Growing numbers of alien plant species threaten natural ecosystems worldwide. Mexico, as a megadiverse country, has lower numbers of alien species than other regions in America. However, there is a lack of information on the factors that determine the diversity patterns of alien species and their relative importance in the vegetation. The diversity of alien roadside herbs was analysed along an elevational gradient in western Mexico, including their relationship with environmental factors. Three hundred and seventeen herbaceous species were found in 37 sampling sites; 10% were aliens. The proportion of alien species in the ruderal herbaceous communities was lower than the average for this vegetation type in the country. Absolute species richness of natives was significantly and positively correlated with elevation. Absolute species richness of aliens was not significantly correlated with elevation, still; higher values were found at lower elevations. Generalised linear models for relative species richness and relative abundance of aliens with elevation had positive estimates near 0, which, though statistically significant, indicated a weak relationship. Other environmental co-factors, such as the distance to roads and highways, climatic variables, and disturbance indicators, were included in a random forest model. No clear correlation patterns were found. This seemingly random distribution of alien herbaceous plants in the region might be caused by the relatively recent introduction of most of the species. They have not yet had time to expand their distribution to their potential maximum. The early stage of the naturalisation process allows us to minimise the further spread of some species with targeted management and prevent them from becoming invasive.

Environmental variables, exotic plant species, ruderal weed species, Sierra de Manantlán

Mexico has far fewer alien species of higher plants, in both relative and absolute numbers, than the USA and Canada, Europe or various South American countries (Espinosa-García et al. 2004b; Villaseñor and Espinosa-García 2004; van Kleunen et al. 2015; Espinosa-García and Villaseñor 2017). A recent survey estimated between 700 and 750 species (Espinosa-García and Villaseñor 2017) which is less than 3% of the flora. Most of them are herbaceous weeds and grasses (Espinosa-García et al. 2004a; Villaseñor and Espinosa-García 2004; Espinosa-García and Villaseñor 2017). They are also, with some exceptions, less successful in terms of the size of their distribution area (Espinosa-García et al. 2004b) or dominance in local weed vegetation.

However, Mexico is a mega-diverse country, and it is risky to generalise results derived from individual local or regional projects (Trejo and Dirzo 2002; CONABIO and SEMADET 2017). Publications on alien plant species in Mexico are scarce, scattered and variable (Espinosa-García and Villaseñor 2017; Ramírez-Albores et al. 2019). Some information is available for the whole country, but most results are restricted to central Mexico or the northern states (Espinosa-García et al. 2004a; Villaseñor and Espinosa-García 2004; Ramírez-Albores et al. 2019). A major effort is needed to study the present-day situation at different scales (Espinosa-García 2009; Espinosa-García and Villaseñor 2017).

Ruderal and roadside vegetation is often the most critical dispersal channel for the initial introduction of alien plants. Roads cross different environmental conditions, such as soils and climate (Kowarik and von der Lippe 2007). After expanding along them, the populations may invade the adjacent vegetation (Kowarik and von der Lippe 2007; Pollnac et al. 2012; Otto et al. 2014; Martínez-De La Cruz et al. 2015; McDougall et al. 2018). Although some studies exist on aliens in roadside vegetation (Otto et al. 2014), this type of vegetation is not well-studied in Mexico (Pollnac et al. 2012; Martínez-De La Cruz et al. 2015; Sánchez-Medrano 2018). In western Mexico, most terrestrial alien plant species grow in highly disturbed areas and roadside vegetation; vegetation without recent disturbances contains only a few aliens (Vázquez-García et al. 1995; Cuevas-Guzmán et al. 2004).

In several studies, low and middle elevations have been shown to host most alien plant species (Pauchard et al. 2009; Alexander et al. 2011; Haider et al. 2018). Temperate regions have their maximum number of alien plants at low elevation, whereas subtropical islands have most aliens at middle elevations (Pauchard et al. 2009). These patterns are not only caused by changes in the abiotic conditions along the elevational gradient and the preadaptation of the alien plants, they also depend on the areas of the primary introduction and highest propagule pressure (Pauchard et al. 2006). In most cases, alien species are introduced at low and middle elevations from where they spread towards higher elevations along roads and other dispersal channels. Anthropogenic disturbances, in general, are also higher at low elevations and foster the establishment of aliens (Pauchard et al. 2006; Alexander et al. 2011; Haider et al. 2018).

Very little literature exists on the distribution of alien plant species along the elevational gradient in tropical or subtropical regions, and the results are highly variable. A study from India found the highest numbers of alien plants at the lowest elevations (Kosaka et al. 2010). An investigation in Ecuador showed a hump-shaped distribution (Sandoya et al. 2017). In contrast, in central Argentina, Paiaro et al. (2010) found that the lowest and highest elevations had more alien plant species than middle elevations. In Mexico, this topic has scarcely been broached (Sánchez-González and López-Mata 2005). One study conducted at elevations from 403 to 1930 m in the north-east of Mexico showed no significant correlation between elevation and alien species number; however, abundance was not analysed (Sánchez-Medrano 2018). This lack of publications and the contradictory results highlight the need to improve the understanding of elevations’ influence on the distribution of alien species in warmer regions.

