Study area

This study was carried out in 6 Andean countries: Argentina, Bolivia, Colombia, Ecuador, Peru, and Venezuela (Table 1), classified as low- and middle-income countries according to the World Bank (n.d.). In this study, we considered every first-order administrative unit taken from the Global Administrative Unit Layers database (FAO 2015) whose boundaries overlapped our study area. Within these countries, we selected the administrative units that belonged to the Andes region following the classification of Tovar et al (2022). The Andes are a mountain range spanning over 9000 km in length and reaching over 6000 m in elevation (Tovar et al 2022) that encompasses diverse ecosystems and hosts almost 10% of the vascular plant species in the world (Pérez-Escobar et al 2022).

TABLE 1

Comparison and characterization of the migration dynamics and demographics between Andean countries.

The socioecological system studied experienced growth of urban areas with increasing concentrations of population and economic activity between 1950 and 2010 (Buytaert and Bievre 2012; Cerrutti and Parrado 2015). The study area encompasses different stages of urban transition (the shift from low to high urban development related to history and the country's economic development; Álvarez-Berríos et al 2013). Urban transitions range from Bolivia and Ecuador, with a low to medium proportion of urban population and high urban growth, to Argentina, Colombia, and Peru, where urban growth has slowed down (Álvarez-Berríos et al 2013; Bernard et al 2017).

Data

An overview of all data used is provided in Table 2. The table also explains their relevance for our study.

TABLE 2

Overview of data selected; their relevance is related not only to prevalence of exotic woody plants but also to the mountain context in which the analysis takes place.

Woody species data: Information on woody species records was collected during February and March of 2023 from several databases (Table 2): the Global Biodiversity Information Facility (GBIF 2023a, 2023b), the Botanical Information and Ecology Network (Enquist et al 2016), Integrated Digitized Biocollections (Michonneau and Collins 2017), and Species Link (Thiers et al 2022). In this study, we only included woody plants (trees and bushes), since these plants tend to thrive in urban areas because of human intervention and management practices (Ossola et al 2020).

To systematize and integrate records from different databases, we used the bdc package of the R software (Ribeiro et al 2022). Species names were standardized according to the Leipzig Vascular Plants Catalogue (Freiberg et al 2020). We classified every plant species based on its origin (native or exotic) using the GlobalTreeSearch Species List (Beech et al 2017). For every first-order administrative unit, we estimated the proportion of exotic woody plant records in relation to the total number of woody species records (Figure 1).

FIGURE 1

Exotic woody plants in proportion to the total number of woody species by first-order administrative unit in the Andean region.

Migration data: We obtained migration data from WorldPop Hub, a portal of open spatial demographic data and dynamics that includes over 40,000 datasets. The dataset on migration flows (Ceauşu et al 2019) comprises migrant stocks obtained from censuses from 2010. Migrants are people whose administrative unit of residence changed in the previous 5 years (Ceauşu et al 2019). We estimated a migration index for each administrative unit that combined internal and international migration. Because of the size of values and variation, we estimated the logarithm of international migrant flows and multiplied it by the logarithm of internal migration for each country to obtain a weighted immigration flow index (Figure 2). This immigration index constitutes an integral indicator of local and international human social mobility (Table 2).

FIGURE 2

Index of weighted immigration flows by first-order administrative unit in the Andean region.

Urban development: We estimated different metrics of urban development that may affect the prevalence of exotic species. We used the most recent census for each administrative unit to estimate its population. The time since the city's foundation is deemed to indicate the period of urban interconnection (Table 2), so we used the foundation date of cities (Grau and Foguet 2021).

We used the harmonized global nighttime light dataset (Li et al 2020) to characterize the urban dynamics from 2000 to 2021 (Table 2). This dataset comprises sensor-detected nighttime light data from 2000 to 2021, making it possible to delineate urban expansion. Nighttime light values range from 0 to 63, and values over 20 are considered reliable urban pixels (Li et al 2020). We estimated the trend in nighttime lights by regressing yearly radiance against the calendar year. Finally, we calculated nighttime lights per inhabitant (Table 2) by dividing the population of each administrative unit by the number of urban pixels in 2020.

