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Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
Floristic composition and potential invasiveness
of alien herbaceous plants in Western Mexico
Isabel Pérez-Postigo
1*
; https://orcid.org/0000-0002-9974-9198
Heike Vibrans²; https://orcid.org/0000-0002-1800-4320
Jörg Bendix³; https://orcid.org/0000-0001-6559-2033
Ramón Cuevas-Guzmán
4
; https://orcid.org/0000-0002-4980-8989
1. Programa de Doctorado en Biosistemática, Ecología, Manejo de Recursos Naturales y Agrícolas, Universidad de
Guadalajara, Autlán de Navarro, Jalisco, Mexico; perezpostigo@gmail.com (*Correspondence)
2. Posgrado en Botánica, Colegio de Postgraduados, Texcoco, Estado de Mexico, Mexico; heike@colpos.mx
3. Faculty of Geography, Philipps Universität Marburg, Marburg, Hessen, Germany; bendix@mailer.uni-marburg.de
4. Departamento de Ecología y Recursos Naturales, Universidad de Guadalajara, Autlán de Navarro, Jalisco, Mexico;
rcuevas@cucsur.udg.mx
Received 16-II-2021. Corrected 30-VII-2021. Accepted 07-IX-2021.
ABSTRACT
Introduction: Numbers of alien plant species are rising around the globe, but not all of them become invasive.
Whereas introductions have been documented for several decades in some regions of the world, knowledge on
alien species in Western Mexico is limited. Here, we study roadside vegetation along an elevational gradient,
which includes a protected area.
Objective: We analysed the floristic composition of herbaceous alien species, their distribution patterns, and
their relationship with various environmental factors. A relative importance value index (IVI) identified the most
important and, therefore, probably invasive taxa.
Methods: During 2017 and 2018, roadside vegetation was documented with 4-6 transects every 300 altitudinal
meters, from 0 to 2 100 m, for a total of 37 transects. Each transect consisted of five 1 m² plots. All herbaceous
species were registered and alien taxa identified. A cluster analysis distinguished grouping of species based on
elevation. The potentially invasive species were identified by their IVI, based on the sum of relative frequency
and density values. The influence of environmental variables was analysed with a canonical correspondence
analysis.
Results: Most alien species were grasses; other families were represented by one or two species. The species
were grouped into three main clusters. The first group included rare species, the second consisted of species
restricted to higher altitudes, and the third group were tropical taxa with a distribution from sea level to medium
altitudes. The most important potentially invasive species were: Urochloa maxima, Melinis repens, Eragrostis
ciliaris and Cynodon dactylon, all African grasses introduced for grazing. The IVI of the species was related to
tree cover, leaf litter depth and surface stone cover for some species and, for others, to soil compaction, distance
to major roads and elevation.
Conclusions: The alien ruderal species clustered according to the general climate (temperate vs. tropical).
Grasses of African origin are of highest concern as invasive species. Although most introductions are related to
human disturbance, each species becomes dominant under certain environmental conditions. Thus, management
programs must be specifically adjusted to each individual invasive alien.
Key words: environmental variables; exotic plants; invasive herbs; ruderal weeds; Sierra de Manantlán.
Pérez-Postigo, I., Vibrans, H., Bendix, J., & Cuevas-Guzmán,
R. (2021). Floristic composition and potential invasiveness
of alien herbaceous plants in Western Mexico. Revista
de Biología Tropical, 69(3), 1037-1054. https://doi.
org/10.15517/rbt.v69i3.45855
https://doi.org/10.15517/rbt.v69i3.45855
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Ecosystems have been influenced by
human activities over several millennia. How-
ever, the most apparent impact has been land-
use change. Over the last 100 years, more than
50 % of the habitable land surface has been
converted into urban areas, crop fields and
rangelands (Ellis et al., 2010). Much more
inconspicuous, but also an important factor of
anthropogenic influence, are introductions of
alien plant and animal species to new regions
(Ellis et al., 2010; Elton, 1958; Mack, 1991).
This human mediated transfer of species is
the largest in the evolutionary history of the
planet (Elton, 1958; Mack et al., 2000), such
that at least 13 168 plant species worldwide
are naturalised outside their original distribu-
tion (van Kleunen et al., 2015a). These species
are considered a major threat to local natural
ecosystems (Espinosa-García & Villaseñor,
2017; Rejmánek et al., 2005; Villaseñor &
Espinosa-García, 2004) as some of them have
been shown to change the structure and func-
tioning of ecosystems (Elton, 1958; Pejchar &
Mooney, 2009).
Not all introduced alien species can natu-
ralise in a new environment, and naturalised
species do not necessarily become invasive
and cause harm (Richardson et al., 2000).
Williamson and Fitter (1996a) proposed the
“rule of 10” that states that only about 10 %
of all introduced species become casual, and
10 % of these naturalise. Of the naturalised
species, around 10 % become invasive (Jarić
& Cvijanovic, 2012; Williamson & Fitter,
1996a). There is much regional variation in the
proportions (Jarić & Cvijanovic, 2012; Jeschke
et al., 2012). Although the “rule of 10” is not
a precise rule and has been criticised, it can
provide guidance (Jarić & Cvijanovic, 2012).
Of course, the other 90 % of the naturalised
species may become invasive in the future,
particularly under changing circumstances, and
their potential impact on natural ecosystems
should therefore not be underestimated (Sim-
berloff, 2011).
