Abstract
Early detection of emerging or sleeper weeds and monitoring of alien plant invasions forms the foundation of effective invasive alien plant management. Using a rapid roadside survey technique, we aimed to (1) establish a baseline of alien plant distribution and abundance along roads in the Garden Route, South Africa, and (2) identify priority species (particularly sleeper weeds) and priority areas to inform appropriate management action. We surveyed along 530 km of roads and recorded 109 alien and/or extralimital species across 1942 point locations. Of these, 35 species were considered to be sleeper weeds on account of displaying estimated dispersal distances distinctive of invasive plants and not being listed by the South African Plant Invaders Atlas (SAPIA) or regulated by South African legislation. Roads along natural forest and fynbos vegetation (often within the Garden Route National Park) displayed lower incidences of alien plants than those associated with degraded or transformed land, with roads along farmland associated with the highest incidences of alien plants. Roads in the Southern Cape region had more species and higher densities of alien plants than roads in the Tsitsikamma region, and a few species were exclusive to either.Conservation implications: Our inventory contributed significant new records and range extensions to SAPIA, while the identified sleeper weeds offered suggestions for species that may be considered for regulation under the National Environmental Management: Biodiversity Act of South Africa. We furthermore provided information to facilitate (1) timely management of emerging invasive alien plants, (2) prioritisation of species and areas for management action, and (3) future monitoring of alien plants in the Garden Route National Park and surrounds.
Highlights
Worldwide, invasive alien plants are considered to be a major threat to biodiversity and ecosystem services, with the threat deemed to be increasing (Maxwell et al 2016)
We identified species as sleeper weeds if a species complied with Criteria 1 and 3, or 2 and 3, of the following: (1) species has not been recorded by South African Plant Invader Atlas (SAPIA) in the study region; (2) species is not listed by NEMBA; (3) species displayed dispersal distances of > 100 m and is potentially invasive
Thirty-five of these species were not NEMBA-listed and 35 species have not been recorded by SAPIA for the study area
Summary
Invasive alien plants are considered to be a major threat to biodiversity and ecosystem services, with the threat deemed to be increasing (Maxwell et al 2016). Called sleeper weeds, are species previously thought to be benign but that may turn invasive; or species that have naturalised but not yet expanded their populations exponentially; or species currently only present in a small area but having the potential to spread widely (Boy & Witt 2013; Cunningham et al 2003; Groves 1999; Thuiller et al 2005; Williams & West 2000) These species should be closely monitored and assessed for their invasion capacity and timeous decisions made on management actions to be taken. Knowledge of the incidence of alien plants and monitoring of changes in their status underpin sound management interventions (Blackburn et al 2014; Wilson et al 2014) and as such are mandated by the National Environmental Management: Biodiversity Act (NEMBA, Act 10 of 2004) in South African protected areas (Foxcroft et al 2017; Wilson et al 2017)
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