Browsing by Author "Fleming, Theodore H."

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    Co-pollinators and specialization in the pollinating seed-consumer mutualism between senita cacti and senita moths
    (Springer-Verlag, 2002) Fleming, Theodore H.; Holland, J. Nathaniel
    Specialization of a plant on a particular pollinator may not evolve if co-pollinators are effective and abundant. This is particularly evident if fruit set is resource limited and cannot be increased above the levels produced by the actions of co-pollinators. The pollinating seed-consuming interaction between senita cacti and senita moths in the Sonoran Desert presents a paradox because it exhibits many traits resembling those of the highly specialized yucca/yucca moth system, but also involves co-pollinators. For 6 years, we studied how contributions of nocturnal senita moths and diurnal co-pollinating bees to fruit set depended on resource and pollen limitation, time of flower closing, and the onset and phenology of flowering. Fruit set was typically resource limited. Fruit set of flowers exposed only to senita moths was not different from resource-limited fruit set of control flowers. When only co-pollinating bees were allowed to visit flowers, however, fruit set became pollen limited. Only in one year when fruit set was pollen limited were bees able to increase fruit set beyond the level resulting from senita moth pollination. High temperatures commonly induced flowers to close before sunrise so that diurnal bees were unable to visit flowers. This was particularly important from 1998 to 2000, when flowering did not begin until late in spring when temperatures were already high enough to induce flowers to close before sunrise. Bees were typically functionally redundant with senita moths; excluding bees from visiting flowers did not alter fruit set. Nevertheless, extreme specialization of floral traits to exclude co-pollinators has not evolved in senita, possibly because there are times when bees do increase fruit set. This can occur when senita moths are rare, fruit set is pollen limited, cool temperatures prevent flowers from closing before sunrise, and flowering begins early in spring.
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    The evolution of obligate pollination mutualisms: senita cactus and senita moth
    (Springer-Verlag, 1998) Holland, J. Nathaniel; Fleming, Theodore H.
    We report a new obligate pollination mu- tualism involving the senita cactus, Lophocereus schottii (Cactaceae, Pachyceereae), and the senita moth, Upiga virescens (Pyralidae, Glaphyriinae) in the Sonoran De- sert and discuss the evolution of specialized pollination mutualisms. L. schottii is a night-blooming, self-incom- patible columnar cactus. Beginning at sunset, its ¯owers are visited by U. virescens females, which collect pollen on specialized abdominal scales, actively deposit pollen on ¯ower stigmas, and oviposit a single egg on a ¯ower petal. Larvae spend 6 days eating ovules before exiting the fruit and pupating in a cactus branch. Hand-polli- nation and pollinator exclusion experiments at our study site near Bahia Kino, Sonora, Mexico, revealed that fruit set in L. schottii is likely to be resource limited. About 50% of hand-outcrossed and open-pollinated senita ¯owers abort by day 6 after ¯ower opening. Results of exclusion experiments indicated that senita moths accounted for 75% of open-pollinated fruit set in 1995 with two species of halictid bees accounting for the remaining fruit set. In 1996, ¯owers usually closed be- fore sunrise, and senita moths accounted for at least 90% of open-pollinated fruit set. The net outcome of the senita/senita moth interaction is mutualistic, with senita larvae destroying about 30% of the seeds resulting from pollination by senita moths. Comparison of the senita system with the yucca/yucca moth mutualism reveals many similarities, including reduced nectar production, active pollination, and limited seed destruction. The in- dependent evolution of many of the same features in the two systems suggests that a common pathway exists for the evolution of these highly specialized pollination mutualisms. Nocturnal ¯ower opening, self-incompati- ble breeding systems, and resource-limited fruit pro- duction appear to be important during this evolution
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    Geographic and population variation in pollinating seed-consuming interactions between senita cacti (Lophocereus schottii) and senita moths (Upiga virescens)
    (springer-Verlag, 1999) Fleming, Theodore H.; Holland, J. Nathaniel
    Interspecific interactions can vary within and among populations and geographic locations. This variation can subsequently influence the evolution and coevolution of species interactions. We investigated population and geographic variation in traits important to pollinating seed-consuming interactions between the senita cactus (Lophocereus schottii) and its obligate pollinating moth (Upiga virescens), both of which are geographically restricted to the Sonoran Desert. Female moths actively pollinate senita flowers and oviposit onto flowers. Their larvae consume developing seeds and fruit of flowers pollinated by females. Traits important to this interaction include fruit set from moth pollination, fruit survivorship, and costs of fruit consumption by larvae. We studied these traits for five populations at two widely separated geographic locations. On average, 37% of flowers set fruit, 22% of flowers produced mature fruit, and larvae consumed 25% of immature fruit pollinated by female senita moths. Senita cactus and senita moth interactions were strongly mutualistic in all populations that we studied. Although one population had statistically lower fruit set and fruit production than the other four, all five populations were qualitatively similar in fruit production, costs, and patterns of fruit survivorship. Hand-pollination experiments suggested that fruit set was resource-limited in all but this one population. Apparent pollen limitation in the one population explains the quantitative differences in fruit set and fruit survivorship among the populations. As predicted by theory and exemplified by the senita mutualism, specialized and/or obligate interactions vary little among populations and geographic locations.