We address the following questions in the paper: 1. How does the invasion level of alien roadside plants vary over an elevation gradient? We expect higher richness at low elevations due to more significant anthropogenic disturbances and shorter distances to seaports and major towns as the initial source of aliens. 2. How does the invasion level relate to other environmental variables representing climate, natural and anthropogenic disturbances? We assume the invasion level to be higher at sites closer to main roads and highways, as well as other anthropogenic disturbances such as fire or grazing intensity. Also, soil compaction should influence the abundance of these plants. We limited our study to herbaceous plants as they are much more common and species-rich in Mexico, difficult to manage, and with different dynamics than woody species.

We identified 317 herbaceous species in the transects, of which 285 were natives and 32 aliens (see Suppl. material 6: Table S2). Thirty-five morpho-species could only be identified to genus level and were not included in further analysis. Based on the “specaccum” analysis our sampling effort covered an estimated 75% to 90% of all roadside species in the area. For each elevation level, the proportion of regional species represented in the transects varied between 64% and 88%, depending on the type of analysis (see the “Data analysis” section). Species richness clustering was highly significant for native species with a z-value of .04 (p-value of .0063; Moran’s I index). For alien species, richness appeared to be random (z-value of -.06267 and a non-significant p-value of .525).

Elevation was significantly and positively correlated with the richness of native species, though it only explained a small part of the data (p = .007, Fig. 3A). We found no significance for absolute alien species richness (p = .92), though there were more species at lower elevations (Fig. 3B). The generalised linear model for relative alien species richness and relative abundance showed a decrease with elevation, the tendency was significant but the models’ estimates were near zero (relative alien species richness: estimate = -.001 and p-value = .028, 35 degrees of freedom, relative alien abundance: estimate = -.001, p-value = .008, 35 degrees of freedom). In natives both relationships were slightly positive and significant. Spatial autocorrelation could be rejected due to high p-values at the Moran’s I test of all generalised linear models. Scatter and box-plots (Fig. 3C,D) show the relative species richness per transect along the elevation gradient and the variation within the elevation levels for alien species.

Figure 3.

Absolute and relative species richness over elevation in m. Blue lines showing glm predicted values with the 95% confidence interval A total species richness of native herbaceous plants over elevation. Glm coefficient = .0003, p-value = .007, 35 degrees of freedom B total species richness of alien herbaceous plants over elevation. Glm coefficient = -2.386e-05, p-value = .92, 36 degrees of freedom C relative species richness of alien herbaceous plants over the elevation. Glm coefficient = -.0009, p-value = .0276, 35 degrees of freedom D relative species richness of alien herbaceous plants, represented by boxplots for each elevation level. They show the median per transect (5 m²), third and first quartile and extreme values.

For relative alien species richness, the most influential variables were (i) the distance to highways, (ii) the elevation and (iii) tree cover, followed by (iv) the annual mean temperature, (v) the slope and (vi) distance to roads (Fig. 4). The relation shown by the partial dependence plots (Fig. 5A–C) of relative species richness to distance to highways was non-linear positive, the relation to elevation was also positive, whereas the relation to tree cover was slightly U-shaped. Both random forest models were significant with low mean squared errors (mean of squared residuals) for relative alien species richness and relative alien abundance. But the variance explained by the models was very low in both cases (see Table 1).

Figure 4.

Variable importance for relative alien species richness according to the random forest calculations. The figure shows the increase in mean of the error (%IncMSE), also called permutation importance index, on the left side; and the total decrease of node impurity (IncNodePurity) on the right side (distance to highways in m, calculated from open data Jalisco; elevation above sea level in m; tree cover measured per plot; mean annual temperature, obtained from Worldclim data; slope of the transect measured in %; distance to paved roads in m, calculated from open data Jalisco; mean annual precipitation sum, obtained from Worldclim data; soil compaction, measured in kg/m²; grazing intensity estimated as an index from 0/none to 5/severe; leaf litter cover measured in percent of the plot area; stone cover measured in percent of the plot area; leaf litter depth in cm measured in the field; fire intensity estimated as an index from 0/none to 5/severe).

The most important variables for the relative abundance of alien species were (i) the annual mean temperature, (ii) elevation, and (iii) the leaf litter depth, followed by (iv) slope, (v) the distance to highways, and (vi) the tree cover (Suppl. material 1: Fig. S1). The relation of relative alien species abundance to the mean temperature was negative, the relation to elevation was positive and the relation to leaf litter depth was also negative (see Suppl. material 2: Fig. S2, Suppl. material 3: Fig. S3, Suppl. material 4: Fig. S4).

