Native-alien populations—an apparent oxymoron that requires specific conservation attention

Abstract

Many countries define nativity at a country-level—taxa are categorised as either alien species or native species. However, there are often substantial within-country biogeographical barriers and so a taxon can be native and alien to different parts of the same country. Here, we use the term ‘native-alien populations’ as a short-hand for populations that result from the human-mediated dispersal of individuals of a species beyond a biogeographical barrier to a point beyond that species’ native range, but that is still within the same political entity as parts of the species’ native range. Based on these criteria, we consider native-alien populations to be biological invasions. However, we argue that, in comparison to other alien populations, native-alien populations: 1) are likely to be closer geographically to their native range; 2) are likely to be phylogenetically and ecologically more similar to native species in their introduced range; and 3) options to control their introduction or manage them will likely be more limited. We argue this means native-alien populations tend to differ from other alien populations in the likelihood of invasion, the types of impacts they have, and in how they can be most effectively managed. We also argue that native-alien populations are similarly a distinct phenomenon from native populations that are increasing in abundance or range extent. And note that native-alien populations are expected to be particularly common in large, ecologically diverse countries with disjunct biomes and ecoregions. Reporting, monitoring, regulating and managing native-alien populations will, we believe, become an increasingly important component of managing global change.

An inventory of native-alien populations in South Africa

“…Terms that have been used to refer to this phenomenon (see Box 1 of Nelufule et al . 4 ) were used as search strings, for example, “domestic exotics”, “intra-country established alien species” , “home-grown exotic” , “extralimital species” and “native-alien species” . Additional searches were performed by adding “AND South Africa” to these terms.…”

“…The processes that lead to the introduction of alien species can act within political entities and, therefore, a species can be both native and alien within the same political entity 1 – 3 , a phenomenon for which the term ‘native-alien populations’ has been proposed 4 . This term was adapted from the term ‘native-alien species’ that is currently used in the Global Register of Introduced and Invasive Species, and by the IUCN SSC Invasive Species Specialist Group 5 , 6 .…”

“…This term was adapted from the term ‘native-alien species’ that is currently used in the Global Register of Introduced and Invasive Species, and by the IUCN SSC Invasive Species Specialist Group 5 , 6 . A native-alien population is defined as a population that results from the human-mediated dispersal of individuals of a species over a biogeographical boundary to a point beyond that species’ native range that is still within the same political entity as some parts of the species’ native range 4 . This definition differs slightly from others that are used to define this phenomenon in South Africa, and in other countries [e.g.…”

“…However, few countries (e.g., Spain and USA) have included native-alien populations in these inventories 6 . This is concerning as native-alien populations pose specific regulatory and management challenges and tend to differ from alien populations introduced from other countries in the impacts caused 4 . This is, in part, as the regulation of biological invasions is often done at the country level, and consequently, native-alien populations are often regarded as native 11 .…”

Considerations for developing and implementing a safe list for alien taxa

“…Where different climatic zones or biomes provide varied opportunities for invasion within a singular region (van Wilgen et al 2020 ), the taxa should be considered safe in all environments within the region to qualify for listing. This is particularly important because there are generally limited capabilities and insufficient barriers to restrict the movement of individuals (and taxa) within countries (Nelufule et al 2022 ). Similarly, although all the environments within a country might be regarded as unsuitable for a specific alien taxon, and the taxon is therefore considered safe at a national level, neighboring countries can still be at risk of invasion (e.g., Faulkner et al 2020 ).…”