, Volume 136, Issue 4, pp 551–557 | Cite as

Density-dependent seed set in the Haleakala silversword: evidence for an Allee effect

Population Ecology


Plant species may be subject to Allee effects if individuals experience a reduction in pollination services when populations are small or sparse. I examined temporal variation in reproductive success of the monocarpic Haleakala silversword (Argyroxiphium sandwicense subsp. macrocephalum) over five years, to determine if plants flowering out of synchrony with most of the population (i.e., in low flowering years) exhibited lower percent seed set than synchronously-flowering plants (i.e., those flowering in high flowering years). Through two pollination experiments conducted over multiple years, I also measured pollen limitation and self-incompatibility in this species. The number of flowering plants varied greatly among years, as did reproductive success. Percent seed set was significantly correlated with the number of plants flowering annually, such that plants flowering in high flowering years (1997 and 2001) exhibited significantly higher percent seed set than did plants flowering in low flowering years (1998–2000). In the 3-year pollen limitation study, plants flowering asynchronously were pollen-limited, whereas plants flowering synchronously were not. This species is strongly self-incompatible. Results of this study demonstrate that the Haleakala silversword experiences reduced reproductive success in low flowering years, and suggest that this Allee effect is pollinator-mediated. Allee effects in plants are an understudied yet potentially important force with implications for the population dynamics and conservation of rare species.


Argyroxiphium Monocarpy Pollen limitation Self-incompatibility Synchronous flowering 



I thank Clare Aslan for assistance in the field and laboratory, and Regis Ferriere for inspiring discussion. Much thanks to Lloyd Loope and Ron Nagata for logistical support at Haleakala National Park. Rob Robichaux, Judie Bronstein, Lucinda McDade, Molly Hunter, Nick Waser and an anonymous reviewer provided valuable feedback on earlier drafts of this manuscript. This project was funded by the National Science Foundation (Graduate Research Fellowship), the National Parks Conservation Association, and the Department of EEB at the University of Arizona. This manuscript was submitted to the University of Arizona in partial fulfillment of requirements for the degree of Doctor of Philosophy.


