Population trends of the mantled howler groups of La Pacifica Guanacaste Costa Rica.код для вставкиСкачать
American Journal of Primatology 11:79-88 (1986) Population Trends of the Mantled Howler Groups of La Pacifica, Guanacaste, Costa Rica MARGARET R. CLARKE,l EVAN L. ZUCKER,l AND NORMAN J. SCOlT, JRZ 'Department of Psychology, Loyola Uniuersity, New Orleans, Louisiana; 'U.S. Fish and Wildlife Seruice, Denver Wildlife Research Center, Museum of Southwestern Biology, Uniuersity of New Mexico, Albuquerque, New Mexico A complete survey of La Pacifica in Guanacaste Province, Costa Rica was conducted in July 1984 in order to determine whether the howler (Alouatta palliata) population had declined since 1972 as a result of deforestation. During the 6-day survey, 257 howlers were located, representing 16 different social groupings and nine solitary animals. The total number of howlers, the number and location of groups, and the age-sex composition were very similar to a 1972-1976 survey of the same population. Age-sex composition of La Pacifica howler groups was similar to those of another population of mantled howlers in Costa Rica and of populations in Mexico and in Panama, although La Pacifica had a higher mean number of adult females per group. There was no evidence to support the hypothesis that the La Pacifica howler population has declined in recent times. Key words: Alouatta palliata, deforestation, demography, survey INTRODUCTION Diminishing nonhuman primate populations are of concern to conservationists and field biologists alike. As human populations increase, a greater number of nonhuman primates are hunted for food, killed as cropraiding pests, or have had their habitats reduced by deforestation for agricultural expansion. The export of nonhuman primates for medical purposes, while reduced in recent years, also continues [Heltne & Thorington, 1976; Southwick & Smith, 1986; Wolfheim, 19831. Although some New World monkey populations are being threatened by hunting for export trade as well as for food, mantled howling monkeys (Alouatta palliata) in Guanacaste Province, Costa Rica appear to be threatened primarily by the conversion of forest habitats to grassland for raising cattle [Boucher et al, 19831. Although the Costa Rican government is attempting to slow deforestation by establishing national parks and strictly regulating deforestation of private property, habitat destruction continues [Gomez & Savage, 19831. Received September 16, 1985; revision accepted March 23, 1986. Please address reprint requests to Margaret R. Clarke, Delta Regional Primate Research Center of Tulane University, Three Rivers Road, Covington, LA 70433. 0 1986 Alan R. Liss, Inc. 80 I Clark, Zucker, and Scott Surveys of howler populations were carried out a t La Pacifica and at Taboga in Guanacaste Province between 1966 and 1971 and were compared to similar surveys of the howler population at Barro Colorado Island in Panama [Heltne et al, 19761. The conclusion was reached that the Costa Rican populations represented “a distressed and declining population” [Heltne et al, 1976:17] while the Barro Colorado Island population was considered relatively stable. La Pacifica and Taboga have been deforested, while Barro Colorado Island has been protected for over 60 years [Carpenter, 1934; Milton, 19821. Since a relatively continuous population decline would be expected to affect group size and structure as well as patterns of migration, competition, and group formation [Altmann et al, 19851, it also might affect the results of studies of social behavior, which have been carried out on howlers at La Pacifica since 1970 [eg, Clarke, 1982; Clarke & Glander, 1984; Glander, 1980, 1981; Jones, 1978,19801. In order to estimate what changes actually have occurred in this howler population, Clarke and Zucker conducted a complete survey of La Pacifica in 1984 in order to compare the results with the population data collected by Scott between 1972 and 1976 [Scott et al, 1978, unpublished data]. The total number of howlers, as well as the number, location, and age-sex composition of howling monkey groups, could be compared over this 10-year period in order to determine if the population decline predicted by Heltne et a1 , based on 1966-1971 estimates, actually had occurred. In addition, the results of the La Pacifica survey allow comparisons with data and census results from three other populations of A. palliata. METHODS Study Site The study site is located 