Consequences for understanding drivers of biological invasions
Increasing economic activities and human population exert a positive influence on the number of alien species among taxa (Taylor & Irwin, 2004; Westphal et al., 2008). To the best of our knowledge, our study is the first to compare the variable importance of absolute and relative metrics of economic and human population. Relative socioeconomic variables (i.e., GDPc) provide insights into standards of livings (Pyšek et al., 2010; Moser et al., 2018), while absolute variables (i.e., GDP) offer information about the intensity of economic activities related to commercial trade, industrialization, and urbanization (Taylor & Irwin, 2004; Leprieur et al., 2008). This distinction becomes apparent in Scandinavian countries, characterized by high values of GDPc alongside low human populations, whereas densely populated countries such as China or India exhibit high GDP but lower values of GDPc (Fig. S2). These differences could be related to different introduction pathways, such as the pet trade, involving intentional introductions through the deliberate release of birds and mammals and being more important in countries with higher GDPc (Hulme et al., 2008) and overall commercial trade, more related to GDP and to unintentional introduction processes (Chapple et al., 2012). Our HP results (e.g., Fig. S9) suggest that both absolute and relative measures play a role in alien richness, but that the former are more important and that future studies should carefully consider the difference between relative and absolute factors when investigating underlying drivers and predicting the distribution of alien species. Our findings highlight the predominant role of socioeconomic variables in elucidating patterns of alien species compared to human population variables. However, some caution is needed because we were unable to quantify other important invasion processes, such as propagule or colonization pressures (Simberloff, 2009; Lockwood et al., 2009). This suggests a potential overestimation of the impact of human population variables in analyses exploring factors influencing macroecological invasion patterns. The current state of invasive research lacks a comprehensive comparison among major groups of drivers, with only a few studies considering both environmental and economic factors (McKinney, 2002; Pyšek et al., 2010). However, these studies did not conduct statistical analyses aimed at determining the net effects of individual variables and their relative importance.