Secondary invasion and weedy native species dominance after clearing invasive alien plants in South Africa: Status quo and prognosis

Mlungele M Nsikani,Sjirk Geerts,Sheunesu Ruwanza,David M Richardson

doi:10.1016/j.sajb.2020.05.009

Abstract

Clearing invasive alien plants often facilitates secondary invasion and/or weedy native species dominance instead of native biodiversity recovery. Secondary invasion and/or weedy native species dominance in turn can present significant barriers to restoration by hindering the recovery of key native species. The problem of secondary invasion and weedy native species dominance is ubiquitous and well appreciated globally, but poorly understood in the context of restoration ecology in South Africa. This study uses a two-pronged approach – a literature review plus an expert workshop – to evaluate the knowledge on secondary invasion and/or weedy native species dominance after clearing invasive alien plants in South Africa. Focus is placed on the definition, habits, biomes, target invaders, factors leading to, effects and management of secondary invasion and/or weedy native species dominance. Results suggest that secondary invasion and/or weedy native species dominance are often observed after clearing target invaders but is seldom reported, focused on, identified by name and/or correctly defined. The occurrence of secondary invasion and/or weedy native species dominance is not biome specific and is mediated by factors such as soil physico-chemical legacies of target invaders, availability of propagules in the soil seed bank and surrounding areas, and side effects of the technique used to clear target invaders. Ferns, grasses, herbs, sedges, shrubs, and trees can be secondary invaders and/or weedy native species. Few or no management interventions currently target secondary invasion and/or weedy native species dominance in South Africa. Given the paucity of knowledge on secondary invasion and/or weedy native species dominance in South Africa, there is clearly a need for more research. Practitioners should integrate the management of secondary invasion and/or weedy native species dominance with their overall invasive alien plant clearing efforts. Relevant steps should be taken to include mechanisms and incentives of dealing with secondary invasion and/or weedy native species dominance in the policy on invasive alien plants in South Africa.

Highlights

Invasive alien plants commonly exert negative ecological impacts on the ecosystems they invade, disrupt ecosystem services and impose a significant cost to the global economy (Pejchar and Mooney, 2009; Vila et al, 2010, 2011)
We aim to (1) determine how secondary invasion and/or weedy native species dominance have been defined in South Africa; (2) identify habits of secondary invaders and/ or weedy native species; (3) identify biomes where secondary invasion and/or weedy native species dominance have been observed; (4) identify the target invaders that were cleared before secondary invasion and/or weedy native species dominance; (5) determine the factors that facilitated secondary invasion and/or weedy native species dominance; (6) determine the effects of secondary invasion and/or weedy native species dominance; (7) identify management approaches for secondary invasion and/or weedy native species dominance and evaluate their outcomes; and (8) provide management and research recommendations on secondary invasion and/or weedy native species dominance
Many aspects are only described by a few studies and there is a strong bias in favour of a few biomes, notably the fynbos. We believe that this is in no way an affirmation that ecosystems in South African are less at risk from secondary invasion and/or weedy native species dominance than those in countries such as Australia and the United States of America (USA) where the phenomenon has been more comprehensively documented (Reid et al, 2009; Abella, 2014; Nsikani et al, 2018b)

Summary

Introduction

Invasive alien plants commonly exert negative ecological impacts on the ecosystems they invade, disrupt ecosystem services and impose a significant cost to the global economy (Pejchar and Mooney, 2009; Vila et al, 2010, 2011). Management of invasive alien plants is necessary to mitigate these negative impacts (ordinarily this involves their removal from invaded ecosystems) and promote native biodiversity recovery (Hulme, 2006; Pysek and Richardson, 2010). It is often assumed that native biodiversity recovery will follow the removal of invasive alien plants from invaded ecosystems (Wittenberg and Cock, 2005; Blanchard and Holmes, 2008). Invasive alien plant management is inherently complex, and it is well known that the removal of invaders may not always translate to full or even partial recovery of native biodiversity (Zavaleta et al, 2001; Pearson et al, 2016; Mangachena and Geerts, 2017, 2019). Nsikani et al / South African Journal of Botany 132 (2020) 338À345

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Conclusion