5 km northwest of Canas in Guanacaste Province, Costa Rica. This region has been classified as dry, deciduous tropical forest [Holdridge, 19671 with little or no rainfall during 5 months of the year. Rainfall amounts vary between 1,300 and 1,600 mm during the wet season, which extends from midMay to mid-December. La Pacifica is a working cattle ranch that was converted to pastureland in four stages between 1960 and 1973. The ranch is presently over 1,330 hectares in size, and about one-quarter of the original forest remains. La Pacifica has been deforested in a conservative manner: riparian forests were retained, and windbreaks and patches of upland forest remain interconnected. In addition, areas used for short-term agriculture are rotated, and the unused areas are allowed to revert to natural growth. Some of the ranch is irrigated, which may stabilize some of the forest area during the dry season, and cattle are methodically rotated from pasture to pasture to maximize natural regeneration of vegetation. Howlers are found in both the riparian and the upland forest habitats. Survey Plan The forests of La Pacifica were surveyed for howling monkeys between 0800 and 1300 h on 6 consecutive days in July 1984. The survey plan was a modified quadrat approach [National Research Council, 19811, and the 14 fieldworkers were divided into survey parties of three to five individuals. Approximately one-sixth of the total forest was surveyed per day, and each forest patch, river side, or windbreak section was assigned to a different survey party. The forest canopy ranges between 15 and 50 m in height, which is low enough for howler groups to be spotted relatively easily. Surveying began on the southern side of the ranch and systematically progressed northward to minimize chances of counting the same animals more than once. Survey party members slowly walked their designated forest area at evenly spaced intervals, which ranged between 10 and 30 meters. When howlers were Howler Demography / 81 sighted, all survey party members left their positions on their transects, and a minimum of one-half hour was spent counting and identifying howlers once a sighting occurred. The count was considered complete when all survey party members agreed on the sex, age, and total number of animals in that area. When the count was complete, the location of the animals was marked on a map, and the observers returned to their locations on the transects to resume the survey. During the actual survey, animals 75 m or more apart were considered as separate sightings. At the end of each day, the counts and locations of howlers were entered into a permanent log. Data Analysis Groups were defined as bisexual associations of three or more animals, wit,h a t least one fully adult male. For the purpose of analysis, groups counted as separate sightings but located within 150 m of each other were considered part of the same social group. Group size, distribution, and composition from both the 1984 survey and the 1972-1976 counts were analyzed using descriptive statistics and were compared to each other. The previous count was the result of a series of capture sessions in which all animals received tattoos and/or were ear notched [Scott et al, 1976,1978, unpublished data]. One adjustment was made during the present study for comparative purposes. When a female loses a n infant, she ceases lactation within 1to 3 days [Clarke, 19821; it can be assumed that a lactating female actually does have a n infant, whether it was captured with her or not. Therefore, one infant (immature) was added to the 1972-1976 count whenever a lactating female was captured without a n infant. Survey data from other mantled howler populations located in Costa Rica, Mexico, and Panama also were analyzed using the same descriptive statistics in order to allow for a direct comparison with the La Pacifica population. RESULTS A total of 248 animals were located in 16 distinct social groupings (Table I). Each group included one to four adult males (2 = 2.56), 2 to 15 adult females (x = 7.63), one to four juveniles ( j t = 2.19), and zero to seven infants (% = 2.13). Total group size ranged from 4 to 29 (% = 15.5). Nine solitary howlers were also located five males, three females, and one juvenile (unknown sex). Thus, 257 animals were sighted during the 6-day survey. “Density” of howlers was estimated a t 77.3 per km2. This was calculated by dividing the number of howlers counted by the area of