Exploring the social-ecological drivers and impacts of invasive alien plant induced regime shifts in the grasslands of the Eastern Cape, South Africa
Date
2023-03
Authors
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Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Invasive alien plant species (IAPs) are currently invading South Africa’s grasslands. In this
study, I explore the drivers and social-ecological impacts of invasive alien plant induced-regime
shifts on the grasslands of the Eastern Cape Province in South Africa. This thesis consists of two
data chapters. The first chapter (Chapter 2) synthesizes literature of the social-ecological
drivers and processes underlying the invasive alien plant-induced regime shift based on the Regime
Shift Database framework (RSDB). In the second data chapter (Chapter 3), I used semi-structured
interviews to understand how the regime shift is perceived to affect the ecosystem services
provided by grasslands and the well-being of the people who live around the Upper Tsitsana
Catchment, Eastern
Cape.
Chapter 2 identified drivers that promote the invasion of IAPs that potentially push grassland
across a critical threshold to cause a shift from a grass-dominated regime (grassland) into a woody
invasive alien plant-dominated regime. These drivers included (1) the introduction of certain alien
plants for various agricultural and aesthetic purposes that are now problematic; which reproduce,
naturalize, become invasive, and disrupt grassland structures and functions, (2) land and soil
disturbance caused by non-sustainable land uses and inappropriate land management practices, timber
extraction and overgrazing, which facilitate the expansion of IAPs, (3) increased carbon dioxide
(CO2) levels that promote invasive plant growth due to their leaf-level traits associated with
carbon capture, (4) land abandonment that creates less competitive conditions for native grasses,
and thus more opportunities for IAPs to invade and dominate, (5) high rainfall, which increases
water content in the surface layer, and allows IAPs to absorb water better than native vegetation
due to their high infiltration rates and superficial root system which favours their invasion
success, and (6) clearing of invasive alien plants which can promote their growth due to
re-sprouting seedlings. In this chapter, I have also highlighted the key feedback mechanisms that
maintain the grassland regime (i.e., fire) and invasive alien plant-dominated regime (i.e., high
rate of germination and seed production) using a causal loop diagram (CLD). Understanding the
interaction between the key drivers and feedback dynamics can inform integrated planning processes
to monitor, reduce, and prevent the ability of IAPs to cause regime shifts.
Chapter 3 reports interview results amongst the respondents (n=50) of the Upper Tsitsana Catchment
that are dependent on the grasslands for their everyday life for farming, grazing and cultivation
in the Eastern Cape Province. More than 90% of the respondents stated that invasive alien plants,
especially Acacia dealbata (silver wattle) and Acacia mearnsii (black wattle) have caused
substantive changes and now dominate the grasslands of Upper Tsitsana Catchment. On the one hand,
respondents stated that the invasion by these woody IAPs have many negative impacts such as reduced
water supply, poor grazing areas, crop yields and providing refuge for
criminals, and would like to see a decrease in Acacia mearnsii and Acacia dealbata dense stands.
The respondents mentioned benefits provided A. mearnsii and A. dealbata such as fuelwood, building
material, medicine, and livestock feed. The benefits of IAPs create conflicts and debates on
whether these species should be controlled to provide the mentioned benefits. Policymakers and
researchers should engage with land-users to understand their perceptions towards IAPs to address
these conflicts. This chapter also highlighted that the fight against IAPs is currently spearheaded
by the Working for Water (WfW) program through a tree debarking and chemicals approach. Despite the
current control measures, these plants are re-establishing, forming dense stands, and invading
larger areas, which makes it increasingly difficult to restore the native grassland. Rapid
strategic management options are required in addition to the control methods by WfW to prevent
further shifts
that will drive the grassland ecosystem and human livelihoods into even more vulnerable conditions.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MScFoodSc)--Stellenbosch University, 2023.