Hadleigh Country Park is a Site of Special Scientific Interest or SSSI. As such the site is protected from all development. Work is carried out to preserve, maintain and improve these valuable habitats in the park. SSSI status is given to sites that are important to wildlife and need to be protected to ensure their continued role in providing habitats to many rare or endangered species.

The park has four main habitat areas of note. Each will be explained and species that it supports listed.

1) Deciduous Woodland

Towards the north of the park there are some areas of open woodland containing mature stands of oak, ash, elm, field maple and hazel.The wooded areas have several glades where the canopy is broken allowing for the development of a grassy and diverse understorey. The woodland areas provide great habitats for foxes and badgers, in addition to many invertebrates and plant species. Birds are also an important part of the woodland habitat. Some of the birds commonly found to be nesting in the parks woodland include: jay, tawny owl, green woodpecker, great spotted woodpecker, mistle thrush, great tit, treecreeper and kestrel.

2) Grassland and Scrub

The sloping ground in the centre of the park is covered with grassland communities, many areas of which have been invaded by scrub due to undergrazing in the past. The area also contains many mature hedgerows. This habitat is very rare in this area of South Essex to the extent that the park is a last refuge for many species that may otherwise be locally extinct due to a lack of habitat.

The grasslands have neutral to acidic soils dominated by mixtures of red fescue, bent-grass and cocks foot. The grasslands also contain several uncommon and rare species of plants including: hairy vetchling, deptford pink, bithynian vetch, slender tare, wild catmint and nitgrass.

The areas of shrub are dominated by hawthorn, blackthorn, bramble and dog rose and as such provide a wide range of habitat for invertebrates such as: rose-plume moth, white-letter hairstreak, great green grasshopper, marbled white butterfly, six-belted clearwing, cloaked carpet and beautiful hooktip. The birdlife of the scrub area includes: greenfinch, linnet, bullfinch, lesser whitethroat, whitethroat, chiffchaff, willow warbler and blackcap.

3) Grazing Marshland

The cattle grazed marshes within the park and their associated dyke system create a fantastic habitat for marshland plants, insects and birds. The range of habitat is further increased by the differences in the water's salt content, ranging from freshwater to slightly brackish water. This means that there are more species that are compatible with the habitat.

Vegetation on the near marsh is mainly freshwater tolerant such as; eye-grass, common foxtail, fleabane, hardheads and soft brome grass. The areas nearer the sea wall contain plants adapted to withstand the increased saline conditions; sea couch grass, mud rush and sea-beat. The dykes contain plants such as; sea clubrush, ruppia maritima and zannichellia palustria.

The marshes contain an extensive insect community including aquatic insects, butterflies, moths, dragonflies, beetles and spiders. Adders, slow worms and grass snakes are also present on the marshes. Other reptiles include great crested newts which are endangered and protected. The marshes also provide ample feeding for many species of bird which have the added scrub cover from the rest of the park to make perfect habitat.

4) Salt Marsh and Inter-Tidal Flats

Although the area of mud flats is relatively small it provides superb habitat for many estuarine species. The salt marsh contains many plant species including: sea purslane, thrift, sea lavender and various glassworts.The mudflats provide great habitat for marine invertebrates and therefore food for many wading birds. Invertebrates found in the mudflats include; ragworms, lugworms, shore crab and the bivalve mollusc.