Three main drivers of invasion success
exist: (i) invasibility of the site, (ii) propagule
pressure and (iii) traits of the species related to
their invasiveness (Barney & Whitlow, 2008;
Catford et al., 2009). The invasibility of a habi-
tat is related to different factors. In general, dis-
turbed or anthropogenically modified habitats
are more prone to invasions than natural sites
(Barney & Whitlow, 2008; Hierro et al., 2005).
To form a population large enough to survive,
reproduce and naturalise, a species needs to
be introduced in large numbers or be able to
produce sufficient propagule pressure (van
Kleunen et al., 2015b; Williamson & Fitter,
1996b). Although scientists agree that the inva-
sion success of the species depends on their
biological traits, no general set of traits respon-
sible for invasiveness has been determined.
Traits seem to be different for each ecosystem
and vary depending on the stages of the inva-
sion process (Sol, 2007). The distribution and
abundance of the species in their native range
can also influence invasion success. Species
with a wide environmental aptitude in their
native range can be expected to adapt to a wide
range of conditions in the new environment
(Dawson et al., 2009).
Alien species richness is often high at low
elevations and decreases with altitude (Alex-
ander et al., 2011; Pauchard & Alaback, 2004).
Most alien species are introduced in high-den-
sity areas of human population, which is gener-
ally concentrated at low and medium elevations
(Alexander et al., 2011; Pauchard, et al., 2009).
A unidirectional expansion of alien species to
higher elevations causes directional filtering of
species with narrow climatic tolerances. Envi-
ronmental conditions at high elevations may
be extreme; nevertheless, alien species here
are generally not highly specialised but gen-
eralists (Alexander et al., 2011). Even though
fewer introduced species are present at higher
elevations, those species may pose a relatively
greater risk to native ecosystems (Alexander et
al., 2011; Pauchard et al., 2009) because gen-
eralists can be expected to be less affected by
disturbance and climate change than native and
highly specialised species. However, whether
this applies to Mexico with its high-altitude
population centres remains to be seen.
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The present-day number of documented
alien plant species in Mexico is between 700
and 750. Most of them are herbaceous weeds
and grasses (Espinosa-García et al., 2004;
Espinosa-García & Villaseñor, 2017). Between
58 and 180 weed species are invasive and may
be causing environmental or socioeconomic
damage (Espinosa-García & Villaseñor, 2017).
A relationship of alien species richness and
abundance to altitude and other environmen-
tal factors was not found (Pérez-Postigo et
al., 2021; Sánchez Medrano 2018). However,
knowledge of single species distribution pat-
terns and floristic groups is still lacking.
We analyse the floristic composition of
alien herbaceous species and their clustering
over an elevation gradient in Western Mexico.
We expect them to form floristic groups related
to climate, and the species’ origin (Dawson et
al., 2009). Also, we study their relationship to
different environmental variables. We then aim
to identify the alien herbaceous species which
might be invasive in the region. We indicate
the origin of these invasive taxa and discuss
their traits as well as their invasion success in
other introduced ranges. We expect paleotropi-
cal grasses to be the most invasive taxonomic
group. They are known invaders in other tropi-
cal American regions, growing at a wide range
of sites that includes different environmental
conditions (Pauchard et al., 2009; Williams &
Baruch, 2000).
MATERIALS AND METHODS
Study site: We worked in the Sierra
de Manantlán and the surrounding areas in
the states of Colima and Jalisco in Western
Mexico. The Sierra runs from Northeast to
Southwest and is part of the Sierra Madre del
Sur. The subtropical climate has a dry and a
rainy season. The precipitation patterns vary
with exposure: the Southern and South-Eastern
slopes are windward and humid; the Northern
ones are dry with semi-desertic conditions
(Vázquez García et al., 1995).
The region has a wide range of natural
and human-modified vegetation types (INE,
2000; Vázquez García et al., 1995). Coast-
al mangroves are dominated by Conocarpus
erectus L., Laguncularia racemosa (L.) C.F.
Gaertn, and Rhizophora mangle L., which are
sometimes displaced by plantations of Cocos
nucifera L., with herbaceous plants in the
understory. Tropical dry and subhumid forests
are the natural vegetation of the coastal plain
and elevations up to 1 700 m where the her-
baceous vegetation is dominated by geophytes
(Cruz Angón et al., 2017; Vázquez García et
al., 1995). Cloud forests with high numbers of
epiphytes, lianas and other herbaceous species
cover humid locations at elevations between
700 and 2 600 m (Rzedowski, 1978; Rze-
dowski & McVaugh, 1966). Quercus and Pinus
forests are found at higher elevations, with
Abies and Cupressus covering smaller patches
(Rzedowski, 1978; Rzedowski & McVaugh,
1966; Vázquez García et al., 1995). The region
is known to harbour a relatively high number of
endemic plants (Vázquez García et al., 1995).
Humans have influenced the region since pre-
Columbian times (Kelly, 1945).
Data: We focused on ruderal vegetation
along an elevational gradient of over 2 100 m,
from sea level near the village of La Manzanilla
(La Huerta, Jalisco) to upper parts of the Sierra
de Manantlán, during the years 2017 and 2018.