remaining forest (3.3 km2, estimated from aerial photographs). The overall ratio of adult males to adult females was 1:3.2, and that of adults to immatures was 1:0.4. Comparisons of the results of this survey with the results of the survey conducted during 1972-1976 showed no difference in totals and age-sex distributions (Table ID. The only notable difference was in the adult ma1e:adult female sex ratio. Even if the unidentified adults from the present survey were included as females, the 1984 adult ma1e:adult female ratio would be 1:3.2 as compared to 1:4.2 recorded in 1972-1976. This difference, however, was not statistically significant (x21 1.56, n.s.). Group locations were also remarkably similar: eight groups were found in the same locations as 10 years ago, and most groups were sighted within the same home ranges estimated a decade ago (Fig. 1). A comparison of La Pacifica survey data with data from 1972 and 1983184 surveys a t Santa Rosa National Park in Costa Rica [Fedigan et al, 1985; Freese, 19761, from a long-term study at Los Tuxtlas in Mexico [Estrada, 19821, and from estimates and counts at Barro Colorado Island in Panama [Milton, 1982; Mittermeier, 19731 disclosed differences in sex ratios and densities. Ranges for age-sex *= 82 I Clark, Zucker, and Scott TABLE 1. Survey Results-Howlers in Groups (La Pacifica, July 1984) Group” 2 3 5 6 7 8 9 11 12 13 14 16 Cabina Roea Corral Tiki Total Adult males 4 4 3 2 1 4 1 2 Subadult males Adult females Juveniles Infants Total 3 8 2 6 0 1 2 0 0 1 0 0 1 0 0 0 0 4 0 0 3 2 4 2 2 1 1 2 2 1 3 2 2 2 2 4 3 7 1 2 3 2 0 1 7 3 2 1 0 0 0 2 22 27 16 15 11 23 4 15 29 15 17 10 6 18 6 14 122 9 35 34 248 10 13 6 8 3 3 3 1 3 1 2 2 0 0 1 1 0 0 1 0 0 0 0 0 1 1 0 41 7 4 Unidentified adults 4 15 2 9 15 8 9 4 aGroup number based on Scott records plus four groups named during the 1984 survey. TABLE 11. Ten Year Comparison of Howler Survey Data (La Pacifica 1972/76 and 1984) Adult males Totals Scott 1972176 32 ClarkelZucker 1984 41 Means per group Scott 1972176 2.1 ClarkelZucker 1984 2.6 Subadult males Adult Unidentified Population Grand females adults Immatures in groups Solitaries total 8 134 1 60a 235 12 247 7 122 9 69 248 9 257 0.4 8.9 4.0 15.7 0.5 7.6 4.3 15.5 No. of x Adults adults per group A u :A 9 Immatures: adults No. of groups Range of group size “Density” Scott 1972176 175 11.7 1:4.2 1:2.9 15 4-31 74.3 per km2 Clarke/Zucker 1984 179 11.1 1:3.0 1:2.6 16 4-29 77.3 wer km2 aIncludes infants on adult females plus one additional infant for each lactating female without a n infant. Howler Demography I 83 Fig. 1. Map of Hacienda La Pacifica indicating locations of howler groups surveyed by Scott in 19721976 and by Clarke and Zucker in 1984. classes and group sizes overlapped, but the group size and mean number of females per group were smaller at Santa Rosa in 1972 and at Los k t l a s (Table 111).Groups a t La Pacifica had a higher proportion of females than those a t the other sites. Other striking differences were seen in the comparative population parameters presented in Table IV.Population densities ranged from approximately one to about 90 howlers per km2, and the ratios of females to males varied from 1:1.5 to 1:4.0. The ratios of adults to immatures also varied. It should be noted that the numbers for the 1984 survey represent the number of animals actually located and identified during the 6-day survey and should not be construed as the total number of howlers on La Pacifica. We estimate that we counted 75% of the howlers, making the total population closer to 350 animals. We arrived at this estimate for the following reasons: on Day 1of the survey, 35 animals were counted in a n area where we know we missed 12; during July and August of 1984, we located and identified 43 additional howlers that were not seen during the survey; and it is probable that some of the smaller animals (juveniles, young females) also were missed. The use of this 75% estimate does not weaken the comparisons with the population data from the 1972-1976 capture sessions, as the same sampling limitations apply to both survey methods. 