Supporting Information Notes S1 & Tables S1–S5
Notes S1 Description of datasets and analyses
Data
The state of Victoria is located in south-eastern Australia and extends from 34°4’ S to 39°6’ S
and 140°59’ E to 149°42’ E, covering just under 230,000 km2. Home to 5.7 million people, it
has a temperate climate with the majority of annual rainfall occurring in the cooler winter
months (mean varies between 300 mm to 2300 mm across the region, BOM 2007).
Vegetation in the state has been categorised into 20 broad vegetation groups, known as
Ecological Vegetation Classes (Table S1, DSE 2009). These can be broadly thought of as
ecosystem types. We use plant occupancy (i.e. presence/ absence) and cover abundance data
collected by the Victorian Department of Environment and Primary Industries (DEPI,
formerly DSE) between 1970 and 2011. Foliar projective cover of plant taxa was estimated in
29,991 30 m² quadrats using the Braun-Blanquet scale and converted to midpoint averages
prior to analysis. The mean number of quadrats surveyed per ecosystem was 1,501 (SE 287),
with the least surveyed ecosystem type being Rocky Outcrop or Escarpment Scrubs (189
quadrats; Table S1). Of 4,609 taxa recorded across the state, 3,575 were native to Victoria and
1,036 were non-native (or alien) to Victoria (967 taxa were alien to Australia). Alien species
were recorded in 21,871 of the 29,991 quadrats surveyed. Native species were recorded in
29,983 quadrats.
The Global Invasive Species Database is managed by the Invasive Species Specialist Group
of the IUCN Species Survival Commission and contains information about 965 invasive taxa,
458 of which are higher plants (ISSG 2013). The GISD focuses on alien species that may
negatively impact native biodiversity and is compiled from contributions by ISSG members
and invasive species experts. The database covers a broad range of taxonomic groups that
invade a broad range of environments and includes qualitative information about the types of
habitat that each species invades (16 habitat types included). Despite a slight geographical
bias that stems from differences in funding (ISSG 2013), the GISD provides an opportunity to
examine global trends in invasive plant species invasion. We restrict our examination to
higher plants.
We use the GISD to assess Hypotheses 1 and 2 and the Victorian data to assess Hypotheses 1,
3 and 4.
Analysis
Recognising that definitions of riparian habitats vary and that species occupying these zones
may be found in other aquatic-terrestrial transition zones that require similar ecophysiological
1
adaptations, we take two approaches for comparing among habitat types. 1) We use the
classifications as supplied by the two databases (GISD: ‘Riparian zones’, Victorian data:
‘Riparian Scrubs or Swampy Scrubs and Woodlands’, hereon Riparian Scrubs) and refer to
these simply as riparian zones. 2) We use a broader category of riparian zone by grouping
several ecosystem types (GISD: ‘Riparian zones’ and ‘Wetlands’; Victorian data: ‘Riparian
Scrubs or Swampy Scrubs and Woodlands’, ‘Wetlands’ and ‘Riverine Grassy Woodlands or
Forests’) and refer to this as ‘Broad Riparian’.
We assessed the likelihood of each of the four hypotheses proposed in the following ways.
Hypothesis 1: A high proportion of the alien species that invade riparian zones exclusively
invade riparian ecosystems
Of the entire alien species pool recorded in Victoria and in the GISD, we calculated the
proportion that exclusively invades riparian ecosystems. This gives an indication of
ecosystem specialisation or habitat fidelity. We did this for both datasets. For the Victorian
data, we also determined the proportion of native species that are only recorded in riparian
ecosystems. Because the native data is not subject to introduction bias (though it could be
subject to differences in frequency, condition and ecological threat), it provides a point of
comparison allowing us to determine whether the alien species pool is relatively more biased
towards riparian species.
Hypothesis 2: Riparian invaders have higher propagule pressure and more dispersal
opportunities than species that invader other types of ecosystems
We used the GISD for this analysis. We did not have information about the fecundity of
individual species nor their dispersal ability, so had to rely on information about species’
dispersal modes to test this hypothesis instead. The GISD lists the types of dispersal modes
used by each species. There are 12 categories: animals, vegetative, water currents, wind,
agriculture, ornamental, garden escape, contaminated soil, hikers, vehicles, forestry,
unspecified. We assume that species that use a greater variety of dispersal modes have the
potential for more effective and widespread dispersal. Using information about the dispersal
modes used by higher plants, we compared the number of dispersal modes used by species
that invade riparian ecosystems versus those that do not. Given the importance of hydrochory
for riparian plant dispersal, we also examined trends in hydrochory (i.e. species recorded as
using water currents as a means of local dispersal).
To test whether riparian invaders have a closer association with humans than species that
invade other ecosystem types, we created an additional category indicating human-mediated
dispersal. If species were recorded as dispersing by at least one of seven categories
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(agriculture, ornamental, garden escape, contaminated soil, hikers, vehicles and forestry) they
were listed as being dispersed by humans.
We compared the prevalence of human-mediated dispersal and the number of dispersal modes
used by riparian and non-riparian invaders using one-way ANOVAs in Minitab 16.2.4. Data
met the assumptions of the tests.
