The Exclosure Plots at Saguaro National Park: A 40-Year Study Shapefiles

Three mammal-proof exclosures (15m x 15m), each with an adjacent non-fenced control area, were established in the Rincon Mountain District of Saguaro National Park in 1965. The purpose of the plots was to monitor the long-term influence of large and small mammal herbivory and granivory on survival of saguaros. In the 1960s, mammals were believed to be a major factor on survival of small saguaros in the park. All plants on each of the six plots were mapped and measured in 1965 and re-sampled in 1974. In 2005, we digitized the original 1965 maps and repeated the surveys using a total station. This report summarizes the changes in vegetation on treatment and controls plots over the 40-year period. We found no statistical differences between number of plants and total plant cover on control and treatments during the 40-year period, probably due to the small sample size of 3 paired plots. In addition, one pair of plots was burned over by a fire in 1994, which reduced diversity on both treatments and controls, and one plot appears to have been breached by small mammals. Overall, the data suggest that mammal herbivory may exert an observable influence on plant communities at the park. For example, number and cover of succulents, and cover of grasses, increased in exclosures compared to non-fenced control plots. In general it appears that factors that change with time (e.g. climate) have a larger impact on these vegetation communities than herbivory. The total number of plants per plot decreased on all plots between 1965 and 2005, while total cover either increased or stayed nearly the same. We found notable changes in certain families and species. Interestingly, one of the most dramatic examples of a single species change was Englemann’s prickly pear, which increased its cover nearly 1200% during the 40 year period. The influx of prickly pear, which appears to be related to climate change, may be an important factor in explaining the apparent decline in saguaro herbivory since the 1960s, as this species is a preferred food source for white-throated woodrats and other rodent herbivores.