FEATURE
In some cases, landscape incision can
create new local habitat elements such
as scrub-lined channels in grasslands
(Pringle et al., 2013; Tinley, 2015). The
inclusion of a new patch type would cer-
tainly increase habitat diversity, but that
might be at the cost of the function of
a vulnerable ecosystem (e.g. structural
grasslands) with the addition of an as-
semblage of plant species thriving on
human disturbance regimes and their
commensals (e.g. birds that use that
habitat patch)?
Some major causes of nested
drought-buffering wetland decline:
Landscape incision by gullies is not
the sole cause of wetland dysfunction.
Overgrazing can destroy local wetlands
by flattening them out or filling them up
with sediment without any involvement
of gullies. A good example of this is on
alluvial plains or plateaux with cracking
clay soils that form gilgai ponds when
healthy due to the wetting and drying
cycle of clay minerals. When consistent-
ly overgrazed these wetland patches be-
come part of flattened, water-shedding
surfaces through compaction, but are
recognisable as pale coloured patches
in a darker mosaic of fine ironstone lag.
Similarly in-channel pools can be filled
with sediment from accelerated erosion
in the hinterland.
The most biologically-impoverishing
process is gully development and ex-
pansion because it targets all of the
most ecologically important compo-
nents of most rangelands that are active
catchments (as opposed to vast, deep
sand plains without much surface drain-
age) (Pringle & Tinley, 2003; Pringle et
al., 2006; Pringle et al., 2011). The cause
is usually a cut into the natural land-
scape base level, be that a broken rock
bar across a river (Figure 4) , a poorly lo-
cated watering point or farm road and
the accelerator mechanism is the lack of
ground cover that feeds gully heads and
accelerates their expansion through in-
creased runoff (Pringle et al., 2011).
Any wetland that overflows to another
system via a soft sill, the feature that
acts as a natural dam wall, is vulnerable
to reduced productivity and biodiversi-
ty through losing its capacity to harvest
and pond water through breaching by
gully heads (“pulling the plug out of the
bath”). This can occur at small or large
scales, for instance a small drainage ba-
sin (Figure 5) or a major wetland such as
the Urema Lake at the base of the Great
African Rift Valley in the Gorongoza Na-
tional Park (Tinley, 1977). In both cases,
animal paths or other linear incisions
such as roads can “unplug” the system.
At Urema Lake, thousands of hippo-
potami travelling between the lake and
Grassroots
Vol 20
No 1
Figure 2: How most gullies are formed by cutting upslope from any type of incision.
Figure 3: An example of how upland tributary flow spreads at the key line and then
hydrates the bottomlands in a healthy system. This isn’t always so simple in real,
large catchments.
pools in the Pungwe River, not livestock,
initiated the gullying that breached the
wetland. In increasingly fragmented
“wild” landscapes dedicated to biodi-
versity conservation (Fynn, 2012), active
biodiversity management may need to
deal proactively with these issues. So
March 2020
do we need a more holistic understand-
ing to solve these issues? Is erosion
the particular problem of farmers and
pastoralists or also of those who aspire
to manage biodiversity values in an in-
creasingly fragmented landscape (Fynn
et al., 2015)?
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