of the whooping recovery of the wild population has been slow and crane population for the past 35 years. We feel that somewhat erratic. the growth of this population is best viewed as the 106 Biological Conservation,Vol.6, No. 2, April1974--0 AppliedSciencePublishersLtd, England,1974~Printedin GreatBritain
Miller, Botkin & Mendelssohn." Whooping Crane (Grus americana) Population of North America realization of a linear stochastic birth-death process (Chiang, 1968), in which the expected value for this model is the same as that for a deterministic model of exponential growth: N t = N Oexp (rt) where No is the initial population, t is time, and r is the net rate of increase (births-deaths). An advantage of the stochastic, rather than a deterministic, model is that it can account for variations that occur in the observed fluctuations in population growth; if the variations are within one or two standard deviations, the model can be accepted as an accurate representation of observed events (Chiang, 1968). The stochastic model also provides projections of population size and estimates of the probability of extinction. The modelling was carried out with the APL programming language-system on an IBM 360/67 computer. TABLE I
Whooping Cranes Counted at Aransas National Wildlife Refuge from 1938-1972 (unpublished data .from R. C. Erickson) Year
Adults
Young
Total
1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972
10 16 21 13 15 16 15 14 22 25 27 30 26 20 19 21 21 20 22 22 23 31 30 33 32 26 32 36 38 39 44 48 51 51 46
4 6 5 2 4 5 3 3 3 6 3 4 5 5 2 3 0 8 2 4 9 2 6 5 0 7 10 8 5 9 6 8 6 5 5
14 22 26 15 19 21 18 17 25 31 30 34 31 25 21 24 21 28 24 26 32 33 36 38 32 33 42 44 43 48 50 56 57 56 51
107
RESULTS
Population Growth Table I shows the numbers of adult and young whooping cranes counted at Aransas each fall since 1938. The numbers for 1938 and 1945 are obviously in error, in view of the unaccountably larger numbers of cranes that were counted in succeeding years. No aircraft were available for surveys in those years, which apparently resulted in an incomplete ground census (R. C. Erickson, pers. comm.). The mean value of the birth rate during the 35-year period was 0-20, and the death rate 0.12. A graph of three-year moving averages (with equal weight for each year) shows that the birth rate has tended mostly to oscillate around the mean value, but has decreased quite markedly in recent years (Fig. l(a)). Other short-term declines have occurred and, except for slight increases around 1960 and 1965, the overall trend in the birth rate shows a slight decline. The death rate shows a somewhat different pattern (Fig. l(b)). The oscillations that were present in early years appear to be damping out, and the death rate seems to be stabilizing at about 0.1. The mean values for the 35-year period of b = 0.20 and d = 0-12 give a net rate of increase (r) of 0-08. If we apply this value to the model, the expected values for total numbers are much too large, and a better fit to the observed values is obtained with a death rate of 0.16 and a net rate of increase of 0.04. The value of d = 0.16 is heavily weighted by the death rate during early years, which suggests that early fluctuations in the death rate strongly affected the subsequent rate of growth of the population, and that this effect is still present. A possible explanation of this phenomenon might be that these early fluctuations in the death rate influenced the present age structure of the population. It was noted earlier that there is no method for determining the age, in years, of a whooping crane, at least from external characteristics, and as none of the birds has been banded, there are no data available on the age structure of the population. Nor do we know the maximum life span of a whooping crane or its expected natural longevity. Some species of cranes have lived for as long as 40 years in captivity, but the maximum for most species is between 20 and 30 years (Walkinshaw, 1949), and the record longevity for a captive sandhill crane (G. canadensis) is 24 years. Walkinshaw (1949) estimates that the maximum natural longevity of a sandhill crane is from 20 to 25 years, and it is reasonable to assume that the larger whooping crane would have a longer life-span. If this is the case, it is not unlikely that fluctuations in the death rate during the first several years of the 35-year period from 1938 to 1972, and particularly
108
Biological Conservation
b~
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(b)
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Fig. 1.
