explained by proxies for environmental factors relating to habitat and climatic
heterogeneity, favourableness of rainfall and temperature, energy, seasonality of
rainfall and temperature metrics, and rainfall irregularity (Cowling et al. 1997).
Richardson et al. (2005) and Thuiller et al. (2006) used similar metrics to contrast
the relationship between plant species richness for native and alien species (using
SAPIA data up to 2004) with indicators of environmental and human-mediated
disturbance. We used updated distribution data for naturalised plant taxa (SAPIA
data up to 2018) and a similar range of variables to revisit this question with respect
to the alien plant species assemblages defined in Fig. 3.4. Results show that
determinants of native and naturalised species richness is similar in most zones,
although there are some interesting differences (Fig. 3.5). Native plant species
richness in the “fynbos-specific invaders” zone is strongly associated with levels
of soil water stress. Areas with low moisture stress support higher native species
richness than areas with high levels of moisture stress overall. For the “semi-arid
invaders” zone, Mean Growing Temperature (GTemp) is important for native
species richness, whereas mean Annual Precipitation and the coefficient of variation
in rainfall are important determinants of naturalised species richness. For the “grassland-specific invaders” zone, Human Footprint and Mean Temperature are important
for naturalised but not native species richness. Interestingly, patterns in naturalised
species richness in all zones is largely explained by environmental factors, and
human-mediated disturbance is not a major determinant at the QDGC scale. This
supports the results of previous research that showed that environmental drivers
predict invasion patterns at broad spatial scales, whereas disturbance is important for
explaining patterns only at the landscape scale (Rouget and Richardson 2003a, b; see
also Wilson et al. 2020, Chap. 13).
3.6 Conclusions
South Africa has a long history of plant introductions and invasions, some aspects of
which have been well documented and studied. As with all invasions, the current
biogeographical patterns offer a snapshot of the outcomes of the ongoing interplay
among many factors. These factors include the socio-historical processes that have
determined which species have been introduced, and to which sites, the traits of the
alien species, and features of the recipient ecosystems, and in many cases the multifaceted role of humans in influencing invasions. The study of the biogeography of
South African terrestrial plant invasions has been highly productive, but many questions
remain. For example, research is needed to better understand the introduction dynamics
and how processes of introduction, cultivation and dissemination interact with environmental features to shape major plant “invasion syndromes” (sensu Kueffer et al. 2013) in
South Africa. Understanding the biogeography of plant invasions is a crucial prerequisite for effective planning. In this regard, we suggest several priorities for future research.
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heterogeneity, favourableness of rainfall and temperature, energy, seasonality of
rainfall and temperature metrics, and rainfall irregularity (Cowling et al. 1997).
Richardson et al. (2005) and Thuiller et al. (2006) used similar metrics to contrast
the relationship between plant species richness for native and alien species (using
SAPIA data up to 2004) with indicators of environmental and human-mediated
disturbance. We used updated distribution data for naturalised plant taxa (SAPIA
data up to 2018) and a similar range of variables to revisit this question with respect
to the alien plant species assemblages defined in Fig. 3.4. Results show that
determinants of native and naturalised species richness is similar in most zones,
although there are some interesting differences (Fig. 3.5). Native plant species
richness in the “fynbos-specific invaders” zone is strongly associated with levels
of soil water stress. Areas with low moisture stress support higher native species
richness than areas with high levels of moisture stress overall. For the “semi-arid
invaders” zone, Mean Growing Temperature (GTemp) is important for native
species richness, whereas mean Annual Precipitation and the coefficient of variation
in rainfall are important determinants of naturalised species richness. For the “grassland-specific invaders” zone, Human Footprint and Mean Temperature are important
for naturalised but not native species richness. Interestingly, patterns in naturalised
species richness in all zones is largely explained by environmental factors, and
human-mediated disturbance is not a major determinant at the QDGC scale. This
supports the results of previous research that showed that environmental drivers
predict invasion patterns at broad spatial scales, whereas disturbance is important for
explaining patterns only at the landscape scale (Rouget and Richardson 2003a, b; see
also Wilson et al. 2020, Chap. 13).
3.6 Conclusions
South Africa has a long history of plant introductions and invasions, some aspects of
which have been well documented and studied. As with all invasions, the current
biogeographical patterns offer a snapshot of the outcomes of the ongoing interplay
among many factors. These factors include the socio-historical processes that have
determined which species have been introduced, and to which sites, the traits of the
alien species, and features of the recipient ecosystems, and in many cases the multifaceted role of humans in influencing invasions. The study of the biogeography of
South African terrestrial plant invasions has been highly productive, but many questions
remain. For example, research is needed to better understand the introduction dynamics
and how processes of introduction, cultivation and dissemination interact with environmental features to shape major plant “invasion syndromes” (sensu Kueffer et al. 2013) in
South Africa. Understanding the biogeography of plant invasions is a crucial prerequisite for effective planning. In this regard, we suggest several priorities for future research.
88
D. M. Richardson et al.