JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecological Monographs.Abstract. In cloud forest at Monteverde, Costa Rica, I investigated the reproductive consequences of avian seed dispersal for three species of gap-dependent plants: Phytolacca rivinoides (Phytolaccaceae), Witheringia solanacea, and W. coccoloboides (Solanaceae). Of six bird species that consumed fruits of these plants, only three (Myadestes melanops [Muscicapidae], Phainoptila melanoxantha [Ptilogonatidae], and Semnornisfrantzii [Capitonidae]), dispersed seeds in viable condition. Other species discarded most seeds before swallowing fruit pulp, or destroyed seeds in the gut. I estimated the quality of dispersal service provided by the "legitimate" disperser species by comparing the seed shadows they produced with the spatial and temporal distributions of establishment sites for the plants.I estimated seed shadows from data on gut passage rates of seeds and on movement patterns of radio-tracked birds. Seed shadows produced by all three effective dispersers were extensive, with few seeds deposited near the parent plant, and some seeds moved >500 m.Seeds of the species examined establish in forest gaps formed by treefalls or landslides. Establishment success varies with gap size and age, but the relationship is different for each species; both Witheringia species establish well in gaps as small as 15 m2 or as old as 6 mo, whereas P. rivinoides establishes well only in gaps > 70 m2 or < 4 mo old. Consequently, establishment sites for all three plants are both rare and ephemeral, but to differing degrees.Seeds that are not dispersed to suitable habitat patches can remain dormant in the soil until a gap is formed overhead; seed dormancy experiments showed no significant decrease in viability of seeds buried for up to 27 mo.To determine consequences of dispersal and dormancy for plant reproductive success, I developed a simulation model that uses data on seed shadows, germination requirements, seed dormancy, and forest dynamic processes to estimate reproductive output (total offspring produced during an individual plant's lifetime) and relative "fitness" (an estimator that discounts the contribution of offspring produced after a long period of dormancy). Results show that (1) dispersal by any of the three legitimate dispersers increases reproductive output 16-36 times, even without seed dormancy. (2) Dormancy capabilities up to 2 yr enhance both reproductive output and "fitness," but greater capabilities increase only reproductive output. (3) Without dispersal, dormancy has little effect on either reproductive output or fitness. Thus, both dispersal and dormancy ("d...

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The Association for Tropical Biology and Conservation is collaborating with JSTOR to digitize, preserve and extend access to Biotropica.

Abstract. In a cloud forest at Monteverde, Costa Rica, we examined pollen loads received by self-compatible flowers of two pairs of plant species pollinated by hummingbirds: Hansteinia blepharorachis and Razisea spicata (Acanthaceae), and Besleria triflora and Drymonia rubra (Gesneriaceae). Each pair consisted of one species (Hansteinia or Besleria) pollinated by short-billed hummingbirds and a related species (Razisea or Drymonia) pollinated by long-billed hummingbirds. At three different times per species, separated by 1-3 mo, we examined flowers on 28-40 focal plants from a wide variety of floral neighborhoods, ranging from plants isolated from conspecifics, either by distance or by other flowering species pollinated by the same hummingbirds, to plants surrounded by conspecifics.Because short-billed hummingbirds often restrict foraging to areas ofhigh flower density, and because short-tubed flowers adapted for hummingbirds often have similar pollen placement, we predicted that short-tubed flowers isolated from conspecifics would receive fewer conspecific grains and more heterospecific grains than short-tubed flowers surrounded by conspecifics. Because long-billed hummingbirds often forage over large areas and because long-tubed flowers adapted for hummingbirds tend to diverge in pollen placement, we predicted that pollination of long-tubed flowers would be relatively unaffected by floral neighborhood.Effects on pollen loads of floral neighborhood (nearness to or isolation from other flowers) followed few patterns consistent with our prediction or with conventional theory. (1) There were no consistent effects of floral neighborhoods on numbers of heterospecific grains deposited on stigmas; in all four species, regardless of corolla length, effects of particular neighborhood variables (as determined with stepwise multiple regression) were as likely to run exactly counter to conventional models as to corroborate models. (2) In none of the 12 sampling runs did increases in absolute densities of neighboring heterospecific flowers adversely affect pollination. (3) However, in two runs, loads of conspecific grains increased with increases in the absolute density of neighboring conspeci:fic flowers, and/or (in three runs) with increases in their relative density (proportion of conspecifics among neighboring flowers). These runs all involved short-flowered species rather than long-flowered species, tending to confirm our initial prediction, but half the sampling runs, even of short-flowered species, failed to show any density-dependent effects from neighboring flowers pollinated by the same hummingbirds.Flowers frequently received fewer conspeci:fic grains than they had ovules to be fertilized. Therefore, the potential existed for floral neighborhoods to affect seed set and fitness of plants. Nevertheless, even though neotropical hummingbird-pollinated flowers have been cited as examples of species whose flowering peaks are displaced through competition for pollination, competitive effects from neighboring heterospeci...

To explore how plants may influence dispersal of their own seeds by manipulating the behavior and physiology of their dispersers, we studied the effect of a soluble chemical (or chemicals) in the fruits of Witheringia solanacea (Solanaceae), a Costa Rican cloud forest shrub, on passage of its seeds through the guts of one of its major dispersers, the Black—faced Solitaire, Myadestes melanops (Muscicapidae: Turdinae). Using artificial fruits containing natural seeds, we found that the presence of a crude pulp extract reduced the median seed retention time by nearly 50%. Estimation of seed dispersal distance as a function of retention time suggested that more rapid seed passage results in shorter average dispersal distances, especially for seeds retained <20 min. At the same time, germination trials revealed that seeds voided rapidly were far more likely to germinate than those remaining longer in Myadestes guts. We propose that “laxative” chemical(s) in Witheringia fruits balance these positive and negative consequences of ingestion by Myadestes.

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