I 1945
I 1950
I 1955
I 1960
I 1965
I 1970
Three-year moving averages of birth and death rates.
those that followed the population low in 1941, would have a significant effect on the present age structure of the population. With a net rate of increase of 0.04, the observed values for population numbers fall mostly within one standard deviation of the predicted values (Fig. 2). By fitting the model against a three-year moving average, a Pierson goodness-of-fit test gives a chi-
square of 23.3 with 32 degrees of freedom, indicating that the model cannot be rejected with 90 per cent confidence. These results suggest that the whooping crane population is undergoing an exponential increase, and the graph of population growth (Fig. 2) represents an early stage in a logistic curve. The inflection point and upper asymptote of this curve cannot be estimated from present data, but it is of
60"
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20
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Fig. 2.
I
19~5
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1950
I
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1965
1970
Three-year moving average of population growth (solid line) and smooth curve (dotted line) of population growth.
Miller, Botkin & Mendelssohn: Whooping Crane (Grus americana) Population of North America considerable interest to note that the exponential increase in this population is the result of a decreasing birth rate and a stabilizing death rate.
Breeding Success Table II shows the number of nesting pairs that have been counted at Wood Buffalo Park since 1954. The data for 1954-1965 are from Novakowski (1966) and the data for 1966-1972 were provided by E. Kuyt from unpublished records of the Canadian Wildlife Service. TABLE II
Number of Nesting Pairs Counted at the Sass River, Klewi River, and Nyarling Areas of Wood Buffalo Park, 1954-1972 Year
Sass River
1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972
3 4 1 5 2 2 4 3 0 2 5 5 5 6 6 5 6 7 8
Klewi River
3 4 7 8 5 7
Nyarling
Total
I 1 1
3 4 1 5 2 2 4 3 0 2 5 5 5 9 I0 12 15 13 16
It is evident that many, if not all, of the earlier counts (1954-1966) were incomplete, as the observed number of nesting pairs could not account for all of the annual production of young recorded at Aransas (Table I). The normal clutch of a whooping crane is 2 eggs, and a pair seldom raises more than 1 young (Novakowski, 1966; Miller, 1973), so that it is highly unlikely that the 2 pairs recorded in the Sass River area in 1958 produced the 9 young that arrived at Aransas that fall, or that 2 pairs raised 7 young in 1963. In fact, during the period 1954-1966, an average of only 3.2 nesting pairs were counted each year at Wood Buffalo Park, but an average of 5.1 young reached Aransas, where it is unusual to see twin juveniles, so that we must assume that a significant number of breeding pairs were not located during those years (Novakowski, 1966).
109
The number of nesting pairs counted at Wood Buffalo Park increased from 5 in 1966 to 9 in 1967 and reached a high of 16 in 1972, partly because of more complete surveys, especially in the Klewi and Nyarling River areas (Table II). All of the annual recruitment of young during the period 1967-1972 can be accounted for by the observed number of nesting pairs. The number of breeding birds during this period ranged from 46 to 70 per cent of the number of adults and subadults observed to leave Aransas that spring, and the average for the period is approximately 53 per cent. The series of observations from 1967 to 1972 is too short for reliable statistical treatment, but it would appear that the total number of breeding birds has not changed significantly, except possibly for 1972, and the absolute number of young produced has remained relatively constant, with an average of 5.6 for the 19-year period from 1954 through 1972. In 1972, when the maximum number of nesting pairs was recorded, 16 pairs laid between 30 and 32 eggs, but only 5 young were produced (E. Kuyt, pers. comm.). Thus, even in a year of apparent breeding success, as measured by the number of pairs found to be nesting, annual production remained near the long-term average. There is no obvious reason why the number of nesting pairs could not increase more significantly than it has, or why nesting success is so low, but it appears that one or more unidentified factors are acting to keep annual recruitment at a constant value, regardless of population size. If this continues, we can expect a continued decline in the birth rate and a correspondingly slower rate of population increase.
