Humans have a long history of protecting certain elements of nature. The concepts and values underlying conservation initiatives, however, have changed repeatedly [1]. Conservation efforts in the 20th century focused primarily on preserving landscapes free of human influence and on preventing the erosion of biodiversity, with an emphasis on protecting rare species from extinction. The last 20 years have seen the emergence of additional concepts that emphasise the resilience of nature and the ‘services’ that nature contributes to human well-being [2,3]. These novel approaches are promoted by some conservation leaders under the assumption that they will broaden the social support for conservation goals [4,5]. The advent of more socially inclusive approaches to conservation biology raises interesting and profound questions regarding the dimensions of the living world we seek to preserve and the political process that is used to specify appropriate conservation objectives.

Non-native species, and how they are valued, are at the heart of these ongoing debates. For the last several decades, non-native species have been portrayed by scientists primarily as a threat to society because a subset can cause economic harm, human-health issues, or the loss of native biodiversity [6,7]. The view that non-native species are potentially undesirable persists in indicators used to track progress towards targets of the Convention on Biological Diversity (CBD), where they only appear as a numerical predictor for future invasion events (Aichi Target 9) [8].

More recently, scientists have also documented the potential positive contributions of non-native species to regional species richness [9,10], conservation goals [11], and to the ecosystem services they contribute to certain stakeholders within society [12–14]. In some instances, non-native species are rapidly appreciated for their cultural ecosystem services. For example, citizen groups have lobbied for the protection of non-native Eucalyptus trees in California and non-native dingos in Australia [11]. The potentially positive contributions of non-native species to biodiversity and to the long-term welfare of humans are missing from current biodiversity indicators (Table 1). This raises the following questions: Are non-native species part of ‘nature’ or ‘biodiversity’ that we wish to preserve? If so, can they be integrated into a conservation planning process in a way that recognises their potential for undesirable effects but also captures their potential positive contributions to biodiversity and society?

Name of Indicator or Study	Ref.	Use	NNS	Comment
Global
Living Planet Index	[15,16]	To inform Aichi Biodiversity Target 12	Included	Non-native populations make up 1.5% of tracked populations (286/18,427; July 2017 email from Stefanie Deinet to me, unreferenced citation. See Acknowledgments).
IUCN Global Red List	[17]	To identify globally threatened species	Excluded	The IUCN protocol does not normally consider populations outside of a species’ native range in the evaluations of a species’ extinction risk. Consequently, the extinction risk of species with significant non-native populations [18] will be overestimated. Furthermore, when the global IUCN data set is disaggregated regionally [19], it does not include non-native species in each region and thus likely results in an overestimate of the percentages of all species (native + non-native) that are threatened.
BII	[20]	To inform Planetary Limit of biodiversity	Excluded	BII value increased by 10% when ‘novel’ species are assumed to be functionally equivalent to ‘native’ species [21].
Global Study on State of Biodiversity	[22]	To project future biodiversity, by biome	Excluded	Defines biodiversity as ‘all terrestrial and freshwater organisms’, yet excludes NNS.
Wild Bird Index	[23,24]	To inform Aichi Biodiversity Target 12	Excluded	Listed on the Biodiversity Indicators Partnership site as an indicator applicable for national use and included in CBD indicators (https://www.bipindicators.net/indicators/wild-bird-index, Accessed 4 April 2018).
City Biodiversity Index (Singapore)	[25]	To measure biodiversity in cities, under CBD	Excluded	Five indicators focus on species richness of different taxa. All focus exclusively on native species and no rationale is provided for excluding NNS.
Regional
EU Common Birds Indicator	[26]	To measure health of environment, sustainability and to inform effectiveness of European Union Directives and Common Agricultural Policy	Excluded	Rationale for excluding NNS: ‘Non-native species are excluded, being an unnatural component that doesn´t contribute to the quality of the avifauna’. (http://www.ebcc.info/index.php?ID=491, Accessed 13 March 2018).
IUCN Regional and National Red Lists	[27]	To identify regionally threatened species	Excluded	NNS are assigned the Not Applicable (NA) code and therefore are not eligible for evaluation. This likely leads to a regional overestimate of the percentages of total species threatened.

Biodiversity is a complex notion and the indicators that are used to track it [16] are a reflection of available information, objectives, and values, which all can vary culturally and with time [3,28]. However, it is important to ask under what circumstances it might be scientifically and politically desirable to modify some biodiversity indicators to include all species in light of ongoing preparations for the Post-2020 Strategic Plan of CBD, ongoing assessments by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), and forthcoming national implementation of the Sustainable Development Goals (SDGs).

Non-native species as an integral component of biodiversity

There are several reasons why non-native species should be considered part of biodiversity and included in biodiversity and sustainability indices.

First, the absence of non-native species from biodiversity indices stands in contradiction to the CBD and SDGs. The CBD definition of biodiversity (Article 2) encompasses the biological dimensions of the world (genes, species, ecosystems and their interactions), but it makes no distinction between native and non-native life forms, nor does it refer to notions of ‘intactness’, which forms the basis for excluding non-native species from the Biodiversity Intactness Index (Table 1). Aichi Target 2 and Goal 15.9 of the SDGs require national and local governments to account for ‘the diverse values of biodiversity’.

Second, non-native species should be included in key biodiversity indices because they represent large fractions of modern ecosystems and regional species-pools. Non-native plants and birds can make up 50% or more of species in some urban [29,30], insular [9,31,32], and old-field [33] environments. There is a risk that regional policy makers in areas in which non-native species comprise a significant component of the landscape will perceive biodiversity indices to be irrelevant if they are based solely on native species.

Finally, and perhaps most importantly, society’s motivations for the conservation of biodiversity are evolving and the indicators used to measure the state of the environment and progress towards our goals should too. Biodiversity indices will need to encompass all species if they are to remain socially relevant and illustrate the full gamut of what are now called ecosystem services (and disservices), or nature’s contributions to people [3] (Fig 1).