Distribution and Abundance of Native Orchids on Roadside Trees in a Global Biodiversity Hotspot

Abstract

Trees play a vital role in supporting biodiversity, particularly in regions where human demand for resources is increasing and many species are experiencing population decline. Native orchids, especially those that are endemic, are particularly vulnerable to population decreases in biodiversity hotspot areas, with anthropogenic factors playing an increasingly significant role in this trend. A substantial portion of the northern district of Kerala, situated in southern India, falls within the biodiversity hotspot of the Western Ghats. The road network traversing the Western Ghats provides refuge for native orchids on various tree species. The present study examined a 60 km perimeter area encompassing 15 different sites located near small towns and built-up areas, regions where trees have already been lost due to settlement and infrastructure development. These roadside areas are lined with a mixture of native and exotic trees, including both naturally recruited fruit trees and exotic species. Approximately 600 trees, representing 72 different tree taxa, were recorded from the sampled areas in Wayanad. Nearly 10,000 orchids encompassing 13 species were observed, with 6 of these species, constituting 46%, being endemic to the region. This study revealed variations between sites, with some locations exhibiting high diversity and abundance of both trees and orchids. The diversity and abundance of native orchids, risks, and prospects of development mitigation are discussed in detail.

1. Introduction

Orchids are among the most dominant groups of vascular epiphytes [1]. In India, the state of Kerala hosts the second-highest number of threatened species of flowering plants according to the IUCN classification. India is home to 1251 orchid species across 160 genera, with 293 species being endemic [2]. Although India is not exceptionally rich in orchid species relative to its size (about 0.38 species per km²), it has a high rate of endemism at approximately 24%, surpassing most other continental Asian countries [2]. Kerala, a southwestern state bordered by the Western Ghats biodiversity hotspot, is a significant orchid hotspot with 271 species, primarily found in the Western Ghats [2]. Kerala and Arunachal Pradesh both host about seven orchid species per 1000 km², a figure rivalled only by global orchid hotspots like Ecuador and Costa Rica [2].

The Western Ghats have experienced a significant decline in forest cover [3], losing around 35% of their total area between 1920 and 2013 [4]. Historically, populations of epiphytic orchids disappeared from their native ranges due to the clearance of forests for other uses, and the current decline is more worrying as many endemic populations are under threat [5,6,7,8]. A study in Peninsular Malaysia found that fragmentation led to the ability of phorophytes to sustain orchid colonisation in the disturbed forests [7]. The state of Kerala, which comprises approximately 17% of the forest within the Western Ghats area, has witnessed the most substantial historical loss of natural forests among Indian states, with a recorded decline of more than 60% [4]. However, this percentage might be considerably higher in Kerala, although the available data is inconsistent. The Western Ghats in Kerala, especially the district of Wayanad, are home to rich biodiversity for both native trees and orchids. As part of the recent and ongoing infrastructure improvements, the widening of existing roads and the creation of new highways resulted in the removal of trees [6]. The populations of native trees and orchids are more under threat from removal/destruction.

Infrastructure projects, such as road widening, lead to the removal of trees, which, in turn, causes the large-scale destruction of native orchid populations within the state. Local extinction is bound to happen in a hotspot like Kerala, as an abundance of some of the native orchids is localised. Unfortunately, there are no reliable statistics available regarding the number of trees and orchids lost because of these projects [6]. Orchids that are lost due to tree-felling vanish permanently, as there is currently no mechanism in place to preserve these plants for replanting in suitable locations elsewhere. As part of a strategic conservation approach grounded in scientific evidence, these native trees are not being replaced.

This paper explores the distribution and abundance of native orchids in selected sites (within the Western Ghats area) in Wayanad, within the state of Kerala in India. The potential for saving orchids and planting them in safe havens as part of development mitigation to benefit both orchids and people as part of conservation–nature connectedness projects will be discussed.

2. Materials and Methods

Study Area

The study areas were on both sides of a large town in Wayanad called Sultan Bathery, 15 km to the west and 10 km to the east (14 sites, called Plots), and a single site 16 km to the south (Figure 1). All these sites are on the National Highway 766 (NH 766) in southern India connecting Kerala State with the adjoining states Karnatka and Tamilnadu. The highway passes through dense forests of the Western Ghats of India where all the study sites are located. The NH-766 passes through 19.7 km of Bandipur National Park and Wayanad Wildlife Sanctuary.

The first two plots were close together, only 310 m away. Plots 4–10 were grouped within an area of about 3 km. Plots 11–15 were within an area of about 6 km. A single survey was performed with trees and orchids counted on one side of the road.

