Pollinator Effectiveness, Pollinator Importance, and Pollen Dispersal in Star Cactus (Astrophytum asterias)




Blair, Andrew W.

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Star cactus (Astrophytum asterias) is a federally endangered plant and from known records is restricted to a single south Texas county and a small number of sites in northeastern Mexico. Star cactus is an obligate outcrosser that does not reproduce vegetatively, so all reproduction is the result of inter-plant pollen transfer by insects. By measuring seed set resulting from single pollinator visits, I evaluated the pollinator effectiveness (mean seed set/visit) and pollinator importance (effectiveness * visitation frequency) of insect species visiting flowers of star cactus. Results indicate that the most common visitor, Macrotera lobata, is a relatively ineffective pollinator, while the less common Diadasia rinconis is the most effective and important pollinator of star cactus. Two behavioral variables (visit duration, whether or not visitors landed on the stigma) were assessed as possible predictors of fruit set. While visit duration was not predictive of fruit set, there was a positive correlation between fruit set and whether a visitor landed on the stigma when entering the flower. In an additional study, fluorescent dye was used as a pollen analogue to track the distribution of pollen dispersal within a 1.9 hectare patch of star cactus. Dispersal distances between source and recipient plants were used to calculate estimates of genetic neighborhood size and area using Wright's neighborhood model. These neighborhood estimates (neighborhood size = 41.8 individuals, neighborhood area = 0.094 hectares) indicate the potential for population subdivision within the larger patch due to restricted pollen dispersal. The results of these studies will be used by conservation officials to inform management decisions.



haworthia, pollination, endangered plants, Texas, Mexico


Blair, A. W. (2007). Pollinator effectiveness, pollinator importance, and pollen dispersal in star cactus (Astrophytum asterias) (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.


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