Culture and Ecology of Chaco Canyon and the San Juan Basin

44 Chaco Project Synthesisflavescens) was most common, followed by the deermouse (Peromyscus maniculatus), and then by thegrasshopper mouse (Onychomys leucogaster), andanother pocket mouse (Perognathusflavus). In 1976,populations peaked in June and August; in contrast,the numbers were highest in the fall months during1975 and 1977. On sampling plots, rodent populationsrecovered from one month to the next. In anexperiment with desert seeds that are part of therodent's natural diet, the seeds proved not to be agood food for these animals; a more balanced diet wasneeded to sustain them. For the foraging ant populations,both species of Pogonomyrmex (P. rugosus andP. occidentalis), activity depended on temperature.The ants foraged between May and October; activitywas highest in August and September. Plant partsutilized included seeds, leaves of Mormon tea(Ephedra torryeana), and twigs. P. occidentalis enjoyedthe fruits of Chenopodium freemontii andOryzopsis hymenoides. P. raguosus also gatheredthese species, plus considerable numbers of Hilariajamesii fruits. Several of these species of plants havebeen recovered in archaeological sites; no studiesindicate the effect of competition between the humanand animal species.Jack Cully (1985a, 1985b) examined bird androdent communities in five areas: the bench, thepinon-juniper woodland, two floodplain areas (CasaChiquita and Puelo Bonito), and the wash. Data werecompared to determine differences among the numbersof animals and species. He recognized that birds arehighly mobile and that changes in their communitiesmay reflect changes in larger areas; e.g., the entirepark or areas outside of the park. Rodents, on theother hand, do not migrate, and therefore react to localconditions. Any changes in their populations probablyrelate to events within the park.Results of the trapping of mammalian speciesindicated that there were three conununities ofmanunals. The pinon-juniper woodland was composedpredominantly of cricetines and sciurids. Itconsistently had the highest number and diversity ofspecies. It also had several species not found in otherhabitats. The rock grasslands along the bench containedthe fewest species and had the lowestpopulations. It acted as a sink for excess productionin other areas. The wash complex, which includedboth floodplain areas, exhibited a mix of heteromyid,cricetine, and sciurid rodents.For birds, J. Cully (1985a) listed 132 speciesthat either he identified or were recorded in park files.Descriptions included whether the birds were residentsor migrants, and breeding species or not. The majoritywere migrants. Due to the presence of trees in theChaco Wash, he thought that the wash functioned asan oasis for migrants crossing the San Juan Basin.The lower diversity of breeding birds along the ChacoWash, when compared with either the San Juan Riveror the Gila River, was probably due to the limitedamounts of water in the intermittent stream (see alsoJ. Cully 1984a, 1984b). When he eliminated birdspecies that live only in deciduous riparian habitats,the results of his study were similar to those reportedfor similar habitats. They were also similar to resultsof the Chaco floodplain habitats in terms of diversityof species.Within the vegetative habits he studied in thecanyon, J. Cully (l985b:281) found that higherdiversity occurred during breeding season. The twofloodplain sites were similar in number of species(45). The pinon-juniper woodland had slightly fewerspecies (41). It also had the second highest diversityduring the breeding season and was low during wintermonths. The bench, with 12 species observed, had thelowest diversity in both winter and summer and wasunique when compared to the other habitation sites.When evaluating the results of bird and mammalstudies, J. Cully noted that bird populations andmammal popUlations could be both assigned to threehabitat-related communities: wash-floodplain, pinon/juniper woodland, and rock grassland of the bench.The diversity index that allowed him to distinguishamong bird popUlations did not work well for mammals.For the latter, a major within-habitat popUlationchange followed tremendous plant production in 1979.The bird popUlations were lower than the mammals,and the mammals were more closely tied to place.The high variance in popUlations of manunals,however, was not easily monitored in the two-yearstudy; long-term studies were needed to understandchanges in the ecology of this diverse area.The results of these studies suggest that speciesmay have been available to earlier inhabitants of thecanyon during different seasons and that variations in