Transects were located as close as possible to
eight predetermined elevation levels, 300 m
apart in altitude. Sites with ruderal vegetation,
at least 5 m wide and 25 m long parallel to
major or minor roads, were selected. Also, we
considered personal security when selecting the
sites. We surveyed between four to six transects
per elevation level. Each transect with a length
of 20 m contained 1 m² plots every five meters
(for further details of sampling methods, see
Pérez-Postigo et al., 2021).
All herbaceous species within the 1 m²
plots were registered and all individuals count-
ed. For species with stolons or rhizomes, groups
of culms were counted as separate individuals.
The species were identified in the field if pos-
sible; one of the authors (RCG) is a specialist
of the regional flora. We collected several
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individuals from every unknown morpho-spe-
cies for identification and three individuals
from every known species for documenta-
tion. The vouchers were deposited at the ZEA
Herbarium of the University of Guadalajara in
Autlán de Navarro. We consulted specialised
literature on regional flora, identification keys
and the ZEA Herbarium for identification
of the 500 collection numbers, resulting in
317 identified species. One specimen was a
new species, not yet described, and thirty-five
specimens could be determined only to the
genus level. To identify the alien species, we
considered the publications of Villaseñor and
Espinosa-García (2004) and Espinosa-García
and Villaseñor (2017). Synonymy was vetted
with the Tropicos database (www.tropicos.org).
Environmental information on nutrient,
water and light resources with possible explan-
atory value for the herbaceous diversity distri-
bution was documented in the field (Chapin et
al., 1987). Geographic position and elevation
were measured with a Garmin etrex GPS, and
the slope of the transect was determined in
% with a Haga clinometer. Slope affects soil
drainage and depth, which influence vegetation
types, dispersal, diversity, richness and growth.
On steeper slopes, rainwater runoff is higher
and soil erosion also increases (Neal, 1938).
For each plot, the surface stone cover percent-
age was estimated in five categories. This is
an important variable since stone cover acts as
protection for the soil against erosion (Mandal
et al., 2005). Leaf litter depth was measured
in cm and the leaf litter cover estimated as a
percentage. Both values were captured per plot
and was expected to positively correlate with
nutrient availability in the soil (Bastida et al.,
2008). Compaction of topsoil as the uncon-
fined strength of kg/cm² was measured for
every plot using a pocket penetrometer from
Soil Test Inc. Topsoil density is an important
variable for plant water and nutrient uptake
and an indicator of disturbance such as grazing
or vehicle movement (Passioura, 1991; Wood-
ward, 1996). To measure the available light
from solar radiation at each plot, we estimated
the percentage of tree cover with a spherical
densiometer from Robert E. Lemmon Forest
Densiometers. We classified the influence of
grazing and fire intensity of every plot in five
categories, from none to severe. Mean values
per transect were calculated using all registered
data on the 1 m
2
plots.
The distance to paved federal and state
roads and highways was calculated in QGIS
(version 3.6.2, QGIS.org, 2019) as the 2D dis-
tance between transects and the nearest point on
these major roads or highways; in some cases,
the transects were along such roads and so the
distance was 3-5 m. The data for roads and high-
ways were obtained from the Atlas de Caminos
y Carreteras del Estado de Jalisco from the year
2012 published by the Government of the State
of Jalisco (Gobierno de México, 2018). Tem-
perature and precipitation regimes are known
to be essential for plant species distribution pat-
terns, thus we included temperature and precipi-
tation variables (Chapin et al., 1987). Monthly
temperature and precipitation data at a 30-arc s
resolution (1 km²) were downloaded from the
Worldclim database, which offers mean values
of the period 1950 to 2000, interpolated from
data of different sources (Hijmans et al., 2005).
We calculated the annual mean temperature and
the annual precipitation sum, using QGIS to
extract the monthly data and R (version 4.0.2,
R Core Team, 2020) to calculate the annual sum
and mean.
Data analysis: We used the importance
value index (IVI) (Curtis & McIntosh, 1951)
as a measure of invasiveness of a species. IVI
is considered a good indicator of the functional
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importance of a species in a community, as it is
based on relative density and frequency values
expressed as percentages per transect (Catford
et al., 2012; Curtis & McIntosh, 1950):
A cluster analysis identified floristic
groups along the elevation gradient. For this,
the species were standardised by dividing their
IVI in each transect by the sum of their IVI
in all transects in which the species is found
and multiplying the result by 100. With the
matrix of standardised values, we obtained the
Bray-Curtis similarity between pairs of spe-
cies, known as the Whittaker index of species
association (Clarke et al., 2014). The cluster
analysis was based on these values, using
group averages as the joining method, accom-
panied by a similarity profile test (SIMPROF)
type 2 and 3. The type 2 test is based on the null
hypothesis of the non-existence of associations
between the species. The type 3 test examines
the non-existence of differences between the
groups of species with a statistical permutation
test for which more information can be found
in Clarke et al. (2014). A heatmap-plot (“heat-
map” function stats package, R Core Team,
2020) of the IVI natural logarithm showed the
IVI of all species.
