Most protected areas are shaped by argument — where a revenue boundary fell, which estate objected, what was left over once everything valuable had been claimed. Neyyar is unusually legible, because it is drawn around a river.
The catchment as a boundary
The sanctuary covers roughly 128 square kilometres, and what it covers is essentially the upper catchment of the Neyyar. The river rises at about 1,860 metres on Agastya Mala and runs only some 56 kilometres in all before reaching the sea at the Poovar estuary, fed on the way by the Kallar, the Mullayar and the Karavaliyar.
Almost all of the ground that collects the water above the dam is inside the protected area. That is not sentiment about wilderness; it is water engineering. The dam was completed in 1958 to supply irrigation and drinking water, and the sanctuary was constituted in the same year. Protecting the forest that feeds a reservoir is the cheapest way to keep the reservoir working — forest cover slows runoff, holds soil, and keeps silt out of the storage you have just paid to build.
It is the same logic that produced Peppara up the road, where the catchment of Thiruvananthapuram’s drinking-water supply is protected for exactly that reason. In both cases the biodiversity value is real and was substantially a by-product.
What the altitude buys
Because the sanctuary follows a river from near sea level to a summit, it captures an altitudinal range that very few protected areas of this size can match: from around 100 metres at the reservoir to 1,868 metres at Agasthyakoodam at its far end.
Climb that range and the forest changes underneath you. Tropical wet evergreen at the bottom, moist deciduous and semi-evergreen through the middle, and montane forest and grassland at the top. Within one boundary you pass through most of the habitat types the southern Western Ghats have to offer, which is why the species list is as long as it is.
One clarification worth making, because it is routinely fudged. The shola–grassland system is generally described as a habitat of roughly 1,200 metres and upwards, with many authors putting its working floor nearer 1,500. Agasthyakoodam at 1,868 m genuinely reaches it — unlike Ponmudi down the road, which tops out around 1,100 and holds only a low-lying outlier of it. But at Neyyar that habitat sits at the permit-only, seasonal far end. Everything an ordinary visitor walks through here is forest.
A continuous landscape
Neyyar does not stop at its own fence. It adjoins the Peppara Wildlife Sanctuary to the north and forms part of the Agasthyamala Biosphere Reserve, and the protected forest runs on east to the Tamil Nadu border and into the Kalakad–Mundanthurai country beyond. For animals that need room — elephant above all — that continuity is worth more than the 128 square kilometres on its own.
A word on the UNESCO label, which gets applied loosely here. Two separate designations overlap: the Agasthyamala Biosphere Reserve, declared by India in 2001 and added to UNESCO’s World Network of Biosphere Reserves in 2016, and the Western Ghats World Heritage Site, a serial inscription made in 2012. A biosphere reserve is a management framework for a lived-in landscape, with core, buffer and transition zones; World Heritage is a heritage listing over specified sites. The first is the one that governs this forest.
An inhabited forest
Which brings up the thing the word sanctuary tends to hide. Kani communities, indigenous to these hills, have settlements within and around this forest, and their knowledge of its plants is well documented. This is not empty wilderness that people were kept out of; it is a landscape with a long human history that a boundary was later drawn around, and the guides who walk you into it are frequently from those communities.
Downstream, the same river that the sanctuary exists to protect has been heavily sand-mined along its lower thirty kilometres, and in 2015 it was one of six Kerala rivers placed under an outright mining ban that did not hold. The catchment is protected; the channel below it is not. It is a useful reminder that a boundary drawn around water only ever covers part of a river.