Introduction - University of the Western Cape

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The introduction of alien species is currently one of the major threats to biodiversity, only
second to habitat destruction. Globally, 20 % of vertebrates that are in danger of
extinction are threatened directly or indirectly by invasive species (McNeely. 2001). The
introduction of species into new areas via human travel has occurred throughout historic
times. The Australian aborigines brought the dingo and Polynesians sailed with pigs, taro
and yams (McNeely. 2001). Over the last century increased global trade and travel have
allowed many species to overcome the geographical barriers that previously restricted
them (Chapin et al. 2000). Introduction of alien species can be accidental (for example
associated with cargo), natural (for example, warming allows the poleward spread of
exotic species; flooding), or intentional (for example, as pets, ornamentals, agriculture,
aquaculture). These alien species become naturalized in their new environment and may
come to dominate native ecosystems. Invasive species have a competitive edge over
native species, because these aliens do not have natural enemies (parasites and predators)
in their new environment. They frequently outcompete native species for natural
resources such as water, nutrients, space and prey on indigenous species or prey on native
species (Chapin et al. 2000). Many invasive species become pests as they cause harm to
the environment into which they are introduced. The rate of invasions seems to increase
where human disturbance to habitats create vacant niches for colonization.

The introduction of alien species into new areas has both biological and economic
application. Many species are introduced into an area for economic gain. For example,
fish introductions for sport fishing; introduction of plants for timber, food etc. However
these introductions come at a cost (economic and biological) that is often not weighted
against benefits. It is estimated that the annual cost for controlling invasives in the US is
$136 billion (McNeely.2001).

Additional notes

Alien species
Species spread (intentionally introduced or accidental or natural spread) beyond their
native geographical range, not necessarily harmful.
Synonyms: Introduced species, exotic species, non-indigenous species [9]

Biological invasion
A broad term, that refers to both human-assisted introductions and natural range
expansions [9]

Indigenous species
A species that occurs naturally in an area
Synonym: Native species [9]
Invasive species

A subset of organisms, introduced into an area, that is able to survive and reproduce in
that area, and cause economic/environmental harm within the new area. These invasive
species may displace native species and have the ability to dominate an ecosystem [9]

Naturalized species
A species introduced from a different biogeographical area, but now behaves like a native
species; because it is able to maintain itself within the new area without further human
intervention; it grows and reproduces in native communities, but does not necessarily
cause harm [9]

These are native or non-native plant species that cause injury or damage to the
environment [9]

A native or alien plant or animal species that is destructive to human health, the
environment or the economy [9]

Invasive Argentinean ants in the Fynbos Biome

Invasive Iridomyrmex humilis have displaced native ant species such as Pheidole
capensis, Anaplolepis steingroeveri and A. custodiens in the Fynbos shrubland in the
South Western Cape of South Africa (Bond and Slingsby. 1984). Myrmecochorous
Fynbos species such as Mimetes cucullatus (also Leucospermum, Leucadendron,
Paranomus, and Orothamnus) rely on ants for seed dispersal. These ants are attracted to
fat bodies, called elaiosomes, on the seeds. Indigenous ants respond rapidly to seeds that
contain elaiosomes and carry seeds some distance away from the parent plant, to their
well-protected nests before feeding on the elaiosomes. In contrast, invasive ants respond
much slower to seeds. This results in greater rates of seed predation. Invasive ants only
move seeds less than 100 mm away from the parent plant and do not take seeds to their
nests. Iridomyrmex builds superficial nests under leaves, stones or logs. As a result,
seeds are exposed to the soil surface and vulnerable to predators and fire. In addition,
newly germinated seedlings compete with parent plants for resources due to their close
locality. Mimetes seedling growth is 50 times higher in native ant sites, compared to
Iridomyrmex infected sites. The latter is primarily due to limited seed predation and
favourable nest conditions for germination in native ant sites. Bond and Slingsby (1984)
established that the effect of invasive ants on Proteaceae communities may only be
observed decades after Iridomyrmex invasion, because myrmecochorous Proteaceae
seeds may remain viable in the soil for some 15 years. Argentinean ants need to be kept
in check. The slow destruction of seed reserves could drive a large proportion of the
Cape’s Proteaceae to extinction.