Population Fluctuations Inspection of the curve of population growth (Fig. 2) suggests that marked decreases in total numbers occur at approximately 10-year intervals. However, a timeseries analysis of the data shows little correlation between events 10 years apart, and there is a change from only a slightly negative to a slightly positive correlation around these 10-year intervals. There is, therefore, no reason to regard these fluctuations as non-random at the present time. However, when these periodic decreases do occur they are quite important. For example, a total of 59 whooping cranes left Aransas in the spring of 1972, but only 46 'adults' returned in the fall. Thus, there was an actual loss of 13 adult and subadult birds during the 1972 breeding season and, with the addition of 5 young, a net loss of 8 to the total population. Although these decreases do not have a significant periodicity, they might be explained by the effect of early oscillations in the death rate on the age structure of the population, and particularly high
Biological Conservation
110
mortality in certain year classes. This would mean, as suggested earlier, that the present population is still experiencing the effects of early perturbations in the system. A more immediate explanation is offered by Novakowski (1966), who suggests that large decreases tend to follow years of high production, and that the observed losses are suffered mostly by young birds that have recently left the family group and are inexperienced. We can find no significant correlation between high production one year and unusually high mortality the next, but this is a logical hypothesis.
With an initial population of 51 birds in 1972, the total number can be expected to reach 76 in 10 years and about 114 individuals in 20 years, with a doubling time of approximately 18 years. However, if the number of breeding pairs remains more or less constant and the birth rate continues to decrease, or if the carrying capacity of the wintering grounds becomes limiting, these projections will have to be revised downward.
DISCUSSION
Probability of Extinction The model predicts a probability of extinction of only 0.018 for 1972, which is consistent with observed events in the preceding years, and the predicted values for future years of course diminish with increasing population size. Given a population of 51 birds in 1972, the model predicts a probability of extinction of only 5.12 x 10 -9 in 1992, and it is obvious that there is little probability that the whooping crane will become extinct because of intrinsic population events in the foreseeable future, without a natural or manmade catastrophe of a magnitude that has not occurred in the period 1938-1972. Population Projections Assuming that the whooping crane population follows a linear stochastic birth-death process, and no major environmental changes occur, we can make the population projections shown in Table III. TABLE III
Population Projections from 1972-1992 Year 1972 1973 1974 1975 1976 1977
1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992
Predicted population 51 53 55 58 60 62 65 68 70 73 76 79 82 85 89 93 97 100 104 109 114
SD
4'4 6"4 8'1 9-2 11.1 12-6 14"0 15'4 16"9 18"4 19-8 21"4 22-6 24'6 26"2 27"9 29'7 31.5 33.4 35"4
This model shows that the current rate of increase in the whooping crane population of North America is due mostly to a stabilizing death rate which has allowed the survival of an increasing number of adult and subadult birds, while the absolute number of breeding pairs does not appear to have increased significantly, and the birth rate has shown an overall decline in the period 1938-1972. The probability of extinction of the population is very low, unless some unforeseen natural or man-made catastrophe occurs, but the projections for future population levels are based on current parameters which might not be sustained. The failure of the number of breeding pairs and annual recruitment of young to show significant increases could be due to limits in the carrying capacity of the breeding grounds, or to the present agestructure of the population. Whooping cranes are strongly territorial and their nests are usually a mile or more apart (Novakowski, 1966), but the total amount of suitable habitat in Wood Buffalo Park seems capable of supporting far more nesting pairs than now use this area, and there is no apparent reason why other regions in the sub-arctic could not serve as adequate breeding areas. The most accurate surveys of the breeding grounds (1967-1972) show that between 46 and 70 per cent of the population of adults and subadults that left Aransas in the spring of those years were breeding birds that nested at Wood Buffalo Park. We do not know the age at which whooping cranes become reproductively mature, but it is estimated that sandhill cranes usually defer breeding until they are 4 to 5 years old (Walkinshaw, 1949), and it is reasonable to assume that whooping cranes defer breeding until at least this age, or perhaps longer. It is possible that the number of nesting pairs observed at Wood Buffalo Park constitutes all--or nearly all--of the reproductively-mature birds in the population, and that the relatively constant size of the breeding population is a product of deferred breeding and the existing age structure of the total population, rather than limits to
