The site's nature conservation importance (the reason for SSSI designation), is due to its plant communities, its community of breeding birds and its invertebrate communities.
Thin-spiked wood sedge (
Carex strigosa), which is scarce in Surrey, is present in woodlands at the site. Two species of rose which are scarce in Surrey,
Rosa micrantha and
Rosa stylosa are found in the scrub on Little Bookham Common. The
bryophyte flora in the site's woodland is rich and includes one of only two Surrey localities for the moss
Zygodon conoideus. Notable plants found in the grassland of Little Bookham Common include
southern marsh-orchid (
Dactylorhiza praetermissa),
pepper-saxifrage (
Silaum silaus),
spiked sedge (
Carex spicata) and
adder’s-tongue fern (
Ophioglossum vulgatum). The flora of the site's open water habitats includes three plants which are scarce in Surrey:
greater duckweed (
Lemna polyrhiza),
fat duckweed (
Lemna gibba) and
thread-leaved water-crowfoot (
Ranunculus trichophyllus), while
tall-herb fen communities here support two plants which are rare in Surrey, the grass
orange foxtail (
Alopecurus aequalis) and
eared willow (
Salix aurita). Breeding birds which are associated with woodland at this site include
hawfinch,
woodcock and
lesser spotted woodpecker, whilst those breeding in scrub areas include
nightingale and
grasshopper warbler. The site has a very well-recorded invertebrate fauna, which includes 611 species of
beetle, 1140 species of
fly, 146
true bugs, 201
spiders, 17
dragonflies and over 300 species of butterflies and moths. Dead oak trees provide habitat for several beetles which are scarce in Surrey including
Nemadus colonoides and
Aridius nodifer. Two moths which occur, the
toadflax brocade and the
broad-bordered bee hawkmoth are
nationally rare. This is a well-known site for the
purple emperor, and other scarce butterflies which are present include
white-letter and
purple hairstreaks and the
white admiral. ==References==