Natural environment: Since environmental conditions, such as climate and topography, can influence plant distributions and establishment, we included mean elevation, the standard deviation of elevation, mean precipitation, precipitation range, and mean temperature of each administrative unit in the analysis (Table 2).

Data analysis

To identify the most important variables to explain exotic woody species' prevalence, we performed multiple linear regression models. We considered all the linear combinations of explanatory variables (mean temperature, mean precipitation, precipitation range, mean elevation, standard deviation of elevation, foundation date, nighttime lights per inhabitant, nighttime light trends, total population, weighted immigration flows; Table 2) to explain the prevalence of exotic woody plants within each administrative unit. We conserved the most parsimonious models (the ones that were within 2 units of the Akaike information criterion [AIC]; Vrieze 2012) with respect to the best model. AIC is a metric that indicates the balance between data adjustment and model complexity. We averaged the most parsimonious models to assess the effect of each explaining variable on exotic prevalence (Symonds and Moussalli 2010). We also evaluated the frequency of each variable in the most parsimonious models to assess their consistency. All these analyses were performed using R software (R Core Development Team 2021).

Migrants do not come empty-handed

Since woody plants are long-lived organisms that can persist for many years, the current species composition and their dynamics are a legacy of anthropogenic disturbance and human mobility (Kowarik and von der Lippe 2008; Pemberton and Liu 2009). Our results showed that social connectivity plays a significant role in shaping species composition and abundance in Andean ecosystems. Human mobility, assessed through an immigration indicator, was a better predictor of exotic species prevalence in woody vegetation communities than other plausible socioeconomic indicators such as population or the date the city was founded (although they also played some role). The effect of migration on species composition is likely explained by 3 different processes: (1) Increased human mobility is associated with more frequent introductions of exotic species, which can establish viable populations and eventually become invaders (Mack and Lonsdale 2001; Hulme 2009). (2) Immigrating people can influence the cultural values of local communities and promote or suppress the use of certain species in managed urban landscapes (Reichard and White 2001; García Lerena et al 2018). (3) Immigrating people may foster urban expansion and increase disturbance frequency and intensity (eg road construction, natural cover fragmentation), which provide new places where pioneer species with high propagule pressure may thrive (Mehraj et al 2018).

Migration patterns in the Andes have undergone a shift from rural–urban to interurban migration, primarily from small towns to larger cities (Medina et al 2016). This transformation is propelled by the pursuit of better opportunities, although in certain instances, migration is influenced by the expansion of agricultural frontiers. For example, in Ecuador, migration flows are concentrated toward the Amazonian lowlands (driven by the search for farmlands) and close to the capital city, Quito, in the highlands, motivated by urban amenities (Medina et al 2016). In Colombia, human mobility has been shaped by political and economic changes, such as the eradication of armed conflict and improved economic growth (Ramírez 2012). Although immigration is usually considered a relevant driver of population distribution and landscape modification (Bernard et al 2017; Royuela and Ordóñez 2018), the assessment of the association of migration flows and the prevalence of exotic plant species remains relatively unexplored. Our results highlight that human mobility, an indicator of the connectivity of a region, is associated with higher proportions of exotic species records (Figure 4). Moreover, using the combination of data on internal and international migration, we were able to overcome the problem associated with scarcity and reduced accessibility to information, which usually hinders studies on the association between human migration and species composition.

FIGURE 4

Correlation between the proportion of woody plants and weighted immigration flows. The solid line represents the fitted regression line. Bars indicate the deviations (residuals) between the observed values and those predicted by the regression. Smaller deviations represent a more accurate fit.

Invasibility patterns in the Andes: climate and people moving plants

The Andean region presents a high diversity of biophysical conditions (eg topography, climate, and land cover) that interact with Andean societies (eg through exotic species introductions and changes in land use and disturbance regimes) to determine their invasibility. Invasibility refers to traits of the receiving communities that enable the establishment and expansion of exotic species (Vicente et al 2010). Most arrivals of exotic plants involve connections with remote places; however, the establishment of new plant populations is moderated by the interaction of species characteristics and biophysical conditions (Furlan et al 2016). For instance, in mountain areas, the prevalence of exotic species decreases with elevation because of adverse biophysical conditions and reduced accessibility (Marini et al 2013). The latitudinal gradient also plays a role, for example, in low latitudes, characterized by less-stressing environmental conditions (eg high temperature and precipitation) and diverse native communities that may act as biological barriers reducing invasibility (Naeem et al 2000; Sax and Gaines 2008).