Miller, Botkin & Mendelssohn: Whooping Crane (Grus americana) Population o f North America
suitable breeding habitat. If this is the case, it could have important consequences, especially if a fairly large number of breeding adults reach maximum longevity at more or less the same time. In other words, if most of the annual mortality occurs among young, more inexperienced birds, as Novakowski (1966) has suggested, and relatively small numbers of these birds survive to reach reproductive maturity, and if a large proportion of the breeding population consists of older birds that might die from old age more or less simultaneously, the population and its reproductive capacity could conceivably suffer drastic losses at some point in the future. The breeding population might, for example, consist mostly of birds that survived the population low of 1941 and those that entered the population in the next decade or so, in which case a large proportion of the breeding population might be in the 20-30 year age class. Unfortunately, we can only speculate about the existing age structure of the population, but it is clearly an important parameter in the welfare of the species, and we know from the principles of population ecology that it is a major determinant of population growth. The question of the carrying capacity of the wintering range is currently under investigation (D. Blankinship, pers. comm.). Competition for food and space are potentially important factors among the wintering birds, and we do not know how many whooping cranes the Aransas refuge can support. If the population increases to a level where competition forces some of the wintering birds out of the refuge, their protection might become a difficult problem and winter mortality, which is not now an important factor, might become important. In fact, if the population continues to increase at its present rate, the carrying capacity of the wintering range could become the most immediate problem in the welfare and management of the species. Obviously, the whooping crane cannot continue to increase and, even though we cannot estimate the upper asymptote of its population curve from present
111
data or the model developed in this paper, it is unlikely that the species will ever again become abundant and widely distributed. It is evident that the ecology of the species places rather severe limits on the habitats it can occupy, and that its upper level of population will be determined by the amount and suitability of breeding areas available, by factors affecting nesting success and annual recruitment of young, and by the ultimate carrying capacity of the winter range. This model also shows that the rate of recovery of the population has been affected by population events in its past history, which might still play a role in its future.
ACKNOWLEDGEMENTS
We wish to thank R. C. Erickson, Bureau of Sport, Fisheries and Wildlife, and E. Kuyt, Canadian Wildlife Service, for permission to use unpublished data. References ALLEN, R. P. (1952). The whooping crane. Nat. Audubon Soc. Res. Rept. No. 3, pp. 1-246. ALLEN, R. P. (1957). A report on the whooping crane's northern breeding grounds. Nat. Audubon Soc. Suppl. Res. Rept., No. 3, pp. 1-60. CmANG, C. L. (1968). Introduction to Stochastic Processes in Biostatistics. John Wiley & Sons, Inc., New York: 313 pp. HOWARD,J. A. (1954). Aransas, a national wildlife refuge. US Dept. Interior Fish & Wildl. Serv. No. 11, pp. 1-12. MILLER, R. S. (1973). The brood size of cranes. Wilson Bull., 85, pp. 436-441. NOVAKOWSKI, N. S. (1966). Whooping crane population dynamics on the nesting grounds, Wood Buffalo Park, Northwest Territories, Canada. Canad. Wildl. Serv. Rept. Series, No. 1, pp. 1-20. WALKINSHAW, L. H. (1949). The Sandhill Cranes. Cranbrook Institute of Science, Bloomfield Hills, Michigan, Bull. 29, pp. 1-202.
DANIEL B. BOTKIN Assistant Professor o f Systems Ecology, School o f Forestry and Env&onmental Studies, Yale University, New Haven, Connecticut, 06511, USA &
ROY MENDELSSOHN School o f Forestry and Environmental Studies, Yale University, New Haven, Connecticut, 06511, USA
ABSTRACT A model o f the whooping crane population o f North America shows that its rate o f increase has been the result o f a stabilized death rate, in spite o f an overall decrease in the birth rate and no significant increase in the breeding population. Projections--based on current trends--predict a total population o f about 76 individuals in 10 years and 114 in 20 years, with a doubling time o f approximately 18 years, but these projections will have to be revised downward i f the birth rate continues to decline, or i f the carrying capacity o f the wintering grounds becomes limiting.
Obviously, conservationists and wildlife biologists would prefer to see a more dramatic increase to a safer population level, but the fact that the recovery of the whooping crane has been so slow, regardless of the protection it has received, suggests that its ecology and demographic properties are strong contributors to its endangered status. The purpose o f this paper is to analyse the population data that are available and to model the past performance and possible future of the species.
