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Agricuhurol Admrnurralion tl(1981) 473-484 ECOLOGICAL AND ENVIRONMENTAL INDICATORS FOR THE RAPID APPRAISAL OF NATURAL RESOURCES MICHAEL STOCKING and NICK ABEL School of Development Studies, University of East Anglia, Norwich NR4 7TJ, Great Britain (Received: 22 December, 1980) SUMMARY Natural resource assessment is traditionally of a long-term nature. This paper examines some of the underlying assumptions andproxy measures involved in their rapid appraisal. Three case studies on soil colour, plant indicators and soil erosion illustrate a range of possibilities in using ecological and environmental indicators to appraise aspects of the physical environment which might normally be assessed by longer methods or not at all. It is concluded that the interdependence of environmental fhctors is high and hence suitable proxy measures can befound. Rapid-and thus low costPmonitoring of change is discussed. The importance of a clear statement of assumptions is stressed. INTRODUCTION Increasingly the appraisal of natural resources is becoming an important element in development programmes. In part this is because of a greater emphasis on rural development with rural areas relying more directly on the natural resource base. But also, pressure from environmentalists now ensures that many aspects of the physical environment are considered before the implementation of a project. However, it is often the case that these surveys are peripheral to the main economic aims and purpose of the final project. For this reason the appraisal of natural resources is subject to considerable constraints in time and manpower. To the purist the appraisal of natural resources demands a combination of lengthy fieldwork, detailed air photo interpretation and laboratory analyses of soils and water, as well as a host of multidisciplinary studies integrating two or more factors such as the relationship between vegetation and wildlife habitats. Recent Land Resources Study reports, published by the Land Resources Development 473 Agricultural Administration 0309-586X/8 l/0008-0473/$02.50 0 Applied Science Publishers Ltd, England, 1981 Printed in Great Britain

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Page 1: Ecological and environmental indicators for the rapid appraisal of natural resources

Agricuhurol Admrnurralion tl(1981) 473-484

ECOLOGICAL AND ENVIRONMENTAL INDICATORS FOR THE RAPID APPRAISAL OF NATURAL RESOURCES

MICHAEL STOCKING and NICK ABEL

School of Development Studies, University of East Anglia, Norwich NR4 7TJ, Great Britain

(Received: 22 December, 1980)

SUMMARY

Natural resource assessment is traditionally of a long-term nature. This paper examines some of the underlying assumptions andproxy measures involved in their rapid appraisal. Three case studies on soil colour, plant indicators and soil erosion illustrate a range of possibilities in using ecological and environmental indicators to appraise aspects of the physical environment which might normally be assessed by longer methods or not at all. It is concluded that the interdependence of environmental fhctors is high and hence suitable proxy measures can befound. Rapid-and thus low costPmonitoring of change is discussed. The importance of a clear statement of

assumptions is stressed.

INTRODUCTION

Increasingly the appraisal of natural resources is becoming an important element in development programmes. In part this is because of a greater emphasis on rural development with rural areas relying more directly on the natural resource base. But also, pressure from environmentalists now ensures that many aspects of the physical environment are considered before the implementation of a project. However, it is often the case that these surveys are peripheral to the main economic aims and purpose of the final project. For this reason the appraisal of natural resources is subject to considerable constraints in time and manpower.

To the purist the appraisal of natural resources demands a combination of lengthy fieldwork, detailed air photo interpretation and laboratory analyses of soils and water, as well as a host of multidisciplinary studies integrating two or more factors such as the relationship between vegetation and wildlife habitats. Recent Land Resources Study reports, published by the Land Resources Development

473 Agricultural Administration 0309-586X/8 l/0008-0473/$02.50 0 Applied Science Publishers Ltd, England, 1981 Printed in Great Britain

Page 2: Ecological and environmental indicators for the rapid appraisal of natural resources

474 MICHAEL STOCKING, NICK ABEL

Centre of the British Overseas Development Administration, are good examples of this type of thorough work. Furthermore, it is often demanded that field trials of crops-and the assessment of management practices and untested forms of land use-be undertaken. It is, for example, considered that a minimum of about ten years’ plot experimentation is required to assess accurately the danger from soil erosion of particular land use practices. Clearly, such time-consuming and expensive methods are out of the question for the majority of rural development plans. The need arises for short-cut, rapid methods to be tested and accepted.

