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
; 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
; 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;
Received 16-II-2021. Corrected 30-VII-2021. Accepted 07-IX-2021.
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
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.
Revista de Biología Tropical, ISSN: 2215-2075 Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
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.
Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
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).
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
Revista de Biología Tropical, ISSN: 2215-2075 Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
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
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
Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
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.
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
Revista de Biología Tropical, ISSN: 2215-2075 Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
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
Alien species registered in 37 transects in the study area (Pérez-Postigo et al., 2021) and their importance
Family Species Acronym
Total abundance/
number of
density in %
frequency in %
Achyranthes aspera L.
Achasp 1.01 33 0.51 0.50
Apium graveolens L.
Apigrav 0.13 6 0.02 0.11
Zantedeschia aethiopica (L.) Spreng.
Zanaet 0.07 10 0.03 0.04
Sonchus oleraceus L.
Sonole 0.03 1 0.00 0.03
Taraxacum officinale L.
Taroff 0.22 34 0.10 0.12
Raphanus raphanistrum L.
Raprap 0.03 1 0.00 0.02
Canna indica L.
Canind 0.07 5 0.03 0.04
Momordica charantia L.
Momcha 0.17 1 0.02 0.14
Mollugo verticillata L.
Molver 0.10 6 0.05 0.05
Andropogon gayanus Kunth
Andgay 0.76 56 0.38 0.37
Bromus catharticus Vahl
Brocat 0.05 2 0.01 0.04
Cenchrus ciliaris L.
Cencil 2.24 1 005 1.75 0.49
Chloris gayana Kunth
Chlgay 0.11 18 0.05 0.07
Chloris inflata Link
Chlinf 0.20 15 0.05 0.15
Cynodon dactylon (L.) Pers.
Cyndac 2.88 250 2.14 0.74
Cynodon nlemfuensis Vanderyst
Cynnle 1.98 190 1.26 0.72
Dactyloctenium aegyptium (L.) Willd.
Dacaeg 0.55 79 0.28 0.28
Digitaria bicornis (Lam.) Roem. & Schult.
Digbic 0.55 99 0.19 0.35
Digitaria ciliaris (Retz.) Koeler
Digcil 2.02 154 1.48 0.54
Echinochloa colona (L.) Link
Echcol 0.16 5 0.02 0.15
Eleusine indica (L.) Gaertn.
Eleind 0.74 33 0.14 0.61
Eragrostis ciliaris (L.) R. Br.
Eracil 3.85 305 3.09 0.76
Melinis repens (Willd.) Zizka
Melrep 4.94 1 138 3.42 1.52
Poa annua L.
Poaann 0.51 75 0.21 0.30
Setaria adhaerens (Forsk.) Chiov.
Setadh 0.42 18 0.12 0.30
Sorghum halepense (L.) Pers.
Sorhal 1.01 31 0.64 0.38
Urochloa maxima (Jacq.) R. D. Webster
Uromax 11.46 235 6.20 5.26
Urochloa mutica (Forssk.) T.Q.Nguyen
Uromut 0.11 4 0.03 0.08
Vulpia myuros (L.) C.C. Gmel.
Vulmyu 0.85 231 0.60 0.25
Rumex crispus L.
Rumcri 0.09 5 0.01 0.08
Anagallis arvensis L.
Anaarv 0.41 23 0.12 0.28
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.
Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
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.
Revista de Biología Tropical, ISSN: 2215-2075 Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
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.
Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 1037-1054, July-September 2021 (Published Sep. 24, 2021)
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.
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.