6. Insects
The huge variety of plants able to grow in limestone grassland supports an even larger variety of insect life. Many feed directly on the plants at some stage in their life cycle whilst others are predators or parasites feeding on other insects. The insect community of Independent Quarry is currently very sparse, but will develop along with the colonisation of plant species. Some species of insects are highly mobile and are likely to appear as soon as suitable plants are present, whilst others will take several years to colonize new areas.
Several species of insects that are likely to live in Independent Quarry are the subjects of UK Biodiversity Action Plans. These plans are the UK government's response to the 1992 Rio Earth Summit and attempt to preserve and enhance biodiversity by coordinating the actions of the various groups involved.The tables in this section list those species which are currently found in old quarries on Portland and could be expected to appear in Independent Quarry once the correct conditions are established. For many insects these conditions will largely consist of a particular plant species but for some species such as the Silver-studded Blue precise environmental conditions will also be required. These requirements are best catered for by providing a variety of environments within the quarry, with a range of aspects from hot, dry south-facing slopes to cool, shady corners. The pond area in particular will provide many insects with a range of habitats, which will vary as the water level rises and falls. Some groups of insects do not appear to have been studied in any detail on Portland and consequently do not feature in this survey. The coverage of insect groups is summarised in Table 17 below.
click here to view the list of Insect groups for which information is available in a new window
6a. Dragonflies
This group includes some of our largest and best-known insects. All have aquatic larval stages and so will require a freshwater area in order to breed in Independent Quarry, one of the main reasons behind the planned pond development. Until recently Portland has not been known for its dragonfly population, largely due to the lack of water on the island. Then in 2001 Scarce Blue-tailed Damselflies were found on the island, followed in 2002 by the even rarer Red-veined Darter. Both were probably breeding in a disused quarry which has now been filled in, making the planned Independent Quarry pond even more important.
6b. True Flies
This enormous group of insects includes animals as diverse as the familiar Cranefly or Daddy-long-legs and the parasitic Flatflies living under the feathers of birds. Many members of this group are flower-feeders and as such are important pollinating insects. Their other main ecological importance is as a food supply for birds and other insects. Some species lay their eggs on carrion and are an important stage in the decomposition of dead animals.
Only one group appears to have been studied to any extent on Portland - the Hoverflies. This group has a particular ecological and economic importance in that several common species feed on aphids and act as a major biological control of this pest. In summer huge swarms often arrive from Europe, and the adults can be seen around Sycamores feeding on moulds growing on the leaves and bark. They in turn provide food for the migrant birds that are moving through Portland at the same time of year. The list below shows the entire Portland list of hoverflies, any of which could be found in Independent Quarry once the vegetation has recovered sufficiently. Many species require trees and shrubs to complete their life-cycle, so it is important that planting should take this into account. Other species are attracted to marshy areas and will be most likely to be found around the pond area, which may well add new species to the Portland list once it has become established with marginal plants.
click here to view the list of Hoverflies likely to occur in Independent Quarry in a new window
6c. Butterflies
Table 18, listing butterfly species, contains some of the most famous representatives of Portland's wildlife. The Silver-studded Blue is particularly well-known as the Portland population is one of only 3 limestone populations of the species in Britain, and is the subject of a UK Biodiversity Action Plan.
The presence of any population of butterflies is entirely dependent on the larval foodplant being available, many species being faithful to just one species. Beyond this, some species also require particular surroundings to provide the correct temperature for the development of larvae. For some this may be a shaded spot to keep the larvae cool and for others the warmth of a site in full sun is required. Most species of blue butterfly also show a dependence on ants, which have been proved to be essential in several stages of the butterflies larval stages. The ants protect the larvae from the attentions of parasitic wasps and in some species provide shelter overnight and carry the pupae into their nests to over-winter. In return the larvae secrete a sugary fluid from a special gland on their abdomens. For a full account of these complex relationships see ref 12, The Butterflies of Britain and Ireland, by Jeremy Thomas.
click here to view the list of Butterflies likely to colonize Independent Quarry in a new window
6d. Moths
Portland's moth population is, if anything, even more famous than its butterfly population, at least amongst entomologists. It includes one species, the Portland Riband Wave, found nowhere else in Britain and 2 species, the Chalk Carpet and the Four-spotted, that are the subjects of UK Biodiversity Action Plans. All are quite likely to find suitable habitats in Independent Quarry in time. The wide variety of foodplants used by moth species show how important it will be to provide a mix of habitats in Independent Quarry. This list covers species thought to breed in quarries on Portland and does not include the huge number of species that occur only after migrating to the island from Europe.
click here to view the list of Moth species likely to colonize Independent Quarry in a new window
6e. Micro-moths
The group of insects known as Micro-moths is a fairly arbitrary collection of families of small moths. Few have English names and amongst the only ones at all familiar to most people are the Clothes Moths. Although not conspicuous, they are a heavily-studied group, particularly in areas popular with entomologists such as Portland. Many species migrate huge distances and some are serious agricultural pests. Their main ecological importance is as a food source for larger animals, particularly birds. As with the larger moths, the key to encouraging their success in Independent Quarry lies in providing a wide range of habitats, as many have very particular foodplant requirements. This list is by no means complete, but includes most species thought to breed on the island; many other migrant species may also breed sporadically.
6f. Other insects
Included in this section are various groups for which only partial information is available. Many are from my own personal observations on Portland (referenced as pers. obs.).
References for Section 6.
3. Dorset Environmental Records Centre web site http://www.derc.org.uk
8. Butterfly Conservation (2004), web site http://www.butterflyconservation.co.uk
9. de Whalley, L. pers.comm. (Broadcroft Moths and Foodplants 1997 & 1998)
10. Cade, M. (1998) An annotated checklist of the moths and butterflies of Portland, in Portland Bird Observatory Reports for 1996, 1997 and 1998.
11. Waring, P. and Townsend, M. (2003), Field Guide to the Moths of Great Britain and Ireland, British Wildlife Publishing
12. Thomas, J. (1991) The Butterflies of Britain and Ireland, Dorling Kindersley
13. Levy, E. T. and Levy, D. A. (1998) Portland Hoverflies 1998, in Portland Bird Observatory Report for 1998.
14. Cade, M. (1995) The Dragonflies of Portland Bill, in Portland Bird Observatory Report for 1995.
15. Green, G. (1978) A study of aquatic invertebrates from Culverwell and the Reedy Ditch, in Portland Bird Observatory Report for 1978.
16. Joint Nature Conservation Committee (2004), UK Biodiversity Action Plan Web Site http://www.ukbap.org.uk