Figure 5.

Relation of the relative alien species richness to the most important variables. Partial dependence plots for the distance to highways (A), the elevation (B) and tree cover (C).

The 317 identified species were the equivalent of approximately 12% of the total vascular flora of the Sierra de Manantlán (Vázquez-García et al. 1995). This proportion matches earlier publications on the weed vegetation of Mexico, which show that 10–12% of the Mexican flora are agrestal or ruderal weeds (Espinosa-García et al. 2004b; Vibrans 2016).

The 32 alien species we identified constitute 10% of the herbaceous flora of the samples. This share is low compared to other regions in the country, where the mean percentage of alien species in weedy vegetation is around 20% (Espinosa-García et al. 2004a, b; Vibrans 2016). The maize field weed vegetation of Puebla and Tlaxcala in central Mexico had 16.4% alien species (Vibrans 1998). A share of 26% of 125 ruderal plants in the urban area of Malinalco in the State of Mexico were aliens (Martínez-De La Cruz et al. 2015). In a study in Mexico City during the 1990s, 37% of 256 ruderal weed species had been introduced from other continents (Vibrans 1998).

The results of the studies are consistent with the remarkably low number of alien plant species that Mexico has in general when seen in a global context, and despite a long history of commercial exchange with Europe, other parts of North America, and Asia (Philippines). The low proportion may be related to the long history of human disturbance and agriculture in the Mesoamerican cultural region. This history promoted the development of native weed communities, which in turn may prevent an excessive invasion of alien plants (Vibrans 1998; Rejmánek et al. 2005; Kühn and Klotz 2007; Vibrans 2016). With further study of the local natural history, we might be able to improve our understanding of this relatively low number of aliens.

Though more alien species were recorded at lower elevations, no statistically clear relationship was found. Spatial autocorrelation could be rejected for all models. However, the tendency partly reflects what the literature has shown for other cases: there are more alien species at lower elevations, mainly due to lower human population densities at high elevations (Alexander et al. 2011; Pollnac et al. 2012; Otto et al. 2014; Haider et al. 2018), but confounding factors may complicate the relationship. In contrast, native species richness of herbaceous plants tends to rise with elevation (Rzedowski and Calderon de Rzedowski 1989), a tendency we also found. Another study in north-eastern Mexico (Sánchez-Medrano 2018) found a similar tendency, but also not significant. The absence of aliens close to the coast might be related to the environmental conditions, where only highly specialised species, such as mangrove species or halophytes can survive (Vázquez-García et al. 1995; CONABIO and SEMADET 2017; Rodríguez-Contreras 2017).

Relative alien species richness and abundance patterns were closely related, which means that dominant species react in the same way to environmental variables as less dominant species. The RF models for relative species richness and abundance had a low mean of squared residuals, indicating a good performance. But low (negative) variances explained by the models showed that variances of the species data could not be explained by the environmental variables.

The distance to highways was the most significant variable for relative species richness. Up to a distance of around 1 km the relation was negative. After this it was, unexpectedly, mainly positive and contradicted our hypothesis as well as previous literature (Otto et al. 2014; Haider et al. 2018; McDougall et al. 2018). This variable not only represents the distance to major transport routes where aliens are moved by traffic and can establish first (Kowarik and von der Lippe 2007), but also the distance to human-influenced areas in general. We don’t yet have an explanation for this curious phenomenon.

The results from the generalised linear models and the random forest models were contradictory, but in both cases the relations were not very strong. The generalised linear models showed a weak negative relation of the invasion level to elevation. For both random forest models the invasion level was positively related to elevation and negatively to temperature. In other studies alien species richness (not relative species richness) decreased with increasing elevation (Pollnac et al. 2012; Otto et al. 2014; Haider et al. 2018).

McDougall et al. (2018) showed that world-wide species richness patterns vary in function of elevation, depending on certain traits. Perennial species showed an increase with elevation in the roadside vegetation, annual species decreased, but the total of ruderal species richness did not change over elevation (McDougall et al. 2018). In our case, classification into annuals / perennials or ruderal / non-ruderal was not possible because of lack of published information. However, the data suggest intriguing elevational patterns related to biological traits may exist.

The relationship between tree cover and leaf litter depth seems to contradict each other, as both can be considered indicators for disturbance. For the relative species richness model, tree cover was the third most important variable, with a positive relation. For relative abundance leaf litter depth was at the third position, showing a negative relation. However, this contradiction may be explained by some herbaceous alien species requiring sufficient light to grow, although they may germinate below leaf litter (Veldman and Putz 2010).

