Towards understanding alien floristics within an urban matrix: the case of Durban, South Africa.
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Date
2021
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Abstract
Invasive alien plant species (IAPs) represent a major threat to biodiversity loss and ecosystem
functioning globally. Alien species can take advantage of changes in ecosystems brought
about by natural and non-natural disturbances and compete with indigenous species for
resources. Urban areas are becoming increasingly susceptible to plant invasions due to
increasing anthropogenic activity levels as urban human populations increase and changing
climatic conditions that favour alien species. If uncontrolled, IAP impacts can bring about the
transformation of natural habitats and exclusion of indigenous species. Given the limited
financial and human resources available for IAP monitoring and control in developing
countries like South Africa, increased efforts to monitor the prevalence of IAPs more
efficiently and identify the drivers of invasiveness within cities is urgently needed to
prioritise urban green spaces and species for monitoring and control interventions.
Frameworks to monitor alien invasions, identify their drivers and impacts and
prioritise sites and species for control of alien plants have been limited in the urban context,
particularly within developing countries such as South Africa. Where these frameworks have
been developed, there are challenges of them not being equally effective at different
geographic scales and across different habitats. Furthermore, these frameworks often also
suffer the weakness of not capturing the multi-dimensionality of plant invasiveness. This
inspired the current study, which aimed to inform the design of an evidence-based framework
that aids in prioritising urban green spaces and alien species for monitoring and control
interventions by carrying out a set of inter-related investigations that addressed the following
research questions: (1) What are the major environmental drivers of alien species
(particularly IAPs) distribution? (2) Are alien and indigenous plant functional diversity (FD)
and alien-indigenous co-occurrence patterns influenced by non-natural disturbance? (3) What
is the influence of non-natural disturbance on the alien and indigenous soil seed bank (SB)
floristics? and (4) Can selected seed physical and/or chemical traits be used as potential
indicators of IAP persistence in natural SBs? These research questions were addressed using
a case study approach: plants occurring within selected natural green spaces in an urban
matrix in the rapidly developing city of Durban (eThekwini Metropolitan Area [EMA]),
located within the Maputaland-Pondoland-Albany biodiversity hotspot in subtropical
KwaZulu-Natal, South Africa.
Classical vegetation survey techniques were used to identify and quantify (in terms of
richness, density and diversity) aliens within 30 natural green spaces in the study area. Levels
of non-natural disturbance were quantified using a scoring matrix, and soil SB samples were
collected from each site. Cumulatively, 80 alien plant species were identified, of which 35 are
presently categorised as IAPs in the EMA. Once it was established that IAP species richness
and density were significantly positively related to disturbance level, selected parameters
measured (viz. alien species richness and density) were used to develop an Alien Invasive
Index (AII), the utility of which was validated in terms of its ability to discriminate between
sites with low and high levels of invasiveness. The findings demonstrate the value of
integrating the data generated using vegetation surveys and Geographic Information Systems
to monitor and prioritise urban green spaces for alien control interventions. Importantly, the
results suggest that the AII could assist in identifying invasive plant hotspots within urban
areas.
In a related study, alien and indigenous floristic patterns were probed further by
comparing alien and indigenous species richness, density, diversity (alpha and functional),
and species co-occurrence levels at the 30 sites in relation to non-natural disturbance levels.
The ratio of alien to indigenous species was 1:1.5, with Asteraceae being the most dominant
family. The relationship between species richness and alpha diversity differed for alien and
indigenous species. Alien species were found to have higher FD, except for reproductive
mode. Additionally, FD was significantly related to disturbance levels, alien species richness,
and alien plant density. Co-occurrence data showed that alien-indigenous species pairs cooccur
at high levels in urban spaces. Three notable alien-indigenous pairs (Centella asiatica-
Conyza sumatrensis, Centella asiatica-Solanum mauritianum and Bidens pilosa-Commelina
erecta) co-occurred at more than 40% of sites, while two alien-alien pairs (Solanum
mauritianum-Lantana camara and Conyza sumatrensis-Tagetes minuata) co-occurred at
more than 50% of sites. The positive interactions between different alien species identified
here contribute to the growing amount of evidence that supports the Invasional Meltdown
Hypothesis (IMH). The results also showed that non-natural disturbance might lead to high
levels of alien plant species diversity and facilitatory alien-alien and alien-indigenous species
interactions. Understanding co-occurrence patterns could help design alien control
programmes that focus on reducing alien-alien facilitation.
