Shack Elton Et Prins 1992

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Journal of Archaeological Science 1992, 19,63 l-637

Charcoal Analysis and the Principle of Least

Effort -A Conceptual Model

C. M. Shackleton and F. Prinsh

(Received 25 Apr il 1991, revised manuscript accepted 10 January 1992)

The applicability of the Princip le of Least Effort (PLE) to interpretations of palaeo-

climatic data i s considered, and found lacking in some instances. A conceptual model

is presented to determine situations in which the PLE may, or may not, apply. This

helps identify when the PLE may be a useful model for interpretation of appropriate

data sets.

Keywords:

ABUNDANCE, EFFORT, FUELWOOD, SELECTIVITY.

Introduction

Interpretation of palaeoenvironmental changes is an important facet of many disc iplines

including archaeology, botany and geography. Each of these disciplines discerns past

changes in different ways. The prevailing conditions during particular palaeoenviron-

ments are archaeologically inferred by several means, including phytoliths, micro and

macro-fauna1 remains, seed material, pollen and charcoal as sources of information

(Brothwell Higgs, 1963; Shackley, 198 1). Credence is lent to particular interpretations

and conc lusions if similar trends are evident from independent, although complementary,

forms of investigation (Deacon Lancaster, 1988).

The taxonomic identification of charcoal in particular yields data depicting changes in

the floral species composition of the area under scrutiny. Combined with ana lysis of

anatomical features such as vessel diameter (Deacon Lancaster, 1988; Scholtz, 1990)

and botanical knowledge of the ecological requirements of extant spec ies (also rep-

resented in the charcoal sample) it is possible to predict some details of the climatic

conditions preva iling during the different periods (e.g. Deacon et al., 1983; Prio r, 1983;

Deacon

et

al., 1984; Prior Price-Williams, 1985; Prior Tuohy, 1987; Dowson, 1988;

Tusenius, 1989).

Trends of increasing or decreasing abundance of chosen indicator species should be

accorded greater emphasis than erratic changes in the abundance of rarer species. A time

sequence of dates and species composition data should be set up to emphasize differences

in the abundance of species (e.g. Prior, 1983).

Functional interpretation of charcoal data sets rests upon acceptance of the Princip le

of least effort (Scholtz, 1986; Tusenius, 1986). It is the val idity of the underlying

assumptions that are the focus on this artic le.

OWits Rural Facility, P.O. Box 7, Klaserie 1381, South Africa.

bDepartmen t of Botany, University of Transkei, P.B . Xl, Unitra 5100, South

Africa.

631

03054403/92/06063 I+07 08.00/O 0 1992 Academic Press Limited


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632 C. M. SHACKLETON AND F . PRINS

Theoretical Developments and the Princip le of Least Effort (PLE)

The majority of charcoal studies undertaken by archaeologists in southern Africa were

conducted within the framework of the ecological paradigm. This approach characterized

Stone Age studies undertaken between the 1960s and 1980s and archaeological develop-

ments elsewhere. This paradigm viewed environmental change as primary in cultural

variability and consequently humans were seen as rational but passive actors subject to

external forces. People merely adapted to factors beyond their control and some

researchers explained changes in the archaeological record as being stimulated primarily

by fluctuating environmental circumstances. Although proponents of the ecological

paradigm warned against environmental determinism, this was the position ultimately

adopted by those interested in charcoal analysis. Most of these studies assumed a one-to-

one causal relationship between prehistoric fuelwood collectors and the woody vegetation

utilized by them. However, some palaeoethnobotanists voiced the opinion that plant

materials were collected according to cultural rules and that their proportions in the

archaeological context were not a simple reflection of their abundance in the prehistoric

environment (Ford, 1979). Unfortunately these ideas, which are supported by ethno-

graphic observations (Ford, 1979) were largely ignored in favour of general law-like

statements applying to all cultures and contexts. This is characteristic of much of the

so-called Processual Archaeology (Hodder, 1986). Such a general law was developed

by charcoal researchers in the form of the Principle of least effort (PLE).