To explore the relationship between spe-
cies IVI and environmental variables for all
transects, we used the canonical or constrained
correspondence analysis (CCA) by applying
the “cca” function from the vegan package
in R (Oksanen et al., 2019). Direct ordination
techniques such as CCA are widely used to
explore relationships between environmental
variables and the distribution and abundance of
species (Borcard et al., 1992). We reduced the
set of environmental variables using the “find-
Correlation” function of the caret package
(Kuhn, 2020). We identified and excluded the
following four highly intercorrelated variables:
mean temperature, distance to highways, fire
intensity and leaf cover. To test the significance
of the CCA, an ANOVA-like permutation test
(“anova.cca” function) was run for the whole
CCA and for each axis separately (Oksanen et
al., 2019).
We considered the four most dominant and
widespread species to be invasive, based on
the tens rule (only about 10 % of naturalised
species become invasive) (Jarić & Cvijanovic,
2012; Williamson & Fitter, 1996b) and the IVI.
A CCA was run for these four species, using the
vegan package in R (Oksanen et al., 2019). For
the CCA, five highly correlated variables were
excluded: mean temperature, fire intensity, leaf
cover, slope and the distance to roads.
RESULTS
Floristic composition: Of the 317 identi-
fied species, 285 were natives belonging to 175
genera and 45 families. The 32 alien species
from 27 genera and 11 families represented
10.09 % of all collected species, but they
accounted for 16.59 % of all plant individuals.
Along the entire elevation gradient Asteraceae
(41), Poaceae (40) and Fabaceae (38) had the
most native species, followed by Malvaceae
(21), Cyperaceae (15), Convolvulaceae (12),
Commelinaceae (10) and Euphorbiaceae (10).
The other families had fewer than 10 species,
most of them only one. There were 20 548
native plant individuals in the transects. Native
species of Asteraceae were the most abundant
with over 5 500 individuals; native Poaceae
and Fabaceae had around 3 850 individu-
als. The families Convolvulaceae, Malvaceae,
Lamiaceae and Caryophyllaceae had abun-
dances of over 500 individuals each.
The family with the highest number of
alien species was Poaceae with 20 species and
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16 genera. The family Asteraceae had two alien
species from two genera and Primulaceae two
Anagallis species. All the other 10 families
were represented by only one alien species and
one genus. Poaceae was the most abundant
family with 3 943 individuals of alien spe-
cies, followed by Primulaceae with 43 plants
and Asteraceae with 35 (Table 1). The 12
most abundant alien species were grasses, fol-
lowed by Taraxacum officinale (Asteraceae),
Achyranthes aspera (Amaranthaceae), and
other grasses. The median of native abundances
was 14 individuals per species.
The average IVI, which is the sum of
the average relative density and the average
relative frequency of the alien species over
all transects, varied from 11.46 for Urochloa
maxima to 0.03 for Raphanus raphanistrum.
The species with the highest total abundance
(number of individuals over all transects) was
TABLE 1
Alien species registered in 37 transects in the study area (Pérez-Postigo et al., 2021) and their importance
Family Species Acronym
Average
IVI
Total abundance/
number of
individuals
Average
relative
density in %
Average
relative
frequency in %
Amaranthaceae
Achyranthes aspera L.
Achasp 1.01 33 0.51 0.50
Apiaceae
Apium graveolens L.
Apigrav 0.13 6 0.02 0.11
Araceae
Zantedeschia aethiopica (L.) Spreng.
Zanaet 0.07 10 0.03 0.04
Asteraceae
Sonchus oleraceus L.
Sonole 0.03 1 0.00 0.03
Asteraceae
Taraxacum officinale L.
Taroff 0.22 34 0.10 0.12
Brassicaceae
Raphanus raphanistrum L.
Raprap 0.03 1 0.00 0.02
Cannaceae
Canna indica L.
Canind 0.07 5 0.03 0.04
Cucurbitaceae
Momordica charantia L.
Momcha 0.17 1 0.02 0.14
Molluginaceae
Mollugo verticillata L.
Molver 0.10 6 0.05 0.05
Poaceae
Andropogon gayanus Kunth
Andgay 0.76 56 0.38 0.37
Poaceae
Bromus catharticus Vahl
Brocat 0.05 2 0.01 0.04
Poaceae
Cenchrus ciliaris L.
Cencil 2.24 1 005 1.75 0.49
Poaceae
Chloris gayana Kunth
Chlgay 0.11 18 0.05 0.07
Poaceae
Chloris inflata Link
Chlinf 0.20 15 0.05 0.15
Poaceae
Cynodon dactylon (L.) Pers.
Cyndac 2.88 250 2.14 0.74
Poaceae
Cynodon nlemfuensis Vanderyst
Cynnle 1.98 190 1.26 0.72
Poaceae
Dactyloctenium aegyptium (L.) Willd.
Dacaeg 0.55 79 0.28 0.28
Poaceae
Digitaria bicornis (Lam.) Roem. & Schult.
Digbic 0.55 99 0.19 0.35
Poaceae
Digitaria ciliaris (Retz.) Koeler
Digcil 2.02 154 1.48 0.54
Poaceae
Echinochloa colona (L.) Link
Echcol 0.16 5 0.02 0.15
Poaceae
Eleusine indica (L.) Gaertn.
Eleind 0.74 33 0.14 0.61
Poaceae
Eragrostis ciliaris (L.) R. Br.
Eracil 3.85 305 3.09 0.76
Poaceae
Melinis repens (Willd.) Zizka
Melrep 4.94 1 138 3.42 1.52
Poaceae
Poa annua L.
Poaann 0.51 75 0.21 0.30
Poaceae
Setaria adhaerens (Forsk.) Chiov.
