Towards understanding alien floristics within an urban matrix: the case of Durban, South Africa.

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 asiaticaConyza 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.