This paper examines some of the underlying assumptions involved in the rapid appraisal of natural resources. Three case studies of varying complexity illustrate a range of possibilities in using ecological and environmental indicators to appraise aspects of the physical environment which might normally be assessed by longer methods, or not assessed at all.

GENERAL ISSUES

Any methodology for the rapid appraisal of natural resources cannot be seen as distinct from the general problems of the measurement of environmental parameters. Rapid methods merely accept procedures that might not have been used had cost and time not been limiting factors. However, with continual refinement of techniques there is no reason why rapid methods should not be both efficient and the most academically acceptable. The use of remote sensing as a powerful tool in surveys of natural resources exemplifies rapidity combined with sound, accurate methodology.

Nevertheless, in rapid rural appraisal we are normally concerned with methods that short-cut existing, established methodology; i.e. they are quicker and cheaper than the method that would have been chosen had choice been free. Two means can be employed :

(i) Use of proxy variables. On the basis that the measurement of one parameter can give an accurate indication of another parameter (to which it is assumed that the first is related) which is less easily measureable.

(ii) Use of ‘small sample’ methods. In practice most natural resources information is based on samples that would be described as unacceptably small by a statistician. Indeed, statistical purity is rarely a virtue of the applied natural scientist. An extremely dense raingauge network of ten Standard Meteorological Office gauges per km2 collects a sample of 0~00000014 %. Considering known variabilities in the spatial distribution of rainfall and other natural resource phenomena, this size of sample is unlikely to appeal to the statistician. The use of even smaller samples in rapid rural appraisal might be argued as a comparatively trivial sin against the already mortal sin of ignoring statistical direction.

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THE RAPID APPRAISAL OF NATURAL RESOURCES 475

This paper will particularly examine the first of these means, the use of proxy variables-a subject which has not received much attention, probably because it crosses disciplinary boundaries and intimately involves the total environment, not just the plants, or the soils or the hydrology or whatever.

Use of proxy variables Proxy variables are intended for parameters that are either not directly amenable

to measurement or are inconvenient to measure. Nearly all measurements are proxy; the simple act of measuring the length of an object is indirect in that the length in question is compared with another object of known length. For the purposes of our discussion, however, proxy measurements will be considered as ones at least two steps removed from the variables of interest. Thus, for example, the measurement of vegetation cover using rainfall quantities as a surrogate parameter is definitely a proxy measure whereas the use of a quadrat frame is not.

The use of proxy variables also incorporates what might be termed the ‘explanation problem’. It must be decided what level of explanation is required of the variable under study. The choice lies between deterministic and empirical (parametric) modelling. For the former, the precise nature and function of all operative variables is required; for the latter, a lower level of information is necessary-a level usually based on statistical inference. Because of the inevitable complexities of modelling any aspect of the environment, it is difficult to escape the adoption of empirical models using proxy variables. In the modelling of soil erosion, parameters of rainfall are used to find the best description of the erosion output in terms of the rainfall input. These parameters integrate the sum effect of the many process interactions between the moment of fall of a raindrop and the subsequent movement of sediment into a river (Stocking’). Whilst this form of integration destroys a lot of explanation for individual process-response steps, it is possibly the only means of obtaining practical results. As scientists we have to compromise our quest for accuracy and explanation by adopting short-cut methods.

CASE STUDIES

Three case studies have been chosen from our own experience to illustrate the range in complexity that might be encountered in the rapid appraisal of natural resources. Whilst only the barest of experimental detail is given, the cases exemplify the type of information that must be processed by agricultural planners before the implementation of new projects.