Setadh 0.42 18 0.12 0.30
Poaceae
Sorghum halepense (L.) Pers.
Sorhal 1.01 31 0.64 0.38
Poaceae
Urochloa maxima (Jacq.) R. D. Webster
Uromax 11.46 235 6.20 5.26
Poaceae
Urochloa mutica (Forssk.) T.Q.Nguyen
Uromut 0.11 4 0.03 0.08
Poaceae
Vulpia myuros (L.) C.C. Gmel.
Vulmyu 0.85 231 0.60 0.25
Polygonaceae
Rumex crispus L.
Rumcri 0.09 5 0.01 0.08
Primulaceae
Anagallis arvensis L.
Anaarv 0.41 23 0.12 0.28
Primulaceae
Anagallis minima (L.) E.H.L. Krause
Anamin 0.09 20 0.04 0.05
IVI (importance value index) = relative density of a species + relative frequency of a species.
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Melinis repens with 1 138 individuals, followed
by Cenchrus ciliaris with 1 005. Three species,
Momordica charantia, Sonchus oleraceus and
Raphanus raphanistrum, had only one indi-
vidual each. The median of alien abundances
was 27 individuals per species. Although total
abundance was highest for M. repens and C.
ciliaris, the average relative density as well as
the average relative frequency were highest for
Urochloa maxima.
Floristic groups: The cluster analysis
showed three groups that were significantly
different from each other (Fig. 1). The dif-
ference between groups two and three had a
p-value of 0.003, and the difference between
both (2 and 3 together) and the first group was
P = 0.004 (marked as A and B in Fig. 1). All
other subgroups had higher p-values and thus
were not statistically different from each other
(Fig. 1). The first group consisted of Anagal-
lis minima with Raphanus raphanistrum, and
Andropogon gayanus with Setaria adhaerens,
all of which were species that appeared only
once. The second group included Anagal-
lis arvensis, Cynodon nlemfuensis, Bromus
catharticus, Rumex crispus, Taraxacum offici-
nale, Zantedeschia aethiopica, Apium graveo-
lens, Poa annua, Sonchus oleraceus and Vulpia
myurus, which were found at higher elevations
(Fig. 1 and Fig. 2). All the other 18 species
were grouped in the third cluster. They were
tropical species widely distributed over most of
the elevation gradient.
The IVI of each species in each transect
over the elevation gradient is shown in Fig.
2. The data were log transformed in order to
better show low IVIs. Although some tran-
sects had no alien species, all elevation levels
had alien species with high (200) or medium
(100) IVI.
Relationship to environmental vari-
ables: The CCA showed that some species
had no relation to the documented environ-
mental variables (Fig. 3). However, Melinis
repens, Eragrostis ciliaris, Setaria adhearens
and Andropogon gayanus were positively
Fig. 1. Dendrogram showing the association of alien species. Solid lines delimit statistically different groups of species.
Letters represent the statistical differences between groups. Differences between groups two and three are shown by A π
= 2.34, P = 0.003. The difference between the group two and three combination and group one is shown by B π = 2.1, P =
0.004. For the acronyms, see Table 1.
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associated with soil compaction, stone cover
and distance to roads. Achyranthes aspera,
Momordica charantia, Urochloa maxima and
Sorghum halepense were related positively to
leaf litter depth and tree cover. A large group
of species, plotted at the lower portion of Fig.
3, was positively related to the elevation and
slope of the transects. Annual precipitation sum
and grazing intensity were the variables with
least impact on the IVI of the species.
Potentially invasive species: The spe-
cies with the highest mean IVI were Urochloa
maxima (Guinea grass), Melinis repens (Natal
grass), Eragrostis ciliaris and Cynodon dac-
tylon (Bermuda grass). Urochloa maxima not
only had the highest IVI but was also the most
abundant species, followed by other grasses,
but in a different order: Melinis repens, Cyn-
odon dactylon and Eragrostis ciliaris. Total
abundance varied from 1 to over 1 000 indi-
viduals, and no species was present in more
than 31 of the 185 plots.
These four species were mainly found at
medium elevations (Fig. 2). However, their
abundances varied between species and eleva-
tions, and were influenced by different vari-
ables (Fig. 4). Urochloa maxima was positively
related to leaf litter depth and tree cover, and
negatively to soil compaction, distance to high-
ways and elevation. Melinis repens had a posi-
tive relation to stone cover. Eragrostis ciliaris
Fig. 2. Heat map, showing the log-transformed IVI of alien species over the elevation gradient. Horizontal lines divide the
eight elevation groups and vertical lines divide the three groups of species determined by the cluster analysis. Darker colours
show higher values; white squares indicate absence of the alien species. The four most important species with the highest
IVI are circled. For the acronyms see Table 1.
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was negatively related to tree cover and leaf
depth but positively to elevation, soil compac-
tion and distance to highways. Cynodon dacty-
lon was negatively related to the second axis,
representing grazing intensity and elevation.
