Evol Ecol Res 9: 599-617 (2007)     Full PDF if your library subscribes.

Optimum size and number of seeds when seeds suffer pre-dispersal predation

Satoki Sakai1* and Yasushi Harada2

1Department of Ecology and Evolutionary Biology, Graduate School of Life Sciences, Tohoku University, Aoba, Sendai 980-8578 and  2Faculty of Bioresources, Mie University, 1515 Kamihama-cho, Tsu, Mie Prefecture 514-0008, Japan

Author to whom all correspondence should be addressed.
e-mail: sakai@mail.tains.tohoku.ac.jp


Question: What are the optimum size and number of seeds when plants suffer pre-dispersal seed predation?

Key assumptions of the model: There is a trade-off between the size and the number of seeds. A certain number of seeds is eaten at a certain stage of seed development from ovules to seeds. After seed predation, a plant may abort a certain number of uneaten seeds to concentrate its resources on the rest. Thus, the plant can adjust the size of its seeds by changing the numbers of seeds initially produced and flexibly aborted after seed predation.

Predictions: If the number of seeds eaten is unpredictable, surplus seeds are produced or not produced dependent on parameter values such as the resource cost of aborting uneaten seeds. When surplus seeds are produced, there is a limit to the number of seeds that develop successfully, and also seeds that develop successfully will be at least a certain minimum size. If the number of uneaten seeds is greater than the maximum number, the plant reduces these by aborting the number in excess of this limit and all seeds will attain the minimum size. All uneaten seeds develop without abortion if the number of uneaten seeds is smaller than the maximum number. When no surplus seed are produced, all uneaten seeds always develop without abortion irrespective of the number of seeds eaten. Plants are not likely to produce surplus seeds if only a small proportion of seeds is expected to be eaten or if each seed that does get eaten or aborted costs a large amount of resources.

Keywords: optimum seed number, optimum seed size, pre-dispersal predation, size–number trade-off, unpredictable predation.

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