Hypothesis 3: A greater proportion of the alien species pool invades riparian ecosystems
compared to other ecosystem types.
We calculated the proportion of invaders recorded in each ecosystem. For the GISD, this was
based on information about the habitats each individual species invades. For the Victorian
data, we specified that a species must occur in at least 10 quadrats for a given habitat type to
be considered an invader of that ecosystem (i.e. excluded the equivalent of 5% of quadrats of
the least sampled vegetation type). This was to avoid census errors in species observations
and difficulties associated with ecosystem classification (e.g. where a site is effectively in a
transition zone between two types of vegetation or has recently changed from one vegetation
type to another).
Hypothesis 4: Invaders of riparian zones are more successful and invasive than those of other
ecosystems
Using the Victorian data, we determine whether alien species in riparian zones contribute
higher cover relative to their richness in comparison with other types of ecosystems. We
calculated the proportion of total vegetation cover and total species richness in each quadrat
that was made up of species that are alien to Victoria (proportional alien cover and
proportional alien species richness). In doing this, we use relative alien cover as a measure of
invasion level. Proportional cover of alien taxa indicates the relative contribution that they
make to the local plant community. Examining relative alien cover in relation to relative alien
species richness indicates whether alien species, on average, are more or less abundant than
the average native species. If relative alien cover is greater than relative alien richness, alien
species are effectively ‘punching above their weight’ in terms of abundance (Catford et al.
2012). We consider ecosystems that have a higher ratio of relative alien cover to relative alien
richness as containing more successful invaders.
References
BOM. 2007. Climate of Australia. URL
http://www.bom.gov.au/lam/climate/levelthree/ausclim/ausclim.htm
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Catford JA, Vesk PA, Richardson DM, Pyšek P. 2012. Quantifying levels of biological
invasion: towards the objective classification of invaded and invasible ecosystems. Global
Change Biology 18: 44–62.
DSE. 2009. Accessing Native Vegetation data. URL
http://www.dse.vic.gov.au/DSE/nrence.nsf/LinkView/51B859D762D586FBCA25709C007F1
14E522C816829EBF3F7CA25700C00240E63 - desc
ISSG. 2013. Global Invasive Species Database (GISD) In. Invasive Species Specialist Group,
IUCN (The World Conservation Union). http://www.issg.org/database
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Table S1 Number of quadrats surveyed for each ecosystem type (or Ecological Vegetation
Class) in Victoria between 1970 and 2011.
Ecological Vegetation Class
Quadrats surveyed
Wet or Damp Forests
4965
Dry Forests
4578
Lowland Forests
2788
Plains Woodlands or Forests
1851
Heathy Woodlands
1724
Riparian Scrubs or Swampy Scrubs and Woodlands
1693
Heathlands
1568
Sub-alpine Grasslands
1558
Mallee
1549
Plains Grasslands and Chenopod Shrublands
1283
Lower Slopes or Hills Woodlands
1032
Riverine Grassy Woodlands or Forests
923
Coastal Scrubs Grasslands and Woodlands
815
Montane Grasslands
704
Box Ironbark Forests or dry/lower fertility Woodlands 649
Wetlands
615
Rainforests
537
Herb-rich Woodlands
506
Salt-tolerant and/or succulent Shrublands
464
Rocky Outcrop or Escarpment Scrubs
189
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Table S2 Our approach for testing four hypotheses proposed to explain high levels of riparian
invasion based on characteristics of invading species. Datasets: GISD, Global Invasive
Species Database; Vic, Victorian Department of Environment and Primary Industries
vegetation quadrat data.
Hypothesis
Test
Dataset
Our findings
1) More riparian specialists
High proportion of the alien
GISD, Vic
No support
in alien species pool
species pool exclusively
GISD
Support
Vic
Support
Vic
Little support
invades riparian zones
2) Higher colonisation and
Greater prevalence of
propagule pressure of
human-mediated dispersal
riparian invaders
and higher number of
dispersal modes used by
species that invade riparian
ecosystems compared to
species that invade other
ecosystems
3) More alien species can
High proportion of the alien
pass the environmental
species pool occurs in
filters of riparian zones
riparian ecosystems
compared to other
ecosystems
4) Riparian invaders are
Relative abundance of
more competitive and
species invading riparian
invasive than other invaders
ecosystems is higher than
invaders of other ecosystems
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Table S3 Results of a one-way ANOVA comparing the number of dispersal modes used by
species that invade and do not invade riparian zones.
Degrees of
Sum of
Mean
freedom
squares
squares
Riparian
1
97.03
97.03
Error
456
1258.17
2.76
F
P
35.17
<0.001
Table S4 Results of a one-way ANOVA comparing the prevalence of dispersal using water
currents among species that invade and do not invade riparian zones.
Degrees of
Sum of
Mean
freedom
squares
squares
Riparian
1
2.941
2.941
Error
456
93.873
0.206
F
P
14.29
<0.001
Table S5 Results of a one-way ANOVA comparing the prevalence of human-mediated
dispersal among species that invade and do not invade riparian zones.
Degrees of
Sum of
Mean
freedom
squares
squares
Riparian
1
3.066
3.066
Error
456
111.327
0.244
F
P
12.56
<0.001
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