DISCUSSION
Floristic composition and groups: Com-
position of the native ruderal flora was similar
to that found in other studies in central Mexico,
with Asteraceae, Poaceae and Fabaceae as the
most species-rich families in ruderal vegeta-
tion (Flores-Huitzil et al., 2020; Vibrans, 1998;
Villaseñor & Espinosa-García, 2004). In our
study, Poaceae accounted for 62.5 % of all
registered alien species. Even though this study
concentrated on herbaceous species only, Poa-
ceae had a remarkably large number of alien
species. Villaseñor and Espinosa-García (2004)
found that 27.7 % of all alien species in Mexico
belong to Poaceae, a tendency which was con-
firmed by Cuevas-Guzmán et al. (2004) for
the Estación Científica Las Joyas (ECLJ) in
the Biosphere Reserve Sierra de Manantlán
where 26.3 % of the alien species were grasses
(Cuevas-Guzmán et al., 2004). Globally, only
around 15 % of alien species belong to the
family Poaceae (Villaseñor & Espinosa-García,
2004). Other authors also emphasize that most
Fig. 3. Relationship of all alien species to environmental variables, as shown by a canonical correspondence analysis (CCA).
The constrained inertia (= weighted variance) of 4.4619 and a significance of 0.001 in the CCA show that the species were
related to different variables. The first axis had a relative constrained inertia of 0.8227 and the second 0.4060. Species
acronyms can be found in Table 1.
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of the invasive (as opposed to merely natu-
ralised) alien weeds in Mexico were grasses
(Espinosa-García et al., 2004; Vibrans, 2016).
This may be due to the effects of forage grass
introductions, as explained in the next para-
graph. In contrast, Asteraceae, the most species
rich family for native species, had only two
alien species.
The cluster analysis showed elevation-
based division between species found at higher
elevations and species at low and middle eleva-
tions; there is some overlap around 1 700 m.
This division reflects changes in the natural
vegetation (from pine-oak to tropical forests)
and is mainly due to the presence or absence
of winter frosts. Most of the ruderal species of
the temperate group originated in Europe and
Asia, whereas the other group was dominated
by African species, mainly grasses (Espinosa-
García & Villaseñor, 2017; Villaseñor & Espi-
nosa-García, 2004) introduced as forage plants
for cattle ranching. Some escaped cultivation
and turned into highly invasive species which
are a threat to the natural vegetation, as in other
parts of the Neotropics (Williams & Baruch,
2000). Though there is no published evidence
of African grass invasions in the study region,
the authorities responsible for the protected
areas in the region reported problems with
invasive grasses that appeared only a few years
ago and already cover large areas (P. Ramos-
Suárez, pers. comm., 2018).
Environmental variables: Only a few
species correlated clearly with environmen-
tal variables. Three sets of variables define
Fig. 4. Canonical correspondence analysis showing the relation of (invasive) alien species with the highest IVI to
environmental variables, with a constrained inertia (= weighted variance) of 2.17 and a significance of 0.001. The ANOVA
showed a constrained inertia of 0.89 for the first axis and 0.66 for the second. Melrep = Melinis repens, Eracil = Eragrostis
ciliaris, Uromax = Urochloa maxima, Cyndac = Cynodon dactylon.
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different types of habitats: tree cover and leaf
litter depth, stone cover and soil compaction,
and elevation and slope. Sites with high tree
cover and leaf litter depth can be considered
less disturbed. Thus, species such as Achy-
ranthes aspera and Urochloa maxima, which
are related to those variables, could prob-
ably invade natural habitats more easily. Spe-
cies positively related to stone cover and soil
compaction might be more representative of
recently disturbed sites, and surviving extreme
conditions (Aziz & Khan, 1993), thus they may
have a lower risk of invasiveness. Taraxacum
officinale, Rumex crispus, Sonchus oleraceus
and others are associated with higher elevation
and steeper slopes and can therefore be expected
to survive lower temperatures and less sunlight.
They are mainly European species adapted to
temperate climates (Schmeil et al., 1959).
Potentially invasive species: As our study
is a snapshot of the current situation, it is dif-
ficult to reliably identify invasive species. The
definition of invasive species by Richardson
et al. (2000) is based on the increase of spatial
distribution over time and its impact. To be
able to capture such a process, the species’ dis-
tribution would have to be monitored for some
time. Considering the small numbers of alien
herbaceous species in the study area and the
lack of correlation with different environmental
variables, we suggest that most species are in
an early stage of invasion (Aikio et al., 2010;
Pérez-Postigo et al., 2021). However, species
with a high IVI are possibly already causing
significant changes to the ecosystems. The four
species identified as potentially invasive not
only have the highest IVIs in some transects
but also a broad elevational range that excludes
only the highest elevations (Fig. 2).
The species with the highest IVI, Urochloa
maxima, commonly known as Guinea grass,
was introduced to the Americas from Africa
for cattle grazing and in some cases for ero-
sion control (Moore, 2010; Overholt & Franck,
2017; Ramirez-Yañez et al., 2007). This spe-
cies is very tall, up to 3 m in height (Veldman
et al., 2009), taller than most native herbaceous
species. This can be an advantage in the com-
petition for light and other resources. In South-
ern USA it has become invasive, displacing
valuable native plants and forming extensive
monospecific populations (Ramirez-Yañez et
al., 2007). Guinea grass was also introduced
into the Mediterranean and temperate regions
where it became invasive (Clark et al., 2008;
Verloove, 2014).
