Evidence of invader decline
To determine the prevalence of novel pathogen accumulation on invasive
plants, we compiled examples of plant invader disease from the
literature. We searched Web of Science with the following terms: (”plant
inva*” OR ”invasive plant*”) AND (dieback* OR disease* OR pathogen* OR
infect*) on March 16, 2020. This search produced 661 results. References
cited in articles found through this search were also analyzed. The
following conditions were required for inclusion: (1) the plant species
must be considered non-native invasive (based on the CABI database or
local invasive plant lists); (2) disease must have been observed on the
invader in its non-native range; (3) the cause of the disease must have
been identified by inoculating the invader with the pathogen and
observing disease symptoms; and (4) the disease agent must be native to
the non-native range of the plant invader or globally distributed.
Because information on the natural ranges of plant diseases can be
scarce, we included plant diseases where the available information
suggests they are likely native, newly identified taxa on invaders in
their non-native ranges, as well as globally distributed pathogens of
unknown origin. While globally distributed pathogens may have
encountered invasive plants in their native range, they are not subject
to stochastic introduction processes as they are already widely
distributed.
We found 20 invasive plant-pathogen combinations with 16 different
invasive plant species experiencing native/novel pathogen effects from
17 unique pathogens (Table 2). The species of plants came from a wide
variety of functional groups including grasses (3), forbs (6),
shrubs/vines (4), leguminous shrubs (2), and a tree (1). Of these
records, 13 were from North America, 2 from South Africa, 3 from Europe,
and 2 from Australia. Seven of these examples had globally distributed
pathogens of unknown origin and 13 were native or newly described
pathogens. We recorded the approximate year of introduction of the plant
invader and the year the pathogen was first observed on the invader. The
amount of time for disease to develop on the invaders ranged from
~60 years to over 200 years, with the average
approximately 120 years. This range is consistent with evidence of
reduced enemy release after similar residence time
(Hawkes 2007). It is
possible that potential invaders that are impacted by native diseases
after short residence time while still in the lag phase do not become
problematic invaders.
Some of the pathogens in our survey had major impacts on invader
populations in the field such as Pseudolagarobasidium acaciicolaon Acacia cyclops , while others showed high levels of
pathogenicity in laboratory trials but had low to moderate effects in
the field such as Erysiphe cruciferarum on Alliaria
petiolata (Cipollini et al.
2020). There are a variety of reasons that effects in the field could
be lower than in laboratory trials. For example, pathogen inoculum
density in the field may be too low to cause severe disease symptoms
(De la Cruz et al.2018), the pathogen may have poor dispersal ability in the field
preventing widespread impacts on the invader, the pathogen may only
cause symptoms in combination with certain environmental conditions
making its impact variable either spatially or temporally
(Aghighi et al.2014), or there may be differences in population level susceptibility
of the invader (Meyeret al. 2001). While we focus primarily on interspecific
differences in invader traits, it is also important to recognize that
differences in population level susceptibility are also likely related
to plant traits (Cipolliniet al. 2020). In some of the cases we identified, the pathogen
is thought to be vectored by insects. For example, rose rosette disease
is vectored by mites
(Pemberton et al.2018) and Pseudomonas syringae pv. syringae disease on C.stoebe ssp. micranthos and Botryodiplodia theobromae onMimosa pigra require wounding to infect plants and are thought to
be associated with introduced biocontrol insects
(Wilson &
Pitkethley 1992; Kearing 1996; Kearing et al. 1997). Plant
traits related to defense against herbivores could also influence the
interactions between invaders and insect vectors.
On the whole, we found relatively few examples of novel pathogen
accumulation compared with the total number of invasive plant species,
but our review also highlights areas where further research is needed.
Additional cases of novel pathogen accumulation may exist that are not
recorded in peer-reviewed literature, or are excluded from our review
because, for example, it is difficult to identify the cause of decline
or because of the limited information on the natural ranges of microbial
pathogens (Aghighi et
al. 2014). There are also strong geographic biases in the study of
invasive plants and there could be additional cases of novel pathogen
interactions in understudied regions
(Pyšek et al. 2008).
With these limitations in mind, how can we better allocate resources to
predict what invasive species may be more susceptible to decline?