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How to count kangaroos PDF

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How to count kangaroos H. I. McCdum Department of Zoology, University of Queensland, Queensland 4072 ABSTRACT The fundamental reason for counting kangaroos is to ensure that harvests are demonstrably sustainable. Given this goal, it is essential that the survey method should be able to detect broad scale trends in population size. Aerial counts will detect trends only if the constant ("correction factor") that relates the number of sightings to actual population size either remains constant, or the way in which it changes depending on environmental conditions is known. Actual counts, and not an index, are essential if harvesting policy is to be proactive, rather than simply reacting to previous changes in population size. It is evident from other papers in this volume that there is a particular problem in estimating appropriate correction factors for grey kangaroos. Several contributors suggested a research programme to produce better correction factors, which would then be used in the future to correct raw sighting data. A better solution may be to use 'double sampling", which is a well-established sampling method. Each time an aerial survey is conducted, a subsample of the transects surveyed would be re-surveyed using helicopter counts, which are more accurate, but more expensive. The ratio of the helicopter to fixed-wing counts is then used to produce a survey-specific correction factor. Double sampling is more expensive than fixed-wing surveys alone, so it would be necessary to decrease the intensity of the fixed-wing surveys to allow for the extra cost of the helicopter surveys. The experiments necessary to see whether double sampling is worthwhile are verv similar to those that are necessarv to develoo better correction factors. I suggest that any eiperlrnenls deslgneo to Improve cor;ectlon factors snould also be analySe0 to nvest gate tne feaston ty of Jslng oo~bles ampl ng to estlmate kangaroo popu atlon slze WHY COUNT KANGAROOS? harvests sustainably, particularly when social and political issues are important. There is, There is never much point in collecting however, one very significant difference data without thinking about what those data between managing fish populations and are to be used for. The underlying reason managing populations of large terrestrial that all Australian mainland States (except vertebrates, such as kangaroos. Most of the Victoria and the Northern Territory) spend data used to monitor fish populations are considerable resources in counting kangaroos catch data. They come from the commercial is to ensure that the kangaroo harvest is exploitation itself. Many of the problems managed appropriately. So, any discussion of that have occurred in fisheries management the adequacy of current methods of counting are direct results of the tendency for catch kangaroos must start with a discussion of the per unit effort to remain high as abundance objectives of the kangaroo management, and decreases through overexploitation (Hilborn the way in which data on kangaroo popula- and Walters 1992). In contrast, the Australian tions relate to these management objectives. kangaroo harvest is monitored primarily The general consensus of the workshop was by surveys, independent of the harvest. that the fundamental objective of kangaroo Independent survey data do not have most management is to maintain viable populations of the inherent problems of catch per unit of all exploited species over their current effort data. range, while allowing for both a sustainable I do not propose to discuss sustainability in and viable kangaroo industry and for reductions detail, other than to note that a sustainable in populations where they contribute to over- harvest is one that does not lead to a grazing. continuing decline in the population being Sustainability of harvests of wild animals is exploited, and that sustainable harvests may discussed in great detail in the fisheries be at, below, or above, the point of maximum literature (Hilborn and Walters 1992). However, sustainable yield. To my knowledge, no-one few of the major exploited fish stocks have has ever suggested that an objective of been managed sustainably (Beddington and kangaroo harvesting should be to extract the Basson 1994; Hutchings and Myers 1994). maximum sustainable yield from the popula- The Grand Banks cod fishery was arguably the tion. A viable harvest is not necessarily the most important in the world, from a historical same thing as a sustainable one, although and social perspective (Kurlansky 1997). It it is obvious that an unsustainable harvest has now collapsed entirely. We should not be cannot be via6le in the long term. Viability overconfident about our ability to manage means that the harvest must be capable of June 1999 Australian Zoologist 31(1) 309 economically supporting participants in it. For Pollock 1995), the use of an index relies on the specific purposes of this discussion, it is an implicit assumption that essential to recognize that a viable harvest depends crucially on the existence of continuing markets for kangaroo products. where N is the actual population size, I is Those markets will be threatened if the the index, and a is a constant of pmportion- harvest cannot he demonstrated