Feeding preferences of generalist herbivores on invasive versus non-invasive seedlings in Hawaii

Background/Question/Methods

Identifying and predicting which non-indigenous plants will become invasive is a central question in both invasion biology and invasive species management.  Whether an introduced plant becomes invasive (i.e. reproducing, spreading, and potentially causing economic and/or ecological impact) is ultimately influenced by many factors including the plant’s defenses and ability to resist attack by generalist enemies.  The most dramatic impacts of herbivores may occur at the seedling stage, especially in long lived species.  However, many studies have focused instead on herbivory of well established plants, leaving a gap in our understanding of plant-animal interactions and invasion biology.  Here we compared herbivore susceptibility of seedlings of invasive and non-invasive plants found in Hawaii.  We expected seedlings of invasive plants to experience lower attack rates and less overall damage from generalist herbivores, compared to seedlings of non-invasive plants, which were predicted to be more strongly limited by herbivores.  Invasive and non-invasive species on the Island of Oahu were paired by family.  Seeds were planted in growth chambers kept at on a 12h day/12h night cycle at temperatures of 25°C/21°C respectively.  In order to measure plant resistance to herbivore attack, 1-2 week old seedlings were used in no-choice laboratory feeding trials with orthopteran and molluscan generalist herbivores. The herbivore’s preference was compared between the invasive and non-invasive plant species.  

Results/Conclusions

Comparing non-native species differing in invasiveness is the most direct test of the determinants of invasiveness.  This approach is rarely taken with regards to seedling herbivory.  In our feeding trials, seedling attack rates and extent of damage were highly variable, and there was not a clear difference in feeding preference between seedlings of invasive and non-invasive plants.  These results suggest that resistance of seedlings to generalist herbivory is not the main factor driving successful plant invasion in Hawaii.  Future studies of plant defenses and invasiveness should also examine tolerance mechanisms that reduce the impact of seedling herbivore damage once it has occurred and escape strategies that reduce the probability of seedlings being found by herbivores in the field.