Plant tolerance to herbivory affect plant growth and also tolerance are water, light, carbon dioxide and soil nutrients. Water and light levels are generally assumed to be positively associated with tolerance (Strauss and Agrawal 1999). However, there are exceptions such as evidence of decreased tolerance in "Madia sativa" with increased water availability (Wise and Abrahamson 2007, Gonzales "et al." 2008). Many studies have found CO levels to decrease tolerance in plants (Lau and Tiffin 2009). Increased nutrient levels are also commonly found to be negatively associated with tolerance (Wise and Abrahamson 2007). There are currently three prominent models that predict how resource levels may
Eutrophication Humankind has increased the rate of phosphorus cycling on Earth by four times, mainly due to agricultural fertilizer production and application. Between 1950 and 1995, an estimated 600,000,000 tonnes of phosphorus was applied to Earth's surface, primarily on croplands. Policy changes to control point sources of phosphorus have resulted in rapid control of eutrophication. Although eutrophication is commonly caused by human activities, it can also be a natural process, particularly in lakes. Eutrophy occurs in many lakes in temperate grasslands, for instance. Paleolimnologists now recognise that climate change, geology, and other external influences are critical in regulating the natural productivity