to be sustain- ality connecting the index to the actual able, in the face of substantial hostility from population size. certain organizations. The monitoring system There are two implicit assumptions in eqn must not only ensure that the harvest is (1). First, there is the assumption of direct sustainable, it must ensure that the harvest is proportionality. If there is a curvilinear seen to be sustainable. relationship between the index and population There is no general consensus about size, or if there is a non-zero intercept, the the extent to which kangaroos actually do index cannot be used to compare relative contribute to overgrazing, with the con- density through space or time. However, sequences to either Land degradation or if the nonlinear relationship between the competition with livestock. This is an index and actual population size can be important debate, but is beyond the scope of described, the index can he still be converted this article, and was not considered in detail into an estimate of population size. Cairns at the workshop. Most primary producers (1999) provided evidence that the problem believe that kangaroo populations do need of nonlinearity is not a major one if aerial to he reduced, at least some of the time, sightings are used as an index of kangaroo to prevent overgrazing. They are supported population. in this belief by most state departments responsible for primary industries, and by The second, and more serious, implicit some biologists outside such departments assumption is that the constant a is indeed (W~lsone l al. 1984; Freudenberger el al. 1997). constant, both through time and across However, other biologists, particularly those space. This assumption is impossible to test who have undertaken controlled experiments, if only the index I is available. If it can be doubt that overgrazing by kangaroos is often tested and verified against the true population a significant land management issue. For size, there is no need to use the index anyway, example, Edwards el al. (1996) suggested that as eqn (1) could then be used to obtain an competition between sheep and kangaroos estimate of actual population size. In some rarely occurred. Whatever the actual reality, cases, this second assumption can be circum- the political reality is that kangaroo shooting vented by estimating a series of constants. for "pest reduction" purposes will continue Each might be appropriate for a particular for the foreseeable future in most Australian habitat or season, or some combination States. An objective of kangaroo monitoring is of both. To estimate these constants, or therefore to ensure that such "pest reduction" "correction factors", it would be necessary is consistent with the overall objective of to have a series of index values and actual maintaining kangaroo populations as viable counts in various habitats, seasons etc. across their range. To determine whether Whenever an index is used alone, instead kangaroo populations are at a level such that of being converted to an actual estimate, what they do cause land degradation is not a major is being asserted is that, while the constant objective of current monitoring schemes, is unknown, it is known to be constant. This although South Australia is moving in this is a difficult assertion to maintain. In the direction (Alexander 1997). It cannot become context of kangaroo counting, the index is a central objective until the relationship typically an uncorrected aerial count, and between kangaroo grazing and land degrada- the constant is the "correction factor". For tion is better established. red kangaroos, there is good evidence that correction factors do not change markedly AN INDEX OR AN ESTIMATE through time (Pople 1999). The 20 years OF ABUNDANCE? of red kangaroo survey data from South An important issue, discussed at some length Australia (Grigg et al. 1999) are therefore a in the workshop, is whether it is essential to regular and consistent record of the dynamics produce an estimate of the actual number of of a large, wild herbivore over a spatial scale kangaroos, or whether an index of abundance and length of time that is probably unique. is adequate. It is often assumed that an index The only comparable data are for caribou is an easier quantity to obtain and work with in the Arctic (Caughley and Gunn 1993), or than an actual population estimate. As has possibly for wildebeest in Africa (Campbell been pointed out a number of times (e.g., and Borner 1995), although neither of these 310 Australian Zoologist 31(1) covers the spatial scale and has the regularity methods (see Seher 1982 for a review). of the red kangaroo data. Although mark-recapture or resight methods have been successful in estimating the local Assuming that a is constant through time ahundance of smaller macropods, there is and space (something I will discuss later), the little doubt that only direct visual counts are question that then can be asked is whether able to be used to estimate the abundance of it is necessary to know its value. If we have the larger macropods over the spatial scales convinced ourselves (and hopefully others) necessary to set harvest policies. It is also that aerial counts are directly proportional generally accepted that, of all current visual to kangaroo ahundance, do we really need census methods, line transects carried out on