ROLE OF FIELD MARGINS IN CONSERVATION AND AS HABITATS FOR SPECIES USEFUL TO THE
Field margins come in many forms; fences, walls, ditches, raised banks,
etc., but the most common is the hedgerow. My task was to consider the
usefulness of field margins as habitats mainly for insects, the benefits to the
farmer of field margins, the habitat requirements of insects and the management
and establishment of hedgerows. I was also to include information on small
mammals and birds. Most work has been done on hedgerows, and hedgerows were
used on the poster, but in this essay I have included some information about
Hedges, walls and uncultivated strips of vegetation around the edges
of fields serve many purposes not only agricultural, but also environmental,
landscape and sporting. An ideal field margin should:-
1. clearly define
the field boundary;
2. provide a stock-proof barrier;
shelter for stock;
4. provide protection to crops and soil by preventing
soil erosion by acting as a windbreak or snow barrier;
5. provide shelter,
nest sites, overwintering and hibernation sites, and food to beneficial
insects, game and other useful animals;
6. field margins may also provide a
screen against the drift of sprayed chemicals.
Many beneficial predatory
invertebrates thrive in field margins. These invertebrates may move into the
cultivated area once the crop provides suitable cover, and hunt harmful insects
such as aphids, or they may play a part in pollinating the crop. Some field
margins can also provide good habitat for game species, e.g., grey partridges,
red-legged partridges, pheasants and hares.
Most British native small
mammals have evolved in woodland habitats. In many intensively farmed areas
hedgerows and field margins may be the only semi-natural habitats left
available. Shrews and voles will only stray from the field margin into the
fields when there is enough cover.
It has been shown that some butterflies
preferentially make use of field margins as flight corridors. Males of
territorial species such as the small tortoiseshell and peacock prefer to take
up territories along stretches of field margins, as such areas increase their
chances of meeting a female looking for oviposition sites.
For over a hundred years many farmers have seen hedges as
being unproductive, but necessary as stock-proof barriers. In the east of the
country cereal growing greatly increased since the Second World War, so hedges
were no longer needed for their stock-proofing and sheltering abilities, and
they were a hindrance to the larger farm machinery. So miles of hedgerows were
removed and small fields were joined together. Until 1974 grants for the
removal of hedges were still available to farmers. In intensively cultivated
areas many of the hedges had become island habitats. Once the hedge was removed
there would be no other suitable habitat available to populations with limited
dispersal abilities, so they would die out.
Variety of habitats
a very short area a hedge and uncultivated strip can provide a large number of
1. a linear habitat of shrubs and bushes;
2. isolated trees;
3. a herbaceous verge, relatively undisturbed;
4. ditches providing a
temporary or permanent aquatic habitat.
In the U. K. about 1000 plant
species have been recorded in hedgerow habitats, and about 250 are closely
associated with it, so there is great structural and foodplant variety. Hedges
are often dismissed as "edge" habitats, and they do contain more "edge" species
than "interior" species. However a good thick hedge can be considered an
"interior" habitat to some insects, and even to some small mammals such as the
dormouse, which rarely strays out of the cover of the hedge. If a hedge has a
ditch with water this can provide a habitat for dragonflies, mayflies,
stoneflies and water beetles, as well as frogs, newts and toads.
Hedgerows with grassy verges and raised banks are important
overwintering habitats for and aphid eating carabid and staphylinid beetles, e.g. Tachyporus hypnorum. Of the 35 carabids found in
agricultural settings 24 overwinter in and around hedgerows. Hedgerows are also
important nesting and hibernating sites for bumblebees. Bumblebees require south facing
banks for their nests, and colder north or north-west facing banks for
hibernation sites for the queens. The long-tongued bumblebee Bombus pascuorum suffers when
grass is cut right up to the edge of the hedge as this destroys its nest which
is found in tussocky grass. This bee is particularly valuable in pollinating
red clover. B. sylvarum, a bumblebee found only in the south, which has
similar habitat requirements to B. pascuorum, has decreased in numbers
to such an extent that captive breeding and re-introductions have been
It has been found that increasing the number of flowers in the
hedge bank and grassy strip of cereal fields, and allowing them to flower leads
to an increase in numbers and diversity of hoverflies and parasitic wasps.
Hoverfly adult females need to eat pollen in order for their ovaries to
develop, then they lay their eggs in the middle of groups of aphids, which the
larvae eat. The numbers of overwintering spiders will
also increase in untreated grassy and clover strips.