Soil colour First, soil colour is a simple and fairly direct case to illustrate how one easily

measured parameter of a soil can be used to indicate other important properties

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476 MICHAEL STOCKING. NICK ABEL

which could only be determined directly by detailed and systematic fieldwork, sampling and laboratory analysis.

Soil colour is controlled by a combination of factors which include soil parent material; clay structure; wetness; organic matter and particle size distribution. An acid igneous rock such as granite contains light-coloured quartz and feldspar crystals with a few darker ferromagnesian minerals. Under tropical and subtropical climates quartz is resistant to weathering and remains as the major constituent of the soil while the feldspars and ferromagnesian minerals weather to clays. These latter dark minerals can, under conditions of good drainage, oxidise to red clays. The redness of well-drained upland soils in the tropics depends on the amount of ferromagnesian minerals; the higher the quantity, the greater the inherent fertility of the soil.

Basic igneous rocks, in contrast, have little quartz. Therefore the predominant dark minerals give a high clay content and deep reddish and brownish colours. Other soil types developed from different parent materials also have typical colours and patterns.

The occurrence of associations of soil colours from hill-top to valley-bottom is particularly diagnostic. These catenal associations indicate varying properties inherited by the soil from its position in the relief (Watson’ ’ ; Stocking7). Sodic soils which, because of the presence of a high exchangeable sodium content, are poor agriculturally and present extreme dangers from erosion, occur as light-coloured patches on air photographs (Stocking4; Wendelaar13).

TABLE 1 RELATIONSHIPS BETWEEN SOIL COLOUR, SOIL GROUPS, ATTERBERG LIMITS AND LINEAL SHRINKAGE FOR THREE

SOIL SAMPLES DERIVED FROM BASEMENT COMPLEX ROCKS-DATA FROM ELWELLl

Soil colour Soil group Atterberg limits % lineal shrinkage

Plastic Liquid Plasticity at liquid index limit

Black

Brown Red

Calcic hydromorphic (vertisol)

Siallitic Fersiallitic

36.6 110.8 14.2 21.3

29.2 74.0 448 16.7 28.3 60.8 32.5 14.0

A useful property that soil colour may, therefore, indicate is soil type, probably at the soil group level in most major classifications. This can, in a general fashion, be related to agricultural development. More detailed properties of the soil may also be inferred from colour. For engineering, as well as agricultural, purposes it is desirable to have a knowledge of particle-size distribution of the major soil types. Soil colour, being an indicator of soil type, can therefore be an accurate predictor of particle-size distribution (Fig. 1) and of some important engineering properties such as plasticity and volumetric activity (Table 1).

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THE RAPID APPRAISAL OF NATURAL RESOURCES 477

0 0.002 0.02 0.2 2.0

Particle Diameter (mm.)

Fig 1. Particle size distribution curves for soils derived from acid and basic igneous rocks in south- central Africa (after Elwell’). Curve 1: Red Clay Soil from epidiorile (basicthigh clay. Curve 2: Red ClaySoil from dolerite (basic)-high clay. Curve 3: Yellowish Red Soil from granite (acid)-well drained

granite. Curve 4: Pale Gray Soils from granite (acid)+lay fraction leached by groundwater.

As with all indirect methods there are important limitations. Soil colour will probably be surveyed from colour aerial photography and ground check using Munsell Soil Colour Charts. Organic matter and the presence of thick vegetation cover can upset the determination of colour. A further limitation is that imposed by the coarseness of the information; surface soil colour is only indicative of the soil type and not absolutely diagnostic. However, if enough local experience is available and if soil colour surveys are used with care, they allow the rapid appraisal of a range of agricultural and engineering properties of soil.