to know what the constant of proportionality foot produce estimates that are closest to is? Given that a sustainable harvest is one actual kangaroo abundance (Southwell 1994). that does not lead to a continuing decline in While they are valuable to calibrate other kangaroo numbers, an index that does not methods, foot transects are not possible at decline through time demonstrates that the the large scales of state-wide surveys. States previous harvests have been sustainable. One therefore base their population estimates on could go further and suggest that if current aerial surveys. harvests are set on the same policy as before, then an index that maintains its Queensland currently uses annual helicopter value through time gives confidence that surveys using line transect methods (Lundie- current harvests are sustainable too. The Jenkins et al. 1999). A line transect (Buckland difficulty is that kangaroo populations et al. 1993) uses the frequency distribution vary greatly through time, largely due to of distances of sighted individuals from the influence of rainfall, hut the periods of the transect line to estimate ahundance. All increase and decrease generated by rainfall individuals on the line are assumed to be variation may he many years long (Caughley seen, but the method assumes that some 1987a; Grigg et al. 1999). This means that animals away from the line will be missed. A evidence that a harvest level (whether defined strip transect, in contrast, simply counts all as effort, numbers or proportion) is not individuals seen within a certain distance of sustainable may not be clear for a number of the line). Evidence from Queensland research years. Any setting of harvest levels only with (Clancy et al. 1997) suggests that helicopter respect to an index therefore operates with a surveys record similar population densities substantial lag. to walked transects, for red and eastern grey kangaroos, but still undercount wallaroos Currently, kangaroo harvest levels are (euros). Helicopter surveys are expensive, approved by the Commonwealth as quotas: although it may be that ultralight aircraft actual numbers that may he taken in a given can be used to produce similar results for year. Quotas are currently set and justified less cost (Grigg et al. 1997). The expense as proportions of the total population. This and short range of helicopters means that is because determining the impact that a Queensland monitors populations on several given quota will have on a population is a fixed monitor blocks only (Lundie-Jenkins matter of determining the per capita level of et al. 1999), rather than extensively across the mortality that the quota is equivalent to. This entire state. requires an estimate of the actual population size, not simply an index, connected to the The remaining states count kangaroos using population size by some unknown constant. strip transects surveyed from fixed-wing Without an absolute population estimate, aircraft. New South Wales and South Australia any harvest policy can only be reactive, rather use extensive annual surveys with two (New than pro-active. It may be possible to measure South Wales) or four (South Australia) transect the impact that a harvest with a given quota lines being flown per degree square through- has had on a population, but not to predict out the main harvest area (Grigg et al. 1999; what effect it will have on a population, except Gilroy 1999), covering a large part of the to conjecture that it may be similar to kangaroo range within each state. Western the effect of a previous quota of similar size Australia uses similar extensive surveys, but on in the past. It will also be difficult to defend a less frequent basis. All three states adjust the publicly a quota based purely on an index. actual numbers seen to produce an estimated actual population, using correction factors originally derived by Graeme Caughley for HOW KANGAROOS ARE COUNTED red kangaroos (Caughley et al. 1976). There CURRENTLY is consensus that the correction factors for red Population size of animals can be estimated kangaroos return reasonable estimates, but in a number of ways, including mark-recapture that grey kangaroo and wallaroo populations methods, visual counts and a variety of indirect are grossly underestimated. Australian Zoologist 31(1) 311 SIGHTABILITY OF KANGAROOS survey, and experiments to ensure that 100 m FROM THE AIR is not closer to this optimum than 200 m are warranted. Stuart Cairns (University of New All kangaroos present on the ground will England) and Josh Gilroy (NSW National not be seen from the air, particularly in a Parks and Wildlife Service) will undertake fixed-wing survey. To translate an aerial count a large-scale experimental programme to into an estimated population size, it is address this issue, among others. necessary to estimate the sightability, which is the probability of seeing an animal, given A related suggestion to increase the sight- that it is present within the area being ability of kangaroos on the transect being counted. As Pople (1999) pointed out, visibility surveyed, at the cost of surveying less area is a function of species, behaviour, habitat for the same flight time, is to count from one and observer. The likelihood is that it also side of the aircraft only. It is clear that "away depends on interactions between these factors, from the