Techniques for the Study of Primate Population Ecology (1981) / Chapter Skim
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CENSUS METHODS FOR ESTIMATING DENSITIES
CENSUS METHODS FOR ESTIMATING DENSITIES
Pages 36-80
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From page 36... ...
. For sample areas based on transects, / = the length of the transect line and 2w = the strip width, which is twice the transect width -- that is, the width lo36
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From page 37... ...
Usually a measurement is made only for the perpendicular distances or for both the sighting angle and the sighting distances from which the perpendicular distances can be computed. Different methods for determining a valid transect width have led to a variety of formulas, usually through the substitution of w by some specifically defined value.
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From page 38... ...
It is obvious from the initial equation, TV -- nA/2-lw, that decreasing the size of the sample area relative to the sample population, n, has the effect of decreasing the denominator and thereby increasing the population estimate, N Thus, formulas that utilize mean sighting or perpendicular distances will produce smaller estimates of the sample area and larger population estimates than will equations that utilize maximum values for these estimates of transect width.
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From page 39... ...
Webb's method uses the mean sighting distances and mean sighting angles to determine perpendicular distances; the other two methods use the mean perpendicular distances. The Gates I method also uses n -- 1 for «.
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From page 40... ...
Anderson and Pospahala also developed a correction factor for animals not observed. ASSUMPTIONS FOR LINE TRANSECT SAMPLING The theory of line transect sampling depends on four assumptions outlined by Burnham et al.
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From page 41... ...
-( T \ -- - / s= Ex2 n -- 1 A census method that can be used for data that are not normally distributed but follow a Poisson distribution is given on p.
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From page 42... ...
are close to each other. To obtain a census that is both accurate and precise, it is necessary to determine the sample size, the number or length of transect lines, and the number of sightings (sample points)
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From page 43... ...
(1980) point out that there are valid statistical methods for estimating densities on the basis of grouped perpendicular distances, but none have been developed for analyzing grouped sighting distances and angles.
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From page 44... ...
(1974) also recommend the use of methods based on perpendicular distances but note that there are some conditions, such as uneven terrain, in which sighting distances are useful because measurement of perpendicular distances from the transect lines is difficult.
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From page 45... ...
Density expressed as numbers of animals per volumetric units are common in fisheries but have seldom been applied to terrestrial vertebrates. In most primate surveys of limited scope, the observer-to-animal distance will probably yield the most useful results for estimating density per unit of area, but the recording of perpendicular distances as well as sighting distances and angles will provide the greatest flexibility in later statistical analyses of the data.
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From page 46... ...
When beginning each transect, the observer should record several standard items including the location, date and starting time, weather, census method, and participating personnel. These items can be recorded on a data sheet modified as necessary from the sample provided in Appendix C for all types of transects, from broad surveys to random compass line transects.
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From page 47... ...
Both the sighting distance and the perpendicular distance should be checked by measuring or by pacing until confidence and reliability in estimating distances are built up through experience. • Height of the first animal sighted.
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From page 48... ...
A field investigator should keep in mind that more precise methods of data gathering and analysis can be used in protected areas than in hunted areas where reliable estimates of depleted populations may be beyond the scope of an initial survey. When entering an unprotected census area in which animals are hunted, investigators will want to know what species are hunted, for what purpose primates are hunted, and about how many animals are killed each year.
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From page 49... ...
As could be predicted from this information, the only primate species seen during diurnal boat surveys were the less favored species Saimiri and Cebus. The other larger species and the highly prized bearded saki were not detected by normal survey techniques.
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From page 50... ...
APPLICATIONS OF LINE TRANSECT CENSUSES IN AN AFRICAN FOREST Many of the data identified in the previous section can be used in a comparison of relative abundance of animals in study sites. However, because results of primate censuses along the same route are extremely variable from one day to the next, the data from broad surveys cannot give an accurate estimate of the absolute density.
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From page 51... ...
In most cases the technique has been of limited value in censuses of tropical forest primates because this kind of information has not been available. Observers should keep accurate notes on the exact locations of sightings along the transect line and the type of vegetation so that it is possible to zone the census transect after the censuses have been completed.
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From page 52... ...
Such zonation would not only increase the precision of density estimates by reducing variability between censuses for each zone but would also reveal habitat preferences of the primates being censused. Two outstanding problems are posed by the line transect method: (1)
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From page 53... ...
No samples were made in 4 mo, and 2-4 censuses were conducted during each of the remaining 14 mo. TABLE 4-1 Results from 44 Censuses of Red Colobus Along the Same Transect Census Number Number of Red Colobus Groups Seen (x)
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From page 54... ...
An example of the calculations for the s of the first group of 10 red colobus samples is shown below from the original census data provided in Table 4-1.
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From page 55... ...
95% confidence limits The % precision = -- X 100 Mean number ot groups seen for that unit of census 1.218 X 100 3.7 = 32.9%. The results are graphed in Figure 4-4 and clearly show that for both species precision began to level off after the first 20-30 censuses.
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From page 56... ...
of the mean number of red colobus groups seen per census, which is 0.46; thus, the minimum sample size required would be 4 X 0.46\2 0.25 + 1 = 55 groups of red colobus, which would indicate 15-20 censuses. Janson and Terborgh suggest, however, that because the data are usually not normally distributed, the samples should be at least 10-20% larger than predicted from their formulas.
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From page 57... ...
This will be affected by the perceptiveness of the observer, visibility and audibility in the 10 Red Colobus 20 Cumulative number of censuses FIGURE 4-4 Precision of estimated mean number of social groups per census as a function of number of censuses completed. Vertical axis is the 95% confidence limits expressed as the percentage of the mean number of groups per census, i.e., 95% confidence limits divided by the estimated mean number of social groups per census times 100.
