2.3 Ecology niche analysis
Using observations to compare the differences in environmental
attributes of recorded sites between the native and exotic ranges in
environmental space is a main approach for quantifying niche changes
(Guisan et al., 2014). Environmental context similarity is a premise of
ENM niche analysis, and a valid conclusion can only be drawn under the
same or similar environmental conditions (Escobar et al., 2016;
Petitpierre et al., 2012). To determine the analogy environments between
the native and new distribution ranges, we adopted three metrics to test
as Qiao et al, (2017). We used Mobility-Oriented Parity (MOP) (Owens et
al., 2013) and multivariate environmental similarity surface (MESS) to
measures environmental similarity (Elith et al., 2010). Both methods are
implemented in the NicheToolbox (Osorio-Olvera et al., 2016). Then we
employed ExDet to identify similar or novel environments between native
and new distribution areas (Mesgaran et al., 2014). ExDet identifies
areas with novel univariate (novelty Type 1) or combinations (novelty
Type 2) of environmental parameters, and can identify the most
influential environmental variables leading to non-analogous environments
between the compared areas (Mesgaran et al., 2014).
To identify niche overlap and population utilization, Niche A and
Ecospat were used. We used the first three components in Niche A
software (Qiao et al., 2016) to display minimum volume ellipsoid (MVE)
generated from observations of three study areas, analyze the overlap of
three ellipsoids, and determine analogous and non-analogous
environments, then projected these environments in geographical space to
identify population utilization under analogous conditions according to
a species distribution map (Elliott et al., 2020).
After determining the occupation of niche and similar environments, the
first two PCA axes were selected at a resolution of 100×100 to compare
species density differences between the native and new distribution
areas. Meanwhile, we used the Schoener’s D metric to calculate the
degree of overlap in the ecospat package (Di Cola et al., 2017) in R (R
Core Team, 2013), including four indices overlap (D), unfilling,
stability, and expansion (Guisan et al., 2014; Petitpierre et al.,
2012). Overlap (D) measures niche overall match between two entities,
from 0 (no Overlap) to 1 (complete Overlap) (Broennimann et al., 2012).
When we overlapped the native and exotic niches, the proportion of
exotic niche that did not overlap with the native niche was termed
expansion, the exotic niche overlapping with the native niche was termed
stability, and the native niche that did not overlap with the exotic
niche was termed unfilling (Guisan et al., 2014). In addition, in order
to test whether or not the niche is equivalent and similar, we used
equivalency and similarity tests in ecospat (Broennimann et al., 2012;
Warren et al., 2008). Niche equivalency tests were performed to
determine whether the native and invaded niche generated by occurrences
were identical, and sample data were randomly run 1000 times to
calculate the overlap scores and actual overlap. When the actual overlap
value was within 95% of the simulated value, the niche equivalency
hypothesis could not be rejected. Niche similarity assesses whether the
niches of two regions are higher or lower than the random expectation
generated when the niche of one study area overlaps with the background
of another study area after 1000 randomizations. If the actual overlap
value was greater than 95% of the simulated value, the actual overlap
was significantly greater than expected. According to the results of
Niche A, we decided to compare the new southern and present niches to
the original native population as niche conservatism (the alternative =
”greater”), and to test the new northern and original native population
niches as niche divergence (the alternative = ”lower”). Finally, the
ecospat package was used to identify response range differences of the
10 environmental variables between the new northern and original native
populations.