Plant indicators for natural resource survey The second case study shows that the complexity of natural systems and the

interrelationships between factors can be utilised so that one variable can effectively integrate several other variables and present useful land resources information in a simplified manner. Plant indicators are the most powerful tool here since:

(i) They are obvious features of the landscape. (ii) They can react dramatically to sometimes small changes in environmental

conditions. (iii) They are the end products of soil, water, geomorphological and climatic

inputs, and past land use and fire. (iv) If the environmental conditions change such that the vegetation changes,

then it is likely that these same environmental conditions will affect decisions in agriculture and land use.

However, by its very complexity and sensitivity, the use of plant indicators is fraught with difficulties. A local knowledge is invaluable in interpreting and understanding the cause of changes in vegetation species and distribution. But, once established, plant indicators can be an extremely reliable, efficient and rapid indicator of natural resources information.

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478 MICHAEL STOCKING. NICK ABEL

This case study briefly reports work undertaken in Zimbabwe to associate vegetation with soil types, soil catenas, sources of material for dam construction and agricultural potential.

Tree and bush species are the most important indicators because they are most easily recognised and theyreflect the soil characteristics to a greater depth than grasses. Table 2 shows some of the associations between trees and soils that have been recognised.

From Table 2 it is clear that vegetation can be indicative of certain aspects of soil climate and drainage, both of which are influenced by the relief of the landscape. Figure 2 illustrates two typical catenary sequences on soils derived from granite. Whilst the mineralogical composition of the granite in both cases is virtually identical, the structure of the granite that forms ‘dwala’ landforms is massive and poorly drained. The granite that forms ‘castle-kopjes’ (a tor-like landform) is well jointed and freely drained. Under high water table conditions (Fig. 2(A)) groundwater movements have leached out the clay fraction to leave pale sands with an open grassland type of vegetation. Where drainage is a little better in mid-catena the vegetation changes perceptibly to species that accept temporary waterlogging. At the top of the catena mnondoimsasa woodland may appear. The castle-kopje catena (Fig. 2(B)) displays soils that are deeper and redder, having a higher clay content and greater agricultural potential.

The use of plant indicators can, therefore, be a useful method of rapid appraisal and for the delimitation of problem soils and environments. For example, the occurrence of Parinari curatellaefolia on the tops of ridges immediately warns of impending problems for any proposed changes in land use. This tree flourishes in zones with a fluctuating seasonal watertable where the roots are usually in saturated soil during summer. Such conditions encourage the formation of non-calcic hydromorphic soils which have a structure consisting of large quartz particles with narrow bridges ofclay in between. It is a very strong structure when dry, but it suffers collapse under load with increased moisture content. This holds especial danger for roads and railway lines built on these soils.

Another more sophisticated example is the overall assessment of agricultural potential from forest vegetation through surveys of key indicator species. Webb et al.,12 in rainforest in eastern Australia, have demonstrated that through statistical analysis of vegetation and canonical procedures they can identify particular forms of suitable agricultural exploitation.

Soil erosion survey Knowledge of the distribution of erosion hazard throughout a country or region

is an important aid to‘broad-scale resource and land use planning. However, erosion information is diverse and difficult to assess. There are many erosional forms (such as gullying, tunnelling, sheet) and it takes years to set up the necessary experiments to monitor rates of erosion in order to assess the severity of the danger.

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THE RAPID APPRAISAL OF NATURAL RESOURCES 479

TABLE 2 SOME ASSOCIATIONS BETWEEN VEGETATION AND SOIL TYPES. NOTE THAT THE ASSOCIATION REFERS TO

DOMINANT SPECIES AND NOT ISOLATED OCCURRENCES. (AFTER THOMPSON”)

Tree species

Brachystegia spiciformis Julbernardia globiflora Acacia albida Combretum apicularum Hyphaene crinita

Msasa Mnondo J White thorn

Vegetable ivory

Guibourria coleosperna African mahogany Baikiaea plurijuga African teak Protea abysinica Protea Parinari curatellaefolia Muhacha Syzygium guineense Waterberry Colophosphermum mopane Mopane Uapaca kirkianu Mahobohobo