Figure: Illustration of the invasive Argentinean ant (Iridomyrmex humilis)
Image credited:

Weeds in Australian rangelands

Australian rangelands constitute 70 % of the Australian continent. By the 1900’s 1500 -
2000 alien species had been naturalized in these rangelands at a rate of 10 species per
year. These weeds are of different growth forms: 50-60% is annual forbs (eg. Xanthium
occidentale, Hyptis suaveolens and Parthenium hysterophorus), 17 % are annual grasses
(eg. Themeda quadrivalvis), 12 -19 % are perennial forbs (Bryophyllum tubiflorum and
Stachytarpheta spp), 5-8 % are perennial grasses (eg. Pennisetum polystachion and
Sporobolus pyramidalis), 5 % are shrubs (eg. Mimosa pigra and Lantana camara), 1- 2
% are trees (eg. Tamarix aphylla and Acacia nilotica) (Grice. 2004)

These weeds are able to reach high densities and biomass and come to dominate the
landscape. The invasive tree Prosopsis pallida is known to reach densities as high as
1600-1800 plants per hectare. Shrubs such as Mimosa pigra reaches densities of up to 30
000 plants per hectare and annual forbs such as Parthenium hysterophorus have reached
densities of up to 14 000 plants per hectare. These weeds sequester nutrients, water and
sunlight that would otherwise be available to native species. The high densities of these
invasives may cause landscape dysfunction, because there is excessive flow of resources
out of the landscape. There are also various ramifications for food webs, especially where
specialist herbivores and detritivores are concerned. Herbivores may only feed on
specific plant species and weed detritus may vary greatly from the detritus of native plant
species (Grice. 2004).

Weeds alter fire regimes. Weeds influence fuel loads, fuel distribution and hence
influences the intensity, frequency and timing of fires. The presence of weeds decreases
native plant species richness but may influence native animal species richness positively
or negatively (Grice. 2004).

Marine Invasives: The Black Striped Mussel (Mytilopsis sallei), Australia

The Black Striped Mussel (Mytilopsis sallei) was detected in Darwin Habour (1999) in
Australia’s Northern Territory [1]. This mussel species is native to tropical and
subtropical western Atlantic waters (Colombia to the Gulf of Mexico). Introduction was
most likely due vessel hull fouling, but could also be due to fouling of aquiculture,
marine debris, and ballast [1]. Vessel hull fouling involves the growth of marine
organisms (in this case bivalve larvae) on artificial surfaces. These bivalves were able to
form dense fast growing (produces 50 000 offspring per individual and matures within
one month of settlement) monocultural mats. The latter led to a reduction in biodiversity,
fouling of wharves, seawater systems and marine farms [1]. This case study was an
example of a situation where effective management and control was taken of a situation
that could have had much more devastating effects. Invasives were effectively treated
with copper sulphate and sodium hypochlorite (total cost excluding personnel costs was
estimated at $2.2 million) [4]. If immediate action was not taken, this invasive species
could have devastated northern Australia’s pearling industry (estimated worth $225
million per annum) by colonizing pearl oyster culture structures such as nets and ropes

The study identified key methods for the fast and successful detection of marine pests,
namely developing monitoring and surveillance programs; stimulating voluntary
reporting by promoting public awareness; systematic risk management [1].

Figure 2: Illustration of the Black striped mussel (Mytilopsis sallei)
Image credited:

Additional notes
Four major methods can be distinguished for the control or eradication of aquatic
invasive species. Physical removal, involves the physical removal of the invasive species
from infested area. The latter method is usually used in the early stages of the invasion
and it may be cost and labour intensive. Ecological control, involves manipulating
environmental factors to disrupt the life cycle of the invasive species. Chemical control is
successful in controlling pest species but may be expensive. In addition, by adding
chemicals to the system, native species may also be harmed. The application or
introduction of biological control agents may be used, however these organisms may not
survive in their new environment or may themselves become invasive [4].

