Ecological Sampling Design and Observer Bias: An Example from Toad- Headed Agama (Phrynocephalus versicolor) in the Southern Gobi, Mongolia
Kate R. Oddie*y**, Togloom Ariunaa* and Davaadorj Enkhnasan*
*Faculty ofBiology, National University of Mongolia. **Centre d'Ecologie Functionelle and Evolutive.
Currently Mongolia faces great challenges in ecological research, with ecological studies relatively neglected during Soviet administration. The development of ecological studies requires an appreciation of sampling bias and how this can be avoided. Here we present acase where observer biases are impossible to disentangle from site effects because sample site data are confounded with observer sampling activity. Three volunteers were collected simple body mass and length measures of toad-headed agama, Phrynocephalus versicolor, as part of a wider ecological survey programnle in Borzongiin Gobi, South Mongolia. Field data potentially reveal a difference in morphological size in toad-headed aganla inhabiting different habitats, but this can only be ascertained through further sampling effort. We give recommendations for future studies.
Keyword: Phrynocephalus versicolor,ecological field techniques,sampling,observer bias,Borzongiin Gobi,Mongolia
Throughout Soviet dominance until 1990, the biology education system in Mongolia focused on systematics and taxonomy, with very little ecological research. Consequently, although a wealth of systematic knowledge has been gathered for Mongolian species, little is known about dynamic processes occurring in Mongolian ecosystems, population sizes and interactive processes between species (Anonymous, 1998). Studies of ecosystem functioning are developing, but resources for such research are scarce and frequently involve untrained individuals in the collection of data. Furthermore, the scale of the ecological work required over such a large, ecologically diverse and unstudied country often necessitates a large workforce to carry out biological sampling.
Techniques for ecological censusing, either for pure biological study or habitat management and conservation purposes, require unbiased estimates to be collected (e.g. Bibby et al., 1992; Sutherland 1996; Begon et al., 1996), involving for example random sampling and sampling replication (Greenwood, 1996). One possibility for the introduction of bias into ecological data is the use of a number of observers to measure the same factor. Variation in data is generated by observer differences rather than representing true biological variation (e.g. Sauer et al., 1994, Robinson et al., 2000, Spaulding et al., 2000).
We document a case of difficulty encountered in disentangling a potential biological phenomenon from a biased sampling effect during a Mongolian field study of toad-headed agama, Phrynocephalus versicolor. Field work was intended to compare ecological parameters at two sites in the Little Gobi Strictly Protected Area A. Species richness, abundance and population distributions in this area are little known and simple inventory work as well as more complex ecological and behavioral studies is urgently needed. Three volunteers from Mongolia and overseas were involved with agama data collection as part of a larger survey programme by international non-governmental organization (NGO) volunteers. Data were collected as a simple training exercise, but demonstrate well the problems encountered with inadequate experimental sampling design and serve as a lesson for future studies.
Material and Methods
Field work was carried out in Borzongiin Gobi, South Gobi Province, within Little Gobi Strictly Strictly Protected Area A, during 03- 15 June 2001. The environment within the protected area is patchy, with wildlife distributed unevenly between distinct habitats. Two areas of particular interest in a zone of mainly sand and gravel plains are (i) natural springs that provide surface water, and (ii) stands of relict elm, Ulmus sp. Data were collected from 5 km transects walked across one such spring (N 42"29'02", E 105"14'06") and through a stand of 60-70 well developed Ulmus trees (N 42"11730", E 1 05"20740"). The sites are 42 km apart. The spring provides year-round surface water that in spring and summer results in a grassy flood plain used for domestic livestock grazing. In both areas the plants of the genera Zygophyllum, Stipa and Salsola are ubiquitous (though heavily grazed at the spring). The spring is characterised by the presence of Chenopodium, Iris and Oxytropis. Prevalent in the shrubby desert and dry river beds of the Ulmus were Kallidium, Nitratia, Cargana and Anabasis. Generally there was a greater density of, and variation in, vegetation at the Ulmus grove (Oddie, 2001 unpublished report and own data). Three volunteers collected mass and body length morphometric data on the toad-headed agama. Field studies on agama were part of a larger survey exercise also comparing vegetation and terrestrial insects between sites. This influenced the logistics of data collection. Eight and ten N-S transects of 5km were completed at the spring and Ulmus respectively, beginning at 07:15 and ending between 11:00 and 13:00. The number of agamas caught by different three observers at different sites varied, as demonstrated in Table 1. Agamas were caught by covering individuals with a cupped hand. They were marked with a small spot of coloured nail varnish on their dorsal surface between their rear legs in order to prevent pseudoreplication by distinguishing those already captured (recaptured individuals are not included in analyses).
Agamas were measured to the nearest point from nose to tail by using a metal ruler attached to a wooden platform with one raised end at right angles to the surface, an arrangement commonly used for measuring bird wing lengths. With the nose just touching the perpendicular section, the body was stretched out flat with the tail being pulled straight and flat next to the ruler before the measurement being read at the tip of the tail. Masses were recorded by suspending agamas in paper cones from pesola spring balances, accurate to 0.1 g. Table 1. Number of agamas caught by three different observers at two sites in Borzongiin Gobi (date is day of June).