Some of the herbaceous
plants that grow at the bottom of the hedge overwinter as rosettes, e.g.
foxglove; these provide valuable hibernation sites for insects such as
ladybirds. Ladybirds in both the larval and adult stages consume large numbers
Hedges provide sheltered nesting and breeding sites for a wide
variety of vertebrates including, the linnet, yellowhammer, whitethroat, and
the dormouse. In a study 84% of whitethroat and 97% of yellowhammer nests were
found in field margins, and both were significantly more numerous in hedges
Plots of arable land containing ditches were
found to contain twice as many species of bird, and three times the density
than plots without ditches. Many of these are game birds whose young eat the
insects found in the grassy verges of hedges.
Hedges as corridors
Hedges can provide links between what would otherwise be isolated
populations; some capture-recapture studies have shown considerable movement,
and probably genetic exchange in some butterfly species, e.g. the gatekeeper,
ringlet and meadow brown. However studies in Norway have shown that hedges can
act as a barrier to butterfly movement, as some butterflies will not fly over
obstacles of a certain height. The best type of hedge for some butterflies
would be one with large gaps in it.
Shape and size
It is stated
throughout the literature that for most types of hedge an "A" shape is better
than a hedge with parallel sides, though there does not seem to have been any
real scientific study to justify this. The best height for a hedge is about 2 m
by 1.5 m wide. However in 1992 The Institute of Terrestrial Ecology found that
the best type of hedge for birds, invertebrates and small mammals was an
unmanaged hedge about 5 m high and 4 m wide. This may take up more space and
cause more shading of crop than the farmer would wish.
A hedge is like a
wood, there will be no one "best" shape and size, and there will probably be a
whole range of management regimes that will suit different groups of insects
and different sites. The study of hedges and their contribution to conservation
is still in its infancy, and there may be a danger that increasing legislation
will produce a standard hedge for the whole country.
Insects often have different habitat requirements at the different stages
in their life-cycle, the requirements of each stage must be met, if they are
not then the insects will die. The current methods of trimming and cutting
hedges can lead to the loss of habitat for some stages in the life cycle of
If the hedge is allowed to flower and is cut on a rotational
two or three year basis, the flowers will provide out-of-season pollen and
nectar to pollinating insects until the crop flowers. Also the berries and nuts
are a valuable food source for rodents and birds.
For some insects that
survive as eggs or pupae on twigs and branches over winter, yearly hedge
cutting can lead to serious population reductions. Insects or eggs on the
branches that are cut off will almost certainly die, others may have lost the
shelter and protection they need, and others may become dislodged by the
violence of the operation. Annual hedge trimming is thought to be the cause of
the decrease in numbers of the brown hairstreak butterfly which lays its eggs
on blackthorn twigs in August, and they hatch the following spring. Also the
small eggar moth which overwinters as a cocoon on twigs. This moth is listed in
the Red Data Book as being vulnerable; indiscriminate hedge trimming,
destruction of hedgerows and pollution by agricultural sprays and motor
vehicles are the threats to the reduced populations that remain.
recommended that hedges be trimmed every two or three years, preferably on one
side at a time, and no more than one third of the length of the hedge should be
trimmed in any one year. The best time for trimming is said to be late winter.
It has also been found that trimming with a mechanical flail causes great
damage to the hedge itself as the branches are not cut cleanly; this encourages
diseases and fungal infections.
Establishing new hedgerows
now available for the establishment of new hedgerows; however these should not
be planted at the expense of existing herb-rich field margins. Ditches should
not be dug if they might drain existing damp or marshy areas of meadow. And at
all times native shrub and tree species should be used, as these will best
provide for the local fauna. A raised bank will provide overwintering sites for
beetles, and nest and hibernation sites for bumblebees.
Conservation headlands is a management technique where pesticide
input to the outer 6 m or 12 m of cereal fields is reduced and selective. This
technique has been shown to benefit butterflies. Ideally such an expanded field
margin can act as a buffer zone between the hedge and the crop.
The main difference between walls and hedges as far as insects and small
mammals are concerned is that a badly neglected and damaged wall will still
provide a perfectly good habitat. Stone walls, especially dry stone walls with
lots of spaces are excellent nest sites for Bombus lapidarius (a bumblebee)
and many solitary bees. In fields bordered by walls it was found that the
highest density of carabid beetles and spiders was in the tussocky grass at the
sides of walls. Some Linyphiidae spiders are only found next to walls and
require stones for hunting, shelter and support for their webs. Also in areas
of low pollution the lichens on walls may provide a background for cryptic
insects to rest and bask.