In essence the PLE assumes that past peoples collected fuelwood that was closest to the

homestead, and that all species were collected in direct proportion to their occurrence in

the surrounding environment. They moved further afield only when fuelwood within the

immediate surrounds was depleted. In other words, they collected what was easiest-

hence the name. Accepting this, then the frequency of species in the charcoal sample would

accurately reflect their abundance in the environment at that time.

Scholtz (1986) and Tusenius (1986) identified several possible sources of error in the

accepted assumptions underlying the PLE, and hence possible errors in palaeoclimatic

interpretations based on charcoal analysis. These sources of error are: (1) potential

selection of particular species over and above others in the vicinity ; (2) possible differential

preservation of charcoals from different species; (3) sampling problems both during

excavation and in the laboratory; and (4) potential non-uniform deposition of charcoal

throughout the time period under consideration. The authors suggest that another may be

added, this being the difference in yield ofcharcoal between species (this is currently under

investigation). Thus, although researchers were aware of the potential non-validity of

PLE, research projects were, in the absence of any alternative frameworks, stil l based on

this general principle. In this paper we are interested in investigating the implications for

archaeological interpretations if the first of the sources of error listed above is found to be

true.

Preference for particular fuelwood species

The PLE assumes that all species were collected as fuelwood in direct proportion to their

abundance in the environment. If, however, particular species were actively avoided or

selected, then the charcoal remains would fail to provide an accurate reflection of the

composition and abundance of the species at that time. Although Prior Price-Williams

(1985) Scholtz (1986) and Tusenius (1986) amongst others, have questioned the validity

of this assumption it cannot be falsified directly through archaeological excavation.

Several ethnographic and biological studies, however, suggest that the assumption may

indeed be false (Best, 1979; Ford, 1979; Jelenic van Vegten, 1981; Gandar, 1983).

Unpublished work by Shackleton (in press) shows that the avoidance of several species


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CHARCOAL ANALYSIS

633

1

Fresh wood

4

Dry wood

I I

b

*

I I

Strong

6

5 4

I

I

I

I

I

Wood abundance per person

1

High

Figure 1. Degree of species selectivity as a function of dry and fresh wood

availabil ity (the numbers lL6 refer to model areas described in the text).

by fuelwood collectors was as marked as their active selection of others. A considerable

number of species were neither avoided nor preferred and so the PLE may be applied to

these.

A conceptual model

The model proposed (Figure 1) ideally applies to fuelwood collected for domestic use but

it will also apply to those species selected for specialist functions like smithing and the

firing of ceramics. It describes the degree of select ivity for different fuelwood species

relative to the total abundance of wood in the environment. As such, it can be used to

predict when the PLE may or may not be in force. It rests on two conditions. Firstly , if

wood is plentiful then inhabitants wil l select species that are the most desirable for the

purposes on hand. Initially, selection will be for dry wood of a particular species-it being

preferred to fresh wood that is heavier to carry and takes time to dry before it can be used.

Secondly, if wood is in short supply, fuelwood collectors will probably exploit whatever

wood is available to them. In this instance the PLE will apply. Additional factors such as

the annual burning of the woody vegetation and clearance of large tracts of woody

vegetation for agricultural fields may influence the process. However, these factors are

not considered here and they require further research. For ease of interpretation four

generalized areas are described in the model. Most settlements probably experienced a

sequential transition from one to the next during the course of occupation.