The total number of agamas caught was 242, and mass and length were recorded in 232 and 235 cases respectively. Mass varied between 0.9 and 8.6g7 with a mean of 3.54 (f 1.47) g.
Plotting the lengths and masses of agamas caught according to observer (Fig. 2) shows that observer 1 measured consistently smaller than the representation of agamas caught by observer 3 at the spring, may account for the differences in measurements between sites.
Simple morphometric data collected for the toad-headed agama in Borzongiin Gobi revealed individuals caught at a spring to be both larger and heavier than those caught at an Ulmus grove. However, whether this difference represents a true biological phenomenon or is due to measurement biases between multiple observers remain to be determined, since with current data we are unable to discern if this is due to sampling and observer biases. Three different individuals measured agamas with significant variation between observers, and sites were sampled unevenly by observers. Given the short distance of only 42km between sites, such large morphological differences as recorded here seem unlikely. It may be that a greater abundance of insect prey at the spring influenced agama measures, resulting in heavier individuals caught at the spring. Differential feeding activity is likely to cause variation in body mass within a species, yet here we also record a surprisingly large variation in body length also. We speculate that differences between sites may be due to two problems involved in experimental design: uneven sampling and use of inexperienced personnel. It is likely that consistent differences in measures between observers bias these data. Observer differences in nose-tail lengths may be expected as individuals become accustomed to holding agamas in their own particular way, but using a spring balance should produce consistent results across observers. Thus we may expect some error in length measurements, whereas weight measures are generally thought to be more reliable. In this study we find greater variation in mass than in length measures. This could be considered evidence to support body length differences between the two sites. However, it is also true that inexperienced users may have inadequately sheltered spring balances from the frequent Gobi winds, resulting in some variation. Furthermore, it may be that, for at least part of the study, spring balances may not have been correctly zeroed. We found that the number of agamas caught at each site was not consistent across observers. This represents the second major problem of the study: uneven sampling. In any ecological study it is important to remove as many sources of bias as possible (e.g. Begon et al., 1996; Sutherland, 1999, 2000), or in a study such as this comparing sites it may not be critical if counts at each site are consistently biased. Here the fact that measures were not distributed evenly across sites by observers meant that measurement biases were not evenly represented across sites. In this case, biased effort across sites could not be avoided because of logistical constraints and nature of the work. Agama surveys were carried out within teams of five or six young volunteer also collecting data on plants and insects, and groups had to be maintained for safety reasons. Other sources of error such as daily weather differences (that affect agama activity and likelihood of capture) were minimised by carrying out surveys during the same time period each day. Furthermore, sampling was repeated over a number of days at each site to reduce error, rather than relying on data from just one or two sampling days. These data call into question the usefulness of volunteers to collect data. The use of untrained personnel limits biological studies in both the complexity of data that can be collected and the reliability, hence its usefulness. In this study volunteers received a two-day training period and required reminding of methods and motivating throughout. Using non-trained personnel to collect data may be tempting given that they can cover a wide survey area in a short time, however, variation in observer effort and ability justifies great caution in the use of non-trained personnel (Kendall et al., 1996; Robinson et al., 2000). Consequently, their use in serious biological field studies, especially those upon which conservation decisions rest, should be questioned. If volunteers are to be used, we recommend extensive training, pilot studies and that observer differences are quantified (for example here a sample of at least 20 agamas are measured by all three observers to reveal inconsistencies in measurements) and caution in results interpretation. Inter-observer variation should always be investigated. In this case the work fulfilled concurrent social and educational project aims as observers were involved in a youth expedition to promote a sense of awareness of the natural environment, encourage interest in environmental processes and cross-cultural exchange. Hence, although biological findings may be questioned, the project upheld other aims. Future studies in Mongolia using untrained students or volunteers must determine at the outset the motives for their work and design adequate training, sampling and monitoring activities accordingly. For the agama, and indeed for other species in Borzongiin Gobi, further investigations are encouraged. These could be coupled with, for example, behavioral observations to record prey items consumed at each site and complementary studies of prey species once identified.
Inadequacies in the current agama data mean we are unable to say with certainty that body size differences are caused by biological variation between sites, neither can we conclude they are caused by observers. In future, measurements made by one observer at both sitks are required. Furthermore, any observer collecting data should measure at least 20 individual agama two times, in order to calculate a repeatability estimate for his/her own measures (see Lessells & Boag, 1987). If observers cannot measure reliably then the validity of any data are questioned; here further training and practice may be advocated.
Volunteers are useful to collect pilot data, perhaps maybe more so in collecting count data rather than measures. However, more experienced scientific personnel with an understanding of rigorous scientific sampling methods are required to produce reliable data for Mongolian populations.
This is especially true for those upon which conservation decisions rest.
We thank volunteers and staff of Raleigh International and Blandine Doligez for field work, and the National University of Mongolia and National Parks Office in Dalanzadgad city for support. We especially thank herding families within the study area for their endless help. Work in the Little Gobi SPA was permitted by the Protected Areas Bureau of the Ministry of Nature and Environment of Mongolia. The senior author was supported by a grant from The Leverhulme Trust, London, with start-up sponsorship from the Association for the Study of Animal Behavior, British Ornithologists Union, Dan Lipscombe Memorial Trust, Edinburgh Davies Trust, Percy Sladen Memorial Trust, Sokkia Communications and the Winston Churchill Memorial Trust.
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