Two small Sonoran Desert cacti, Mammillaria microcarpa and Echinocereus englemannii, are commonly found beneath canopies of the larger, tree-like cactus Opuntia fulgida. The mechanism leading to this distribution pattern is incidental to the mode of reproduction in O. fulgida. Opuntia fulgida propagates by means of easily-detached, spine-covered stem joints that accumulate beneath the parent plant. These accumulations of spines apparently deter mammalian herbivores that otherwise consume succulent tissues of the smaller cacti. Such incidental effects are little studied, but they may contribute substantially to structure within plant communities.