Because of their geographic isolation and steep slopes, which are associated with wide gradients of environmental conditions, mountains are extremely vulnerable to climate change. Climate warming and human interventions are likely to eliminate the barriers to the upward migration of exotic woody plants. This phenomenon is particularly evident in lowland cities, where changing temperature and precipitation patterns are weakening the limits to exotic plant migration upward (Payne et al 2020; Jimenez et al 2021). In addition, in higher latitudes there is a widespread use of exotic species (Egerer et al 2019) for aesthetic, recreational, and productive purposes, especially when native communities are present in small patches or when native species are undervalued for human uses (Sjöman et al 2016). For example, in harsh regions, cultivated cosmopolitan plants, such as Pinus, Ulmus, and Eucalyptus, are more likely to be used (Furlan et al 2016) to overcome resource scarcity with regard to specific purposes, such as silviculture (Thuiller et al 2006) or agriculture, as with Prunus persica in the puna region (Stampella et al 2013). In conclusion, it is likely that at least part of the increased prevalence of exotic species in harsher environments is explained by their adaptive traits, acclimatation capacity, and tolerance to adverse conditions that have been widely used for human purposes.

Ecosystem services and disservices of exotic plants in the Andes

The functioning of Andean ecosystems and the provision of ecosystem services depends on their identity and structural complexity. Alteration in species composition can disrupt the predictability of these ecosystem services and increase the vulnerability of local communities (eg exotic plants associated with fire regimes because of their traits; Contreras et al 2011) through alteration of dynamics in native plant communities (increased fire frequency) that are valuable for ecosystem services provision, such as water regulation.

At present, exotic species do not reach high densities in high elevations, but their presence in these environments, even at low densities, may disrupt ecosystem functions (Morales and Aizen 2002; Pauchard et al 2009). It is important to note that exotic plants can also offer ecosystem benefits, as they can host certain invertebrates (Salisbury et al 2017) or provide additional services. Exotic forest expansion in the surroundings of urban centers and low-residential areas enhances water regulation services and the capacity to sequester atmospheric carbon dioxide, showcasing the complex interactions between exotic and native species in Andean ecosystems (Jimenez et al 2023).

Big data: advantages and biases

Our study explored the role of socioeconomic development on the prevalence of exotic woody species using a novel approach that combines indicators of human migration with massive biodiversity data. Migration data are scarce, so we built an indicator of human mobility using the highest-resolution regional dataset available and combining it with emerging datasets on biodiversity.

Recent years have witnessed a significant increase in available biodiversity data, providing new opportunities to explore patterns and processes governing exotic plant dynamics. Biodiversity records, georeferenced by museums, herbaria, and citizens, are aggregated in regional and global datasets (Franklin et al 2017). However, challenges persist in handling these data, including geolocation and taxonomic uncertainties and sampling biases. For example, sampling is uneven across geographical space: with most sampling concentrated near roads and cities in the lowlands, the Andes tend to be underrepresented (Hughes et al 2021). Quantitative modifications to avoid these biases, such as evaluating the proportion of exotic species, rather than their raw abundance, must be implemented. In addition, the northern Andes (eg Colombia with 60% of exotic occurrences), which has higher native biodiversity, has more information about species occurrences. It is crucial to recognize the limitations of biodiversity records to avoid drawing inaccurate conclusions about vegetation patterns and dynamics; for example, range maps or diversity maps with erroneous records may lead to inaccurate results. These biases may be explained by a combination of factors: (1) inventories are usually restricted to forest vegetation, (2) exotic species lists underestimate the number of exotic species in each country (eg Global Register of Introduced and Invasive Species [GRIIS]; Seebens and Kaplan 2022), and (3) there is an overrepresentation of urban trees in citizen science studies (Ossola et al 2020). In this study, the inclusion of all the exotic woody species rather than only those included in GRIIS allowed us to generate a more complete database.