The Reserve is a quarry infilled by limestone spoil and in places capped by clay. The limestone areas are SSSI (Site of Special Scientific Interest) and support a herb rich limestone flora. The scrub and clay areas add species diversity. Over 130 plant species have been recorded on the Reserve including Bee Orchid, Ivy Broomrape and Autumn Gentian.

There are 15 breeding species of butterflies on the Reserve. An additional 13 visitors include Clouded Yellow, Large White, Small White, Green-veined White, Brown Argus, Chalkhill Blue, Adonis Blue, Holly Blue, Red Admiral, Painted Lady, Small Tortoiseshell, Peacock and Comma.

Silver-studded Blues here form one of the three largest colonies on the island. Their caterpillars feed on Birds-foot Trefoil and are attended by ants (Lasius alienus) who also host the chrysalids in their nests.

Small Blues had by far the highest ever transect count (206) on Broadcroft in 2004. The larvae are also tended by ants.

Lulworth Skipper is a recent coloniser.

Management is mainly scrub clearance by volunteers and contractor. Bare ground patches were scraped in 2002 to expose less rich soil to favour low growing plants and ants, this work being supported by an English Nature AGSL grant. At the south end of the Reserve beyond Sector 3 near the fence is a strip recapped with Portland limestone in 1997.

Over 70 macro moths have been recorded on site including Four-spotted and Chalk Carpet. Look out for the irridescent red day flying micro moth Oncocera semirubella during June/July.

Section 1: RAISED BEACH AREA (150m)

Walk into the Butterfly Reserve by passing by the side of the noticeboard, and start the walk by following the path away from the fence, between the Brambles, towards the Beach (raised dumped quarry spoil). On the edge of the brambles, by the side of the path, you may see Ivy Broomrape, which is a parasitic plant, and also the lovely small pink and white flowers of Restharrow. After the Brambles, on the flat area before the valley and summit, is a good Silver-studded Blue and Small Blue area.

Follow the valley going to the summit of the Beach. The vivid blue flower you will see here is Vipers Bugloss, which contrasts wonderfully with the yellow of the Birdsfoot Trefoil, the foodplant of the Siver-studded Blues. Small Blue butterflies are found here, laying their eggs on the Kidney Vetch. At the top look out for Ringlets and Common Blues. Small Skippers and Meadow Browns are plentiful here on a sunny July day, so take a rest here and observe their active flights and aerial battles. To the right of the track are some fine clumps of Red Valerian.

From the summit you can get a good view of the Reserve area and other landmarks: the Young Offenders Institution, St. Georges Church (west) and the Bill lighthouse. Red Admirals arriving from the continent in large numbers frequently land on Portland initially, and the viewpoint provides an excellent chance to watch out for the strong purposeful flight of these butterflies. The viewpoint ends Section 1.

Section 2: SHELTERED WEST AND SOUTH SLOPES (150m)

Turn back on yourself, keeping the Wild Privet and Bramble scrub to your right and descend a sloping path to a sheltered enclosed area between the Beach and the fence. Plants to look out for here include Bee Orchid, Kidney Vetch, Ivy Broomrape and Yellow-wort. Small Blues have now colonised this area, egg laying on the Kidney Vetch.

Meadow Browns and Skippers like the sheltered grassy slopes and Marbled Whites flit round the Bramble and Red Valerian.

Walk around the Beach keeping it top your left. If you are walking here in July, the sides of this path should have an abundance of Small Skippers, Meadow Browns, Marbled Whites and also some Ringlets. You should also be able to find plenty of Six-spot Burnet moths newly hatched from their cocoons, which are attached to grass stems. Along the left hand side are yellow clusters of Kidney Vetch. The large Buddleia bushes are always worth a second glance as the flowers are very attractive to many butterflies.

Note the interesting stonework at the corner, which in summer is covered with masses of butterfly attracting Red Valerian. On the ground below Teasel can be seen. Ahead of you is the horse field.

Walk between the Beach and the field, along the path passing between two large stones, until you come to the reclaimed area on your right. Another plant, dear to butterflies, Hemp Agrimony, is found here together with Red Bartsia and Common Vetch. The line of stones mark the end of Section 2.

Section 3: MEADOW (150m)

Turn right through the stones and bear left away from the horse field across grassland, which contains Kidney Vetch, Restharrow and Birds-foot Trefoil. Patches of Bramble have been left as shelter for bunnies, birds and nectar sources. This area is good for Marbled White, Small Skippers, Common Blue, Small Heath and Meadow Brown. Look out for Small Blue, as they may be moving in.
The sector ends at a steel gate to the quarry track near the southernmost part of the Reserve.
Look at the strip of land running beside the fence leading to the houses. This had clay removed and a layer of Portland limestone caps the surface to help small plants compete against their coarser rivals.
Take care crossing the track.

Section 4: QUARRY MARGIN (300m)

Walk eastwards into the boulder-strewn area, still heading for the Young Offenders Institution, and walk towards the quarry edge but keep safely from the fence and edge. This is the main Silver-studded Blue area, and during late June and July a sunny day will bring out dozens of these beautiful butterflies, which will rise up as you walk through, and then settle back down to the serious task of egg laying on the Birdsfoot Trefoil. Each individual adult butterfly lives only four or five days, so fast procreation is a necessity. The average range of the butterfly is a mere 18 metres (60 feet) which is one of the prime reasons for the rarity of the species – change the habitat and the butterfly dies because it is unable to fly far enough to find a new home.

Continue until your way is blocked by a mound and dense scrub. Admire the view of the biggest hole on Portland (soon to begin the long process of infilling, hence the road). At the scrub turn back, bear right, continue around and soon you will approach the edge again. Do not penetrate the scrub, and eventually you will be walking west, back to the roadway; aim for the left-hand end of the Beaches.

Section 5: TRACK (40m)

Cross the road with care, and follow the track west, going between the reclaimed and the beaches areas. This is a short section and soon a valley will be evident running north (on your right) into the Beaches. The pretty pink flower you may see is Common Centaury. The section ends at the junction.

Section 6: SHELTERED BEACH (120m)

Turn right and walk along the valley, taking a good look at the Buddleia bush, then along a scrub-lined footpath after passing between some large stones. This will lead you again to the lorry track, close to the wire fence bordering the recreation ground and the end of Section 6. This is the best place on the walk for Dingy Skippers in late spring and early summer, and Common Blues and Large Skippers are also found here in good numbers. Follow the track back to the starting point and walk out of the Reserve, looking to your right for the dense covering of the yellow flowers of the Ribbed Melilot to the area opposite the noticeboard.

Section 7: BRAMBLE SCRUB WITH GRASS (150m)

Walk a rectangular circuit of this area (not part of the formal Reserve). Turn right and first walk east along the path towards the Young Offenders Institution, then turn left (north) towards Grove Terrace; turn left again (westwards) towards the recreation ground, and finally, turn left again (south).

This is a Small Blue area, but also boast several other species of butterfly, including Meadow Brown, Marbled White and Large Skipper. Plants include Horseshoe Vetch, Mallow, Scarlet Pimpernel and Teasel. The walk ends when you return to the main entrance to the Reserve.