  1. Ågren J (1996) Population size, pollinator limitation, and seed set in the self-incompatible herb Lythrum salicaria. Ecology 77:1779–1790Google Scholar
  2. Aizen MA, Feinsinger P (1994) Forest fragmentation, pollination, and plant reproduction in a chaco dry forest, Argentina. Ecology 75:330–351Google Scholar
  3. Allee WC (1931) Animal aggregations, a study in general sociology. University of Chicago Press, ChicagoGoogle Scholar
  4. Allee WC (1938) The social life of animals. W.W. Norton, New YorkGoogle Scholar
  5. Augspurger CK (1980) Mass-flowering of a tropical shrub (Hybanthus prunifolius): influence on pollinator attraction and movement. Evolution 34:475–488Google Scholar
  6. Augspurger CK (1981) Reproductive synchrony of a tropical shrub: experimental studies on effects of pollinators and seed predators on Hybanthus prunifolius (Violaceae). Ecology 62:775–788Google Scholar
  7. Bosch M, Waser NM (1999) Effects of local density on pollination and reproduction in Delphinium nuttallianum and Aconitum columbianum (Ranunculaceae). Am J Bot 86:871–879PubMedGoogle Scholar
  8. Bosch M, Waser NM (2001) Experimental manipulation of plant density and its effect on pollination and reproduction of two confamilial montane herbs. Oecologia 126:76–83CrossRefGoogle Scholar
  9. Carr GD, Powell EA, Kyhos DW (1986) Self-incompatibility in the Hawaiian Madiinae (Compositae): an exception to Baker's Rule. Evolution 40:430–434Google Scholar
  10. Cole FR, Medeiros AC, Loope LL, Zuehlke WW (1992) Effects of the Argentine ant on arthropod fauna of Hawaiian high-elevation shrubland. Ecology 73:1313–1322Google Scholar
  11. Courchamp F, Macdonald DW (2001) Crucial importance of pack size in the African wild dog Lycaon pictus. Anim Conserv 4:169–174CrossRefGoogle Scholar
  12. Courchamp F, Clutton-Brock T, Grenfell B (1999) Inverse density dependence and the Allee effect. Trends Ecol Evol 14:405–410CrossRefPubMedGoogle Scholar
  13. de Jong TJ, Waser NM, Klinkhamer PGL (1993) Geitonogamy: the neglected side of selfing. Trends Ecol Evol 8:321–325Google Scholar
  14. Feinsinger P, Tiebout HM III, Young BE (1991) Do tropical bird-pollinated plants exhibit density-dependent interactions? Field experiments. Ecology 72:1953–1963Google Scholar
  15. Forsyth SA (2002) Demographic modeling of Hawaiian silverswords, and its implications for conservation. Ph.D. dissertation, Department of Ecology and Evolutionary Biology, University of Arizona, TucsonGoogle Scholar
  16. Foster RB (1977) Tachigalia versicolor is a suicidal neotropical tree. Nature 268:624–626Google Scholar
  17. Gilpin ME, Soule ME (1986) Minimum viable populations: processes of species extinction. In: Soule ME (ed) Conservation biology: the science of scarcity and diversity. Sinauer, Sunderland, Mass., pp 19–34Google Scholar
  18. Groom MJ (1998) Allee effects limit population viability of an annual plant. Am Nat 151:487–496CrossRefGoogle Scholar
  19. Hackney EE, McGraw JB (2001) Experimental demonstration of an Allee effect in American ginseng. Conserv Biol 15:129–136CrossRefGoogle Scholar
  20. Hainsworth FR, Wolf LL, Mercier T (1985) Pollen limitation in a monocarpic species, Ipomopsis aggregata. J Ecol 73:263–270Google Scholar
  21. Harder LD, Barrett SCH (1995) Mating cost of large floral displays in hermaphrodite plants. Nature 373:512–515Google Scholar
  22. Huenneke LF (1991) Ecological implications of genetic variation in plant populations. In: Falk DA, Holsinger KE (eds) Genetics and conservation of rare plants. Oxford University Press, New York, pp 31–44Google Scholar
  23. Ingvarsson PK, Lundberg S (1995) Pollinator functional response and plant population dynamics: pollinators as a limiting resource. Evol Ecol 9:421–428Google Scholar
  24. Janzen DH (1976) Why bamboos wait so long to flower. Annu Rev Ecol Syst 7:347–391Google Scholar
  25. Jennersten O (1988) Pollination in Dianthus deltoides (Caryophyllaceae): effects of habitat fragmentation on visitation and seed set. Conserv Biol 2:359–366Google Scholar
  26. Juvik SP, Juvik JO (1998) Atlas of Hawaii, 3rd edn. University of Hawaii Press, HonoluluGoogle Scholar
  27. Kearns CA, Inouye DW (1993) Techniques for pollination biologists. University Press of Colorado, Niwot, Colo.Google Scholar
  28. Kelly D (1994) The evolutionary ecology of mast seeding. Trends Ecol Evol 9:465–470Google Scholar