Our results show that relative alien species richness and abundance were not strongly related to any of the environmental variables included in this study. Different factors may cause this seemingly random distribution. Previous studies show that the influence of environmental variables on the invasion process varies with the ecosystem (Lonsdale 1999; Rejmánek et al. 2005; Richardson and Pyšek 2006; Kühn and Klotz 2007). The number of species and abundance of natives also influences the results, as we are working with relative values. At the 1 m² scale higher native species richness can be expected to reduce the probability of finding alien species; at larger scales this relation is inverted (Kühn and Klotz 2007; Pauchard and Shea 2016). As we focused only on roadside vegetation which represent dispersion channels for primary invasion, we suggest that the alien species have not yet concluded the phase of the primary introduction (McDougall et al. 2018).

The conversion from traditional to modern agriculture and urbanisation has caused land-use change on a large scale (Jardel-Peláez 1998; Instituto Nacional de Ecología 2000; Graf-Montero et al. 2003). This change, particularly from agriculture to cattle ranching, has taken place mainly in the last five decades. With this the introduction and propagation of new alien species have risen significantly (Villaseñor and Espinosa-García 2004; Vibrans 2016; Ramirez-Albores et al. 2019). This period is relatively short for the introduced species to establish in their new habitats, and to expand significantly and reach the potential maximum distribution that reflect their biology (Kowarik and von der Lippe 2007; McDougall et al. 2018). Many alien species are known to have a lag-phase during the first decades after introduction, where they remain at low numbers and in few localities before their populations start to increase (Aikio et al. 2010). Therefore, the present-day distribution might represent the lag-phase distribution and thus depend mainly on the location and intensity of the initial introduction. To confirm this, further studies should include similar regions in central and western Mexico. Species records from herbarium data could be used to analyse the temporal trends of the invasion, although sampling biases and other potential errors could hamper this (Aikio et al. 2010).

Many African grasses have been planted on large areas as forage for cattle, but only in the last 100 years, and often even more recently (Rzedowski 1990; Williams and Baruch 2000). The intensity of the introduction or propagule pressure is a crucial factor for alien plant distribution patterns. It is difficult to measure and was not included in this study (Richardson and Pyšek 2006). Even though initial introductions of all alien plants are directly linked to human activities (Reichard and White 2001; Kowarik 2005; Kowarik and von der Lippe 2007), the number, distribution, and frequency of repeated introductions can vary considerably.

For eradication success, the size of the distribution area of the alien species and the time since the initial introduction are highly relevant (Coulston 2004; Mack and Lonsdale 2004; West 2004; Gardener et al. 2009; Kettenring and Adams 2011; Pluess et al. 2012). In the case of western Mexico with low total numbers of alien species, which have been introduced recently and not yet expanded, targeted management could make containment possible (Bruce and Johnson 2014). The large number of native weedy plants may help with management. After local eradication of alien species, it is important to re-establish the native flora, and these strong native competitors could be used to initiate appropriate succession (Kettenring and Adams 2011).

Western Mexico has relatively few alien herbaceous roadside species. They can be found mainly at lower elevations but not directly at the coast. This pattern was expected as introductions occur mainly at lower elevations, where a large part of the land is converted to agricultural land and other uses. Although the relationship was statistically significant, low values of the estimates showed a weak relationship. The result may be related to another finding: the lack of clustering of alien species richness. Both results could be attributed to the fact that most introductions have been relatively recent. The region’s alien plant species have probably not yet expanded sufficiently to show clear distribution patterns. For comparison and in order to confirm this assumption, similar studies would have to be conducted in central and western Mexico. It would be interesting to repeat the study in the future and include further plots in the natural adjacent vegetation to see how the situation develops. In the future, and especially if further introductions are not limited to a minimum, and no targeted management is applied, the number and abundance of alien herbaceous plant species will probably increase. This situation can be viewed as an opportunity to prevent the expansion of introduced species and prevent large-scale invasions through management. Identifying the potential invaders and their potential distribution helps to concentrate management strategies on threatened areas.

This study was conducted in the context of the Doctorate Program in Sciences in Biosystematics, Ecology, and Management of Natural and Agricultural Resources (BEMARENA) of the Universidad de Guadalajara, Mexico. The Consejo Nacional de Ciencia y Tecnología (CONACyT), Mexico, funded the research through a doctoral thesis grant to the first author. We received financial support from the University of Guadalajara through the program Fortalecimiento a la investigación, provided by the Coordinación de Investigación, Posgrado y Vinculación. Members of the Laboratory of the Botany Department at the Centro Universitario de la Costa Sur of the Universidad de Guadalajara helped with the field work and species identification. For the data analysis, we were assisted by the members of the Laboratory for Climatology and Remote Sensing (LCRS) of the Philipps University of Marburg, Germany. The authors declare that no competing interests exist.