Invasive alien trees (Acacia mearnsii) in South Africa

Acacia mearnsii is native to Australia and was introduced into South Africa about 150
years ago as a source of bark products [2]. It has since become invasive in South Africa
and other countries such as Hawaii, China, Japan, New Zealand, France and the United
States [3]. The combating of this alien in South Africa seems to have a conflict of
interest, because these trees have economic value (provide timber and tannin)[2].
However the severity of the effects of this weed on indigenous South African ecosystems
requires us to take immediate action. It threatens conservation areas, dune ecosystems
and potential agricultural land.

A. mearnsii is able to produce large amounts of long-lived seeds (which germinate in
large masses after a fire) [2][3] and develops a monospecific crown, which shades other
vegetation [2]. Also, this invasive competes with important grazing grasses and causes a
reduction in grass cover. This severely affects rural communities in grassland areas (such
as the E- Cape, Mpumalanga, Kwazulu-Natal) because the carrying capacity of their land
is reduced. In addition the leaves and branches of A. mearnsii may have allelopathic
properties [2].
This invasive tree species is able to tap into South Africa’s already dwindling water
supply with an estimated use of 577 million cubic meters of water annually (equivalent to
$ 1.4 billion annually). Hence, water resources that would otherwise have been available
to indigenous species are used, surface runoff and streamflow is reduced [2].
Deep channeling on riverbanks results in slumping during floods; erosion and
destabilization of stream banks.
A.mearnsii increases fuel loads and as a result also fire intensity (Ehrenfeld. 2006). The
nitrogen-fixing bacteria associated with the legumes of A. mearnsii, causes increased soil
nitrogen levels. The latter makes the habitat unsuitable for indigenous species and more
susceptible to invasion by other exotic species (Ehrenfeld. 2006).

In conclusion, A. mearnsii has been shown to decrease species richness of birds and total
ground plant cover in infested areas. This invasive tree is able to displace diverse
indigenous plant communities by forming large monospecific stands.

Figure 3: Illustration of an Acacia mearnsii (Black wattle)
Image credited:

The Cane Toad (Bufo marinus) in Australia

The Cane toad is indigenous to South America, Central America, Mexico and some parts
of the Southern United States [4]. The cane toad was deliberately introduced into
Australia in 1935 as a biological control agent to stop French’s Cane Beetles and the
Greyback Cane Beetle from destroying sugar cane in Northern Queensland. The cane
toad failed in controlling cane beetles, and subsequently became invasive [4].
Bufo marinus was able to spread over most of Australia due to the fact that it is a very
fecund species (females producing 8000 – 35 000 eggs at a time, usually twice a year). In
addition, this species is extremely adaptable in that they are able to loose up to 50 % of
its body water, and tolerate salinities up to 15%. B. marinus adults are 10 – 15 cm long,
feeding on insects, marine snails, small native frogs, small snakes and mammals. They
inhabit a range of habitats (urban areas, disturbed habitats, lakes, riparian zones, natural
forests). However this invasive species competes with native amphibians for breeding
ground and food. B. marinus is toxic at all life stages (the skin produces are toxic
exudates), and frequently cause death in indigenous animals [4]. When the human body
absorbs toxins, it can induce intense pain, temporary blindness or inflammation [4].
Current research for controlling cane toads involves creating a biological control from a
native amphibian virus. This virus, which is used as a vector, would be used to transfer a
manipulated native toad gene to invasive toad genetics. This specific gene would prevent
tadpoles from maturing and hence reproducing (i.e. metamorphosis is interrupted) [4].

Figure 4: Illustration of the cane toad (Bufo marinus)
Image credited:

The invasive Red deer (Cervus elaphus)
The red deer is native to parts of Europe and was introduced into Australia, New Zealand,
North America and South America [5]. This invasive species inhabits natural and planted
forests, rangelands and grasslands, shrublands and tundra. A major effect of this invasive
species on biodiversity is that it prevents regeneration of preferred plant species (in
forests and woodlands it may cause canopy collapse) [5]. Hence changes in the structure
and composition of native ecosystems are brought about. In Chile, C. elaphus competes
with endangered endemic deer species such as Hippocamelus bisulcus and pudu, Pudu
pudu. In Argentina, invasive red deer has invaded numerous National Parks, and have
negatively influenced diversity of native fauna and flora in addition to disrupting
ecosystem processes [5]. In the highlands of Scotland, red deer prevent regeneration of
woodland by browsing. This has consequently led to the loss of extensive heather cover
(Clutton-Brock et al. 2004).