In some areas field margins are
the only semi-natural habitats left. With agricultural over production farmers
are being encouraged to set aside land, this could be used as an opportunity to
expand the field margin area, or even to allow whole fields to revert to
woodland. Field margins provide habitats to many useful insects, their
populations could be increased with just a little more knowledge about their
requirements, particularaly their life-cycles. This would decrease the
necessity for pesticides and some herbicides, leading to savings in finances
Boatman, N. (Ed.) (1994). Field margins:
integrating agriculture and conservation. British Crop Protection Council,
Monograph No. 58.
Brodie, L. (1996).
Bumblebee foraging preferences: species and individuals. BSc. Thesis Aberdeen
Chinnery, M. (1993). Insects of Britain and Northern Europe.
Jones, S., Fry, R. & Lonsdale, D. (1991). Hedgerows and arable
field margins. In: Habitat conservation for insects - a neglected green issue.
The Amateur Entomologists' Society.
Marshall, J. (1994). Life on the edge:
managing field margins. Enact. 2: 7-9. Park, J.R. (Ed.). (1988). Environmental
Management in Agriculture. Belhaven Press.
Robb, K. (1996). The
invertebrate assemblage of a dry stone wall and adjacent field systems. BSc.
Thesis Aberdeen University.
Roberts, M.J. (1995). Spiders of Britain and
Northern Europe. Collins.
Shirt, D.B. (Ed.). (1987). British red data
books; 2 insects. Nature Concervancy Council.
Sladen, F.L.W. (1912). The
humble bee. MacMillian.
Way, J.M. & Greig-Smith, P.W. (Eds.). (1987).
Field margins. British Crop Protection Council, Monograph No. 35.
R. (1994). Hedges in decline. Enact. 2: 18-20.
Buffer zones are, "Areas peripheral to National Parks or reserves which
have restrictions placed on their use to give an added layer of protection to
the nature reserve itself and to compensate villagers for the loss of access to
strict reserve areas".
Tropical forests probably harbour over 50% of the
world's species of animals and plants, but they are being opened up by logging,
and converted to agriculture so rapidly that there is widespread concern that
the entire biome will be irredeemably damaged in the next few decades. The
conservation of these ecosystems requires the establishment of protected areas.
The problem in the past however, has been that legal protection alone has not
maintained the integrity of these areas as conservation zones. The view often
held by the local people in the region surrounding the parks and reserves was
that such protected zones were an infringement on their traditional rights.
Despite the demarcation of boundaries, inadequate patrolling by guards and
minimal management all too often resulted in illegal hunting, the gathering of
forest products and the encroachment of agriculture. It soon became clear that
laws without support cannot be enforced and it was this realisation that
prompted the MAB/UNESCO Biosphere Reserves Action Plan adopted in Minsk, in the
USSR, in 1984. The purpose of the Action Plan was to incorporate "buffer zone"
activities that involve the interaction with local communities into the
management of protected areas.
2 Planning buffer zones
2.1 The two main
kinds of buffer zone are: Extension buffers, this extends the habitat of the
reserve, allowing the animals and plants a larger area. There may also be
selective logging, hunting and firewood collection by the local villagers.
Socio-buffering is where the main importance of the buffer zone is to provide
the locals with cash crops etc. This involves the planting of species that the
animals in the reserve would find unattractive in order to keep them within the
2.2 Requirements and restrictions When planning a buffer
zone the following should be taken into consideration:
The natural history
of any species within the reserve.
The need for protection against fire,
soil erosion, water conservation etc.
The need to contain species that may
wander out of the reserve, e.g. elephant.
The needs of the locals for
forest products and cash crops.
The amount of land available and its
The suitability of particular crops for the soil and climatic
conditions, and also whether the animals in the reserve would find the crop
attractive, e.g. bananas or maize near elephants and monkeys.
restrictions should be considered:
No permanent settlement within the zone.
No burning of vegetation except where species are adapted to a regular fire
No hunting of endangered species or species that may threaten the
food acquisition of endangered species.
No planting of crops likely to
entice wildlife out of the reserve.
2.3 Types of buffer zone
Traditional Use Zones. Where no suitable land exists or is available
outside the reserve, some part of the reserve is made available to the locals
to satisfy the need for forest products, e.g. fishing, traditional hunting,
fruit gathering, firewood collection, rattan cutting, some grazing of animals
if it does not conflict with the grazing of wild animals.