For the study examining alien and indigenous species soil SBs in relation to nonnatural
disturbance across the 30 sites, samples collected were processed using a modified
seedling emergence method, incubated under greenhouse conditions (with irrigation), and
monitored for one year. Germinants were identified and quantified, and comparisons were
made between the soil SB and standing vegetation (SV). Cumulatively, 70 species belonging
to 20 families were identified within the SB, with a higher presence of indigenous (60%) than
alien species (40%). Overall, the SB flora was dominated by graminoids. Of the 70 species
found in the SB, 69 were shared with the SV. Irrespective of the disturbance level, indigenous
was higher than alien plant density within the SB. The findings of this study have
implications for managers of urban green spaces since alien species in SBs could exploit
niches created by disturbances, promoting urban invasions.
In the final investigation, seeds of five IAPs were buried for two years at an
experimental site to mimic seed burial within natural SBs. Additionally, the seeds of these
species were characterised in terms of selected morphological and anatomical (seed mass,
seed size, seed coat thickness) and chemical traits (estimated lipid content and changes in
lipid melting properties). This study was designed to assess whether seed physical and/or
chemical traits can be used as potential indicators of IAP persistence in natural SBs. Batches
of buried seeds were exhumed every three/six months for viability (germinated seeds + seeds
that stained positively following a Tetrazolium Chloride Test). Seed mass, size and coat
thickness differed significantly across species, with Canna indica having the biggest and
heaviest seeds of all species and the thickest seed coat. Solanum mauritianum had the
smallest and lightest seed, with one of the thinnest seed coats. Results showed decreases in
viability for four species (Canna indica, Melia azedarach, Senna didymobotrya, and Ricinus
communis), while Solanum mauritianum maintained a 100% viability throughout the
experimental period. However, while the viability of C. indica, R. communis, and S.
didymobotrya was dominated by germinable seeds before burial, as viability declined with an
increase in burial time, viability was dominated by seeds that were not germinable but stained
positively. Melia azedarach differed slightly, where viability declined with burial time, but
the majority of the seeds that were viable remained germinable. The longest ageing rate based
on P50 was observed for C. indica (lowest estimated lipid content), while M. azedarach had
the shortest ageing rate (highest estimated lipid content). Decreases in germination over time
for C. indica, R. communis, M. azedarach, and S. didymobotrya led to changes in either/both
enthalpy of melting of the lipid, and the temperature of the lipid melt, which could be related
to different seed deterioration mechanisms when buried.
This study did not identify significant relationships between the seed physical and/or
chemical traits and SB longevity using the traits investigated. This could be explained by the
relatively low number of species and traits studied and/or the phenotypic plasticity associated
with seed post-harvest physiology in wild species. Nevertheless, the study gave rise to the
beginnings of a conceptual continuum of IAP seed bank longevity, which, once populated
with data for more species, could supplement the criteria currently used for assessing species
invasiveness.
As mentioned earlier, alien plant invasions are a major challenge for developing
countries, particularly in urban settings. In countries like South Africa, the management of
IAPs is complicated by heterogeneous combinations of non-natural disturbances, site-specific
levels of disturbance intensity, and diverse vegetation types and associated species richness.
The results clearly show that IAPs thrive under high levels of non-natural disturbance in urban
green spaces and that alien and indigenous species can co-occur in various vegetation types.
The AII was able to identify alien hotspots within the study area and may represent a useful
tool for conservation practitioners/land managers. Lantana camara and Solanum mauritianum
emerged as the most problematic IAPs currently in the study area. It was also evident that
disturbance levels play a role in soil SBs of alien and indigenous species richness and plant
density, and some problematic IAPs, notably S. mauritianum, can persist in SBs for long
periods due to morphological, anatomical, and chemical traits. The results generated were
used to design a multi-disciplinary and multi-dimensional framework that aids in prioritising
sites and species for alien plant monitoring and control within urban areas. The methods
proposed for assessing the prevalence and drivers of alien plant species and the indicators
identified as potential indicators of their impacts in the framework can assist land managers,
practitioners, and researchers develop recommendations and actions for alien plant monitoring
and control in urban areas. The framework is designed to supplement and aid, and not replace,
existing EMA policies/guidelines for alien plant control by preceding their planning and
preparation phases. It could also help in developing new approaches to monitoring alien plant
species in urban spaces.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.