Timothy P. Spira

The Ecology of Geitonogamous Pollination

Working as a natural historian in the 1700s, C.K. Sprengel wrote a pioneering book demonstrating that many hermaphroditic species require pollinator visits in order to produce seed (see Chapters 1 and 2). He did not provide a scientific explanation as to why cross-pollination is important, but in the next century Darwin, H. and F. Muller, and others proposed that various outcrossing mechanisms have evolved to avoid selfing and the consequences of inbreeding (Darwin, 1876; see Baker, 1983). Darwin also recognized that the potential for selfing is greatest in species with massive floral displays because having many flowers promotes the transfer of self-pollen to other flowers on the same genetic individual (geitonogamy). Following Darwin’s lead, many authors have suggested that the avoidance of selfing has been a major factor in the evolution of traits such as dioecy, self-incompatibility, monoecy, temporal separation of male and female organs (dichogamy), spatial separation of anthers and stigmas within flowers (herkogamy), and having few open flowers per day (see reviews by Arroyo, 1976; Lloyd, 1979; Bawa and Beach, 1981; Willson, 1983; Wyatt, 1983; Richards, 1986; Charlesworth and Charlesworth, 1987; Thomson and Brunet, 1990; de Jong, et al., 1992a; Harder and Barrett 1995; Hodges, 1995; also see Chapters 6, 8, and 14).

POLLEN-TUBE COMPETITION AND MALE FITNESS IN HIBISCUS MOSCHEUTOS

The stigmas of animal‐pollinated flowers often capture more pollen than is needed to fertilize all available ovules, and mixed‐donor pollen loads are probably common. When this is the case, variation in average pollen‐tube growth rates can potentially affect the number of seeds sired by a given plant. Despite considerable interest in effects of postpollination processes on male fitness, little is known about the extent of variation in pollen performance among plants from natural populations. To examine this question in Hibiscus moscheutos (rose mallow), we conducted mixed‐donor hand‐pollination experiments with 39 pollen donors bearing distinctive isozyme markers. Pairs of competing donors were compared on sets of 11 to 15 recipient plants per pair. These donors often differed in the proportions of seeds they sired, with the maximum deviation from an expected ratio of 50:50 being 68:32. Furthermore, three intensively studied plants exhibited consistent trends in relative pollen performance when each was tested against (1) the same three competitors, and (2) groups of 14 competitors chosen at random from the study population. In a separate experiment, we investigated the effects of salinity stress and high soil nutrients on pollen performance. These environmental factors had anticipated effects on leaf production, flower production, and petal length, but style length and (most importantly) the number of seeds sired relative to a standard pollen donor were not affected. In summary, this study provides the strongest evidence to date that pollen‐tube competitive ability varies among coexisting plants and may be an important component of male fitness in plants.

Ant-seed mutualisms: Can red imported fire ants sour the relationship?

Invasion by the red imported fire ant, Solenopsis invicta, has had negative impacts on individual animal and plant species, but little is known about how S. invicta affects complex mutualistic relationships. In some eastern forests of North America, 30% of herbaceous species have ant-dispersed seeds. We conducted experiments to determine if fire ants are attracted to seeds of these plant species and assessed the amount of scarification or damage that results from handling by fire ants. Fire ants removed nearly 100% of seeds of the ant-dispersed plants Trillium undulatum, T. discolor, T. catesbaei, Viola rotundifolia, and Sanguinaria canadensis. In recovered seeds fed to ant colonies, fire ants scarified 80% of S. canadensis seeds and destroyed 86% of V. rotundifolia seeds. Our study is the first to document that red imported fire ants are attracted to and remove seeds of species adapted for ant dispersal. Moreover, fire ants might damage these seeds and discard them in sites unfavorable for germination and seedling establishment.

Consequences of Forest Clear-Cuts for Native and Nonindigenous Ants (Hymenoptera: Formicidae)

Abstract Currently, the southern United States produces more timber than any other region in the world. Entire timber stands are removed through a harvesting method called clear-cutting. This common forestry practice may lead to the replacement of native ant communities with invasive, nonindigenous species. In four deciduous forest sites in South Carolina, we monitored the change in ant species richness, diversity, and abundance immediately after forest clearing for a period of 15 mo to 2 yr and determined the incidence of colonization of the red imported fire ant Solenopsis invicta into these four newly disturbed sites. Each site consisted of an uncut, forested plot and a logged, pine-planted plot. Fire ants were collected in clear-cuts as early as 3 mo postcutting, and by the end of the experiment, they were found in all four treatment sites. Our study is the first to document, through a controlled experiment, that clear-cutting alters ant species assemblages by increasing S. invicta and Pheidole spp. populations and significantly reducing native ant numbers. Long-term studies are needed to assess how replacing native deciduous forests with pine monocultures affects ant assemblages.

Comparative reproductive biology of alpine biennial and perennial gentians (Gentiana: Gentianaceae) in California

Reproductive characteristics of three sympatric species of Gentiana exhibiting perennial and non-perennial life histories were studied in alpine meadows of the White Mountains of California during three consecutive years. High fruit and seed set and the production of a relatively large number of seeds characterized the alpine biennials, Gentiana tenella and G. prostrata. In contrast, fruit and seed set were considerably reduced and yearly seed production was relatively low in the alpine perennial, G. newberryi. Successful seed production in the biennial species was a result of low rates of flower predation, self-pollination, and the allocation of a comparatively high proportion of biomass to reproductive structures. Seed production in the perennial species was limited by high rates of flower predation, insufficient amounts of pollen reaching stigmas, an unusually late flowering period, and by the relatively small proportion of biomass allocated to reproduction.

