Abstract
Recently, the importance of seed predation by crabs on mangrove species distributions and densities has been established by several studies. In a tropical coastal terrestrial forest in Costa Rica, we investigated the relative importance of predation by land crabs, Gecarcinus quadratus, and hermit crabs, Coenobita compressus, on measured forest composition through a series of seed removal and seedling establishment experiments. We also used natural light-gaps and adjacent non-gap sites to test how canopy cover affects crab predation (seed removal) and seedling establishment. We found fewer tree species (S=18) and lower densities (seedlings, saplings, and adults) in the coastal zone within 100 m of coastline, than in the inland zone (S=59). Land crab densities were higher in the coastal zone (3.03±1.44 crabs m−2) than in the inland zone (0.76±0.78 crabs m−2), and hermit crabs were not present in the inland zone. Seed removal and seedling mortality also were higher in the coastal zone than in the inland zone, and in the open controls than in the crab exclosures. Mortality of seeds and seedlings was two to six times higher in the controls than exclosures for four of the five experiments. Crabs preferred seeds and younger seedlings over older seedlings but showed no species preferences in the seed (Anacardium excelsum, Enterolobium cyclocarpum, and Terminalia oblonga) and seedling (Pachira quinata and E. cyclocarpum) stages. We conclude that the observed differences in tree densities were caused by differential crab predation pressure along the coastal gradient, while the differences in species composition were due to predator escape (satiation) by seed quantity. Canopy cover did not affect seed removal rates, but did affect seedling survival with higher mortality in the non-gap versus gap environments. In summary, crab predation of seeds and seedlings, and secondarily canopy cover, are important factors affecting tree establishment in terrestrial coastal forests.
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Acknowledgements
Our work was supported by the NSF Dissertation Improvement Grant (DEB-0104791), Sigma Xi, and various field and travel award grants from the University of Georgia. We are grateful to the Costa Rica MINAE administration of San Miguel Biological Station, Cabo Blanco Absolute Nature Reserve for allowing this study to be conducted in such a restricted area, and expressing interest in the study’s findings. This study would not have been possible without the unselfish field help of numerous field assistants, with particular thanks to K. Brewster for her painstaking dedication to every seedling in our study dead or alive. We would also like to thank M. Ardón, R. Cooper, M. Hunter, K. Krauss, D. Lieberman, A. Parker, C. Peterson, P. Sherman, and two anonymous reviewers for their constructive comments on earlier versions of the manuscript. All experiments conducted in this study complied with the current laws of Costa Rica and the United States of America.
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Lindquist, E.S., Carroll, C.R. Differential seed and seedling predation by crabs: impacts on tropical coastal forest composition. Oecologia 141, 661–671 (2004). https://doi.org/10.1007/s00442-004-1673-5
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DOI: https://doi.org/10.1007/s00442-004-1673-5