It was long believed that conventional tea plantations, while no replacement for natural forest cover, could support healthy populations of birds. New research suggests that tea plantations which lack a variety of native trees are much poorer in bird diversity.
This finding was made by scientists from the Nature Conservation Foundation (NCF), Mysuru, following field studies in the Anamalai Hills in winter and spring 2016, during which the researchers recorded 103 species of birds and their prevalence across three types of tea plantations and two forest types.
These included conventional tea plantations which used agrochemicals and employed Australian silver oak for shade cover; organic tea plantations with used organic fertilisers and silver oak for shade; mixed shade plantations that used agrochemicals but utilised a variety of native trees, plus some non-native trees including silver oak. The last two habitats studied were forest fragments and contiguous forest in and around the Anamalai Tiger Reserve
Shade trees are vital for plantations because they keep leaf temperatures down to ensure maximum photosynthesis while simultaneously preventing sun burning. The trees also help conserve soil moisture. However, non-native silver oak trees have become the vogue instead of native trees because they are easier to prune and because they can be easily sold for lumber.
“This widespread use of silver oak has created unintentional effects from an ecological point of view,” explained ecologist Chayant Gonsalves of NCF and a co-author of the study, which was published in Current Science.
In 225 point count surveys, the researchers found that overall bird abundance was lowest in conventional tea plantations, with numbers being 33% higher in organic tea plantations and 35% higher in rainforest fragments.
Remarkably, mixed-shade tea plantations, with their inclusion of native and wide-canopied tree species, were found to have 40% higher species richness than organic tea plantations did. They also had 83% more birds than a conventional tea plantation.
Gonsalves said that while mixed-shade tea plantations, using a variety of fig, jackfruit and other native forest trees are better than organic and conventional plantations, but that they are nevertheless poorer than forests.
“Our study focused on bird populations which thrive in the upper canopies of trees but there is an entire middle canopy layer that is present in rainforests but which is missing even in mixed-shade plantations. Here is where birds such as thrushes and babblers live, alongside small mammals and other wildlife. Additional studies are required to gauge the ecological impact of these missing creatures,” he said.
“Tea plantations, even under mixed native shade, cannot be a replacement for fragmented or intact forests. This is because there is a higher diversity of rainforest birds in forest spaces,” he added.
The researchers added, however, that retaining or promoting native shade trees in tea plantations “will increase bird diversity and abundance, including of forest-affiliated species.”
The finding is pertinent as the space occupied by tea plantations in India has continued to grow. In 2003, for example, tea plantations occupied an area of 5.21 lakh hectares across India. As of 2020, this area has expanded to 6.35 lakh hectares.
Beyond bird abundance, the researchers also catalogued types of birds. Of the 103 species, 59 were forest-dwelling species of birds and 44 were open country species which had moved into areas previously inhabited by forest resident species. The two different bird groups showed significant variation across environments.
Rainforest bird populations were lowest in conventional and organic tea plantations and 572% higher in mixed-shade plantations and 591% higher in rainforest fragments compared to conventional plantations. Open country bird populations, in contrast, were 31% more in conventional tea plantations and 72% lower in rainforest fragments and 87% lower in continuous forests.
The researchers wrote that their “findings also indicate that forest fragments, despite degradation and ongoing anthropogenic pressure, are important refuges for rainforest birds, with their species richness fairly similar to that in continuous forest sites.”
Already, the entry of open country birds in other Asian forests has been described as “biological infiltration.”
Loss of rare birds
The change in habitat has conservation implications, as it triggers the loss of rarer, forest-dwelling or specialist birds. “What happens is that already hardy open-country birds thrive at the cost of rare rainforest birds. They end up filtering out the rainforest birds,” Gonsalves pointed out.
Ultimately, there is said to be a long-term commercial and environmental benefit if plantation owners go organic or native. “Mix-shade plantations could experience increased yields, improved soil nutrition through decomposition of shade tree leaf litter and higher native biodiversity,” the scientists wrote.
“As mixed-shade plantations support higher species richness and abundance of birds this is likely to help in pest control as evidenced in coffee plantations,” they added.
Scientists also believe that incorporating multi-species native shade trees could help in carbon sequestration.
The bird populations were used as a proxy for change in habitat and agrochemicals, although the researchers said that further studies are required to determine what effect agrochemicals are having on insect prey, ground cover, herbs and birds of the low-level trees and plants in plantations.