Woodland buffers help maintain bird communities in forest edges, finds study

A forest-agriculture matrix in Mawphlang, Meghalaya. Photo by Ashwin Kumar / Wikimedia Commons.

  • While converting forest lands for agriculture is usually considered harmful for biodiversity, there is growing acknowledgement that the agricultural matrix can supplement protected areas.
  • A recent study looked at the wintering bird community’s usage of woodlands within agriculture fields around Nongkhyllem wildlife sanctuary in Meghalaya and found that more than half the birds recorded used both agricultural matrix and protected areas.
  • Given the complementary roles, it is critical that protected areas and agricultural matrix around them are considered together for designing effective landscape-scale conservation plans.

Woodlands in agricultural fields surrounding a protected area maintain functionality of a wintering bird community, according to a recent study.

Agriculture is usually considered to be harmful to biodiversity as it involves conversion of huge tracts of forests into cultivation fields. Protected areas are formed to protect threatened species and conserve forests from land conversions for developmental activities like agriculture.

However, there is also growing acknowledgement that protected areas are not enclosed entities. The agricultural matrix has the potential to supplement protected areas as animals use areas inside wildlife parks as well as surrounding farms.

The agricultural matrix is a mosaic of managed land-use types (for example, farms and plantations) interspersed with natural and semi-natural habitats that vary in their ability to support biodiversity. Its potential to supplement protected areas is highly contextual though. It depends on how various elements of the matrix are composed (i.e., habitat type, structure of vegetation) and spread out (i.e., distance from protected areas) and how biodiversity navigates through them.

The structure of the vegetation determines the foraging areas and resources available to birds. It also provides protection from predation. Along with this, the distance of the matrix from protected areas also determines the presence and absence of bird species, as farther the distances fewer the species. The study, by researchers from National Centre for Biological Sciences (NCBS) and Wildlife Conservation Society India (WCS), finds how vegetation structure and distance from protected area determines the wintering bird community’s usage of woodlands within agriculture fields around Nongkhyllem wildlife sanctuary in Meghalaya.

“The agricultural landscape can have an important role to play in biodiversity conservation,” said Biang Syiem, first author of the study and alumnus of masters in wildlife biology programme run by NCBS. “Increasing biodiversity-friendly land-use types, such as woodlands, in the agricultural landscape can supplement protected areas in conservation, especially relevant in places where the expansion or establishment of new protected areas is difficult.”

Betel leaf cultivation in an agroforestry matrix. Photo by Biang Syiem.

Influence of canopy cover in determining bird species

The researchers grouped different bird species into eleven guilds based on their size, the areas they foraged in, diet and the manner in which the birds foraged, which involved gleaning and sallying, used exclusively by insectivorous birds.

The guilds were classified as those of (1) insectivorous, (2) granivores, (3) omnivores, (4) frugivores, (5) large high-canopy gleaning insectivores, (6) large understory gleaning insectivores, (7) large high-canopy sallying insectivores, (8) small mid-canopy gleaning insectivores, (9) small understory gleaning insectivores, (10) small mid-canopy sallying insectivores and (11) large woodpeckers.

To assess the effects of vegetation and proximity to protected areas on these guilds, the researchers counted the bird species detected in the identified sites. They compared the areas used by different guilds inside the protected area to that in the neighbouring matrix. They also compared the number of species detected in the protected area to those in the matrix.

The researchers found that the chances of detecting granivorous and small understory gleaning insectivorous birds decreased with more canopy cover. But increase in canopy cover augmented the detection chances of large high-canopy sallying insectivores, small mid-canopy sallying insectivores and large woodpeckers.

As areas with canopy cover increased, fewer granivores, omnivores, large high-canopy gleaning insectivores and small understory gleaning insectivores were found in them. In areas with bamboo too, fewer granivores and omnivores frequented them. But small mid-canopy sallying insectivores did well under bamboo.