Figure 5: Illustration of the Red deer (Cervus elaphus)
Image credited:

The Chinese Mitten crab (Eriocheir sinensis)

The Chinese mitten crab has been categorized as one of the world’s top 100 invaders by
the ISSG (Invasive species specialist group), which forms part of the IUCN [7].

Eriocheir sinensis is native to the coastal rivers and estuaries of the Yellow Sea in South
West China and Korea [6]. This species is distinctive in that it has dense patches of hair
on its white-tipped claws [8]. Adults reproduce in estuaries during winter while juveniles
migrate to freshwater in spring where they may stay for up to five years. The Chinese
mitten crab has spread widely over the US (including the Great Lakes, California, and
mouth of the Mississippi River, Louisiana, and Pacific Northwest, Hawaii) and has
become well established in Northern Europe. Ways of introduction may be through the
live trade of seafood, ballast water or ship hull fouling [7].

The success of these invasives is in part due to their tolerance to a wide range of
hydrological conditions, including varying water temperatures and salinity, as well as
elevated pollutant levels [7]. Females are highly fecund, and may produce up to 1 million
eggs per reproductive event) [8].

E. sinensis is a well-known pest in rice paddies, where they consume young rice shoots.
Furthermore, they affect commercial fishing operations as they may cause damage and
clog fish nets [6]. This invasive may even be a human health concern as it serves as an
immediate host for the Asian lung fluke (Paragonimus ringeri) [8].

E. sinensis has major effects on the landscape and biodiversity in the introduced area.
These species degrade estuarine and freshwater food webs due to the fact that they
compete directly with indigenous invertebrates (juvenile Chinese mitten crabs are
herbivorous while adults are carnivorous). Successive burrowing by these animals causes
collapse of support structures, i.e. bank erosion and consequent destabilization of banks,
dikes and levees [7].

Figure 6: Illustration of the Chinese mitten crab (Eriocheir sinensis); image credited:

Figure 7: Map illustrating areas where E. sinensis has been introduced
Image credited:

Invasive Red Mangrove (Rhizophora mangle) and pickleweed (Batis maritima) in
Hawaiian wetlands

A study by Rauzon and Drigot (2001) looked at the effect of invasive Red mangrove
trees and the invasive pickleweed in Hawaiian wetlands, more specifically Nu’upia
Ponds, and historic Hawaiian fishpond complex. The Nu’upia Ponds are home to 10 % of
Hawaii’s endemic and endangered black-necked stilt (Himantopus mixicanus knudseni)
and about 16 species of native fish (Rauzon and Drigot. 2001).

Hawaii’s coastal wetlands have been remarkably changes because of activities such as
aquaculture, agriculture, grazing and urban development, and have consequently become
extremely vulnerable to alien species encroachment. Polynesian settlers changed plant
succession by the introduction of taro (Colocasia), seaweed and the creation saltpans. In
addition, during the late 19th and 20th century cattle grazing in the Mokapu Peninsula
caused erosion and sedimentation, creating mudflats that have since been extensively
colonized by alien vegetation (Rauzon and Drigot. 2001).

Pickleweed (Batis maritima) was introduced into Hawaii around 1859 from South
America. Today, these salt-tolerant plants dominate the shores of Nu’upia Ponds by
forming thick low growing vegetation mats. Pickleweed exclude shorebirds and
waterbirds from feeding and nesting on the mudflats. Extensive pickleweed cover also
provides cover for alien mammalian predators, which prey on native birds (Rauzon and
Drigot. 2001).

