Geography Reference
In-Depth Information
Gaston
et al
. (1998) produced a GIS-based
macro-scale study of changes in ecosystem carbon
pools caused by land conversion in Africa's
tropical forests. They estimate that the
aboveground forest biomass accounts for 75 per
cent of the total carbon, below-ground forest
biomass for 21 per cent, and grass/shrub savannahs
for 4 per cent. Mean biomass C densities are
reported as 180 Mg ha
-1
for lowland moist forests,
82 Mg ha
-1
for all forests, and 6 Mg ha
-1
for grass
savannahs. Forest conversion, 1980-90, caused a 13
per cent decrease in the above-ground forest
carbon pool, including 5.6 per cent due to
deforestation and 7.4 per cent to biomass
reduction by other human activities
(ibid.)
. I n
Brazil, deforestation and burning has increased
carbon monoxide and ozone concentrations in
the lower atmosphere (Kirchoff 1996).
The microbial coenoses of tropical forest soils
remain largely unexplored. A recent Amazonian
study described 100 sequences of genes, ninety-
eight of which were bacterial and two archaean.
No duplicate sequences were found, and none of
the sequences had been previously described
(Bornemann and Triplett 1997). Eighteen per cent
of the bacterial sequences could not be classified in
any known bacterial family. There were significant
microbial population differences between a mature
forest soil and an adjacent pasture soil.
Vesicular arbuscular mycorrhiza are endophytic
fungal symbionts that aid plant growth by
increasing the uptake of soil nutrients. Johnson and
Wedin (1997) examined mycorrhizae during
conversion of dry tropical forest to grassland in
Costa Rica. They found that while the beta
diversity of mycorrhizal spore communities was
lower in the grassland plots than in the forest plots,
total spore density and alpha diversity of
mycorrhizal spore communities were unaffected by
conversion to pasture or by subsequent burning.
These results suggest that forest regeneration would
not be constrained by any lack of mycorrhizal
symbionts. Johnson and Wedin suggest that the
grasslands are sustainable, alternative stable states for
these former forest areas. Positive feedback between
the grassland vegetation, fire and nutrient cycling
systems reinforce this condition.
BIOGEOGRAPHICAL IMPACTS OF
DEFORESTATION
It is well known that tropical rain forests are large
and complex ecosystems. They rank among the
world's greatest reserves for biodiversity and offer
huge pools of potentially useful species and genes
(Reading
et al
. 1995:151-5; Wilson 1992).
Environmentalists argue that we may be losing
50-200 species each day, and that current rates of
extinction are five orders or magnitude above the
geological norm (Myers 1995:179
et seq.
). Myers
continues to suggest that the biodiversity of the
Earth could be halved by the middle of the next
century and that, if this happens, tropical
deforestation will have been the reason. Naturally,
these numbers are contested.
However, loss of biodiversity is not the only
biogeographical impact. Biogeographical changes
also result from ecological invasions and habitat
fragmentation. For example, in the Himalaya,
mined areas, roadsides and degraded vegetated
areas have provided avenues for the invasions of
exotic species such as
Celosia argentea, Lantana
camara,
and
Eupatorium glandulosum
(Rajwar 1998).
Once established,
Lantana
spreads into shrub land
and forest, and
Eupatorium
into montane grassland.
Their competitive advantage is secured by high
primary productivity and by non-palatability to
grazing animals
(ibid.)
. Once established in
disturbed sites, these exotics often spread into less
disturbed areas. Many more dramatic tales come
from tropical Oceania and Australasia, where
ecological invasions by species like the cane toads
in Queensland and mynah birds on Rarotonga
have had major impacts on the numbers and range
of indigenous species.
Deforestation can also influence the
epidemiology of disease. For example, when coffee
prices doubled in 1986, this prompted large-scale
deforestation for coffee plantations in southern
Thailand. The increased area of standing water
favoured
Anopheles minimus,
a highly efficient
vector of malaria. The new plantations also
attracted migrants from endemic and non-
endemic areas. In 1986-7, an epidemic wreaked
havoc among the non-immune migrants
