• Biological invasions represent one of the leading threats to natural ecosystems and biodiversity, second only to habitat transformation.

• Alien species affect native species and ecosystems in numerous ways, with effects that are usually irreversible.

http://www.angelfire.com/nt/57pipesanddrums/57.html

Impacts of invasive species

• The impact of invasive species is difficult to measure.

• Parker et al (1999) devised the following equation to measure impact:

• Range and/or abundance are often used individually to infer impact but there are flaws with this: it is assumed that impact increases with range size, but impacts on an island can be greater than those on the mainland. Impact is also assumed to increase with abundance but this relationship is not always linear or true for all species.

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ECOLOGICAL IMPACTS

I (impact) = R (range) x A (abundance) x E (effect)

This equation, though simplified, gives the best representation of quantification of impact that can be compared between species.

Ecological impacts

• NB impact, rather than establishment that threatens indigenous species. The impact of an invader can be measured at 5 levels:

1. Effects on individuals:

ECOLOGICAL IMPACTS cont.

changes in demographic rates, viz. mortality, and reduced growth or reproduction

morphology of individual organisms may also change

behavioural impacts occur when individuals change habitat use or activity patterns in response to new predators or competitors

Effects on individuals

2. Genetic effects – either direct, as a result of altered patterns of natural selection or gene flow within indigenous populations, or indirect, through hybridization and introgression.

3. Population dynamic effects – impact on the abundance and dynamics of particular indigenous species, the most extreme being complete extinction.

http://www.wmnh.com/wmiab07.htm

ECOLOGICAL IMPACTS cont.

e.g. when gypsy moth outbreaks occur in North America, the white-footed mouse populations that feed on them also peak, increasing tick populations - consequentlythe incidence of lyme disease also peaks

Genetic & population dynamic effects

• Alien species have contributed to the decline of 42% of US endangered and threatened species (Schmitz and Simberloff, 1997). They are the main cause of recent bird extinctions. Over the last 500 years, alien spp. have been partly or wholly responsible for the extinction of at least 65 bird species. Island species are particularly susceptible (Birdlife International, 2004).

ECOLOGICAL IMPACTS cont.

Alien birds

4. Community effects – high profile invaders are assumed to cause overall reductions in biodiversity. However, many invasions may actually increase total species richness

5. Effects on ecosystem processes – changes in species composition (additions of a new species, substitution of the invader for one or more indigenous species, or loss of indigenous species diversity) and disturbance regimes (Parker et al, 1999).

ECOLOGICAL IMPACTS cont.

Community & ecosystem processes

• Aliens can promote or suppress fire, thereby changing the disturbance regime leading to sometimes dramatic effects on the indigenous plants.

ECOLOGICAL IMPACTS cont.

• Aliens can be discrete-trait invaders (DTIs) or continuous-trait invaders (CTIs). DTIs add one or more new functions to an ecosystem (e.g. improvig nutrient levels in nutrient-poor soils), CTIs differ slightly from indigenous species in providing one or more functions that already exist (i.e. no new functions)

Chapin et al (1996).

• Transformer species: Aliens can use resources excessively eg. water, light and oxygen so that they are no longer available for the indigenous plants. They can also add resources eg. nitrogen to the soil.

Transformers, DTIs & CTIs

• Disturbance regimes: Aliens either introduce, enhance or suppress events such as fire and erosion, or are themselves agents of disturbance – disrupting soil or sediment, or killing a significant amount of the community in a relatively short period of time (Mack and D’Antonio, 1998).

• The spread of fire-adapted plants that burn easily increase the frequency and severity of fires, to the detriment of property, human safety, and indigenous fauna and flora (Schmitz and Simberloff, 1997). Fire is usually more intense when burning in alien vegetation. Remember the 2000 fires in Cape Town?

ECOLOGICAL IMPACTS cont.

Disturbance regimes

Alteration Example

Fire enhancement Grasses increase rates of fire frequency, spread, and/or spatial extent

Fire suppression Trees decrease fine fuel load and fire spread in grassland or open woodland

Increased erosion Trees increase stream bank collapse in riparian zones

Decreased erosion Plants stabilize mobile substrates

Increased biotic disturbance Pigs increase soil disturbance

Increased biotic disturbance Pests or pathogens cause stand-level or population-level dieback

Change in consequences of disturbance

Invader changes susceptibility of community to physical forces of disturbance

Table 1. How invaders alter disturbance regimes (Mack and D’Antonio, 1998)

Invaders alter disturbance regimes

For example, here follow some effects of alien species in aquatic environments:

• changes in resource competition (food, space, spawning areas)• physical changes in habitat (reduced water movement, erosion,

alteration of substrate)• limitation of resources (nutrients, light, oxygen)• detrimental changes in trophic web due to introduction of new

functional group• harmful algal blooms• genetic effects on indigenous species (hybridisation, change in

gene pool, loss of indigenous genotypes)• drastic reduction of population size or even extinction of

indigenous species (BioMare, 2004).

