Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 43considered possible site indicators (Young and Potter1975:Table 4); the best indicators wV}re Western tansymustard (Descurainia pinnata), greasewood (Sarcobatusvermiculatus), winterfat (Eurotia lanata), alkalisacaton (Sporobolus airoides), and wolfberry (Lyciumpalladium).In summary, in the contemporary environment ofChaco Canyon, the amount of rainfall and the timingof precipitation events are crucial to the growth andproduction of many plants. Even minor variationsaffect the successful maturation of com plants, and, byinference, other plants that would have supplementedan agricultural diet or have been important to huntergathererpopulations.FaunaBy combining evidence from archaeological andhistorical studies, Brand (1937c:47) concluded thatthere were few, if any, mammalian species presenttoday that were not there prehistorically. The converse,however, was not true. Antelope had not beenseen for at least 50 years. Mule deer and elk weremissing for at least one generation. Brand postulatedthat these mostly herbivorous animals left due tohunting and the introduction of domesticated grazinganimals (Historic period). He also proposed that thecarnivores, or Felidae, that preyed upon these herbivoresprobably moved out at about the same time. Asa result, smaller animals, especially rodents, movedinto the empty niche. After the monument boundarieswere fenced (beginning in the 1930s and completed in1948 [Brugge 1980)), vegetation slowly recovered.Gordon Vivian and Mathews (1965: 16) indicated thatwithin the next few decades mule deer popUlations hadbecome more common. Wapiti or elk reappeared.Vivian and Mathews doubted, however, ifbison, bear,and mountain sheep were ever part of the local scene.(Pronghorn were last sighted moving through the parkin the late 1980s.) Over the years, park personnelcompiled lists of other animal species. Scurlock(1969) realized that his list of birds was not comprehensive,and that baseline biological inventorieswere lacking. More detailed studies were needed tounderstand the movements of various animal speciesacross the landscape.Initial baseline studies began in 1968, whenKirtland L. Jones initiated the first structured study ofherpetofauna of the area (Jones 1970). After establishingtransects across six physiographic areas, he founddifferences in herpetofauna associations. For example,there were no reptiles or amphibians associatedwith rabbitbrush (Chrysothamnus nauseosus) in anarea where water stands for long periods. Toads wererecorded in riparian and four-wing saltbush/greasewood(Atriplex canescens-Sarcobatus vermiculatus)associations, but not in the mixed grassland, mixedshrub, or pinon-juniper associations. Jones concludedthat herpetofauna were distributed according tophysiographic areas in which there was a distinctcomplement of plants. Within the physiographicareas, the herpetofauna were segregated by nicherequirements. For lizards, the niche distribution wasby substrate; for snakes, there was no nichedistribution, but species segregated, in part, byactivity periods and feeding habits.During his two-year study of small rodents,Jones recorded 163 species. Those areas that had ahigher plant diversity also had higher numbers andgreater diversity of small rodents (Potter 1974). Withfew exceptions, the number and species recoveredduring the second year were similar to those trappedduring the first year. In one sampling unit (PuebloAlto South), however, no grasshopper mice werepresent during the second year, although they comprisedthe majority of the species in the previous year.Instead, kangaroo rats and pocket mice, species notpresent a year earlier, were found. When Potter(1974: 114) reviewed data from both years, hesuggested that the density of small rodents was approximately1192.9 m 2 (1/1,000 fe), with the highestdensity (2/92.9 m 2 [2/1,000 feD associated withmixed grasslands and the lowest (0.2/92.9 m 2[0.2/1,000 fe)) with rabbitbrush. There had been adecline in the number of Northern grasshopper miceand an increase in other small mice. These studiessuggest how minor changes in vegetation affect smallmammalbehavior; the differences between years alsoindicates the ease with which populations moved aboutin their environment.Research reported in Scott (1980) evaluated thebehavior of insectivorous bird populations, rodentpopulations, and seed-harvesting ant popUlations tounderstand their effects on perennial plant coverage.Rodents demonstrated differences in population duringthree years of study. The pocket mouse (Perogllathus