Geology Reference
In-Depth Information
Pickett & Thompson (1978) discuss ideas of
reserve design in the light of resource fragmen-
taion. Janzen (1983) emphasises the dangers of
small patches of prisine forest being colonised by
pioneer species from peripheral buffer zones of
secondary forest. A recent symposium volume
from Australia emphasises the coninuing role of
remnant forest vegetaion patches in conserving
plants and smaller animals (Saunders et al., 1987).
However, they stress the need for informed and
coninued management efforts.
Harris (1984) summarised the above by saying
that the 'efecive' size of forest patches is
determined by (i) actual size, (ii) distance between
patches and (iii) the hosility of the intervening
matrix. All three consraints can be lessened by
efecive management.
The ecological consequences of forest frag-
mentaion have been the subject of much recent
study (e.g. Burgess & Sharpe, 1981; Saunders et
al., 1991). Hill (1985) makes the disincion
between edge and interior forest types and says
that forest patches below 8 ha in extent probably
will not survive as primary forest (they certainly
will not hold their vertebrate populaions, as
shown by Lovejoy (1986) in South America).
Ranney, Bruner & Levenson (1981) liken edge
communiies to highly disturbed forests, and sug-
gest edges are usually over 20 m wide and will not
support a climax community. Whitcomb (1981)
stated that 'edge' -dominated forest will have a
large number of generalist edge species but a very
small subset of habitat specialist forest-dwelling
species. These authors go on to say that forest
racts that are sufficiently large to preserve
populaions of most plant species may be
inadequate as reserves for bird species. The
maintenance of forest bird communiies requires
forests of hundreds or even thousands of hectares
and the management of these forests should be
aimed at minimising disturbances of the forest
interior. Mathiae & Steans (1981) show that
excessive fragmentaion leads to a remnant auna
of 'opportunist and generalist' species; much of
the 'specialist' original fauna will disappear.
The effect of intenal patch dynamics and con-
servaion values has been little studied in African
forest systems. The effects of shifing culivaion,
of clearings from logging operaions and of
general overexploitaion of fuelwood, pole and
ree resources are to create a series of intenal
gaps or edges. Whilst small-scale gaps from
natural treefall and minor perturbations are con-
sidered to play a major role in the pattens of
community dynamics and maintenance of diver-
sity (Mabberley, 1983), larger gaps may cause
more lasing change towards secondary forest
condiions (Skorupa & Kasenene, 1984).
Although some species may achieve higher
densiies in secondary forest, the overall levels of
species diversity will be reduced, and conserva-
ion goals of maintaining climax forest systems
will be lost. Some ecological principles for trop-
ical forest conservaion have been put orward by
Ng (1983), based on work in Malaysia. Major
findings are that forest exploitaion is likely to lead
to reducion in height and biomass of succeeding
forest cover, resuling in less niche space for spe-
cies occupancy. Patterns of reproducion of most
forest species dictate that conserved forests must
be large, undisturbed and numerous to be fully
effecive. The effects of logging on the conserva-
ion values of forest have been examined in a
major review by F AO (l 982c): 'Selecive logging
of superior trees may cause geneic erosion and
damage to residual vegetaion. Seed trees may not
survive isolaion shock.' Simplificaion of forest to
enhance imber values by selecive cuting and
enrichment planing may reduce overall resilience
to pests and pathogens (e.g. Way, 1977; White-
head, 1982). Deecive trees, usually removed,
may be ecologically valuable. This finding has led
to a rapidly growing literature from the USA on
the great conservaion importance of such 'snags',
often removed as over-mature individuals: see,
for example, Thomas (1979). Such old growth
may be important in maintaining Rhizobium
populaions. Logging may make adjacent stands
more vulnerable to windthrow, fire, species intru-
sion, illegal felling and encroachment.
The sustainable utilisation of forest resources
Whilst most consevationists are convinced of the
need to integrate geneic resources protecion
with increased invesment and consideraions to
Search WWH ::




Custom Search