These reduce the need for the locals to take resources from the reserve itself,
and may include, cash crops, wildlife hunting permits for use outside the
reserve, or even a share of the income from the reserve.
These provide the locals with firewood. The area may be a part of the reserve,
or secondary forest, or even plantations to maximise growth.
buffers. Where there is no land available, or no suitable buffer area fences,
ditches, canals etc. discourage wildlife from leaving the reserve and people
and domestic animals from entering.
Therefore it can be seen that several
types of buffer zones exist and involve the consideration of many aspects of
conservation in their planning. Two case studies will now be examined to
demonstrate firstly the different types of buffer zone used in practice. And
secondly to assess their relative success or failure.
3 Case study - Lake
Manyara National Park
Lake Manyara National Park is
part of the Tarangire-Simanjro ecosystem situated south of the Tanzania/Kenya
border. The area of the national park is very small, only 110km2 of park land.
It has been awarded "Biosphere Reserve Status" due to its unique qualities.
Wildlife. The park has high animal diversity with the
highest animal biomass in Africa. Migratory species include zebra, wildebeest,
Grant's gazelle, buffalo, as well as elephants and lions.
escarpments, saline lakes and hot springs.
3.3 Society. Rural area dwellers
and tourists. Land uses include agriculture, pastoralism, wildlife,
conservation, grazing, there is also a phosphate mine and game controlled
3.4 Problems Population increase. In 1976 the study area had 6.5
persons/km2, which by 1988 had increased to 37.9 persons/km2, with a total
population of 25,985. The population growth rate is 3.4% (much higher than the
national average), due to an increase in immigration into the area, especially
Low returns from agriculture. As the returns from agriculture are
low crop land has been greatly increased. In 1958 cropland accounted for 36km2,
and by 1988 185km2 (Mowambu cropland increased from 218ha in 1957 to 1093 ha in
1980, a difference of 401% in 23 years).
Game migration and crop
destruction. Game migration after the rains coincides with crop ripening (May -
June), this leads to crop destruction which creates conflict and tension with
Game movement is restricted. this is caused by
increased human numbers and therefore settlements with expansion of
Elimination of grazing and water resource. Enlargement of
settlements has meant encroachment onto dry season grazing land and water
resources for the wild animals. The area is also used as an escape route when
the lake floods along the shoreline.
The blocking of this area has also
meant there is a risk of populations being wiped out if a natural catastrophe
hits the area, e.g. rinderpest epidemic in buffalo in 1959; severe flooding of
the lake in 1970 which led to the death of many wildebeest and zebra through
drowning of marooned animals.
Game corridor. This will
enable migratory species to have a route for dispersion, and allow a connection
between Lake Manyara National Park and Tarangire National Park.
zone. To minimise conflict on boundaries with partly restricted land use, which
will protect the national park and gives benefits to the villages.
Proposed plan for Lake Manyara National Park A 2km wide buffer zone between
Lake Manyara Hotel and Marang forest reserve, within which fuel wood and
livestock grazing will be allowed. The existing park boundaries will serve as a
buffer strengthened by trenches, electrical fences and spike barriers to a)
discourage animals from destroying crops, b) prevent people and livestock from
entering the reserve. And plans to convert part of the whole forest reserve
into a National Park.
3.8 Conservation incentives Rural populations to be
involved in decision making on such issues as game corridors, buffer zones,
anti-poaching and prevention of fires. Public awareness, information and
education. Programmes for sustainable utilisation of wildlife within village
boundaries, some allowance to exploit some resources of the park, and
compensation for damage to crops and property caused by wild animals.
case study highlights proposed plans made in 1991 by R. Mwalyosi. An original
plan was proposed in 1987 by H. Prins, "Nature conservation as an integral part
of optimal land use in E. Africa: The care of the Masai ecosystem of northern
Tanzania". After the 1987 plan it was decided that more field data were
necessary. The 1991 plan was produced, but again it has been decided that more
field data are necessary.