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From page 58... ...
On the basis of detailed studies of individual groups, the red colobus density in the cenused area of the Kibale Forest was estimated at 6 groups per km2. Maximum Perpendicular Distance In this formula, the maximum perpendicular distance of all first sightings of a particular species is used to estimate transect width (unpublished report by K
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From page 59... ...
(1) Maximum perpendicular distance from transect to a red colobus group was 110 m or 0.11 km.
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From page 60... ...
In such cases it may be advisable to compute density estimates for more than one possible maximum reliable distance and to present the range of resulting density estimates. Take, for example, the case of the red colobus census data.
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From page 61... ...
. In the 44 censuses at Kibale, Uganda, referred to above, nearly 40% of the 166 sightings of red colobus were over the census transect and were scored as zero meters from the trail, although usually they were more than 20 m from the observer.
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From page 63... ...
Observer-to-Animal Distance Estimates of the initial distance between the observer and the first animal seen overcome the problem of zero distance inherent in the transect-to-animal method, although Janson and Terborgh (in press) discuss some of the theoretical difficulties.
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From page 64... ...
The same method applied to the redtail data gave an underestimate of 30.5%, but for blue monkeys it yielded an overestimate of about 2-17.4%. Maximum Reliable Sighting Distance More accurate estimates were obtained by inspecting the frequency distribution of observer-to-animal distances and, as with the Kelker method, determining the cutoff distance or maximum reliable sighting distance within which all animals were probably observed, but some missed (e.g., Ghiglieri, 1979)
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From page 65... ...
In this way one can begin to focus on the number of animals in a group and the approximate size of its daily foraging range. Of course, in so short a time it is not possible to estimate the entire home range of group or to obtain a very accurate idea of the mean group size, unless the sample is very large.
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From page 66... ...
The preliminary data are thus a useful indicator of relative abundance. The number of transect samples may be increased or reanalyzed according to the following procedure: Set each transect length so that for a known detection width one will cover an area equal to 0.5 the average home range of the species in question.
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From page 67... ...
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From page 68... ...
. This method was studied further in Venezuela and was found to generate density estimates comparable with those estimated from detailed studies (Eisenberg, 1979)
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From page 69... ...
A major deficiency of this method is that it fails to resolve the problem of individuals or groups living partly in the sample quadrat and partly outside it. (This causes no difficulty where quadTABLE 4-5 Use of the Nonlinear Density Plot Method to Estimate the Density of Alouatta palliata in Panama" Conditions: Transect length = 1 km; transect width = 50 m Plot Number Animal Counts Group Counts Frequency of Plots with Sightings 1 7 2 1 2 3 1 1 3 0 0 0 4 0 0 0 5 3 1 1 6 4 1 1 7 2 1 1 8 0 0 0 9 3 1 1 10 0 0 0 11 0 0 0 12 5 1 1 13 0 0 0 14 4 1 1 15 0 0 0 16 5 1 1 17 3 1 1 18 0 0 0 Total number of animals per 0.90 km2 Mean number of animals per 0.05 km2 s Estimated density of animals per km2 39 2.167 + 2.256 43 11 0.611 ±0.608 122 10 160 "Data from J
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From page 70... ...
Grids are drawn on maps of very large areas to subdivide the areas into quadrats until the sample quadrats represent about 15% of the study area. Each choice of quadrat is made randomly.
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From page 71... ...
Calculations for population estimates using the quadrat method are shown in Table 4-6. SPECIALIZED CENSUS METHODS FIXED-POINT COUNT In this method the observer remains at one point and records all primate groups seen or heard.
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From page 72... ...
, censusing by direct observation may be inadequate. In such cases, marking in combination with radio tracking may be the only way of estimating densities and home range size.
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From page 73... ...
marked animals Total no. animals seen Total population of animals _25_-30 110 ~ X X= 132 The above calculation assumes that the 30 marked animals are from randomly selected groups.
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From page 74... ...
This formula can be further refined by computing a weighted mean nest life span that takes into account differences in nest life span. Life spans differ according to the kinds of trees or other vegetation in which the nests are built.
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From page 75... ...
Figure 4-9 clearly shows major interspecific differences within the same study plot. A group of 15 mangabeys at the Kanyawara study site in the Kibale Forest had an enourmous home range -- some 400 ha.
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From page 76... ...
Home range increase was plotted in increments of 0.25-ha quadrats. Data are from compartment 30 of the Kibale Forest, Uganda.
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From page 77... ...
In reality, however, the situation is generally more complex. Usually the home range of the main study group is only partly shared, and the home ranges of neighboring groups are incompletely known.
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From page 78... ...
SUMMARY OF IMPORTANT CENSUS METHODS FOR DIURNAL PRIMATES When time to cover large areas is limited and replication of censuses along the same route is not plausible, emphasis should be placed on obtaining indices of relative abundance of social groups. These indices are usually expressed as numbers of groups seen per kilometer of transect and per hour of searching.
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From page 79... ...
It is also the method that gives the best data on group size and age/sex composition. EXTRAPOLATION OF DENSITY ESTIMATES FROM CENSUS AREA TO OTHER AREAS After determining estimates of density from the more accurate methods of repeated line transects and detailed studies, one would like to be able to extrapolate densities in similar habitats.
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From page 80... ...
Census methods, especially data analysis, still need much more study to improve their accuracy and general applicability. Extrapolation can be a dangerous guess.
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