The Essex Naturalist
67
evidently have limited powers of dispersal or are not able to enter established
ecosystems and compete successfully. As suitable habitats and populations
become fragmented re-colonisation of isolated patches becomes less likely.
Although little is known about the specific complex ecological needs of most
invertebrates it is evident that populations cannot survive in sites or areas where
all their requirements are not met. Solitary aculeate Hymenoptera for example,
even at the simplest level of understanding require a combination of suitable
nesting sites and nearby foraging or hunting areas and many species are
associated with specific forage plants or prey. The smaller and more fragmented
wildlife habitat becomes the less likely it is that areas will provide the complex
of conditions required for each species, resulting in the extinction of local
populations. There is increasing evidence that many of the species which have
undergone dramatic declines in the last half of this century require large areas of
countryside containing relatively intact habitats.
Archer (in Archer and Burn, 1995 & Archer 1996) has shown that there is a
clear relationship between the number of aculeate Hymenoptera species
recorded at a site and its area. Thus the larger the area of the locality the more
species are present. For sites where the species lists are more or less complete this
relationship is linear and represented by the regression line given by the
equation In S = C + z ln A where S is the number of species, A is the area in
hectares, C the interception on the Y-axis and z the slope or regression
coefficient of the relationship. Reduction in the area of a site will reduce the
species complement and there will be a resultant loss to biodiversity. Despite the
difficulty of defining a 'site' it is evident that the isolation of sites and the
reduction in habitat area drastically affects the ability of populations to survive.
Wildlife Corridors
The use of 'Wildlife Corridors' has become fashionable as a means to attempt to
prevent isolation of wildlife habitats. At least some planners and local authorities
are incorporating the concept of wildlife corridors in their local plans. The author
urges caution however and an appreciation of the limitations of such corridors
(Harvey 1997) which are not in themselves viable substitutes for the networks of
habitat required by many invertebrates including the scarcer ant species.
Invertebrates often have very specific habitat requirements, e.g. bees may
need south-facing slopes to provide a warm microclimate, areas of bare ground
for nesting, flower-rich areas for foraging and many species even need specific
plants to provide pollen. Solitary wasp species also have different requirements,
for example nesting habits may demand different kinds of dead wood or various
sandy, chalky or clay substrates. Spiders and ants often have very specific habitat
requirements, ranging from pioneer habitat such as open ground to scrub,
ancient woodland, leaf litter, old grassland, heathland, etc. Many invertebrates
have particular prey requirements and so will only he found where these occur.