Hypothesis Summary of hypothesis Suggested relevant factors(s) Suggested relevant ecological context(s) Explanation for how factors and contexts interact to produce evidence for hypothesis
Enemy-mediated biotic resistance (BR) Enemies in the invaded range limit invasive ability (Levine et al. 2004) Enemy diversity; Enemy impact Relatedness to native species; Time since introduction Closely-related invaders are likely to be targeted effectively by specialists in the native range, reducing invasive capacity. There may not be an immediate loss of specialists, and generalists may accumulate faster through time because they are used to targeting similar species. Alternatively, high enemy impact and diversity on exotics providing evidence for BR would be expected in well-developed invasions.
Enemy inversion (EI) An exotic’s natural enemies have reduced effect in invaded range (Colautti et al. 2004) Enemy diversity; Enemy impact; Host adaptation Resource availability in native and invaded range; Temporal and spatial asynchronicity Resources may be higher in the invaded range, meaning invasive species can better compensate for any losses. Natural enemies of the invader may not be able to succeed in all invasive areas due to physiological or other constraints, leading to areas of reduced enemy pressure.
Enemy of my enemy (EE) Enemies affect native species more than exotics (Colautti et al. 2004) Enemy impact Relatedness to native species; Temporal and spatial asynchronicity Invasive species may be more resistant to enemies (e.g. due to novel defences), which is more likely if they are less closely related. Invaders may have a wider realised niche than native enemies (or co-introduced enemies), leading to areas of reduced enemy pressure for the invader.
Enemy reduction (ERD) A partial (rather than a total) loss of enemies (Colautti et al. 2004) Enemy diversity Relatedness to native species; Number of invasion events; Time since introduction The more related an invader is to natives, the more specialists in the invaded range that will be able to target it. With an increasing number of invasion events comes an increased likelihood of co-introducing natural enemies of the invader. Generalists are also likely to accumulate with time.
Evolution of increased competitive ability (EICA) Loss of enemies leads to loss of defences and upregulation of growth (Blossey & Notzold 1995) Enemy diversity; Host adaptation Existence of defence/performance trade-offs; Resource availability in native and invaded range; Time since introduction Reducing investment in defence will be more likely to increase growth if there is a strong growth-defence trade-off. High resources in the invaded range relative to the home range could allow for upregulation of both growth and defence. Such evolutionary changes will only be seen after a number of generations.
Increased susceptibility (IS)/New associations (NAS) Low genetic diversity increases susceptibility to enemies in invaded region (Colautti et al. 2004) Enemy impact Number of invasion events; Type of enemy; Time since introduction Genetic bottlenecks reduce adaptive capacity to deal with enemies; this effect will be stronger with fewer or smaller invasion events. The degree to which this occurs will also depend on the type of enemy (e.g. pathogen vs. herbivore) and the complexity of defences marshalled against them. This is more likely to be observed earlier in invasions.
Resource-Enemy release (R-ER) Resource-acquisitive species have more enemies to lose, and will benefit most from that loss (Blumenthal 2006) Enemy diversity; Enemy impact Resource availability in native and invaded range Resource-acquisitive invaders that come from high-resource environments (relative to the invaded range) likely have a greater diversity of enemies, and thus experience a larger loss of enemies upon invasion. They are also better-equipped to benefit from high resource availability or disturbance in the invaded range, and can effectively compensate for enemy damage. Invaders from low-resource environments likely show the opposite trend.
Shifting defence (SDH) Loss of specialists stimulates investment in cheaper generalist defences which frees resources for growth (Müller-Schärer et al. 2004) Enemy diversity; Host adaptation Existence of defence/performance trade-offs; Resource availability in native and invaded range; Time since introduction Reducing investment in defence will be more likely to increase growth if there is a strong growth-defence trade-off. The degree to which specialists are lost (determined by other contexts), and the limiting nature of resources, will determine whether there is a clear evolutionary switch to cheaper defences and thus capacity for increased investment in growth. Such evolutionary changes will only be seen after a number of generations.