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    Mutualistic interactions between Upiga virescens (Pyralidae), a pollinating seed-consumer, and Lophocereus schottii (Cactaceae)
    (Ecological Society of America, 1999) Fleming, Theodore H.; Holland, J. Nathaniel
    Pollinating seed-consuming interactions are rare, but include fig–fig wasp and yucca–yucca moth interactions, both of which are thought to be coevolved. Conditions favoring such mutualisms are poorly known but likely include plants and pollinators whose life cycles are synchronized. In this paper, we describe a new pollinating seed-consumer mutualism between a Sonoran Desert cactus, Lophocereus schottii (senita cactus), and a pyralid moth, Upiga virescens (senita moth). We compare this mutualism with the yucca mutualism in terms of life history traits, active pollination, and selective abortion. Senita cactus flowers were pollinated nearly exclusively by nocturnal senita moths, but a few halictid bees also pollinated flowers. Only 40% of flowers set fruit during the years of study, apparently due to resource limitation. All phases of the senita moth’s life history were associated with the senita cactus. During flower visitation, female senita moths collected pollen, actively pollinated flowers, and oviposited one egg. After flowers closed, emerging larvae bored into the tops of developing fruit, where they consumed seeds and fruit tissue. However, not all seeds/fruit were consumed by larvae because only 20% of eggs produced larvae that survived to be seed/fruit consumers. Senita cactus and senita moth interactions were mutualistic. Moths received food resources (seeds, fruit) for their progeny, and cacti had a 4.8 benefit-to-cost ratio; only 21% of developing fruit were destroyed by larvae. Life history traits important to this mutualism included low survival of senita moth eggs/larvae, several moth generations per flowering season, host specificity of senita moths, active pollination, oviposition into flowers, and limited seed/fruit consumption. Active pollination by senita moths in the presence of co-pollinators supports the prediction that active pollination can evolve during a period of coexistence with co-pollinators. The specialization of both senita and senita moths in the presence of co-pollinators makes the senita mutualism quite remarkable in comparison with fig–fig wasp and yucca– yucca moth mutualisms.
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    Sonoran Desert columnar cacti and the evolution of generalized pollination systems
    (Ecological Society of America, 2001) Hamrick, J.L.; Nason, John D.; Holland, J. Nathaniel; Sahley, Catherine T.; Fleming, Theodore H.
    We studied variation in flowering phenology, fruit and seed set, and the abundance of the pollinators of four species of night-blooming Sonoran Desert columnar cacti for up to eight years at one site in Mexico and one year at one site in Arizona. We determined how spatiotemporal variation in plant–pollinator interactions affects the evolution of generalized pollination systems. We conducted pollinator exclusion and hand pollination experiments to document annual variability in pollinator reliability and to determine whether pollination systems were redundant (different species are partially or totally substitutable) or complementary (different species have an additive effect on fruit set). The cacti we studied included three species with generalized pollination systems involving bats, birds, and bees (cardon, Pachycereus pringlei; saguaro, Carnegiea gigantea; and organ pipe, Stenocereus thurberi) and one specialized moth-pollinated species (senita, Lophocereus schottii). We predicted that the migratory lesser long-nosed bat, Leptonycteris curasoae, is a less reliable pollinator than birds and bees, and that cacti with generalized pollination systems have more variable flowering phenologies than the specialized species. Annual time of peak flowering and mean size of flower crops were relatively invariant in saguaro and organ pipe. Time of peak flowering in cardon varied by as much as six weeks, and mean flower crop size varied threefold over six years. In senita, peak flowering varied by as much as 5–8 wk among years. Peak numbers of the nectar bat L. curasoae varied among years, and bat density (0.9/ha) was an order of magnitude lower than that of cactus-visiting birds at both study sites. The abundance of migratory hummingbirds was also highly variable among years. Pollinator exclusion experiments indicated that bats were major pollinators of cardon, whereas diurnal visitors accounted for most fruit set in saguaro (except in 1995 when bats were most important) and organ pipe at our Mexican site; honeybees accounted for 64–87% of diurnal fruit set in these species. Annual variation in the contribution to fruit set by bats was substantially higher than that of diurnal pollinators in saguaro and organ pipe, but not in cardon. There was little geographic variation in the relative importance of nocturnal vs. diurnal pollinators in saguaro and senita, but bats were much more important for fruit set in organ pipe in Arizona than in Mexico. We generally detected no effect of different pollinators on number of seeds per fruit in any species. Annual variation in fruit set was lowest in saguaro, the species with the most diurnal pollination system, and highest in organ pipe, the species with the most generalized pollination system. Fruit set was strongly pollen limited only in females of cardon (a trioecious species) and in organ pipe (at both sites). The ‘‘missing’’ pollinators in both species are likely Leptonycteris bats. The pollination systems of saguaro and cardon were partially redundant, whereas that of organ pipe was complementary. The four species of cactus that we studied occur at the northern geographic limits of Mexican columnar cacti where many vertebrate pollinators are seasonal migrants. In the Sonoran Desert, variation in rainfall and spring temperatures affects timing of flowering and the extent of competition between cacti for pollinator visits and causes the relative importance of particular pollinators, especially Leptonycteris bats, for fruit set to vary annually. Under such conditions, selection has favored generalized pollination systems (as seen in organ pipe) or shifts from reliance primarily on nocturnal pollinators (as seen in cardon) to reliance primarily on diurnal pollinators (as seen in saguaro). Nonetheless, as exemplified by the senita–senita moth system, highly specialized pollination mutualisms can also evolve in this habitat in plants that rely on sedentary insects rather than migratory bats and birds for pollination