Of the 15 sites, Plot 3 is the control site, which has no human settlement and is located within the dense Western Ghats Forest area, while all other areas have human settlements of varying degrees. Data collection for this entire study was conducted in the months of March and April 2025. Trees that were found on the roadside of the study areas, each measuring 3 m wide and 500 m long (named Plots) on one side of the road, were sampled (Figure 1 and Supplementary Materials Figure S1a–c). Diversity and abundance of both trees and orchids were recorded, covering all 15 study areas. Mature trees that have a circumference of at least one metre were counted. The total perimeter of the study area was 60 km (connecting all fifteen sites along the 766), with three groupings of study areas (consisting of two, seven, and five sites) and a single plot, as shown in Figure 1. Details of each plot are described in

Table 1. Details of the 15 plots surveyed as part of the study explaining vegetation type and nearby landscape characteristics.

Plot No.	Plot Details	Approximate Percent Fragmentation
Plot 1	The plot is close to a paddy field, with the roadside hosting both native and deciduous trees planted as part of social forestry.	92
Plot 2	The original forest landscape is partially replaced with introduced social forestry, with many of the original forest trees still existing.	7
Plot 3	The original forest landscape.	4
Plot 4	The original forest landscape is partially replaced with social forestry trees, with some original forest trees still existing.	7
Plot 5	Mixture of native and social forestry origin trees.	9
Plot 6	Roadside and fallowed coffee plantation with mixture of native and plantation trees.	11
Plot 7	Close to the urban area with small number of native trees.	15
Plot 8	Within the urban area which is dry with abundance of exotic trees orchid species.	19
Plot 9	Within the urban areas, with sampled areas which are dry.	94
Plot 10	Urban area and dry area abundant with social forestry trees.	24
Plot 11	Close to the Wayanad wildlife sanctuary with a high proportion of native trees.	7
Plot 12	Social forestry trees interspersed with native forest trees.	10
Plot 13	Sampled areas host more social forestry taxa than native trees.	16
Plot 14	Mix of domesticated trees and social forestry trees with few native trees.	20
Plot 15	Mix of social forestry and domesticated trees with coffee plantations nearby.	14

. Flowering data gathered over the past three years were used to identify species that were not in flower during the data collection period, March–April.

Counting of orchids on each tree was performed using photographs taken from different angles. In some cases, orchids with large clumps were counted based on an error value of ±3 plants, as some plants were difficult to count due to the close packing of the individual orchid plants within each clump.

Fragmentation in the sample plots studied was recorded based on the baseline data from Plot 3, which has <5 percent fragmentation. Loss of native trees and presence of social forestry trees, and domesticated fruit trees, and other introduced trees were taken into consideration to assess the percentage of fragmentation that occurred.

3. Results

The study sites were selected based on the presence of a rich diversity of trees, native and endemic orchids, and the historic and recent fragmentation. National Highway 766, which runs through the Western Ghats region in Wayanad, adjacent to Bandipur National Park and Wayanad Wildlife Sanctuary, is a busy highway that may be widened because of the ever-increasing transport needs for the region. There were nearly 600 trees under 72 tree taxa recorded from the sampled areas in Wayanad (Supplementary Materials, Figure S2a–d). Of these trees, nine species are endemic, which is about 12%. Plot 11 was found to host the most diverse number of tree species, followed by Plot 12, while Plot 4 hosts the least diverse number of tree species. Lagerstroemia macrocarpa and Stereospermum colais are the most abundant native trees recorded, while Mangifera indica is the domesticated tree species found to be the most abundant, followed by Artocarpus heterophyllus. Of the endemic species, Pterocarpus marsupium is the most abundant, while Hopea ponga and Ficus tsjahela are the least abundant endemic species (Figure 2 and Figure 3).

