Ecosystem Guidelines for Environmental Assessment

MAINLAND THICKET
INCORPORATING THICKET, VALLEY THICKET & ARID THICKET
MAINLAND THICKET
ECOSYSTEMS
MAINLAND THICKET
Doug Euston-Brown
What are the key ecological “drivers” in terms of
maintaining ecosystem function, pattern or structure
Herbivory is an integral part of thicket evolution and therefore function, pattern and structure.
Thicket represents the backbone resource for browsing animals in Southern Africa: it is drought tolerant,
reliable and enduring. Being further from the savannas and associated animals, thicket in the Western
Cape may have been less influenced by herbivory than in the Eastern Cape. However, many animals
such as kudu continue to make seasonal east-west migrations to escape winter droughts in the Eastern
Cape and summer droughts in the Western Cape.
Seed dispersal by animals (especially birds) results in the development of bush clumps around
solitary perch sites such as pioneer trees and termite mounds. Bush clumps enlarge and, depending
on local site conditions, eventually coalesce into dense thickets.
Fires are important in maintaining thicket boundaries when they occur in the adjacent grassland and
fynbos biomes. Thicket mosaics tend to develop in these boundary areas. Solid or uniform thicket tends
to be associated with topographically-determined fire refugia - such as deep kloofs, cliffs and scree
- or climatically-determined fire refugia, i.e. areas too arid to support flammable vegetation. Heavy
grazing can reduce fuel loads, which can retard the spread of fire and allow the establishment and
spread of thicket clumps.
Climate variability - droughts, floods and heat waves - has little impact on thicket vegetation.
Relative to the other biomes in the Western Cape, thicket is probably the most resilient to climate
change. Thicket is therefore critically important for the health and persistence of broader ecosystems.
Arid Thicket is probably the most resilient to climate change, but is most vulnerable to disturbance
resulting from heavy browsing or grazing.
Winter versus summer rainfall ratios (≥ 20% annual rain in winter, April-August) strongly influence
the distribution of thicket. The western limit of Mainland Thicket correlates closely with the transition
to a higher proportion of winter rainfall, and dominance by the Fynbos or Succulent Karoo biomes.
Coastal Thicket only penetrates inland in the Western Cape where the maritime influence on the climate
ameliorates summer drought.
Topography, geology and soil type also drive the distribution of thicket vegetation. Mainland Thicket
in the Western Cape is usually associated with the base of, or valleys within, the Cape Fold Mountains.
This correlates with a transition to a more arid climate and a change from sandstone to non-sandstone
soils. Thus, thicket acts as a buffer or barrier between the Fynbos Biome on the mountains and the
Succulent Karoo Biome on the drier hills and flats.
What are the key issues, vulnerabilities and threats
Overgrazing, especially by goats, in combination with drought has led to the demise of spekboom
Portulacaria afra. Spekboom is an essential component of thicket, especially in the Western Cape, where
it is often the dominant species at the base of north-facing sandstone slopes. It appears to be a pioneer
species that may facilitate the establishment of other thicket species. Once spekboom is lost, many
other thicket species also die, and the slope becomes barren and prone to erosion. Such areas do not
JAN VLOK
62 : MAINLAND THICKET ECOSYSTEMS