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. |