The role of metapopulation dynamics in the persistence of rare species

Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




William T. Starmer


Dipodascus starmeri, Pichia cactophila, Stenocereus gummosus, cacti, yeast, Drosophila mojavensis

Subject Categories

Ecology and Evolutionary Biology


The relative importance of factors determining the distribution of a common and a rarer cactophilic yeast inhabiting the stem necroses of Stenocereus gummosus were investigated. In nature, Candida ingens (now known as Dipodascus starmeri), occurs in a low but steady proportion of rots, while Pichia cactophila is both locally abundant and widespread. Metapopulation theory can be used to understand those types of rarity in which local populations are distributed across patches of habitat.

Variability in species occurrence in nature was found primarily between plants in a locality. Yeast community composition and physiological abilities were persistent across time. Region, seasonality, and negative effects of other yeast species are not important determinants of the presence of C. ingens in S. gummosus necroses. Candida ingens was more frequent in rots of neutral pH.

Relative attractiveness, vector acquisition and deposition, and mortality of yeasts during transport by D. mojavensis were investigated. Flies are attracted by the necrosis-initiating bacterium and P. cactophila, but C. ingens was less attractive. Mortality did not differ between yeast species. Acquisition and deposition were generally proportionate to species presence in the substrate, however P. cactophila was deposited more variably than C. ingens.

Potential sources of differences in colonization success between a rarer and a common yeast species, including the presence of other species, cactus chemistry, and bacterial species presence were investigated. Colonization by C. ingens is unaffected by P. cactophila. Candida ingens' carrying capacity is highest when it arrives early and in large numbers. Variation in stem chemical composition influences the carrying capacity of P. cactophila but not C. ingens; initial growth rates of both yeasts are unaffected. The rate of increase and the carrying capacity of both yeasts are significantly affected by the bacterial species initiating the rot.

A simulation model detected small effects of differential attractiveness and cactus chemistry on the distribution of C. ingens. The proportion of patches inhabited by C. ingens was more sensitive to growth conditions, especially carrying capacity, under different bacterial regimes. Bacterial succession, along with deposition variability, are likely to contribute to the metapopulation persistence of a rarer yeast species.