Abstract
AbstractAimThe number of alien species has been increasing for centuries world‐wide, but temporal changes in the dynamics of their inter‐regional spread remain unclear. Here, we analyse changes in the rate and extent of inter‐regional spread of alien species over time and how these dynamics vary among major taxonomic groups.LocationGlobal.Time period1500–2010.Major taxa studiedVascular plants, mammals, birds, fishes, arthropods and other invertebrates.MethodsOur analysis is based on the Alien Species First Record Database, which comprises >60,000 entries describing the year when an alien species was first recorded in a region (mostly countries and large islands) where it later established as an alien species. Based on the number and distribution of first records, we calculated metrics of spread between regions, which we termed “inter‐regional spread”, and conducted statistical analyses to assess variations over time and across taxonomic groups.ResultsAlmost all (>90%) species introduced before 1700 are found in more than one region today. Inter‐regional spread often took centuries and is ongoing for many species. The intensity of inter‐regional spread increased over time, with particularly steep increases after 1800. Rates of spread peaked for plants in the late 19th century, for birds and invertebrates in the late 20th century, and remained largely constant for mammals and fishes. Inter‐regional spread for individual species showed hump‐shaped temporal patterns, with the highest rates of spread at intermediate alien range sizes. Approximately 50% of widespread species showed signs of declines in spread rates.Main conclusionsOur results show that, although rates of spread have declined for many widespread species, for entire taxonomic groups they have tended to increase continuously over time. The large numbers of alien species that are currently observed in only a single region are anticipated to be found in many other regions in the future.
Highlights
The numbers of alien species are rising continuously across all continents and in most taxonomic groups (Seebens et al, 2017)
We have shown that the inter-regional spread of individual alien species has extended historically over long time periods, often >100 years (Figure 1)
Our data suggest that spreading dynamics intensified after 1800, resulting in higher numbers of first records per species (Figure 4), with a wider distribution (Figure 3), this result might reflect the paucity of first record data from earlier centuries
Summary
The numbers of alien species are rising continuously across all continents and in most taxonomic groups (Seebens et al, 2017). The overall dynamics of the spread of alien species across the globe can be expected to have accelerated in recent times owing to increased human pressures, the rate of spread might have decelerated for some alien species as they reach the limits to the number of regions in which they can establish It remains unclear how these processes have developed over time at a global scale, how they interact and how temporal trends in proliferation differ among species and across taxonomic groups. A subsequent study of temporal dynamics of newly recorded alien species showed a surprisingly high proportion of the so-called emerging alien species in recent years, which could be related to a continuous increase in the sizes of source pools from which the species originated (Seebens et al, 2018) These studies focused on total numbers of alien species, and it remains unclear how the dynamics of inter-regional spread of individual species has changed over time. How has the rate of spread changed over time, both for individual alien species and for whole taxonomic groups?
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