Information and Advsiory Note Number 78, February 1997
1.1 Heather moorland is a semi-natural habitat of considerable European conservation importance. It supports characteristic assemblages of plants and animals and represents a distinctive cultural landscape. Although disappearing within Europe as a whole it is still well represented in the Scottish uplands.
1.2 Common heather or ling (Calluna vulgaris) is one of the most important components of open moorland vegetation communities and habitats, being dominant over about 17% of Scotland (approximately 1.35 million ha) and present over some 30% of the land surface. It is an important food source for red grouse, and to a lesser extent black grouse, and important winter forage for mountain hares, red deer and sheep. It provides important nesting habitat and hunting grounds for a number of rare raptors such as merlin and hen harrier. However, it should not be forgotten that, within a moorland context, it also supports a considerable diversity of invertebrates.
1.3 This Note provides information and advice on one very characteristic and functionally important group of invertebrates of heather moorland, the butterflies and moths (Lepidoptera), and the effects of grazing and burning (or cutting) on this group.
2.1 On the drier types of heather moorland Lepidoptera contribute more to the above-ground invertebrate biomass than any other invertebrate group. Lepidopteran caterpillars are an important food source for above-ground predators and they are a very important component in the diet of young game birds. However, despite their functional importance other groups are often more diverse or abundant. There are usually more species of true flies (Diptera), spiders (Araneae) or beetles (Coleoptera), while springtails (Collembola), mites (Acari) and bugs (Hemiptera) occur at very much higher population densities, but most of these are less closely associated with Calluna than are Lepidopteran caterpillars.
2.2 The total invertebrate biomass of heather moorland (including that below ground) is likely to be about 15-30 kg dry weight ha−1. Lepidopteran caterpillars make up about a third of this. There are usually about 10-15 caterpillars m−2 (sometimes up to 50 individuals m−2), but densities can vary considerably both seasonally and from year to year. The distribution of caterpillars over a heather stand is often patchy rather than uniform.
2.3 Certain species such as Winter moth (Operophtera brumata), Vapourer moth (Orgyia antiqua), and Magpie moth (Abraxas grossulariata) show extreme population fluctuations. During population peaks caterpillar densities of more than 1000 m−2 may occur. Serious browning of heather, in early summer, can result from the feeding activities of the caterpillars. Such “outbreaks” are most commonly due to the Winter moth which can occur almost anywhere. Outbreaks of Magpie moth mostly occur in the north-west Highlands and Islands. Outbreaks can vary in size from a few hundred square metres to, exceptionally, many square kilometres.
2.4 Collated feeding records suggest that slightly in excess of 100 species of Lepidoptera may feed on Calluna vulgaris over the whole of Britain. All of these are moths, mostly “macros”. (Macrolepidoptera is an artificial group of mostly larger species belonging to the superfamilies Bombycoidea, Geometroidea, Noctuoidea, Hepialoidea, Cossoidea, and Zygaenoidea).
2.5 This number is about what is expected for a plant with such as wide geographical distribution, regional and local abundance, relatively large living biomass per unit stand area, and long history of presence throughout most of the post-glacial period. The number of macrolepidoptera species is comparable with the number associated with widespread trees like birch, oak and willow (though these also support many more species in other herbivorous invertebrate groups).
2.6 Some of these species which feed on Calluna vulgaris are specific to it but many can feed on a range of food plants. Not all of the moth species which have been recorded feeding on Calluna will be found in any one geographical area, on any one site, at any one time: some are lowland and southern, others are rare except within particular altitudinal zones, while the abundance of some fluctuates so much that they seem to effectively disappear for periods.
2.7 Typically, 10-30 species of moth caterpillar will be found feeding on Calluna at sites within any geographical area of several hundred square kilometres.
2.8 The core Calluna fauna of widespread, and normally abundant, species includes:
Northern eggar (Lasiocampa quercus callunae)
Fox moth (Macrothylacia rubi)
Northern spinach (Eulithis populata)
July highflyer (Hydriomena furcata)
Ling pug (Eupithecia goossensiata)
Narrow-winged pug (Eupithecia nanata)
Common heath (Ematurga atomaria)
True lover’s knot (Lycophotia porphyrea)
Beautiful yellow underwing (Anarta myrtilli)
2.9 The assemblage of moth caterpillars usually comprises a small number of very abundant species and a larger number of species which are represented by a few individuals.
2.10 Some of the larger species, such as the Emperor moth (Pavonia pavonia) and the Northern eggar (Lasiocampa quercus callunae), can be quite conspicuous even at low densities since both caterpillars and adults are large, colourful and active during the day.
2.11 The much larger leafed and deciduous blaeberry (Vaccinium myrtillus), a common associate of Calluna in dwarf-shrub heaths and in the field layer of acid woodlands in Scotland, also supports a similar number of macrolepidoptera but about three times more “microlepidoptera” species. The caterpillars of the latter are often leaf miners, flower feeders, or feed on young expanding leaves, sometimes rolling or tying leaves together. Other, less widespread dwarf-shrub species probably support fewer moth species, but rare moth species do occur and the species lists might increase with more intensive study.
3.1 Heather stands with a variety of associated plant species will support a larger number of moth species, for the following reasons.
Other plant species can provide some insurance against an absence of suitable food at a critical time even for those species which preferentially feed on Calluna. For example, spring shoot growth starts several weeks sooner in blaeberry than in Calluna which may avert starvation in years when hatching occurs before heather shoot growth begins.
