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• The integrity of aquatic ecosystems
is being challenged worldwide by
invading species.
• Affected aquatic ecosystems include
estuaries, fresh water, marine and
• Species are introduced by means of
ballast water, hull fouling,
infrastructure (canals), aquaculture
and many other pathways.
• Some examples include the Nile perch, sea lamprey, zebra
mussel and brown trout.
• Tolerant of a wide range of environments, the brown trout
continues to be stocked for recreational fisheries in rivers
and streams in the USA. The species reproduces well, and
predates or competes with many indigenous fish and
amphibian species.
A significant invasive in Europe is Leidy’s comb jelly
(Mnemiopsis leidyi). It was probably released from ballast
waters. It feeds on zooplankton, and since there were no
predators in the Black Sea, food webs were damaged, leading
to the collapse of anchovy and other fisheries – a significant
ecological as well as economic impact.
• The US receives more than 79 million
tons of ballast water from overseas
each year. Ballast tanks carry a diverse
community of organisms, resulting in
many biological invasions. Pathogens
are common in coastal waters and can
also be transferred in ballast water
(Ruiz et al, 2000).
• Ballast water is surprisingly not the
most significant pathway of IAS
introduction for the US. Stocking and
aquarium release are the most
significant pathways for fishes, with
baitfish and food importation close
Ballast water
technologies include:
Ozonation treatment
UV Irradiation
Natural Product
• Barnacles, mussels, sponges,
algae, sea squirts etc. attach
themselves to the hulls of ships,
fouling them.
• Invasions can occur when these
fouling organisms come in
contact with structures in a new
port or harbour, or release their
larvae into new waters.
• Historically hull fouling was considered a primary vector for
transporting species. However, the modern use of metal hulls
and anti-fouling paints, decreased port residency times and
faster ship speeds, contribute to reduced hull fouling (Marine
Invasions Research Lab).
• The invasion of water hyacinths in Zambia is choking lake,
river and wetland systems such that local fishing economies
are devastated and hydroelectric facilities have been
• Most of the previously forested lowland systems on the drier
sides of Hawaii are now dominated by the invasive fountain
grass (Pennisetum setaceum). This grass has severely altered
the ecosystem dynamics of these arid regions by suppressing
native vegetation and promoting fires that have proved
devastating to the native flora (Cabin et al, 2000).
• The rapid invasion and spread of alien trees and shrubs of
Acacia, Hakea and Pinus species over large areas of fynbos
in South Africa, threaten hundreds of native plant species
with extinction. They change fire and nutrient-cycling
regimes, and greatly reduce streamflow from watersheds
(Richardson, 2001).
Pathogens can undermine local food
and livestock production, thereby
causing hunger and famine. They
often damage or kill indigenous
species because these species have
no defences against them.
Chestnut blight eliminated American
chestnuts from approximately 180
million acres in eastern USA. Ten
moth species that could survive only
on chestnut trees also suffered.
Currently another introduced microbe
(Phytopthora sp.) is causing the
decline of most of the dominant
hardwood forest species along the
California coast.
Rinderpest was introduced
from Asia to Africa in 1889. The
disease travelled 5000km in 10
years, killing more than 90% of
Kenya’s buffalo population,
(Daszak et al, 2000).
Some species populations still
remain depleted or at risk.
Rinderpest also wiped out most
of the cattle in several regions,
leading to widespread famine,
huge economic losses and
social unrest.
The bubonic plague spread from central
Asia through north Africa, Europe and
China using a flea vector on an invasive
species of rat (Rattus rattus) that
originally came from India.
The viruses carrying smallpox and
measles spread from Europe into the
western hemisphere shortly following
European colonisation (McNeely, 1996).
Cholera was introduced into Peru in
1991 from Asia, from ship ballast water.
In this case it coincided with an algal
bloom, and then a redtide outbreak
(dinoflagellates, which are the
intermediate hosts of Vibrio cholerae),
and sickened more than 300 000 people.