Common name Soil

Well drained para- ferrallitics and fersiallitics Alluvial siallitics Sandy siallitic soils Siallitics or sodic soils

(if stunted) Kalahari sands Kalahari sands Temporarily wet soils Temporarily wet sands Permanently wet sands Sodic soils Shallow, well drained gravel

A. “DWALA” CATENA

viei - wetland (open grassland)

grassland and tree clumps scattered Syzygm with Brachystegg and Parinari and Julbernardia curatellaefolia

B. “CASTLE” KOPJE” CATENA

mixed Brachystegk anr

Fig. 2. Contrasting granite catenas in the high rainfall (1000 nn) area. A: On massive granite. B: On well jointed granite (from Elwell’ and Stocking6).

Page 8: Ecological and environmental indicators for the rapid appraisal of natural resources

480 MICHAEL STOCKING, NICK ABEL

An indirect and relatively rapid method of overcoming these problems is the survey of thefactors that control erosion. Full details of an erosion hazard survey of Rhodesia employing existing knowledge of the relationship between erosion and individual factors are given by Stocking and Elwel18. Mention here will be confined mainly to the basic procedure and practical problems and limitations of the method.

Soil erosion can be assessed as the product of five main factors-erosivity of rainfall, land slope, vegetation cover, erodibility of the soil and human influences (Table 3). Within each one of these categories there is a complex of factors controlling actual rates of erosion, but for simplicity the five main factors provide the basic model for erosion hazard assessment. For two of the factors-erosivity and slope-sufficient qualitative data was available to derive the best parameters to describe their individual effects on soil erosion. This knowledge was the product of a lengthy research programme to investigate a wide range of possible parameters (e.g. Elwell and Stocking2), the results of which have general applicability. Less precise data was available for the other factors and generalised parameters were adopted. For example, the influence of man on erosion is highly complex and, at the present time, is neither fully understood nor readily quantifiable. In such cases recourse was made to indirect parameters which are known to affect erosion in broad terms. A parameter to describe human influence might include density of population and standards or types of agriculture.

In the next phase of the analysis the best parameters were quantified and their distribution mapped on a geographical basis. Thus, for example, considerable information could be plotted on a parameter representing the kinetic energy of rainfall, bringing out remarkable differences in the power of rainfall to cause erosion (Stocking and Elwell’).

Finally, the spatial information on the individual factors was brought together in a simple factor analysis with the end result, shown in Fig. 3, giving an indication of the relative hazard and the likely cause for it. Such information is now being used by conservation planners (Stocking3).

Beneath such an approach, that employs both broad survey techniques and the use of proxy variables, there lies severe assumptions and potential limitations which ought to be stressed if the information derived by the method is to be used sensibly. These may be summarised as follows.

(i) Each factor in erosion can be treated separately. Natural systems do not function in this way. There is, for example, strong interdependency between rainfall and vegetation. However, this type of assumption, common in the sciences, has to be made given the poor level of information available.

(ii) For simplicity, linearity between erosion and the individual factors was assumed. In some cases an approximate linear relationship can be proved but in others-such as the influence of vegetation cover on erosion-the relationship is definitely curvilinear.

Page 9: Ecological and environmental indicators for the rapid appraisal of natural resources

TABL

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Page 10: Ecological and environmental indicators for the rapid appraisal of natural resources

482 MICHAEL STOCKING, NICK ABEL

Fig. 3. Soil erosion hazard over Rhodesia-factorised erosion survey.

(iii) The level of information available for each of the factors varies. The accuracy of the final result can be no better than the level of information for the least known factor. This is why the key to Fig. 3 is partially in qualitative form to indicate relative, rather than absolute, hazard.