METHODS
The endangered status of the whooping crane (Grus Aerial and ground surveys have been conducted americana) is widely known and its past history has been well documented by Allen (1952, 1957). The every year since 1938 of the number of adult and young species was placed under the protection of the whooping cranes that arrive at the wintering grounds Migratory Bird Treaty in 1916, but the population of the Aransas National Wildlife Refuge at the terminacontinued to decrease from 47 individuals in 1918 to tion of fall migration. Juveniles can be distinguished a low of 15 in 1941. When the wintering ground of the from older birds by their size and plumage, but all whooping crane was discovered on the Blackjack whooping cranes have identical 'adult' plumage after Peninsula of Texas, 47,261 acres of this area were their first complete moult. It is, therefore, impossible purchased by the federal government and the Aransas to distinguish between subadults and reproductivelyNational Wildlife Refuge was established in 1937 mature individuals, and all birds older than juveniles (Howard, 1954). Following World War II, intensive will usually be referred to as 'adults' in this analysis. conservation efforts by private and government Annual surveys are also conducted by Canadian agencies in the United States and Canada resulted in Wildlife Service personnel of the number of nesting better protection during spring and fall migration pairs and production of young on the breeding (Novakowski, 1966), and in 1954 a team of Canadian grounds in Wood Buffalo Park. These surveys began Wildlife Service biologists discovered the last remain- in 1954, when the breeding population was discovered, ing breeding area of the species in Wood Buffalo Park, and have continued to the present time. Northwest Territories (Allen, 1957). This area is In spite of our lack of knowledge of many critical remote and relatively inaccessible (Novakowski, 1966), parameters, it is unusual to have an accurate census and nesting pairs are well isolated from human of the total numbers, annual production, and annual disturbance. Nevertheless, in spite of unprecedented mortality of a species, and we have used these data conservation efforts and almost total protection, the to develop a model of the growth of the whooping recovery of the wild population has been slow and crane population for the past 35 years. We feel that somewhat erratic. the growth of this population is best viewed as the 106 Biological Conservation,Vol.6, No. 2, April1974--0 AppliedSciencePublishersLtd, England,1974~Printedin GreatBritain
Miller, Botkin & Mendelssohn." Whooping Crane (Grus americana) Population of North America realization of a linear stochastic birth-death process (Chiang, 1968), in which the expected value for this model is the same as that for a deterministic model of exponential growth: N t = N Oexp (rt) where No is the initial population, t is time, and r is the net rate of increase (births-deaths). An advantage of the stochastic, rather than a deterministic, model is that it can account for variations that occur in the observed fluctuations in population growth; if the variations are within one or two standard deviations, the model can be accepted as an accurate representation of observed events (Chiang, 1968). The stochastic model also provides projections of population size and estimates of the probability of extinction. The modelling was carried out with the APL programming language-system on an IBM 360/67 computer. TABLE I
Whooping Cranes Counted at Aransas National Wildlife Refuge from 1938-1972 (unpublished data .from R. C. Erickson) Year
Adults
Young
Total
1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972
10 16 21 13 15 16 15 14 22 25 27 30 26 20 19 21 21 20 22 22 23 31 30 33 32 26 32 36 38 39 44 48 51 51 46
4 6 5 2 4 5 3 3 3 6 3 4 5 5 2 3 0 8 2 4 9 2 6 5 0 7 10 8 5 9 6 8 6 5 5
14 22 26 15 19 21 18 17 25 31 30 34 31 25 21 24 21 28 24 26 32 33 36 38 32 33 42 44 43 48 50 56 57 56 51
107
RESULTS
Population Growth Table I shows the numbers of adult and young whooping cranes counted at Aransas each fall since 1938. The numbers for 1938 and 1945 are obviously in error, in view of the unaccountably larger numbers of cranes that were counted in succeeding years. No aircraft were available for surveys in those years, which apparently resulted in an incomplete ground census (R. C. Erickson, pers. comm.). The mean value of the birth rate during the 35-year period was 0-20, and the death rate 0.12. A graph of three-year moving averages (with equal weight for each year) shows that the birth rate has tended mostly to oscillate around the mean value, but has decreased quite markedly in recent years (Fig. l(a)). Other short-term declines have occurred and, except for slight increases around 1960 and 1965, the overall trend in the birth rate shows a slight decline. The death rate shows a somewhat different pattern (Fig. l(b)). The oscillations that were present in early years appear to be damping out, and the death rate seems to be stabilizing at about 0.1. The mean values for the 35-year period of b = 0.20 and d = 0-12 give a net rate of increase (r) of 0-08. If we apply this value to the model, the expected values for total numbers are much too large, and a better fit to the observed values is obtained with a death rate of 0.16 and a net rate of increase of 0.04. The value of d = 0.16 is heavily weighted by the death rate during early years, which suggests that early fluctuations in the death rate strongly affected the subsequent rate of growth of the population, and that this effect is still present. A possible explanation of this phenomenon might be that these early fluctuations in the death rate influenced the present age structure of the population. It was noted earlier that there is no method for determining the age, in years, of a whooping crane, at least from external characteristics, and as none of the birds has been banded, there are no data available on the age structure of the population. Nor do we know the maximum life span of a whooping crane or its expected natural longevity. Some species of cranes have lived for as long as 40 years in captivity, but the maximum for most species is between 20 and 30 years (Walkinshaw, 1949), and the record longevity for a captive sandhill crane (G. canadensis) is 24 years. Walkinshaw (1949) estimates that the maximum natural longevity of a sandhill crane is from 20 to 25 years, and it is reasonable to assume that the larger whooping crane would have a longer life-span. If this is the case, it is not unlikely that fluctuations in the death rate during the first several years of the 35-year period from 1938 to 1972, and particularly
108
Biological Conservation
b~
(a) ! h r~
nl
r¢
(b)
2~ T_ w
(~
,lO
.05 o
I 1940
Fig. 1.
