Many studies have looked for correlations between drought related environmental stress and population
dynamics (Larson and Michaelson; 1990, Larsen, 1996; and Larsen et al 1996)


CASE OF THE VIRGIN BRANCH (Larson and Michaelson, 1990,Larson, 1996)
Two major drought episodes A.D. 1000 to 1015 and A.D. 1120 to 1150, contribute to significant
change in adaptive strategies of the Virgin Branch Anasazi.

The virgin Anasazi occupied the Southwestern Great Basin between A.D. 100 and A.D. 1150.

Larson and Michaelson found that the drought of 1000 to 1015 promoted the adoption of alternative
buffering strategies that included:

•    Intensive agriculture,
•    Food storage, and
•    Organization of large laborious groups,

whereas the drought of A.D. 1120 to 1150 resulted in the complete abandonment of the region. The study
explores possible explanations to this two different strategies after severe environmental stress. The purpose
of the study was to evaluate the roles of population growth and climatic variability in producing cultural change
among the Virgin Anasazi. Cross-cultural studies suggest that the level of stress experience by a population
due to change in climatic conditions will depend on (Colson, 1979; DeGarine and Harrison 1988; Dirks, 1980;
Hitchconck, 1979; Maclachlan, 1983; Minnis, 1985; Winterhalder, 1980): (1) the magnitude, duration,
and frequency of the climatic shifts; (2) preexisting adaptive strategies of the human population ; (3) the natural
response of highly ranked resources to such events; and (4) the population size and density of the effected group
(Larson and Michaelson, 1990).

STUDY AREA:

The study area lays between the Death Valley on the west, the Colorado River on the south and east, and
numerous ranges and valleys on the north (Figure 1).  Elevations range between 150 and 3500 m. The two main
tributaries are the Virgin and the Muddy rivers. The source of the former is snowmelt from the mountains
whereas the former is a perennial spring-fed river. Therefore the stream flow in the Muddy River is more
consistent than in the Virgin River.
 





ARCHEOLOGICAL METHODS

The authors conducted a systematic archaeological 32- site survey and surface-artifact-collection program
along the Virgin River (Nevada). Site dating was based on attribute-based ceramics analysis. The strongest
temporal patterning in the Virgin Branch Anasazi ceramics were exterior color, temper and surface treatment.
The authors consider the presence of corrugated and red warer ceramics to discriminated sited temporally. 
They analyzed a total of 28,647 shreds from the 32 sites.  They basically order he sites on the basis if changing
frequencies of corrugated pottery and presence absence of olivine-tempered pottery.  They verified the dating
based on 7 radiocarbon dating.

In order to establish population growth they counted the number of rooms and then assumed a typical household
per room. Rooms were assigned to 5 different periods based on the pottery analysis results.


CLIMATIC RECONSTRUCTION METHODS
The climatic analysis was based on tree-ring chronologies. The data was obtain from a stand of low-altitude
bristlecone pine located at approx.  100 km. southwest from the study area. The record extends from A.D.
966 to 1965.  They used the tree ring data couple with the available stream flow record for the Virgin River
(USGS record from 1932 to 1965) to predict the stream flow at the time the Anasazi occupied the area.


 




Bistlecone pine



RESULTS


Demographic Reconstruction


  






There is clear evidence of dramatic increase in population from periods 1 to IV, followed by a dramatic decline in Period V (Larson and Michaelson, 1990).

Agriculture Intensification

The archeological  investigations also indicate changes in agricultural practices and community organization,.
The findings are summarized in the following table:





Climatic Reconstruction Results
The reconstruction of climate based on the regression of tree-rings against Virgin River Flows shows that
tree rings could explain 61% of the variance in stream flow (figure 7).  Figure 8 presents the reconstructed
and actual stream values for the calibration period between 1932 and 1965.  For the most part,
the reconstruction  captures the actual stream flow (Larson and Michaelson, 1990).





Climatic results are shown in Figure 9 and can be summarized in:

•    Drought dominated period from A.D. 966 - 1015
•    Moderate flow volumes from with neither wet nor dry extremes A.D. 1020 -1060
•    Very wet period from A.D. 1060 to 1120.
•    Drought between 1120 to 1150

The conditions change from A.D. 966 up to 1150 from DRY to MODERATE to WET to DRY


CONCLUSIONS (FOR PERIODS III TO V)

Period III (A.D. 850)      - Rate of population increase began to accelerate
                                        - Competition for wild resources between communities
                                        - Anasazi become highly dependent in agriculture
                                        - Increase reliance on storage

Period IV                         - Favorable climatic conditions after A.D. 1020 resulted in accelerate population growth
(A.D1000-1300)             - Greater reliance on agriculture associated with this demographic growth  
                                        - More sophisticated agriculture practices
                                        - Population grouped in large labor communities
                                        - Population reached a level that could not be supported by hunting and gathering even during
                                          favorable climatic periods.

CLIMATE AND POPULATION SIZE ARE BOTH IMPORTANT FACTORS



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