constraints as these can potentially be replaced by novel interactions through
ecological fitting (see Heleno et al. 2012 for seed dispersal example).
As posited by the ERH, the liberation from highly specialised parasitic interactions (such as herbivores or pathogens) will aid establishment success. More than a
century of biological control of invasive plants in South Africa provides strong
support for the role of enemy release in plant invasiveness (Zachariades et al. 2017;
also see Hill et al. 2020, Chap. 19), although the high levels of control observed can
equally be explained by the biological control agents having been released from their
natural enemies. Levels of interaction specificity are also important when considering interactions between the alien species and resident species (e.g. symbionts) in the
new range, i.e. ecological fitting. That is, establishment success and invasive performance are expected to be enhanced when resident antagonists or predators are
highly specialised and/or resident mutualists are generalist (so called SpecialistGeneralist Hypothesis, Catford et al. 2009).
The different pathways for interaction reassembly (ecological fitting,
co-introduction vs. co-xenic) can have distinct impacts on the establishment success
of aliens, and many of these have been formally described as hypotheses in invasion
ecology. For example, ecological fitting may either enhance or impede the performance of introduced species (so-called New Associations Hypothesis; Catford et al.
2009), while co-xenic associations may lead to invasional meltdown, whereby
positive interactions among different invasive species initiate feedbacks that intensify their ecosystem impacts and/or promote secondary invasions by other species
(Simberloff and Von Holle 1999). Co-introduction of mutualists almost always
benefits invaders. In some instances, co-introduced enemies may be less effective,
or may even have an opposite effect, in the new environment (i.e. Enemy Inversion
Hypothesis; Catford et al. 2009). The Enemy of my Enemy Hypothesis can operate
through apparent competition, whereby the enemy ends up causing more damage to
maladapted native species than the alien species, potentially reducing inter-specific
competition between invasive and resident species (Catford et al. 2009). The outcomes of an introduction (i.e. invasiveness) will therefore to a large degree depend
on the structure of ecological interaction networks in both native and non-native
communities (Fig. 14.2).
14.1.2 The Structure of Ecological Interaction Networks
and Their Infiltration by Invasive Species
Ecological networks with interactions varying in their specificity can show high
levels of nestedness, e.g. if specialist plants in a community only interact with a
subset of the pollinators with which generalist plants interact (Bascompte 2009).
Specialisation also means that species pairs may not have the same chances for
interacting. For example, networks will become modular when host plants only
14 Biotic Interactions as Mediators of Biological Invasions
393
ecological fitting (see Heleno et al. 2012 for seed dispersal example).
As posited by the ERH, the liberation from highly specialised parasitic interactions (such as herbivores or pathogens) will aid establishment success. More than a
century of biological control of invasive plants in South Africa provides strong
support for the role of enemy release in plant invasiveness (Zachariades et al. 2017;
also see Hill et al. 2020, Chap. 19), although the high levels of control observed can
equally be explained by the biological control agents having been released from their
natural enemies. Levels of interaction specificity are also important when considering interactions between the alien species and resident species (e.g. symbionts) in the
new range, i.e. ecological fitting. That is, establishment success and invasive performance are expected to be enhanced when resident antagonists or predators are
highly specialised and/or resident mutualists are generalist (so called SpecialistGeneralist Hypothesis, Catford et al. 2009).
The different pathways for interaction reassembly (ecological fitting,
co-introduction vs. co-xenic) can have distinct impacts on the establishment success
of aliens, and many of these have been formally described as hypotheses in invasion
ecology. For example, ecological fitting may either enhance or impede the performance of introduced species (so-called New Associations Hypothesis; Catford et al.
2009), while co-xenic associations may lead to invasional meltdown, whereby
positive interactions among different invasive species initiate feedbacks that intensify their ecosystem impacts and/or promote secondary invasions by other species
(Simberloff and Von Holle 1999). Co-introduction of mutualists almost always
benefits invaders. In some instances, co-introduced enemies may be less effective,
or may even have an opposite effect, in the new environment (i.e. Enemy Inversion
Hypothesis; Catford et al. 2009). The Enemy of my Enemy Hypothesis can operate
through apparent competition, whereby the enemy ends up causing more damage to
maladapted native species than the alien species, potentially reducing inter-specific
competition between invasive and resident species (Catford et al. 2009). The outcomes of an introduction (i.e. invasiveness) will therefore to a large degree depend
on the structure of ecological interaction networks in both native and non-native
communities (Fig. 14.2).
14.1.2 The Structure of Ecological Interaction Networks
and Their Infiltration by Invasive Species
Ecological networks with interactions varying in their specificity can show high
levels of nestedness, e.g. if specialist plants in a community only interact with a
subset of the pollinators with which generalist plants interact (Bascompte 2009).
Specialisation also means that species pairs may not have the same chances for
interacting. For example, networks will become modular when host plants only
14 Biotic Interactions as Mediators of Biological Invasions
393