Area 1. This represents the typical situation encountered at a new settlement in a relatively

unexploited area. There is plenty of fuelwood, both dead and live, with a considerable

diversity of species available. Thus, there is maximum wood abundance per person and

hence individuals may select dry wood freely, taking that which is most suited to their

needs. Even though there is maximum selectivity there is very little effort involved in

fuelwood collection since total abundance is high, i.e. inhabitants do not have to go far,

nor search for long, to obtain dry wood of the particular species that they are seeking. In

summary, therefore, area one is characterized by a high dry wood availability, maximum


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634 C. M. SHACKLETON AND F. PRINS

selectivity and low effort. Modern analogues are scarce but this situation may apply in

some nature reserves where controlled exploitation of natural resources by indigenous

peoples is allowed (Brunton, 1980).

Area 2. This area is defined by a medium abundance of dry wood, declining selectivity and

maximum effort. This situation applies near settlements with a declining dry fuelwood

resource. This may be a new site in a previously exploited area. More likely however, it is

the same settlement as in (1) above, after several decades of occupation. The size of the

settlement has increased and the cumulative effects of fuelwood collection over time have

decreased the total availability of dry wood. The degree of selectivity is decreasing since

the abundance of the most sought-after species is also decreasing. Some individuals are

using increasing amounts of previously unfavourable species. Maximum effort is required

at this stage, since it takes time and energy to seek out the few remaining dry branches of

the most preferred species. Situations with declining dry fuelwood resources may even

occur in relatively low population pressure areas where traditional settlement patterns are

maintained such as was observed in tribal areas near Willowvale in the Transkei (Prins,

unpubl.).

Area 3. As time progresses, dry wood of the preferred species is depleted to levels where

the effort required to seek it out is too great (although it is not necessari ly exhausted

completely). This stage is characterized by low wood abundance, little selectivity and

minimal effort. In the most extreme situation any wood present is removed, even if its

fuelwood properties are poorly rated. In this situation the PLE would apply. Resettlement

areas in Transkei and Gazankulu, with a relatively high population pressure on the

available natural resources, exhibit this pattern.

Area 4. Demand for dry wood far exceeds supply, even if the least favourable species are

included. Supplementary supplies are obtained through the collection of live wood. Since

this is a new source of supply, many different species are available and the degree of

select ivity will therefore be high. The situation is essentially a repetition of Area 1 in the

model but applied to fresh wood. The model follows the same course for live wood as

described for Area 2 and 3 pertaining to dry wood. A modern analogue would be long-

established areas of resettlement in tribal areas such as were observed in northern

Transkei and the eastern Transvaal Lowveld. However, it should be noted that some

overlap between Areas 3 and 4 is likely to occur as certain highly favoured species could be

removed as green wood, even though dry wood of less desirable species occurs abundantly

near the homestead. This would especially be the case where wood is required for specialist

functions or ceremonies. Nevertheless, the transition from Area 3 to 4 represents both an

anthropological and ecological threshold.

Predictions and implications

Acceptance of the proposed model has several important implications.

1. Upon occupation of a site, per-capita wood availability would be high (Area 1). Here

the PLE does not hold. Therefore the species and abundance of charcoal from this period

would not be representative of the species composition of the area.

2. During the later phases of occupation characterized by reduced abundance of preferred

species, inhabitants would (a) walk further to obtain the species they wanted and/or (b)

swap to alternative species. There is likely to be a gradual transition from the former to the


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CHARC OAL ANALYSIS 635

latter. Ultimately, whatever is available will be collected. Should inhabitants adopt the

first strategy, species not present in the area may be imported into the charcoal deposits

leading to erroneous interpretation. Even if (a) is adopted, under conditions of high

fuelwood demands and long occupancy option (b) wi ll ultimately be taken. In this way

PLE

may apply during the later occupational phases.

3. Highly preferred species may be selectively reduced with increasing harvesting pressure.

This will come about through either an increase in the number of people in the settlement

and, therefore, a greater fuelwood demand, or continuous removal over long periods.

Thus, the relative abundance of less desirable species will increase. At some threshold

abundance, the amount of effort required to seek out the most favourable spec ies is too

great even though a few sparsely distributed individuals may remain. At th is point an

increasing proportion of the less desirable species would be evident in the charcoal record.