Genetic bottlenecks
Colonization of introduced species often
involves a population bottleneck because
the number of initial colonists is often
small. However, the loss of genetic
variation through genetic drift and the
inbreeding effect of small populations are
thought to contribute to the increased
extinction rate of small populations
(Frankham and Ralls, 1998).
So, a newly established population is likely
to be much less genetically diverse than
the population from which it is derived. The
reduced genetic diversity can have two
1. Inbreeding depression may limit population growth and lower the
probability that the population will persist
2. Reduced genetic diversity will limit the ability of introduced
populations to evolve in their new environments
? ?
If population bottlenecks are harmful (often leading to extinction),
then why are invasive species that have gone through a founding
bottleneck so successful? (Allendorf and Lundquist, 2003).
Many species cannot be assigned to
either indigenous or alien status
because of their global movements
and lack of data to support or dismiss
movements of species. These are
defined as cryptogenic species
(Carlton, 1996).
Over the last 150 years, Phragmites
australis’ distribution and abundance
has increased dramatically in North
11 halotypes were native to North America, 2 being
widespread, with one of those being the most common
(ancestral type). This type replaced native types and expanded
into areas where historically P. australis did not grow.
Japanese red alga (Polysiphonia harveyi) occurs on
North American coasts of the North Atlantic. It is
regarded as an alien in the British Isles and Atlantic
Europe. Two separate invasive halotypes of P. harveyi
were discovered, which originated from two separate
Displacement of indigenous by invasive
species through hybridisation is called
‘genetic assimilation’
The invasive taxon is integrated into
the gene pool of the indigenous
species (Petit, 2004).
Should hybrid plants resulting from the
cross between an indigenous and alien
plant be termed indigenous or alien?
According to Petit (2004), hybrids that
have at least one alien parent should not be
considered as indigenous since they would
not be present without human intervention.
This is the anthropocentric view. If the
biogeographical view were accepted,
hybrid taxa would be termed indigenous.
The loss of native fish due to
hybridisation between indigenous and
alien species has been known for more
than 20 years (Miller et al, 1989).
Other hybrids also occur e.g. between
different alien species, between their
ecotypes, and between previously
isolated native species brought
together by human activity.
The mallard duck was introduced into
SA from Europe, and they are capable
of breeding with most species in their
genus. They have been known to breed
with the yellow-billed duck, which
compromises the yellow-bills’ genetic
integrity (Dean, 2000).
In the USA, the most
abundant genotype of the
alien Tamarix (which has
invaded over 600 000ha of
riparian and wetland
habitats) is a novel hybrid
between two introduced
Eurasian species (Gaskin
and Schaal, 2002).
anglica represents an example of a fixed hybrid form.
Spartina alterniflora from eastern America was introduced in
southern England, where hybridisation with the local Spartina
maritima resulted in a sterile hybrid. Chromosome doubling in
this hybrid gave rise to a new fertile species, Spartina anglica, a
vigorous and aggressive perennial plant that has actively been
colonising British and western Europe salt marshes since its
formation (Petit, 2004).
and Rogers (2001) found that an invasive tree species,
the Chinese tallow tree (Sapium sebiferum), evolved increased
competitive ability in their introduced range. Invasive genotypes
were larger than native genotypes and produced more seeds, but
they had lower quality leaves and invested fewer resources in
defending them. Thus, there are a number of reasons why
introduced genotypes may fare well even though indigenous
genotypes may be locally adapted.
Chapter 1 Definitions
Chapter 2 History, globalisation and GMOs
Chapter 3 The human dimension
Chapter 4 Pathways of introduction
Chapter 5 Characteristics of invasive alien species
Chapter 6 The ecology of biological invasions
Chapter 7 Impacts of invasive alien species
Chapter 8
Chapter 9
Chapter 10
Chapter 11
Chapter 12
I hope that you found chapter 6 informative and that
you will enjoy chapter seven!