The timing and effectiveness of sequential pollinations in Hibiscus moscheutos

In many species with animal-pollinated flowers, pollen arrives on the stigma in pulses and late-arriving pollen may be precluded from fertilizing ovules. When seed set per fruit is not pollen limited, the fate of pollen from later cohorts is affected by the time between pollinator visits, variation in pollen tube growth rates, and the amounts of pollen deposited relative to the availability of stigmatic area and unfertilized ovules. In a natural population of Hibiscus moscheutos, we found that consecutive pollinator visits to individual flowers occurred within 15 min of each other in more than half of our observations. We then conducted hand-pollination experiments using equal doses of early and late pollen bearing unique allozyme markers for paternity analysis (each dose was more than sufficient to result in full seed set). When pollen was applied 15 or 30 min after an earlier pollen load, the proportions of seeds sired by late-arriving pollen were reduced by 13–30% and 21–57%, respecitively. A few pollen-tubes from the late pollen load sired seeds even when a large dose of pollen was applied 1–2 h earlier, suggesting that the performance of pollen grains within each cohort was highly variable. The advantage of arriving early was greater when the first pollen load was applied at 0930 hours as compared with 1130 hours. We found no effect of previous pollination on the growth rates of late-arriving pollen-tubes. Taken together, these results demonstrate that pollen from later visits can compete with pollen from earlier visits, but the success of late-arriving pollen typically declines to very low levels after an interval of about 2 h. Given that “surplus” pollen often arrives on stigmas by mid-morning, we conclude that male reproductive success is more likely to be affected by the timing of pollen dispersal and pollen-tube competitive ability than by the total amount of pollen that is exported from flowers.

Germination, recruitment and survival in the weedy annual Medicago polymorpha in successive wet and dry years

-Germination, establishment and survival in Medicago polymorpha, a weedy winter annual, were studied during 2 successive seasons in California. Seeds were dormant when produced in spring and remained largely dormant following exposure to summer conditions. Approximately 40% of viable seeds in the seed bank were carried over from year to year. Consequently, autumn recruitment of seedlings is due in large part to germination of seeds produced in previous years. Seedling emergence was mostly simultaneous following the first substantial autumn rains. Survival to reproduction differed between years; few individuals produced seeds prior to senescence in the dry year, whereas in the wet year most overwintering individuals successfully produced fruits.

Biodiversity of Fungi in Red Imported Fire Ant (Hymenoptera: Formicidae) Mounds

Abstract The red imported fire ant, Solenopsis invicta Buren, became established in North America more than 70 yr ago, and it currently occupies most of the southeastern United States. Fire ants change the physical and chemical components of soil, which likely influence soil fungi in ant mounds. To determine the effects of fire ants on soil fungi, we sampled soil from fire ant mounds and the surrounding nonmound soil. In addition, we sampled soil from the nests of the native ant Aphaenogaster texana carolinensis Wheeler. We found that both fire ant mounds and native ant nests had greater fungal abundance but lower species richness and diversity than nonmound soil. Fire ant mounds contained 19 times more colony forming units (cfu g−1) than adjacent soil; however, nonmound soil had more than twice the number of fungal species. Two species (Papulaspora byssina Hotson and Penicillium janthinellum Biourge) made up the majority (54.5 and 19.2% relative colony frequencies, respectively) of fungi in fire ant mounds. These high proportions of limited numbers of fungal species in fire ant mounds indicate that only some species are tolerant to and thrive in mound conditions. Alternatively, fire ants might not selectively remove these fungi from their mounds. Given the high densities of fire ants and their frequent mound movements, changes in soil fungal communities might have lasting impacts on soil conditions. In addition, we suggest that differences between fungal communities in soil from native and non-native ant colonies might indirectly influence ant-mediated seed dispersal by affecting seedling survival.

YELLOW JACKETS MAY BE AN UNDERESTIMATED COMPONENT OF AN ANT-SEED MUTUALISM

Abstract Yellow jackets (Hymenoptera: Vespidae) are attracted to the typically ant-dispersed seeds of trilliums and will take seeds from ants in the genus Aphaenogaster. To determine if yellow jacket, Vespula maculifrons (Buysson), presence interferes with seed foraging by ants, we presented seeds of Trillium discolor Wray to three species (A. texana carolinensis Wheeler, Formica schaufussi Mayr, and Solenopsis invicta Buren) of seed-carrying ants in areas where vespids were present or excluded. We found that interspecific aggression between yellow jackets and ants is species specific. Vespid presence decreased average foraging time and increased foraging efficiency of two of the three ant species studied, a situation that might reflect competition for a limited food source. We also found that yellow jackets removed more seeds than ants, suggestive that vespids are important, albeit underestimated, components of ant-seed mutualisms.

Germination as a Component of Pollen Competitive Ability in Hibiscus moscheutos

Pollen tube competition is widely cited as a possible mechanism for nonrandom fertilization and gametophytic selection (see recent reviews by Snow, 1986; Mulcahy and Mulcahy, 1987; Walsh and Charlesworth, 1991). Two general approaches have been used to vary the intensity of pollen competition and study effects on progeny fitness. One involves varying the stylar distance which pollen tubes must traverse to reach the ovules. This technique has been extremely useful in studies of maize, for example, which has silks of vastly different lengths on the same ear (e.g., Sari-Gorla and coworkers, this volume). Competition among pollen tubes should be strongest in the longest silks.