sun" counting is much easier than rather than simply being able to be estimated "towards the sun" counting. Using only one as a multiple of individual probabilities for observer to count away from the sun would each of the above factors. reduce personnel requirements considerably, and possibly increase the reliability of the In ground counts and helicopter surveys counts, but it would also reduce the total area using line transect methods, the distance to surveyed. the sighted individual or group is recorded. Provided all animals actually on the transect Observer training is a crucial issue in both line (i.e., zero distance) are seen, the use increasing the mean sightability and reducing of appropriate software (Laake et al. 1993) its coefficient of variation. Beard (1999) enables these counts to be corrected for produced data showing that the number of sightability. Strip transect methods, however, kangaroos seen by a trainee observer, relative require sightability to be estimated from to that seen by a trained observer, does not information additional to that provided by the reach an asymptote until after at least 10 survey itself. flights. Similarly, the number of kangaroos seen per transect by a trainee is much more To improve the accuracy of fixed-wing aerial variable than that counted by an experienced counts, three things can be done with sight- observer. Some observers, despite experience ability. It can be improved, to increase the with kangaroos, do not seem to approach mean value of the raw count. It can be the sighting levels of experienced observers standardized, to reduce the variation in at all. Clearly, there is a major problem counts between replicate aerial transects. in setting fixed "correction factors" unless Finally, if it can be estimated, it can be all observers have been calibrated against used to correct the raw sighting index, and a single "experienced observer" standard. translate it into an actual estimate. Improving Such a standard probably does not exist, as mean sightability and reducing its variation even experienced observers differ in the (or coefficient of variation) are issues that mean number of kangaroos that they will see cannot be discussed separately. Some measures on the same transect. may improve both, whereas others may trade one off against the other. Few ways of improving mean sightability have no STRATIFICATION consequences for its coefficient of variation. One way to produce major improvements in First, there is the possibility of reducing the precision of overall estimates of kangaroo the strip width over which kangaroos are density is to use stratified sampling methods. counted from the current 200 m to 100 m. The idea is extremely simple. If some This is likely to increase the probability of mappable characteristic of the environment is an animal on the strip being sighted, as thought to influence kangaroo density, then the search pattern of the observers can be the total area for which the estimate is concentrated, and there will no longer be required is divided into a small number of attempts to count animals away from the separate subareas (or strata) on the basis of track of the aircraft. On the other hand, this characteristic. The population density, fewer animals will be on a strip of half and its standard error, is then estimated the width, and it is likely that fewer will separately for each stratum, and the results am be counted for the same expense and effort. combined to give an overall estimate. Strata All other things being equal, the larger the should be selected so that variation between total number of individual animals counted them in density is maximized, with the in a survey, the lower will be the resulting consequence that variation within them is standard error in the final estimated density. minimized. The obvious mappable character- There is an optimum strip width for any visual istic to use in this particular situation would 312 Australian Zwlogist 31(1) be vegetation type. Precision is maximized in others. Second, it is well known that if sampling effort is allocated between kangaroos have different habitat preferences in strata proportional to the overall kangaroo different environmental conditions (Caughley population in each stratum, but substantial 198713; Hill et al. 1987; Pople 1989; Southwell gains in precision are possible if sampling is 1989). If such habitat shifts occur at a large random or systematic across the entire study scale, then habitat-dependent correction area, or even if stratification is done after factors may allow for consistent estimation of sampling is completed. population size in different environmental conditions. However, correction factors would necessarily be applied at the transect scale, THE PROBLEM WITH GREY of several kilometres. If habitat selection KANGAROOS occurs at the fine scale of patches within The biggest problems with aerial strip transects, then habitat-specific correction transects occur when counting grey kangaroos factors would not assist. Third, drought is (both eastern and western). There are similar likely to change sightability within particular diff~cultiesw ith wallaroos, but as the com- habitat types. Fourth, a fixed set of correction mercial harvest of this species is relatively factors does not allow for differences between small, the implications are not as