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.