We performed a comparative analysis of defensive and nutritional plant traits responsible for differential herbivory in a series of experimental feeding trials with generalist herbivores. We measured three defensive traits (leaf strength, leaf mass per unit area and endophytic fungal infection) and two nutritional traits (foliar nitrogen and water) for 26 native and eight non-native plant species from coastal California shrublands. Our feeding trials involved three species of generalist herbivore (beet armyworm, cabbage looper and the garden snail) in two types of laboratory feeding trial (single plant species and preference tests). All traits were significantly related to the amount of leaf area consumed, with foliar nitrogen followed by leaf strength explaining most of the variation in herbivore damage. Defensive and nutritional traits were tightly correlated with one another. These correlations were still apparent after incorporation of the phylogenetic relationships of species using independent contrasts, suggesting that there has been repeated selection for certain trait combinations. Non-native species had lower defensive traits and greater nutrient content and therefore experienced greater herbivory damage than natives. Poorly defended, nutrient-rich species (like most of the non-natives in our study) may be better suited for rapid growth and nutrient acquisition, thus reducing the cost of replenishing leaf material lost to herbivores.
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