Cymbidium aloifolium was the most dominant orchid species in the majority of plots surveyed (11 out of 15 sites), followed by Pholidota imbricata (Figure 4, Figure S3a). Plot 3 is the control site with the least amount of fragmentation, that is located within the dense Western Ghats forest area (Table 1). All other areas have varying degrees of human settlements. Plot 1, the most urbanised area, hosts the least number of orchid species. Plot 1 hosts only 4 species of orchids in comparison to 10 and 13 species in Plot 2 and Plot 4, respectively (Figure 5). Plot 3, the only site with extremely low habitat fragmentation and without any human settlements, hosts 11 species of orchids. Plot 1 hosts Aerides ringens as the only endemic species found on Mangifera indica. Dendrobium herbaceum, D. macrostachyum, and Ploystachya concreta are found only on one type of tree in Plot 2. D. herbaceum in Plot 3, P. concreta in Plot 4, and Vanda thwaitesii in Plot 5 are found only on Pterocarpus marspium. Plots 5–8 host 13 species each, with most orchids present in all plots (Figure 6). Plot 7 is characterised by fragmentation and dominance of non-native trees because of human settlement, similar to Plot 1. Pholidota imbricata colonised most trees while most of the endemic species were found on only one tree in some plots. The two endemic orchids D. herbaceum and Oberonia brunoniana are found only on a single species of trees each. In Plot 9, within the urban area, the sampled areas are dry with a less diverse number of orchids, but the endemic orchids A. ringens, L. zeylanica, and V. thwaitesii are found on native trees. Plots 10, 11, 12, and 13 are more diverse, with orchids found on more than one tree species (Figure 7). Interestingly, the endemic V. thwaitesii is recorded on 10 tree species. In Plot 14, V. thwaitesii is recorded on 66% of trees, while D. herbaceum is found only on one native tree. In Plot 15, out of 19 tree species, only M. indica hosted Vanda ovatum, an endemic species of orchid. Except for Plot 14, where a smaller number of trees with more abundance of non-native trees were recorded, all other plots hosted a high number of tree species. However, both non-endemic and endemic orchids were found in Plot 14 (Figure 8). The t-test comparing the top 5 plots (with highest diversity and abundance) against the bottom 5 plots (with lowest diversity and abundance) indicating significant difference (p = 0.0006) in orchid abundance between the most abundant and least diverse plots. Plots with higher diversity tend to have significantly greater abundance (Figure S3b).

A total of 15 plots, covering 7.5 km of roads (sampling only one side), were surveyed within the Western Ghats. These plots were found to host nearly 10,000 orchids (Supplementary Materials, Table S1) on approximately 600 trees. Among the 13 orchid species recorded in the sampled areas, 6 species (representing 46%) are endemic. Cymbidium aloifolium emerged as the most abundant species, with about 3000 individuals, followed by Pholidota imbricata. Among the endemic orchids, Luisia zeylanica was the most prevalent, being recorded in all plots, closely followed by Aerides ringens, which was present in all plots except Plot 1. In contrast, Gastrochilus acaulis exhibited the least widespread distribution, occurring in only 9 out of the 15 plots (Figure 9).

Overall, Plot 1 within an urban area was the least diverse, and Dendrobium spp. are the least abundant taxa compared to other orchid species in all plots. The endemic Pterocarpus marsupium and the fruit tree Mangifera indica hosted most orchid species, both endemic and non-endemic, compared to a small number of orchid species, with some plots hosting just a single species (Figure 4).

4. Discussion

The global biodiversity hotspot of the Western Ghats is known for the high occurrence of endemic plants [9,10]. Tropical trees provide habitats for other forms of biodiversity, such as epiphytes, and facilitate the colonisation and establishment of resilient populations [11,12,13,14,15]. The state of Kerala, with 17% of forest within the Western Ghats area, has undergone a substantial loss of natural forests in the region of 60% which is one of the highest in India [4]. The Western Ghats in Kerala, especially in the district of Wayanad, are unique as this region is home to many native trees and orchids compared to other areas in the state. The current study explored roadside areas within the Western Ghats region, which is already fragmented by human settlement. Abundance of native orchids globally is correlated with mountainous areas, high rainfall, as in the northeastern Himalaya ranges, the Western and Eastern Ghats [2]. Orchids are sensitive to environmental changes as reported from many parts of the world, including orchid hotspots [16,17,18]. Climate change is causing unprecedented changes in rainfall in the Western Ghats area in India, especially in Kerala [19,20,21,22].

An additional challenge facing native trees and orchids in Kerala is the unprecedented removal of trees for infrastructure improvement. In the last decade, this must have contributed to the loss of tens of thousands of trees and hundreds of thousands of native orchids. Only media reports and anecdotal evidence are available to suggest that Kerala is losing substantial biodiversity, including endemic taxa [6]. Removal of trees for widening existing roads and creating new highways is happening in almost all Indian states, but it is a major concern for the northern districts in Kerala, as this area has an abundance of native trees and orchids, many of them endemic.