ECOLOGICAL IMPACTS cont.

Aquatic examples

Feedback loops: Grass invasions can initiate and maintain a grass-fire feedback system that prevents the regeneration of indigenous woody species over large areas. The cycle can begin after purposeful land clearing or after the invasion of woodlands/shrublands by alien grasses.

ECOLOGICAL IMPACTS cont.

woody vegetation

land clearing fire

selection for grass

grassland/savanna

microclimate change

introduction of alien grasses

and/or

Feedback loops

• Globalisation brings many new medical threats. Global environmental and climate change is expected to lead to range expansion of many viruses potentially dangerous to humans.

• Chapter 6 covered much about parasites and pathogens that can affect human (and animal) health and well-being.

• E.g. In India, introduced black rats bite ± 20 000 people per year, resulting in hospital admittance (Pimentel et al, 2001).

SOCIAL IMPACTS

• Human health can also be impacted by species that pollute (decompose or release chemicals into) water resources. Reduced volumes and quality of water and food crops due to many alien pests can cause famine and starvation.

Social impacts

• Many indigenous spp. are used in cultural practices and for medicinal purposes. Losses of these species due to invasive aliens would be a social or cultural impact.

• Benefits and costs of introductions are unevenly distributed among ecosystems, within and across regions, among sectors of society, and across generations. Although an introduction may meet a desired objective in an area, at a certain time and for some sectors of society, unwanted and unplanned effects may also occur (Ewel et al, 1999).

• The relative magnitude of costs and benefits vary in space and time. eg. In SA, some Pinus spp. are commercially important, but cause expensive problems when they spread from plantations into watersheds and conservation areas (Richardson and Higgins,

1998)

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SOCIAL IMPACTS cont.

Social impacts cont.

• Measuring the economic impact of alien species often assists managers and funding organisations to determine priorities for control – the species causing the most economic damage or loss are often the first to be controlled, or at least assessed for control.

• Alien species invasions cause more that US$ 314 billion per year in damage around the world. Many invasive alien species have caused major economic losses in agriculture and forestry (Pimentel et al 2001).

http://www.abesy.de/web/consult1.html

ECONOMIC IMPACTS

Economic impacts

However, many of the impacts relate to change in ecosystem structure, or extinction of individual species, which are very difficult to attach monetary values to.

http://www.kolkatabirds.com/page16.htm

ECONOMIC IMPACTS (Pimentel et al, 2001).

Weeds in US agriculture cause 12% reduction in potential crop yields.It is likely that non-native weedsresult in US$ 27.9 billion in losses.

In South Africa, alien weeds cause total crop losses of about US$ 1.5 billion per year

The house sparrow and the european starling together cause ± US$ 1 billion per year in crop damages in the USA.

Pest insects and mites cause about US$ 16 billion in crop losses in the USA each year.

Crop losses caused by introduced insect and mite pests in South Africa are ± US$ 1 billion per year.

Crop losses in the USA due to alien plant pathogens come to ±US$ 23.5 billion per year.

± 85% of plant pathogens attacking crops in SA are thought to be introduced: US$ 1.8 billion per year in crop losses.

Examples economic impacts

• European purple loosestrife in the US, has changed the structure of most the wetlands it has invaded. ± US$ 45 million is spent on control of purple loosestrife each year (Pimentel et al, 2001).

• Water hyacinth is a major weed in South Africa, where it is reducing already scarce water resources (Richardson et al, 1997). More than US$ 25 million is spent per year on control of water hyacinth.

• More than US$ 5 million per year is spent on preventing future alien plant invasions in South Africa (van Wilgen et al, 1998).

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Loosestrife & Hyacinth

• Domestic mammal introductions include dogs, cats, cattle, horses, sheep, pigs and others. Other introduced species are the house mouse, European rabbit, brown rat and the black rat.

• In the US, it is estimated that an individual adult rat causes US$ 15 in damage per year; so with approximately 1.25 billion rats, the cost is astounding. Rats are also major vectors for and carriers of human and livestock disease (Pimentel et al, 2001).

http://coastal.er.usgs.gov/navassa/hike/lh15.html http://wonderclub.com/Wildlife/mammals/brown_rat.html

ECONOMIC IMPACTS cont.