serious. observers. Finally, the problem with a single There was total consensus among workshop set of correction factors is that they are participants that the correction factors likely to become fixed in usage, and not originally derived by Graeme Caughley grossly reviewed until their faults are gross. The undercount greys, and that they need to be experience with the original correction factors at least doubled. Some participants suggested derived by Graeme Caughley epitomizes quadrupling them was more appropriate. this last point. Whatever the most appropriate correction Is there an alternative to fixed correction factor is, such ad hoc doubling and re-doubling factors? is better suited to a poker game than to the management of a major wildlife harvest. IS DOUBLE SAMPLING A SOLUTION? Sightability of grey kangaroos is much lower than that for reds primarily because they A method known as double sampling offers prefer less open habitats than do reds, and an alternative to the use of fixed correction are therefore more likely to be hidden from factors. The basic idea would be to survey an aerial observer. Several participants in a large number of transects using fixed- the workshop suggested that the solution wing aerial survey methods, and then to to the correction factor problem for grey use a more intensive and accurate sampling kangaroos is to undertake a substantial method (such as helicopter surveys) to obtain research programme to establish improved a more accurate estimate on a subsample correction factors. The idea would be to of those transects. The ratio of helicopter survey grey kangaroos in a variety of habitats, counts to fixed-wing counts is estimated using both aerial strip counts and helicopter from the subsample, and used to correct the counts. Assuming that the helicopter counts counts on the full sample. Details of the are close to being complete counts, an method are in standard textbooks on sampling improved set of correction factors would be strategies (e.g., Thompson 1992) and it is developed. This would represent a consider- widely used in wildlife surveys in North able improvement over the current situation. America (Prenszlow and Lovvorn 1996). I have several misgivings about this as a There are two major advantages in using complete solution to the problem. First, "open" double sampling compared with fixed or "closed" habitats may be quite different correction factors. First, because the correction things in terms of vegetation structure or is recalculated in each survey, changes in composition across the range of grey average sightability that occur through kangaroos. Even if objective definitions in time are allowed for. This is the case whether terms of percentage tree cover were developed, they occur because of changes in actual the grain or patchiness in the habitat will visibility in a given habitat, habitat shift, vary considerably, as will the tree species or inter-observer variability. Second, the making up the canopy. Such factors may well estimated error in the final estimate incor- influence sightability considerably. It would porates a component due to the error in certainly be possible to test whether this the estimation of the correction factor. This problem is substantial, but the costs would means that the precision of the final estimate be considerable. Realistically, the likelihood is better described than it is by applying a is that factors established in one part of fixed correction factor. The error in an the country would be applied uncritically estimate based on a fixed correction factor is Australian Zoologist 31(1) 313 underestimated, as well as the estimate itself surveys, while placing the whole procedure in being biased in an unknown fashion. a more secure theoretical framework. I would see the broad design as operating as follows: There is an obvious disadvantage in double sampling: cost. To add an expensive, but 1. Identify a small number of major zones accurate estimation method on top of the within the state that are geographically current intensity of fixed-wing aerial surveys contiguous, and across which kangaroo will considerably increase the total cost of populations can be expected to vary, in kangaroo surveys. The suggestion should not species composition or density. These are necessarily be that the resources dedicated the basic strata. to kangaroo surveys should be increased to 2. strata, identify (preferably at random) allow for double sampling. It may be that the a small number (5-10) of blocks, or strips, existing resources would be better allocated by within which kangaroo populations are to reducing the intensity of fixed-wing aerial be monitored. Fixed-wing transects would survey to allow for some double sampling. then be flown across these areas. These are then cluster samples. EXTENSIVE VERSUS INTENSIVE 3. A subsample of the fixed-wing transects SURVEYS within each block is then selected for The current fixed-wing aerial surveys are helicopter or ground sampling. This is the expensive. Despite this cost, all participants double sampling step. in the workshop agreed that the estimates For this approach to be feasible financially, of grey kangaroo numbers produced may it is clear that the total number of strip be out by a factor of at least two. This is not transects would need to be fewer than the a satisfactory state of affairs. It is