Local spatial structure in pondweed populations: the role of propagule size
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RU Radboud Universiteit Nijmegen, 26 mei 2004
Promotor : Groenendael, J.M. van Co-promotores : Santamaria, L., Ouborg, N.J.
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Aquatic Ecology and Environmental Biology
In this thesis I focused on the study of factors that determine local spatial structure of an aquatic clonal plant (Potamogeton pectinatus) at both the population and community level. Particular attention was paid to the adaptive value of propagule size under various ecological conditions, with special emphasis on the role of propagule predation by Bewick's swans (Cygnus columbianus bewickii). Within the P. pectinatus population studied in the Lauwersmeer, natural selection on asexual propagule (tuber) size took place. The three prerequisites for natural selection to take place were met: (1) tuber size had a heritable basis, (2) within population variation of this trait was present, (3) variation in the trait resulted in differential fitness. Furthermore, the selection pressures were stable (spatial variation in substrate type and predation pressure were consistent). Deep burial of large tubers can thus be regarded as an adaptive strategy to escape predation by Bewick's swans. However, maternal non-genetic effects also influenced tuber size. This effect slows down the pace of selection, allowing the persistence of polymorphic variation in tuber size, as found in the study population. The spatial structure at the community level (a pondweed-hybrid-complex) was also affected by spatial variation in swan predation. In deep water P. pectinatus escaped predation by deep burial of its relatively large tubers while the smaller hybrid tubers could not escape due to their size. In shallow water where no escape through deep burial could occur the large P. pectinatus tubers were preferred over the small hybrid tubers by swans resulting in the absence of the P. pectinatus in shallow water. This thesis has shown that even at a local scale (i.e. < 800 x 300 m) spatial variation in abundance within and among clonal plant species (i.e. population and community level) may result from evolutionary responses to contrasting ecological conditions
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