Figure 4.4 illustrates the trend in the mean number of identified plant taxa among macro-
72 optimization and risk in southeastern arizona
botanical assemblages at phase or period-contemporaneous sites in southern Arizona. The occupations represented in Figure 4.4 span the interval from the San Pedro phase (1200-800 BC) of the Early Agricultural period (1700 BC-AD 150) through the Rincon phase (AD 950-1150). Assemblages from Early Agricultural period sites are consistently more diverse than the assemblages from later sites, with diet breadth declining by almost half from the San Pedro phase. The trend is statistically significant using Spearman's rank-order correlation when site age is compared against taxo-nomic diversity, with a moderate correlation (Rs = -.44; p < .01).
As Figure 4.5 illustrates, the average number of taxa in southern Arizona faunal assemblages also decreased through time, but it fluctuated to a greater degree than was observed in the macrob-otanical assemblages. It is quite clear that Early Agricultural period assemblages were more diverse than Early Ceramic and Pioneer period assemblages. The complete trend, including Colonial and Sedentary period sites, is not very strong or statistically significant (Rs = -.26; p = .18), owing to an increase in faunal diversity in the Colonial period. However, when Colonial and Sedentary period sites, essentially, sites more recent than AD 750, are excluded from the analysis, the downward trend is both strong (Rs = -.51) and statistically significant (p < .03). We attribute the increase in diet breadth that occurred in the Colonial and Sedentary periods to anthropogeni-cally induced changes in the local landscape. These changes are discussed later.
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