84 I Clark, Zucker, and Scott TABLE 111. Grow ComDosition: ComDarative Howler Data* Costa Rica La Pacifica Scotta Mean Range Clarkeb Mean Range Santa Rosa Freese‘ Mean Range Fedigand Mean Range Mexico Los Tuxtlas Estrada“ Mean Range Panama Barro Colorado Mittermeierf Mean Range Miltong Mean Ranee Adult cr Adult 9 3.0 1-5 9.0 2-17 3.0 1-6 9.8 2-16 2.2 1-4 1.6 1-4 3.6 1-10 3.0 1-8 Juveniles Infants 4.0 1-8 Group size No. groups 15.7 4-31 15 2.2 0-7 15.5 4-29 16 2.0 0-7 0.9 0-3 7.8 3-24 8 5.9 2-18 2.8 2-9 1.9 0-9 13.7 3-40 25 3.0 1-5 4.1 2-6 1.6 0-3 1.5 0-3 9.1 5-16 17 3.2 2-5 5.8 4-8 4.3 3-6 2.3 1-4 16.2 13-23 3.5 NA 9.2 NA 2.6 NA 5.5 NA 20.8 9-32 6 13 *Adulta, includes adult males plus subadult males; Adult Q , includes adult females plus unidentified adults; NA, data not available ‘Scott, 1972/1976 survey. bClarke and Zucker, 1984 survey. ‘Based on Freese . dBased on Fedigan et a1 . eEstrada . fBased on Mittermeier . Wilton  (1977 data). DISCUSSION These results do not support the hypothesis that the howling monkeys at La Pacifica are declining in numbers. The number of howlers may have decreased immediately following deforestation, contributing to the “stressed population” interpretation of Heltne et a1  based on brief surveys made between 1966 and 1971. The present population, however, has remained relatively unchanged, both in size and in group location and composition, between 1972-76 and 1984. All groups except two were multimale, multifemale groups, and one of the one-male groups is known to have formed in 1980 when the then solitary adult male attracted the first adult female to his home range. Thus, the La Pacifica population of howlers continues to exhibit the “typical” mantled howler social structure [Crockett & Eisenberg, in press]. Howler Demography / 85 TABLE IV. Population Parameters: Comparative Howler Data* Total howlers Costa Rica La Pacifica Scotta Clarkeb Santa Rosa Freesec Fedigand Mexico Los Tuxtlas Estrada“ Panama Barro Colorado Mittermeierf Miltong Usable habitat (ha) Estimated Ratios density per km2 Adult o :Adult Q Adu1ts:immatures 247 257 332.5 332.5 74.3 77.3 1:4.2 1:3.0 1:0.34 1:0.38 70-100h 341 10,000 7,560 0.7-1.0h 4.9 12.2’ 12.0 1:0.56’ 1:0.53 155 700 22.1 1:1.4 1:0.27 8001,050h 1,352 1,351 59.277.7h 91.7 1:l.g 1:0.77J NA NA 1,475 *NA, data not available. =Scott, 1972/76 survey. bClarke and Zucker, 1984 survey. ‘Based on Freese . dBased on Fedigan et a1 . eEstrada . fBased on Mittermeier . Wilton  (1977 data). hBased on estimates rather than survey counts. ‘Based on a sample of eight groups. ’Based on a sample of six groups. Although the comparison of the four howler populations reveals similar age-sex compositions of groups, the size of the groups and the density of howlers in the usable habitat are variable. Both surveys a t La Pacifica and the results from B m o Colorado indicate high densities of howlers, while the two surveys at Santa Rosa and the data from Los Tuxtlas reveal considerably lower numbers. This variation could be due to different environmental stresses on the populations, to differences in methods of estimating “density,” or could simply reflect normal variability in population parameters. Despite the fact that La Pacifica was largely deforested, the remaining forest has been relatively stable since 1973. Barro Colorado Island is protected and has a history of population fluctuations [Collias & Southwick, 19521, but even at the lowest estimates Barro Colorado Island has a “density” of howlers that is fairly close to that calculated for La Pacifica. The population at Los Tuxtlas was described by Estrada [19821 as a “stressed” population due to habitat destruction, small groups, and low proportions of females to males and of immatures to adults. Deforestation may account for low population density at Los Tuxtlas, but it would not explain the low density of howlers a t Santa Rosa, a protected National Park since 1970 [Fedigan et al, 19851. A resurgence in the population of monkeys, such as has been documented for other species when they became protected andlor provisioned [Malik et al, 1984; Southwick et al, 19821, has not occurred yet at Santa Rosa. There was an increase in density of howlers between the 1974 and 1984 surveys at Santa Rosa, but part of the increase was due to a change in the definition of 86 I Clark, Zucker, and Scott “usable habitat” as well as to a more thorough survey of the area. Different estimates of “usable habitat” and errors in sampling, including the differential observability of particular age-sex classes, may account for some of the variability in population density estimates from site to site. “Available habitat” may not be “usable” for a monkey [Glander et al, 19841. The actual density of animals in “usable habitat” would be considerably higher than the density for “available habitat.” It is possible that the