Factors Research questions: Proposed methods:
Model accuracy
What is the accuracy of the model developed in this review when applied to additional invasive species? Specific challenges: How can we identify a larger array of pathogens on invaders including low impact pathogens to improve the model? Enhance reporting of pathogens on invasive species by including these data in invasive species monitoring databases. Experimentally test the impacts of different known pathogens on invaders with a range of traits, specifically controlling for pathogen traits as well as inoculation rates to better control for pathogen virulence. Isolate microbial taxa from these plant species. Identify potential pathogens and use these to re-infect plants to determine the impacts of native pathogens.
Plant traits
What are the mechanisms driving differences in pathogen susceptibility with plant traits? Specific challenges: Do native and invasive plants differ in SAR or defense priming? How do EM, AM, and non-mycorrhizal invaders differ in their response to pathogens both above- and belowground? Decouple differences in correlated traits, for example, by testing disease susceptibility in genetically distinct populations of an invader. Compare across congeneric sets of native, non-native, and invasive plants. Expose invaders with different mycorrhiza status to a range of potential pathogens.
Pathogen traits What kinds of pathogens are most likely to cause invader decline? Collect data on pathogen traits and apply a similar model as developed for plant traits here to determine the most influential pathogen traits.
Context dependence
How does incorporating context dependence improve the model? Specific challenges: How do plant/pathogen traits interact with environmental/abiotic conditions to influence disease susceptibility? How do inherent differences in plant tissue nutrient concentrations interact with environmental and habitat differences in nutrient availability?
Collect data on aspects of context dependence discussed previously and re-evaluate the model.
Pathogen/plant trait interactions
How do pathogen traits interact with plant traits to determine species combinations that will result in invader decline? Specific challenges: To what extent do allelopathic defense compounds affect pathogens with different traits? Does the type of pathogen influence whether annual species are less/more susceptible to pathogens than perennials? Combine models with pathogen traits and plant traits to determine interactions. Conduct bioassays using extracts from invasive plants comparing different pathogens such as generalists and specialists. Expose annual and perennial plants with a range of other traits to different types of pathogens (e.g. generalists vs. specialists).
Residence time
How do invader pathogen interactions change over time? Specific challenges: How long is a “weapon” of an invasive species “novel”? I.e. How long does it take for pathogens to evolve resistance to allelopathic chemicals of invaders in the novel range? Survey pathogen impacts on populations of invaders across different residence times. Conduct bioassays using extracts from invasive plants collected along temporal gradients in invader residence time.