No evidence for trade-offs between bird diversity, yield and water table depth on oil palm smallholdings: implications for tropical peatland landscape restoration

Tropical peat swamp forests retain large carbon stocks and support unique
biodiversity, but clearance and drainage for agriculture have resulted in
fires, carbon emissions and biodiversity losses. Initiatives to re-wet
cultivated peatlands may benefit biodiversity if this protects remaining
forests from fire and agricultural encroachment, but there are concerns
that re-wetting could reduce yields and damage livelihoods, as
relationships between drainage, on-farm biodiversity, and crop yields have
not been studied. We examined oil palm fruit yields and bird diversity on
41 smallholder farms in Jambi (Sumatra, Indonesia), which varied in
drainage intensity (12-month mean water table per plot from August 2018 to
August 2019: -52 to -3 cm below ground). We also compared farm bird
diversity with a neighbouring area of protected forest (11,000 ha, 21
plots; mean water table per plot -3 to +15 cm). Bird species richness
(3-18 species per plot), species composition, and oil palm yields
(4.5-19.2 t fresh fruit bunch ha-1 yr-1) varied among farms, but were not
detectably affected by water table depth, although ground-level vegetation
was more complex on wetter farms. Bird richness in oil palm (mean = 10.3
species per plot) was <50% of that in forest (26 species per plot),
and only three out of 35 conservation-priority species found in forest
were recorded in oil palm. Synthesis & applications: Tropical
peatlands in Indonesia have been drained to allow farmer access and
improve farm yields, but we found no trade-offs between drainage depth,
yields or bird diversity on smallholder oil palm farms in our study
landscape. Current restoration initiatives to re-wet peat may benefit
farmers by reducing fire risk, without affecting yields. Wetter farms had
increased understorey vegetation complexity, but this did not affect bird
diversity, so we find no evidence that re-wetting improves on-farm
biodiversity within the studied range of drainage depths. However, on-farm
fire reduction efforts in cultivated peatlands, including re-wetting, will
be vital for reducing the risk of fires escaping into nearby forests,
which contain unique and diverse bird species assemblages. Protection of
remaining peatland forests from fire and clearance is key for biodiversity
conservation, and for providing a source of seed dispersers and genetic
material for future forest and landscape restoration efforts. Restoration
of more biodiversity-friendly land covers will improve landscape
permeability and help conserve species and the ecosystem services they
deliver.