Low levels of fruit production are thought to threaten the persistence of Asclepias meadii (Mead's milkweed) populations. We evaluated four hypotheses explaining the low fruit production in one population (herbivory, pollen limitation, resource shortages and rainfall) by collecting within-year data on stem size, levels of fruit initiation and mature fruit production and by considering among-year data on total fruit production and annual rainfall. In 2002, severe herbivory resulted in the death of about 63% of the population's flowering stems. Managers could increase fruit production by protecting stems from mammal herbivory. A path analysis revealed that leaf width and the number of fruits initiated were the most important predictors of mature fruit production. In 1991-2002 the previous year's total annual precipitation significantly affected the total number of fruits produced only in years of burning. Increasing the persistence of these long-lived plants to years of greater rainfall could have the most dramatic effects on fruit production and on survival of the species.
Milkweeds as a group have low rates of reproduction; abortion of initiated fruits is common and production of mature fruit occurs in 0.33-5.0% of flowers in different Asclepias species (Wyatt and Broyles, 1994). Low fruit production is a special concern for rare species of milkweed whose populations have already been diminished in size and number by habitat loss. The geographic range of A. meadii, a federally threatened tallgrass prairie milkweed, has been reduced substantially and most of the remaining populations are very small (United States Fish and Wildlife Service, 1988). Most of the 150 known populations of A. meadii exist in haymeadows where mowing occurs just before fruit maturation and effectively prevents sexual reproduction (Kettle et al., 2000a). In one of the few populations reproducing successfully, only 0.53% of flowers and 0.06% of stems per year produced fruit over 7 y of observations (Betz, 1989). This low rate of reproduction may be augmented somewhat by occasional asexual reproduction as underground connections between multiple stems of an individual break (Hayworth et al., 2001). However, as this obligate outcrossing species cannot reproduce sexually with genetically identical individuals (Tecic et al., 1998), this asexual reproduction probably does not increase the long-term viability of populations. Some populations may be so genetically homogeneous that sexual reproduction is already impossible (Tecic et al., 1998; Hayworth et al., 2001).
The unique pollination syndrome of milkweeds has encouraged volumes of research on their reproductive behavior (Lynch, 1977; Chaplin and Walker, 1982; Queller, 1985; Wyatt and Broyles, 1994). Because milkweed pollen is distributed in pollinaria (pairs of packets of pollen called pollinia that are joined by corpuscula), both pollen donation and fruit production can be quantified and reproduction can be studied from both the male and female perspectives (Wyatt and Broyles, 1994). Pollinia are inserted into one of five stigmatic chambers in the central portion of the milkweed flower (Wyatt and Broyles, 1994).
Many hypotheses have been investigated to explain the low rates of fruit production prevalent in milkweeds. Larger inflorescences are probably more attractive to pollinators and may receive more pollen (Stephenson, 1981; Broyles and Wyatt, 1995). As a result of this greater number of pollen depositions and initiated fruits, the plant is able to selectively abort fruits developed from inferior pollen or damaged by herbivory and, thus, possibly increase a surviving offspring's chance of success (Stephenson, 1981; Bookman, 1984; Queller, 1985). In years of greater-than-average pollen transfer or resource availability, milkweeds may be able to produce more fruits and utilize the maternal capabilities of more flowers (Stephenson, 1981; …