Determinants of fruit set in Yucca whipplei: Reproductive expenditure vs. pollinator availability

Abstract

The number of flowers produced by inflorescences of Yucca whipplei (Agavaceae) consistently exceeds the number of fruits produced by about one order of magnitude. To determine the factors responsible for low fruit set, the relation between pollinator availability, the amount of resources spent on reproduction (as indicated by inflorescence size), and the number of fruits matured was studied during 1978 and 1979 at 18 locations in chaparral, coastal sage scrub, and desert scrub communities of southern California.The following results support the conclusion that pollinators do not usually limit fruit production in Yucca whipplei. Rather, fruit production is limited by the amount of resources available to support developing fruits. (1) Fruit production is positively correlated with inflorescence size both within and between populations. The average size of inflorescence for a population is an excellent predictor of mean fruit production. Furthermore, 54% of the total variance in fruit production of individual plants can be explained by inflorescence size. (2) In contrast, although fruit production within most populations is positively correlated with an index of the number of pollinator visits to an inflorescence, the relative abundance of pollinators for a population is a poor predictor of mean fruit production, and only 9% of the total variance in fruit production can be explained by the visitation index. Furthermore, at four sites studied for two years, there was little change in average inflorescence size or fruit production from 1978 to 1979, despite large differences in relative abundance of pollinators at each of the sites. (3) Based on geographic proximity, and physiographic and vegetational similarities, study sites were grouped into regional clusters. Both inflorescence size and fruit production varied considerably between regions. Of the total variation in fruit production, 27% can be attributed to differences between regions. Most of this variation is the result of regional differences in inflorescence size, which in turn influence fruit production.Why does Yucca whipplei produce such large inflorescences if so few fruits can be supported? Two relevant hypotheses are discussed: (1) the floral display is the result of selection for pollen dissemination at the expense of fruit set; and (2) the floral display is the result of selection for a bet-hedging strategy either to increase the probability of adequate pollination when pollinators are unusually rare, or to allow individuals to support more fruits when resources are unusually abundant.

Phenotypic variation of flowering and vegetative morphological traits along the distribution for the endemic species Yucca capensis (Agavaceae)

“…and Hesperoyucca whipplei (Torr.) Baker ex Trel., a strong association has been detected between precipitation and infl orescence, fl ower and fruit production (Smith and Ludwig, 1976;Udovic, 1981). This supports the hypothesis that levels of rainfall, in conjunction with humidity retention in microhabitats, could explain the variations in fl owering patterns within and between the populations observed in this study.…”

Geographic and population variation in pollinating seed-consuming interactions between senita cacti ( Lophocereus schottii ) and senita moths ( Upiga virescens )

“…Resource-limited fruit set is common in senita cacti and species of yucca (Udovic 1981;Udovic and Aker 1981;Fuller 1990;Pellmyr and Huth 1994;Richter and Weis 1995;Huth and Pellmyr 1997;Fleming and Holland 1998;Holland and Fleming 1999; Fig. 1).…”

“…On average, only 37% (range 30-46%) of flowers set fruit. Similarly, on average 8% (range 3-14%) of flowers set fruit in different Yucca populations and species throughout their geographic range (Schaffer and Schaffer 1979;Udovic 1981;Udovic and Aker 1981;Aker 1982aAker ,1982bKingsolver 1984;Fuller 1990;James et al 1993James et al , 1994Pellmyr and Huth 1994;Huth and Pellmyr 1997;Addicott 1998).…”

“…It is unlikely that the abundance of resources responsible for limiting fruit set is similar among populations and species across the wide geographic distribution of Yucca or throughout the Sonoran Desert where senita occur. Alternatively, because survival of pollinating seedconsumers is directly influenced by fruit abortions (Richter and Weis 1995;Wilson and Addicott 1998;Holland and Fleming 1999), plants may be able to regulate pollinator populations and fruit production through low levels of fruit set (Udovic 1981;J.N.Holland and D.L. DeAngelis, unpublished work).…”

The ecological function of extrafloral nectaries: herbivore deterrence by visiting ants and reproductive output in Caryocar brasiliense (Caryocaraceae)

“…Finally, the surplus number of flowers and juvenile fruits observed in ant-visited control plants may be advantageous for Caryocar shrubs in three ways. First, a large floral display may increase the plant's attractiveness to pollinators, this being particularly relevant at periods of low pollinator availability (Augspurger 1980;Udovic 1981). Indeed, Gribel & Hay (1993) report that C. brasiliense trees bearing increased numbers of flowers are more attractive to potential bat pollinators in central Brazil.…”