Biogeography of Hanging Gardens on the Colorado Plateau
Infrequent seeps and springs along xeric canyon walls of the Colorado Plateau support hydrophytic habitats called hanging gardens. Vascular plant community heterogeneity, species-area relationship, species-occurrence pattern, and level of endemism were determined for 84 hanging gardens. Vascular plant species importance was used to develop hanging-garden vegetation types. Bee (Superfamily Apoidea) and small-mammal communities also were sampled for species richness, relative abundance, and species-occurrence patterns (the last, bees only). Hanging gardens in Arches, Canyonlands, and Zion National Parks, Dinosaur National Monument, and Glen Canyon National Recreation Area were equally large and species-rich compared to the small, species-poor gardens in Capitol Reef National Park while those in Natural Bridges National Monument were intermediate in size and richness. Hanging gardens that have a relatively high number of plant species and low dominance proportions were judged to be more diverse than hanging gardens with a low number of species and high dominance proportions. The plant species-area relationship was indicative of insular communities. Five hanging-garden vegetation types were developed: fern, fern-columbine, columbine, reedgrass, and fern-thistle. The species-occurrence curves fit a logarithmic series distribution for plants, apoid bees, and Bombus. This does not support the core-satellite hypothesis prediction of bimodal species-occurrence curves. Twelve plant taxa found on hanging gardens were endemic to the Colorado Plateau; seven of these were endemic to hanging garden habitats. Arches NP had a significantly higher number of hanging-garden endemic taxa per garden than the other six parks, but abundance of hanging-garden endemic taxa per garden did not differ among the parks. Endemic plants on Zion NP hanging gardens form a distinct set of species compared to the other six parks. Six bumble bee (Bombus) species were netted as floral visitors, with B. morrisoni being the most frequently collected, most widespread, and most polylectic. Five Bombus species have biogeographic affinities with boreal and montane regions of North America. Apoid bees were more abundant on hanging gardens than off. However, there was no difference in small mammal trapping success between on and off garden.
Fowler, James Floyd, "Biogeography of Hanging Gardens on the Colorado Plateau" (1995). Canyonlands Research Bibliography. Paper 36.