3.2 Heather plants are made up of a variety of plant parts of varying nutritional quality. Growing shoot tips and flowers have the highest nutritional quality and are important diet components for many species e.g. flowers are important for Narrow-winged pug and Ling pug. If these plant parts are poorly developed or missing, e.g. as a consequence of browsing by deer or sheep, then both populations and diversities of moths are likely to be much reduced.
3.3 The most recent shoots of very young Calluna plants generally have higher nitrogen and phosphorus concentrations than the most recent shoots of plants older than about four years, but apart from this food quality of individual plant parts does not significantly change with plant age or height. However, the total amounts of different plant parts does change with plant age.
3.4 The density of most species of moth caterpillar increases progressively with increasing heather height. The effect is most marked for situations where caterpillars are very abundant (for other reasons). On average there is about a 3-4% difference in caterpillar densities for every 1 cm difference in heather height. Plant age, density of green shoots and density of flowers may also be important factors but these are usually positively correlated with heather height.
3.5 Generally, more species have been found in taller stands. Abundant species are not restricted to particular heather heights although the relative abundances of different moth species does change with heather height. It is possible that less abundant species also occur in shorter heather but at such a low densities than they are unlikely to be encountered.
4.1 Information is not available for heather but a study by Dr. David Baines of the Game Conservancy Trust has shown that browsing of the shoots of blaeberry, in pine woods across the Highlands of Scotland, greatly reduces densities of moth caterpillars.
4.2 This study found that densities of caterpillars were 4 times higher in areas protected from grazing than in heavily grazed areas (11-20 red deer km−2). Even with as few as 5 red deer km−2 caterpillars were 41% less abundant than in ungrazed areas.
4.3 The reduction in caterpillars probably occurred because the deer removed blaeberry shoot tips which are also the main food resource for caterpillars. In woodland, nearly all of the shoot tips will be removed at high deer densities of 15-20 red deer km−2, and even at much lower deer densities up to half the shoot tips may be removed. Grazing deer may also inadvertently eat moth eggs and caterpillars if they are near the shoot tips.
4.4 The commonest moth species in this study are also common on heather. It seems likely that the impacts of grazing on the moth fauna of heather will be similar.
4.5 In the short term, burning or cutting destroys the shoots of heather and other plant species which provide food and shelter for moths and their caterpillars, and directly kills the caterpillars and eggs of some species.
4.6 In the longer term, however, muirburn or cutting helps to maintain dense and vigorous stands of heather if carried out responsibly and in appropriate circumstances - see Information and Advisory Notes no 58, Cutting of heather as an alternative to muirburn, and no 35, Heather layering and its management implications, and the SNH booklet A Muirburn Code (Phillips et al., 1993).
4.7 Heather which is both dense and tall can suppress other plant species of importance to the moth fauna. For example, blaeberry is somewhat slower growing than heather and, although quite shade tolerant, it can eventually be suppressed. Blaeberry survives burning well, readily resprouting from underground rhizomes. Regular burning or cutting when the heather reaches 30 cm tall, which corresponds to about a 12-15 year burning cycle for moderately vigorous heather at lower altitudes, will help to ensure the continued presence of blaeberry while still providing areas of taller heather.
4.8 Very frequent burning should be avoided. If it occurs, a large proportion of the site will have little suitable habitat for many moth species (and for other herbivores including sheep and grouse). For at least three years after burning the cover of heather is very low and with a 10 year burning rotation 30% of the ground is in this state at any given time.
5.1 Management aims should be to:
5.2 These goals are most likely to be achieved if heather moorland is managed to maintain habitat mosaics composed of lightly grazed heather stands of different ages and heights. To achieve this:
Note that these critical large herbivore densities are based on very limited evidence and are indicative only - see also Information and Advisory Note No 47 “The grazing behaviour of large Herbivores”.
5.3 These recommendations also should benefit botanical diversity, the diversity and abundance of other heather moorland invertebrates and vertebrates, and the performance of game and domestic livestock.
Baines, D., Sage, R.B. and Baines, M.M. (1994). The implications of red deer grazing to ground vegetation and invertebrate communities of Scottish native pinewoods. Journal of applied ecology 31, 776-783.
Coulson, J.C. (1988). The structure and importance of invertebrate communities on peatlands and moorlands, and effects of environmental and management changes. Usher, M.B. and Thompson, D.B.A. (Eds.) Ecological Change in the Uplands, pp 365-380, Blackwell Scientific Publications, Oxford.
Coulson, J.C., Fielding, C.A. and Goodyer, S.A. (1992). The management of moorland areas to enhance their nature conservation interest. JNCC Report No. 134, Joint Nature Conservation Committee, Peterborough.
Haysom, K.A. (1994). Aspects of the ecology of the Lepidoptera associated with calluna vulgaris on managed northern heath. Unpublished PhD Thesis, University of Durham. (A copy is held in the SNH library, 2 Anderson Place, Edinburgh).
Hobbs, R.J. and Gimingham, C.H. (1987). Vegetation, fire and herbivore interactions in heathland. Advances in ecological research 16, 87-173.
Phillips, J., Watson, A. and MacDonald, A. (1993). A Muirburn Code. Scottish Natural Heritage, Battleby.
Comments were kindly provided by Dr Susan
Hartley (ITE), and SNH colleagues Dr David
Phillips, Dr David Horsfield, Dr Helen
Armstrong and Dr Des Thompson.