Component of ERH Definition Example metrics Key prediction
Outcome of the ERH: increased exotic performance Outcome of the ERH: increased exotic performance Outcome of the ERH: increased exotic performance Outcome of the ERH: increased exotic performance
Population-level performance Enhanced performance of an exotic population after introduction to new range. Population growth rate; population density; rate of spread; area of occupancy. Exotic populations that experience enemy release will demonstrate increased population performance, facilitating naturalisation and/or invasive impact (Keane & Crawley 2002). This increase may be relative to either conspecifics in the home range or native competitors in the invaded range (Box 1).
Three factors influence exotic performance Three factors influence exotic performance Three factors influence exotic performance Three factors influence exotic performance
1: Enemy impact Change in the per-capita effect of enemies on the exotic after introduction to new range Comparison between home and invaded range of exotic: survival; growth; reproduction [i.e. vital rates] The per-capita effect of enemies is reduced on exotic species in their invaded range, as they have novel defences or chemical signatures that generalists in the invaded range are not adapted to (Keane & Crawley 2002).
2: Enemy diversity Change in the diversity of enemies attacking exotic after introduction to new range Comparison between home and invaded range of exotic: enemy richness; enemy abundance; enemy prevalence [direct measures of enemy presence]; level of host damage [indirect measure] Upon invasion, exotic species lose specialist enemies but are still faced with generalist enemies (Müller-Schärer et al. 2004).
3: Host adaptation Adaptive changes to investment in growth and defence by exotic after introduction to new range Common garden comparisons of: host growth/reproduction in home vs. invaded range; host defence in home vs. invaded range Reductions in enemy impact and/or diversity will lead to exotic species reducing investment in defence and increasing investment in growth (Blossey & Notzold 1995).
Seven contexts alter the influence and strength of each factor Seven contexts alter the influence and strength of each factor Seven contexts alter the influence and strength of each factor Seven contexts alter the influence and strength of each factor
i) Time since introduction Time elapsed since a specific local exotic population established in invaded range Number of generations since establishment of an exotic population; years since establishment of an exotic population; minimum residence time of an exotic population Enemy impact on exotics increases through time as generalists become better adapted to invader; enemy diversity increases through time as generalists accumulate; later generations of exotics show adaptive increases in growth and reductions in defence in response to changes in enemy pressure (Hawkes 2007).
ii) Resource availability Difference in the level of resources between the home and invaded range of an exotic population % soil nitrogen; micronutrient availability; light competition from canopy Exotics from high-resource environments have lower defences and experience higher enemy impact and diversity in their home range and so will benefit the most from release, but will also accumulate enemies at a faster rate in their invaded range. High resources in the invaded range can facilitate evolutionary investment into both growth and defence (Blumenthal 2006).
iii) Phylogenetic relatedness of exotic and native species Genetic relatedness of an exotic population to the native (invaded) community Phylogenetic distance of exotic to most closely-related neighbour in invaded community; mean abundance-weighted phylogenetic distance of exotic to entire invaded community Exotics that are closely related to co-occurring natives will experience higher specialist diversity and greater impacts from generalists, as enemies in the invaded range will be more adapted to target them (Mitchell et al. 2006).
iv) Host-enemy asynchronicity in space or time Difference between the observed spatial and temporal occurrences of an exotic species and enemies Number of days between peak host flowering/seeding period and peak activity of floral herbivores; Distance in niche space between optima of a host and a generalist enemy Greater temporal or spatial asynchronicity between exotics and enemies in the invaded range will reduce enemy impact and diversity. For example, an exotic that flowers at a different time of year to native species will escape generalist floral herbivory (Fan et al. 2016).
v) Number of introduction events Propagule pressure of an exotic species to a given location Number of discrete arrival events of an exotic in a country/state; genetic diversity of an exotic An increasing number of introduction events makes it more likely that specialist enemies will be co-introduced with the exotic, increasing enemy diversity and impact. Increasing invasion events (or their size) reduces founder effects, altering exotic adaptive potential (Mitchell & Power 2003).
vi) Type of enemy Functional and taxonomic characteristics of the enemy species that are compared between the home and invaded range of an exotic Specialist vs. generalist; taxonomic kingdom or class (e.g. mammal vs. arthropod vs. fungi) Small parasitic specialists (e.g. fungi) are more likely to be co-introduced than larger specialists (e.g. insects or mammals). In the invaded range, immediate impacts on exotics are most likely from specialists of closely-related natives or from large browsing generalist herbivores (Maron & Vilà 2001).
vii) The strength of growth-defence trade-offs The degree to which individuals with high growth have lower defence and vice versa Slope of the line describing correlations between investment in growth (e.g. growth rate, max height) and investments in defence (e.g. total phytochemical quantity, diversity), at the within-species level Exotics that exhibit a strong growth-defence trade-off are more likely to evolutionarily increase investment in growth and decrease investment in defence if they experience reductions in enemy impact or diversity (Agrawal et al. 2012).