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Effects of Dispersers, Gaps, and Predators on Dormancy and Seedling Emergence in a Tropical Herb

Horvitz, Carol C., Schemske, Douglas W.
Ecology 1994 v.75 no.7 pp. 1949-1958
Calathea, El Nino, Formicidae, field experimentation, germination, granivores, insects, invertebrates, mortality, pathogens, predators, seed dormancy, seed predation, seedling emergence, seedlings, seeds, temporal variation, vertebrates
The influence of biotic and abiotic factors on seed fates are exceptionally difficult to analyze in natural populations because it is not usually possible to identify an emerging seedling with its history as a seed. Large seed size and the unusual feature that the seed body remains attached to the new seedling in the ant—dispersed herb Calathea ovandensis provided an opportunity for field experiments that examine the influence of (1) seed dispersers, (2) treefall gaps, and (3) post—dispersal predators on seed survival, seed dormancy, seedling emergence, and seedling distributions. In a seed depot experiment, seed—dispersing ants significantly affected the seedling emergence patterns in three ways. Seeds with rewards for ants: (a) had improved seedling emergence, by 1.6—fold (17.4 vs. 10.6%); (b) gave rise to seedlings with a mean dispersal distance that was threefold farther (34 vs. 114 cm); and (c) generated a non—leptokurtic seedling shadow (kurtosis: —0.60, not significant vs. 10.84, P < 0.001) in contrast to seeds without ant rewards. There was significant spatial variation in the effect of ant rewards on seedling emergence and also temporal variation in seedling emergence, the greatest number of seedlings emerging in an el Nino year. In two germination box experiments: (1) treefall gaps enhanced seedling emergence 180—fold (72.4 vs 0.4%) compared to the deep shade, and ungerminated seeds in the deep shade were mostly dormant, not dead; (2) post—dispersal seed predation was very low (overall mean of 6.2%), with considerable microspatial variability, and pathogens and small invertebrates accounted for only one—fourth as much seed mortality as larger (vertebrate and large insect) post—dispersal predators. Seedling emergence in this ant—dispersed herb with a relatively long—lived seed bank depends upon a complex of factors, suggesting an interaction of the effects of biotic dispersal to local safe microsites with larger scale effects of spatial and temporal variation in the abiotic environment.