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The influence of forest edge effects on
the ecology of the wood ant, Formica rufa L. (Hymenoptera: Formicidae)
in woodlands in the East Midlands.
1 Introduction.
Habitat fragmentation is a significant threat to the maintenance
of biodiversity in terrestrial ecosystems (Young et al. 1996).
Fragmentation decreases the size of habitat patches, but leads to
the increased isolation of populations, disrupting both individual
and population behaviour, the genetic make up of populations and
a host of other important ecological processes (Margules 1996).
For remnant forest fragments isolated within an agricultural mosaic,
exposure to edge effects is also a serious problem (Murcia 1995;
Didham 1997). The impacts of forest edge effects have received widespread
attention with research covering such factors as the response of
edge vegetation, species composition, avian behaviour and seed predation
(McClanahan & Wolfe 1987; Burkey 1993; Fraver 1994; McCollin
1998). Evidence now suggests that edge effects are a major driving
force behind changes in insect populations in forest fragments (Didham
et al. 1997). This research aims to determine the influence
of forest edge effects on the ecology of a specialist woodland insect,
the wood ant, Formica rufa. L. (Hymenoptera: Formicidae).
1.1 Forest Edge Effects
Forest edge effects occur as a result of two adjacent, but quite
different habitats, being separated by an abrupt divide (Matlack
1994; Murcia 1995; Fox et al. 1997). This abrupt boundary
produces a microclimatic gradient passing across the edge between
the extreme conditions of the non-forested area and the stability
of the interior forest (Chen et al. 1992; Cadenasso et
al. 1997). This gradient includes factors such as air temperature,
air and soil moisture, rates of decomposition, light intensity and
wind penetration and is believed to extend up to three times the
canopy height into the interior. However, in reality the situation
is very complex and this distance varies in response to seasonality,
physiognomy, orientation and successional development (Whitney &
Runkle 1981; Kapos 1989; Matlack 1993; Young & Mitchell 1994;
Camargo & Kapos 1995; Murcia 1995; McCollin 1998). Typically
edge effects influence species via three major pathways (1) the
modification of microclimate, (2) the alteration of biological interactions,
and (3) by changing vegetation structure (Murcia 1995; McCollin
1998).
1.2 Wood Ant Ecology
F. rufa is the largest of the three British species, which
all belong to the sub-genus Formica sensu stricto, the
Formica rufa group (Rosengren & Pamilo 1983). This species
is found most commonly in woodlands where it forages both on the
ground and in the forest canopy within a well defined territory
(Rosengren 1971; Skinner 1980a). Its diet consists of honeydew obtained
from tended aphids (Hemiptera: Aphididae) and proteins derived from
large numbers of invertebrate prey (Brian 1977; Skinner 1980b).
A key factor in the success of F. rufa in woodland habitats
is its ability to maintain favourable microclimatic conditions within
its nest mound (Rosengren et al 1987). These mounds are very
conspicuous features, sometimes reaching a metre in height and are
constructed from conifer needles and resin, twigs and leaf petioles
(Skinner 1987, 1998). They are carefully built so as to trap heat
from sunlight and at times maintain core temperatures of 20ºC
above ambient (Coenen-Stass et al 1980; Pontin 1996; Laakso
& Setala 1998). Nest temperatures are also maintained by the
wood ants' metabolic response to their excessive carbohydrate intake
(Rosengren et al 1987). These elevated temperatures facilitate
colony activity and the production of brood (Skinner & Allen
1996).
The structure of F. rufa colonies vary throughout its European
range (Welch 1978; Rosengren & Pamilo 1983). Colonies may consist
of a single queen occupying a single nest (a monogynous - monodomous
colony), or multiple queens utilising multiple, socially connected
nests (a polygynous - polydomous colony) (Rosengren & Pamilo
1983; Hofener et al 1996). The factors which determine colony
structure are not well understood, but are believed to be linked
to climate and food resource quality (Mabelis 1979; Hofener et
al 1996).
Honeydew obtained from tending aphids accounts for over fifty per
cent of the F. rufa food resource (Skinner 1976) and this
very important relationship is considered to be mutualistic (Brian
1977; Stadler & Dixon 1998). However, F. rufa is selective
in its choice of aphid partner and although several species of aphid
may live on a particular tree, the ant will often only tend one,
utilising the others as prey (Skinner 1980a)
Tree aphids in turn are specific in their choice of host tree,
over half acting as a parasite on their chosen species of tree for
most of their life cycle (Thacker & Hopkins 1998). However,
many also utilise the strategy of 'host alternation'. This involves
dispersal to a secondary host, usually a species of woodland plant
during mid-summer to avoid the period of lower quality tree sap
(Dixon 1985). This strategy allows the aphids to maintain activity
thus avoiding diapause and therefore also to continue to act as
a valuable food resource for F. rufa (Thacker & Hopkins
1998).
Due to the rather complex and inter-related requirements of both
wood ants and tree aphids it seems apparent that the species composition,
structure and density of woodland vegetation are key factors in
their survival. As these aspects of vegetation are directly effected
by the processes of edge effects it is likely that they also critically
influence key aspects of F. rufa ecology.
2 Aims
The aim of this proposed research is to identify the influence
of forest edge effects on the behavioural ecology of F. rufa.
The extent of the abiotic edge effects will be determined from sample
sites at each of the cardinal compass points within the woodlands.
The following biotic factors will then be examined in relation to
the abiotic edge effects:
- responses of vegetation (species composition, structure, density
etc.).
- nest location (in relation to microclimate and vegetation)
- the pattern of nest dispersion
- height of nest in response to microclimate and vegetation
- size of individual colony territories in response to microclimate
and vegetation
- foraging activity
- colony structure
- ant-aphid mutualisms
It is expected that the processes of edge effects will not involve
singular responses, but act so as to cause a 'flow' of interrelated
reactions (Fig 1).
Fig 1: Diagrammatic representation
of the behavioural responses of F. rufa to the major influences
of forest edge effects.
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