Evaluation of the Evolution of Increased Competitive Ability (EICA) Hypothesis: Loss of Defense Against Generalist but not Specialist Herbivores

• Published in: Journal of chemical ecology 2007-04, Vol.33 (4), p.781-799
• Main Authors: Hull-Sanders, Helen M, Clare, Robert, Johnson, Robert H, Meyer, Gretchen A
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Animals; Autoecology; Bioassays; Biological and medical sciences; Biological Assay; Biological control and other methods; chemical constituents of plants; Conspecifics; Continents; Diterpenes - metabolism; diterpenoids; ecological invasion; Feeding Behavior; feeding guilds; Flowers & plants; Fundamental and applied biological sciences. Psychology; herbivore performance; herbivore-induced chemical defense; Herbivores; host specificity; hydrocarbons; Hydrocarbons - metabolism; Indigenous plants; induced resistance; insect growth; Insects; introduced plants; Invasive plants; Invasive species; Larva - physiology; Parasitic plants. Weeds; Phytopathology. Animal pests. Plant and forest protection; phytophagous insects; plant competition; plant defense; Plant species; Plants and fungi; Sesquiterpenes - metabolism; sesquiterpenoids; Solidago; Solidago - chemistry; Solidago - metabolism; Solidago - physiology; Solidago gigantea; Spodoptera - growth & development; Spodoptera - physiology; Spodoptera exigua; Weeds
• ISSN: 0098-0331; 1573-1561
• DOI: 10.1007/s10886-007-9252-y

The evolution of increased competitive ability (EICA) hypothesis predicts that invasive plant species may escape their specialized natural enemies in their introduced range and subsequently evolve with a decrease in investment in anti-herbivore chemical defenses relative to native conspecifics. We compared the chemical profile of 10 populations of US native and 20 populations of European invasive Solidago gigantea. To test for differences in inducibility between native and invasive populations, we measured secondary chemistry in both damaged and undamaged plants. We also performed bioassays with three specialist and two generalist insect herbivores from four different feeding guilds. There was no evidence that invasive populations had reduced concentrations of sesquiterpenes, diterpenes, or short-chain hydrocarbons (SCH), although significant variation among populations was detected. Sesquiterpene and diterpene concentrations were not influenced by damage to the host plant, whereas SCH concentrations were decreased by damage for both native and invasive plants. Performance of the three specialist insects was not affected by the continental origin of the host plant. However, larval mass of the generalist caterpillar Spodoptera exigua was 37% lower on native plants compared to invasive plants. The other generalist insect, a xylem-tapping spittlebug that occurs on both continents, performed equally well on native and invasive plants. These results offer partial support for the defense predictions of the EICA hypothesis: the better growth of Spodoptera caterpillars on European plants shows that some defenses have been lost in the introduced range, even though our measures of secondary chemistry did not detect differences between continents. Our results show significant variation in chemical defenses and herbivore performance across populations on both continents and emphasize the need for analysis across a broad spatial scale and the use of multiple herbivores.