almost current number. It should be possible to certainly the case that the surveys may detect model the alternative strategies, including changes in grey kangaroo numbers with far costs and reasonable assumptions about greater accuracy than they estimate actual kangaroo distributions, to see if the alternative numbers, hut this relies on the assumption approach is feasible. that the uncorrected counts are an adequate index of grey kangaroo population size. As A decision that must be taken is whether the I discuss above, if only changes in numbers blocks and transects should be kept fixed, are estimated, harvest management becomes or re-randomized each year. A general rule reactive, and cannot be proactive. is that fixed sampling units are better able to detect changes through time, but re- It is certainly worth exploring the possibility randomized sampling units will better estimate of changing the basis of the surveys. The population size at any given time. current design in all states except Queensland attempts a complete geographical coverage A problem with the above proposal is that of the main kangaroo populations, but uses the optimal sample allocations to monitor a method that is known to be inaccurate for grey and red kangaroos will be different. grey kangaroos. The alternative is to use a Some sort of compromise would need to be more accurate estimation process, carried out established, but there would still be major over an appropriately selected sample from gains in precision compared with a random the total geographic area. This is essentially allocation. the approach Queensland currently takes, using helicopter surveys over selected monitor WHERE TO FROM HERE? blocks (Lundie-Jenkins et al. 1999). Whether To immediately replace the current monitor- the 10 Queensland monitor blocks can really ing system with double sampling would be be considered an appropriate sample from premature. It is necessary to first demonstrate the total area of kangaroo distribution is that, for the same cost, double sampling doubtful. They bear some resemblance to a will lead to better estimates of kangaroo stratified sample, as there is some replication population size than provided by the current within each of the major biogeographic zones fixed-wing surveys. The previous discussion of the state within which kangaroos are merely suggests that it may. Much of the common. These biogeographic zones could be information necessary to make an informed considered as strata. Clancy (1999) discussed decision is already available. The records of the advantages and disadvantages of the aerial surveys across extensive areas of Queensland approach relative to the approach New South Wales and South Australia for a of the other states. number of years form a database that can A double sampling approach offers the easily be subsampled to simulate stratified prospect of combining some of the best surveys from only a sample of areas. Sub- features of both helicopter and fixed-wing sampling would result in some loss of precision 314 Australian Zoologist 31(1) in the estimate of the uncorrected mean REFERENCES number of sightings per unit area, as fewer Alexander, P., 1997. Kangaroo culling. harvesting individual transects would be used in the and farming in South Australia - an ecological estimate. Simulation could explore the extent approach. Aurt. Biol. 10: 23-29. that the standard error in the mean Beard, L. A,, 1999. Training observers. Awl. Zaol. 31: uncorrected index increased, and more 287-91. importantly, whether the same trends that Beddington, J. R. and Basson, M., 1994. The limits to are observed in the extensive counts can exploitation on land and sea. Phil. Tranr. R. Soc. be consistently picked up in subsamples. It Lond. B 343: 87-92. would not be difficult to cost subsamples Buckland. S. T., Anderson, D. R., Burnham, K. P. and in various locations across a state, including Laake, J. L., 1993. Distance sampling: estimating personnel, flight survey time and ferry time. abundance of biological populations. Chapman and Hall: London. The element of double sampling that cannot easily be investigated using currently available Campbell, K. and Borner, M., 1995. Population trends data is the strength of correlation between and distribution of Serengeti herbivores: implica- tians for management. Pp. 117-45 in Snngcli XI: fixed-wing and helicopter counts in different dynamics, management and carcrvolion of an ccoryrtem locations and at different times. If this ed by A. R. E. Sinclair and P. Arcese. University of correlation is very strong and consistent, then Chicago Press: Chicago. double sampling is unnecessary. All that is Caughley, G. and Gunn, A,, 1993. Dynamics of large needed is a single set of correction factors herbivores in deserts: kangaroos and caribou. Oikor calculated once, and precision would be 67: 47-55. maximized in the future by directing all Caughley, G. C., 1987h. The effect of draught on available resources to fixed-wing surveys. On kangaroo populations: a response. J. Wildl. Manage. the other hand, if the correlation were entirely 51: 603-04. absent, then fixed-wing surveys would be Cairns, S. C., 1999. Accuracy and consistency in the useless, and all resources should be dedicated aerial survey of kangaroos. Aust. Zool. 31: 275-79. to helicopter surveys. 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