I 1945
I 1950
I 1955
I 1960
I 1965
I 1970
Three-year moving averages of birth and death rates.
those that followed the population low in 1941, would have a significant effect on the present age structure of the population. With a net rate of increase of 0.04, the observed values for population numbers fall mostly within one standard deviation of the predicted values (Fig. 2). By fitting the model against a three-year moving average, a Pierson goodness-of-fit test gives a chi-
square of 23.3 with 32 degrees of freedom, indicating that the model cannot be rejected with 90 per cent confidence. These results suggest that the whooping crane population is undergoing an exponential increase, and the graph of population growth (Fig. 2) represents an early stage in a logistic curve. The inflection point and upper asymptote of this curve cannot be estimated from present data, but it is of
60"
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20
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.5 o
I
1940
Fig. 2.
I
19~5
I
1950
I
{955
I
19b0
[
1965
1970
Three-year moving average of population growth (solid line) and smooth curve (dotted line) of population growth.
Miller, Botkin & Mendelssohn: Whooping Crane (Grus americana) Population of North America considerable interest to note that the exponential increase in this population is the result of a decreasing birth rate and a stabilizing death rate.
Breeding Success Table II shows the number of nesting pairs that have been counted at Wood Buffalo Park since 1954. The data for 1954-1965 are from Novakowski (1966) and the data for 1966-1972 were provided by E. Kuyt from unpublished records of the Canadian Wildlife Service. TABLE II
Number of Nesting Pairs Counted at the Sass River, Klewi River, and Nyarling Areas of Wood Buffalo Park, 1954-1972 Year
Sass River
1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972
3 4 1 5 2 2 4 3 0 2 5 5 5 6 6 5 6 7 8
Klewi River
3 4 7 8 5 7
Nyarling
Total
I 1 1
3 4 1 5 2 2 4 3 0 2 5 5 5 9 I0 12 15 13 16
It is evident that many, if not all, of the earlier counts (1954-1966) were incomplete, as the observed number of nesting pairs could not account for all of the annual production of young recorded at Aransas (Table I). The normal clutch of a whooping crane is 2 eggs, and a pair seldom raises more than 1 young (Novakowski, 1966; Miller, 1973), so that it is highly unlikely that the 2 pairs recorded in the Sass River area in 1958 produced the 9 young that arrived at Aransas that fall, or that 2 pairs raised 7 young in 1963. In fact, during the period 1954-1966, an average of only 3.2 nesting pairs were counted each year at Wood Buffalo Park, but an average of 5.1 young reached Aransas, where it is unusual to see twin juveniles, so that we must assume that a significant number of breeding pairs were not located during those years (Novakowski, 1966).
