Discussion
We aimed to document the introduction of the Eucalyptus snout
beetle in Ecuador, determine its taxonomic placement, and model
potentially suitable habitat throughout South America. With this
framework, we provide the first genetic evidence for the presence ofGonipterus platensis in Ecuador. Furthermore, our ecological
niche model analysis suggests areas of suitable habitat throughout a
broad range of climatic and elevational regimes that may have played a
role in the establishment of populations of Eucalyptus snout
beetles in South America and could promote its future expansion into new
areas.
The phylogenetic analysis we showed herein highlighted large genetic
variation across the Gonipterus scutellatus species
complex. However, most of the samples analyzed in this study came from
Australia and Tasmania, with few samples coming from Portugal, Spain and
South Africa (Mapondera et al., 2012), and only the nine samples we
collected coming from South America. DNA barcoding data (mitochondrial
COI sequences) clearly resolved the identity of the beetles found in
Ecuador as G. platensis , which has also been documented in other
South American countries (i.e., Argentina, Brazil, Colombia, Chile, and
Uruguay) (Garcia et al., 2019; Mapondera et al., 2012; Schröder et al.,
2020). The genetic diversity of G. platensis was one of the
lowest among the species analyzed in this study (see values of π for
each clade in Fig. 2), which coincides with the results found by
Mapondera et al. (2012) for this species in Western Australia. In fact,
the Ecuadorian samples share the same haplotypes with individuals from
Tasmania (Garcia et al., 2019), which suggests that the Ecuadorian
population could have originated from a Tasmanian source.
Notwithstanding, an analysis including genetic samples from Colombia and
other South American countries is needed to thoroughly examine whether
the Ecuadorian samples were a product of a direct introduction from the
native range or if they were secondarily introduced from another South
American country. An introduction from Colombia seems plausible given
geographical proximity and the continuity of suitable climatic habitat
predicted by our model (Fig. 3). Moreover, our phylogeny suggested that
there are at least three different geographical origins of the G.
platensis specimens found outside Australia. Indeed, two different
sequences of this species from Spain and Portugal (JN391479_ESP and
JN391480_POR) are the most different compared to the sequences of South
America and Tasmania, as evidenced by their long branches in the
phylogeny (see Gonipterus platensis clade in Fig. 2). Additional
sampling and genetic analysis are necessary from Eucalyptus snout
beetle populations from Southeastern Australia and from other South
American countries to truly pinpoint the origin of invasive populations.
Two main challenges have been identified when producing ecological niche
models (ENM) for invasive species (Lake et al., 2020). First, invasive
species are often in disequilibrium with the novel environment they
occupy, and second, generating ENM projections from native to invaded
ranges may be problematic for correlative model approaches (Elith et
al., 2010). We followed an ENM approach at the species level, including
all unique localities of G. platensis in South America available
in GBIF (GBIF.org, 2022; www.gbif.org;
https://doi.org/10.15468/dl.8mwpht). This approach is justifiable
because few records were available in open-source databases from the
native range of Australia (N = 9). Such a low number of occurrences
would bias ENM predictions, particularly when projected from native
ranges into novel environments where the species was introduced.
Furthermore, using only the native range to estimate the potential
suitable habitat of a species may result in misrepresentation of
predictions, particularly if the colonization into the invaded range is
characterized by niche expansion (Broennimann et al., 2007). To mitigate
the spatial data bias, we rarefied records and used a background extent
that included only potentially reachable areas (following Elith et al.,
2010). This choice of background would reduce the degree of model
extrapolation (Elith et al., 2010). Despite the uneven sampling, our
approach allowed us to produce estimates of the areas where G.
platensis occurs, but also assess regions of suitable habitat where
these beetles could potentially be found or invade (Fig. 3).
The ENM predictions presented herein attained a high true positive
fraction for the currently known observations (Appendix, Fig. A1).
Notably, the westernmost locality in the Buenos Aires province of
Argentina showed the lowest predicted distribution values and was
excluded by the LPT95 threshold. This inland locality of Argentina was
characterized by a narrow range of high temperatures (i.e., little
variation in temperature seasonality) and generally dry conditions
(i.e., lowest precipitation of coldest quarter) that were unique
compared to all other occurrences. Despite the lack of observation
records from GBIF, model predictions showed that suitable habitat forG. platensis exists in southeastern Brazil in the city of
Curitiba, PR, where the beetle was first documented as invasive for the
country (Fig. 3; op. cit. G. scutellatus in Freitas, 1979). This
region of suitability in Brazil spans from the state of Espírito Santo,
south through Rio Grande do Sul, and west into Mato Grosso do Sul, in
which G. platensis is widely recognized as a pest ofEucalyptus forests (de Souza, 2016; Wilcken & Oliveira, 2015).
In the northwestern parts of South America, ENM predicts suitable
habitat across the Andes from Colombia to Bolivia at high elevations
(Fig. 3). In Ecuador, G. platensis was predicted to have
significantly higher elevations than elsewhere in the continent (Fig. 4;
Table 1), with highly suitable habitat existing in and around the cities
of Quito, Ambato, and Cuenca and with at least moderately suitable
habitat occurring all along the Ecuadorian Andes (Fig. 3). Suitable
habitat at high elevations was also predicted throughout Perú, which may
represent potential areas for invasion (Fig. 3). In the southern
countries, the model resulted in high to moderate values of suitability
at lower elevations in Chile, Argentina, and Uruguay, respectively (Fig.
3). Examination of the climate envelopes revealed that G.
platensis in Ecuador occupies significantly less seasonal and drier
conditions than in other parts of the invaded range at similar
elevations (Fig. 4; Table 1). Nonetheless, when considering the entire
distribution, this beetle occurs across a wide elevational range and a
variety of suitable environmental conditions where it could potentially
sustain long term populations. The sampling bias, which likely excluded
areas occupied by G. platensis but not included in GBIF or not
yet documented across South America, renders our ENM predictions
somewhat exploratory. Nonetheless, these results combined with climate
envelope analysis highlights the potential of G. platensis to
occupy a broad range of environmental conditions, which may allow it to
become highly invasive in different parts of South America.
Finally, we make a call to study the ecological and economic impacts of
the invasion of Eucalyptus snout beetle and its hosts (e.g.,Eucalyptus globulus ) in South America, particularly in Colombia
and Ecuador. In northern South America little research has been
conducted on forestry of Eucalyptus species, whereas in Brazil
and Chile, forestry research is quite active. In the particular case of
Ecuador, Eucalyptus trees were planted in the late 19th century,
mainly in volcanic soils, known as ‘cangagua’, in the inter-Andean
valleys (FAO, 1981). These valleys have had human influence for
millennia (Bush et al., 2022; Young, 2009), so perhaps these degraded
environments have lost natural enemies that could exploit these insects
as new hosts. We thus recommend extensive sampling of Eucalyptussnout beetles and potential controllers in Ecuador to test this
hypothesis. Also, as far as we know and according to records in GBIF,Gonipterus platensis remains concentrated in a couple of areas of
the Ecuadorian Andes (around the city of Quito in the North and in the
city of Loja in the South, Appendix, Fig. A1), but our model predicts
the whole range as climatically suitable for invasion. In fact, climatic
suitability expands across the border to the Peruvian Andes, within the
area of distribution of Eucalyptus spp. plantations (Castillo
Vera et al., 2019; Luzar, 2007), where no reports of the insects have
been made so far. All this highlights the urgency of conducting more
studies on the invasion of this pest to design effective control and/or
prevention measures.