The loss of roadside trees and the impacts of climate change are contributing to the localised decline of orchid populations in northern Kerala. Given that Kerala is home to nearly 25% of India’s native orchids and is recognised as an orchid hotspot, a significant population decline is anticipated in the coming decades because of the loss of orchids for development [6]. Conditions such as the availability of the right phorophytes with rugose bark and moisture levels are factors hypothesised as important drivers for successful orchid and orchid mycorrhizal colonisation [11,23]. Nearly 600 trees, representing 72 taxa, were recorded from the studied areas in Wayanad, highlighting the exceptional biodiversity of trees within a relatively small region. Except for Plot 3, the sampled areas are fragmented due to human settlements. Approximately 13% of the recorded trees are endemic, alongside a variety of domesticated species. Notably, the endemic species Pterocarpus marsupium hosts the majority of orchids, followed by the introduced fruit tree Mangifera indica. The rugose bark of these trees appears to provide optimal conditions for orchid colonisation.

Fragmentation is clearly visible in Plot 1 as the landscape has changed due to urbanisation, as this is the least diverse among all the plots. Different plots host both trees and orchids, with some trees hosting only a small number of orchid species, while some host just a single species. Dendrobium spp. are the least abundant compared to other orchid species in all plots, which included some endemic taxa. The abundant species, such as Pholidota imbricata and Cymbidium aloifolium, are colonised on most trees. There was no clear trend in the type of orchid and colonisation on specific trees, except that the rugose bark of Pterocarpus marsupium and Mangifera indica hosted the most orchids. However, some of the sites have certain orchids colonising more on native trees. Plot 1, the most urbanised area, hosted only 4 species of orchids in comparison to 10 and 13 species in Plot 2 and Plot 4, respectively. Plot 3, the only site without obvious habitat fragmentation, hosted 11 species of orchids. However, introduced fruit trees are equally good phorophytes for both endemic and non-endemic species. Plot 1 hosts Aerides ringens as the only endemic species found on Mangifera indica. D. herbaceum in Plot 3 and Vanda thwaitesii in Plot 5 are found only on Pterocarpus marsupium. Most orchid species are found in Plots 5–8, with 13 species apiece, while Plot 7, along with Plot 1, is characterised by fragmentation and dominance of non-native trees due to anthropogenic reasons. Forest fragmentation and introduced crops are known to negatively affect epiphytic orchid diversity [24].

The only dry plot sampled, Plot 9, was found to host a less diverse number of orchids, but the endemic orchids A. ringens, L. zeylanica, and V. thwaitesii were found on native trees here. Plots 10–13 hosted more diversity, with the endemic V. thwaitesii recorded on 10 tree species. In Plot 14, V. thwaitesii is recorded on 66% of trees, while D. herbaceum is found only on one native tree. In a fragmented area as studied here, most orchids started to colonise on available introduced trees due to the historical loss of native trees. The extent of native tree loss over the past one hundred years, during which anthropogenic impacts accelerated, remains unknown due to the absence of records documenting the scale of this loss. In Plot 15, out of 19 tree species, only M. indica hosted Vanda ovatum, an endemic species of orchid.

About 600 trees, representing 72 different tree taxa, and 13 orchid species with nearly 10,000 orchids in the sampled area show a high diversity and density on one side of the road. As more road widening and creation of new roads are expected, with a projected 200 km of road, an estimated loss of half a million native orchids in Wayanad district alone could happen in the next five years. Currently, there is no mitigation plan in place for the orchids when trees are removed. A study carried out recently by Sarasan et al. [6] showed that native orchids can be rehabilitated in protected environments such as wildlife parks, which are open to the public. This is possible in both urban and non-urban settings. The model developed by Sarasan et al. [6] to provide refuge areas for orchids that are facing the threat of destruction due to anthropogenic reasons, such as road widening and infrastructure projects, is proving to be a success. There is a high recovery rate and flowering of the translocated plants after 18 months (Supplementary Materials, Table S2). Conservation strategies should give precedence to the preservation of existing forests and the maintenance of forest fragments. Furthermore, active restoration measures such as the translocation of epiphytes have proven to be more effective than passive restoration approaches [1].

Orchids rely on mycorrhizal fungi for seed germination [25,26,27,28], but the relationship between the phorophyte, orchid, and orchid mycorrhizal fungi enabling the process of seed germination and resilience of colonies on tree bark is not studied in detail. Phorophyte–orchid relationship in the wild and introduced trees depends on many factors and may vary, and this can be species-specific in some cases, but generally it is not critical for the colonising orchid [7,14,15,23,29,30]. In the current study, some species are sparsely distributed and found on native trees, but detailed studies are required to understand the extent of colonisation and resilience of such species. Host plant characteristics are shown to play an important role in the colonisation of host-dependent species [31,32]. Translocation of mature plants saved from destruction can be performed with an evidence-based approach. Native plants and fruit trees in equal proportion could be used, provided they have rugose bark, which will help the recovery process.

