Mapping invasive alien plants through citizen science: shortlisting species of concern for the Nilgiris

: Species introduced from elsewhere are known as alien species. They may be introduced as crop plants or ornamental plants, or for timber. A small proportion of introduced species can become invasive thereby spreading at the cost of native species and habitats, negatively affecting biodiversity, food security, and human wellbeing. Despite the growing recognition of the threat of invasive alien species, we still lack information about the distribution and abundance of species widely accepted to be invasive. To address this information gap regarding invasive alien species distributions, we initiated a pilot citizen science effort to create an atlas of invasive plants in the Moyar-Bhavani landscape of the Nilgiri District. We aimed, through this pilot effort, to develop and test user-friendly mapping protocols and develop an interface for citizen scientists to use. Ultimately, we hope to create a model that can be scaled up to large conservation landscapes, such as the Western Ghats, the Central Indian Highlands, and the Himalaya.


INTRODUCTION
People have moved species around the globe since time immemorial for food, fibre, fuel, sport, and aesthetic reasons.Such species, which have been introduced outside their natural range of distribution, are referred to as 'alien species' (or introduced or exotic species).Most alien species arrive in new environments intentionally, though some can arrive inadvertently as contaminants on known introductions or simply as stowaways.Examples of species introduced intentionally include plants and animals introduced for food (e.g., the African Catfish Clarias gariepinus), for timber and fuelwood (e.g., the Black Wattle Acacia mearnsii), or those introduced as ornamental plants (e.g., Lantana camara), and for the aquarium and pet trade (e.g., the Goldfish Carassius auratus and the Red-eared Slider Trachemys scripta elegans).An example of an inadvertent or accidental introduction is Parthenium hysterophorus, whose seeds are thought to have arrived in India as a seed contaminant of wheat imported from the Americas.
Although the vast majority of introduced species are of great value, a small proportion of these can become invasive.This refers to their becoming widespread and having negative impacts on biodiversity, ecosystem services, food security, or human health and wellbeing.The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) identifies invasive alien species as amongst the greatest threats to biodiversity and ecosystem services globally, comparable with climate change, change in land (and sea) use, species overexploitation, and pollution (IPBES 2019).
In India too, we now recognise the threat of invasive alien species.One of India's National Biodiversity Targets (NBT4) focuses on preventing new invasive species introductions and controlling existing invasive species (MoEFCC 2014).Some states have gone further by formulating policy on invasive species management, Tamil Nadu being the first (TN-PIPER 2022).
Despite this growing recognition of the threat of invasive alien species, we still lack information about the distribution and abundance of species widely accepted to be invasive.Such information is vital to prioritise species and habitats for management interventions and provides a baseline against which to assess future invasive species spread.
To address this information gap regarding invasive alien species distributions, we initiated a pilot citizen science effort to create an atlas of invasive plants in the Moyar-Bhavani landscape of the Nilgiri District.Citizen science, which refers to a partnership between scientists and members of the public, is a growing field both in India and globally.Over the last decade the ubiquitousness of smartphones, plus access to the internet, has made it easier for people to record and share observations, leading to a growing number of such researcher-citizen scientist collaborations (see for example, https://citsciindia.org/projects/).In our specific case, scientists working in partnership with naturalists, students, community members, and forest managers could achieve the task of mapping invasive species at a scale, and within a timeframe, that would be meaningful for both researchers and managers-something that scientists on their own could not do.We aimed, through this pilot effort, to develop and test user-friendly mapping protocols and develop an interface for citizen scientists to use.Ultimately, we hoped to create a model that could be scaled up to large conservation landscapes, such as the Western Ghats, the Central Indian Highlands, and the Himalaya.

Description of the study area
The Nilgiris District has a long history of plant introductions.The cool, temperate upper elevations of the Nilgiris attracted European settlers during the colonial period.They introduced many alien species as garden ornamentals (e.g., Cestrum aurantiacum, Asclepias curassavica, and Cytisus scoparius) and for fuelwood (e.g., Acacia mearnsii and Eucalyptus spp.).Many of these species have since become invasive, suppressing native species and altering habitats.The introduction of species to the Nilgiris continues to date, as the area is still of great horticultural importance and remains a source of exotic fruits, vegetables, and ornamental plants for the rest of southern India.
Our study area, the Moyar-Bhavani watershed of the Nilgiris, straddles two terrestrial ecoregions-southern Western Ghats moist deciduous forests and the southern Western Ghats montane rainforests.We used the ecoregion information included in the Indian Alien Flora Information database (v1.0 available at https://ilora2020.wixsite.com/ilora2020/data)and identified 378 plant species that have been introduced to these two terrestrial ecoregions (Pant et al. 2021).Of these, about 81 can be considered invasive alien species today, based on expert opinion.However, we felt that mapping the distribution and abundance of these many invasive species was an unreasonable ask of citizen scientists (i.e., volunteers, students, and forest department field staff).We therefore prioritised amongst these 81 species to arrive at a more manageable shortlist of widespread and highly invasive species for citizen scientists to record.Here, we describe the process followed to create that priority list of invasive species as a precursor to creating a pilot citizen science atlas of invasive species.

