Results
The standardized protocol for recording plant species communities along
mountain roads has been thoroughly tested in the field on all continents
except Antarctica (Figure 4). The first survey was carried out in eight
regions in 2007 and has been repeated every five years since, resulting
in one baseline historical survey (2007) and up to two resurveys (2012
and 2017). The number of regions has increased since 2007, with 18
regions performing the survey by 2018 (Figure 4). The global database
currently includes circa 2,700 plots and >100,000
observations of >5,000 vascular plant species.
One of the most striking findings of the global MIREN surveys to date
has been to document the importance of roads in facilitating mountain
invasions. Specifically, we have found that non-native species richness
in roadsides decreases with increasing elevation, but generally peaks in
the lower third of the elevation gradient (Alexander et al., 2011).
Moreover, the vast majority of non-native species found at high
elevation are also present at low elevation, with only 2.3% of
high-elevation non-native species being specifically adapted to alpine
environments (Alexander et al., 2016). These findings indicate that
non-native species are first introduced and become established at low
elevation sites, following this they spread to higher elevations
(Alexander et al., 2011). At higher elevation sites, non-native species
generally become increasingly filtered out by environmental pressures,
so fewer warm-adapted perennials reach higher elevations (McDougall et
al., 2018). We have also revealed that the number of non-native species
declines with increasing distance from the road (Seipel et al., 2012;
Haider et al., 2018), indicating that the native plant community serves
as a second environmental filter that selects for more shade- and
moisture-tolerant perennials (McDougall et al., 2018). In addition to
non-native species, the MIREN surveys have shown that native species
also use roads as corridors (Lembrechts et al., 2017). Interestingly,
Lembrechts et al. (2017) found that occurrence optima are higher in
roadside habitats than faraway habitats, and moreover that some alpine
species have shifted their ranges downwards due to altered abiotic
conditions and competitive release in roadside habitats (see also e.g.
Lenoir et al., 2010).
Globally, the MIREN surveys have demonstrated that native plant species
richness does not follow a consistent pattern in non-roadside
(semi-)natural habitat along elevation gradients, suggesting the
existence of additional region-specific mechanisms, such as biome,
vegetation type and human activity. These mechanisms are now the subject
of further study. In contrast, a clearer elevation signal is present on
roadside plots, with total species richness peaking at mid-elevations in
most regions (Haider et al., 2018). Further, we have observed a
reduction in community dissimilarity (beta-diversity) along roadsides
relative to more distant plots, which is amplified by the arrival of
non-native species along mountain roadsides homogenizing plant community
composition (Haider et al., 2018). The MIREN surveys have also provided
insight into the vulnerability of habitats regionally (Pollnac et al.,
2012), the genetic background of successful invasions (Haider et al.,
2012) and the impact and management of local invasions (McDougall et
al., 2011a). For example, in the Greater Yellowstone Ecosystem in the
United States, we found that non-native species emergence varies with
elevation and habitat type, which provided land managers valuable
information for mitigating biological invasions (Pollnac et al., 2012).
Moreover, in the dry Mediterranean Andes in Argentina, which are
characterized by treeless vegetation, the survey demonstrated how
non-native plant species can successfully spread from the roadside into
natural vegetation at low and intermediate elevations, thus highlighting
the susceptibility of these types of ecosystems to invasion (Aschero et
al., 2017). By contrast, the alpine vegetation of northern Norway has
been shown to be more vulnerable to invasion than its low elevation
counterpart, indicating that vegetation structure plays an important
role in community invasibility (Lembrechts et al., 2014). Finally, the
MIREN surveys have already generated information about regional floras.
An excellent example is the discovery of a new species of Poaceae during
MIREN monitoring in Kosciuszko National Park, Australia – this species
was named after the network: Poa mireniana (Walsh & McDougall,
2018).