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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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