Global rise in emerging alien species results from increased accessibility of new source pools

Abstract

Our ability to predict the identity of future invasive alien species is largely based upon knowledge of prior invasion history. Emerging alien species-those never encountered as aliens before-therefore pose a significant challenge to biosecurity interventions worldwide. Understanding their temporal trends, origins, and the drivers of their spread is pivotal to improving prevention and risk assessment tools. Here, we use a database of 45,984 first records of 16,019 established alien species to investigate the temporal dynamics of occurrences of emerging alien species worldwide. Even after many centuries of invasions the rate of emergence of new alien species is still high: One-quarter of first records during 2000-2005 were of species that had not been previously recorded anywhere as alien, though with large variation across taxa. Model results show that the high proportion of emerging alien species cannot be solely explained by increases in well-known drivers such as the amount of imported commodities from historically important source regions. Instead, these dynamics reflect the incorporation of new regions into the pool of potential alien species, likely as a consequence of expanding trade networks and environmental change. This process compensates for the depletion of the historically important source species pool through successive invasions. We estimate that 1-16% of all species on Earth, depending on the taxonomic group, qualify as potential alien species. These results suggest that there remains a high proportion of emerging alien species we have yet to encounter, with future impacts that are difficult to predict.

Keywords: drivers; globalization; invasive species; source species pools; time series.

Fig. 1.

Frequency distribution of the number of first records per alien species.

Fig. 2.

Time series of observed first-record rates (black circles) and simulation results of the invasion model (red dots) for eight taxonomic groups. Observed first-record rates are shown for all alien species (Upper), emerging alien species (Upper Middle), and the proportion of emerging among all alien species (Lower Middle). In the invasion model, species were randomly selected from the predicted candidate species pool according to the first-record rate of all alien species to simulate the dynamics of emerging alien species. The temporal development of the size of the candidate species pool (Lower) is the result of the model fitting and represents the species pools, which are necessary to reproduce the dynamics of emerging alien species best. Black dots indicate the knots, which define the temporal dynamics of the candidate species pool (Materials and Methods). The congruence between observed and predicted proportions of emerging alien species is shown as R² values. Note that the y axes vary in scale.

Fig. 3.

Results of the statistical analysis of time series of first-record rates of emerging and nonemerging alien species using GAMMs for eight taxonomic groups. Nonemerging alien species denote first records of those alien species, which have already established somewhere else in the world, and are shown for comparison. The importance of a single predictor (x axes) is indicated by the size of the dots corresponding to the difference in AIC (ΔAIC) between the full model and the one without the parameter under consideration (Materials and Methods). Significant improvements of the model fit tested by a likelihood ratio test are highlighted by black outer circles. The sample size is given in the first column and the adjusted R² in the last column of each panel. A missing dot denotes that the respective predictor did not enter the GAMM.