Animal examples

• In Australia, feral cats decimate indigenous bird, mammal, marsupial and amphibian populations. There are ± 3 million pet cats and 18 million feral cats in Australia (Anon, 1996).

• Pet and feral cats are also a problem in SA. On Prince Edward Island, feral cats prey on native birds, causing significant problems with the burrowing petrels (Procellaria sp.) (Huntley, 1996).

•Pigeons cause problems in the UK fouling buildings, statues, cars, and sometimes pedestrians. On farms they consume grains, causing production and economic losses. They are also responsible for transmitting at least 3 poultry diseases (Bevan and Bracewell, 1986). Damages in the UK are ± US$ 270 million per year.

http://college.laboetie.free.fr/parcours/pigeon2.htm http://college.laboetie.free.fr/parcours/punk.htm

ECONOMIC IMPACTS cont.

Cats and pigeons

The brown tree snake (BTS) was accidentally introduced to Guam immediately after World War II when military equipment was moved onto Guam (Fritts and Rodda, 1995). Soon the snake population reached densities of 100 per hectare, dramatically reducing indigenous bird, mammal and lizard populations, causing major problems for small farmers and pet owners. The BTS is thought to have resulted in the extinction of 9 endemic bird species, 5 indigenous lizard species and 2 bat species (Armand, 2000). The snake crawls up utility poles and has caused a total of 1500 power outages on the island, amounting to a cost of US$ 1 million per year. The snake is slightly venomous & causes health problems amounting to US$ 25 000 per year (Pimentel et al, 2001).

ECONOMIC IMPACTS cont.

Brown tree snake

• IAS amount to higher agricultural costs, because of losses, damages and control costs. There are also losses to the livestock industry because of poor forage material – most invasive plant species don’t make for suitable forage, as they are unpalatable and often toxic.

• Aquatic IAS can compete with and reduce commercial fish stocks. Toxic blooms can affect aquaculture. Clogging of water intake pipes and fouling of boats and buoys result in cleaning costs and antifouling painting (BioMare, 2004).

• The zebra mussel, Lantana, kudzu, Brazilian pepper and rats are all economic and ecological catastrophes.

ECONOMIC IMPACTS cont.

More economic examples

• Are invasive species facilitating the success of successive invasions?

• Invasional meltdown is defined as the process by which a group of non-indigenous species facilitate one another’s survival in various ways, increasing the likelihood of survival and/or of ecological impact, and possibly the magnitude of the impact (Simberloff and Von Holle, 1999).

http://www.unb.ca/fredericton/science/biology/Fish_key/Clupeidae/Alewife.htm

http://www.nps.gov/isro/NR_Profile_Internal/NR_stills/fish_imag/pages/rainbow%20smelt_jpg.htm

INVASION MELTDOWN

• Multiple invasions can have a synergistic impact, meaning the problems created are bigger and more dramatic. E.g. the sea lamprey paved the way for two other invaders. By the 1950s, the sea lamprey invasion killed off most populations of indigenous trout in the Great Lakes. The removal of this natural predator paved the way for the explosion of smelt and alewife fish populations, which in turn reduced zooplankton and indigenous planktivores, causing a decline in overall fish productivity in the system.

Invasion meltdown

Another example: African crystalline ice plant invades existing plant communities in California through preferential grazing by animals on native plants. Once it is established, it accumulates salt, which is released when the plant dies, through leaching by fog and rain. The salt then produces an osmotic environment intolerable to the indigenous plants (Vivrette and Muller, 1977). When wind or other disturbance creates openings in the ice-plant carpet, these are colonised by the ice-plant itself, or by other alien plants.

http://www.uib.es/depart/dba/botanica/herbari/generes/Mesembryanthemum/crystallinum/Mesembryanthemum_crystalinum(fl)_f_s.jpg

INVASION MELTDOWN

Invasion meltdown cont.

General model of invasion of alien plants facilitated by mutualisms – “invasional meltdown” (Richardson et al, 2000).

Model of invasion

Links to other chapters

I hope that you found chapter 7 informative and that you will enjoy chapter eight!

Chapter 1 Definitions

Chapter 2 History, globalisation and GMOs

Chapter 3 The human dimension

Chapter 4 Pathways of introduction

Chapter 6 The ecology of biological invasions

Chapter 5 Characteristics of invasive alien species

Chapter 7 Impacts of invasive alien species

Next Chapter 8 Invasive species management

Chapter 10

Chapter 9

Chapter 11

Chapter 12