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Quantifying Ecological Restoration by Analyzing Sounds in a Tropical Dry Forest

Updated: Apr 12

Restoration of degraded forests and lands worldwide is given much-needed impetus within the United Nation’s decade of restoration, global agreements, and sustainable development commitments. Therefore, it is crucial to quantify the effects of such restoration efforts on biodiversity to guide restoration efforts. In Central India, many restoration initiatives involve the removal of an invasive shrub, Lantana camara (L. camara). Higher densities of lantana camara are associated with lower densities of native vegetation, often of the species necessary for wildlife and local livelihoods. In a recent study (with many NCCI members as authors), researchers used acoustic technology to examine the bird community composition and the acoustic space used (ASU) across restored, unrestored and naturally low lantana density sites.

The study was conducted in tropical dry forests of the Bicchhiya subdistrict in Madhya Pradesh. There were 55 study sites divided into three broad categories: a) Restored sites - Sites where restoration by way of Lantana camara removal has taken place in the last five years; b) Unrestored Sites - Sites with a high density of L. camara where no restoration has taken place in the last five years; and c) Low Lantana Density (LLD) Sites - Sites which naturally have very few L. camara plants or no L. camara plants in the last five years.

Figure 1. Pictures from unrestored (A, D), low Lantana density (B, E), and restored (C, F) sites. The red arrows point to the Lantana camara in the understory.

The study compared the three categories of sites based on the cumulative number of birds, variations in the bird species, and the ASU in the frequency range of 2-8 kHz. Acoustic recorders were tied to each site approximately 2m above ground on tree trunks. Forty-five minutes of the clips were randomly selected for each sampling location during the morning hours (5:30-9:30).​

No significant differences were observed in the cumulative number of bird species between the sites (median number of species at restored and LLD sites = 38, unrestored sites = 41). Regarding species uniqueness, thirteen unique species were found in the restored sites, followed by unrestored sites having eleven unique species, while LLD sites had only three unique species.

Figure 3. Violin plots displaying (A) the cumulative number of bird species detected, (B) the cumulative number of generalist species detected, and (C) cumulative number of forest- and woodland-affiliated species detected across the sites. Refer to Tables S6 and S7 for the Wilcoxon test of significance results.

The restored sites had considerably higher ASU than LLD and unrestored sites. However, ASU in restored and LLD sites was found to be more similar to each other than unrestored sites.

Figure 4. Acoustic space used over time in 24 hours (frequency range: 2000-8000 Hz) averaged across all days of data collection across all sampling locations

Although there was no significant difference in the cumulative number of species in a site, there was a considerable difference in community composition across the sites. There were some differences observed in the ASU across sites, but the association of ASU with site type was not significant.

The acoustic niche hypothesis (ANH) infers that intact habitats have more acoustic niches than degraded habitats. Contrary to the authors' expectations, removing L. camara did not result in empty acoustic niches or a lower ASU value. Therefore, based on ANH, an association of restoration and ASU is a positive indication of the ecological health of restored sites. The study highlights that people-centric restoration efforts that are not intended to increase faunal diversity has marginal co-benefit to biodiversity over a short timescale. This study provides a baseline to monitor the effects of restoration using ASU and faunal responses, which can be examined on a larger scale and for a longer period. This would help guide future restoration efforts.

Original Paper: Choksi, P., Kotian, M., Biniwale, S., Mourya, P., Korche, D., Agarwala, M., Khanwilkar, S., Ramesh, V. and DeFries, R., Listening for Change: Quantifying the Impact of Ecological Restoration on Soundscapes in a Tropical Dry Forest. Restoration Ecology, p.e13864.

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