  29. Klinkhamer PGL, de Jong TJ, de Bruyn G (1989) Plant size and pollinator visitation in Cynoglossum officinale. Oikos 54:201–204Google Scholar
  30. Kobayashi, HK (1973) Ecology of the silversword, Argyroxiphium sandwicense DC. (Compositae), Haleakala Crater, Hawaii. Dissertation, Department of Botany, University of Hawaii, HonoluluGoogle Scholar
  31. Kunin WE (1993) Sex and the single mustard: population density and pollinator behavior effects on seed set. Ecology 74:2145–2160Google Scholar
  32. Kunin WE (1997) Population size and density effects in pollination: pollinator foraging and plant reproductive success in experimental arrays of Brassica kaber. J Ecol 85:225–234Google Scholar
  33. Kuussaari M, Saccheri I, Camara M, Hanski I (1998) Allee effect and population dynamics in the Glanville fritillary butterfly. Oikos 82:384–392Google Scholar
  34. Lamont BB, Klinkhamer PGL, Witkowski ETF (1993) Population fragmentation may reduce fertility to zero in Banksia goodii- a demonstration of the Allee effect. Oecologia 94:446–450Google Scholar
  35. Lee TD (1988) Patterns of fruit and seed production. In: Lovett-Doust J, Lovett-Doust L (eds) Plant reproductive ecology: patterns and strategies. Oxford University Press, New York, pp 179–202Google Scholar
  36. Loope LL, Crivellone CF (1986) Status of the Haleakala silversword: Past and present. Tech. Rep. 58. National Park Resource Studies Unit, University of Hawaii, HonoluluGoogle Scholar
  37. Loope LL, Medeiros AC (1994) Biotic interactions in Hawaiian high elevation ecosystems. In: Rundel PW, Smith AP, Meinzer FC (eds) Tropical alpine environments: plant form and function. Cambridge University Press, New York, pp 337–354Google Scholar
  38. Mavraganis K, Eckert CG (2001) Effects of population size and isolation on reproductive output in Aquilegia canadensis (Ranunculaceae). Oikos 95:300–310Google Scholar
  39. Mustajärvi K, Siikamäki P, Rytkönen S, Lammi A (2001) Consequences of plant population size and density for plant-pollinator interactions and plant performance. J Ecol 89:80–87CrossRefGoogle Scholar
  40. Ramsey M, Vaughton G (2000) Pollen quality limits seed set in Burchardia umbellata (Colchicaceae). Am J Bot 87:845–852PubMedGoogle Scholar
  41. Robichaux R, Bergfeld S, Bruegmann M, Canfield J, Moriyasu P, Rubenstein T, Tunison T, Warshauer F (2000) Reintroducing Hawaii's silverswords. Endangered Spec Bull 25:22–23Google Scholar
  42. Roll J, Mitchell RJ, Cabin RJ, Marshall DL (1997) Reproductive success increases with local density of conspecifics in a desert mustard (Lesquerella fendleri). Conserv Biol 11:738–746CrossRefGoogle Scholar
  43. Rundel PW, Witter MS (1994) Population dynamics and flowering in a Hawaiian alpine rosette plant, Argyroxiphium sandwicense. In: Rundel PW, Smith AP, Meinzer FC (eds) Tropical alpine environments: plant form and function. Cambridge University Press, New York, pp 295–306Google Scholar
  44. SAS (2000) JMP in statistical software, version 4.0.2 (academic). SAS, Cary, N.C.Google Scholar
  45. Schemske DW (1980) Floral ecology and hummingbird pollination of Combretum farinosum in Costa Rica. Biotropica 12:169–181Google Scholar
  46. Shaffer ML (1981) Minimum population sizes for species conservation. BioScience 31:131–134Google Scholar
  47. Sih A, Baltus MS (1987) Patch size, pollinator behavior, and pollinator limitation in catnip. Ecology 68:1679–1690Google Scholar
  48. Stephens PA, Sutherland WJ, Freckleton RP (1999) What is the Allee effect? Oikos 87:185–190Google Scholar
  49. Taylor OR Jr, Inouye DW (1985) Synchrony and periodicity of flowering in Frasera speciosa (Gentianaceae). Ecology 66:521–527Google Scholar
  50. U.S. Fish and Wildlife Service (USFWS) (1992) Determination of endangered or threatened status for 15 plants from the Island of Maui, HI. Federal Register 57:20772–20787Google Scholar
  51. Wolf AT, Harrison SP (2001) Effects of habitat size and patch isolation on reproductive success of the serpentine morning glory. Conserv Biol 15:111–121CrossRefGoogle Scholar
  52. Zimmerman M, Pyke GH (1988) Reproduction in Polemonium: assessing the factors limiting seed set. Am Nat 131:723–738CrossRefGoogle Scholar

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© Springer-Verlag 2003

Authors and Affiliations

  1. 1.Department of Ecology and Evolutionary Biology, Biological Sciences West 310University of ArizonaTucsonUSA

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