Union for Conservation of Nature (IUCN). Significant impact on a species is more
likely when a pathogen is evolutionarily novel to a susceptible host species, which
most invasive diseases are. Indirect harm is also an important effect, as one host
species may act as a parasite reservoir for another more vulnerable species (Castro
and Bolker 2005; Gerber et al. 2005). A quarter of the IUCN’s “world’s worst”
invasive alien species are associated with the spread of wildlife diseases with
negative environmental effects (Hatcher et al. 2012). Conversely, when population
numbers of predators or competing species are reduced by parasites, it is to the
benefit of other species that can increase in number due to reduced pressure. A
combination of the above effects can subsequently cause vegetation and landscape
changes in an area invaded by a parasite. However, it is often extremely difficult to
predict or assess what damage is occurring. This is because the disease may be a
slowly progressing type, leaving the animal enough time to reproduce, so that
population effects may not manifest, or will manifest only over a long period. It is
clear, however, that the introduction of a parasite into an ecosystem can have wideranging effects comparable to the introduction of any other invasive species.
South Africa is known for its unique biodiversity, and as one of the regions where
certain ecosystems and populations of wildlife species are conserved and protected.
Diversity itself can act as a buffer against threats such as infectious disease, although
there is also the potential for large-scale disease spread where swaths of similar
species exist together. It is therefore evident from the effects discussed above that
invasive diseases could have a major impact in our region. In fact, invasive diseases
have, as we will show, had a substantial impact on the ecology, economy and people
of South Africa. However, it is important not to lose sight of the fact that disease
knows no borders, and should ideally also be considered in a multiscale context.
The introduction of almost any pathogen into a previously naïve ecosystem is
easily facilitated by the increasing trend of international and local human and animal
or animal product movement. The rate of spread for many pathogens would be
partially a function of this travel, and may be slow should travel or trade become
restricted in future. It is highly likely that many novel pathogens have been introduced, even repeatedly, into South Africa, but did not invade. For a pathogen to
progress from introduction to epidemic, the right conditions must be present. Firstly,
susceptible host species must be present in the new ecosystem. Then, sufficient
quantities of the pathogen must be excreted by an infected host for a sufficient time,
and in an appropriate manner, to facilitate transmission to naïve hosts. For this to
happen, there must be a large enough host population with adequate contact rates
between individuals. Further advantages are experienced by adaptable pathogens
that can evolve to infect multiple host species (Jones 2007).
A pathogen that successfully invades in a new geographic area may progress from
causing an outbreak to establishing itself permanently. In epidemiology, a disease
that is maintained in a certain population without needing to be re-introduced is
known as endemic (Centers for Disease Control and Prevention 2012). One may
assume that a high population of susceptible hosts and a high transmission rate
would increase the likelihood of an invasive disease becoming endemic. However,
highly virulent pathogens which produce many copies of themselves for
10 Pathogens of Vertebrate Animals as Invasive Species: Insights from South Africa
251
likely when a pathogen is evolutionarily novel to a susceptible host species, which
most invasive diseases are. Indirect harm is also an important effect, as one host
species may act as a parasite reservoir for another more vulnerable species (Castro
and Bolker 2005; Gerber et al. 2005). A quarter of the IUCN’s “world’s worst”
invasive alien species are associated with the spread of wildlife diseases with
negative environmental effects (Hatcher et al. 2012). Conversely, when population
numbers of predators or competing species are reduced by parasites, it is to the
benefit of other species that can increase in number due to reduced pressure. A
combination of the above effects can subsequently cause vegetation and landscape
changes in an area invaded by a parasite. However, it is often extremely difficult to
predict or assess what damage is occurring. This is because the disease may be a
slowly progressing type, leaving the animal enough time to reproduce, so that
population effects may not manifest, or will manifest only over a long period. It is
clear, however, that the introduction of a parasite into an ecosystem can have wideranging effects comparable to the introduction of any other invasive species.
South Africa is known for its unique biodiversity, and as one of the regions where
certain ecosystems and populations of wildlife species are conserved and protected.
Diversity itself can act as a buffer against threats such as infectious disease, although
there is also the potential for large-scale disease spread where swaths of similar
species exist together. It is therefore evident from the effects discussed above that
invasive diseases could have a major impact in our region. In fact, invasive diseases
have, as we will show, had a substantial impact on the ecology, economy and people
of South Africa. However, it is important not to lose sight of the fact that disease
knows no borders, and should ideally also be considered in a multiscale context.
The introduction of almost any pathogen into a previously naïve ecosystem is
easily facilitated by the increasing trend of international and local human and animal
or animal product movement. The rate of spread for many pathogens would be
partially a function of this travel, and may be slow should travel or trade become
restricted in future. It is highly likely that many novel pathogens have been introduced, even repeatedly, into South Africa, but did not invade. For a pathogen to
progress from introduction to epidemic, the right conditions must be present. Firstly,
susceptible host species must be present in the new ecosystem. Then, sufficient
quantities of the pathogen must be excreted by an infected host for a sufficient time,
and in an appropriate manner, to facilitate transmission to naïve hosts. For this to
happen, there must be a large enough host population with adequate contact rates
between individuals. Further advantages are experienced by adaptable pathogens
that can evolve to infect multiple host species (Jones 2007).
A pathogen that successfully invades in a new geographic area may progress from
causing an outbreak to establishing itself permanently. In epidemiology, a disease
that is maintained in a certain population without needing to be re-introduced is
known as endemic (Centers for Disease Control and Prevention 2012). One may
assume that a high population of susceptible hosts and a high transmission rate
would increase the likelihood of an invasive disease becoming endemic. However,
highly virulent pathogens which produce many copies of themselves for
10 Pathogens of Vertebrate Animals as Invasive Species: Insights from South Africa
251