been termed human commensals). We do, however, discuss how native species
interactions are impacted as alien species are integrated into the communities they
invade. Lastly, we discuss the future directions for biotic interaction research on
biological invasions in South Africa.
Box 14.1 Human Commensals
Humans are clearly a fundamental driver and mediator of invasions. Alien
species, by definition, owe their presence in an area to human-mediated
introduction (Richardson et al. 2000c). But humans play crucial, often dominant, roles at all stages of the introduction-naturalisation-invasion continuum
(sensu Richardson and Pyšek 2012) by, among other things: mediating potential abiotic barriers; reshuffling the biotas of ecosystems (thereby potentially
mediating biotic barriers posed through competition and other factors); affecting within-region dispersal in many ways (through accidental dispersal during
human activities, cultivation and propagation, e.g. for aquaculture, mariculture, ornamental horticulture, forestry, and the pet trade); and by attempts to
manage these species. The roles of humans in disseminating alien species in
different taxonomic groups, and the diverse effects of people in mediating the
abundance, distribution and impacts of these species are detailed in other
chapters of this book (see Potgieter et al. 2020, Chap. 11; Faulkner et al.
2020, Chap. 12). One aspect that requires attention here, however, relates to
urban environments and the confinement of many alien species to such
environments. This implies that many alien species directly or indirectly
interact with humans to ensure their existence in urban environments. Urban
ecosystems in South Africa are, like those elsewhere in the world, hotspots for
the arrival of alien species. These ecosystems also have the highest species
richness of alien taxa of all habitats. Reasons for this include the demand for
alien species for many purposes, high levels of propagule pressure, concentrated opportunities and conduits for dispersal, high levels of disturbance, and
the diversity of habitats and niches provided by human activities. Many
widespread invasive alien species in mainland South Africa are virtually
confined to urban ecosystems. For example, bird invasions in South Africa
are unusual in that all seven alien species with viable populations are strongly
commensal with humans and none of the 48 alien bird species in South Africa
has established viable populations in natural ecosystems (Richardson et al.
2011; Measey et al. 2020, Chap. 5). This is an ongoing natural experiment, but
evidence suggests that while human-built environments provide many opportunities for alien birds, combined biotic and abiotic pressures present a strong
barrier to the invasion of natural ecosystems in South Africa. Most invasive
mammals in South Africa are also strongly commensal with humans; examples include the Grey Squirrel (Sciurus carolinensis), the House Mouse (Mus
musculus), the Norway Rat (Rattus norvegicus), the Black Rat (Rattus rattus),
(continued)
396
J. J. Le Roux et al.
interactions are impacted as alien species are integrated into the communities they
invade. Lastly, we discuss the future directions for biotic interaction research on
biological invasions in South Africa.
Box 14.1 Human Commensals
Humans are clearly a fundamental driver and mediator of invasions. Alien
species, by definition, owe their presence in an area to human-mediated
introduction (Richardson et al. 2000c). But humans play crucial, often dominant, roles at all stages of the introduction-naturalisation-invasion continuum
(sensu Richardson and Pyšek 2012) by, among other things: mediating potential abiotic barriers; reshuffling the biotas of ecosystems (thereby potentially
mediating biotic barriers posed through competition and other factors); affecting within-region dispersal in many ways (through accidental dispersal during
human activities, cultivation and propagation, e.g. for aquaculture, mariculture, ornamental horticulture, forestry, and the pet trade); and by attempts to
manage these species. The roles of humans in disseminating alien species in
different taxonomic groups, and the diverse effects of people in mediating the
abundance, distribution and impacts of these species are detailed in other
chapters of this book (see Potgieter et al. 2020, Chap. 11; Faulkner et al.
2020, Chap. 12). One aspect that requires attention here, however, relates to
urban environments and the confinement of many alien species to such
environments. This implies that many alien species directly or indirectly
interact with humans to ensure their existence in urban environments. Urban
ecosystems in South Africa are, like those elsewhere in the world, hotspots for
the arrival of alien species. These ecosystems also have the highest species
richness of alien taxa of all habitats. Reasons for this include the demand for
alien species for many purposes, high levels of propagule pressure, concentrated opportunities and conduits for dispersal, high levels of disturbance, and
the diversity of habitats and niches provided by human activities. Many
widespread invasive alien species in mainland South Africa are virtually
confined to urban ecosystems. For example, bird invasions in South Africa
are unusual in that all seven alien species with viable populations are strongly
commensal with humans and none of the 48 alien bird species in South Africa
has established viable populations in natural ecosystems (Richardson et al.
2011; Measey et al. 2020, Chap. 5). This is an ongoing natural experiment, but
evidence suggests that while human-built environments provide many opportunities for alien birds, combined biotic and abiotic pressures present a strong
barrier to the invasion of natural ecosystems in South Africa. Most invasive
mammals in South Africa are also strongly commensal with humans; examples include the Grey Squirrel (Sciurus carolinensis), the House Mouse (Mus
musculus), the Norway Rat (Rattus norvegicus), the Black Rat (Rattus rattus),
(continued)
396
J. J. Le Roux et al.