Humans and livestock first introduced red mangrove into Hawaii in 1902 as a way of
controlling extensive erosion that had occurred after the destruction of coastal vegetation.
The Red mangrove subsequently reached the Nu’upia fish complex by 1970 (Rauzon and
Drigot. 2001). By 1977 Red mangrove was estimated to constitute as much as 32 % of all
estuarine intertidal habitats in Hawaii. In Hawaii, the Red mangrove grows as robustly as
in its native range (propagules survive at a significantly higher rate due to a lack of seed
predators), changing patterns of succession and energy flow in infested sites. Due to the
relatively recent introduction mangroves (± 100 years), native fauna have not been able to
take advantage of the high productivity (detritus-based) that this vegetation provides.
Consequently, native fauna give way to exotic fauna, which are pre-adapted to
mangroves. Invasive Red mangroves decreases water circulation, increases algal
production, creates anoxic water conditions, and increases temperature, salinity and water
acidity. During the past 30 years, red mangrove establishment has aided egret (Babulcus
ibis) and heron use of the Nu’upia Ponds and increased the threat of predation on
endangered stilts (Rauzon and Drigot. 2001).

Systematic pickleweed and Red mangrove control programs were initiated during the
1980’s. This involved using Amphibious Assault Vehicles (for pickleweed control) and
volunteer labourers manually cutting down mangrove trees (using chain saws, shovels
and lopping shears). Current research for mangrove control involves developing a
biological control agent such as the alien beetle (Poecilips or Coccotrypes), which
reduces the production of viable Red mangrove seeds (Rauzon and Drigot. 2001).

Figure 8: Illustration of the Red mangrove tree (Rhizophora mangle)
Image credited:

Figure 9: Illustration of the propagules of Rhizophora mangle
Image credited:

The Local and Global effects of invasive species on biodiversity (Is biodiversity
increasing or decreasing?)

As a biodiversity issue, it is not always possible to say that invasive species are inherently
bad. Alien invasives play a central role in human welfare in virtually all parts of the
world. The majority of our dietary needs are met by species that have introduced from
elsewhere. What does this apparent globalization of species have on biodiversity? The
consensus seems to differ depending on the scale that the matter is viewed at. As an
example, the central European flora has undergone an increase in biodiversity over
historical time, mainly due to anthropogenic plant invasions. After the introduction of
Tilapia grahami as a biological control agent (to control mosquito’s) into Lake Nakuru in
1961, the ecosystem has transformed from one of low diversity to one of significantly
higher diversity. According to McNeely (2001) such a dramatic increase in species
numbers at a local level, is in part at the expense of native species. The loss of native
species reduces species and genetic diversity at a global scale.

According to Davis (2003) homogenization is not synonymous to low diversity. He used
coral reefs as an example and argues that these diverse ecosystems are more dispersal
than niche driven. Reefs are homogenized due to high dispersal rates. However the
diverse coral and reef fish communities are fundamentally the product of these high
dispersal or invasion rates. This matter is clearly viewed at a local community level. The
diversity at a metacommunity level is reduced due to the fact that abundant and
widespread species drive rare and local species to extinction (Davis. 2003).
Peer Reviewed References

Davis M (2003) Biotic Globalization: Does Competition from introduced species
Threaten Biodiversity? BioScience 53: 481-488

Bond W, Slingsby P (1984) Collapse of an ant-plant mutualism: The Argentine ant
(Iridomyrmex humilis) and myrmecochorous Proteaceae. Ecology 65:1031-1037

Chapin F, Zavaleta E, Eviner V, Naylor R, Vitousek P, Reynolds H, Hooper D, Lavorel
S, Sala O, Hobbie S, Mack M, Diaz S (2000) Consequences of changing biodiversity.
Nature 405: 234-242

Clutton- Brock T, Coulson T,Milner J (2004) Red deer stocks in the Highlands of
Scotland. Nature 429:261

Ehrenfeld J (2006) A potential novel source of information of screening and monitoring
the impact of exotic plants on ecosystems. Biological Invasions 8: 1511-1521

Grice A (2004) Weeds and the monitoring of biodiversity in Australian rangelands.
Austral Ecology 29: 51-58

McNeely J (2001) Invasive species: a costly catastrophe for native biodiversity. Land Use
and Water Resources Research 1: 1-19

Rauzon M, Drigot D (2001) Turning the tide: the eradication of invasive species. IUCN
SSC Invasive Species Specialist Group, Cambridge p240-248

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