It can be seen that the buffer
zone planned around lake Manyara National Park does not fall into just one
classification of a buffer zone, in fact it relates to three types; economic,
physical and forest buffer zones. The continuation of livestock grazing within
the buffer zone reduces the need for locals to take resources from the reserve
and in this respect it is an economic buffer zone. The introduction of
trenches, electrical fences and spike barriers introduces the physical aspect
to this buffer zone. Lastly the collection of fuel wood within this buffer also
implies that it is a forest buffer. Therefore it can be seen that in practice
buffer zones often incorporate a number of factors. The second case study
examines some problems that may occur in the implementation of a National Park
and buffer zone, the location of this case study is Korup National Park in the
4 Case study- Korup National Park.
Korup national park in Cameroon is situated in the south west province near the
border with Nigeria. It covers 1,250km2. The park was established in 1986 after
preliminary surveys in the early 1980's identified the ecological potential of
4.2 Resources and processes
Environment Wildlife, 17 tree
species previously unknown, were discovered. Korup emerged as potentially the
richest and most diverse forest yet studied in the whole of Africa. A chemical
screening programme identified ninety substances of potential economic value,
38 of which were new to science. Land uses are agriculture, logging, wildlife,
conservation, fuel wood gathering.
Society Inhabitants include the
indigenous population and employees of logging companies.
4.3 Aims of Korup
The aims of Korup National Park were to protect 8000 species
of plant, animal and insects, whilst providing the local population with space
for sustainable rural development in the buffer zone. 1042 people living in six
villages would have to be resettled in the buffer zone.
of Korup National Park Project
The project needed financial backing and in
April 1988 the World Wildlife Fund pledged to raise £2.5 million, the
Overseas Development Agency pledged a further £500 000 and the West
German government pledged a further £600 000 to the project. The W.W.F.
signed a contract with the Cameroon government to establish a protected core
area of 1 200 km2 surrounded by a 4 000 km2 buffer zone. The contract included
helping farmers in the buffer zone to identify the better land and offer advice
on farming techniques. Tree nurseries would be established to provide seedlings
of indigenous food crops, cash crops, fruit trees and fuel wood. The Cameroon
government also agreed to establish guard posts off the buffer zone to stop
Population re-settlement. 1042 people
living within the boundaries of the proposed National Park would have to be
resettled within the buffer zone.
Illegal logging. The Cameroon government
sees Korup National Park as a prime source of foreign exchange to help
alleviate the country's severe economic crisis. Due to ineffective surveillance
and policing mechanisms along the park's boundaries, illegal logging continues.
As well as this, the government have granted logging concessions since February
1990 to international timber companies in areas bordering the park. It has been
alleged that some companies have removed species other than those they have
declared. Directives to consult local agricultural officials and communities
before applying to cut timber are being ignored.
of indigenous population. In the creation of the National Park several villages
will have to change location and alter their lifestyles.
biodiversity. As a result of illegal logging involving bad management policies
the rich biodiversity is under threat.
Therefore in this case study it can
be seen again that the buffer zone falls into several categories, not just one.
These are traditional use zone, economic buffer, forest buffer, and physical
buffer. Korup National Park has been regarded as a model for the conservation
of threatened rain forests world wide. However it has been demonstrated that
despite careful management of the buffer zone many problems still exist, such
as illegal logging and slack implementation of rules. Problems such as these
are difficult to resolve unless the government gives full support to a project.
In conclusion it can be seen that buffer zones represent
an example of resources and processes in environment and society. Diagram 1
summarises these resources and processes. Buffer zones demonstrate how
conservation problems in the tropics such as deforestation and loss of
biodiversity, are directly related to economic activities such as
agro-forestry, livestock production, and logging. For example in the case of
Lake Manyara the buffer zone not only serves to protect the National Park and
therefore preserve conservation but also to aid the economic development of the
local population by allowing fuelwood collection and livestock grazing. In the
case study of Korup National Park it can be seen that trying to satisfy both
economic and conservation needs is not easy and many problems can occur.
Examples are illegal logging and governmental regulations allowing concessions
for international timber companies, these pose threats for the natural
resource. Short term benefit may initially be brought to the local economy, but
in the long term the results may be highly detrimental.
INTRODUCTION HALL, J. B. and RODGERS, W. A. 1992. Rural development
forestry network. Overseas Development Institute, U. K.
OLDFIELD, S. 1988.
Buffer zone management in tropical moist forests. IUCN, U. K.
BUFFER ZONES MacKINNON, J. and MacKINNON, K. (Eds.). 1986. Managing protected
areas in the tropics. IUCN, U.K.
LAKE MANYARA NATIONAL PARK
R. B. B. 1991. Ecological evaluation for wildlife corridors and buffer zones
for Lake Manyara National Park, Tanzania, and its immediate environment.
Biological conservation 57:171-186.
PRINS, H. H. T. 1987. Nature
conservation as an integral part of optimal land use in East Africa: The case
of the Masai Ecosystem of Northern Tanzania. Biological conservation
KORUP PROJECT PARK, C.C. 1992. Tropical rainforests. Routlege,