Avian Seed Dispersal of Three Neotropical Gap‐Dependent Plants

E-ISSN： 1557-7015|58|4|271-298

ISSN： 0012-9615

Source： Ecological Monographs, Vol.58, Iss.4, 1988-02, pp. : 271-298

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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 Semnornis frantzii [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 ("dispersal" in time) are essential to these gap—dependent plants, but long—term seed dormancy may be relatively unimportant.