Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

J. V. Remsen, Jr


I studied the pollination ecology of a widespread desert shrub, ocotillo (Fouquieria splendens), and some of the consequences of geographic variation in availability of pollinators. I studied ocotillo in the Chihuahuan Desert of western Texas (Big Bend National Park) for three flowering seasons, and in the Sonoran Desert of southern California (Anza-Borrego Desert State Park) for parts of two flowering seasons. In both areas, ocotillo flowers profusely for one month each year, and requires outcross pollination to mature more than 5% of its potential seed crop. In Texas, carpenter bees (Xylocopa californica arizonensis) and their nest plants (species of Agave, Dasylirion, and Yucca) are common throughout the habitats occupied by ocotillo, whereas hummingbirds (Calothorax lucifer, Archilochus alexandri) have localized distributions. In two of three years, carpenter bees thoroughly harvested the nectar and pollen of ocotillo, resulting in high fruit set and seed set. Female carpenter bees provisioned spring nests with pollen from F. splendens and mesquite (Prosopis glandulosa) and with nectar sugar; F. splendens accounted for 69% of pollen volume in an average nest. Ocotillo's direct contribution to carpenter bee fecundity and an abundance of nest plants help maintain a high bee density in Texas. In California, carpenter bees nest only in palms (Washingtonia filifera) and are unavailable to many ocotillo populations. Costa's Hummingbird (Calypte costae) is widespread, and migrant Rufous Hummingbirds (Selasphorus rufus) and orioles (Icterus spp.) are sometimes common. Ocotillo populations were poorly pollinated and only a small percentage of their nectar was harvested, except during a brief period when migrant hummingbirds were abundant. Density of breeding Costa's Hummingbirds is much lower than the March nectar supply would support. Two aspects of hummingbird biology may be responsible for the disparity: a low reproductive rate, and high daily energy requirements throughout the year. Ocotillo is engaged in a strong mutualism with carpenter bees in Texas, but interacts weakly with hummingbirds and orioles in California. This suggests that biological idiosyncrasies of flower-visiting animals are important in determining whether interactions between plants and potential pollinators become effective mutualisms.