Compiling, selecting, and shortlisting of invasive species
As a first step, in September 2017, we compiled a database of the 81 invasive plants for the Nilgiris using various sources such as Zarri et al. (2004), Keystone Foundation (2008, 2016), Narasimhan (2009), Khuroo et al. (2012), Hiremath & Sundaram (2013), and from personal observations.For each species, we included additional information on its origin, the range of elevations within which it is found, and its presence (or absence) in various habitats.We created a matrix to indicate species presence in these different habitats, i.e., dry and wet forest, grasslands, plantations, wetlands (marshes, peat bogs), and freshwater habitats (ponds, lakes, rivers, reservoirs) (Annexure 1).
The next step was to select a preliminary short-list of invasive species from amongst this list of 81 species.Our selection was informed by existing definitions of invasive species.The IPBES defines an invasive alien species based on its ecological and socio-economic impacts (IPBES 2019).An alternative definition is proposed by Colautti & MacIsaac (2004), who suggest that an invasive species is one that is both locally abundant, and widespread, distinguishing it from other introduced species.We combined these considerations into the following three criteria: 1. Species that were well known in the landscapes (a measure of the species' impacts and abundance) 2. Species that have spread into multiple habitats (a measure of the species' local abundance).
3. Species that occur over more than one altitudinal zone (a measure of the species' spread).
We shortlisted 34 species that met these three criteria (Annexure 2) and convened a stakeholder workshop later that same month to assess the appropriateness of the shortlist for the Nilgiris landscape.A scoring was done to reflect the presence of the species in different habitats, with '1' denoting a species' presence in only one habitat, '2' denoting its presence in two habitats, and so on.
The stakeholder workshop included participants from local conservation organisations, community-based organisations, academic institutions, and restoration practitioners.We added Pennisetum clandestinum and Polygonum polystachyum to the final shortlist during the workshop as these species were known to be spreading in the Nilgiris.On the other hand, Acanthospermum hispidum, Argemone mexicana, Kalanchoe delagoensis, Opuntia stricta, Synedrella nodiflora, and Tithonia diversifolia were excluded from the list because, despite being invasive, these were not considered widespread by the stakeholders.Later, we replaced Senna alata with Senna spectabilis after observing the rapid spread of Senna spectabilis during a field visit to Sathyamangalam Tiger Reserve, and based on expert opinion that S. spectabilis is of greater conservation concern.
Based on the workshop discussions, we selected 26 of the 34 shortlisted species for mapping invasive alien species in the Nilgiris (Annexure 3).We then prepared a field identification key with images of the plant parts (habit, twigs, leaves, flower, fruit, seeds) to help individuals identify species while mapping invasive species in the field.A hard copy of the field identification key was printed for reference.The naming of the species has followed the International Plant Name Index (IPNI).

RESULTS AND DISCUSSIONS
Over 70% of the species that we prioritised for the Nilgiris were categorised as 'invasive' in existing databases of invasive alien plants in India, namely the 'Khuroo list' (Khuroo et al. 2012) and the ILORA database (Pant et al. 2021) (Table 1).When comparing our list of 26 invasive alien species with Khuroo's, we found 19 species were assigned the status 'invasive', Passiflora mollissima was assigned the status 'naturalised/invasive', while Senna spectabilis and Pennisetum clandestinum, are considered to be 'cultivated' and 'naturalized', respectively.None of the remaining four species-Cestrum aurantiacum, Gamochaeta purpurea, Polygonum polystachyum, and Solanum mauritianum on our list appears on the Khuroo list, though several of their congeners do.
Meanwhile, the ILORA database has additions to the list of alien species that were absent in the Khuroo list.These include C. aurantiacum and S. mauritianum, which are assigned the status 'invasive' and 'naturalised alien,' respectively.Both species are on our priority list of 26 species.Apart from this, the invasion status of some species from the Khuroo list has been revised in the ILORA database.For example, Phragmites australis and Pistia stratiotes (again, both on our priority list of 26 species), considered invasive in the Khuroo list, are now listed as native in the ILORA database (ver. 1) and unlisted altogether in an updated version (1.1); this may be due to their cryptogenic origins.Overall, 24 out of the 26 invasive species shortlisted by us can be found in the ILORA database, with the exception of P. polystachyum and G. purpurea.