smaller sites, such as Barro Colorado Island, La Pacifica, and Los Tuxtlas, can be more thoroughly surveyed with relatively few fieldworkers and the amount of usable habitat can be more accurately assessed. The low ratios of immatures to adults at the Costa Rican and Mexican sites have been interpreted as indicating stressed populations [Estrada, 1982; Heltne et al, 19761. Models of population growth, however, assume that immatures remain in the group until they are adult. As juvenile mantled howler males emigrate as early as 1year of age and females as early as 2 years of age [Clarke, 19841, these solitary juveniles would be easy to miss in a survey and yet should be considered as part of the total population. Indeed, a n inflated estimate of a howling monkey group’s size would be obtained if the group were surveyed during the brief period when new infants have just been born and juveniles are in the process of emigrating. Thus, both a n overestimate of usable habitat and a n underestimate of solitary animals would result in artificially low density estimates. These density estimates also may reflect normal fluctuations in mantled howling monkey population growth cycles. Crockett and Eisenberg (in press) calculated a regression of mean group size on population density for A. palliata, and the results of the present census are consistent with their predictions. The range of population densities reported for the different mantled howler populations are within the range of documented fluctuations for a baboon population (Papw cynocephalus) in response to natural environmental changes [Altmann et al, 19851. Population changes, both in total numbers and in group size and number, also have been documented for red howlers (A. seniculus) in Venezuela [Crockett, 1985; Crockett & Eisenberg, in press]. Changes in density within a population could reflect immediate responses to changes in food supply and competition with other nonhuman populations, as well as to human intervention. This highlights the need to continue to monitor primate populations in order to make accurate assessments of their success in a world of diminishing habitat and to understand the specifics of population dynamics. The howlers at La Pacifica, despite a probable immediate decline in numbers, appear to have adjusted to the altered environment and to have maintained their numbers over a 10-15-year period. This suggests that planned deforestation, including large undisturbed patches of forests with interconnected forest “pathways,” may allow howling monkey populations to persist in areas where deforestation is unavoidable. CONCLUSIONS 1. A complete survey of La Pacifica indicated a population similar to the population surveyed there 10 years earlier. 2. A total of 257 animals were sighted during the 6-day survey. Since 43 animals were known to be missed in the survey, and it was assumed some immatures and solitaries were also missed, the total population was estimated at about 350. 3. The age-sex composition of La Pacifica howler groups was similar to those of the Santa Rosa, Los Tuxtlas, and Barro Colorado Island groups, but the “density” of howlers was quite variable among the sites. 4. Comparisons of population densities across sites are confounded by differing Howler Demography / 87 estimates of usable habitats, differential observability of immature and solitary animals, and the timing of births vis-a-vis emigration by juveniles. ACKNOWLEDGMENTS This research was supported by the School for Field Studies, Cambridge, MA, and by the World Wildlife Fund U.S. We would like to thank our field workers for their enthusiastic help: Aaron Ferster, Betsy Thompson, Romana Prokopiw, Rob Rand, Chuck Rosenthal, Adrian Forman, Ron Phillips, Lieta Comfort, Kate Offutt, Todd Broseghini, Jay Sears, and Albie Nash from the 1984 survey, and Linda Malmgren, Alan Scott, Bruce Woodward, Bob Reynolds, and Ken Glander from the 1972-1976 field sessions. We would also like to thank Lillian and Werner Hagnauer and Vreni Leigh for their gracious hospitality while we were at La Pacifica. We would also like to thank Dr. Michael A. Bogan, Dr. Carolyn M. Crockett, Dr. Kenneth E. Glander, Dr. Curtis H. Halvorson, Dr. Fritz L. Knopf, Dr. Paul A. Opler, Dr. Elizabeth S. Watts, and reviewers for comments on this manuscript. The secretarial assistance of Carolyn Boudreaw and Sharon Nastasi of Delta Primate Center is greatly appreciated. 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