(iv) Finally, each of the five factors was assigned an equal weight and combined in simple arithmetic terms. This is a fundamental weakness in the whole methodology for not only can it be demonstrated that some factors are more important than others in controlling the process of erosion in particular areas but also the combined effect of two hazardous factors is very likely to be much greater than the sum of their individual effects. The factor scores in Fig. 3 are, therefore, artificial and the summation of the scorings even more so.

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THE RAPID APPRAISAL OF NATURAL RESOURCES 483

Despite these limitations accessible information can be brought together to obtain a fairly accurate picture of a parameter, erosion hazard, which otherwise would be virtually impossible to assess.

CONCLUSION

The use of ecological and environmental indicators is an obvious means of speeding the appraisal of natural resources. Through the judicious use of proxy variables, the monitoring of one parameter can provide information on another parameter or set of parameters. Environmental systems are amenable to this treatment because of the high degree of interdependence between factors.

For agricultural planning in developing countries rapid appraisal methods are well established and are undergoing continual refinement. The integration of a number of factors through one variable-particularly natural plant communities- is a method where rapid appraisal is consistent with more accurate appraisal. Successful agriculture is, after all, an amalgam of many natural and human circumstances. Monitoring at least part of that amalgamation is therefore bound to be desirable for accuracy as well as speed. In other circumstances, the disaggregation of complex environmental and ecological interactions into their component parts is necessary. Much of the acceptability of this sort of simplified rapid appraisal of natural resources boils down to the acceptability of a loss in accuracy and the provision of a careful statement of assumptions (cJ: soil erosion survey case study).

In the final analysis the appraisal of natural resources has to be incorporated with the many other appraisals and decisions in the planning and implementation of agricultural projects. Rapid rural appraisal in providing more timely results and answers in simplified form should be more acceptable to the decision-makers.

REFERENCES

1.

2.

3.

4. 5.

6.

ELWEI L, H. A.: Geomorphological factors and on-site indicators as aids to soil surveys. Rhodesian Engineer, Paper No. 169 (1975). ELWELL, H. A. & STOCKING, M. A., Parameters for estimating annual runoff and soil loss from agricultural lands in Rhodesia. Water Resources Research, 11 (1975), pp. 601-5. STOCKING, M. A., Soil erosion potential: The overview. In: Engineering Handbook, Department of Conservation and Extension, Ministry of Agriculture, Salisbury, 1975. STOCKING, M. A., Tunnel erosion, Rhodesia Agricultural Journal, 73 (1976), pp. 35-39. STOCKING, M. A., Rainfall energy in erosion; some problems and prospects. Research Discussion Paper No. 13, Department of Geography, University of Edinburgh, 1977. STOCKING, M. A., Prediction and estimation of erosion in subtropical Africa. Gee-Eco-Trop., 2 (1978), pp. 161-274.

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484 MICHAEL STOCKING, NICK ABEL

7. STOCKING, M. A., Catena of sodium-rich soil, .I. Soil Science, 30 (1979), pp. 139-46. 8. STOCKING, M. A. & ELWELL, H. A., Soil erosion hazard in Rhodesia, Rhodesia Agricultural Journal,

70 (1973), pp.93-m101. 9. STOCKING, M. A. & ELWELL, H. A., Rainfall erosivity over Rhodesia. Transacrions Insrirute oj

British Geographers. N.S., 1 (1976), pp. 231-45. 10. THOMPSON, J. G., Soils and some factors that govern their engineering properties. Rhodesian

Engineer, Paper No. 60 (1966). 11. WATSON, J. P., A soil catena on granite in southern Rhodesia, J. SoilScience, 15 (1964), pp. 238-57. 12. WEBB, L. J., TRACEY, J. G., WILLIAMS, W. T. & LANCE, G. N., Prediction of agricultural potential

from intact forest vegetation, J. uppl. Ecol, 8 (1971), pp. 99-121. 13. WENDELAAR, F. E., Field identification of sodic soils, Rhodesia Agricultural Journal, 73 (1976),

pp. 77-82.