Research Article |
Corresponding author: Kumar Manish ( kumarmanish910@gmail.com ) Academic editor: Pavel Stoev
© 2024 Dinesh C. Nautiyal, Kumar Manish.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Nautiyal DC, Manish K (2024) Anthropogenic disturbance produces divergent effects in the community structure and composition of tropical semi-evergreen forests in the Eastern Himalaya. BioRisk 22: 1-15. https://doi.org/10.3897/biorisk.22.120802
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Studies documenting anthropogenic disturbance-driven changes in forest communities of the Eastern Himalaya, a global biodiversity hot spot, are largely lacking. We studied six forest sites of tropical semi-evergreen forests in Arunachal Pradesh in the Eastern Himalaya to understand the effects of varying disturbance intensities on the forest community structure and composition. Based on the magnitude of disturbance, forest sites were classified as experiencing low, moderate and high disturbance. Mean species richness (SR) of trees and shrubs decreased from low disturbance to high disturbance. Mean SR of herbs was maximum in moderately disturbed forest sites. Maximum values of the Shannon-Wiener Diversity Index (SD) were recorded for trees at sites with low disturbance, for shrubs at sites with high disturbance and for herbs in moderately disturbed forests. Pilelou Evenness Index (EI) values were maximum for trees at sites with high disturbance, while maximum EI values for shrubs and herbs were recorded in the forest sites with low disturbance. The number of tree families decreased from 18 to 13 in the forests with low and high disturbance, respectively. Moderate disturbance led to increased herb species richness and diversity, while increasing disturbance produced contrasting effects on trees. High anthropogenic disturbance led to low species richness, but high diversity amongst shrubs. Our investigations suggest that the magnitude of disturbance elicits differential responses in different physiognomic classes in the forest ecosystems and further our understanding of the effects of disturbance in tropical forest ecosystems of a global biodiversity hotspot.
biodiversity, hotspot, species diversity, species richness
Tropical forests form a major component of the global biodiversity hotspots, as well as the world’s most endangered ecosystems (
The effects of disturbance on the structure and dynamics of vegetation communities have been contentious in ecological literature. Some regard disturbance as a negative influence as it destroys the climax assemblages (
Arunachal Pradesh, the easternmost state of India, comprising the major part of the Eastern Himalaya global biodiversity hotspot, is regarded as one of the most sensitive areas with a high risk of deforestation (
Earlier studies on the effect of disturbance on the vegetation communities in the Arunachal Himalaya have been limited to the effect of disturbance only on the tree layer (
We studied six forest sites located in Aalo Forest Division, West Siang District of Arunachal Pradesh (Fig.
Map of the study area showing the locations of six sampling sites (1-Tatogito, 2-Roying, 3-Poma Basti, 4-Tagurshit, 5-BB Camp Bamboo bridge area, 6-BB Camp Basti area). Each sampling site represents a different forest site. Sampling was undertaken to assess the structure and composition of the vegetation community at each site during pre-monsoon (March-April), monsoon (July) and post-monsoon (October) seasons.
Sampling in six forest sites was undertaken to assess the structure and composition of the vegetation community of the study area. Sampling locations were randomly chosen in each forest site in such a manner that the maximum possible representative vegetation of the respective forest site was covered. In each forest site, three physiognomic classes, i.e. trees, shrubs and herbs were surveyed, sampled and analysed using the standard nested quadrat sampling method. Each nested quadrat design contained subplots of 10 m × 10 m [for recording tree species with circumference at breast height (cbh) > 30 cm], 5 m × 5 m (for recording shrubs and saplings with 10–30 cm cbh), and 1 m × 1 m (for recording herbs with < 10 cm cbh) (
Following the methods of
Relative impacts for each disturbance source in six tropical semi-evergreen forest sites in Arunachal Pradesh.
Disturbance source | Relative impact at forest sites | ||||||
---|---|---|---|---|---|---|---|
Tatogito | Roying | Poma Basti | Tagurshit | BB Camp Bamboo bridge area | BBCamp Basti area | ||
Road | 1 | 20 | 25 | 40 | 50 | 100 | |
Habitation | 1 | 4 | 8 | 1.3 | 80 | 100 | |
Tree cutting/lopping | Relative density | 1 | 1.83 | 4 | 2.67 | 5.17 | 6.17 |
Relative basal area | 1 | 5 | 8 | 12 | 14 | 12 | |
Browsing/Grazing | 1 | 1.93 | 3.67 | 3.97 | 4.65 | 4.78 | |
Total impact | 5 | 32.76 | 48.67 | 59.97 | 153.82 | 222.95 |
After the relative impact of each of the four disturbance sources was estimated, their individual scores were summed to yield a cumulative impact at each of the six sampling sites. This cumulative total impact was taken as a surrogate for the degree of disturbance at each site. Based on the disturbance intensity, we classified different forest sites under the following three anthropogenic disturbance classes: (i) Least disturbed forest site with a total impact of less than 10, (ii) Moderately-disturbed forest site having a total impact of less than 100 and (iii) Highly-disturbed forest site having a total impact of greater than 100. Though there may be other sources of anthropogenic disturbance in the study area like herb collection, firewood cutting and intentional fire, we focused on the relative impacts of only road construction, human habitation, logging/lopping and browsing/grazing as these were the only predominant sources of disturbance as per our field study. There are no records of any rare, endangered or threatened medicinal plant species in the study area.