Theoretically, the change in focus for particular species from the most desirable to those

of intermediate preference (not actively sought initia lly, nor actively avoided) would

provide a respite to the few remaining trees of the most preferred species. This would

provide time for them to increase in relative abundance, whilst the other species experi-

enced a decline in relative abundance similar to that which the former previously under-

went. Since generation times of woody spec ies are long, these changes would take several

decades. Were that the case, then young fuelwood harvesters would be unable to recognize

the resurgence of the most preferred species, having being taught what were the most

desirable from an impoverished assemblage of species. An example of this is the inability

of young fuelwood collects (younger than 16 years old) to recognize the less abundant fruit

trees (e.g. Annona senegalensis) in the Timbavati communal grazing lands-and yet older

community members easily identified it as auseful fruit-bearing species (Shackleton,

in press). However, at some threshold of decreasing abundance of the species recognized

as desirable, an increasing proportion of the initial ly most favoured species would again

be collected.

This theoretical projection could result in cyc lica l changes of increasing and decreasing

abundance of spec ies in both the environment and the charcoal remains. This extra-

polation of the model may be extreme. However, if plausible, it is signi ficant in that it

projects changes in species abundance in the absence of climatic changes. Furthermore,

stable limit cycles are not unknown from biological literature (Whittaker, 1975;

McNaughton Wolf, 1979). If other animal species may induce such cycles why not

humans?

4. If cyc lica l changes in species abundances exist in the environment (and hence

the charcoal record) brought about through human interference important thrusts in

ethnographic research would be to identify the threshold points of the model. Thus, at

what threshold densit ies of wood (total and relative spec ies) is there a change in the

situation from one to which the PLE is not applicable to one where it is?

5. Even if the extreme, theoretical extrapolation (discussed in implication 3) is

unattractive, the model st ill demonstrates (although to a lesser degree) that changes in

relative frequencies of species in the charcoal record through time may not necessarily be a

result of altered abundance induced through climatic changes. Thus, as argued by Scholtz

(1986) they may have been the result of the select ive use of the most favoured species.

For example, data from Deacon et al. (1983,1984) demonstrate changes in abundance

of several species throughout the stratigraphic record. Th is is interpreted as indicating

climatic change. This may well have been the case. However, the possib ility also exists that

these changes were induced or exacerbated by human selection and localized removal of

some species over others, promoting the exploitation of alternative species.


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636 C. M. SHACKLETON AND F. PRINS

Simila rly, data presented by Prior (1983) indicate that the frequency of Dichrostachys

cinerea increases throughout the stratigraphic record, with a concomitant decrease in

Sclerocarya birrea.

There is little discussion of these trends, but they could function

according to the proposed model.

We make no claim to entirely discredit previous interpretations since we cannot

fals ify them with current methods.

Likewise, it is unfortunately equally difficult to falsify

the above model. However ethnographic evidence lends it support. Falsification may

best be achieved through independent avenues of investigation all pointing to a similar

conclusion. Nevertheless, we do suggest that alternative interpretations may exist,

especially for high resolution observations spanning relatively short time spans where

potential climatic changes assume lesser importance, and hope that the above model

makes a contribution towards a rationalization and conceptualization of when and where

certain interpretations may have greater emphasis over others.

Finally, the proposed model suggests that charcoal analysis is not limited to palaeo-

environmental interpretations and that it can be a valuable aid in the understanding

of past human behaviour. This potential has been realized in the analysis of fuelwood

diameter (Salisbury Jane, 1940; Scholtz, 1986) but has not been applied to the taxo-

nomic identification of charcoal and the interpretation of the relative abundances of

various species. The latter approach has dominated charcoal studies in southern Africa

and this study emphasizes the need to give more attention to the development of theory.

Acknowledgements

Thanks are due to the colleague of CS at the Wits Rural Facili ty for comments and

discussion on the penultimate draft of this paper.

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Shack Elton Et Prins 1992