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). |