The Longis nature reserve was designated under a memorandum of understanding in 2003 between the Trust, the States of Alderney and local land owners. It is the largest terrestrial reserve within Alderney, covering approximately 105 hectares. The reserve contains 13 distinct habitats, including marine, intertidal, coastal heathland, grassland, scrub woodland and freshwater ponds, both natural and man-made.

The reserve’s coastal grasslands have a great diversity of plant species, with rarities such as small hare’s-ear, small restharrow, scrambled egg lichen, autumn lady's tresses, bastard toadflax and green winged orchids to be found. The marine and freshwater wetlands within the reserve provide an important site for migratory birds with other habitats also hosting many of the island’s best mammal habitats. There are also numerous insect species, nearly 100 of national importance, many of which have not yet occurred in the UK.