Likewise, small understory gleaning insectivores and small mid-canopy sallying insectivores could be found more under large shrub cover. Whereas, omnivores, frugivores and small mid-canopy gleaning insectivores such as yellow-bellied warbler and velvet-fronted nuthatch didn’t fare well with increasing shrub cover but preferred the forest areas. While oriental white-eye and common iora were found more in the matrix.

Nectivores, granivores and large woodpeckers preferred areas with shrub cover whereas large high-canopy and small mid-canopy sallying insectivores didn’t forage in shrubby areas. Birds like the great slaty woodpecker, greater yellownape, great hornbills, white-browed and speckled piculet were found only in the forests.

The Great Hornbill. Photo by Aparajita Datta/Wikimedia Commons.

Consider agricultural matrix and PAs together for effective conservation plans

The researchers sampled across 100 sites that covered both the protected areas and the agricultural matrix. They recorded 94 species in these 100 sites. Out of the 94 species, 11 were found only in the protected areas and 20 were recorded only in the matrix, while 63 species used both the areas.

“This assessment is likely to be conservative because the core habitat still exists and if it was all converted to matrix habitat then it is likely other forest-dependent species currently found in matrix habitat would be lost because they would not be supported by adjacent core habitat,” according to Peter Cale, manager and senior ecologist, Australian Landscape Trust. “So matrix forests are not adequate in themselves to support the bird assemblage and the authors clearly make this point,” said Cale, who was not part of the study.

Of the 94 species, 49 used the the protected areas more than the ones in the agricultural matrix while 45 preferred the matrix more than the protected areas.

“The most interesting aspect of this study is the finding that use of matrix habitats by all bird guilds was high and comparable to the protected area,” said Hari Sridhar, a post-doctoral fellow at Centre for Ecological Sciences, Indian Institute of Science. Sridhar was not part of the study. “At the same time, the study also found that the species that made up the guilds were often different between matrix and PA.”

Birds like little siper hunter, black crested bulbul, black hooded oriole, scarlet minivet, black drongo, yellow-browed warbler, jungle babbler, Siberian rubythroat, rufescent prinia  and grey headed canary flycatcher foraged highly in areas both in the matrix and the protected forests. On the other hand, frugivores and small low-canopy gleaning insectivores, birds like red-vented bulbul and common tailorbird, used areas in the matrix more.

Such use of the areas could be due to wooded connections between the forests and the matrix, or due to the birds’ preference for particular feeding areas.

The study finds that various groups of birds are not solely dependent on canopy cover to forage. This research also shows that shrub cover and bamboo are vital to some species of birds. But at the same time, they weren’t ideal habitats for some other species. This, perhaps, could be due to constraints of foraging under shrub cover or lack of prey abundance in bamboo.

“The influence of vegetation structure was more nuanced, wherein shrub cover and bamboo cover were important in determining site use by these guilds even when canopy cover was not a limiting factor,” said Varun Goswami, Associate Director, Conservation Science, WCS India and one of the authors of the study. “This points to the subtle differences that vegetation structure may bring about in forage availability and foraging opportunities.”

Other studies have also found that density of shrub cover also determines the number of bird species foraging in woody patches. Bamboo too impacts non-forest birds. And as this study shows, woodland vegetation in an agricultural matrix supports the functioning of a bird community even if not all species are supported. “Thus, the matrix has an important ecological and conservation role to play, but one that is complementary to PAs,” adds Goswami. “It is critical that the two are considered together in designing effective landscape-scale conservation plans.”

Conversion of large swathes of forests for agriculture poses grave threats to biological communities. But as this study highlights, “by increasing wooded land-use types such as regenerating forests, community managed forests and betel leaf cultivation forests around agricultural farms, the functionality of birds communities and their potential roles as pest-controllers and pollinators will be maintained,” said study author Syiem.


Syiem BLN., Goswami VR., Vasudev D., (2018) In a tree by the brook, there’s a songbird who sings: Woodlands in an agricultural matrix maintain functionality of a wintering bird community. PLoS ONE

Banner image: A forest-agriculture matrix in Mawphlang, Meghalaya. Photo by Ashwin Kumar / Wikimedia Commons.

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