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Experimental Tests of the Spatiotemporal Scale of Seed Predation in Mast-Fruiting Dipterocarpaceae

L. M. Curran and C. O. Webb
Ecological Monographs
Vol. 70, No. 1 (Feb., 2000), pp. 129-148
Published by: Wiley
DOI: 10.2307/2657170
Stable URL: http://www.jstor.org/stable/2657170
Page Count: 20
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Experimental Tests of the Spatiotemporal Scale of Seed Predation in Mast-Fruiting Dipterocarpaceae
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Abstract

Mast-fruiting Dipterocarpaceae exhibit highly synchronous, interspecific seedfall at irregular, multiyear intervals. To investigate how the temporal pattern of seedfall affects dipterocarp seed and seedling survival, in both a logged and a primary lowland tropical forest, we planted Shorea stenoptera Burck seeds in the last three weeks of a 12-wk synchronous dipterocarp seedfall during a major community mast-fruiting event in West Kalimantan, Indonesia. As a result of commercial timber harvest of dipterocarp individuals eight years before, total dipterocarp seed production in the logged site was only 23% of that in the primary forest. At both sites, an average of 35 kg of seed was sown across a large area (≥1 km2) to examine the spatial pattern of seed destruction. During the period in which "natural" community mast seed was available to predators, 92% and 99% of experimentally sown seed escaped predators in logged and in primary forest, respectively. After regional seed resources were exhausted, nomadic vertebrates (primarily the bearded pig, Sus barbatus) were observed in both forest areas, and all ungerminated seed was destroyed. Seed predators arrived earlier in the logged area, before most experimentally sown seed had germinated, and the logged site experienced greater seed loss to vertebrates than did the primary forest. Because nomadic seed predators were absent during peak fruit fall of naturally occurring communities at both study sites, there was no evidence of local predator satiation. Rather, experimentally sown seed escaped predation because of rapid germination before predator arrival, as opposed to being ignored by satiated predators. Seed escape was more dependent on the late arrival of pigs than on the amount of local seed production. There was no significant spatial autocorrelation of seed predation. All remaining seed at the scale of the experiment (>1 km2) was destroyed by predators. These findings suggest that satiation of nomadic predators occurs at the landscape scale. Postdispersal seed predators caused significantly greater seed destruction in the experimentally sown seed populations than in naturally dispersed, mast-fruiting communities at both sites. In both logged and primary forests, there was significantly greater loss of experimentally sown seed to predation than was found in the entire natural mast-fruiting Shorea community combined (21 spp.). Moreover, a naturally occurring, but late-fruiting, Shorea species also exhibited greater seed losses to predation than did all other species within each mast-fruiting community, and these proportional losses were similar to those observed in the experimentally sown seeds. Seeds that escaped predation and vertebrate herbivory on post-establishment seedlings displayed high survival, indicating that the availability of suitable microsites did not limit recruitment. In the primary forest, 65% of the germinated experimental seed that survived early causes of mortality was alive 40 mo post-planting, which coincided with the next mast-fruiting event. The spatial distribution of these seedlings was modified primarily by the foraging behavior of vertebrate seed predators in the first two weeks post-planting. The influence of vertebrate predation on seed and seedling survival suggests that foraging behavior by terrestrial vertebrate seed predators may cause directional and/or stabilizing selection for synchronous, interspecific supra-annual dipterocarp seed production across forest regions in Kalimantan.

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