As anthropogenic pressures on ecosystems continue at an increasing rate, as recorded in the past few decades, the practice of translocation has become increasingly widespread in biodiversity conservation efforts globally. However, it is not clear yet whether long-term conservation outcomes can be achieved. A major review on translocations of threatened Australian plants [33] showed that 85% of all translocations have taken place since 2000, with half of these occurring since 2010. There has been a particularly rapid increase in development mitigation translocations, which now constitute 30% of all documented translocations. However, it is important to note that many of these mitigation translocations have been conducted on a small scale, involving fewer than 250 plants. According to the review referenced above, the majority (80%) of mitigation translocations have been carried out in the coastal regions of Australia. Additionally, there is a record of a single translocation involving trees and shrubs for a road widening project in Tasmania. Our previous study [6] of planting nearly 18,000 mature orchids rescued from destruction, as far as we know, is one of the largest development mitigation projects recorded. Therefore, mature orchids, which require decades to become resilient populations on trees facing obliteration for development, could be used as part of conservation translocation, especially in global biodiversity hotspots where threats to biodiversity are many [6].

The value of native orchids in improving nature connectedness and urban conservation is overlooked in India, which is blessed with 1250 orchid species, of which more than 25% are endemic. According to Soanes et al. [34], urban environments are useful for engaging people with nature but are considered to possess little intrinsic conservation value. Undervaluing of urban environments arises from misconceptions regarding the capacity of native species to survive within cities and towns, which in turn impedes effective conservation efforts [34]. The extinction of native orchids in Singapore [8] and the story of translocation [11] is a good example of how this can be achieved for both conservation and connecting people with nature. Soanes et al. [34] call for recognising the value of unconventional habitats, experimenting with creative solutions, and employing scientific evidence to mitigate the impacts of future urban development. The use of native orchids to enhance urban landscapes, whether small or large, in states in India such as Kerala, which is recognised as a hotspot for native orchid species, represents a highly informed choice.

The loss of biodiversity due to development activities is leading to the local extinction of species in many biodiversity hotspot regions [6]. Raising awareness about the need to prevent extinction at the local level can have a positive impact on species preservation over a larger geographic area. This is especially important in countries like India, where there is a high density of endemic species in various parts of the Western Ghats. Implementing conservation translocation as part of development mitigation is crucial for protecting epiphytic orchids, which are experiencing population declines due to increasing deforestation.

5. Conclusions

The present study investigated the removal of orchid-hosting trees and orchids within a small area in the Western Ghats, a region already fragmented by human settlement. Globally, the abundance of native orchids is closely associated with mountainous areas and regions of high rainfall, such as the Western Ghats. In addition to anthropogenic factors, climate change is causing unprecedented alterations in rainfall patterns in the orchid-rich mountainous areas that lie within the state of Kerala, which serves as a hotspot for endemic orchids.

To date, approximately 17% of the natural forests within the Western Ghats region of Kerala have been lost. The ongoing expansion of infrastructure projects, notably road construction, has resulted in the loss of thousands of trees and hundreds of thousands of orchids, with little to no rehabilitation efforts in place. This trend continues, placing additional species at risk, especially in micro-hotspot districts like Wayanad. Roadside trees, frequently targeted for removal during road widening or new road construction, further exacerbate the threat to orchid populations.

Considering these challenges, the implementation of evidence-based mitigation strategies is vital. The mitigation plan proposed by Sarasan et al. [6] offers a potential pragmatic solution to prevent the destruction of orchids and encourage a connection between people and nature. This is particularly important in densely populated states like Kerala, where communal green spaces are scarce and likely to become an even more pressing concern in the coming decades. Facilitating opportunities for different generations to engage with nature, especially by fostering connections with native plants such as orchids, can enhance individual well-being, increase the value of local landscapes, and promote conservation efforts.

While success has been documented in Singapore growing native orchids in urban areas, as previously noted, it is essential to obtain long-term results from this study before this approach can be considered as a reliable tool for conservation of species that are found from high-altitude to lowland landscapes. Ongoing monitoring of flowering and natural recruitment is necessary to properly evaluate the viability of this method as part of a conservation toolkit.