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Effects of herbivory and competition on growth and reproduction of Triglochin maritima in a gradient of primary succession

Egas, M. (1995) Effects of herbivory and competition on growth and reproduction of Triglochin maritima in a gradient of primary succession. Master's Thesis / Essay, Biology.

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Abstract

In this study the effects of competion and disturbance through herbivory are investigated in a gradient of primary succession. The objective was to test two theories on plant community structure (Grime's life-history theory and Tilman's mechanistic resource competition theory). These two theories yield different predictions about the role of competition and other factors (like herbivory) in determining plant community structure. Grime predicts that the importance of competition will increase when primary production increases, whereas Tilman predicts that competition will always be important, but will shift from competition for soil resources to competition for light. These predictions can be tested in a gradient of primary succession. One plant species, Triglochin maritima, was studied in a chronosequence of salt marsh plant communities on Schiermonnikoog, near three transects (transects 1, 2 and 3; age of successional stage increases from 1 to 3). Transplants of Triglochin (both mature plants and seedlings) were subjected to treatments with no neighbours, with only the roots of neighbours or both roots and shoots of neighbours. These treatments were either protected from grazing by Brent Geese, hares and rabbits with an exclosure, with only a cage around the transplant or not at all (control). As it is a precondition that a resource is limiting in order to expect competition for this resource, and because N was earlier found to be growth limiting, N fertilisation was applied in order to check limitation of this nutrient. Growth of the mature plants was measured from May 1995 through July 1995, and biomass of mature plants and seedlings was harvested in July, in order to assess the effects of competition and herbivory. Flowering of transplants was recorded three times. Growth and biomass of mature plants was increased by fertilisation at transects 1 and 2, but not at transect 3. Biomass was not affected by competition treatments, though. The plants showed phenotypic response instead, from short plants with many leaves in no-neighbours treatments to high plants with a few leaves in control treatments. Plants at transect 3 were higher than plants with equal biomass at transects 1 and 2, implying light limitation. Seedling biomass was not increased by fertilisation, yet a competition effect was found at transect 3, indicating competition for light. Seedling survival was not significantly affected by grazing or competition, but survival at transect 1 was very low. Grazing effects on mature plant biomass and growth was found at transect 2 in May (during spring staging of Brent Geese) and at transect 1, where grazed plants weighed three times less than ungrazed plants. Flowering of Triglochin was strongly affected by grazing, also at transect 3, and was reduced in the all-neighbours treatment at transects 2 and 3. Comparing both results of mature plants and seedlings together to the theoretical predictions yields indecisive conclusions on the importance of competition and the possible shift from competition for soil resources to competition for light. Distinguishing between the effect and response components of competition (Goldberg, 1990) in the two theories, yields more precise predictions that dictate a separate evaluation of results for mature plants and seedlings and fit closer to the results in this study. Applying this distinction to the two theories does seem to prove useful and explicitly holds the possibility that both theories are true in their own context. Extending the length of this experiment may provide more insight in the importance of competition and herbivory for the growth and reproduction of Triglochin maritima.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Biology
Thesis type: Master's Thesis / Essay
Language: Dutch
Date Deposited: 15 Feb 2018 07:48
Last Modified: 15 Feb 2018 07:48
URI: http://fse.studenttheses.ub.rug.nl/id/eprint/10104

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