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Modeling how herbivores facilitate lawn and bunch grass coexistence: comparing two hypotheses

E.R. Veldhuis, (2016) Modeling how herbivores facilitate lawn and bunch grass coexistence: comparing two hypotheses. Research Project 2 (major thesis), Ecology and Evolution.

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Abstract

It is generally accepted that herbivores promote lawn grasses and thereby play an important role in the lawn- and bunch- grass coexistence. Whereas lawn grasses are nutrient rich and have a short stature, bunch grasses have a tall erect growth and have high concentrations of unpalatable compounds. Indirect effects of herbivores that promote lawn grasses outweigh direct effects of herbivory that reduce lawn grasses. The mechanism of the indirect effects is controversial and currently two hypotheses exist that explain how herbivores promote lawn grasses. Whereas the nutrient-based hypothesis assumes that herbivores promote lawn grasses by increasing nutrient availability, the water-based hypothesis states that herbivores increase drought stress and lawn grasses are better in the competition for water than bunch grasses. With two models we investigated whether the assumed indirect effects of herbivores of each hypothesis can outweigh the costs of herbivory and thus result in a net benefit for lawn grasses. Our nutrient-limited model shows that herbivores only stimulate the evolution of lawn and bunch grasses when herbivores are externally controlled. Although herbivores increase the nutrient cycling speed, herbivores always suppress lawn grasses because of herbivory. Thus indirect positive effects did not compensate for the direct negative effects. Outcomes of our nutrient-limited model did not correspond with field observations. Our water-limited model shows that herbivores stimulate the evolution of lawn and bunch grasses, regardless of how herbivore numbers are controlled. An increase in the water influx promotes evolution of bunch grasses and bunch patch formation. When competition for light was implemented in our water-limited model herbivores promote lawn grasses. Our water-limited model supports the water-based hypothesis, but only when competition for light was implemented.

Item Type: Thesis (Research Project 2 (major thesis))
Degree programme: Ecology and Evolution
Thesis type: Research Project 2 (major thesis)
Language: English
Date Deposited: 15 Feb 2018 08:11
Last Modified: 15 Feb 2018 08:11
URI: http://fse.studenttheses.ub.rug.nl/id/eprint/13683

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