Habitat
Limestone soils of rocky hills in the desert. Chihuahuan Desert populations are common between 750 and 1675 m elevation of a variety of topographic forms and on both igneous and sedimentary substrates, most commonly on limestone and gypsum soils (Baker 2007). Arizona populations (var. nicholii) occur in Parkinsonia microphylla scrub with Ambrosia deltoidea, Carnegiea gigantea, Cylindropuntia acanthocarpa, Encelia farinosa, Fouquieria splendens, Krameria grayi, and Opuntia phaeacantha; and individuals occur between 750 and 1250 m elevation exclusively on limestone substrates of dissected alluvial fans, inclined terraces and saddles between ridges (Baker 2007).
Reproduction
A small cactus species, growing only 10-12 inches high and about 6 inches wide, Echinocactus horizonthalonius have seeds that are cylindrical and roll down the side of the parent plant. Self dispersal is therefore evident, and what may look like one plant is actually the parent plant, acting as a nurse plant, with offspring surrounding it (Thornton 2007). Long distance dispersal by birds is not suspected given that fruits are heavily encased in a cage of spines (Thornton 2007). Finally, serotiny, or delayed self-dispersal, is documented in the genus Echinocactus, and is a bet-hedging strategy employed by plants in arid environments, so that seeds are dispersed when environmental conditions are favorable for germination (Peters et al. 2009).