Tri-trophic implications of invasive shrubs: Effects on quantity and quality of prey for insectivorous birds

Although ecological impacts of invasive plants have received much attention, the trophic pathway leading from plants to birds via herbivorous insects has largely escaped notice. Many species of insectivorous and omnivorous birds rely largely on herbivorous insects, so that if an invasive plant has truly escaped from herbivory, the consequences of this would likely be severe for these species. However, complete escape from herbivory is rare, with the insect faunas on exotic species generally shaped by their physical, chemical, taxonomic, and ecological distinctiveness from native plants, as well as other factors. Since distinctive exotic plants are likely to host only generalists, this might have implications in terms of prey quality for birds, if specialists have more defenses, such as aposematism and extreme crypsis available to them. I have been examining insect herbivory and faunas, and avian insectivory in two systems, the first comprising one native and one non-native co-occurring shrub; and the second comprising  5 native and 2 invasive co-occurring shrub species.  In each, avian insectivory was examined by means of bird exclosure experiments. Results for the 2-species system have shown that while the native species suffered a much higher rate of herbivory, the rates of avian insectivory did not differ. This could be explained by much of the herbivory on the native species being due to specialized inedible or cryptic insects. In the more complex system, the 5 native species comprise 2 congeners, another 2 species that are confamilial with each other, and 1 species that is confamilial with 1 of the invasives. This research design has permitted comparisons to be made between the faunas found on native and invasives, and between species having differing degrees of taxonomic relatedness, while determining how these variables affect avian insectivory.