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The comparison of our list of priority invasive species for the Nilgiris with the Khuroo list and the ILORA database offers interesting insights.One is that invasion is a dynamic process, and a species' invasion status could change over time.For example, the Khuroo list (published in 2012) considered Senna spectabilis to be 'cultivated', but in the short time since, Senna spectabilis has become widespread and abundant in several parts of the Western Ghats, including the Nilgiris.Recent work by Anoop et al. (2021) suggests that elephants are aiding in its widespread and rapid dispersal.This change in the species' status is reflected in the ILORA database (see Table 1), which was compiled a decade after the Khuroo list.
Another interesting insight is that a species' 'alien' status is determined by biogeographic boundaries, not by geopolitical ones.In this case, Polygonum polystachyum (Himalayan Knotweed), which is native to the Indian Himalaya, does not appear on either the Khuroo list or the ILORA database.However, stakeholders in the upper elevations of the Nilgiris, a region that is biogeographically distinct from the Himalaya, consider the species to be invasive.
Overall, our results highlight the value of the ILORA databases as a starting point for any effort to compile a locally relevant list of invasive species.The ILORA database builds on the earlier Khuroo list, and also incorporates information from other existing databases (see Pant et al. 2021), making it the most comprehensive listing of invasive alien plants for India today.However, this also makes the database unwieldy in smaller regions.For instance, the ILORA database lists a staggering 120 invasive alien species for the Moyar-Bhavani watershed, which barely extends over 4,100 km 2 .This might be because the watershed straddles two terrestrial ecoregions (i.e., the southern Western Ghats moist deciduous forests and the southern Western Ghats montane rainforests).Nevertheless, mapping the distribution and abundance of these many species is a daunting task, even when energised by citizenscientists.Here, our priority list of 26 invasive alien species for the Nilgiris underscores the value of local expert opinion in shortlisting species that are locally relevant.
The ILORA database may still be useful when developing lists for large landscapes or states, but local expertise is invaluable in developing lists for smaller landscapes.
Local expertise is also vital for identifying emerging threats.One illustration of this is the expert inclusion of Cestrum aurantiacum and Solanum mauritianum, neither of which appears on the Khuroo list.Both species are relatively recent additions to the database on alien species in India (even though they are listed as invasive alien species in global databases such as CABI and GISD).Another is the expert inclusion of Senna spectabilis, which was earlier listed as cultivated (in 2012) and has now been listed as invasive (in 2021).Given that published information about invasive species in India is still incipient, and that a large proportion of this information is dominated by a few species (Hiremath & Sundaram 2013), comprehensive databases like ILORA are constrained by the information that they can build on.In such a situation, expert opinion of local community members, forest managers and botanists must continue to inform the listing and prioritising of invasive alien species, in conjunction with existing databases.

CONCLUSION
There are an estimated 220-225 invasive alien plants in India (Khuroo et al. 2012;Pant et al. 2021).A few, such as Lantana camara and Prosopis juliflora are very widespread (Hiremath & Sundaram 2013).Others are more regional in their distribution, though locally abundant and widespread, e.g., Acacia mearnsii in the upper elevation regions of the Western Ghats (Nayak et al. 2023), or Anthemis cotula in the Kashmir Himalaya (Reshi et al. 2012).Yet the distribution and abundance of each invasive alien species, and even the number of invasive alien species, is expected to change in time.A citizenscience approach is best placed to track these changes and build an atlas of invasive alien plants for India.
To enable citizen scientists to contribute easily and continuously to such an atlas, it is important that they are not overwhelmed by the large number of invasive alien plants across India.Instead, they need to work with a shorter list of species that is relevant to the area they live in.The process that we have followed for prioritising invasive plants for the Nilgiris could provide a replicable model for other regions as well.

Table 1 . A comparison of the short-list of 26 priority invasive alien plants of the Nilgiris with existing national invasive species lists (ILORA version 1 and version 1.1). Accepted scientific name of species Family Native region Invasion status (Khuroo list) Invasion status (ILORA)
Cl-Cultivated | Cs-Casual | C/N-Casual/Naturalised | Nt-Naturalised alien | N/I-Naturalised/Invasive | In-Invasive.