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The number of nesting pairs counted at Wood Buffalo Park increased from 5 in 1966 to 9 in 1967 and reached a high of 16 in 1972, partly because of more complete surveys, especially in the Klewi and Nyarling River areas (Table II). All of the annual recruitment of young during the period 1967-1972 can be accounted for by the observed number of nesting pairs. The number of breeding birds during this period ranged from 46 to 70 per cent of the number of adults and subadults observed to leave Aransas that spring, and the average for the period is approximately 53 per cent. The series of observations from 1967 to 1972 is too short for reliable statistical treatment, but it would appear that the total number of breeding birds has not changed significantly, except possibly for 1972, and the absolute number of young produced has remained relatively constant, with an average of 5.6 for the 19-year period from 1954 through 1972. In 1972, when the maximum number of nesting pairs was recorded, 16 pairs laid between 30 and 32 eggs, but only 5 young were produced (E. Kuyt, pers. comm.). Thus, even in a year of apparent breeding success, as measured by the number of pairs found to be nesting, annual production remained near the long-term average. There is no obvious reason why the number of nesting pairs could not increase more significantly than it has, or why nesting success is so low, but it appears that one or more unidentified factors are acting to keep annual recruitment at a constant value, regardless of population size. If this continues, we can expect a continued decline in the birth rate and a correspondingly slower rate of population increase.
Population Fluctuations Inspection of the curve of population growth (Fig. 2) suggests that marked decreases in total numbers occur at approximately 10-year intervals. However, a timeseries analysis of the data shows little correlation between events 10 years apart, and there is a change from only a slightly negative to a slightly positive correlation around these 10-year intervals. There is, therefore, no reason to regard these fluctuations as non-random at the present time. However, when these periodic decreases do occur they are quite important. For example, a total of 59 whooping cranes left Aransas in the spring of 1972, but only 46 'adults' returned in the fall. Thus, there was an actual loss of 13 adult and subadult birds during the 1972 breeding season and, with the addition of 5 young, a net loss of 8 to the total population. Although these decreases do not have a significant periodicity, they might be explained by the effect of early oscillations in the death rate on the age structure of the population, and particularly high
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mortality in certain year classes. This would mean, as suggested earlier, that the present population is still experiencing the effects of early perturbations in the system. A more immediate explanation is offered by Novakowski (1966), who suggests that large decreases tend to follow years of high production, and that the observed losses are suffered mostly by young birds that have recently left the family group and are inexperienced. We can find no significant correlation between high production one year and unusually high mortality the next, but this is a logical hypothesis.
With an initial population of 51 birds in 1972, the total number can be expected to reach 76 in 10 years and about 114 individuals in 20 years, with a doubling time of approximately 18 years. However, if the number of breeding pairs remains more or less constant and the birth rate continues to decrease, or if the carrying capacity of the wintering grounds becomes limiting, these projections will have to be revised downward.
DISCUSSION
Probability of Extinction The model predicts a probability of extinction of only 0.018 for 1972, which is consistent with observed events in the preceding years, and the predicted values for future years of course diminish with increasing population size. Given a population of 51 birds in 1972, the model predicts a probability of extinction of only 5.12 x 10 -9 in 1992, and it is obvious that there is little probability that the whooping crane will become extinct because of intrinsic population events in the foreseeable future, without a natural or manmade catastrophe of a magnitude that has not occurred in the period 1938-1972. Population Projections Assuming that the whooping crane population follows a linear stochastic birth-death process, and no major environmental changes occur, we can make the population projections shown in Table III. TABLE III
Population Projections from 1972-1992 Year 1972 1973 1974 1975 1976 1977
1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992
Predicted population 51 53 55 58 60 62 65 68 70 73 76 79 82 85 89 93 97 100 104 109 114
SD
4'4 6"4 8'1 9-2 11.1 12-6 14"0 15'4 16"9 18"4 19-8 21"4 22-6 24'6 26"2 27"9 29'7 31.5 33.4 35"4
