Discussion
Existing literature reports both, positive as well as negative effects
of non-native plant species on resident biodiversity. However, negative
effects have been noted more frequently especially for insect diversity
(see Wohlgemuth et al. 2022 for a recent meta-analysis on the
effects). Contradicting findings always raise the question about global
drivers of the observable effects, that might help to reconcile such
contrasting findings existent in the published literature. Geographic,
climatic and phylogenetic distance between the non-native plant species
and its native congeners or area of introduction have been named as
three dominant drivers of the strength and direction of the effects of
non-native plant species on biodiversity. In our study, we identified
phylogenetic distance between non-native and native tree species to be
the main diver of resident insect abundance and diversity with lower
insect abundance and diversity being recorded on more phylogenetically
distant, non-native species. This predominance of the phylogenetic
effect in contrast to geographic or climatic factors might relate to the
dominant role of evolutionary forces shaping plant-insect interactions
such as the process of coevolution.
There is increasing consensus, that diversity of phytophagous arthropods
resulted from balanced rates of successful colonization of the new host,
speciation on the respective host and local extinction (Farrell et
al. 1992). The main mechanism driving the positive relationship between
phylogenetic relatedness among host plants and their herbivore
colonization is proposed to be a high similarity of functional traits
involved in host plant selection or host resistance (Pearse et
al. 2013; Ricciardi & Ward 2006). Whether a shared phylogenetic
history between plant species can explain similarities in inhabited
insect communities thereby strongly depends on the degree of
conservatism of such herbivore related traits (Fine et al. 2006).
Insect-plant associations are reported to be generally very conservative
with respect to the evolution of new host affiliations and, thus, may
persist over extensive periods of the phylogenetic history (Farrellet al. 1992; Farrell & Mitter 1994). Closely related host plant
species are therefore expected to be more likely to be colonized by the
same, specialized insect species due to a high degree of phylogenetic
conservatism being reported for morphological and chemical plant traits
that regulate interactions with herbivores (Brändle et al. 2008;
Gilbert & Webb 2007; Whitfeld et al. 2012). Proportion of
specialized, herbivorous insects are generally reported to be high (Ali
& Agrawal 2012; Price et al. 2011). However, many plant
functional traits affecting plant-insect interactions are strongly
shaped by the abiotic environment (e.g. leaf thickness and hairs,
thickness of the cuticula as a response to low water availability and/or
high radiation etc.) – a fact that might challenge plant-insect trait
matching dynamics acting over evolutionary time scales in variable
environments.
Already Darwin proposed in his naturalization hypothesis that non-native
plants are more likely to invade if they lack close relatives in their
new range (Darwin 1859). Several studies highlight a positive
relationship between the abundance and phylogenetic relatedness of
non-native plants with their native congeners and the likelihood that
the non-native species get colonized by native herbivores (Goßneret al. 2009; Kirichenko & Kenis 2016; Neuvonen & Niemelä 1981;
Roques et al. 2006). This phylogenetic effect on plant-herbivore
interactions has been shown to be especially strong for tropical
plant-insect herbivore interactions, where 25% of the variance in
herbivore community similarity between tropical tree species was
explained by the phylogenetic similarity of the host tree species
(Weiblen et al. 2006). Furthermore, host switches of phytophagous
insects have been reported for the temperate region to be more probable
to happen between related host tree species than between more unrelated
host species which co-occurred in the same habitat (Brändle & Brandl
2006). Another impressive though anecdotal example of a high degree of
phylogenetic conservatism in host-plant insect interactions is reported
for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), a
non-native, economically important tree species with no close relatives
in Europe. In the 130 years since this species has been introduced to
Europe, it has recruited only 87 arthropod species, despite being the
most planted, non-native tree species in European forest systems (Roqueset al. 2006). These are only 33.9% of the arthropod species
richness found on Douglas-fir in its native range in North America. Our
results show that such a strong phylogenetic effect on plant-insect
interactions of non-native trees is not a peculiarity of single tree
species but a general phenomenon with extensive geographic and taxonomic
coverage. In our study these effects have been detected for non-native
species of native tree genera, showing that this effect even holds true
on lower taxonomic levels.
Our results furthermore show that the effect of phylogenetic distance on
resident insect abundance, species richness and diversity is visibly
stronger for phytophagous insect species in comparison to
non-phytophagous species. This reflects theoretical assumptions proposed
for the enemy release hypothesis (ERH), one of the most prominent
hypotheses in plant-related invasion biology. It states that
host-specific herbivores are left behind when a plant species is
transferred to a new area that haven’t been occupied before (Keane &
Crawley 2002). This lack of native antagonists in a plant host’s
introduced range can favour its vitality and spread (Adams et al.2009; Cincotta et al. 2009). Lower trait similarity between
phylogenetically distant, non-native plants and their native congeners
in the area of introduction has been proposed as an underlying,
prominent mechanism explaining the establishment success of such
non-natives (Goßner et al. 2009). In this framework, our results
would basically mean, that such release effects might be stronger, the
more phylogenetically distant non-native plant species will be to their
native counterparts on place (for empirical evidence see e.g. Nesset al. 2011).
Although phylogenetic distance turned out to be the predominant, global
driver in our study, other context-specific drivers of the effects of
non-native plants on resident biodiversity have to be expected to be
important on local scale (Wohlgemuth et al. 2022). Local drivers
such as density effects (i.e. trees planted in mono- vs. mixed stands;
Adams et al. 2009; Yang et al. 2019) or differences in
species pools of arthropods (Miles et al. 2019 in the context of
urbanisation) are named to be important. However, these effects might
not play out in a common garden setting such as ours, where all tree
species are experiencing a very similar abiotic and biotic environment
and, thus, genetic differences might predominate the responses (Allenet al. 2017). Local differences in environmental conditions might
overwrite the effects of phylogenetic differences between non-native
species and their native congeners when comparing different, local study
sites. Other exogenic factors such as time since introduction in a new
biogeographic realm (i.e. residence time) can play an additional,
crucial role for native insect - non-native host plant interactions
(Brändle et al. 2008; Sheppard & Brendel 2021). Further
investigations on other tree genera and species, as well as in other
geographical areas will be needed to test the relative importance of
local vs. global drivers of non-native tree species on resident insect
communities. Coordinated, spatially distributed, common garden
experiments would help to advance in this direction. Tree collections in
botanical gardens can provide a fruitful basis for such kind of
investigations.