Each of the six sites was surveyed for angiosperm taxa. The collected species were assigned to their respective plant families and the three physiognomic classes (trees, shrubs and herbs). Each site was analysed for density, abundance, frequency and dominance of constituent flora using standard procedures of vegetation sampling (
In order to characterise the community structure of the investigated sites, the following variables were analysed: (i) SR - determined as the total number of species per sampling unit (
The study sites varied in nature and intensity of disturbance they experienced and a noticeable disturbance gradient between them was discerned (Table
A total of 160 species were recorded at these sites of which 54 were trees, 30 shrubs and 76 herbs (Table
Effect of anthropogenic disturbance on vegetational parameters of different forest sites. (A) Variation of mean species richness with disturbance classes for trees, shrubs and herbs, (B) Mean density as related to disturbance classes for different vegetation layers. Error bars represent the standard deviation.
Density-diameter distribution of trees in different forest sites (TG - Tatogito, RO - Roying, PB – Poma Basti, TAG - Tagurshit, BB1 - BB Camp Bamboo bridge area, BB2 - BB Camp Basti area). In all the forest sites, most trees had maximum density in the lower diameter classes (30.1–90 cm cbh) and minimum density in the higher diameter classes (150.1–210 cm cbh).
Principal Component Analysis (PCA) ordination plot, based on total basal area of tree species in six forest sites (1 - Tatogito, 2 - Roying, 3 - Poma Basti, 4 - Tagurshit, 5 - BB Camp Bamboo bridge area, 6 - BB Camp Basti area). A total of 51.92% variation in species composition was explained by the first two principal components (PCA axis 1 – 27.14%, PCA axis 2 – 24.78%) in the ordination plot.
Shanon-Wiener Diversity Index and Pilelou Evenness Index for different vegetation layers in six forest sites along a disturbance gradient.
Vegetation parameters | Forest sites | ||||||
---|---|---|---|---|---|---|---|
Tatogito | Roying | Poma Basti | Tagurshit | BB Camp Bamboo bridge area | BB Camp Basti area | ||
Shanon-Wiener Diversity Index | Trees | 2.79 | 2.61 | 2.54 | 2.42 | 2.47 | 1.65 |
Shrubs | 1.75 | 1.51 | 1.34 | 1.62 | 2.04 | 1.73 | |
Herbs | 1.9 | 2.17 | 2.55 | 2.96 | 2.42 | 2.03 | |
Pilelou Evenness Index | Trees | 0.13 | 0.15 | 0.16 | 0.17 | 0.18 | 0.24 |
Shrubs | 0.25 | 0.17 | 0.22 | 0.23 | 0.2 | 0.16 | |
Herbs | 0.19 | 0.15 | 0.17 | 0.09 | 0.15 | 0.14 |
Species composition showed varied dominance of different species at different forest sites. Species listed in Suppl. material
Anthropogenic activities are a looming threat to the biodiversity of the Himalayan forests and have been responsible for the significant transformation of its landscape (
All six study sites in the present paper are directly affected due to various human activities including ongoing hydroelectric power projects in the study area. We were able to locate a total of 105 households in the study area. Scheduled tribe population accounts for more than 95% of the total population (
We recorded significant changes in the community composition of the investigated forests with increasing disturbance levels. Moderate disturbance led to increased herb richness and diversity, while high disturbance levels had the opposite effect on trees. Shrubs showed low richness, but high diversity with increasing disturbance. These results confirm earlier findings of
We observed that, with increasing disturbance, tree species richness decreased, while their Evenness increased. For shrubs and herbs, maximum values of the Evenness Index were found in forests with the least disturbance. The negative relationship between richness and Evenness has also been reported elsewhere (
Our results suggest that disturbance impacts tree species more and arrests climax community formation by the proliferation of disturbance-tolerating shrub and exotic herbaceous species. Ageratum conyzoides L., an exotic invasive herb, was absent in the forests with low disturbance, but dominated the sites experiencing high disturbance. Numerous earlier studies have reported the invasion of disturbed sites by exotic invasives (
All five sites, except the one with the least disturbance, experienced varying intensities of biotic pressure in the form of lopping for timber and firewood and grazing by domestic animals. Loss of plant diversity and changes in community structure in terms of composition, species density and population structure have resulted from these anthropogenic disturbances in the tropical semi-evergreen forests of Arunachal Pradesh. The help of local communities can be sought in the conservation of the forests through participatory activities by the grant of title rights, delineating specific areas for browsing/grazing, fixing permits for extraction of timber wood etc. The policy-makers can do well to involve the local communities in conservation programmes and sensitise them about the ill effects of anthropogenic disturbances rather than adopting a top-down conservation approach.
The authors have declared that no competing interests exist.
No ethical statement was reported.
No funding was reported.
Dinesh C. Nautiyal – Conceptualisation, Investigation, Methodology, Supervision; Kumar Manish – Writing original draft, review and editing, Formal analysis, Resources, Software, Validation, Visualisation
All of the data that support the findings of this study are available in the main text or Supplementary Information.
Appendices
Data type: pdf
Explanation note: appendix A. Important vegetational parameters of six forest sites. + – species present at the forest with < 10% IVI. TBA – total basal area, IVI – Importance value index. Units: density (ha-1), TBA (m2 ha-1), cover (m2 ha-1). appendix B. The distribution of tree families and their contribution to total genera and species in six tropical semi-evergreen forest sites. Dashed entries indicate absence of respective genera/species at a sampling site.