Urochloa maxima has been registered as
one of the most important invasive species
of African origin in other parts of Mexico
(Rzedowski & Calderon de Rzedowski, 1990;
Vibrans, 2004). It is also known to be very
abundant in tropical dry forests where it forms
dominant stands (Romero Martins et al., 2007;
Sampaio & Schmidt, 2013; Veldman et al.,
2009). It particularly invades forests disturbed
by logging since, although the species can
germinate below thick layers of leaf litter, it
needs sufficient light to grow (Veldman & Putz,
2010). Our results did not confirm this but
rather showed a positive relationship between
leaf litter depth and tree cover. Vegetation
affected by fire is often dominated by Uro-
chloa maxima (Lu & DeLay, 2016; Veldman,
2010). The considerable biomass produced by
this plant also provides large quantities of fuel
which propagate fires (Veldman et al., 2009).
Its robust and deep root systems and fast-
growing stalks allows it to recover rapidly after
fires (Wied et al., 2020). These traits also make
it very resistant to eradication efforts, such as
mowing and herbicides (Ammondt et al., 2013;
Clark et al., 2008).
Melinis repens, Natal grass, originated in
Southern Africa and has an introduced range
from Africa to Australia, Asia and America
(Moore, 2010; Pyšek et al., 2020; Romero
Martins et al., 2007). It was first recorded in the
USA in the 19
th
century (Stokes, 2010). In Mex-
ico it was first found in 1945 after it escaped
from experimental cultivation (Itié, 1945). It is
one of the most invasive species globally, and
in Mexico it is also causing damage, especially
by modifying fire regimes (Espinosa-García &
Villaseñor, 2017; Gonçalves Rolim et al., 2014;
Martínez-de la Cruz et al., 2015; Pyšek et al.,
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2020; Rzedowski & Calderon de Rzedowski,
1990; Space et al., 2000).
The species was initially introduced to
Mexico as a potential forage grass but was not
suitable for cattle grazing. The lack of grazing
led to an increase in its populations (Gutiérrez
Gutiérrez et al., 2019; Melgoza Castillo et al.,
2014). It is widely distributed in the country,
particularly in arid areas, in tropical dry for-
ests and even in temperate regions of Mexico
(Vibrans, 2004). Different chemical eradica-
tion methods have not been successful. The
longevity of seeds on the surface is one month,
whereas buried seeds can survive for over one
year. Maximum germination was found at tem-
peratures of 20-35 °C (Melgoza Castillo et al.,
2014; Stokes, 2010; Stokes et al., 2011) and
its growth is enhanced by fire, thus threaten-
ing particularly fire sensitive species (Melgoza
Castillo et al., 2014; Overholt & Franck, 2017).
Melinis repens is more abundant, produces
more biomass in ruderal vegetation and is more
likely to invade roadsides than other habitats
(David & Menges, 2011). In our study, Melinis
repens was clearly associated with soil com-
paction and stone cover - typical conditions in
the immediate vicinity of roads. This coincides
with qualitative observations in the region and
with results in the Atlantic forest of Brazil (de
Paula et al., 2015).
Another South African grass introduced to
America as forage is Eragrostis ciliaris. It was
the third most important species in our survey.
This species can adapt to harsh conditions.
It was found to invade extreme sites, such as
desert areas and bare volcanic rocks after erup-
tions (Aziz & Khan 1993; Rejmánek et al.,
1982). Eragrostis ciliaris is known in Africa
and Asia and is widely distributed in North
and South America (Pyšek et al., 2020). The
species is present but not highly invasive and
does not cause damage to natural ecosystems
in many of these regions (Gonçalves Rolim et
al., 2014; Pyšek et al., 2020; Veldman, 2010).
The species has been found in at least 20 of the
32 states in Mexico (Espinosa-García & Vil-
laseñor, 2017), but there are no detailed data
on its effects. We can expect it to form part
of the natural succession of new or disturbed
sites and be replaced by other species during
the succession process. Still, it could be com-
peting with highly specialised species such as
endemic lithophytes.
Bermuda grass, Cynodon dactylon, has
been introduced in tropical and subtropical
regions worldwide (Espinosa-García & Vil-
laseñor, 2017; Gonçalves Rolim et al., 2014;
Mack, 2001; Moore, 2010; Romero Martins et
al., 2007; Sampaio & Schmidt, 2013; Sánchez
Medrano, 2018; Space et al., 2000) and in the
US since the 19
th
century (Mack & Lonsdale,
2001). For the Southern states of the USA, it
is one of the alien species that causes the most
concern (Espinosa-García & Villaseñor, 2017;
Gonçalves Rolim et al., 2014; Mack, 2001;
Moore, 2010; Romero Martins et al., 2007;
Sampaio & Schmidt, 2013; Sánchez Medrano,
2018; Space et al., 2000). In some regions
in Pakistan, Bermuda grass was the most
dominant alien species (Javaid & Riaz, 2012;
Shabbir & Bajwa, 2006). It is highly abun-
dant in Northern and central Mexico, where it
occasionally dominates the natural vegetation
(Espinosa-García & Villaseñor, 2017; Sánchez
Medrano, 2018; Vibrans, 1998).
Cynodon dactylon, a C4 grass, is more
effective than other native C4 grasses in its
introduced range (Mojzes & Kalapos, 2008). It
is also tolerant to burning (Grace et al., 2001).
A climatic niche model showed a potential dis-
tribution that included tropical, subtropical and
temperate regions (Barney & DiTomaso, 2011).