This model shows that the current rate of increase in the whooping crane population of North America is due mostly to a stabilizing death rate which has allowed the survival of an increasing number of adult and subadult birds, while the absolute number of breeding pairs does not appear to have increased significantly, and the birth rate has shown an overall decline in the period 1938-1972. The probability of extinction of the population is very low, unless some unforeseen natural or man-made catastrophe occurs, but the projections for future population levels are based on current parameters which might not be sustained. The failure of the number of breeding pairs and annual recruitment of young to show significant increases could be due to limits in the carrying capacity of the breeding grounds, or to the present agestructure of the population. Whooping cranes are strongly territorial and their nests are usually a mile or more apart (Novakowski, 1966), but the total amount of suitable habitat in Wood Buffalo Park seems capable of supporting far more nesting pairs than now use this area, and there is no apparent reason why other regions in the sub-arctic could not serve as adequate breeding areas. The most accurate surveys of the breeding grounds (1967-1972) show that between 46 and 70 per cent of the population of adults and subadults that left Aransas in the spring of those years were breeding birds that nested at Wood Buffalo Park. We do not know the age at which whooping cranes become reproductively mature, but it is estimated that sandhill cranes usually defer breeding until they are 4 to 5 years old (Walkinshaw, 1949), and it is reasonable to assume that whooping cranes defer breeding until at least this age, or perhaps longer. It is possible that the number of nesting pairs observed at Wood Buffalo Park constitutes all--or nearly all--of the reproductively-mature birds in the population, and that the relatively constant size of the breeding population is a product of deferred breeding and the existing age structure of the total population, rather than limits to
Miller, Botkin & Mendelssohn: Whooping Crane (Grus americana) Population o f North America
suitable breeding habitat. If this is the case, it could have important consequences, especially if a fairly large number of breeding adults reach maximum longevity at more or less the same time. In other words, if most of the annual mortality occurs among young, more inexperienced birds, as Novakowski (1966) has suggested, and relatively small numbers of these birds survive to reach reproductive maturity, and if a large proportion of the breeding population consists of older birds that might die from old age more or less simultaneously, the population and its reproductive capacity could conceivably suffer drastic losses at some point in the future. The breeding population might, for example, consist mostly of birds that survived the population low of 1941 and those that entered the population in the next decade or so, in which case a large proportion of the breeding population might be in the 20-30 year age class. Unfortunately, we can only speculate about the existing age structure of the population, but it is clearly an important parameter in the welfare of the species, and we know from the principles of population ecology that it is a major determinant of population growth. The question of the carrying capacity of the wintering range is currently under investigation (D. Blankinship, pers. comm.). Competition for food and space are potentially important factors among the wintering birds, and we do not know how many whooping cranes the Aransas refuge can support. If the population increases to a level where competition forces some of the wintering birds out of the refuge, their protection might become a difficult problem and winter mortality, which is not now an important factor, might become important. In fact, if the population continues to increase at its present rate, the carrying capacity of the wintering range could become the most immediate problem in the welfare and management of the species. Obviously, the whooping crane cannot continue to increase and, even though we cannot estimate the upper asymptote of its population curve from present
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data or the model developed in this paper, it is unlikely that the species will ever again become abundant and widely distributed. It is evident that the ecology of the species places rather severe limits on the habitats it can occupy, and that its upper level of population will be determined by the amount and suitability of breeding areas available, by factors affecting nesting success and annual recruitment of young, and by the ultimate carrying capacity of the winter range. This model also shows that the rate of recovery of the population has been affected by population events in its past history, which might still play a role in its future.
ACKNOWLEDGEMENTS
We wish to thank R. C. Erickson, Bureau of Sport, Fisheries and Wildlife, and E. Kuyt, Canadian Wildlife Service, for permission to use unpublished data. References ALLEN, R. P. (1952). The whooping crane. Nat. Audubon Soc. Res. Rept. No. 3, pp. 1-246. ALLEN, R. P. (1957). A report on the whooping crane's northern breeding grounds. Nat. Audubon Soc. Suppl. Res. Rept., No. 3, pp. 1-60. CmANG, C. L. (1968). Introduction to Stochastic Processes in Biostatistics. John Wiley & Sons, Inc., New York: 313 pp. HOWARD,J. A. (1954). Aransas, a national wildlife refuge. US Dept. Interior Fish & Wildl. Serv. No. 11, pp. 1-12. MILLER, R. S. (1973). The brood size of cranes. Wilson Bull., 85, pp. 436-441. NOVAKOWSKI, N. S. (1966). Whooping crane population dynamics on the nesting grounds, Wood Buffalo Park, Northwest Territories, Canada. Canad. Wildl. Serv. Rept. Series, No. 1, pp. 1-20. WALKINSHAW, L. H. (1949). The Sandhill Cranes. Cranbrook Institute of Science, Bloomfield Hills, Michigan, Bull. 29, pp. 1-202.