Can interactions between native and non-native plants in urban landscapes influence herbivore diversity?
As urban and suburban regions expand throughout the United States, a large variety of non-native plant species both invasive and non-invasive are becoming increasingly prevalent. While invasive plants have proven to be detrimental to native ecosystems, few studies have investigated the impacts of non-native, non-invasive ornamental species. Most non-native plant species support fewer species of insect herbivores, and suburban landscapes are often planted with a mixture of natives and non-natives. This begs the question: can the association between native and non-native plants influence the abundance and species richness of herbivorous insects in suburban landscapes? Associational Resistance (AR) and Associational Susceptibility (AS) are widely cited concepts in ecology and are used to better understand the relationship between an herbivore and its host plant in various plant community contexts. This study investigates the roles that AR and AS play in mediating caterpillar and natural enemy abundance and taxonomic richness on native red maples (Acer rubrum), when either native or non-native neighboring plants are present. In a 2-year field experiment, we have established 5 x 5 meter gardens located along a naturalized woodland in Tallassee, Alabama. Each garden consists of a focal red maple surrounded by four neighboring trees that vary in origin (native vs. non-native) and relatedness (congener vs. non-congener). In addition to insect abundance and richness, we are also scoring the amount of feeding injury to the focal trees associated with native and non-native neighbors.
We are finding that after one year, significantly fewer morphospecies of caterpillars are present on native red maples when surrounded by crepe myrtle (Lagerstroemia indica) compared to red maples surrounded by other red maples. Red maples surrounded by the non-native congeneric species, Acer platanoides, also support significantly fewer morphospecies. When assessing feeding injury to the focal trees, no significant differences between treatments were found. So far, the major conclusion that can be drawn from this data is that associational interactions between these plant species via herbivore load are occurring. Changing plant communities from native to non-native, or intermixing them, can have dramatic effects on insect communities. Data yielded from this experiment will be important information for urban ecologists, conservation biologists, pest management professionals, and landscape designers alike. We assert the need for people to value landscape plants not only for aesthetic value, but also for ecological function. Data from natural enemy assessments and year two will also be presented and discussed.