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The alignment of projects dealing with wetland restoration and alien control: A challenge for conservation management in South Africa

http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-23532022000100010

Erwin J.J. SiebenI; ŞerbanProcheşI; Aluoneswi C. MashauII; Moleseng C. MoshobaneII, III

ISchool of Agricultural, Earth, and Environmental Sciences, University of KwaZulu-Natal, Durban, South Africa

National Herbarium, South African National Biodiversity Institute (SANBI), Pretoria, South Africa

Department of Biology, Sefako Makgatho Health Sciences University, Pretoria, South Africa

ABSTRACT

An inventory of wetland vegetation across the country generated a list of the most common invasive alien plants across South Africa. Many of the plants on that list do not correspond with the priorities in the programmes for alien control across the country, as they are not listed on a government produced list that guides the priorities for alien control. We explore the reasons for this situation. We argue that because wetlands are such important parts of the landscape, invasive aliens in wetlands are of special concern, and there should be more alignment between alien control programmes and wetland rehabilitation programmes. This alignment starts by considering the full number of species that form a threat to wetland habitats, but also considers which pesticides to use, erosion and recolonisation in wetlands, planting indigenous vegetation after aliens have been removed, and strategising by working from upstream to downstream. Existing alien control programmes for specific grasses (some relatively new to the country and in the phase of early detection) and floating aquatic plants may guide how to tackle the invasions of grasses and forbs that have been established in South African wetlands for an extended period of time.
SIGNIFICANCE:
• Wetlands have a distinct set of alien invasive plants that affect their ecology and functioning and many of these plants are not listed as priorities in alien control programmes.
• Many restoration projects have an element of removing invasive plants and revegetating. Wetland restoration and alien control need to be integrated to preserve water resources.

Keywords: invasive species, Expanded Public Works Programme, ecological restoration, revegetation, wetland plants

Introduction

Two of the biggest conservation challenges that South Africa faces are the control of invasive alien plants and the prevention of the loss of wetland resources against land degradation. The government invests heavily in programmes that address these challenges while employing large numbers of low-skilled workers in public works programmes.^1,2 These programmes profess to pursue the same goals, which are the protection of biodiversity and enhancement of ecosystem services, which, in the context of a semi-arid country, revolve for a large part around protecting the condition of water resources. It is therefore surprising to find that there is little alignment between the programmes that aim to restore wetland habitats and those that remove invasive alien plants from them as most of the species that are targeted for alien control do not correspond with the invasive species that are commonly found in wetlands. Therefore, many of the invasive species that particularly affect wetland ecosystems are considered to have a low priority in alien control programmes.

Many invasive plant species are known to thrive in habitats that are regularly disturbed, such as riverbanks, which are naturally subject to regular flooding and removal of vegetation, and rivers also aid in the dispersal of seeds.³ Rivers and their immediate environments are often among the worst invaded parts of a landscape,⁴ and many of the efforts in alien control have focused on these areas. Therefore, there is an awareness of the connection between drainage networks and alien invasion, but this has not yet translated into systematic on-site control of invasive aliens in wetlands, when we recognise wetlands as distinct habitats within the drainage network, that are different from riverbanks. Invasive aliens growing along riverbanks are known to use excessive amounts of water and this is one of the reasons for their systematic removal from important catchments. They have their own dynamics; and guidelines for managing invasive alien plants in these areas are well developed.^5,6

Wetlands in this study are defined as any terrestrial area where water is present at or close to the surface area for at least part of the year, the depth of which is never higher than 6 m.⁷ This definition includes seepages, rivers, shallow lakes and saline pans, which are all areas that are inundated for extended periods of time and this inundation represents the main stress on vegetation. However, riverbanks are high-energy disturbance-prone environments that often do not get inundated for prolonged times and therefore not all riparian corridors can be regarded as wetlands. Wetlands are limited in extent but have a disproportionate role in many landscape-level ecosystem processes, often being referred to as the ‘kidneys of the landscape’ with reference to their role in biogeochemical cycles.⁸ Invasions in these parts of the landscape should therefore be regarded as having high consequences for the landscape as a whole, even though the invaded areas may be limited in size due to the restrictions imposed by the size of the wetland. There are specific groups of plants that thrive in wetland environments that are defined as such, and these are not necessarily the same species as those that thrive on riverbanks.

When biological invasions take place in wetland environments, it should be a priority to understand what impact they have on wetland functioning and on the broader water cycle in the surrounding landscape. Wetlands are positioned in such a way in the landscape that it is nearly certain that invasions in them will have high social and ecological consequences⁹, and this has implications for strategic planning around invasions in wetlands¹. Species that are capable of surviving and establishing in a wetland environment need to have a certain number of morphological and physiological adaptations, but many of the traits required for this survival are also traits that benefit the invasiveness of organisms (clonal growth, large numbers of light seeds, high rate of resource capture¹⁰) and it is therefore expected that there will be a subset of the invasive species in South Africa that is particularly successful in invading wetland habitats. It also means that combating alien invasives in these habitats must not affect indigenous species with similar properties.

In this paper, we provide an overview of the problem of alien invasive species and how in particular they affect wetland ecosystems, with regard to species composition and ecological restoration. We focus on which species are mostly problematic in wetland habitats, speculate on why they may have been neglected in alien eradication programmes, and discuss some general problems that we have to face when dealing with invasion in wetland habitats.

Species that invade wetland habitats

Within the group of plants that have invaded South Africa so far, a limited number of species have created such problems that they have been prioritised for clearing and targeted for alien control.¹¹ Which species are included on the list of invasive species targeted to this effect, represents a critical decision within every integrated plan for control of invasives.^12,13 Legislation around alien control in South Africa’s natural ecosystems is provided by the National Environmental Management: Biodiversity Act No. 10 of 2004 (NEMBA). The invasive species that are targeted for alien control programmes are listed in the NEMBA legislation, whereby the species are categorised in different classes, with different strategies to prevent their further spread and their eventual eradication.¹⁴ If wetland species are well represented on this NEMBA list, we would expect the majority of the most common alien invaders in wetlands to feature prominently on this list, as widespread occurrence of an alien invader is a suitable indicator for the level of threat that they pose.

The National Wetlands Vegetation Database provided the data on which this assessment of invasion of wetland habitats by invasive aliens is based.¹⁵ Firstly, the most common invasive species in wetlands are listed and ranked in Table 1. Here it can be clearly seen that many of the most common species in this database are not listed in the NEMBA list of species. When all alien plant species (whether invasive or not) found in the wetlands of South Africa are cross-referenced with the NEMBA list for prioritisation for alien control, it appears that only a fraction of the invasive alien species in wetlands are listed as priorities for eradication and that this fraction is independent of their frequency of occurrence (Table 2).This can be tested with a chi-square test and it appears that the likelihood of a species to be found in wetlands and the likelihood for a species to be listed on NEMBA are independent of each other and therefore that the most important wetland invasive alien species are underrepresented when it comes to priorities in alien control.