Although Bermuda grass grows in temperate
regions, its growth is restricted by very low
temperatures (Alexander et al., 2011; Grace
et al., 2001). Treatment with herbicides is not
effective for eradication of Cynodon dactylon
(Burkhart, 2004). It has been shown to alter soil
properties and have a negative impact on avian
and mammalian species (Barnes et al., 2013).
All four species were introduced as for-
age grass and escaped cultivation. They are
known invaders in other regions where they
have displaced the natural vegetation. Although
in the study area the species still have quite
low relative abundances and frequencies along
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roadsides, they are expected to spread into the
natural vegetation in the near future. To prevent
possible damage to the natural ecosystems,
their advance along roadsides and into the natu-
ral vegetation should be prevented, along with
generalised measures to minimise unnecessary
disturbances and new introductions. We sug-
gest that the four potentially invasive species
should be the focus of detection, eradication
and containment efforts in the more natural,
conserved vegetation of the region. Since these
species react to different environmental fac-
tors, management must be adapted to each
species individually.
Conclusion: The most common and abun-
dant alien species were grasses, all other fami-
lies had only one or two species and limited
abundance. The species formed three clusters
- temperate, tropical and rare species - related
to elevation and the geographic origin of
the species.
Consequently, the most invasive species
of the region were grasses, all of them African,
introduced intentionally for ranching. Where-
as Urochloa maxima, Cynodon dactylon and
Melinis repens are known invaders elsewhere,
the impacts of Eragrostis ciliaris are not yet
well-known and should be explored further.
Ethical statement: authors declare that
they all agree with this publication and made
significant contributions; that there is no con-
flict of interest of any kind; and that we fol-
lowed all pertinent ethical and legal procedures
and requirements. All financial sources are
fully and clearly stated in the acknowledge-
ments section. A signed document has been
filed in the journal archives.
ACKNOWLEDGMENTS
This study was conducted in the context of
the Doctoral Program in Sciences in Biosyste-
matics, Ecology, and Management of Natural
and Agricultural Resources (BEMARENA) of
the Universidad de Guadalajara, Mexico. The
Consejo Nacional de Ciencia y Tecnología
(CONACyT) funded the research through a
doctoral thesis grant to the first author. We
received financial support from the University
of Guadalajara through the program Forta-
lecimiento a la investigación. Members of the
Laboratory of Botany at the Centro Universita-
rio de la Costa Sur helped with the field work
and species identification. The authors declare
no conflicts of interest.
RESUMEN
Composición florística y potencial de invasión de
plantas herbáceas exóticas en el occidente de México
Introducción: El número de especies de plantas exóticas
está aumentando en todo el mundo, pero no todas las
especies se convierten en invasoras. Si bien para algunas
regiones la introducción de especies se ha documentado
durante varias décadas, para el occidente de México los
conocimientos sobre especies exóticas son escasos. Aquí,
estudiamos la vegetación de los bordes de las carreteras a lo
largo de un gradiente altitudinal en una región que incluye
un área protegida.
Objetivos: Estudiamos la composición florística de las
herbáceas exóticas, sus patrones de distribución y su rela-
ción con diversos factores ambientales. El índice de valor
de importancia (IVI) de las especies identificó las especies
más importantes, y por lo tanto probablemente invasoras.
Métodos: Durante el 2017 y 2018 se documentó la vege-
tación del borde de la carretera con 4-6 transectos en cada
nivel de elevación de 300 m, de 0 a 2 100 m, para un total
de 37. Cada transecto constó de cinco parcelas de 1 m². Se
registraron todas las especies herbáceas y se identificaron
las especies introducidas. Un análisis de agrupamiento
identificó los grupos de especies basado en sus patrones
de distribución relacionadas con la elevación. Las especies
potencialmente invasoras se identificaron por su IVI, basa-
do en la suma de la frecuencia relativa y la densidad relati-
va. La influencia de las variables ambientales fue estudiada
mediante un análisis de correspondencia canónica.
Resultados: La mayoría de las especies exóticas fueron
pastos, otras familias estuvieron representadas por una o
dos especies. Las especies formaron tres grupos princi-
pales. El primero incluyó especies raras; el segundo las
especies distribuidas principalmente en elevaciones altas, y
el tercero incluyó los taxones con una distribución desde el
nivel del mar hasta elevaciones medias. Las especies más
importantes y potencialmente invasoras fueron: Urochloa
maxima, Melinis repens, Eragrostis ciliaris y Cynodon
dactylon, todas gramíneas africanas introducidas como
forraje para el ganado. El IVI de las especies se relacionó
con la cobertura de árboles, profundidad de hojarasca y
con pedregosidad para algunas especies, y para otras con la
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compactación del suelo, la distancia a carreteras (federales)
y elevación.
Conclusiones: Las especies ruderales exóticas se agrupa-
ron según el clima general (templado vs. tropical). Las gra-
míneas de origen africano son las que más preocupan como
especies invasoras. Aunque la mayoría de las introduccio-
nes están relacionadas con las alteraciones humanas, cada
especie se vuelve dominante en determinadas condiciones
ambientales. Por ello, los programas de manejo deben
ajustarse específicamente a cada especie exótica invasora.
Palabras clave: variables ambientales; plantas exóticas;
hierbas invasoras; malezas ruderales; Sierra de Manantlán.
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