<p>To the main part, this paper is the result of a literature survey and to the minor part of a field survey. The study is found on the question of, if and why unpalatable plant species invade heavily grassed rangelands and if plants change their defence strategy from a mechanical defence to a chemical defence if the herbivory pressure increase. I conclude that defended plants do invade heavily grassed rangelands if the rangelands lose essential recourses (often nutrients) and/or the defended plants are strongly avoided by mammalian herbivores. I also conclude that plants do go from a mechanical defence strategy to a chemical strategy if their environment loses essential recourses under a threshold. This firstly depends on that mechanical defended plants can not develop a complete defence if they suffer from a shortage in the nutrient supply, and secondly of that plants in resources rich environments often have lager possibilities of responding to herbivory with regrowth. They do not therefore have to defend them self as hard as plants in environments with low supply of recourses.</p>

To the main part, this paper is the result of a literature survey and to the minor part of a field survey. The study is found on the question of, if and why unpalatable plant species invade heavily grassed rangelands and if plants change their defence strategy from a mechanical defence to a chemical defence if the herbivory pressure increase. I conclude that defended plants do invade heavily grassed rangelands if the rangelands lose essential recourses (often nutrients) and/or the defended plants are strongly avoided by mammalian herbivores. I also conclude that plants do go from a mechanical defence strategy to a chemical strategy if their environment loses essential recourses under a threshold. This firstly depends on that mechanical defended plants can not develop a complete defence if they suffer from a shortage in the nutrient supply, and secondly of that plants in resources rich environments often have lager possibilities of responding to herbivory with regrowth. They do not therefore have to defend them self as hard as plants in environments with low supply of recourses.